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==Chapter IV Implantation and Placentation==
==Chapter IV Implantation and Placentation==


The Implantation of the Ovum. — The problem of the
==The Implantation of the Ovum==
mechanism of implantation of the human ovum has
presented numerous difficulties in the past, because
there were described only a relatively small number
of good specimens to illustrate the condition. The
investigations of Wislocki and Streeter (1938) gave,
for the first time, a connected account of implantation
in a primate, and the technical methods devised for
their study stimulated other workers, particularly
Hertig and Rock to search for earlier stages in human
development than had hitherto been known. At the
present time our knowledge of the process of implantation in the human has been greatly extended


The problem of the mechanism of implantation of the human ovum has presented numerous difficulties in the past, because there were described only a relatively small number of good specimens to illustrate the condition. The investigations of Wislocki and Streeter (1938) gave, for the first time, a connected account of implantation in a primate, and the technical methods devised for their study stimulated other workers, particularly Hertig and Rock to search for earlier stages in human development than had hitherto been known. At the present time our knowledge of the process of implantation in the human has been greatly extended by the recovery of normal human ova as young as seven and a half days conceptional age. The following account of implantation in the human is based on the work of Hertig and Rock (1944).


IMPLANTATION AND PLACENTATION 27


by the recovery of normal human ova as young as
The human ovum is fertilized at the outer end of the uterine tube, and while segmentation takes place, the ovum passes to the uterine cavity, its journey occupying probably three days. Its transport is accomplished partly by the downward streaming flow of fluid directed by the ciliary action of the tubal epithelium and partly by peristaltic contractions of the tubal musculature. The ovum arrives in the uterine cavity at the morula stage. It was formerly thought that the human ovum remained free in the uterine cavity for the next six days (Teacher, 1926) being nourished by the secretion of the uterine glands (‘ uterine milk ’). It is now known that the human ovum starts to implant about the seventh day since the youngest human embryo thus far studied (seven and a half days), is already superficially attached to the endometrium. It is then in the blastocyst stage ; the zona pellucida has disappeared. The embryonic pole of the trophoblast comes in contact with the epithelium, usually on the anterior or posterior wall of the uterus, and the epithelial cells are broken down by the trophoblastic cell secretion. That part of the trophoblast in contact becomes greatly thickened and shows two kinds of cells : ( a ) peripheral syn cytiotrophoblast cells, that is, a layer where cell boundaries are not distinct, which actively erodes and penetrates the endometrium ; and ( b ) cytotrophoblast cells forming a layer next to the cavity of the blastocyst. The syncytiotrophoblast cells proliferate rapidly, actually erode and digest the endometrial stroma, and soon they form a*thick zone of anastomosing strands with spaces or lacunae between them. The lacunae contain broken down endometrial cells and some maternal blood derived from eroded uterine vessels. While these events take place the developing blastocyst sinks into a cavity in the substance of the endometrium, the implantation cavity. The site of entry into the endometrium becomes obliterated, first by a plug of fibrinous material and later, by reepithelialization from the surrounding intact uterine cells. This mode of implantation is termed interstitial.
seven and a half days conceptional age. The following account of implantation in the human is based on  
the work of Hertig and Rock (1944).  


The human ovum is fertilized at the outer end of
the uterine tube, and while segmentation takes place,
the ovum passes to the uterine cavity, its journey
occupying probably three days. Its transport is
accomplished partly by the downward streaming
flow of fluid directed by the ciliary action of the
tubal epithelium and partly by peristaltic contractions
of the tubal musculature. The ovum arrives in the
uterine cavity at the morula stage. It was formerly
thought that the human ovum remained free in the
uterine cavity for the next six days (Teacher, 1926)
being nourished by the secretion of the uterine glands
(‘ uterine milk ’). It is now known that the human
ovum starts to implant about the seventh day since
the youngest human embryo thus far studied (seven
and a half days), is already superficially attached to
the endometrium. It is then in the blastocyst stage ;
the zona pellucida has disappeared. The embryonic
pole of the trophoblast comes in contact with the
epithelium, usually on the anterior or posterior wall
of the uterus, and the epithelial cells are broken
down by the trophoblastic cell secretion. That part
of the trophoblast in contact becomes greatly thickened
and shows two kinds of cells : ( a ) peripheral syn
cytiotrophoblast cells, that is, a layer where cell
boundaries are not distinct, which actively erodes and
penetrates the endometrium ; and ( b ) cytotrophoblast
cells forming a layer next to the cavity of the blastocyst. The syncytiotrophoblast cells proliferate
rapidly, actually erode and digest the endometrial
stroma, and soon they form a*thick zone of anastomosing strands with spaces or lacunae between them.
The lacunae contain broken down endometrial cells


Continued activity of the cells of the syncytiotrophoblast results in further destruction of maternal tissue and opening up of blood vessels so that the lacunar spaces become progressively filled with maternal blood. A few days after implantation commences there is a slow circulation of maternal blood through the lacunae of the trbphoblast. Before this time the developing embryo depends for its nutrition upon the absorption of broken down endometrial stroma cells. Certain of these, termed decidual cells, contain much glycogen and fat and are prominent around the implantation cavity. This type of nutrition is called embryo trophic. When, however, maternal blood flows through the syncytial lacunae of the implantation cavity a haemotrophic form of nutrition for the embryo is established.


