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Common Traits in Development and Structure of the Organs Originating from the Coelomic Wall

PETER GRUENWALD

Department of Anatomy, The Chicago Medical School, Illinois

FOUR PLATES (TWENTY-SIX FIGURES)

INTRODUCTION

It has long been known that the lining of the coelomic
cavities differs from other epithelia during early periods of
development by its close structural and genetic relations to
the underlying mesenchyme. A persistence of the respective
potencies permanently distinguishes the coelomic epithelium
from other tissues of similar structure, as pointed out in
Maximow and Bloom’s text-book, and in more detail by
Maximow (727 a,b). It is the principal purpose of the present
report to demonstrate that the tissues originating from this
mesothelium in the embryo retain, permanently or for a
limited period of time, one important character of their parent
tissue: a combination of epithelial and mesenchymal properties and potencies. Changes from one of these structural
types to the other can be produced in many tissues with varying case under experimental conditions, such as those of tissue
culture. The mesothelium and its derivatives, however, undergo such changes more readily than most other tissues, even
under normal or only slightly altered conditions. This accounts for many peculiarities in their development, structure,
and pathology, some of which will be discussed in this report.

It is difficult if not impossible to give a clear-cut definition
of epithelial arrangement of cells. Most text-books of histology stress the close connection of the cells, with but little
cement substance present between them. Consequently, the

353
304 PETER GRUENWALD

occurrence of lattice fiber networks surrounding individual
eclls will be considered as indicating non-epithelial arrangement. In addition, it will be seen that the presence of lattice
fibers in cell groups resembling epithelia marks a transition
either from or into distinctly mesenchymal aggregations.

A brief discussion of the structure of the early coelomic
wall may serve as a starting point for the following deseriptions and considerations. Many investigations have established the fact that the lining of the early embryonic coelomic
walls consists of a layer of cells which resembles an epithelium
only on its distal side. In bounding the tissue against the
cavity, the cells necessarily form a continuous layer with an
even surface; this is the epithelial side of the lining. ‘Toward
the solid tissuc, however, these same cells are provided with
tapering processes identical with those of the underlying
mesenchyme (fig. 1). No boundary separates the superficial
and deep layers, and daughter cells of the superficial cells
contribute to the growth of the mesenchyme. Fischel (713) and
Politzer (’386) pointed out that migration of cells from the
superficial layer into the depth may be so abundant in certain
areas of the dorsal coelomic wall that it resembles the early
primordia of the adrenal cortex. Thus the superficial layer
may in these early stages be regarded as a germinal layer of
the adjacent mesenchyme. It is, in this respect as well as
structurally, somewhat similar to the ependyma of the embryonic neural tube; however, the mesenchyme contributes
more to its own growth than does the mantle layer of the
neural primordium. The condition just described for the
coelomic lining gradually changes and a layer with all morphological characteristics of an epithelium develops. In human
embryos of about 8 mm. and mammal embryos of corresponding stages, only limited areas still show the original structure
(fig. 2).

This peculiar condition of the coelomic lining has been
mentioned by various investigators in connection with the
early development of its derivatives, particularly the spleen
(see below). The principal objective of these considerations
ORGAN FORMATION FROM COELOMIC WALL ood

was to point out that mesenchyme and mesothelium are derived
from one souree, and to determine whether the organ in question forms before or after the separation of these layers is
completed. However, very little attention was given to the
impheations of the fact that in the early condition the superficial layer contributes to the growth of the mesenchyme and
that both together form a common unit, particularly with
regard to tissue growth. Observations of limb bud formation
in amphibians (Filatow, ’33) suggest that this early condition
ean be retained or reactivated if special developmental
processes require a rapid localized multiplication of the mesenchyme of the body wall. It will be pointed out soon that a
very similar transformation of the higher differentiated mesothelium and mesenchyme back into a common unit takes place
in the initial stages of the development of various organs from
the coelomic wall. However, the peculiar combination of epithelial and mesenchymal characteristics and the readiness to
change from one type to the other are not limited to these
early stages as will be shown by various examples.

As far as the gonads are concerned, many observations and
considerations related to the present problem were recently
published in a separate article (’42); this publication will be
referred to on numerous occasions throughout the present
report in order to avoid repetitions.

MATERIAL AND METHODS

The importance of an adequate staining technic in investigations of the type to be reported here was emphasized
previously (’42). The use of methods showing collagenous
and lattice fibers is essential whenever relations of mesenchymatous and epithelial tissues are examined. Many misconceptions of development and structure of mesodermal
organs, particularly gonads and adrenal cortex, could have
been avoided by the use of such methods instead of the common hematoxvlin stains which mav be perfectly adequate in
similar investigations of most organs with ectodermal or entodermal epithelial components. As in the previous investiga306 PETER GRUENWALD

tion, azan stain and silver impregnation were employed, except
in a few instances when older material was used. These
methods were supplemented recently by a very useful combination of Gomori’s (’39) silver impregnation with azocarmine or acid fuchsin stain by which nuclei and a few other
cellular details could be shown in the otherwise empty spaces
between the impregnated fibers. The stains were applied after
completion of the impregnation.

Because of the large amount of disparate material used in
the present work, no enumeration of the specimens will be
given. Most of the complete embryos and gonads used are
listed in a previous report (’42). The present investigation
is based mainly on human and mammalian material. Chick
embryos will be used only to demonstrate conditions not so
clearly visible in mammals, as is the case in the changes in
the miillerian duct and tubal ridge described later in this
report.

OBSERVATIONS

In line with the aim of the present work, only such conditions and changes will be described here in detail as are useful
to demonstrate the peculiar common characteristics of mesothelial derivatives. These traits will be found to be most
outspoken and permanently persisting in the gonads and the
adrenal cortex; the miillerian ducts and the spleen show similar behavior only during early developmental stages. As a
supplement, the formative activity of the coelomic lining in
late stages will be demonstrated by means of several examples.

