Download Extra Embryonic Membranes E

Extra Embryonic Membranes,
Types & Physiology of Placenta
Subject:
Zoology
Course:
B.Sc. 3rd Year
Paper No. & Title:
Z-305B
Developmental Biology
Topic No. & Title:
Topic – 5
Extra Embryonic
Membranes, Types &
Physiology of Placenta
Lecture Title:
Extra Embryonic
Membranes, Types &
Physiology of Placenta
Video Index
1. Introduction
2. Extra embryonic membranes in mammals
3. Function of extra embryonic membranes
4. Introduction of Placenta
5. Classification of mammalian placenta
6. Physiology of placenta
7. Summary
Academic Script
Extra embryonic Membranes
In vertebrate embryonic development, only a part of the egg or the
cleavage mass of cells forms the actual embryo, while other parts
lying outside the embryonic territory develop into extra-embryonic
regions, called embryonic or foetal membranes. Embryonic
membranes are auxiliary organs, which have arisen partly for
protection of the embryo, and more specially to provide for its
nutrition, respiration and excretion until the independent existence
is attained.
Extra embryonic membranes in chick
In chick, the presence of an enormous amount of yolk and
embryonic life to be spent within a shell is correlated with the
development of extra-embryonic membranes. Original blastoderm
is a small disc, which spreads by peripheral expansion and
eventually covers the entire surface of the egg. But only the most
central region is directly connected with the formation of the
embryo proper. All the remainder of the blastoderm is extraembryonic. These are mainly four types. They are:
1. Yolk sac
2. Amnion
3. Chorion
4. Allantois
Development of Yolk SAC
In reptiles and birds, the somatopleure and splanchnopleure
develop from the periphery of the blastodisc. These usually spread
peripherally over the yolk mass. Soon afterwards, the embryo
undergoes series of folds, which appear all around the body of the
embryo. These folds are termed as the body folds. The extra
embryonic splanchnopleure (splanchnic mesoderm + endoderm)
constantly spreads over the yolk mass and eventually yolk sac
encloses the mass of yolk in a large measure. The yolk sac,
however, not surrounds the yolk fully. A small passage is left on the
ventral side for the embryo to absorb the remains of albumen at a
later stage. Immediately with the formation of the yolk sac, the
intra embryonic splanchnopleure is subjected to fold resembling
with the more superficial body folds i.e., the intra embryonic folds.
The intra embryonic folds give rise to walled digestive tract, or gut,
in the body of the embryo. The middle of the embryonic gut
remains open to the yolk beneath. At this level, yolk sac is
connected to the digestive tract by a constricted yolk stalk.
Development Of Amnion & Chorion
The amnion and chorion are developed jointly as upward projecting
folds, the amniotic folds of the extra embryonic somatopleure.
The amniotic folds are named according to their location. They are
amniotic head fold and amniotic tail fold.
The amniotic fold first
appears as a transverse fold in front of the head. It is called
amniotic head fold. It grows upwards and then bends backwards,
over the anterior end of the head and covers it as with a hood.
Another fold develops behind the embryo. It is termed as amniotic
tail fold. All these folds finally cover an embryo in two sheets of
somatopleure. The inner somatopleuric sheet becomes the amnion
and the outer, the chorion.
The amnion consists of a layer of extra embryonic ectoderm on the
inside and a layer of extra embryonic somatic mesoderm on the
outside whereas, the chorion is made up of a layer of extra
embryonic ectoderm on the outside and a layer of extra embryonic
somatic mesoderm on the inside. The cavity between the amnion
and the embryo is called the amniotic cavity. In between, the
amnion and the chorion is the chorionic cavity
or extra
embryonic coelom.
DEVELOPMENT OF ALLANTOIS
The allantois arises as a ventral outgrowth of the splanchnopleure
from the hindgut on the third day of incubation. It slowly enlarges
as holosac and expands inside exocoel. Its walls are formed of an
outer splanchnic mesoderm and inner endoderm. The proximal part
of the allantois forms a slender neck or the allantoic stalk with
which it remains connected with the hindgut of the embryo. The
distal part of the allantois expands and penetrates between the
amnion and the yolk sac on one side and the chorion on the other
side. By the middle of the incubation period, the allantois spreads
all around the egg underneath the chorion. The mesoderm on the
external surface of the allantois fuses with that of the chorion
forming a conjoined chorio-allantoic membrane.