28


When the ovum has become implanted in the endometrium the destructive powers of the syncytiotroph oblast gradually diminish and beyond the zone of destruction there is reaction on the part of the maternal tissues to hinder the erosive activities of the growing blastocyst.


AIDS TO EMBRYOLOGY


In the normal course of events the ovum becomes embedded near the fundus of the uterus, but in certain cases it wanders and implants itself in the lower part of the uterus near the cervix. When later the placenta is formed, it will partly or completely cover the internal os of the uterus, and the important clinical condition of placenta praevia results. Some times the ovum may be retarded in its journey towards the uterus and will attempt to embed itself in the tubal mucosa. The embryo develops for a short time but the condition usually terminates by the rupture of the tube, a grave surgical emergency.


and some maternal blood derived from eroded uterine
==The Decidua==
vessels. While these events take place the developing
blastocyst sinks into a cavity in the substance of the
endometrium, the implantation cavity. The site of
entry into the endometrium becomes obliterated,
first by a plug of fibrinous material and later, by reepithelialization from the surrounding intact uterine
cells. This mode of implantation is termed interstitial.


Continued activity of the cells of the syncytiotrophoblast results in further destruction of maternal
During the process of implantation the developing embryo has come to lie in a little cavity in the endometrium which, altered somewhat in character, is now known as the decidua. Different names are applied to various regions of this : thus the decidua immediately deep to the implanted ovum is the decidua basalis, that covering the developing embryo is the decidua capsularis and the remainder which lines the uterine cavity is the decidua parietalis.
tissue and opening up of blood vessels so that the  
lacunar spaces become progressively filled with
maternal blood. A few days after implantation
commences there is a slow circulation of maternal
blood through the lacunae of the trbphoblast. Before
this time the developing embryo depends for its
nutrition upon the absorption of broken down endometrial stroma cells. Certain of these, termed
decidual cells, contain much glycogen and fat and
are prominent around the implantation cavity. This
type of nutrition is called embryo trophic. When,
however, maternal blood flows through the syncytial
lacunae of the implantation cavity a haemotrophic
form of nutrition for the embryo is established.  


When the ovum has become implanted in the endometrium the destructive powers of the syncytiotroph oblast gradually diminish and beyond the zone of
==Formation of Chorionic Villi==
destruction there is reaction on the part of the maternal
tissues to hinder the erosive activities of the growing
blastocyst.


In the normal course of events the ovum becomes
The chorion may be defined as the trophoblast along with its somatopleuric lining of extra-embryonic mesoderm. It is through this simple type of chorion that nourishment passes by diffusion in the early stages of implantation, but, with increasing differentiation and the formation of an early embryonic circulation, the chorion undergoes structural modifications leading eventually to the formation of the placenta. The strands of syncytiotrophoblast which form the boundaries of the lacunae form the primary chorionic villi. These soon show a core of cytotrophoblast and commence to branch. Strands of extra-embryonic mesoderm appear within the cytotrophoblast of the villi and these shortly become continuous with the layer of extra-embryonic mesoderm forming the deeper part of the chorion. It is disputed whether the mesodermal core of the villus arises in situ by delamination from the cytotrophoblast or whether it actively invades the solid villus from the chorionic aspect.
embedded near the fundus of the uterus, but in
certain cases it wanders and implants itself in the  
lower part of the uterus near the cervix. When
later the placenta is formed, it will partly or completely
cover the internal os of the uterus, and the important
clinical condition of placenta praevia results. Some


IMPLANTATION AND PLACENTATION 29


times the ovum may be retarded in its journey towards
About the end of the third week of development (twenty to twenty-one days) blood vessels differentiate in situ in the mesodermal cores • of the villi (Hertig x 935) an d become connected with other vessels which are laid down in the chorionic mesoderm. These in turn become linked up with the primitive intraembryonic circulation by vessels (allantoic, see p. 35 ) which run to the embryo in the body stalk. These villi become more complex by branching. Some of them pass right through the peripheral layer of syncytiotrophoblast which bounds the now confluent lacunae, and are attached to the decidua. These are termed anchoring villi, but the majority of the villi hang free in a blood-filled space, the intervillous space, in which maternal blood slowly circulates. There is no communication between this maternal blood and the embryonic blood cells in the vessels of the villi. The barrier of tissue between the two circulations is the placental membrane (see later).
the uterus and will attempt to embed itself in the  
tubal mucosa. The embryo develops for a short
time but the condition usually terminates by the  
rupture of the tube, a grave surgical emergency.  


The Decidua. — During the process of implantation
the developing embryo has come to lie in a little
cavity in the endometrium which, altered somewhat
in character, is now known as the decidua. Different
names are applied to various regions of this : thus
the decidua immediately deep to the implanted ovum
is the decidua basalis, that covering the developing
embryo is the decidua capsularis and the remainder
which lines the uterine cavity is the decidua parietalis.