The adrenal cortex

A. review of the literature on adrenal development shows
a fair agreement as to the statement that the epithelial cell
cords of the cortex grow into the retroperitoneal mesenchyme
from the coelomic epithelium. The investigation of Politzer
(736), based on well preserved human material, is in line with
this coneeption. Only Uotila (’40) recently admitted that ‘‘the
formation of distinet cell buds has not been observed’’. Many
ORGAN FORMATION FROM COELOMIC WALL 357

investigators probably desired to trace epithelium back to
epithelium; however, the actual conditions to be seen in sections offer very little in support of the description advocated
by the majority of related reports. When trying to refrain
from any prejudice as to the epithelial nature of the adrenal
cortex and its parent tissue, one can, according to the conditions seen in human and mammalian embryos, give the following description of early adrenal development.

At the time and location thoroughly described by Politzer
for human embryos, cell masses appear in the retroperitoneal
mesenchyme, reaching from the peritoneal surface dorsad
(fig. 3). The arrangement of their cells indicates that the
superficial layer is active in their production, but neither the
lining of the coelomic cavity in this area, nor the cells apparently moving dorsad from it, show an epithelial arrangement. Investigation of the sections themselves does not reveal
more epithelial characteristics than does the photograph of
our figure 3 or similar figures in Politzer’s and Uotila’s reports. Soon after the stage just described, the primordium
of the adrenal cortex is separated from the coelomic lining
by loose mesenchyme, and its cells begin to differ from the
surroundings by their round, large nuclei and more darkly
staining cytoplasm. An indistinct arrangement in cords can
also be noticed, but it is not epithelial. Proper staining
methods, as silver impregnation, show reticulum fibers surrounding each individual cell and separating it, from its neighbor in the same cord (fig. 4). This condition persists long and
in great parts of the adrenal cortex even throughout life.
There is considerable variation as to the appearance of these
reticulum fibrils in the adult adrenal, among different mammalian species as well as among individuals of the same
species. Figures 6 to 12 may serve to illustrate this. It was
found that in general the zona glomerulosa loses most completely the reticulum separating individual cells, and assumes
the structure of epithelial cords, whereas the zona reticularis
and the adjacent part of the zona fasciculata are always found
to retain this reticulum. The outer zona fasciculata shows the
308 PETER GRUENWALD

largest variability as to the presence of reticulum fibrils
within its cords; an outer portion of varying extent may be
free of them.

The reticulum of the adrenal cortex was the subject of an
investigation by Corner (’20). He found that there are no
connective-tissue cells between the cortico-adrenal cords to
account for the rich reticulum. Assuming that the adrenal
cells proper do not form lattice fibers, Corner designated the
cells of the capillary endothelium as the only possible source
of the reticulum. Knowing the cortico-adrenal cells as descendants of cells of the coelomic wall with potencies of
embryoni¢e connective tissue, we cannot decline a priori to
consider them as possible producers of reticulum. According
to Plenk (’27), many derivatives of the embryonic connective
tissue, such as endothelial and fat cells and muscle fibers,
smooth as well as striated, may form argyrophil fibers. This
may very well hold for cortico-adrenal cells as well. It is not
surprising to find these cells enveloped by lattice fibers during
early stages of embryonic development, when they still resemble their parent tissue, that is, the coclomic wall (fig. 3).
A persistence of this condition accounts for part of the
reticulum, particularly that within the cords and not in contact
with capillary walls. It is interesting to notice that Corner
found a very similar condition in the corpus Inteum which,
being also a derivative of the coelomic wall, is very similar in
its origin to the adrenal cortex. In other organs investigated
by Corner (hypophysis, thyroid, kidney), the conditions are
different since there are always at least a small number of
connective tissue eclls that can be held responsible for the
production of the reticulum.

Plenk (’27) and Bachmann (’37 and ’39) also investigated
the argyrophil connective tissue of the adrenal cortex. Both
authors consider its gradual increase in the deeper layers as
an expression of a decrease in function and vitality of the
cortico-adrenal cells;.Bachman tried to corroborate this by
a comparison with the corpus luteum where the lattice fibers
inerease with its age. Recent work of Bennet (’40) corORGAN FORMATION FROM COELOMIC WALL 359

roborates this statement as far as the age of the .cells is
concerned. As to the relation of appearance of the reticulum
to function, Bachmann’s suggestion is not confirmed by Bennet
who localizes the zone of maximal function of the cortical cords
to an area central to the outer cortex devoid of reticulum.

Tn spite of these embryological and histological observations, the adrenal cortex is still widely believed to consist
of epithelial cell cords. This is expected in analogy with most
of the other endocrine organs; the cord-like arrangement of
the cells so rich in cytoplasm also seems to point to an epithelial structure. Most of the stains ordinarily employed
either do not show reticulum separating the eclls, or make it
appear as unusually well-marked cell boundaries. Our own
observations, as well as previous reports of Plenk and Bachmann, however, show that the cords deserve to be called epithelial only in a smaller outer portion comprising the zona
elomerulosa and a part of varying height of the zona fasciculata (figs. 6 to 12). There is a gradual transition to
non-epithelial structure as we follow the cords toward the
center of the organ, and in early embryonic stages is there no
epithelial zone at all (fig. 4).

With this fact and our characterization of the mesothelium
in mind, we are able to arrive at a more satisfactory conception
of development and structure of the adrenal cortex. In its
carly stages, large numbers of cells develop from the coelomic
wall, and mesenchyme as well as mesothelium actively participate in this process as parts of one uniform tissue comparable to the carly embryonic coelomic wall. The blastema thus
formed has a dense framework of argyrophil fibrils like the
surrounding mesenchyme; as its cells swell and form cords,
this reticulum remains between them and the changes in its
distribution are only due to the widening of the meshes by
the growth of the cell bodies (fig. 4). Only in the periphery
of the organ do the reticulum fibrils eventually disappear
within the cords, thus allowing the cells to assume an epithelial
arrangement. This behavior of the reticulum was described
here in detail because it is a good indicator of the nature
360 PETER GRUENWALD

of the cortico-adrenal cells: they are swollen cells of the
coelomic wall with the combination of epithelial and non-epithelial properties characteristic of their mother tissue.