Extra embryonic membranes in Mammals
The developing embryo of rabbit and other eutherian mammals are
provided with four foetal or extra-embryonic membranes namely,
amnion, chorion, allantois and yolk sac as in chick. But in
marsupial and placental mammals, embryo depends on the mother
for food and oxygen and elimination of its own wastes. The foetal
membranes begin to develop while the contact with the uterine wall
is being established. Different extra-embryonic membranes of
mammals develop in the following manner:
DEVELOPMENT OF AMNION & CHORION
Amnion and chorion develops simultaneously as upward projecting
folds of somatopleure called amniotic folds. Two types of
amniotic folds develop. They are named according to their location.
They are the amniotic head fold, and the amniotic tail fold. First
of all, the amniotic tail fold appears. It grows over the embryo
dorsally and then grows forward. Afterwards, a transverse fold
appears in front of the head. It is known as amniotic head fold. It
grows upwards and then backwards over the embryo. Lastly the
head fold and the tail fold meet and fuse together. The place of
their final fold develops. First fusion is marked by a provisional
connection called seroamniotic connection or seroamniotic
raphe.
After the fusion of the folds, the inner and outer layers separate
and the inner layer becomes the chorion. The further development
is same as in chick.
DEVELOPMENT OF YOLK SAC
There is very less or no yolk in marsupial and eutherian eggs, yet a
yolk sac develops in their embryos which points to their reptilian
ancestor. In the mammals, yolk sac begins to form during early
gastrulation. In the blastocyst stage, the hypoblast endoderm
cells found in the inner cell mass starts to extend along the inside
surface of the trophoblast and encircles the inner cavity of the
blastocyst. The cavity of the blastocyst within the enveloping
endodermal layer is known as the yolk sac.
DEVELOPMENT OF ALLANTOIS
The development of allantois in mammals is same as in chick.
FUNCTIONS OF EXTRA EMBRYONIC MEMBRANES
FUNCTION OF YOLK SAC IN BIRDS & MAMMALS:
The function of yolk sac in birds is to digest the yolk and transfer
the product of digestion to the embryo. In mammals, the yolk sac
is very little or with no yolk.
FUNCTION OF AMNION IN BIRDS & MAMMALS:
The primary function of amnion is to protect the embryo from
dessication and provides “private salty” to an embryo to float.
Amniotic fluid serves as an efficient shock absorber.
FUNCTION OF CHORION IN BIRDS AND MAMMALS:
A part of chorion forms finger like out growths known as chorionic
villi that penetrate into the wall of uterus for exchange of
substances between embryo and uterus in the body of the mother.
FUNCTION OF ALLANTOIS IN BIRDS & MAMMALS:
In birds, cavity of allantois serves as an urinary bladder. In
mammals, the original function of allantois as urinary bladder
becomes all together lost. The carbon dioxide produced by the
embryo diffuses into the maternal blood and is excreted by the
kidney of the mother.
Types & physiology of Placenta
The
placenta
is
a
composite
structure
produced
by
the
development and apposition of the extra embryonic membranes
with the uterine endometrium for the purpose of physiological
exchange. In between these two parallel plates, a huge blood sinus,
the intervillous space, contains an enormous number of chorionic
villi. The placenta consists of two parts:
1. Foetal placenta
2. Maternal placenta
1. Foetal placenta:
It is formed by extra embryonic membrane
chorion which is the principal component of foetal placenta. It
establishes a vascular link between the embryo and the
maternal tissues.
2. Maternal
placenta:
Uterine
endometrium
is
a
solitary
component of maternal placenta.
Placenta is also found in diverse groups of the animal kingdom such
as in Peripatus, Salpa, Elasmobranchia and certain lizards. In each
case, mode of origin and the structure of placenta is different.
CLASSIFICATION OF MAMMALIAN PLACENTA
The mammalian placenta is classified into four different types.
A. TYPES OF PLACENTA ACCORDING TO THE NATURE OF THE
FOETAL MEMBRANES TAKING PART IN THE FORMATION OF
PLACENTA
1. Chorio-vitelline or “Yolk-sac” placenta-It is a primitive type of
placenta found in some of the marsupials. E.g., Opossum and
Kangaroo.
In
this
type
of
placenta,
the
allantois
remains
comparatively tiny and never makes fusion with the chorion, while
the yolk sac becomes very huge and combines broadly with the
chorion.