Formation of Chorionic Villi. — The chorion may be
defined as the trophoblast along with its somatopleuric lining of extra-embryonic mesoderm. It is
through this simple type of chorion that nourishment
passes by diffusion in the early stages of implantation,
but, with increasing differentiation and the formation
of an early embryonic circulation, the chorion undergoes structural modifications leading eventually to
the formation of the placenta. The strands of
syncytiotrophoblast which form the boundaries of
the lacunae form the primary chorionic villi. These
soon show a core of cytotrophoblast and commence
to branch. Strands of extra-embryonic mesoderm
appear within the cytotrophoblast of the villi and
these shortly become continuous with the layer of
extra-embryonic mesoderm forming the deeper part of
the chorion. It is disputed whether the mesodermal
core of the villus arises in situ by delamination from
the cytotrophoblast or whether it actively invades
the solid villus from the chorionic aspect.


About the end of the third week of development
(twenty to twenty-one days) blood vessels differentiate
in situ in the mesodermal cores • of the villi (Hertig*


Fig. 8. Longitudinal Section of early Embryo and Membranes.


3 o
1, Extra-embryonic coelom ; 2, amniotic cavity ; 3, yolk sac ; 4, body stalk ; 5, allantoic duct ; 6, embryonic disc ;


7, chorionic villi.


AIDS TO EMBRYOLOGY


The villi are at first scattered over the surface of the entire chorion, and it is then known as the chorion frondosum. During the fourth month the villi disappear from that part of the chorion related to the decidua capsularis, this portion being termed the chorion laeve. The persisting part of the chorion frondosum becomes transformed into the mature discoidal placenta.


x 935) an d become connected with other vessels which
==The Placenta==
are laid down in the chorionic mesoderm. These in
turn become linked up with the primitive intraembryonic circulation by vessels (allantoic, see p. 35 )
which run to the embryo in the body stalk. These
villi become more complex by branching. Some of


In the human subject the placenta is a disc-like, flattened cake with the chorion attached to its margins. The foetal surface is loosely covered by the amnio tic sac and gives attachment to the umbilical cord. Between the amnion and the foetal surface of the placenta are found large vessels, the major branches of the umbilical (allantoic) arteries and vein. From the uterine aspect it can be seen that the placenta is subdivided into a number (15 or more) of areas by septa. These areas are called cotyledons and each corresponds to a bunch of chorionic villi. At birth the placenta is about 20 centimetres in diameter, some 3 centimetres in thickness and weighs about 500 grams. The attachment of the umbilical cord is usually eccentric. Sometimes the attachment of the cord is to the outer margin of the placenta forming the so-called battledore placenta ; in other cases the cord sub-divides before reaching the placenta giving rise to the placenta furcata. The vessels too, may spread out in the investing membranes instead of at the placental area, and form what is known as the velamentous placenta. Accessory lobules of the placenta separated from the main mass are not uncommon (placenta succenturiata) . They probably arise from abnormal persistence of a group of villi of the chorion laeve.


==Functions of the Placenta==


Fig. 8. Longitudinal Section of early Embryo and  
The placenta functions in a triple capacity ; it isolates the foetal from the maternal organism so that the blood stream of each remains distinct ; it permits the exchange of nutritive substances from the maternal to the foetal circulation, and of waste products from the foetal to the maternal ; hormones probably also pass from the mother to the child ; and the placenta itself elaborates hormones (oestrogens) which pass into the maternal bloodstream, and of which large quantities are found in the urine of pregnant women.


Membranes.


1, Extra-embryonic coelom ; 2, amniotic cavity ; 3, yolk sac ;
The cellular layers which separate the maternal blood in the intervillous spaces of the placenta and the foetal blood in the chorionic capillaries obviously determine the rate of transmission of substances from the one circulation to the other. These cellular layers are collectively known as the placental membrane. During the latter part of gestation there is a progressive thinning of this placental membrane due to disappearance of the cytotrophoblastic layer of the villi, which are then clothed only by thinned-out syncytium. Without any great increase in size the placenta becomes functionally more efficient. Gellhorn, Flexner and Heilman (1943) have been able to study the transfer of radio-active sodium across the human placenta at various stages and find that the permeability to this substance is increased about seven-fold in the full-term placenta as compared with that of the tenth week of gestation.
4, body stalk ; 5, allantoic duct ; 6, embryonic disc ;


7, chorionic villi.
==The Amnion==


them pass right through the peripheral layer of  
The amniotic cavity is commencing its formation in the eight day human ovum (p. 15). In the twelve-day ovum it is seen to lie between the ectodermal cells of the embryonic disc and a layer of flattened epithelial cells overlying this and continuous with its margin. The outer surface becomes covered with extra-embryonic splanchnopleuric mesoderm. Extension of the extra-embryonic coelom separates the amnion from the inner surface of the chorion except at the caudal end of the embryonic disc where a mass, the body stalk, remains (see Fig. 8). When the embryonic head, tail and lateral folds arise, the line of attachment of the amnion is carried ventrally, so that it appears to become constricted and the embryo appears to rise up in the amniotic cavity. The amnion is growing actively and its cavity is increasing in size. Finally, its walls come in contact with the inner aspect of the chorion and the extraembryonic coelom is thus obliterated. With this growth the amnion comes to cover the outer aspect of the body stalk and its contained structures (yolk sac, allantois and allantoic vessels) which is now known as the umbilical cord.
syncytiotrophoblast which bounds the now confluent
lacunae, and are attached to the decidua. These are
termed anchoring villi, but the majority of the villi
hang free in a blood-filled space, the intervillous
space, in which maternal blood slowly circulates.  
There is no communication between this maternal