The close developmental relations of the adrenal cortex to
the surrounding mesenchyme of the coelomic wall are not
limited to early embryonic stages. Zwemer, Wotton and
Norkus (’38) called attention to cell types in the capsule of
the adrenal, showing all transitional stages between connective
tissue cells and typical cortico-adrenal cells, and they concluded that the capsule participates in replacing cortical cells
worn out by use. This was recently confirmed by Salmon and
ZAwemer (’41) when they traced cells laden with trypan blue
from the capsule to the cortical cords. It was also recognized
that presence of the capsule is essential for adrenal grafts to
survive; most of the transplanted differentiated cortical cells
become necrotic and a new organ is formed from the capsule
(Higgins and Ingle, ’88; Ingle and Higgins, 389; Baker and
Baillif, ’39). The author’s own observations fully confirm the
report of Zwemer and his co-workers. Particularly in adrenals
of man and the rhesus monkey, but also in other mammals,
tapering cortical cell cords reach into the capsule, assuming
there a course parallel to the surface of the adrenal (figs. 13
to 15). Their appearance may very well be the expression
of a new-formation of cortico-adrenal cells from the capsule.
It is, however, difficult to imagine how such new-formation
should take place in the many species with a more distinctly
differentiated zona glomerulosa. In these cases, the cell
groups of the adrenal cortex are most sharply bounded and
epithelium-like just where they face the capsule (figs. 9 to 12).
With the few observations at hand it is diffienlt to reconcile
this with the possibility of physiological replacement of cells
from the capsule. However, even in adrenals with a very
distinct zona glomerulosa, occasional small cortical cell groups
can be encountered in the inner part of the capsule, sometimes
counected with cords of the zona glomerulosa, so that even
in these organs a participation of the capsule in regeneration
eannot be excluded. Bachmann (’39 b) rejects the conclusions
ORGAN FORMATION FLOM COELOMIC WALL 361

of Zwemer and his coworkers and traces the regeneration of
cortico-adrenal cells to a sub-capsular blastema. This seems
to be a dispute about terminology rather than facts; both
authors probably saw the same histological pictures. The layer
peripheral to the zona glomerulosa, not consisting of compact
cortical tissue, was called a blastema by Bachmann, and a
loosely woven inner portion of the capsule by Zwemer and
his coworkers,

Another indication of a special activity of the capsule of
the adrenal was found by H. Popper (personal communication). When investigating adrenals by means of fluorescence
microscopy, Popper found in the eapsule lipoid droplets containing vitamin A, as they occur in cortico-adrenal cells, but
not in ordinary connective tissue. This, too, suggests that the
capsule of the adrenal is more than just a protecting layer
of connective tissue and should be considered in all investigations on the adrenal cortex.

It is commonly agreed upon that mitosis of cortico-adrenal
cells, particularly in the outer portion of the zona fasciculata,
contributes to the production of new cortical cells. Tt is unknown to which extent this process and the before-mentioned
transformation of capsular cells cooperate in the regeneration
of the organ, and whether there are conditions causing the
oue or the other process to prevail.

The gonads

The conditions found in early gonad primordia, although
similar to the initial stages of the adrenal cortex, have caused
a great deal of controversy. There is hardly any theoretically
possible mode of early gonad development that has not been
suggested by several investigators. The majority of authors
hold that epithelial cell cords growing into the deeper lavers
from the covering mesothelium give rise to the primary sex
cords; in the female, these were said to be supplemented later
by secondary cords which also originate from the surface
epithelium. Other authors (Van Vloten, ’27; Higuchi, ’32)
trace the primary sex cords back to a blastema composed of
362 PETER GRUENWALD

a mixture of epithelial and mesenchymal cells. Fischel (730)
disputes any participation of the mesothelium in the development of the sex cords. According to his view the primary
cords originate from the mesenchyme of the early gonad
primordium without any contribution from the surface epithelium, and no new cords are formed during later stages.
The author’s own results were recently published in detail
(°42). It was pointed out that gonad development starts after
the lining of the respective part of the coelomic cavity has
been differentiated as an epithelium for a short period of
time. Both the epithelium and the underlying mesenchyme
participate in the following differentiations, but as a common
umt rather than separately. The newly formed basal membrane of the mesothelium disappears (fig. 5; for more figures,
see °42) and, just as in early adrenal development, the uniform condition of the coelomic wall reappears to form the
gonad blastema. Here, however, the newly formed organ does
not sever its relations with the coelomic lining as soon as
does the adrenal, but considerable differentiation goes on before a truly epithelial lining is re-established on the surface
of the organ. When the cells of the gonad blastema first group
themselves in radial rows to form the primary sex cords, the
zone of differentiation is sharply bounded neither from the later
surface epithelium nor from the mesenchyme at the basis of
the organ. Only considerably later, in human embryos of over
20 mm., is the mesothelium restored as a typical epithelium,
retaining connections with the sex cords to an extent varying
according to sex and species. It follows from this description
that the primary sex cords are derivatives of neither the
mesothelium nor the mesenchyme alone; they are furnished
by a blastema in which these two parts of the coelomic wall
cannot be distinguished, just as is generally the case in the
early embryonic coeloma. Closest to the mode of development
outlined here comes the conception of origin of the sex cords

*The immigrated primordial sex cells are not included in these considerations ;

the question whether or not they give risc to the sex eclls of the mature gonads
will not be discussed here,
Vey

ORGAN FORMATION FROM COKLOMIG WAIT, 363

from a mixture of epithelial and mesenchymal cells. This
theory, however, suggests the presence of morphologically and
potentially different mesodermal cells in the gonad blastema,
an assumption which is not justified according to the conception outlined here. As to the secondary sex cords, the
author agrees in general with earlier investigators on their
origin from the surface epithelium of the gonad. For a detailed description of this process the reader is referred to
the previous report (’42).