2. Chorio-allantoic placenta- In chorio-allantoic placenta, the yolk
sac remains undeveloped. The fusion found between the uterine
wall and the embryo is lined by chorion and allantois. So, allantois
furnishes the chorionic circulation. Since the placenta is formed of
chorion and allantois, it is termed as chorio-allantoic placenta.
E.g., Parameles, Dasyurus.
3. Chorionic placenta- The chorionic placenta is formed of thickened
layer of chorion containing sinuses filled with maternal blood.
Chorionic “villi” containing foetal connective tissue and capillaries
expand and cross the chorionic lacunae. Chorionic type of placenta
is found in human beings. The portion of the trophoblast which is
nearer to the embryo is known as cytotrophoblast. The more
external lying part of the trophoblast is called syncytiotrophoblast
as it is a syncytium of irregular strands with interstices in between.
The part of the uterine wall to which the placenta is joined is
termed as decidua basalis. The portion of uterine mucosa and
epithelium which is over the blastocyst forming a capsule, is known
as decidua capsularis, the left over part of the uterine wall with
which the chorion finally comes in touch, is called as
decidua
vera.
B. TYPES OF PLACENTA ACCORDING TO THE DEGREE OF
INTIMACY BETWEEN THE FOETAL AND MATERNAL TISSUE
Five sub-types of placenta can be distingushed.
1. Epithelio-chorial placenta- It’s the simplest type of placenta
where the villi of the chorion dip into the crypts of the uterine wall.
Eg., All marsupials, some ungulates and lemurs. Therefore, the
molecules of oxygen and other nutrients diffuse from the blood of
the mother to that of the embryo through
I. Maternal endothelium
II. Maternal connective tissue
III. Maternal uterine epithelium
IV. The Epithelium of chorion
V. Foetal connective tissues and
VI. Foetal endothelium
2. Syndesmochorial Placenta- In this type of placenta, the uterine
epithelium is eroded. The chorion comes in contact with maternal
connective tissue. Thus the nutrients pass through maternal
endothelium, maternal connective tissue, chorion, foetal connective
tissue and foetal endothelium. This type of placenta is found in
ruminant ungulates.
3. Endotheliochorial placenta-In this type, uterine epithelium and
maternal
connective
tissues
are
eroded.
Thus
the
nutritive
materials pass through the maternal endothelium, chorion, the
foetal connective tissues and foetal endothelium. This condition is
found in carnivores.
4. Haemochorical
epithelium
and
placenta- Here, in addition to the uterine
maternal
connective
tissues,
the
maternal
endothelium is also eroded. Thus the chorionic villi directly dip in
maternal blood. This type of placenta is found in lower rodents,
insectivores, bats and man.
5. Haemoendothelial placenta- In this type of placenta all the three
maternal tissues and two foetal tissues i.e., chorionic epithelium
and chorionic tissue are completely eroded. The foetal blood vessels
dip into blood lacunae of the uterus. The number of barriers
between the maternal and foetal blood streams, therefore is
reduced to just one. E.g., Higher rodents and rabbit.
C. TYPES OF PLACENTA ACCORDING TO THE DEGREE OF
CONTACT
BETWEEN
CHORIONIC
VILLI
AND
THE
ENDOMETRIUM
Two sub-types of placenta may be recognized.
1. Non-deciduate
placenta-
The
chorionic
villi
are
simple
projections. They have loose connections with crypts in the uterine
epithelium. At the time of birth, the chorion peels off from the
uterine wall by pulling the villi out of the crypts. So no bleeding
occurs at parturition. It is found in pig, cattle, horse and other
ruminants.
2. Deciduate placenta- In higher Eutherian mammals including dog,
rabbit and man, the union between the chorion and the uterine
epithelium is much more close. The villi are so closely united with
the uterine wall that at parturition, a large part of the uterine tissue
is lost along with the foetal membranes. A large amount of bleeding
also occurs. As the uterine wall participates in the formation of
placenta, such a placenta is called decidua.
The decidua has three types as follows:
(i) Decidua basalis: The upper part of uterine wall to which the
embryo becomes attached is called decidua basalis.
(ii) Decidua capsularis: The part which surrounds the blastocyst and
separates it from the cavity of the uterus is called decidua
capsularis.
(iii) Decidua parietalis: The parts which form the inner lining of the
uterine wall is called decidua parietalis.
D. TYPES OF PLACENTA ACCORDING TO THE DISTRUBUTION
PATTERN OF CHORIONIC VILLI
The placenta is classified into four sub- types according to the
distribution and arrangement of villi on the surface of the chorion.