IMPLANTATION AND PLACENTATION 31
==The Amniotic Fluid==


blood and the embryonic blood cells in the vessels of  
The amniotic cavity contains fluid from the time of its first formation. This fluid is derived from the walls of the cavity. In the later stages of pregnancy foetal urine is added to the amniotic fluid. The body stalk elongates considerably in the formation of the umbilical cord so that the embryo floats in the amniotic fluid suspended by it, and is thus mechanically protected against sudden shocks, blows or pressure. The fluid assists in the maintenance of a constant environmental temperature for the foetus and allows of its free movements in utero. During parturition, the fluid helps to dilate the cervix uteri, acting mechanically within the amniotic sac as a fluid wedge.
the villi. The barrier of tissue between the two
circulations is the placental membrane (see later).  


The villi are at first scattered over the surface of the  
At full term about 1*5 litres of amniotic fluid are present, but the volume relative to the volume of the foetus has diminished somewhat in the latter onethird of pregnancy. Flexner and Gellhorn (1942) using radioactive sodium and heavy water have shown that, in the guinea-pig, at least, there is constant exchange of these substances between the maternal circulation and the amniotic fluid. The exchange of water is such that a volume equal to that of the amniotic fluid, is exchanged every hour.
entire chorion, and it is then known as the chorion
frondosum. During the fourth month the villi
disappear from that part of the chorion related to the
decidua capsularis, this portion being termed the  
chorion laeve. The persisting part of the chorion
frondosum becomes transformed into the mature
discoidal placenta.  


The Placenta. — In the human subject the placenta
is a disc-like, flattened cake with the chorion attached
to its margins. The foetal surface is loosely covered
by the amnio tic sac and gives attachment to the
umbilical cord. Between the amnion and the foetal
surface of the placenta are found large vessels, the
major branches of the umbilical (allantoic) arteries
and vein. From the uterine aspect it can be seen
that the placenta is subdivided into a number (15 or
more) of areas by septa. These areas are called cotyledons and each corresponds to a bunch of chorionic
villi. At birth the placenta is about 20 centimetres
in diameter, some 3 centimetres in thickness and
weighs about 500 grams. The attachment of the
umbilical cord is usually eccentric. Sometimes the
attachment of the cord is to the outer margin of the
placenta forming the so-called battledore placenta ;
in other cases the cord sub-divides before reaching
the placenta giving rise to the placenta furcata. The
vessels too, may spread out in the investing membranes instead of at the placental area, and form what
is known as the velamentous placenta. Accessory
lobules of the placenta separated from the main mass
are not uncommon (placenta succenturiata) . They
probably arise from abnormal persistence of a group
of villi of the chorion laeve.


While the normal volume of the amniotic fluid is about i*5 litres, an increase (hydramnios) or a decrease (oligamnios) may be found. In the latter event difficulty in parturition may arise.


32
==The Yolk Sac==


The formation of the primary yolk sac and its later diminution in size with the development of the extra-embryonic coelom has already been mentioned (p. 19). In the later part of the third week of development areas of angiogenesis are found in the splanchnopleuric mesoderm (extra-embryonic) which clothes the yolk sac, and soon a network of vessels covers its surface. These become organized as paired vitelline arteries and veins which link up with the early intra-embryonic circulation. The yolk sac is concerned during early development with transfer of nutritive material from the trophoblast to the embryo, but this function is only transient until the chorionic villi become vascularized. The further fate of the vitelline vessels is considered on page 114.


AIDS TO EMBRYOLOGY


When the embryonic disc becomes folded into a cylindrical embryo the large yolk sac naturally becomes constricted at the site of the future umbilicus and a portion of it becomes incorporated in the body of the embryo as the entodermal lining of the primitive gut. This part is connected with the remainder of the yolk sac (definitive yolk sac) by an elongated duct buried in the mesoderm of the body stalk (vitelline duct ; see Fig. 8). These two structures later degenerate and disappear ; the yolk sac may usually be found, however, as a little vesicle situated towards the placental end of the umbilical cord until the fifth or sixth month of pregnancy.


Functions of the Placenta. — The placenta functions
==The Allantois==
in a triple capacity ; it isolates the foetal from the
maternal organism so that the blood stream of each
remains distinct ; it permits the exchange of nutritive
substances from the maternal to the foetal circulation,
and of waste products from the foetal to the maternal ;
hormones probably also pass from the mother to the
child ; and the placenta itself elaborates hormones
(oestrogens) which pass into the maternal bloodstream,
and of which large quantities are found in the urine
of pregnant women.


The cellular layers which separate the maternal
During the third week of development, a diverticulum grows into the mesoderm of the body stalk from the caudal wall of the yolk sac.
blood in the intervillous spaces of the placenta and
the foetal blood in the chorionic capillaries obviously
determine the rate of transmission of substances from
the one circulation to the other. These cellular
layers are collectively known as the placental membrane. During the latter part of gestation there is a  
progressive thinning of this placental membrane due
to disappearance of the cytotrophoblastic layer of the  
villi, which are then clothed only by thinned-out
syncytium. Without any great increase in size
the placenta becomes functionally more efficient.
Gellhorn, Flexner and Heilman (1943) have been able
to study the transfer of radio-active sodium across
the human placenta at various stages and find that
the permeability to this substance is increased about
seven-fold in the full-term placenta as compared with
that of the tenth week of gestation.  