While the primary sex cords are developing, the rete differentiates in a similar manner near the hilus of the organ,
and interstitial cells appear, at least in the testis. Most
authors take it for granted that the interstitial cells differentiate from the mesenchyme. It would then appear as if a
permanent separation of epithelial and mesenchymal tissues
were gradually established within the gonad, with the epithelial line represented by sex cords, rete, and surface lining,
and the mesenchymal portion by the interstitial tissue and
stroma of the organ. There are, however, indications of close
developmental similarities of sex cords and the interstitial
tissue; the latter develops in many species in the form of
cell cords from the gonad blastema much like the primary
sex cords, and many other analogies as well as transitions
between both tissues are on record (see Gruenwald, ’42). The
most impressive example is the regular and abundant transformation of sex cords into interstitial cells in the embryonic
horse ovary, as described by Kohn (’26) and Petten (’33).
These relations of the epithelial sex cords and the mesenchymal interstitial tissue again recall the peculiar character
of the derivatives of the coelomic wall. Further evidence is
the fate of the granulose cells after rupture of the follicle:
these epithelial cells cooperate with the mesenchymal theca
cells in the formation of the corpus luteum to such an extent
that both types cannot be distinguished any longer. These
changes of the follicle cells during corpus luteum formation
have been compared with the succession of forms from the
periphery toward the center of the adrenal cortex (Bach364 PETER GRUENWALD

mann, ’37). In both instances fluent transitions exist between
epithelial and mesenchymal types of tissue.

The miillerian ducts

The miillerian duets develop from the coclomic wall at
about the same time as the gonads and adrenal cortex, but
without disturbing the distinctly epithelial structure of the
mesothelium. The ducts themselves grow caudad from thickened portions of the mesothelium, and closely follow the
wolffan ducts in their course. Details of this process were
previously described (Gruenwald, ’41); it was pointed out
that the growing end of the miillerian duct is not separated
from the wolffian duct by either mesenchyme or a basal membrane. It grows toward the cloaca as a wedge between the
cells and the basal membrane of the wolffian duct, and somewhat later this peculiar process is followed by separation of
the ducts by basal membranes and mesenchyme. Neither this
previous investigation, nor any other related reports in the
literature showed any evidence of such transitions between
epithelial and mesenchymal structure as were just described
in the gonad and adrenal. However, the following observation
will demonstrate that the miillerian duct normally forms
mesenchyme, at least in its caudal portion. This process will
be briefly described here as it oceurs in chick embryos, but
there are indications of similar changes occuring to a varying extent in mammal embryos as well. A detailed investigation of this subject will be reported in the near future.

Figures 16 to 18 show cross sections of wolffian and
miillerian ducts of a 6-day, 38-hour chick embryo. In the region
in which the solid growing end of the miillerian duct is found
within the basal membrane of the wolffian duct (fig. 16), the
mass of miillerian cells is almost homogeneous; only a slight
difference exists between a cell group in the center and the
rest of the mass, and even this difference is absent at the
very end of the miillerian wedge. The wolffian duct can be
distinguished fairly well from the miillerian cells although no
basal membrane separates the two tissues at the most caudal
ORGAN FORMATION FROM COELOMIC WALL 365

level. Figure 17 shows a section 110 » cranial to the one just
deseribed; here the development is slightly farther advanced
and lattice fibers have begun to form in the arca enveloped
in the common basal membrane of the two ducts. These fibers
gradually complete the basal membrane of the wolffian duct,
thus separating it from the miillerian cell mass, and at the
same time appear within that cell mass, thus dividing it into
two portions. One part, in the center of this mass, remains
free of lattice fibers and ean now be seen to form the miillerian
duct proper; the rest of the cell mass, reaching between the
two duets from both sides in the form of wedges (fig. 17),
develops an increasing amount of fibers. This latter portion,
although it is a part of the original miillerian primordiun, is
gradually transformed into typical embryonic connective
tissue, and cannot be distinguished from the surrounding
mesenchyme when the original common basal membrane disappears. Figure 18, 280 u cranial in the series to figure 17,
shows the beginnmg dissolution of this membrane. This
peculiar type of mesenchyme formation from the miillerian
primordium was not observed in the cranial portion of the
miillerian duct. A condition similar to these findings was seen
in a 22-mm. cat embryo (fig. 19).

The present observation shows that the growing end of
the miillerian duct forms, im the caudal part of the urogenital
ridge, considerable amounts of mesenchyme in addition to the
duct proper. This mesenchyme soon after its formation can
not be distinguished from the surrounding tissue of the ridge.
We do not know at the present time whether the miillerian
duct and its epithelial derivatives permanently retain their
potency to form cells morphologically identical with the surrounding mesenchyme, nor is there any indication on record
of miilerian potencies of the embryonic connective tissue
formed from the miillerian primordium. However, it is safe
to assume that in the stages described here the cells of the
miillerian ducts show a similar readiness to change from epithelial to mesenchymal arrangement as those of the mesothelium itself and the gonads and adrenal cortex.
366 PETER GRUENWALD

The spleen

Early spleen development has repeatedly been reviewed
with regard to its relation to the mesothelium. Thiel and
Downey (’21), Bergel and Gut (734), and Holyoke (736) agree
that the mesothelium participates in spleen formation only
indirectly insofar as it forms part of the mesenchyme from
which the spleen is to originate. The author’s own observations fully confirm these reports. At no time after the appearance of the spleen is there any indication of considerable cell
migration from the mesothelium that might form more than
an occasional cell of the capsule. This is indicated by the
position of cells and lattice fibers even in places where the
epithelium is not bounded by a definite basal membrane. Figure 20 was taken from the spleen of a 12-mm. human embryo,
at a stage when massive proliferation from the surface lining
has ceased. It shows the presence of a basal membrane of the
mesothelium along part of the surface, and in the remaining
portion an irregular, but fairly complete boundary of lattice
fibers at the basis of the superficial cells. In this latter area,
a few cells were probably being given off by the mesothelium.
Soon after this stage more and more of the surface epithelium
has a complete basal membrane. In later periods only occasional areas may be seen devoid of a distinct basal membrane
(fig. 21). These areas are usually located in places where
trabeculae are connected with the capsule, and this indicates
that the few cells that may leave the mesothelium on its basal
border, are developing into fibroblasts rather than specific
elements of the spleen.

