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NORMAL HEMOPOIESIS IN INTRA-UTERIINe AND NEONATAL LIFE
 
J. R. GILMOUR
 
From the Bernhard Baron Institute of the London Hospital
 
(PLATES VII—X)
 
During a study of the morbid histology of erythroblastosis foetalis
I realised that the literature upon foetal haemopoiesis in man was
insufficient for a reliable control. The classical Works of Van der
Stricht (1891 and 1892), Saxer (1895-96) and Maximow (1909) on
haemopoiesis in small mammals were valuable, but the latter differs
from that in man in certain respects. The present work was
therefore undertaken to establish a control but was extended to
cover early embryonic life and the first weeks of extra-uterine life.
 
MATERIAL
Embryos and foetuses
 
(1) Presomite embryo (A), (Frazer). Ovum 2-1 X 1-95 x 1 mm. (measured
from the chorionic epithelium excluding the rudimentary villi). Amniotic
vesicle 0-17 X0-143 ><0-1 mm. Entodermic vesicle 0-127 ><0-11 mm.
Embryonic plate 0-065><0-045 mm. Age about 16 days, probably slightly
younger than Peters’ embryo (1899). '
 
(2) Presomite embryo (B), (Frazer). Ovum 2-2 X 2-1 X 1-7 mm. Yolk sac
0-225 >< 0-190 mm. Embryonic plate 0-323 mm. Age about 19 days,
probably between the Jung (1908) and Meyer (1924) embryos in development.
 
(3) Presomite embryo (C), (Frazer). Ovum 4-66x4-5 X 3 mm. Amnion
0-55 X0-5 mm. Yolk sac 1-2 X0-75 mm. Embryonic plate 0-55 X0-36 mm.
Age about 19 days, probably between the V011 Spee (1896) and J ones-Brewer
(1935) embryos in development.
 
(4) Embryo of about 20 pairs of somites, (Frazer). Yolk sac only. Age
 
3 to 4 weeks.
(5-43) Embryos and foetuses 3-200 mm. crown-rump, 4-5 weeks to 23
 
weeks. Nine from Professor Frazer-’s collection. Yolk sac only in the 12-5,
 
15-5 and 26-9 mm. specimens.
(44-57) Foetuses 320-546 mm. crown-heel, 24-25 weeks to full term.
 
One post-mature.
(58-68) New-born infants, 2-21 days old, 406-533 mm. crown-heel. Five
 
premature.
The ages of the presomite embryos are derived from Grosser (1924),
 
those of the older specimens from the table of Mall (1910) according to the
crown-rump measurement in embryos and foetuses up to 200 mm., and the crown-heel measurement in longer foetuses. The specimens under 9 mm.
long were measured by serial section when embedded, the others when fresh
or fixed. The specimens are referred to according to their length, whether
crown-rrump (C-R) or crown-heel (C-H).
 
Methods of preparation
 
My specimens were fixed in 4 per cent. saline-formaldehyde, dehydrated
in alcohol, cleared in chloroform and embedded in paraffin.
 
The number of tissues examined and the stains applied varied with each
specimen. Much reliance had to be placed upon Ehrlich’s heematoxylin and
eosin, but most specimens were stained also by Jenner’s method according to
Turnbull (1931). The prussian-blue reaction was done on sections from
most specimens. The tissues examined are mentioned in the description of
haemopoiesis.
 
The 13 Frazer embryos had been treated with clove oil between the
alcohol and chloroform and in some instances a little celloidin had been
 
added to the clove oil. Most of the sections had been stained by Heidenhairfs.
iron haematoxylin.
 
GENERAL DESCRIPTION OF HZEMOPOIESIS, WITH NOMENCLATURE
 
In embryos and foetuses there is a system of mesodermal cells
of many potentialitie corresponding to the reticulo—endothelia1
system of post-foetal life. Within this system in embryos must
be included the vascular endothelium and, to a lesser extent, the
interstitial mesoderm of the yolk sac. From the fixed cells of this
system free cells arise in vessels or solid tissues.
 
One type is‘the haemocytoblast (fig. 1). It is the most primitive cell, whose
potentiality of differentiation is directed to haemopoiesis only. It forms red
blood corpuscles, granular leucocytes, lymphocytes and megakaryocytes.
Its appearance has been described by Turnbull (1934). In foetal and postfcetal life the heemocytoblast does not show evidence of migration but in
early embryos it is frequently amoeboid. Occasionally in blood Vessels and
very often in the connective tissues of early embryos it shows pseudopodia
and in the process of migration in the tissues may appear distorted. Some
of these early haemocytoblasts are slightly smaller and more lymphocytoid
in appearance than those in fig. 1, but still larger than the large lymphocyte.
The early haemocytoblast appears also to be able to form histiocytes. These
slight differences in the early heemocytoblasts do not warrant giving them a
special name. They correspond to the primary wandering cell of Saxer
(1895-96), the lymphocytoid wandering cell of Maximow (1909, 1927) and the
mesamoeboid cell of Minot (1912). Maximow (1927) regarded his lymphocytoid wandering cell as identical with the haemocytoblast and sometimes
called it such.
 
The other type of cell that may arise directly from the reticulo-endothelial
cell is the free kistéocyte. As already mentioned, it appears to arise also from
early heemocytoblasts in embryonic life. It has a round, oval or kidneyshaped nucleus and a moderate or abundant amount of pale-staining cytoplasm which may be vacuolated. Its chief potentiality is to take up Various
substances or cells into its cytoplasm and become a phagocyte. In the
cedematous rarefied mesoderm of the chorion and in places in the embryo
are to be found cells not unlike histiocytes. They differ in having a more
abundant, more Vacuolated pale-staining cytoplasm. These are the same as the Hofbauer cells of the placenta (Hofbauer, 1905). The potentiality
of this cell is not obvious. It is probably connected in some way with the
tissue fluids, as they are numerous in oedernatous tissues.
 
In young embryos the megalca/ryocytes differ from those in older specimens
and I call these early megakaryocytes. They are smaller and have one, two
or three separate nuclei or one bi- or trilobed nucleus. They do not possess
pseudopodia, whereas in later embryonic and middle foetal life those in the
tissues are often surrounded by numerous minute pseudopodia.
 
The nomenclature in connection with the development of the red blood
eorpuscles is, with minor differences, that used by Tumbull (1934).
 
The term erythroblast (fig. 1) is used for any nucleated cell whose potentiality of differentiation is directed only towards the formation of erythrocytes.
It is not used in the sense of Lowit (1886), who introduced the term for
haemoglobin-free precursors of nucleated red blood cells. The term erythrocyte
is used for a nucleus-free cell containing haemoglobin stainable by eosin,
not, as is sometimes used, for an eosinophilic red blood corpuscle Whether
nucleated or not.
 
During the differentiation of the erythroblast the nuclei show all gradations
of diminution in size, condensation of chromatin structure and diminution in
size and number of nucleoli from that of the haemocytoblast, till eventually
the nucleus becomes shrunken and pyknotic. The pyknotic nucleus is then
extruded and the cell becomes an erythrocyte. The erythroblasts may be
subdivided into early, intermediate and late, those with pyknotic "nuclei
having a similar size of cell and nucleus to those in the intermediate or late
stage. Nuclei do not as a rule become pyknotic until they have become
very small and reached the late stage. Sometimes larger nuclei in the
intermediate stage of development become pyknotic (fig. 1, cells 4a and 10a).
During their development, erythroblasts multiply by mitotic division. In a
late stage of development they probably do not multiply and those with
pyknotic nuclei certainly do not. Mitotic division is usually accompanied
by variable shrinkage in size of the cell. The nucleus of the erythroblast is
usually round, but late and pyknotic forms are frequently deforIned-Tu1'nbull’s crenated, moniliform or rosette-like distortion, probably from
lowering of the surface tension between nucleus and cell body, as suggested
by Albrecht (Weidenreich, 1904).
 
During the development of the erythroblasts, haemoglobin stainable by
eosin appears in the cytoplasm. The nucleus is not lost until haemoglobin
is thus made visible. The stage of development of the erythroblast at which
visible haemoglobin appears varies, and accordingly there are two types of
erythrocyte formation. In one type haemoglobination is late and begins in
small erythroblasts with pyknotic or almost pyknotic nuclei and the resulting
erythrocyte is within the normal size for adult blood. This is nownoblaetelc
erythropoiesis and a normocyte is formed (fig. 1, cells 3-8). In the other
type heemoglobination occurs in larger erythroblasts with larger nuclei
having still a Well preserved nuclear structure. This is megaloblastic erythropoicsis (fig. 1, cells 9-13 or 14-18). The resulting erythrocyte is usually a
megalocyte, larger than the normocyte found in normal adult blood, but
during the evolution of a megaloblast so great a reduction in size may occur
that a normocyte is formed. An erythroblast with basophil cytoplasm and
its nuclear structure still preserved can develop along either the normoblastic
or the megaloblastic series. Such a cell has been called by Turnbull a primary
erythroblast. The term had been applied previously by Schridde (1907) to
the erythroblasts in embryos of 1-9 mm., later called by Maximow
“ primitive ”. Since MaximoW’s terminology in this respect has been generally
adopted Schridde’s priority to the use of the term primary erythroblast has been waived. The remaining erythroblasts develop from the primary and
may be called secondary erythroblasts. Schridde used the term “ secondary
erythroblasts ” for those first appearing in the liver in a 12-5 mm. embryo,
corresponding to Maximow’s definitive erythroblasts, and Maximow used
the term as an alternative for his definitive erythroblasts. ' The term is not
now used for these cells and is available for erythroblasts which are not
primary. It is useful to call secondary erythroblasts nomwblasts when they
are scarcely larger than a norrnocyte and develop only into normocytes, and
megaloblasts when haemoglobin is visible in larger cells which will form as a
rule megalocytes but sometimes norinoeytes. The nucleus of the normoblast
is always small and pyknotic or almost pyknotic, while that of the megaloblast
has variable degrees of preservation of nuclear structure (Ehrlich and Lazarus,
1909) or is pylmotic.
 
The normoblasts are small cells with pyknotic or almost pyknotic nuclei
and are subdivided into basophil if there is no visible heemoglobination of the
cytoplasm, and polychromatic or orthochromatic if there is incomplete or
complete heemoglobination as revealed by eosin staining. The normoblast in
foetal tissues which have been dehydrated in alcohol, cleared in chloroform
and embedded in paraffin is not more than 5 [L and the normocyte not
more than 4-5 p, in diameter. They are slightly smaller than the corresponding
cells in adults.
 