The Amnion. — The amniotic cavity is commencing
its formation in the eight day human ovum (p. 15).
In the twelve-day ovum it is seen to lie between the
ectodermal cells of the embryonic disc and a layer of
flattened epithelial cells overlying this and continuous
with its margin. The outer surface becomes covered
with extra-embryonic splanchnopleuric mesoderm.
Extension of the extra-embryonic coelom separates


This is known as the allanto-enteric diverticulum or allantois. This grows towards, but does not reach, the chorion, since it is a rudimentary structure in the human. Paired arteries and veins of great importance arise alongside it in the mesoderm of the body stalk. These form a connection between the vessels of the chorion and those of the embryo and are termed the umbilical arteries and veins.


IMPLANTATION AND PLACENTATION 33
The Umbilical Cord. — The umbilical cord arises by the elongation of the body stalk. Fig. 8 shows how it is formed. It is formed mainly of mesoderm covered externally by the amnion. In section this mesoderm is seen to be loosely arranged in a jellylike mass (Wharton’s jelly) and has embedded in it the umbilical vessels and the vitelline and allantoic ducts. One of the umbilical veins (the right one) and the vitelline duct soon disappear. The allantoic duct persists as a microscopic structure in the proximal part of the cord until full term. At this time the cord is a long twisted rope-like structure about 55 centimetres in length. It shows well-marked spirals running from left to right ; these spirals are probably caused by the vessels contained within it, growing more rapidly than the matrix of the cord. The length of the cord varies and may be as little as 10 or 15 centimetres, but sometimes it is excessively long and may measure as much as 105 centimetres.
 
the amnion from the inner surface of the chorion
except at the caudal end of the embryonic disc where
a mass, the body stalk, remains (see Fig. 8). When
the embryonic head, tail and lateral folds arise, the
line of attachment of the amnion is carried ventrally,
so that it appears to become constricted and the
embryo appears to rise up in the amniotic cavity.
The amnion is growing actively and its cavity is
increasing in size. Finally, its walls come in contact
with the inner aspect of the chorion and the extraembryonic coelom is thus obliterated. With this
growth the amnion comes to cover the outer aspect of
the body stalk and its contained structures (yolk sac,
allantois and allantoic vessels) which is now known as
the umbilical cord.
 
The Amniotic Fluid. — The amniotic cavity contains
fluid from the time of its first formation. This fluid
is derived from the walls of the cavity. In the
later stages of pregnancy foetal urine is added to the
amniotic fluid. The body stalk elongates considerably in the formation of the umbilical cord so that
the embryo floats in the amniotic fluid suspended by
it, and is thus mechanically protected against sudden
shocks, blows or pressure. The fluid assists in the
maintenance of a constant environmental temperature for the foetus and allows of its free movements
in utero. During parturition, the fluid helps to
dilate the cervix uteri, acting mechanically within
the amniotic sac as a fluid wedge.
 
At full term about 1*5 litres of amniotic fluid are
present, but the volume relative to the volume of the
foetus has diminished somewhat in the latter onethird of pregnancy. Flexner and Gellhorn (1942)
using radioactive sodium and heavy water have
shown that, in the guinea-pig, at least, there is constant
exchange of these substances between the maternal
circulation and the amniotic fluid. The exchange of
3
 
 
34 AIDS TO EMBRYOLOGY
 
water is such that a volume equal to that of the
amniotic fluid, is exchanged every hour.
 
While the normal volume of the amniotic fluid is
about i*5 litres, an increase (hydramnios) or a decrease (oligamnios) may be found. In the latter
event difficulty in parturition may arise.
 
The Yolk Sac. — The formation of the primary yolk
sac and its later diminution in size with the development of the extra-embryonic coelom has already been
mentioned (p. 19). In the later part of the third
week of development areas of angiogenesis are found
in the splanchnopleuric mesoderm (extra-embryonic)
which clothes the yolk sac, and soon a network of
vessels covers its surface. These become organized
as paired vitelline arteries and veins which link up
with the early intra-embryonic circulation. The yolk
sac is concerned during early development with
transfer of nutritive material from the trophoblast to
the embryo, but this function is only transient until
the chorionic villi become vascularized. The further
fate of the vitelline vessels is considered on page 114.
 
When the embryonic disc becomes folded into a
cylindrical embryo the large yolk sac naturally
becomes constricted at the site of the future umbilicus
and a portion of it becomes incorporated in the body
of the embryo as the entodermal lining of the primitive
gut. This part is connected with the remainder of the
yolk sac (definitive yolk sac) by an elongated duct
buried in the mesoderm of the body stalk (vitelline
duct ; see Fig. 8). These two structures later degenerate and disappear ; the yolk sac may usually
be found, however, as a little vesicle situated towards
the placental end of the umbilical cord until the fifth
or sixth month of pregnancy.
 