1. Diffuse Placenta- The villi are uniformly distributed all over the
surface of the blastocyst. It is found in pig, horse and lemurs.
2. Cotyledonary placenta- The villi are arranged in patches. Each
patch of villi is known as cotyledon. The uterine wall is provided
with thickened sockets into which the cotyledons fit. E.g., Sheep,
cattle and deer.
3. Zonary Placenta- The villi are arranged in the form of a belt
around the middle of the chorionic sac. E.g., Cat and dog.
4. Discoidal Placenta- The villi are restricted to a small disc shaped
area of the blastocyst and hence known as discoidal placenta. E.g.,
Insectivores, rodents, anthropoid apes and bats.
Physiology of placenta
1. Placenta forms a physiological barrier and a semi-permeable
membrane between the mother and the foetus. It prevents the
straight mixing of the maternal and the foetal blood. It prevents the
entry of harmful materials.
2. It allows smaller molecules to diffuse. It provides nourishment and
oxygen to the embryo. Oxygen, water and small molecules such as
monosaccharides, salts of sodium, potassium and magnesium
diffuse from the maternal blood into the foetal blood through the
placenta. Macromolecules of polysaccharides, lipids and proteins
may be engulfed by trophoblast cells by pinocytosis.
3. The placenta provides immunity to the foetus against certain
diseases such as diphtheria, scarlet fever, small pox and measles.
The antibodies which have developed in the blood of mother
against these diseases are passed to the foetal placenta. Similarly
Rh antibody also pass through placenta. Blood proteins cannot pass
through the placenta because they are large molecules, so they are
broken into aminoacids and transmitted. The foetus rebuilds
complex proteins.
4. The most important function of placenta is the transfer of food
stuffs from the mother to the foetus.
5. Many drugs consumed by the mother penetrate the placental
barrier and cause most adverse effects in the embryo.
Eg- Thalidomide used as a sedative by women in early pregnancy
causes extensive deficiencies.
6. Certain pathogenic
organisms
and
viruses
can penetrate
through the placenta and infect the foetus if the mother is infected.
This is known to happen in infections with syphilis, smallpox,
chickenpox and measles.
SUMMARY
The extra embryonic membranes are protective membranous
structure that appear in parallel with the embryo and play an
important role in embryonic development. It consists of chorion,
amnion, yolk sac and allantois. The chorion participates in
exchange of gases between the embryo and its surroundings
whereas the amnion protects the embryo. The yolk sac is sole
source of food containing yolk and allantois stores the metabolic
waste of an embryo. Thus, the developing embryo is able to carry
on essential metabolism while sealed within the egg and womb.
The placenta is a materno-foetal temporary organ that develops at
the implantation and is required for development of embryo and
foetus. Its principal activities are metabolism, respiratory gas
exchange, transfer of nutrients, elimination of waste products and
endocrine secretion for maintenance of foetus during pregnancy.
Objective
 To study extra embryonic membranes in chick and mammals.
 To discuss functions of extra embryonic membranes.
 To classify mammalian placenta in depth.
 To elaborate physiology of placenta.
Frequently Asked Questions
Q1. Why is the human placenta designated as haemochorial
type of placenta?
A1. As the placental villi are bathed in maternal blood, the human
placenta is designated as haemochorial type of placenta.
Q2. What type of cells invades the maternal spiral arteries
and reduces the flow of blood from their open ends?
A2. Cytotrophoblast cells.
Q3. What are extra embryonic membranes?
A3. Extra embryonic membranes are membranous structures that
appear in parallel with the embryo and play important role in
embryonic development. They form from the embryo, but do not
become part of the individual organism after its birth.
Q4. What are extra embryonic membranes present in
vertebrates?
A4. The extra embryonic membranes that may be present in
vertebrates are the yolk sac, the amnion, the chorion, the allantois
and the placenta.
Q5. Are the extra embryonic membranes the same in all
vertebrates?
A5. The presence of each extra embryonic membrane varies
according to the vertebrate class. In fishes and amphibians, only
the yolk sac is present. In reptiles and birds, besides the yolk sac,
there are also the amnion, the chorion and the allantois. In
placental mammals, besides all these membranes the placenta is
also present.
Q6. Which is an extra embryonic membrane whose function
is to store nitrogen wastes of the embryo? Is this function
present in placental mammalian embryos?