Megaloblasts are either polychromatic or orthochromatiti/»——there are no
basophil megaloblasts. The megaloblasts may be divided into early, intermediate and late according to the degree of preservation of nuclear structure,
corresponding to similar stages of the primary erythroblast. The final stage
is one with a pyknotic nucleus and may be called in short a pyknotic
megaloblast (Turnbull, 1934). The megaloblasts are larger than the normoblasts. A late megaloblast can either develop from an intermediate
megaloblast or a late primary erythroblast. An intermediate megaloblast
can either develop from an early megaloblast or from an intermediate
primary orythroblast. As has been mentioned, a megaloblast during maturation of its nucleus may show considerable reduction of its cytoplasm, so that
when the nucleus is pyknotic the cell is small and identical with a normoblast.
This may be referred to as megalo-normoblastic erythropoiesis (fig. 1—change
from cell 12 to cell 7——and fig. 2).
 
After the first two or three weeks of post-foetal life normal erythropoiesis
is entirely normoblastic. In embryoni.c life "it is entirely megaloblastic ;
in early foetal life it is megaloblastic and megalo-normoblastic, while in later
foetal life it is partly megaloblastic and partly normoblastic.
 
The terms primitive and definitive erythroblaets are used in the sense of
Maximow to describe the erythroblasts of two distinct erythropoietic families
in the embryo. The term primitive erythroblast was first used by Bryce
(1906) for the first erythroblasts in Lepidosiren paradoxa but these differ
from those in man. As has been mentioned, Schridde used the terms primary
and secondary erythroblasts for the cells Maximow later called primitive
and definitive erythroblasts. The primitive erythroblasts (fig. 1, cells 14-17)
are first formed in the yolk sac and later are present and multiply in the blood
of early embryos but soon disappear. The definitive erytlmoblasts are
formed in the yolk sac and embryo later than the primitive. There is no
essential difference between the two types of erythroblasts. The primitive
erythroblast and the megalocyte it forms are, however, as a rule considerably
larger than the definitive forms, though small varieties of about the same
size as the latter occur. Late and pyknotic late primitive erythroblasts
often swell up to a large size, apparently by imbibition of fluid, and usually
have an elliptical shape. The nuclei of the primitive erythroblasts usually
PLATE VII
 
FIG. 1.--Red blood cells and their precursors. 1 and 2, haemoeyteblaets ; 3, 4 and
5, early, intermediate and late primary erythroblaets; 6 and 7, basophil and
erthochrematic normoblasta; 8, normocyte; 9, 10, l1 and 12, early, intermediate, late and pyknotie megaloblaate; 13, megalocyte ; 14, 15, 16 and 17
early, intermediate, late and pyknotic primitive megaloblaets; 18, primitive
erythrocyte; 4a, basophil normoblaet With prematurely pyknotie nucleus;
10a, megaloblaet with prematurely pyknotie nueleue. Jenner.
 
FIG. 2.-—-125—rm:n. foetus. Focus of megalo-normoblaetic erythropoieeis in neck.
One early and several intermediate megaloblaete forming pyknotic megaloblaets
and—--with 105 of nucleue-—a Inegalpcyte, and——from shrinkage in size——orthochrematic normoblaete and-—by loss of nucleue—-—a norrneeyte. Jenner.
JOURNAL OF PATHOLOG-Y——VOL. LII PLATE V11
 
 
FIG. 2 .
 
differ slightly from those of the definitive. They are paler and the cl'J.romatin
threads more delicate in the primitive early and intermediate erythroblasts
than in the definitive, and nucleoli may be seen in the primitive intermediate
erythroblasts but never in the definitive. The primitive erythroblasts are all
heemoglobinated and megaloblasts. The cytoplasm is usually more deeply
stained by eosin than in definitive secondary erythroblasts. Primitive
erythroblasts arise from hsemocytoblasts, probably directly. I have not been
able to recognise primitive primary erythroblasts.
 
The primitive erythroblasts were first seen by Erb (1865), who likened
them to the blood cells of the frog because of their great size and elliptical
 
shape. Howell (1891) also recognised them and called them ancestral
oorpuscles because they resembled those of reptilia and amphibia.
 
DEVELOPMENT or BLOOD VESSELS AND CELLS BEFORE A COMPLETE CIRCULATION Is ESTABLISHED (EMBRYOS UP TO 3 MM.)
 
The origin and development of the first blood vessels is very
obscure owing to the small number of well preserved and stained
very young human embryos available for study by any one observer
and the scant attention given to the vascular development in
many of the original descriptions. The three presomite embryos
of Frazer which I have examined are by themselves insufficient
for me to draw conclusions ; the following account therefore takes
into consideration the observations in the literature.
 
The presomite embryos that will be mentioned are the Teacher-Bryce
no. 1 (1908)*, Miller (1913, quoted by Streeter, 1920), Linzenmeier (1914)*,
Frazer (A), Peters (1899), Mollendorff (l921)*, Fetzer (l9l0)*, Sch1agenhauferVerocay (1916), Tea-cher-Bryce no. 2 (1908)*, Jung (1908), Frazer (B), Meyer
(1924), Strahl-Beneke (l9l0)*, Herzog (1909), von Spec (1896), Frazer (C),
Jones-Brewer (1-935), Minot (1912), Streeter (1920), Grosser (1931), I-Ieuser
(1932), Grosser (1913), Ingalls (1918), McIntyre (1926-28), Eternod (1898-99),
Boerner-Schwarzacher (1923), von Spec (1889) and Triepel (1917). They
are placed in order of development according to Grosser (1924), if included
in his list, and according to the individual author in specimens subsequently
described. I have placed the Frazer embryos according to their probable
development. The somite embryos are those of Ingalls (1920) 2-3 pairs of
somites, Jagerroos (1934) about 6 pairs of somites, Dandy (1910) 7 pairs of
somites, Low (1907-08) 13-14 pairs of somites, 2-6 mm., Thompson (1906-07) 23
pairs of somites, 2'5 mm. and Frazer, about 20 pairs of somites. Descriptions
of other embryos were examined but found to be of no value.
 
Yolk sec
 
The first vessels arise from solid cellular masses which have
proliferated from the mesothelium. The latter is the single layer
of flattened mesodermal cells which first covers the yolk sac after
the formation of the chorionic cavity. They had not appeared in
the Teacher—Bryce (no. 1), Linzenmeier or Fetzer embryos according
to McIntyre, the Miller embryo according to Streeter, Peters’s
 
* Quoted by McIntyre (1926-98).
 
embryo according to Lewis (1912) or the Jung or Herzog embryos.
A single mass of this kind is first seen in the Frazer presomite
(16-day) embryo. In this the yolk sac consists of two layers of
endothelial-like cells, the inner entodermal, the outer mesodermal.
On the ventral aspect of the sac there is a solid rounded proliferation
of mesodermal cells about 4 cells wide, projecting about 40 p. high
into the chorionic cavity (fig. 3). This is probably angeioblastic.
 
‘Mesodermal cell masses were present in the Mollendorif, Teacher
Bryce (no. 2) and Strahl-—Beneke embryos according to McIntyre
and in the Schlagenhaufer—Verocay embryo. They are present in
the Frazer presomite 19-day embryos. Here the masses project
as papillae into the chorionic cavity. I believe vessels arise from
the masses by loss of cells in the centre while the peripheral cells
become endothelial. From the endothelial cells hmmoglobin-free
basophil cells arise and become free in the lumen. These are
haemocytoblasts. Isolated vessels containing free cells constitute
the blood islands of Pander (1817), observed by him in chick
embryos. One of the Frazer presomite 19-day embryos appears
to be the youngest with blood islands. In it several proliferations
of mesodermal cells on the ventral aspect of the yolk sac project
as papillae from the outer surface. Some papillae contain empty
spaces, like vessels, some of which are lined with flattened cells
(fig. 4). The spaces appear to have arisen by vacuolation and
loss of cells in the centre of the papillae. Deeper in the mesoderm
beneath the papillae are a few well formed endothelial-lined vessels.
Four of these contain groups of free cells in the lumen and constitute
blood islands (fig. 5). The cells are basophil and most can be
identified as haemocytoblasts. Others cannot be identified because
of crowding. The cells probably arose from the vessel wall, since
some plump endothelial cells are present as if in process of becoming
free. It appears certain that blood vessel precedes blood cell
formation. Connections between the different spaces and vessels
could not be made out but in the absence of reconstructions they
cannot be excluded.) In the Frazer presomite 19-day embryo (0)
similar appearances are present except that the papilla and blood
islands are more numerous and occupy about the ventral half of
the yolk sac. Further, while some of the free cells are typical
haemocytoblasts others are slightly smaller and more lymphocytoid.
Blood islands are present in the Meyer (1924) and von Spee ( 1896)
embryos and are probably constant in all older embryos. The
haemocytoblasts differentiate into haemoglobinated primitive megaloblasts, but a few persist and a few apparently differentiate into
histiocytes. The J ones-Brewer embryo is the youngest containing
haemoglobinated cells; Bloom ( 1938) describes some of the free
cells as haemoglobinated primitive erythroblasts. When haen1o—
globinated cells become constantly present is diflicult to estimate.
 
 
 
 
FIG. 3.——Embryo 1, Frazer, presomite (A). Small mass of
proliferated mesodermal cells
on Ventral pole of yolk sac.
I-Ieidenhain’s iron heeInatoxy
'1 _ I lin. X570.
 
FIG. 4.—-Embryo 2, Frazer,
presomite (B). Papillary
projections of mesoderm
on ventral part of yolk
sac, some containing
 
spaces. I-Ieidenhairfs
iron haematoxylin.
X 480.
 
FIG. 5.——-Embryo 2, Frazer, prosomite (B). Blood island in mesoderm of ventral part of yolk sac.
I-Ieidenhairfs iron haematoxylin.
x 750.
 