The Allantois. — During the third week of development, a diverticulum grows into the mesoderm of the
body stalk from the caudal wall of the yolk sac.
 
 
IMPLANTATION AND PL A CEN TATI ON 35
 
This is known as the allanto-enteric diverticulum or
allantois. This grows towards, but does not reach,
the chorion, since it is a rudimentary structure in the
human. Paired arteries and veins of great importance
arise alongside it in the mesoderm of the body stalk.
These form a connection between the vessels of the
chorion and those of the embryo and are termed the
umbilical arteries and veins.
 
The Umbilical Cord. — The umbilical cord arises by  
the elongation of the body stalk. Fig. 8 shows how  
it is formed. It is formed mainly of mesoderm  
covered externally by the amnion. In section this  
mesoderm is seen to be loosely arranged in a jellylike mass (Wharton’s jelly) and has embedded in it  
the umbilical vessels and the vitelline and allantoic  
ducts. One of the umbilical veins (the right one)  
and the vitelline duct soon disappear. The allantoic  
duct persists as a microscopic structure in the proximal  
part of the cord until full term. At this time the  
cord is a long twisted rope-like structure about 55  
centimetres in length. It shows well-marked spirals  
running from left to right ; these spirals are probably  
caused by the vessels contained within it, growing  
more rapidly than the matrix of the cord. The  
length of the cord varies and may be as little as 10  
or 15 centimetres, but sometimes it is excessively  
long and may measure as much as 105 centimetres.
 






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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.

   Aids to Embryology 1948: 1. Germ Cells | 2. Segmentation and Germ Layer Formation | 3. Changes in Female Genital Tract | 4. Implantation and Placentation | 5. Formation of the Embryo | 6. Skin and Accessory Structures | 7. Nervous System | 8. Special Sense | 9. Alimentary Canal | 10. Circulatory System | 11. Coelomic Cavities | 12. Urogenital System | 13. Muscular and Skeletal Systems | 14. Hereditary
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Chapter IV Implantation and Placentation

The Implantation of the Ovum

The problem of the mechanism of implantation of the human ovum has presented numerous difficulties in the past, because there were described only a relatively small number of good specimens to illustrate the condition. The investigations of Wislocki and Streeter (1938) gave, for the first time, a connected account of implantation in a primate, and the technical methods devised for their study stimulated other workers, particularly Hertig and Rock to search for earlier stages in human development than had hitherto been known. At the present time our knowledge of the process of implantation in the human has been greatly extended by the recovery of normal human ova as young as seven and a half days conceptional age. The following account of implantation in the human is based on the work of Hertig and Rock (1944).


The human ovum is fertilized at the outer end of the uterine tube, and while segmentation takes place, the ovum passes to the uterine cavity, its journey occupying probably three days. Its transport is accomplished partly by the downward streaming flow of fluid directed by the ciliary action of the tubal epithelium and partly by peristaltic contractions of the tubal musculature. The ovum arrives in the uterine cavity at the morula stage. It was formerly thought that the human ovum remained free in the uterine cavity for the next six days (Teacher, 1926) being nourished by the secretion of the uterine glands (‘ uterine milk ’). It is now known that the human ovum starts to implant about the seventh day since the youngest human embryo thus far studied (seven and a half days), is already superficially attached to the endometrium. It is then in the blastocyst stage ; the zona pellucida has disappeared. The embryonic pole of the trophoblast comes in contact with the epithelium, usually on the anterior or posterior wall of the uterus, and the epithelial cells are broken down by the trophoblastic cell secretion. That part of the trophoblast in contact becomes greatly thickened and shows two kinds of cells : ( a ) peripheral syn cytiotrophoblast cells, that is, a layer where cell boundaries are not distinct, which actively erodes and penetrates the endometrium ; and ( b ) cytotrophoblast cells forming a layer next to the cavity of the blastocyst. The syncytiotrophoblast cells proliferate rapidly, actually erode and digest the endometrial stroma, and soon they form a*thick zone of anastomosing strands with spaces or lacunae between them. The lacunae contain broken down endometrial cells and some maternal blood derived from eroded uterine vessels. While these events take place the developing blastocyst sinks into a cavity in the substance of the endometrium, the implantation cavity. The site of entry into the endometrium becomes obliterated, first by a plug of fibrinous material and later, by reepithelialization from the surrounding intact uterine cells. This mode of implantation is termed interstitial.


Continued activity of the cells of the syncytiotrophoblast results in further destruction of maternal tissue and opening up of blood vessels so that the lacunar spaces become progressively filled with maternal blood. A few days after implantation commences there is a slow circulation of maternal blood through the lacunae of the trbphoblast. Before this time the developing embryo depends for its nutrition upon the absorption of broken down endometrial stroma cells. Certain of these, termed decidual cells, contain much glycogen and fat and are prominent around the implantation cavity. This type of nutrition is called embryo trophic. When, however, maternal blood flows through the syncytial lacunae of the implantation cavity a haemotrophic form of nutrition for the embryo is established.


When the ovum has become implanted in the endometrium the destructive powers of the syncytiotroph oblast gradually diminish and beyond the zone of destruction there is reaction on the part of the maternal tissues to hinder the erosive activities of the growing blastocyst.