A6. The allantois is the extra embryonic membrane whose function
is to store the excreted matter of the embryo. In placental
mammals, the allantois is present but it does not exert that
function, since the embryonic wastes are collected by the mother’s
body through the placenta.
Q7. What is the difference between amnion and chorion?
A7. Amnion is the membrane that covers the embryo. Chorion is
the membrane that covers the amnion, yolk sac and allantois. The
space delimited by the chorion and the amnion is called amniotic
activity and is filled with amniotic fluid.
Q8. Why can amnion also be considered as an adaptation to
terrestrial life?
A8. The amnion is also an adaptation to dry land since one of its
functions is to prevent desiccation of the embryo.
Q9. What is the chorioallantois membrane present in the
embryonic development of reptiles and birds? How does this
membrane participate in the energetic metabolism of the
embryo?
A9. The chorioallantois membrane is formed by juxtaposition of
some regions of the chorion and the allantois. Since it is porous,
the chorioallantois membrane allows the passage of gases between
the
embryo
and
the
exterior
thus
making
aerobic
cellular
respiration possible.
Q10. In which type of animals does the placenta exist?
Mention its main function?
A10. True placenta is present in placental mammals. The placenta
is formed from the chorion of the embryo and from the mother’s
endometrium. Its main function is to allow the exchange of
substances between the fetus and mother’s body.
Q11. What are the main substances transferred from the
mother to the fetus through the placenta and vice versa?
A11.
From the mother to the fetus, the main substances
transferred through placenta are water, oxygen, nutrients and
antibodies. From the fetus to the mother, metabolic wastes,
including urea (nitrogen waste) and carbon dioxide are transferred
through placenta.
Quiz
Q1. Yolk sac is not found in _____.
(A) Amphioxus
(B) Mammals
(C) Chick
(D) Birds
Q2. In placenta, if the allantois is not present, it is called ______.
(A) Chorionic
(B) Allantoic
(C) Chorio-vitelline
(D) Vitelline
Q3. Diffuse placenta occurs in ______.
(A) Pig
(B) Sheep
(C) Cat
(D) None
Q4. Foetal membranes are found only in _______ .
(A) Invertebrates
(B) Vertebrates
(C) Both of these
(D) None of these.
Q5. In chick embryo, enormous amount of _____ is present in a
shell.
(A) Chorion
(B) Amnion
(C) Allantois
(D) Yolk sac
Q6. In ______ placenta, the villi are arranged in form of a belt
around the middle of chorionic sac.
(A) Discoidal
(B) Diffuse
(C) Cotyledonary
(D) Zonary
Q7. The process of pinocytosis is employed by ______cells.
(A) Trophoblast
(B) Hypoblast
(C) Meroblast
(D) None of these
Q8. _____covers an embryo in two sheets of somatopleure.
(A) Yolk sac
(B) Allantois
(C) Amnion and chorion
(D) Amnion alone.
Q9. Blastodisc is found in ______.
(A) Chick
(B) Amphioxus
(C) Frog
(D) Mammal
Q10. Ruminants provide an example of____ placenta.
(A) Cotyledonary
(B) Discoidal
(C) Zonary
(D) None of these.
Q11. In the mature placenta, which foetal tissue directly interferes
with the maternal connective tissue?
(A) Cytotrophoblast
(B) Syncytiotrophoblast
(C) Extraembryonic mesoderm
(D) Decidual cells.
Q12. Blood vessels associated with which structure enter the foetal
component of the placenta?
(A) Decidua basalis
(B) Allantois
(C) Amnion
(D) Yolk sac
Answer
Q1. Yolk sac is not found in _____.
A1. Mammals
Q2. In placenta, if the allantois is not present, it is called ______.
A2. Chorionic
Q3. Diffuse placenta occurs in ______.
A3. Pig
Q4. Foetal membranes are found only in _______ .
A4. Vertebrates
Q5. In chick embryo, enormous amount of _____ is present in a
shell.
A5. Yolk sac
Q6. In ______ placenta, the villi are arranged in form of a belt
around the middle of chorionic sac.
A6. Zonary
Q7. The process of pinocytosis is employed by ______cells.
A7. Trophoblast
Q8. _____covers an embryo in two sheets of somatopleure.
A8. Amnion and chorion
Q9. Blastodisc is found in ______.
A9. Chick
Q10. Ruminants provide an example of____ placenta.
A10. Cotyledonary
Q11. In the mature placenta, which foetal tissue directly interferes
with the maternal connective tissue?