They are present in the McIntyre and probably in all older embryos
but are undoubtedly present in some of the embryos intermediate
in development between the J ones-Brewer and McIntyre, such as
the Minot, Grosser (1931) and Heuser embryos. The persistence
of hsemocytoblasts after the formation of erythroblasts is mentioned
only in the Jones—Brewer embryo by Bloom (1938), and in the
Minot embryo, where they are called mesamoeboids. They are
also present in the Frazer embryo of about 20 pairs of somites.
Their presence, however, can be presumed in embryos of intermediate
development. The first appearance of histiocytes is apparently in
the Frazer embryo of about 20 pairs of somites, but no doubt they
are present in younger somite and perhaps some presomite embryos.
In this embryo the yolk sac in rather more than its ventral half
shows numerous vessels distended with cells (fig. 6). The vessels
project from both the outer and inner aspects of the sac, especially
the former. The great majority of the cells are haemoglobinated
early and intermediate megaloblasts, and less numerous late and
pyknotic late megaloblasts, all of the primitive type. The cells
stain deeply eosinophil with haematoxylin and eosin, but Jennerstained sections are not available to show the degree of
chromatophilia. Many are in mitosis. Besides these there are
a very few hsemocytoblasts and phagocytic histiocytes. The
vessels are probably united to each other to a considerable degree.
The blood vessels and islands first appear at the ventral pole
of the yolk sac and later involve the ventral half of the sac.
Fewer and smaller islands may be found at a later date in the
other hemisphere of the sac. Little can be concluded about the
union of the separate vessels to form a net. Probably union of
blood islands occurs early but the net is not complete till late,
possibly not till the yolk sac vessels have joined the embryonic.
The above view of the histiogenesis of blood islands differs
from that of Jones and Brewer, Streeter, and McIntyre. In their
embryos they describe mesodermal cell masses in which central
cells become in part free cells while the peripheral cells become
endothelial. McIntyre describes syncytial masses of mesodermal
 
cells the centres of which become haemoglobinated and subsequently
break up into free cells.
 
Okorion and body stalk
 
In the chorion, Bremer’s theory (1914) of the histiogenesis of vessels has not been disproved.
 
He states that the Vessels are derived from the mesothelium of the body stalk. The mesothelium is at first limited to the yolk sac, as in the Frazer presomite (16-day) embryo, but later spreads as a continuous or interrupted
layer over the body stalk and finally lines the chorionic cavity completely.
From the mesothelium funnel-shaped ingrowths pass into the body stalk mesoderm. By closure of the lumen. of a funnel at some point near its opening
into the chorionic cavity, isolated spaces lined with mesothelium are formed
inside the body stalk. The space may remain connected by solid cellular
cords with the orifice of the funnel or with the surface mesothelium. From
the inner ends of funnels or from the spaces formed from them, multiple
nets of tubes pass out into the body stalk and extend into the remainder of
the chorion. The tubes have no definite endothelial lining and he calls them
unli_ned spaces. By a process which he calls delamination, solid cellular
cords—-his angioblastic cords_arise from the walls of the unlined spaces and
lie in the lumen. Angioblastic cords may also grow out from the inner ends
of the mesothelial funnels and no_t be enclosed in unlined spaces. At an early
stage spaces, called angiocysts, may form in places in the angioblastic cords
and later a continuous lumen changes the cords into vessels. The separate
nets unite to form a continuous vascular net in the body stalk and remainder
of the chorion. His theory was formed from a study chiefly of the Grosser
(1913), Minot, and Herzog embryos. McIntyre found similar structures in
his own and the Teacher-Bryce (no. 2) embryo and Grosser in his 1931
embryo.
 
The scantiness of the material I have examined does not permit
me to dispute Bremer’s theory but two observations appear to
disagree with it. Firstly, angioblastic activity is first present in
the Frazer presomite (16~day) embryo. The chorionic mescderm
is in general poor in cells, stellate and spindle, but adjacent to
the embryo, in the part that would become the body stalk, it is
more cellular than elsewhere and contains a few small groups,
either syncytial or bounding very small spaces. There are also a
few strands of elongated cells, in single or double layer, which pass
out into the neighbouring mesoderm. One of the strands (fig. 7),
separated from the chorionic cavity by only a very few cells, has
developed a lumen and resembles a capillary. As the mesothelium
is limited to the yolk sac these angioblastic structures probably
arose from undifferentiated mesodermal cells. Secondly, in the
Frazer presomite ( 19-day) embryos, lined capillary-like empty
tubes are present in the body stalk and chorion, very few in the
first and many in the second embryo. Some strands two cells
wide without lumina are also present but no solid cellular cords
enclosed in unlined spaces. In the body stalk of these embryos a
few larger end0thelial—lined spaces are also present. On the other
hand in the Frazer embryo of about 20 pairs of somites, in addition
to lined capil1ary—like empty tubes, there are several unlined spaces
containing short cellular cords either solid or with minute lumina.
 
Angioblastic activity in the body stalk and chorion is absent from the
Teacher-Bryce (no. 1), Linzenmeier and Fetzer embryos according to
McIntyre, from the Miller embryo according to Streeter and from the Peters
and Schlagenhaufer-Verocay embryos. In the Mollendorff embryo according
to McIntyre there are channels lined with flattened cells in the chorionic
mesoderm which do not join up to form a continuous system. Angioblastic
activity is present in the Teacher-Bryce (no. 2) embryo according to McIntyre
and is probably constant in older embryos.
 
Heemopoiesis occurs in the body stalk independently of that
in the yolk sac. In the present state of knowledge this cannot
be said to be constant. In the Minot, Grosser (1931), Ingalls (1918),
1VIcIntyre and Triepel presomite embryos and Ingalls’s (1920) embryo
of 2-3 pairs of somites and in Dandy’s embryo of 7 pairs of somites
free cells are present in some body stalk vessels which have no
connection with yolk sac vessels. The cells are probably for the
most part haemoglobinated primitive erythroblasts but the presence
of haemoglobin is 11ot stated in all instances. It is probable that
the cells arose from heemocytoblasts derived from the endothelium
but their histiogenesis is not described. Dandy and McIntyre
describe blood cell formation in chorionic vessels apparently apart
from the body stalk. J agerroos in his embryo of about 6 pairs of
somites describes blood cells in isolated chorionic vessels, the cells
having been formed extravascularly and subsequently included in
vessels. These are however only isolated observations of haemo
'poiesis in the chorion apart from the body stalk. Besides the
 
intravascular formation of blood cells in the body stalk the Minot,
Heuser, Grosser (1913), McIntyre and Dandy embryos are said to
show blood islands consisting of groups of cells apparently free in
the body stalk mesoderm. McIntyre describes the cells as haemoglobinated and perhaps haemoglobin was present in the cells of
some of the others. The nature of these so-called blood islands
is very doubtful as they are very poorly described. I doubt whether
they represent an extravascular formation of blood cells derived
from the mesodermal cells. I think also that the term blood
island should be reserved for blood cell formations inside isolated
vessels. If blood cells were present inside body stalk vessels then
perhaps some of the extravascular groups of cells could be explained
by continued multiplication of cells which had escaped from the
vessels. This undoubtedly occurs in older embryos as I will
describe. In the J ones—Brevver and Boerner—SchWarzacker embryos
blood islands are described in the body stalk but their relation to
vessels is not mentioned.
 
The embryo
 
The number of observations on the histiogenesis of vessels
Within the embryo itself is few. The appearances in von Spee’s
(1889, 1896) and Triepel’s presomite embryos and Ingalls’s (1920)
embryo of 2-3 pairs of somites suggest that cardiac and vascular
endothelium arises in situ from mesodermal cells. Ingalls believed
that the vessels arise in multiple sites. In the present state of
knowledge it must be presumed that blood islands consisting of
blood cell formation in isolated vessels do not occur. The blood
island Ingalls describes in the posterior end of the right aorta of his embryo of 2-3 pairs of somites is an isolated observation
and the nature of the cells and their mode of origin are not
described.
 
The establishment of the circulation
 
It is certain that an umbilical circulation between the body
stalk and the embryo is established before a vitelline between
the yolk sac and the embryo. This is seen in Eternod’s presomite
embryo, somewhat precociously, and in Dandy’s. The blood in
the circulation I was probably derived from blood islands in the
body stalk. In Ingal.ls’s embryo of 2-3 pairs of somites a similar
condition is developing, the yolk sac vessels being still unconnected
with the embryo while the right umbilical artery communicates
with the right aorta. Whether or not this is the rule cannot be
stated but there are no observations of the establishment of the
vitelline before the umbilical circulation. Both umbilical and
vitelline circulations were established in Low’s embryo of 13-14
pairs of somites (2-6 mm.) and Thompson’s embryo of 23 pairs
of somites (2-5 mm.). At this time it may be presumed that the
cells of the blood are for the most part derived from the yolk sac
but they will have joined cells from the body stalk in embryos
with blood islands in this site. In a few embryos some may have
come from chorionic vessels apart from the body stalk and if
Ingalls’s observation is correct some may have been derived from.
the embryo itself. It is not possible to state the time when the
chorionic vessels fill with blood but it is probably about the time
of the formation of the complete circulation.
 
Summary
 
It may be concluded that vessels arise from mesodermal cells independently in three areas, the yolk sac, the chorion——-perhaps at first limited to the body stalk—and the embryo. The vessels
in each of these areas unite to form nets or systems. The three
systems later unite with each other and the complete circulation
is established. In some if not all embryos the circulation between
body stalk and embryo begins before that between yolk sac and
embryo. Blood islands form constantly in the yolk sac and consist
of separate vascular units containing blood cells. The cells are
first hsemocytoblasts which arise from the vessel wall. Later the
cells are almost entirely primitive erythroblasts but a few haemocytoblasts persist and a few differentiate into histiocytes. Intravascular blood formation occurs frequently, perhaps constantly
in the body stalk, and when the umbilical but not the vitelline
circulation has been established it will supply the embryonic
vessels with blood. Doubtful blood formation has been recorded
in the chorionic and embryonic vessels and in the mesoderm of
JOURNAL OF PA’I‘HOLOGY—VoL. Ln PLATE IX
 
NORMAL HIAEMOPOIESIS
 
FIG. 6.—-—Enr_1b1-yo 4, Frazer, about 20 pairs of somites. Blood vessels in yolk sac
' full of primitive erythroblasts. H. and E. Dufay process. X 550.
 
FIG. 8.—-10-mm. embryo (Frazer). Two haemocytoblasts among epithelial cells of
yolk sac. Primitive erythroblasts in yolk sac vessel. Eht-lich’s haernatoxylin
and eosin. Dufay process. X 1100.
NORMAL H EM OPOI ESI S 35
 
the body stalk. The cytology and histiogenesis of blood formation
outside the yolk sac at this period has unfortunately been neglected
or very inadequately described.
 
This theory of the histiogenesis of blood cells and vessels is in opposition
to that of His who in 1900, according to Bloom, deri.ved the blood cells and
vessels from a special angioblastic tissue which arose from the yolk sac
ontoderm. Minot (1912) accepts this theory and states that the angioblast
maintains its independence throughout life. According to this theory the
embryonic and chorionic vessels are extensions from those of the yolk sac.
Minot derived the cells of the reticulo-endothelial system from the angioblast but the work of Maximow has shown that these cells are mesodermal
and their potentialities can be assumed by undifferentiated mesodermal cells.
 
DEVELOPMENT on THE BLOOD AFTER ESTABLISHMENT OF THE COMPLETE CIRCULATION (EMBRYOS FROM 3 To 12 MM.)
 