In the normal course of events the ovum becomes embedded near the fundus of the uterus, but in certain cases it wanders and implants itself in the lower part of the uterus near the cervix. When later the placenta is formed, it will partly or completely cover the internal os of the uterus, and the important clinical condition of placenta praevia results. Some times the ovum may be retarded in its journey towards the uterus and will attempt to embed itself in the tubal mucosa. The embryo develops for a short time but the condition usually terminates by the rupture of the tube, a grave surgical emergency.

The Decidua

During the process of implantation the developing embryo has come to lie in a little cavity in the endometrium which, altered somewhat in character, is now known as the decidua. Different names are applied to various regions of this : thus the decidua immediately deep to the implanted ovum is the decidua basalis, that covering the developing embryo is the decidua capsularis and the remainder which lines the uterine cavity is the decidua parietalis.

Formation of Chorionic Villi

The chorion may be defined as the trophoblast along with its somatopleuric lining of extra-embryonic mesoderm. It is through this simple type of chorion that nourishment passes by diffusion in the early stages of implantation, but, with increasing differentiation and the formation of an early embryonic circulation, the chorion undergoes structural modifications leading eventually to the formation of the placenta. The strands of syncytiotrophoblast which form the boundaries of the lacunae form the primary chorionic villi. These soon show a core of cytotrophoblast and commence to branch. Strands of extra-embryonic mesoderm appear within the cytotrophoblast of the villi and these shortly become continuous with the layer of extra-embryonic mesoderm forming the deeper part of the chorion. It is disputed whether the mesodermal core of the villus arises in situ by delamination from the cytotrophoblast or whether it actively invades the solid villus from the chorionic aspect.


About the end of the third week of development (twenty to twenty-one days) blood vessels differentiate in situ in the mesodermal cores • of the villi (Hertig x 935) an d become connected with other vessels which are laid down in the chorionic mesoderm. These in turn become linked up with the primitive intraembryonic circulation by vessels (allantoic, see p. 35 ) which run to the embryo in the body stalk. These villi become more complex by branching. Some of them pass right through the peripheral layer of syncytiotrophoblast which bounds the now confluent lacunae, and are attached to the decidua. These are termed anchoring villi, but the majority of the villi hang free in a blood-filled space, the intervillous space, in which maternal blood slowly circulates. There is no communication between this maternal blood and the embryonic blood cells in the vessels of the villi. The barrier of tissue between the two circulations is the placental membrane (see later).



Fig. 8. Longitudinal Section of early Embryo and Membranes.

1, Extra-embryonic coelom ; 2, amniotic cavity ; 3, yolk sac ; 4, body stalk ; 5, allantoic duct ; 6, embryonic disc ;

7, chorionic villi.


The villi are at first scattered over the surface of the entire chorion, and it is then known as the chorion frondosum. During the fourth month the villi disappear from that part of the chorion related to the decidua capsularis, this portion being termed the chorion laeve. The persisting part of the chorion frondosum becomes transformed into the mature discoidal placenta.

The Placenta

In the human subject the placenta is a disc-like, flattened cake with the chorion attached to its margins. The foetal surface is loosely covered by the amnio tic sac and gives attachment to the umbilical cord. Between the amnion and the foetal surface of the placenta are found large vessels, the major branches of the umbilical (allantoic) arteries and vein. From the uterine aspect it can be seen that the placenta is subdivided into a number (15 or more) of areas by septa. These areas are called cotyledons and each corresponds to a bunch of chorionic villi. At birth the placenta is about 20 centimetres in diameter, some 3 centimetres in thickness and weighs about 500 grams. The attachment of the umbilical cord is usually eccentric. Sometimes the attachment of the cord is to the outer margin of the placenta forming the so-called battledore placenta ; in other cases the cord sub-divides before reaching the placenta giving rise to the placenta furcata. The vessels too, may spread out in the investing membranes instead of at the placental area, and form what is known as the velamentous placenta. Accessory lobules of the placenta separated from the main mass are not uncommon (placenta succenturiata) . They probably arise from abnormal persistence of a group of villi of the chorion laeve.

Functions of the Placenta

The placenta functions in a triple capacity ; it isolates the foetal from the maternal organism so that the blood stream of each remains distinct ; it permits the exchange of nutritive substances from the maternal to the foetal circulation, and of waste products from the foetal to the maternal ; hormones probably also pass from the mother to the child ; and the placenta itself elaborates hormones (oestrogens) which pass into the maternal bloodstream, and of which large quantities are found in the urine of pregnant women.


The cellular layers which separate the maternal blood in the intervillous spaces of the placenta and the foetal blood in the chorionic capillaries obviously determine the rate of transmission of substances from the one circulation to the other. These cellular layers are collectively known as the placental membrane. During the latter part of gestation there is a progressive thinning of this placental membrane due to disappearance of the cytotrophoblastic layer of the villi, which are then clothed only by thinned-out syncytium. Without any great increase in size the placenta becomes functionally more efficient. Gellhorn, Flexner and Heilman (1943) have been able to study the transfer of radio-active sodium across the human placenta at various stages and find that the permeability to this substance is increased about seven-fold in the full-term placenta as compared with that of the tenth week of gestation.