It follows from these observations and corresponding reports in the literature that the mesothelium does not to any
considerable extent contribute to the splenic tissue proper
after the appearance of the primordium of the spleen. Oceasional cells given off in later embryonic periods probably become part of the capsule or adjacent trabeculae.
ORGAN FORMATION FROM COELOMIC WALL 36

=)

The coelomic walls in later stages

The mesothelial lining of the serous cavities retains a remarkable activity for a long time after acquiring a distinctly
epithelial form. It was mentioned earlier that on the surface
of the gonad, particularly the ovary, the mesothelium forms
sex cords during the greater part of embryonic life. In the
testis there is far less of this activity, although it is not
entirely missing. Hett (’27, ’30, ’32) published detailed
descriptions of proliferations of the testicular mesothelium in
various mammals, but failed to distinguish those related to
sex-cord formation from others which, according to the
author’s earlier results (’37), have nothing to do with sex
cords, but follow in their distribution so closely the arrangement of the large blood vessels in the tunica albuginea that
a connection of some kind has to be assumed. Even after
birth, the surface epithelium. of the gonad is not reduced to
an inactive lining. There are several reports of new formation of follicles from proliferations of the surface lining in
the ovary of various adult mammals. For details and references see Swezy (’33) and Duke (’41). Experiments on mice
revealed that after x-ray irradiation the epithelium lining the
surface of the ovary proliferates abundantly, as if attempting
to regenerate the parenchyma destroyed by the experiment
(Parkes, ’26; Geller, ’30). No ova were found in these newly
formed cell groups, but luteinization was reported. In the dog
ovary, similar proliferations can be observed under normal
conditions. The ‘‘anovular follicles’? resulting from them
were thoroughly described by Jonckheere (’80).-These formations may be considered as the expression of an exceptionally
great potency of sex-cord production in this species, causing
the mesothelium to proliferate even in the absence.of sex cells.
Groups of mushroom-shaped projections were seen on the
surface of the ovaries of adult rabbits. In this case, however,
is there no reason to bring the outward directed proliferation
in any connection with sex-cord formation.
368 PETER GRUENWALD

Considerable activity of the embryonic mesothelium can
also be noticed in areas adjoining the gonad, particularly those
covering the mesonephros. Hett (’30) found the remarkable
thickenings of the mesothelium which he indiscriminately considered as evidence of persistent germinal epithelium, extending from the testis onto the mesenephros. Another type of
proliferations of the mesonephric mesothelium can be seen in
many mammal embryos as, e.g., in human embryos of about
14 to 21mm. The lining of the dorso-lateral surface of the
mesonephros is thrown into folds perfectly resembling external glomeruli on section (fig. 22). It must be noticed that
this was found.so regularly and so far caudally that remnants
of the pronephros can be excluded. No evidence of nephrostomes near these ‘‘glomeruli’’ could be obtained, and their
blood supply is, as far as study of sections allows such conclusions, not large enough to indicate an excretory function
of considerable extent. In figure 22, one of these structures
may be compared with a typical mesonephric glomerulus. In
pig embryos, ridges separated by mesothelium-lined clefts
were also found on the lateral surface of the mesonephros.
These, however, bear little resemblance to glomeruli. Similar
uncharacteristie clefts and villi were observed in embryos of
other mammalian species as well. The difference in the appearance of proliferations on the surface of the mesonephros
is important to notice. It shows that the simuarity with
glomeruli as found particularly in human embryos, is probably incidental and is in no way sufficient to identify these
structures as functional or vestigial excretory organs.

Another area of great activity of the mesothelial covering
of the mesonephros is the tubal ridge. This strip of high
mesothelium develops as a caudal continuation of the area
of the later ostium tubae, along the entire course of the
wolffian and miillerian ducts in the mesonephric ridge. It loses
its characteristic appearance shortly after the millerian duct
has completed its caudal growth. The continuity of the tubal
ride with the ostium of the miillerian duct, and its close proximity to this duct suggest that it might have to do with its
ORGAN FORMATION FROM COELOMIC WALL 369

formation. Careful investigation of serial sections, however,
revealed nothing to indicate a contribution of cells of the
tubal ridge to the developing miillerian duct. Whereas the
ridge may be indistinctly bounded, the basal membrane of
the duct is always intact and separates the miillerian epithelium from the coelomic lining in all places except the
ostium. Even in the absence of the miillerian duct in consequence of experimental destruction of the wolffian duct does
the tubal ridge not regenerate a new miillerian duet (Gruenwald, ’87). The basal boundary of this strip of high mesothelium is indistinct in places, and the basal membrane
interrupted. The histological pictures in these areas indicate
new-formation of mesenchymal cells from the proximal layers
of the mesothelium.