In embryos of 3-9 mm., after the circulation is established,
new blood cells are formed everywhere in the circulation equally
by mitotic division of blood cells previously present. Perhaps to
a very slight extent erythroblasts are formed by differentiation
of haemocytoblasts in the circulation but the great majority are
derived from preformed erythroblasts. At this stage almost all
the blood cells are primitive erythroblasts in early, intermediate,
late or pyknotic late stages: most are intermediate. With
haematoxylin and eosin they appear fully haemoglobinated but
Jenner sections are not available to show the degree of chromatophilia. A very few erythroblasts are binucleate. Very rarely one
may be seen with vacuolated cytoplasm. Mitoses are numerous.
In a 3—mm. embryo the erythroblasts average about 7-5 pt in
diameter. The other embryos of this group (3-9 mm.) belong to
Professor Frazer’s collection and in them the erythroblasts average
about 10-5 p, in diameter. A few are small-—about 6 p.—or very
large——about 16 u. The difference in average diameters is probably
due to difl°erent methods of embedding. Some of the small
erythroblasts might form erythrocytes within the normal variation
of diameter for adult blood, 23.6. normocytes. This would constitute
megalo—normoblastic erythropoiesis. Knoll (1929) describes a small
proportion of definitive erythroblasts in the blood at this stage,
calling them second generation cells, but these are undoubtedly
only mall primitive erythroblasts. Besides the erythroblasts there
are a few megalocytes, histiocytes—~—some phagocytic-———and haemoeytoblasts, most of which are slightly smaller and more lymphocytoid
than the typical hsemocytoblast- The histiocytes contain nuclear
fragments or brown granular pigment. In the aorta, groups of
histiocytes may be present. The cells forming a cluster in the
aorta of a 9-4.—mm. embryo of Minot (his figure 368) I would call
histiocytes but he calls them mesamoeboids, which I believe correspond to heemocytoblasts. In the Frazer 4-5~mm. embryo
(fig. 11) there are many more heemocytoblasts and histiocytes in
the yolk sac vessels than in the embryonic vessels but in the other
embryos at this stage this diiference is very slight.
 
In all the embryos of this stage (3-9 mm.) the chorionic vessels
are filled with blood. In all, primitive erythroblasts are scattered
singly or in groups in the mesoderm of the embryo, chorion and
umbilical cord. These undoubtedly result from hsemorrhage, but
continued multiplication occurs, as is shown by the presence of
mitotic figures. Haemorrhage is present in the yolk sac cavity in
two and chorionic cavity in one.
 
The Frazer 10wmm. embryo is similar as regards the circulating
blood and the presence of haemorrhages in the mesoderm, but there
is, in addition, activity of the reticulo—endothelial system, chiefly
directed to the formation of a new haemopoietic family, the definitive.
Hsemocytoblasts, isolated or in groups, appear in the liver among
the liver cells outside the sinusoids. Some lie in deep bays (lacunae
of Neumann, 1874) within liver cells. In the sinusoids besides
primitive erythroblasts there are a few haemocytoblasts and one
early trilobed megakaryocyte is seen, while scattered or in groups
are numerous histiocytes, many containing brown granular pigment
or less commonly nuclear fragments or degenerated primitive
erythroblasts. In the yolk sac are a few haemocytoblasts, singly
or in groups, among the large entodermal epithelial cells (fig. 8).
Some appear to lie in lacunae, as in the liver. Two early megakaryocytes are also present among the epithelial cells. In the yolk
sac vessels, besides primitive erythroblasts and a few haemocytoblasts
and histiocytes, there are a very few early megakaryocytes, groups
of early and intermediate primary erythroblasts and intermediate
megaloblasts, all of definitive type.
 
The brain and spinal cord are surrounded by a zone of very
delicate connective tissue rich in young capillaries. These contain
histiocytes and heemocytoblasts of the early, smaller and more
lymphocytoid type in greater number than the general circulation.
A few of the heemocytoblasts have small pseudopodia. Some of
the histiocytes contain brown granular pigment, one a pyknotic
primitive megaloblast. Some early forms of megakaryocytes are
also present. Some of these are no larger than a hsemocytoblast,
others are 2-3 times as large. They have one or two nuclei which
are round, oval or bilobed. The cytoplasm is eosinophil and as a
rule abundant. Outside the capillaries there are many amoeboid
haemocytoblasts——some with pseudopodia, many histiocytes———some
pigmented, and a few early megakaryocytes. This activity is
especially marked at the base of the brain.
 
In the Frazer 12-mm. embryo the distribution is similar except
that extravascular heemopoiesis is more extensive in the liver.
NORMAL H EM OPOI ESI S 37
 
Besides many hsemocytoblasts there are numerous cells of the
definitive series, namely early and intermediate primary erythroblasts, some intermediate and fewer early and late megaloblasts
 
and a few early megakaryocytes. The latter lie outside and inside
the sinusoids.
 
Summary
 
In embryos of 3-9 mm. the blood cells in the vessels are of the
primitive series ; almost all are formed by mitosis of erythroblasts
previously formed ; some of the erythroblasts are of the diameter
of normoblasts. A few erythroblasts, have reached the tissues by
haemorrhage and multiply there by mitosis. In embryos of 10-12
mm. the blood vessels still contain cells of the primitive series but
in the liver and yolk sac there is now active proliferation of
haemocytoblasts, early megakaryocytes and erythroblasts of the
definitive series.
 
FURTHER DEVELOPMENT on THE BLOOD (SPEOIMENS OVER 12 MM.)
 
(1) General circulation, excluding capillaries
 
In the 18- and 19-5-mm. embryos the blood cells are almost
entirely erythroblasts and erythrocytes in about equal numbers.
Of the former, all or almost all are primitive, all are megaloblasts
and the majority orthoohromatic ; a very few are early, many are
intermediate but the majority late or pyknotic. Mitotic figures
are numerous. A few heemocytoblasts, histiocytes and phagocytic
histiocytes are also present. The first are usually slightly smaller
than typical hsemocytoblasts and are lymphocytoid in appearance
but larger than large lymphocytes. A few show small pseudopodia.
 
In the 26- and 28—mm. embryos about a quarter of the red blood
corpuscles are nucleated. They are megaloblasts and the majority
are primitive. A very few are early, a few intermediate, the
majority late or pyknotic. Mitotic figures are very sparse. Haemocytoblasts are absent but there are a few lymphocytes which were
probably formed directly from haemocytoblasts. Histiocytes are
absent. In the 35-min. embryo the blood is similar except that
there are no early and only a few intermediate megaloblasts and
only one mitotic figure was seen.
 
In one of the 48—mm. embryos about 10 per cent. only of the
red blood corpuscles are nucleated. Most are late and pyknotic
primitive megaloblasts. The great majority are fully haemoglobinated; a few of the definitive are polychromatic. Mitotic
figures are absent. Besides these cells there are a very few
lymphocytes and neutrophil myelocytes and leucocytes.
 
In the 65-mm. and all subsequent embryos the blood cells
appear to be definitive. Not more than 5 per cent. of the red
cells are nucleated. Most are late and pyknotic megaloblasts, a few are intermediate megaloblasts and normoblasts (4-5 p. in
diameter). A few are polychromatic. Mitotic figures are absent.
There are a Very few lymphocytes and neutrophfl myeloeytes and
leucocytes. In the 7 6—mm. embryo the blood is similar except
that a Very few intermediate primary erythroblasts are present.
A very few of these are in mitosis. One eosinophil leucocyte
was seen.
 
In the lO4.—mm. embryo the blood is similar except that now
there are Very few early primary erythroblasts : a mitotic figure
in a megaloblast was seen. In the 120- and 125-mm. embryos the
blood is the same except that no early primary erythroblasts are
present.
 
In the 146—mm. embryo a film of fresh blood from the umbilical
cord shows about 0-6 per cent. of the red cells to be nucleated.
In 200 nucleated cells 5 per cent. are intermediate, 18 per cent.
late and 34-5 per cent. pyknotic megaloblasts, 10 per cent. are
normoblasts (not larger than 8 )u. in diameter), 2 per cent. inter~
mediate primary erythroblasts, 30 per cent. lymphocytes, 1 per
cent. eosinophil and 0-5 per cent. neutrophil myelocytes. Almost
all the erythrocytes are megalocytes, a few are normocytes (6-5
to 7-5 p, in diameter). Some of the nucleated and non-nucleated
red cells are polychromatic. Two megaloblasts showed mitotic
figures. Some leucocytes are present in the blood in sections but
not in the film.
 
In the 170-mm. embryo the blood in sections is similar to that
in the 120- and 125-mm. embryos, except that only a very few
intermediate megaloblasts are present and primary erythroblasts
and mitoses are absent. In longer foetuses the blood was not studied
in detail. There appeared to be no marked alteration and probably
there was a gradual change to the state at birth.
 
(2) Yolk sac
 
In the Frazer 12-5-mm. embryo the vessels contain a few early,
some late and pylmotic and many intermediate megaloblasts and a
very few megaloeytes, all of the primitive series. A very few
and small vessels contain small groups of early and intermediate
primary erythroblasts and intermediate megaloblasts of the definitive
generation. A very few small haemocytoblasts are also present:
one early bilobed megakaryocyte was seen. Only three haemocytoblasts were seen, lying outside vessels among the epithelial
cells.
 
The Frazer 15-5-mm. embryo is similar except that a few late
megaloblasts are present in the few small groups of definitive
erythroblasts. Six early megakaryocytes and one histiocyte
containing a pyknotic megaloblast were also seen. Amongst the
epithelial cells outside vessels are a few groups of about a dozen cells consisting of early and intermediate primary erythroblasts
of the definitive generation and one or two heemocytoblasts.
 
The 26°9-mm. embryo is similar except that there are now
numerous megalocytes. There are several histiocytes and phagocytic histiocytes but no megakaryocytes. The histiocytes contain
pyknotic nuclear fragments or nucleated red cells : one contained
brown granular pigment. A few scattered groups of early and
intermediate primary erythroblasts with a very few intermediate
megaloblasts are present outside vessels among the epithelial cells.
 
In a 48-mm. embryo both the mesoderm and entoderm of the
yolk sac are very atrophic and the wall of the sac is usually very
thin. The blood in the vessels consists of megalocytes with a few
late and pyknotic megaloblasts and a very few lymphocytes. There
are no histiocytes and no megakaryocytes. There are now no foci
of heemopoiesis in the vessels or among the epithelial cells.
 
