Download Extra Embryonic Membranes

[Academic Script]
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
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 extra-embryonic. These are mainly four
types. They are:
1. Yolk sac
2. Amnion
3. Chorion
4. Allantois
Development of Yolk SAC
In
reptiles
and
splanchnopleure
birds,
develop
the
from
somatopleure
the
periphery
of
and
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 placenta- Here, in addition to the uterine
epithelium and 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 semipermeable 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.