The clearest evidence of extensive mesenchyme formation
from the tubal ridge was gained in chick embryos of the
seventh and eighth day. Two sections of this area from a
7-day, 17-hour embryo are shown in figures 23 and 24. In the
more caudal one (fig. 24), the tall epithelium of the tubal
ridge is bounded proximally by a distinct basal membrane.
The greater proximal portion of the epithelium shows early
stages of lattice fiber formation between its cells. A view of
a later stage of this same process may be obtained in a more
cranial section of the same structures (fig. 23). A dense framework of lattice fibers has now developed in the tubal ridge,
sparing only its most distal part. The old basal membrane
can hardly be recognized between the lattice fibers of the tubal
ridge and those of the underlying mesenchyme; it is apparently becoming part of this framework. In still later stages
of this transformation a new basal membrane forms underneath the distal portion of the former tubal ridge, and only
this portion turns out to remain epithclial whereas the greater
basal part of the original epithelium of the tubal ridge is now
completely transformed into mesenchyme surrounding the
miillerian duet. (It will be remembered that another portion
of this mesenchyme is furnished by the miillerian duct itself,
see above.) In human embryos the epithelium of the tubal
370 PETER GRUENWALD

ridge never reaches the height of that in the chick embryo,
and consequently mesenchyme formation is not so exteusive
and obvious. In a 13-mm. hedge hog embryo, however, stages
of transformation of the tubal ridge were seen well comparable
to those in the chick. In one portion, the basal boundary of
the original tall epithelium is still visible, but much of the
tissue distal to it 1s obviously being transformed into mesenchyme; only a thin mesothelium remains on the surface, and
a new membrane is formed at its base.

Another area of quite close relations of mesothelimn and
adjacent mesenchyme may be seen in chick embryos at the
caudal end of the abdominal cavity, near the points where the
wolffian ducts join the cloaca. There is an area on either side
of the body with large groups of buds, with or without lumen,
extending from the mesothelium into the body wall (fig. 25).
Near the abdominal cavity, these structures are epithelial and
have distinct basal membranes; however, with increasing
distance from the coelomic cavity the basal boundaries become
indistinct, the lumina disappear, and the entire mass of these
buds gradually passes over into the mesenchyme. The significance of these structures is unknown. The only reference
to their existence known to the author, was found in Lillie’s
book (719) as a legend to a figure, describing them as follows:
‘‘Posterior angle of the body cavity; the epithelium is invaginated and folded so as to simulate a glandular strueture.”’
Tt is possible that these peculiar buds represent vestigal
caudal continuations of the coelomic cavities, or that they are
related to some change, increase or decrease, in the volume of
the body cavity. The only observation in mammals faintly
similar to these findings was made in a 15-mm. dog embryo
(fig. 26). In a large area on the dorsal wall of the urogenital
sinus the basal membrane of the mesothelium is lacking, and
the lattice fiber structure suggests moderate mesenchyme
formation by the mesothelium. In two areas near the midplane, however, grooves extend into the tissue and there the
mesothelium is continuous with the deeper layers without any
trace of a boundary. Delicate lattice fibers radiate into this
ORGAN FORMATION FROM COELOMIC WALL 371

tissue from the deeper layers. Here, too, gradual transitions
exist between mesothelium and mesenchyme.

The proliferating capacity of the mesothelium is not limited
to the peritoneum. Patches of thickened mesothelium with
formation of mesenchyme from their basis were described in
the parietal pleura of newborns by Kampmeier (’28). They
are, just like the above mentioned thickenings of the testicular
epithelium, associated with blood vessels. Kampmeier is certainly correct in saying about these patches: ‘‘We must look
upon that phenomenon as a continuance into postnatal life of
the primitive activity of the mesoderm lining the body cavities
—the generation of mesenchymal and many other cells with
all the potencies which they possess.’’ Another observation
of Kampmeier (37) concerns fold-like projections of the pericardium to both sides of the aorta. I can, from my own
observations, confirm Kampmeier’s statement that these
structures are regularly present in human embryos of about
20 to 830mm. Their significance is unknown.

A number of observations were compiled in this chapter
concerning proliferations of the mesothelium in distal or proximal direction, ranging from true sex-cord formation contributing essentially to the development of an organ to
apparently insignificant folds and villi. These observations
not only indicate a great morphogenctie activity of the mesothelium, but also show that we have to be extremely careful
in interpreting such findings. In view of so many proliferations of the mesothelium not related to the development of any
organ, we are not entitled to consider any thickening of the
mesothelium of the gonad as evidence of a potency of sex-cord
formation, nor can we conclude from the presence of projections similar to glomeruli that we are dealing with a vestigial
excretory organ. Other characteristics have to be found in
addition, to justify such conclusions. The present observations
are evidence of the fact that the mesothelium, apart from
participating in the formation of organs in early embryonic
periods, has a great capacity of nonspecific proliferation,
forming epithelial projections as well as mesenchyme.
ws
“I
lo

PETER GRUENWALD

DISCUSSION AND CONCLUSIONS

The present investigation revealed a remarkable conformity
of the derivatives of the coelomic wall and their parent tissue
in one important respect. The capacity of the cells of the
coclomic wall to change from epithelial to mesenchymal arrangement or vice versa, was found in the tissues of the
gonads, adrenal cortex, and miillerian ducts as well. During
early development of the gonads and the adrenal cortex, the
coelomic wall again assumes its early embryonic structure
with the surface lining acting as a germinal layer. This has
repeatedly been mistaken for proliferations of a coelomic epithelium; proper staining methods show that no epithelium
exists in these areas during the period of rapid proliferation.
At this time the superficial layer has essentially the same
structure as is generally the case in carlier stages: its cells
have on their basal side the same structure as the underlying
mesenchyme, and no natural boundary exists between the two.
These findings, along with similar observations concerning
early development of the extremities in amphibians (Filatow),
suggest that the capacity of reversion to the primitive condition is a general property of the embryonic coelomic wall,
activated whenever organ development necessitates abundant
cell proliferation.