(3) Liver
 
In an 18-mm. embryo there is a conspicuous increase in amount
of haemopoiesis from that in the l2—mm. Frazer embryo and the
blood formation is definitive. The second 18-mm., 19-5- and 26mm. (fig. 9) embryos show further increases and the maximum
amount found is in the last. The liver now shows great diffuse
infiltration with hsemopoietic cells, with numerous foci of increased
density of infiltration. There are numerous haemocytoblasts, early
and intermediate primary erythroblasts and intermediate 1negalo—
blasts, and a few late primary erythroblasts and early, late or
pyknotic megaloblasts. The primary erythroblasts and haemocytoblasts outnumber the megaloblasts. The cells for the most
part lie in the parenchyma outside the sinusoids and many,
especially heemocytoblasts, lie in bays or lacunae in the liver cells.
Similar cells are found in the sinusoids, scattered or in small groups
among the Inegalocytes or megaloblasts of the primitive series.
A few eosinophil and neutrophil myelocytes and leucooytes and
megakaryocytes lie within thesinusoids or scattered outside in the
parenchyma. The portal systems are not formed but some neutro~
phil and eosinophil myelocytes and leucocytes are present in the
connective tissue of the hilum.
 
The degree of haemopoiesis relative to the parenchyma in the
26~mm. embryo is maintained in the 20 longer specimens up to
190 mm. in which the liver was examined. The heemopoiesis shows
slight changes with increasing age. The early megaloblasts soon
disappear and towards the end of this group the late megaloblasts
outnumber the intermediate. Primary erythroblasts and heen1ocytoblasts still outnumber the haemoglobinated cells. Orthochromatic
normoblasts (about 4-5 pr. in diameter) appear in scanty numbers
in the 48-mm. and longer embryos. Basophil normoblasts do not appear until the 190—mm. embryos and are then very scanty. In
embryos of 48-180 mm. inclusive, therefore, the normoblasts are
derived from megaloblasts by considerable shrinkage of cytoplasm
during development, that is, by megalo —norm oblastic erythropoiesis.
The amount of leucopoiesis in the parench yma shows little increase,
but in the connective tissue of the hilum it is increased and it is
abundant in the portal systems, which are flrst well developed in
the 48—mm. embryos. In six specimens of 65-190 mm., focal
erythropoiesis similar to that in the parenchyma is present in the
portal systems. In six specimens of 76-190 mm., scattered tissue
mast cells lie in the portal systems and in four (65-190—mm.) foci
of lymphocytes are also present there. In the 170-mm. foetus a
very few tissue mast cells lie free in the sinusoids.
 
In five foetuses of 200-457 mm. (fig. 10) a progressive decrease
in the amount of heemopoiesis relative to parenchyma is apparent.
It is still diffuse, with foci of increased density, but the density of
the infiltration becomes progressively less. Intermediate and late
primary erythroblasts increase relatively to the early forms and
late megaloblasts outnumber intermediate forms. Basophil and
orthochromatic normoblasts are scanty. The leucopoiesis remains
the same in the portal systems and parenchyma. A few scattered
tissue mast cells are present in the sinusoids of the 444- and
457-mm. foetuses and in the portal systems of all. In the 200-,
343— and 457 -mm. foetuses foci of erythropoiesis, and in the 330-,
343- and 457-mm. foetuses foci of lymphopoiesis, occupy portal
systems.
 
In nine older (470-546-mm.) foetuses the haemopoiesis in the
liver has decreased conspicuously (fig. l 1). It is new entirely
focal. In the parenchyma outside the sinusoids are to be found
widely separated small foci of intermediate and late primary
erythroblasts, occasionally including some early forms or normoblasts and late and pyknotic megaloblasts. Some foci consist
entirely of normoblasts and late and pyknotic megaloblasts. A
very few heemocytoblasts are present outside the sinusoids. In
the sinusoids are a few isolated or small groups of primary
erythroblasts, normoblasts or late megaloblasts and a few isolated
heemocytoblasts. A very few eosinophil myelocytes and leucocytes
are present in the parenchyma or in the sinusoids. Megakaryocytes,
constantly present in the younger foetuses, are now found in only
3 of the 9 older specimens and then in very scanty numbers.
Haemopoiesis is considerably reduced in the portal systems. A
few scattered or small groups of lymphocytes and eosinophil
myelocytes and leucocytes and a few scattered tissue mast cells
are present in most systems. In two specimens (508 and 533 mm.)
one or two tissue mast cells lie free in sinusoids. A 546 mm. foetus
is considerably post-mature but the amount of haemopoiesis is the
 
 
PLATE X
 
 
FIG. 7.—Embryo 1, Frazer, presornite (A). Ca.pi11a.ry- FIG. 9.—~26 mm. embryo. Amount
like structure in chorionie mesoderm HeideI1hai11’s of hzmnopoiesis in liver. Ehr1ich’s
haematoxylin and oosin.
 
X 155.
 
iron haematoxylin. X 440.
 
FIG. 10.—-444-mm. foetus. Amount of hzaemopoiesis FIG. 1l.—546—mm. fmt-us. Amount
Ehrlic-h’s hae.xI1a.t0xy11'n and eosin.
 
of hwmopoiesis in liver. Ehrlich’s
 
in liver.
X 155.
 
haematoxylin. and cosin.
 
x I55.
 
same as in the others of the group. In all but two (495 and 540
mm.), the liver cells are to a variable degree water-clear. This is
undoubtedly due to glycogenic infiltration.
 
In the 1 1 newly born infants the amount of haemopoiesis in
the liver shows a considerable diminution from that in late foetal
life. A very few leucocytes and lymphocytes are present in some
portal systems in most cases. In the 4-day infants and one of the
5-day infants some portal systems contain numerous myelocytes
and leucocytes. Two or three small foci of late primary erythroblasts and normoblasts are present outside the sinusoids in the
2-day, one of the 3-day and the 4-day infants. In the remainder,
including all infants over five days, erythropoiesis is absent. In
the. 4- and 15-day infants a few usually degenerate -looking
megakaryocytes are present. The water-clear appearance of the
liver cells seen in the full-term foetuses is absent in the infants.
A very few small bile thrombi are present in dilated intercellular
canaliculi in one of the 5- and one of the 14-day infants, neither
of which was jaundiced. Bile thrombi are not present in the
liver of the 4-day infant, which was jaundiced.
 
Free iron is present in the liver cells of all the embryos of 18 mm. and
over except two. It is present either in the form of brown granular haemosiderin or as a diffuse prussian blue reaction. Both forms occur together
except in the 70-mm. embryo, in which granules are absent. Only a minority
of liver cells are affected. In very young embryos, before the formation of
portal systems, the iron is localised as a rule to the cells around veins. In
older subjects the usual site is in the cells bordering the portal systems and
the cells of the bile ductules near their junction with the liver columns.
The iron may be confined to these sites or may also occupy scattered cells
anywhere in the lobule. In a few specimens in which intercellular bile
canaliculi are sharply outlined, the iron favours that part of the liver cell
which lies between the canalicular lumen and the nucleus. The total amount
of intracellular iron is never great and does not show much variation in
different subjects. However in nine, all 28-17 O-mm. specimens, the amount of
iron is more abundant than in the remaining 30 livers examined. Towards
the end of pregnancy (457 mm." onwards) iron is relatively scanty. In seven
infants of 2-15 days the amount of iron in the liver cells is in keeping
with that in late foetal life ; in the remainder, it is slightly more
abundant.
 
Several foetuses of 146 mm. or more show some intra- and extracellular
haemosiderin granules in the portal systems.
 
Iron pigment is present in a variable number of Kupffer cells in all but
three (80-, 190- and 343-mm.) embryos and foetuses. It is usually in the form
of granular hsemosiderin, but in some it shows as a diffuse prussian blue
reaction in the cytoplasm. In all but two (2- and 4-day) infants the Kupffer
cells contain more iron than in foetal life. The cells are often enlarged and
sharply defined by a deep prussian blue reaction in the cytoplasm. Granules
or globules of iron pigment are also present in some. In the other three
infants the amount of iron in the Kupffer cells is about the same as in foetal
life. This intense iron staining of Kupifer cells is probably related to the
normal post-natal heemolysis.
 
Phagocytic Kupffer cells containing erythrocytes, nucleated red cells or
free nuclei are present in the sinusoids of all the livers of embryos, foetuses
and infants. Sometimes only one, sometimes many such cells are seen.
The number in infants does not differ from that in foetuses.
 
(4) Connective tissue, including its capillaries
 
In the two 18-mm. embryos and in the 19-5 mm. embryo, foci
of a few intermediate and more late and pyknotic primitive
megaloblasts and megalocytes are scattered in many connective
tissues. These are undoubtedly similar to those seen in the earlier
embryos and probably resulted from multiplication of primitive
megaloblasts which had escaped from blood vessels. These foci
are also present in the 26-, 28~ and 35-mm. embryos, but contain
increasing numbers of erythrocytes, so that nearly all the cells in
the last embryo are erythrocytes.
 
In four embryos of 18-26 mm. many amoeboid haemocytoblasts,
megakaryocytes and some histiocytes, most of them containing
brown granular iron pigment and a few containing free nuclei or
nucleated red cells, lie scattered in connective tissues. They are
most numerous in delicate tissues such as the very vascular
meninges, especially at the base of the brain. The megakaryocytes
are usually early, with one, two or three nuclei or bi- or trilobed
nuclei, and rarely have pseudopodia. Similar cells are present
in greater number in the meningeal capillaries than in the general
circulation. Several megakaryocytes are present in the capillaries
of various tissues and organs throughout these embryos. The intravascular megakaryocytes do not have pseudopodia. There are also
a few megakaryocytes, haemocytoblasts and histiocytes within the
lumen of and in the tissue around small veins forming plexuses in the
retroperitoneal tissue and neck in the two 18- and the 19-5—mm. but
not in older embryos. In the 28- and 35—mm. embryos the capillaries
and connective tissue of the meninges contain fewer of these cells,
but iron-containing phagocytes are still found outside the capillaries.
In the 48-mm. embryos these cells are absent from the meninges,
while megakaryocytes are absent from the connective tissues and
capillaries elsewhere and are confined to special sites—-—liver, marrow,
spleen, lymph glands and stroma of lymph plexuses. In some
connective tissues, especially the loose tissues of the neck, some
amceboid haemocytoblasts and iron—containing histiocytes are
present in decreasing numbers in foetuses up to 190 mm. Thereafter no special examination was made of the connective tissues
in general.
 