The Amnion

The amniotic cavity is commencing its formation in the eight day human ovum (p. 15). In the twelve-day ovum it is seen to lie between the ectodermal cells of the embryonic disc and a layer of flattened epithelial cells overlying this and continuous with its margin. The outer surface becomes covered with extra-embryonic splanchnopleuric mesoderm. Extension of the extra-embryonic coelom separates the amnion from the inner surface of the chorion except at the caudal end of the embryonic disc where a mass, the body stalk, remains (see Fig. 8). When the embryonic head, tail and lateral folds arise, the line of attachment of the amnion is carried ventrally, so that it appears to become constricted and the embryo appears to rise up in the amniotic cavity. The amnion is growing actively and its cavity is increasing in size. Finally, its walls come in contact with the inner aspect of the chorion and the extraembryonic coelom is thus obliterated. With this growth the amnion comes to cover the outer aspect of the body stalk and its contained structures (yolk sac, allantois and allantoic vessels) which is now known as the umbilical cord.


The Amniotic Fluid

The amniotic cavity contains fluid from the time of its first formation. This fluid is derived from the walls of the cavity. In the later stages of pregnancy foetal urine is added to the amniotic fluid. The body stalk elongates considerably in the formation of the umbilical cord so that the embryo floats in the amniotic fluid suspended by it, and is thus mechanically protected against sudden shocks, blows or pressure. The fluid assists in the maintenance of a constant environmental temperature for the foetus and allows of its free movements in utero. During parturition, the fluid helps to dilate the cervix uteri, acting mechanically within the amniotic sac as a fluid wedge.

At full term about 1*5 litres of amniotic fluid are present, but the volume relative to the volume of the foetus has diminished somewhat in the latter onethird of pregnancy. Flexner and Gellhorn (1942) using radioactive sodium and heavy water have shown that, in the guinea-pig, at least, there is constant exchange of these substances between the maternal circulation and the amniotic fluid. The exchange of water is such that a volume equal to that of the amniotic fluid, is exchanged every hour.


While the normal volume of the amniotic fluid is about i*5 litres, an increase (hydramnios) or a decrease (oligamnios) may be found. In the latter event difficulty in parturition may arise.

The Yolk Sac

The formation of the primary yolk sac and its later diminution in size with the development of the extra-embryonic coelom has already been mentioned (p. 19). In the later part of the third week of development areas of angiogenesis are found in the splanchnopleuric mesoderm (extra-embryonic) which clothes the yolk sac, and soon a network of vessels covers its surface. These become organized as paired vitelline arteries and veins which link up with the early intra-embryonic circulation. The yolk sac is concerned during early development with transfer of nutritive material from the trophoblast to the embryo, but this function is only transient until the chorionic villi become vascularized. The further fate of the vitelline vessels is considered on page 114.


When the embryonic disc becomes folded into a cylindrical embryo the large yolk sac naturally becomes constricted at the site of the future umbilicus and a portion of it becomes incorporated in the body of the embryo as the entodermal lining of the primitive gut. This part is connected with the remainder of the yolk sac (definitive yolk sac) by an elongated duct buried in the mesoderm of the body stalk (vitelline duct ; see Fig. 8). These two structures later degenerate and disappear ; the yolk sac may usually be found, however, as a little vesicle situated towards the placental end of the umbilical cord until the fifth or sixth month of pregnancy.

The Allantois

During the third week of development, a diverticulum grows into the mesoderm of the body stalk from the caudal wall of the yolk sac.


This is known as the allanto-enteric diverticulum or allantois. This grows towards, but does not reach, the chorion, since it is a rudimentary structure in the human. Paired arteries and veins of great importance arise alongside it in the mesoderm of the body stalk. These form a connection between the vessels of the chorion and those of the embryo and are termed the umbilical arteries and veins.

The Umbilical Cord. — The umbilical cord arises by the elongation of the body stalk. Fig. 8 shows how it is formed. It is formed mainly of mesoderm covered externally by the amnion. In section this mesoderm is seen to be loosely arranged in a jellylike mass (Wharton’s jelly) and has embedded in it the umbilical vessels and the vitelline and allantoic ducts. One of the umbilical veins (the right one) and the vitelline duct soon disappear. The allantoic duct persists as a microscopic structure in the proximal part of the cord until full term. At this time the cord is a long twisted rope-like structure about 55 centimetres in length. It shows well-marked spirals running from left to right ; these spirals are probably caused by the vessels contained within it, growing more rapidly than the matrix of the cord. The length of the cord varies and may be as little as 10 or 15 centimetres, but sometimes it is excessively long and may measure as much as 105 centimetres.



Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
   Aids to Embryology 1948: 1. Germ Cells | 2. Segmentation and Germ Layer Formation | 3. Changes in Female Genital Tract | 4. Implantation and Placentation | 5. Formation of the Embryo | 6. Skin and Accessory Structures | 7. Nervous System | 8. Special Sense | 9. Alimentary Canal | 10. Circulatory System | 11. Coelomic Cavities | 12. Urogenital System | 13. Muscular and Skeletal Systems | 14. Hereditary

Cite this page: Hill, M.A. (2024, May 28) Embryology Book - Aids to Embryology (1948) 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Aids_to_Embryology_(1948)_4

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