The above mentioned capacity of structural transformation
does not end in the primordia of gonads and adrenal cortex
when the coelomic wall returns to its resting condition with
a well defined surface epithelium. In the gonads, developmental relationships as well as transitions between the epithelial sex cords and the mesenchymal interstitial tissue may
be observed for a long time (see Gruenwald, *42), and the
formation of the corpus luteum from granulosa and theca cells
recalls this peculiarity even in the adult. In the adrenal cortex,
gradual transitions from epithelial to non-cpithelial structure
are permanently found in all cell cords within the outer portion
of the zona fasciculata, and stages of formation of epithelial
cortico-adrenal cell cords from mesenchymal cells may be scen
in the capsule of the organ.
ORGAN FORMATION FROM COELOMIC WALL 373

Not quite so obvious are corresponding findings in the
miillerian ducts. The material of these ducts is not laid down
simultaneously as a large blastema as happens in the gonads
and adrenal cortex; these ducts rather develop from small
initial primordia, and without further contribution by the
coelomic wall. Accordingly, no radical structural changes of
the coelomic wall seem to be necessary and we find the
miillerian ducts arising from a distinctly epithelial part of
the coelomic lining. The ducts remain purely epithelial in
their cranial parts. Caudally, however, their primordia differentiate so that only the central portions develop into continuations of the ducts, whereas the rest forms typical
embryonie connective tissue. This process can be traced distinctly in certain species (chick, cat), and is indistinct or
entirely absent in others. There are, wp to the present time,
no indications of similar mesenchyme formation in later
embryonic or postnatal stages.

In addition to these observations it was shown that during
later embryonic periods the mesothelium itself participates
more actively in mesenchyme formation than is usually believed. The area of greatest activity is the tubal ridge where
the greater part of the stratified mesothelium is transformed
into embryonic connective tissue. On the surface of the spleen
of older embryos, the mesothelium may occasionally show
pictures indicating contribution of a few cells to the connective
tissue capsule.

The classical conception of histology held that a definite
separation of epithelial and mesenchymal tissues is established
soon after the earliest periods of embryonic development.
Processes such as differentiation of kidney epithelia from a
mesenchymal blastema were considered as rather exceptional
and in that case, too, the epithelial differentiation, once established, was thought to be irrevocable. During the past decades
experimental histology, particularly tissue culture, revealed
that various factors entered into the determination of epithelial or mesenchymal structure, and that experimental
changes of these factors may be followed by structural
374 PETER GRUENWALD

changes not in aceord with the above mentioned rigid conception. With these results in mind, the question must be
raised whether the derivatives of the coelomic wall are essentially, or gradually, or not at all different from other tissues
as far as their above described structural lability is concerned.

In a summarizing review of the results of tissue culture
regarding differentiation, Bloom (’37) quotes reports demonstrating the effect of consistency of the medium and similar
physical factors on the arrangement of cells in vitro, and eoncludes that ‘‘.... There seems to be no fundamental difference between epithelium and connective tissue’’. These
results and conclusions are doubtless correct; they concern,
however, only one of the many groups of factors involved
in determining tissue differentiation. Many well known developmental processes show that other, probably intrinsic
factors are sufficient to produce the changes in question in the
absence of such environmental changes as active in experiments in vitro. A number of examples may be found in the
present report. Neither the formation of epithelial sex cords
from the mesenchymal gonad blastema, nor the transformation
of the greater part of the epithelial tubal ridge into embryonic
connective tissue, are preceded by extrinsic changes of consistency, surface conditions, or similar factors in the environment. It is evident from these considerations that the results
of tissue culture and those of the present investigation concern different factors affecting tissue differentiation; each of
these groups of factors may alone or combined with others
affect a tissue sufficiently to produce a change in its
differentiation.

Perhaps more important than the investigation of the environmental conditions themselves in the above mentioned in
vitro experiments, is the fact that the number of potencies
left in the body cells after the early periods of development
is much larger than was originally believed. Most or all of
the body cells seem to have the capacity to form epithelial as
well as mesenchymal aggregations, and in this respect the
ORGAN FORMATION FROM COELOMIC WALL alo

derivatives of the coelomic wall are probably not essentially
different from other tissues. The difference to be pointed out
here is a quantitative one. In ectodermal, entodermal, and
probably certain mesodermal tissues, changes from epithelial
to mesenchymal structure or vice versa are rare under normal
conditions. One example was given above when it was mentioned that the early coelomic wall bears certain structural
parallels to the developing neural tube; the nervous tissue,
however, is so peculiar in its structure that it is usually not
included in the system worked out for the majority of the
tissues. Other examples are the enamel pulp and possibly
the thymus; these, however, do not compare with the variety
and duration of transitions recorded in the present report.
One is, therefore, justified to assume that, although all cells
of the body may have the necessary potencies, those of the
coelomic derivatives show the greatest readiness to undergo
changes from epithelial to mesenchymal structure or vice
versa. Transformations comparable to those normally oceurring in the coelomic derivatives, could be produced only by
radical experimental procedures in most other tissues.

The present observations were gathered from a limited
selection of material, mostly embryonic and comprising only
a few species. Further work of histologists and embryologists
will doubtless reveal many more examples to illustrate the
peculiar behavior of the tissues derived from the coelomic
wall. The structural changeability of the coelomic derivatives
will also have to be considered in explaining pathological
growth and differentiation of these tissues. It is certainly not
incidental that so many diffieult problems have arisen for the
pathologist from the study of abnormal growth in the gonads,
or tumors related to the adrenal cortex. It will have to be
remembered that the clear-cut separation of epithelial and
mesenchymal tumors cannot be kept up in these instances and
that the tissues arising from the coelomic wall can produce
an unusually great variety of pathological growth types, due
to their peculiar wide range of differentiation.
376 PETER GRUENWALD

SUMMARY

1. The capacity of the cells of the early embryonic coelomic
wall to change from epithelial to mesenchymal arrangement
or vice versa, was traced through the development of their
derivatives, particularly gonads, adrenal cortex and miillerian
ducts, as well as through later stages of the mesothelium
itself.

2. When large masses of cells are to proliferate for the
formation of new organs, the coelomic wall meets the demand
by reverting to the early embryonic condition in which the
superficial layer takes essential part in producing mesenchymal cells. This process, characterized by abolishment of
the basal boundary of the mesothelium, occurs during the
earliest stages of gonad and adrenal formation. No true epithelial proliferations occur in either instance.