In one of the 18-mm. embryos in the dense connective tissues
of the head below the brain there are a few small scattered groups
 
of heemocytoblasts and early primary erythroblasts of the definitive
series. In the other 18-mm. embryo in the same site, especially about the eyes and cranial nerves, are more numerous and larger
groups of either hsemocytoblasts and early primary crythroblasts
alone or of intermediate and late megaloblasts alone or of mixtures
of these cells. Many groups of megaloblasts with occasional
intermediate primary erythroblasts are present in the meninges at
the base of the brain. In the 19-5-mm. embryo there are similar
groups in the meninges and a few elsewhere ; one was seen in the
mesentery and two near the vertebral column. In the 26- and 28mm. embryos there are several scattered foci of primary erythroblasts
of all stages, some having in addition intermediate and late megaloblasts, and more numerous foci of intermediate, late and pyknotic
megaloblasts. Most lie in the tissues of the head and neck below
the brain, fewer in the meninges at the base of the brain and in
other tissues such as the choroid plexus, mesentery and limbs.
In six 35-125—mm. specimens small foci of intermediate and late
primary erythroblasts alone, or of intermediate, late and pyknotic
megaloblasts alone or mixtures of these cells, are present in
connective tissues in various places. In this group a very few
orthochromatic cells of 4-5 p. diameter with pyknotic nuclei and
thus identical with normoblasts are present in association with
megaloblasts in embryos of 48 mm. or longer. As there are no
basophil normoblasts, they undoubtedly arose from megaloblasts
(megalo—normoblastic erythropoiesismfig. 2). In the 170- and one
of the 190—mm. foetuses the number of erythropoietic foci in the
connective tissues is considerably reduced and only a very few
are present, consisting of late and pyknotic megaloblasts and
orthochromatic normoblasts, the latter derived from the former.
In the other 190-mm. and longer foetuses only special organs were
examined, but probably erythropoiesis in connective tissues soon
disappeared, as the foci in the 170- and first of the 190-mm. foetuses
are very few and composed of erythrocytes and secondary erythroblasts only ; there are no groups of primary erythroblasts to suggest
recent formation of foci.
 
In five 18-28-mm. embryos a few eosinophil and neutrophil
myelocytes and leucocytes lie scattered or in small groups in
various connective tissues, especially the meninges, mesentery and
retroperitoneal tissue. In the longer specimens up to 190 mm.
it is rarer to find similar cells scattered in any connective tissue.
 
In five 26-48—mm. embryos a few small haemocytoblasts and
eosinophil and neutrophil myelocytes and leucocytes lie in the
adventitia of a few arteries in various parts of the body. In six
65-125-mm. specimens a few lymphocytes are present as well.
Similar cells are also present in the carotid bodies of some of these
foetuses. In the 170- and one of the 190-mm. foetuses these
infiltrations are absent.
 
(5) Lymph plexuses
 
In one 18 -«mm. embryo a few neutrophil and eosinophil
myelocytes and leucocytes and amoeboid haemocytoblasts are
present in the connective tissue of developing lymph plexuses
in the neck, and a few haemocytoblasts and histiocytes lie in blood
capillaries therein. In the 19 '5 — mm. embryo this infiltration is
increased and the stroma of cervical lymph plexuses shows many
eosinophil and neutrophil myelocytes and leucocytes, amoeboid
haemocytoblasts, histiocytes—some containing brown granular iron
pigmcnt—and a few megakaryocytes. A few hjstiocytes and
haemocytoblasts are also present in the blood capillaries. In seven 26-75 - mm. embryos there are also a few lymphocytes.
 
Histiocytes and haemocytoblasts are not present in the capillaries
in the lymph plexuses in the 48-mm. and longer embryos. In five
28—125—mm. specimens a very few small foci of erythropoiesis
occupy the connective tissue of cervical lymph plexuses. In the
 
I 170- and one of the 190-mm. foetuses there is no special haemopoietic activity in the plexuses.
 
(6) Lymph glands
 
In one 48 mm. embryo definite lymph glands have formed and
consist of local proliferations of stellate and spindle cells in the
connective tissue stroma of lymph plexuses. The areas of pro1ifera~
tion are infiltrated with lymphocytes, amoeboid haemocytoblasts
and a very few histiocytes and eosinophil and neutrophil myelocytes
and leucocytes. A very few lymphocytes have entered the lymph
vessels. In the 65- and 70-mm. embryos the glands are further
developed and lymphocytes have increased greatly. They now
contain numerous myelocytes and leucocytes and a few megakaryo—
cytes. Some lymph vessels contain numerous lymphocytes. In
the 75-, 76- and 120—mm. embryos several of the glands contain in
addition small foci of late megaloblasts and normoblasts or of
intermediate primary erythroblasts and late megaloblasts. In the
125 — mm. foetus leucopoiesis is considerably less, and in seven
17 0-546-mm. foetuses it is still less, amounting to a few eosinophil
and fewer neutrophil myelocytes and leucocytes in the sinuses or
gland tissue, especially in the peripheral parts. Megakaryocytes
are absent in one 190 - mm. and longer foetuses examined. One
small focus of megaloblasts is present in a lymph gland in the
125- and 170—mm. foetuses but all erythropoiesis is absent from
glands in one of the 190-mm. and longer foetuses. A few tissue
mast cells lie free in the sinuses or in the gland tissue in four
470-546—mm. foetuses.
 
Erythrophages are rare in lymph glands in foetal life. Two
ZVTORMAL H EMU OPOI ESI S 45
 
were present in sinuses in one 546 mm. foetus. Some free sinus
cells in full—term foetuses contain diffuse brown pigment which does
not give the prussian blue reaction.
 
('7 ) Marrow
 
A clavicle was examined in 13 specimens (18-120 mm.), a femur
in 26 (18-540 mm.), a humerus in 9 (18-75 mm.) and a rib in 8
(1041-5340 mm.). Heemopoiesis made its first appearance in the
skeletal system in the clavicle in the 43—mm. embryo. It then
appeared in the humerus in the 57-mm. embryo and in the femur
in the 75-mm. embryo. Cytological studies were difficult because
of the effect of decalcification upon staining.
 
In the 43-min. embryo in one place between capillary sinuses
in the loose connective tissue marrow of the clavicle there are a
very few amoeboid heemocytoblasts and neutrophil myelocytes and
leucocytes and in another place a few am oeboid haemocytoblasts
alone. In the clavicular marrow of the 48-mm. embryo a few
hsemocytoblasts, lymphocytes and neutrophil myelocytes and
leucocytes and one or two histiocytes are present. In the clavicle
of the 57 —mm. embryo a few eosinophil myelocytes and leucocytes
and a small focus of definitive erythroblasts, apparently intermediate
primary I erythroblasts and megaloblasts, are present in addition.
In the humerus of this embryo there is a small group of amoeboid
haemocytoblasts. In the clavicle in the 65—mm. embryo haemopoiesis
has developed considerably, forming larger and denser areas between
the sinuses, most dense around the arteries. There are many
haemocytoblasts, numerous eosinophil and neutrophil myelocytes
and leucooytes, a few early and intermediate primary erythroblasts
and intermediate and late megaloblasts and a few lymphocytes.
There are also a few scattered well developed megakaryocytes.
In the "3'5—mm. foetus the marrow in the clavicle, humerus, femur,
radius, ulna, tibia, fibula and pelvis presents a similar appearance.
The haemopoietic tissue occupies only part of the marrow and in
the long bones there is no haemopoiesis for quite a distance from
the cartilaginous epiphyses. The areas of haemopoiesis, although
often diffuse between the sinuses, tend to surround the developing
arteries. In older foetuses the maximum density is reached in the
160~mm. foetus. The cytology of the marrow was not followed.
It is probable, however, that erythropoiesis was of the same type
as elsewhere, becoming in part normoblastic in middle foetal life
and entirely normoblastic in post-foetal life.
 
In the femur in six foetuses of 80—170 mm. no heemopoiesis is
present for distances of 0-8 to 1-1 mm. from the zones of provisional
calcification of cartilage. In the 320- and 343—mm. foetuses hwmopoiesis reaches to 0-3 mm. from the zone. In five foetuses of 444-540 mm. it reaches up to the zone but is less dense and in
places absent in the metaphysis just below the zone. In the rib
the haemopoietic marrow has not reached the zone in one 508and in the 540—mm. foetus. No haemopoiesis has occurred in any
specimen in primary medullary spaces, periosteum or chondral
canals. Towards the end of foetal life some adipose tissue cells appear in the marrow in the lower half of the femur in some
specimens.
 
(8) Spleen
 
The spleen was examined in 28 embryos and foetuses and in
7 infants. It is first present in the 28-mm. embryo. In the 28and 35-mm. embryos it consists of a mass of undifferentiated cells.
In the 35—mm. embryo one or two early megakaryocytes are present
in capillaries in this mass. In four specimens of 55-80 mm. the
spleen consists of vessels surrounded by spindle cells, lying in a
pulp of widely meshed delicate connective tissue with an occasional
recognisable sinus. In the pulp is a lake of blood. A few early
or well formed megakaryocytes are present, usually in sinuses.
In the lake of blood are a very few haemocytoblasts, eosinophil
myelocytes and leucocytes, a few orthochromatic normoblasts
and scattered megaloblasts, usually late and pyknotic, but a few
intermediate. The erythroblasts were probably not formed in s-itu
from precursors but carried in from the blood stream and probably
most are definitive. Five specimens of 70-162 mm. show in addition
a few scattered foci of definitive erythropoiesis, some in sinuses,
most in the pulp. They consist of early and intermediate primary
erythroblasts with occasionally one or two haemocytoblasts, or of
 
intermediate and late and sometimes one or two early megaloblasts,
 
or of primary erythroblasts and megaloblasts mixed. A few
normoblasts formed from megaloblasts are also present. In the
146-mm. foetus a very few heemocytoblasts are present in the cellular
connectivet issue around the arteries. In the 162—mm. foetus
heemocytoblasts are more numerous and many lymphocytes have
been formed from them, constituting small Malpighian bodies. In
the remaining foetuses there is very little variation. The total
amount of haemopoiesis is always slight and near the end of
pregnancy is very slight. Early megaloblasts are absent and
intermediate forms less numerous. Megakaryocytes decrease but
one or more are always‘ found. In the 470-mm. and longer foetuses
a very few neutrophil myelocytes and leucocytes have appeared
in the pulp. A few tissue mast cells lie in the trabeculae in the
444- and 533—mm. and in the pulp in the 457 - and 533-mm. foetuses.
The spleen at full term shows a few scattered heemocytoblasts,
intermediate and late megaloblasts, normoblasts, eosinophil (and
fewer neutrophil) myelocytes and leucocytes, a few small groups of primary erythroblasts and one or two megakaryocytes, often
appearing degenerated. These cells lie chiefly in the pulp but are
also present in the sinuses. Quite frequently intermediate megaloblasts were absent from the liver at full term but were readily
found in the spleen.
 