A11. Cytotrophoblast
Q12. Blood vessels associated with which structure enter the foetal
component of the placenta?
A12. Allantois
Summary
The extra embryonic membranes are protective membranous
structure that appear in parallel with the embryo and play an
important role in embryonic development. It consists of chorion,
amnion, yolk sac and allantois. The chorion participates in
exchange of gases between the embryo and its surroundings
whereas the amnion protects the embryo. The yolk sac is sole
source of food containing yolk and Allantois stores the metabolic
waste of an embryo. Thus, the developing embryo is able to carry
on essential metabolism while sealed within the egg and womb.
The placenta is a materno-foetal temporary organ that develops at
the implantation and is required for development of embryo and
foetus. Its principal activities are metabolism, respiratory gas
exchange, transfer of nutrients, elimination of waste products and
endocrine secretion for maintenance of foetus during pregnancy.
Tutorials
No tutorial for this lecture.
Assignments
1. What
are
extra-embryonic
membranes?
Describe
the
origin,
development, structure, functions and fate of chorion and amnion
in chick.
2. What are the major differences in the foetal membranes of chick
and mammals?
3. Justify why allantois is known as the urinary bladder of the
developing chick.
4. Define placenta. Describe its formation, structure and function in
mammals.
5. Classify the different types of placenta in various vertebrates.
References
Books
 Text Book of Embryology – D. R. Khanna.
 Developmental biology - Scott F. Gilbert.
 Developmental biology – Dr. K. V. Sastry & Dr. Vinita Shukal,
Rastogi Publications.
Links
 https://www.us.elsevierhealth.com/media/us/samplechapters/978
0323053853/Chapter%2007.pdf
 www.med.umich.edu/Irc/coursepages/m1/embryology/embryo/06
placenta.htm
 education-portal.com/academy/lesson/the-placenta-and-the-fetusstructure-and-function.html#lesson
 en.wikipedia.org/wiki/Placenta
 www.glowm.com/section_view/heading/Placental%20Physiology/it
em/195
Glossary
Allantoic stalk
The narrow connection between the intra embryonic portion of the
allantois and the extra embryonic allantoic vessel.
Allantois
It is the foetal membrane formed as an outgrowth of the embryo’s
gut. It lies below the chorion in many vertebrates. In birds and
reptiles, it grows to surround the embryo; in eutherian mammals,
it forms part of the placenta.
Amnion
The innermost extra embryonic membrane that encloses the
embryo of a mammal, bird or reptile.
Blastoderm
A blastula having the form of a disc on top of the yolk.
Chorion
The outermost extra embryonic membrane surrounding of a reptile,
bird, or mammal. In mammals, it contributes to the formation of
the placenta.
Desiccation
It is the state of extreme dryness, or the process of extreme
drying.
Insectivores
An animal that feeds on insects, worms, and other invertebrates.
Parturition
The action of giving birth to young; childbirth.
Pathogenic organism
An organism usually bacteria or virus, that cause disease.
Pinocytosis
The ingestion of liquid into a cell by the budding of small vesicles
from the cell membrane.
Ruminants
An even-toed ungulate mammal that chews the cud regurgitated
from its rumen. The ruminants comprise the cattle, sheep,
antelopes, deer, giraffes, and other relatives.
Sedative
A drug taken for its calming or sleep-inducing effect.
Semi-permeable membrane
It is a type of selectively permeable membrane that will allow
certain molecules or ions to pass through it by diffusion.
Somatopleure
A layer of tissue in a vertebrate embryo comprising the ectoderm
and the outer layer of mesoderm, and giving rise to amnion,
chorion, and part of the body wall.
Splanchnopleure
A layer of tissue in a vertebrate embryo comprising the endoderm
and the inner layer of mesoderm, and giving rise to the gut, lungs
and yolk sac.
Synctium
A single cell or cytoplasmic mass containing several nuclei, formed
by fusion of cells or by division of nuclei.
Trophoblast
A layer of tissue on the outside of a mammalian blastula,
supplying the embryo with nourishment and later forming the
major part of the placenta.
Yolk sac
A membranous sac containing yolk attached to the embryos of
reptiles and birds and the larva of some fishes. A sac lacking yolk
in the early embryo of a mammal.
Yolk stalk
In humans, it is a long narrow tube that joints the yolk sac to the
midgut lumen of the developing foetus. It appears at the end of
the fourth week, when the yolk sac presents the appearance of a
small-pear shaped vesicle.