3. In the gonads, the thus formed blastema differentiates
into the epithelial primary sex cords and the separating
mesenchyme. Transitional forms are represented by the
interstitial tissue which may develop in the embryo from epithelial sex cords as well as from the unspecific mesenchyme ;
this is parallelled in the adult by corpus luteum formation
from both granulosa and theca cells.

4. The adrenal cortex also develops from a mesenchymal
blastema, and remains entirely mesenchymal for a considerable period of embryonic life. Only its most peripheral layers
then acquire epithelial structure, and gradual transitions
exist between these and the permanently non-epithelial central
portion. Reports of new-formation of cortico-adrenal cell
cords in the capsule of the organ are confirmed.

5. In certain species the epithelial primordia of the
miillerian ducts form, in their candal parts, considerable
amounts of embryonic connective tissue in addition to the
duets proper.

6. Several examples of the proliferating activity of the
coelomic epithelium in later embryonic stages are described.
In some of these, the boundary of epithelium and mesenchyme
remains intact, whereas in others the epithelium adds cells
ORGAN FORMATION FROM CORLOMIC WALL 377

to the underlying mesenchyme. This can best be demonstrated
in the tubal ridge, but it also happens in other places as, for
instance, on the surface of the spleen.

7. The present results are compared with those of tissue
culture indicating that all or most of the bodv cells are capable
of assuming epithelial as well as mesenchymal arrangement.
The conclusion is reached that there is a gradual difference
between the tissues developing from the coelomic wall and
those of other parts of the body, to the effect that the former
have under normal conditions a far greater tendency toward
changes between epithelial and mesenchymal structure than
the latter.

8. It is to be expected that the elimination of the prejudice
as to the stability of epithelial or mesenchymal structure in
the organs discussed here, will help the understanding of their
normal and pathological changes.

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EXPLANATION OF FIGURES

PLATE 1

1 Coelomic wall of a human embryo of the third week. The cells of the surface
lining have, on their proximal sides, the same structure as those of the underlying mesenchyme. Hemalum-eosin stain.

2 Coelomic wall at the lateral border of a mesonephros of a 7.5-mm. human
embryo. Part of the surface lining still shows the same structure as in the earlier
embryo (fig. 1). Azan stain.

3 Primordium of the adrenal cortex of a 9-mm. human embryo. Azan stain.

4 Part of the adrenal cortex (lower portion of fig.) and adjacent tissues of
a human embryo of 48mm. CRL. Each cortico-adrenal cell is surrounded by a
network of lattice fibers. Gomori’s silver impregnation.

5 Gonad region of a 9-mm. rabbit embryo. The basal membrane of the mesothelium is interrupted in the area of gonad formation (arrows). Azan stain,

PLATE 1


PLATE 2

EXPLANATION OF FIGURES

Lattice fiber structure in the postnatal adrenal cortex

6, 7, 8 Iluman adrenals. Gomori’s silver impregnation.

Dog adrenal. Gomori’s silver impregnation.

9
Same specimen and technic as shown in figure 9, at highcr magnification.

10
11 Horse adrenal, Gomori’s silver impreguation.
12 Same specimen and techuie as shown in figure 11, at higher magnification.


PLATE 2


PLATE 3
EXPLANATION OF FIGURES

18 Adult guinea pig, zona glomerulosa (below) and capsule of the adrenal.
Groups of cortico-adrenal cells are present in the capsule. Azan stain.

14 Adult human adrenal. Cell cords of the cortex radiate into the capsule.
Azan stain.

15° Adult human adrenal. The outlines of cortical cell cords radiating into
the capsule are shown by Gomori’s silver impregnation.

16 Miillerian duct and adjacent structures of a chick embryo of 6 days and
3 hours, M, miillerian duct; W, wolffian duct. Gomori’s silver impregnation.

17 Section of tho same series, 110 cranial to that shown in figure 16.
Arrows point through the basal membrane of the original miillerian primordium to
the portions being transformed into mesenchyme.

18 Section of the same scrics, 280 4% eranial to that shown in figure 17, The
basal mombrane of the original miillerian primordium is disappearing (lower side
of fig.), and the miillerian duct acquires a new basal membrane,

19 Miillerian duct and adjacent structures of a 22-mm. eat embryo, in a stage
of transformation comparable to figure 17. Azan stain.


PLATE 2
3
PLATE 4


20) Spleen of a 12-mm, human embryo. The surface lining is distinetly separated from the underlying mesenchyme only along part of the surface. Gomori’s
silver impregnation.

21 Spleen of a 160-mm. human cmbryo. No distinct basal membrane separates
the mesothelium from the capsule. In the center of the figure a trabecula connects
with the eapsule, and there lattice fibers scem to reach into the mesothelium.
Gomori’s silver impregnation.

22 Mesonephros of a 14.5-mm. human embryo, A structure resembling an
external glomerulus is attached to the dorsal surface of the organ.

23 and 24 Tubal ridge of a chick embryo of 7 days and J7 hours. M,
miillerian duct; W, wolffian duct. Lattice fibers uppear in the tall mesothelium
eovering the miillerian duct (fig. 24). 6804 cranial to this level (fig. 23), the
greater proximal part of the tubal ridge is transformed into mesenchyme coating
the miilllerian duct. The old basal membrane can no longer be distinguished.
Gomori’s silver impregnation. .

25 Ducts connected with the caudal end of the coelomic eavity of a 5-day
chick embryo. No definite boundary between cpithelium and mesenchyme ean be
established. Gomori’s silver impregnation.

26 Posterior wall of the urogenital sinus of a 15-mm. dog ombryo. The mesothelium lines symmetrical invaginations. At their bottom no boundary between
epithelium and mesenchyme ean be determined. Gomori’s silver impregnation.