In five infants of 3-5 days a few myelocytes, leucocytes, normoblasts and late megaloblasts are present scattered in the pulp.
In one 14-day and in the 15-day infant a few leucocytes but no
myelocytes, megaloblasts or normoblasts are present. One megakaryocyte was seen in a 3-day infant but none in the other four.
 
Erythrophages are visible in the pulp in a third of the embryos
and foetuses and in some of the infants. Never numerous, they
are difficult to see because of the large number of red blood cells
free in the pulp.
 
In eleven 55-190-mm. specimens and in the 320- and 330-mm. foetuses, free
iron pigment is present in the pulp in small amount, either free or in reticulum
cells in the form of haemosiderin granules and spheres. In one 190- and in the
343-mm. foetus no iron pigment is present. In the 444-mm. and older
foetuses iron is present in granules or globules or diffusely in the cytoplasm
of reticulum cells and sometimes in sinus cells. It varies greatly in amount.
In four cases (470-508 mm.) the amount is abundant, but in three others
(495-546 mm.) it is very slight. This considerable variability in iron content
in older foetuses probably. depends upon the degree of congestion of the
pulp. In infants the amount of iron varies considerably, as in older foetuses,
and probably for the same reason. In one of the 3-day infants and in the
15-days infant the amount is slight. In three infants of 3-14 days it is very
abundant and in two of 4 and 5 days a moderate amount is present.
 
(9) Kidneys
 
The kidneys were examined in 26 embryos and foetuses and
in 7 infants. In the 104-mm. foetus one small group, and in three
foetuses of 146-180 mm. a few small groups of primary erythroblasts
and megaloblasts or of megaloblasts and normoblasts are present
about the tubules in the upper part of the medulla. In one 190
mm. foetus several very small foci of late megaloblasts and normoblasts and one or two myelocytes lie around tubules and vessels
in the intermediate zone between cortex and medulla. In the
200—mm. fcetus two foci, and in the 320- and 495-mm. foetuses
single small foci of erythropoiesis occupy the intermediate zone.
In one 3-day infant there is one focus of normoblasts in the
peripelvic tissue ; in the very premature 15-days infant there are
three in the connective tissue of the intermediate zone.
 
In one of the 180-mm. foetuses and in the 200-mm. foetus a few
h2em0siderin—containing phagocytes are present in the connective tissue of the
upper part of the medulla and intermediate zone. These are probably the
result of slight extravasations of blood. No free iron is present in the
epithelium of the kidney in any specimen.
 
(10) Thymus
 
This was examined in 22 embryos and foetuses. In the 28-mm.
embryo a few amoeboid haemocytoblasts infiltrate the otherwise
epithelial thymus. In the 35-mm. and longer specimens lymphopoiesis is established. Leucopoiesis begins in the 55-mm. embryo,
and in this and in the four specimens of 75-120 mm. is shown by
the presence of a Very few small groups of eosinophil promyelocytes
and myelocytes in the interlobular septa. In the 70- and 140-mm.
specimens, besides these cells there are a few eosinophil myelocytes
in the medulla and a few tissue mast cells in the capsule and interlobular tissue. In the 125—mm. foetus a few eosinophil myelocytes
and leucocytes are present in the capsule and interlobular tissue
and in the cortex‘ ; two mast cells were present in the capsule.
In the 162-mm. foetus leucopoiesis has considerably increased.
There are numerous eosinophil and a few neutrophil myelocytes
and leucocytes and a few promyelocytes in several interlobular
 
septa, while many eosinophil myelocytes and leucocytes infiltrate
the medulla adjacent to the septa and a few lie in the cortex.
 
Tissue mast cells are also present in the interlobular septa and
capsule. In eight foetuses of 17 0-540 mm. there is similar
leucopoiesis.
 
In eleven specimens of 28-170 mm., but not in longer, a little
erythropoiesis is represented in the younger specimens by a few
small groups of intermediate and late megaloblasts and in the
older by late megaloblasts and normoblasts. The groups lie in
 
the interlobular septa or in the perithymic tissue. Erythropoiesis
is never present within the thymic tissue.
 
SUMMARY OF DIFFERENT TYPES OF BLOOD CELLS FROM THEIR EARLIEST APPEARANCE
 
Hcemocytoblasts and histiocytes
 
Heemocytoblasts first arose from the endothelium of the yolk
sac Vessels and were first seen in the Frazer embryo (B). For a
time they were the only free cells in the blood islands but soon
most differentiated into erythroblasts—first in the Jones—Brewer
presomite embryo, a few into histiocytes. The latter were first
seen in the yolk sac vessels in the Frazer embryo of about 20 pairs
of somites but were probably present in some younger embryos.
After the establishment of the circulation, hwmocytoblasts and
histiocytes, some of them phagocytic, persisted in the yolk sac
vessels and were present in the general circulation up to the 196mm. embryo ; a few histiocytes however persisted in the yolk sac
vessels in the 26-9—mm. embryo. In the 26—mm. and longer
 
specimens the heemocytoblasts had become smaller and were
identical with lymphocytes.
 
 
Beginning in the 10-mm. embryo and persisting up to the
35-mm. embryo there was a concentration of haemocytoblasts and
histiocytes in the tissues around the central nervous system,
especially at the base of the brain. Similar cells were present
in many connective tissues—especial.ly loose tissuesmin embryos
of 18-190 mm. and in and about small veins forming plexuses in the
neck and retroperitoneal tissue in the 18- and 19-5-mm. embryos,
in the connective tissue in lymph plexuses in embryos of 18-75
mm. and in the capillaries of this connective tissue in embryos
of 18-48 mm. Heemocytoblasts were also present in the adventitia
of scattered small arteries in embryos of 26-125 mm. They were
present in the developing lymph glands in embryos of 48 mm.
or more and were associated with histiocytes. Some haemocytoblasts appeared in the connective tissue of the clavicular marrow
in the 43—mm. embryo. Infiltration of the epithelial thymus by
heemocytoblasts occurred in the 28-mm. embryo. The haem0cytoblasts in these sites differentiated into the various blood cells.
In certain sites such as connective tissue, especially that around
the central nervous system and in lymph plexuses, the haemocytoblasts appeared also to form histiocytes. In young embryos
hsemocytoblasts migrating in connective tissues or the thymus were often distorted from their usual more or less rounded shape and often showed pseudopodia. Small pseudopodia were also occasionally seen on the rounded forms in the blood.
 
Erythropoiesis
 
In early embryos the erythroblasts belonged to the primitive
family. They appeared first in the J ones-Brewer presomite embryo
in the blood islands of the yolk sac. They were megaloblasts and
were derived directly from haemocytoblasts without passing through
a stage comparable with the primary erythroblasts in the definitive
family. According to the literature erythroblasts are also formed
in many older presomite and younger somite embryos in isolated
vessels in the body stalk and possibly in a few embryos in the
chorion. Blood islands are also described in the mesoderm of the
body stalk in a few embryos and in one embryo one island was
found in the embryo itself. Since the cellular content and histiogenesis of these formations were not described they must be regarded
as very doubtful. With the establishment of the circulation the
primitive erythroblasts entered the embryonic vessels and continued
to multiply there. With increasing age the number of early and
intermediate primitive erythroblasts decreased and the late and
pyknotic late forms and megalocytes increased, accompanied by
diminished multiplication. Mitotic division of primitive erythroblasts had ceased in the 35-mm. embryo, and in the 48-mm. embryo
these cells were all in a late or pyknotic late stage. In older embryos no primitive erythroblasts were recognisable in the blood,
but primitive megalocytes undoubtedly persisted longer. In
embryos of 3-35 mm. multiplication and development of primitive
erythroblasts continued in places in many connective tissues, the
original cells having reached the tissues by haemorrhage. In the
35mm. embryo most of these cells had developed into megalocytes
and the erythroblasts were all late. Apart from this observation
and the doubtful description in the literature of blood islands
in the mesoderm of the body stalk, primitive erythropoiesis was
essentially intravascular. Primitive erythropoiesis was also
essentially megaloblastic but in embryos of 3 mm. or more a very
few small erythroblasts of the size of normoblasts had been formed
from megaloblasts (megalo-normoblastic erythropoiesis).
 
Definitive erythropoiesis appeared first in the 10-mm. embryo
in the yolk sac vessels as groups of primary erythroblasts and
megaloblasts. These groups persisted but had disappeared in the
48—mm. embryo. Heemocytoblasts had appeared among the hepatic
and yolk sac epithelial cells in the 10-mm. embryo but extravascular
definitive erythropoiesis did not begin in the liver until the 12-mm.
embryo and in the yolk sac till the 15-5-mm. embryo. In the
yolk sac it was still present in the 26-9- but had disappeared in
the 48—mm. embryo. In the liver it increased in amount and
reached a maximum relative to the parenchyma in the 26-mm.
embryo. This maximum persisted until in foetuses of 200-457 mm.
there was a progressive decrease to a slight amount which remained
constant in foetuses of 470 mm. and more. In infants 5 or more
days old such erythropoiesis was absent. Definitive erythropoiesis
was also present in embryos and foetuses of 18 mm. or more in the
sinusoids of the liver, in amounts that were relatively slight but in
proportion to the amounts outside the sinusoids. In some foetuses of 65-457 mm. focal definitive erythropoiesis was also present in the connective tissue of the portal systems. In many connective
tissues it was present focally in specimens of 18-190 mm. and
probably did not persist in longer foetuses. The sites of election,
especially in the younger embryos, were the dense tissue at the
base of the brain and in the upper part of the neck, the delicate
tissue surrounding the central nervous system, especially at the
base of the brain, and the mesentery, but any connective tissue
in the body or limbs sometimes showed it. In the spleen focal
definitive erythropoiesis in the pulp and to a less degree in the
sinuses began in the 70-mm. embryo and first disappeared in
infants over 5 days old. It was always slight and decreased in
late foetal life. In the connective tissue of the marrow definitive
erythropoiesis began in the clavicle in the 57—mm. embryo and
subsequently increased considerably in amount. In the connective
tissue in lymph plexuses it formed a few foci in some specimens of 28-125 mm. It was also present in slight amount in a few lymph
glands of a few foetuses of 75-170 mm. In the kidney a few foci
were found in some foetuses and infants of 104 mm. or more and
were last observed in the 15-days-old infant. In the thymus
there were a few foci in the connective tissue of the capsule and
interlobular septa in some specimens of 28-170 mm. In the blood
definitive megaloblasts were present in small numbers in the 28and 35-mm. embryos and were almost as numerous as the primitive
in the 48—mm. embryo. All erythroblasts in the blood were definitive
in the 65~mm. and older specimens. In foetuses of 76-146 mm.
there appeared to be exceptional erythropoietic activity in the blood,
as some intermediate and in one a few early primary erythroblasts
appeared and mitotic figures reappeared. In the circulation primary
erythroblasts were not seen previously nor subsequently.
 
TABLE
Stages of erythropoiesis
 
Stage of v§;%1;;1}e%a"%Er%%i%r Within or outside Heemopoietic Type of _
development tissues? vessels family erythropoiesls
J ones-Brewer preso- In blood islands in Intravascular: pos- Primitive Megaloblastic
mite embryo till yolk sac, in some sibly extravasestablishment of cases in body stalk cular in body stalk
circulation (about and possibly rarely in a few cases
2-5 mm.) in the chorion or '
the embryo
From establishment In general circulation Intravascular except Primitive Megaloblastic
of circulation (of embryo, yolk for. continued
(about 2-5 mm.) to sac and chorion) multiplication of
9 mm. cells in memorrhages
10 mm. up to less In embryo, yolk sac Intra- and extra~ Partly primitiv e, Megaloblastic
than 48 mm. a n d g e n e r a l vascular (marrow partly definitive in
circulation first in 57-mm. eneral circulation;
embryo) cfinitive elsewhere
48 mm. to between 15 I n t r a c 0 r p or e a 1 , Extravascular, except Definitive Partly megaloblastic,
and 21 days after in many organs and sinuses of liver and partly megalo-nob
birth tissues spleen and multi- moblastic (48-190
plication of cells in mm.); partly me- i
c i 1‘ c 111 a t i o n ('76- galoblastic, partly
160 mm.) normoblastic (190
mm. onwards)
21 days or more after I n t r a c o r p o r e a 1 , Extravascular Definitive Normoblastic
birth in marrow only
 
 
It is safe to state that after the 15th day of life erythropoiesis is confined to the marrow. Definitive erythropoiesis was essentially
extravascular apart from that in the sinusoids of the liver and
sinuses of the spleen and that in the circulation in foetuses of
76-146 mm. It was entirely megaloblastic in specimens up to
48 mm. and partly megaloblastic subsequently up to the infant
of 5 days old. In specimens of 48-180 mm. orthochromatic but
no basophil normoblasts were present in some sites and had been
formed from megaloblasts (megalo-normoblastic erythropoiesis).
Such formation of normoblasts occurred also in older foetuses to
a less degree. In the 190-mm. and older specimens basophil normoblasts were present so that the erythropoiesis became in
part truly normoblastic. With increasing age the number of
earlier forms of megaloblasts decreased. Early megaloblasts were
very rarely seen after 48 and never after 125 mm. and towards the
end of foetal life the intermediate megaloblasts usually disappeared.
The last secondary erythroblasts seen in the tissues examined
microscopically were in the kidneys of the 15-day infant. They’
were normoblasts and it is probable that erythropoiesis in the
marrow is, as found by Turnbull (1934), entirely normoblastic in
older infants and children as in adults.
 
Erythropoiesis can be divided into five stages in embryos,
foetuses and post-foetal life according to its site and character.
The stages are summarised in the table (p. 51).
 
Leucopoiesis
 
Leucopoiesis with the formation of eosinophil and neutrophil
myelocytes and leucocytes was first seen in one of the 18 mm.
embryos in the parenchyma of the liver and in various connective
tissues such as the meninges, mesentery and stroma of lymph
plexuses. It persisted up to birth, never in great amount, in the
hepatic parenchyma and in various connective tissues up to 190
mm., and in the lymph plexuses up to 125 mm. It was fairly
abundant in the hilum of the liver in early embryos such as the
18 mm. and in the portal systems in 48--495—mm. foetuses, after
which it became reduced. It was present in some portal systems
in infants up to 5 days old, and a very few leucocytes but no
myelocytes persisted in the older infants. A little was seen in the
adventitia of a few scattered arteries in 26-125-mm. specimens.
It was present in lymph glands from 48mm. embryos up to fullterm foetuses, fairly abundantly in the 65- and 70-mm. embryos,
but at other times in scanty amount. It began in the interlobular
septa of the thymus in the 55-mm. foetus and in the glandular
tissue in the 7 0-mm. foetus ; it was scanty but increased considerably
in amount in the 162-mm. foetus and subsequently remained about
the same until birth. In the spleen slight leucopoiesis was present
in infants over 5 days old. Leucopoiesis began in the clavicular marrow in the 43-mm. embryo and subsequently increased con
siderably. Leucopoiesis in the yolk sac, as observed by Maximow
(1927) and figured by Bloom, was not seen. Myelocytes and
leucocytes first appeared in the blood stream in scanty numbers
in the 48-mm. embryo.
 
Lymphopoiesis
 
Lymphopoiesis began in the connective tissue of lymph plexuses
in very scanty amount in the 26—mm. embryo and in the adventitia
of a few arteries in the 65-mm. embryo and was continued in these sites until the 125-mm. embryo. It began in lymph glands in the
48-mm. embryo, at which time lymphatic vessels contained some
lymphocytes. It began in the thymus of the 35-mm. embryo and
in the Malpighian bodies of the spleen in the 162-mm. foetus. It
was present in some portal systems in the liver in several foetuses
of 65-457 mm., and some portal systems contained a few lymphocytes in older foetuses and infants. Lymphocytes were present in
the marrow in embryos of 48-65 mm. and probably persisted but
could not be recognised because of the effect of decalcification on
staining and because of the abundance of erythropoiesis and
leucopoiesis. No lymphopoiesis occurred in the yolk sac. In the
blood, lymphocytes were first seen in the 26-mm. embryo and they
probably arose directly from the haemocytoblasts in the circulation.
 
Formation of megakaryocytes
 
Intravasoular and a little extravascular formation of megakaryocytes was present in the yolk sac in the 10-mm. embryo.
Intravascular but not extravascular formation continued in the
12-5- and 15-5—mm. embryos but not in older. Intravascular or
extravascular formation of megakaryocytes occurred in the liver
of all specimens from 10 mm. up to 457 mm. but in only 3 of 9 older
foetuses. Megakaryocytes, usually degenerated, were present in the
liver of 3 of the 12 infants, the oldest being 15 days. In the spleen
they were constant in specimens from 35 mm. to birth; at full
time they were often degenerated ; only one was seen after birth,
in an infant of 3 days. They were constantly present in the marrow
of foetuses of 65 mm. or more. In the capillaries and connective
tissues they were especially numerous at the base of the brain,
where they were first seen in the 10-mm. embryo ; in the rest of
the body and limbs they appeared in the 18-mm. embryos; in
both sites they persisted to the 35—mm. embryo. Some were
present in small veins forming plexuses in the neck and retroperitoneal tissue in the 18- and 19-5—mm. embryos, in the connective
tissues in lymph plexuses in specimens of 195-125 mm. and in
lymph glands in foetuses of 65-170 mm. No megakaryocytes were
seen in the general circulation in any specimen. In the early
embryos (10-35 mm.) the megakaryocytes usually differed from
the typical form of adults in being smaller and having one, two
or three separate nuclei. These early forms did not have the
pseudopodia which were often abundant in the well developed
forms of early and middle foetal life.
 
SUMMARY
 
The development of the blood vessels and cells until a complete
circulation has been established is described from 3 presomite and
1 somite embryo kindly lent by Professor J. E. S. Frazer, supplemented by 30 presomite and somite embryos selected from the
literature. The subsequent development of the blood cells in the
Vessels and tissues is described from the examination of 53 embryos
and foetuses ranging from between 4 and 5 weeks to full term,
and of 11 new-born infants 221 days old. The development of
histiocytes and phagocytes and the distribution of iron are included.
 
I wish to thank Professor H. M. Turnbull for help in the preparation of
this paper. I am deeply indebted to Professor J. E. S. Frazer for permission
to examine some of his young embryos.
 
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J UNG, P. J. . . . . . . Beitrage zur friihesten Ei-Einbettung beim
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1912, vol. ii, p. 295.
 
Low, A. . . . . . . . J. Anat., 1907-08, X111, 237.
 
LCWIT, M. . . . . . . Sitzungsber. d. Kais. Akad. d. Wissen. z.
Wien, Abt. III, 1886, Xcii, 22.
 
MALL, F. P. . . . . . . In Keibel and Ma11’s Manual of human embryology, Philadelphia and London, 1910, vol. i, p. 199.
 
MAXIMOW, A.
 
3!
 
MCINTYRE, D.
MEYER, P. .
Mnsroaz, C‘. S. .
 
NEUMANN, E .
PANDEB, C.
 
PETERS, H.
 
SAXER, F. . . . . .
SOHLAGENHAUFER AND VEROGAY
SCHBIDDE, H. .
VON SPEE, F. GRAF .
 
33 3.’!
 
STEEETEE, G. L.
 
VAN DER STRICHT, O.
 
35 93 93
 
THOMPSON, P.
TBIEPEL, H. . .
TURNBULL, H. M.
 
)9
 
WEIDENEEICH, F.
 
Arch. mikros. Arlal., 1909, lxxiii, 444.
 
I rl V011 M<'5llendorff’s Handbuch der mikr0skopischen Anatomic der Menschen, Berlin,
1927, V01. ii, part I, p. 232.
 
Trans. Roy. Soc. Ealinb., 1926-28, lv, 77.
 
Arch. Gymilc., 1924, cxxii, 38.
 
I re. Keibel and Ma.11’s Manual of human
embryology, Philadelphia and London,
1912, vol. ii, p. 498.
 
Arch. Hcilla, 1874, xv, 441.
 
Beitriige zur Entwickelungsgeschichte des
Hiihnchens irn Eye, Wilrzburg, 1817.
 
fiber die Einbettung des menschlichen
Eies, Leipzig and Vienna, 1899-.
 
Anal. Hefle, 1895-96, Vi, 347.
 
Arch. Gyra.alc., 1916, cv, 151.
 
Verh. cllsch. path. G'es., 1907 (1908), xi, 360.
 
Arch. Anal. Phys-iol., Anat. Abt., 1889,
p. 159.
 
Ibid., 1896, p. 1.
 
Contributions to embryology, Carnegie Institution of Washington, 1920, ix, 389.
 
Arch. Biol., 1891, xi, 19.
 
Ibid., 1892, xii, 199.
 
J. Anal, 1906-07, xli, 159.
 
Anal. Hcftc, 1917, liv, 149.
 
This Journal, 1931, xxxiv, 277.
 
I rl The anaemias, by J. M. Vamtghan,
London, 1934, p. 8, (2nd ed., 1936, p. 11).
 
Ergcbn. Anal. E’nlwiclccl., 1904, xiv, 345.
 
 
[[Category:Draft]]

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