The attachment of developing embryo to the appropriate body layer or surface to obtain nutrition is called implantation. This phenomenon is a common event in most mammals (except prototheria) in which embryo (blastocyst stage) after reaching in uterus attaches itself with the wall of the uterus. In other animals like fishes, reptiles, birds, prototherian mammals etc., this nutritive connection is established with the yolk present in egg. In higher mammals including men, the blastocyst on its contact with endometrium of uterus gets completely buried in the wall of the uterus.
Process of Implantation
Mechanism of Implantation
Initially the oocyte after its release from ovary, comes into falopian tube where the process of fertilization is completed. Just after fertilization, embryonic development starts and a blastocyst is formed after cleavage and morulation. In human being, the blastocyst gets attached with the uterine endometrium in about four days after entering in uterus. At the same time , the cells of endometrium of implantation area separate out and adhere with embryonic cells with the help of certain enzymes secreted by the cells of trophoblast. In human, the site of implantation is generally mid-dorsal/fundus part of uterus. Implantation of blastocyst takes about 7-8 days after fertilization in human and by 12th day it is completely buried in the wall of the uterus. The place of entry through which the embryo enters into the wall, is completely closed by a fibrous and cellular plug, known as closing coagulum.
Types of Implantation
On the basis of the position of attachment in the uterus, implantation is of three types-
1. Central or Superficial implantation- In this type the blastocyst attaches superficially with the wall of uterus, and remains suspended in the lumen of the uterus.
Example: cow, pig, dog etc.
Fig: Types of implanatation
2. Interstitial implantation- The blastocyst is buries deeply inside the wall of uterus and covered by the endometrial tissues lying under epithelium. This type of implantation occurs in human being.
3. Eccentric implantation- It occur in rat, squirrel etc. In this type of implantation, the blastocyst settles in in the folds of epithelium of uterus. After some time it is completely surrounded by these folds.
Summary of developmental stages in human
Day 1 - Fertilization; the diameter of fertilized egg is about 0.15 mm.
Day 2 - Two cell stage
Day 3 - 16 cell stage, morula
Day 4 - Entry of blastocyst in the lumen of uterus, disappearance of zona pellucida, diameter of blastocyst is about 0.3 mm.
7-8 Days - Partial entry of blastocyst inside the endometrium of uterus; implantation
Day 12 - Complete entry of blastocyst into endoderm, extra-embryonic mesoderm, amnion and yolk sac.
Day 14 - primitive streak formation.
Day 18 - Formation of 3-5 pairs of somites
The above mentioned timing are approximate time periods. Some times, due to some reasons, certain babies are born before stipulated time. The babies born in 7th month may also survive as normal babies.
EXTRA EMBRYONIC MEMBRANES AND PLACENTA
Extra embryonic membranes
In chordates like reptiles, birds and prototherian mammals, blastula is a disc shaped structure called as blastodisc.
Fig: Extraembryonic membrane in human
The cellular layer formed of blastomeres remains as blastoderm. The central part of blastoderm gives rise to embryo proper, while the peripheral portion does not take part in the formation of embryo. This peripheral part is known as extra embryonic region. This region takes part in the formation of certain membranes called extra embryonic membranes. These extra embryonic membranes provide facilities for nutrition, respiration and excretion to the embryo. Extra embryonic membranes are of four types-
3. Yolk sac
On the basis of presence or absence of amnion, two groups of vertebrates are categorized
1. Amniota - This group is characterized with the presence of amnion in the embryos of its members.
For example members of class Reptilia, Aves and Mammalia.
2. Anamniota - Animals of this group are devoid of amnion in their embryos. For example class cyclostomata, pisces and amphibia.
Extra embryonic membranes in human
The process of gastrulation in embryo results into the formation of endoderm or hypoblast, ectoderm or epiblast, amniotic cavity, yolk sac, extra embryonic parietal and visceral mesoderm, connecting stalk etc. Extra embryonic membranes are also formed during this process. Each extra embryonic membrane is derived from two layers.
1. Amnion- It is formed by the layer of amniogenic cells present around the amniotic cavity and the extra embryonic mesoderm. Extra embryonic mesoderm layer surrounds the amnion. The connecting stalk is also attached with it. With a gradual increase in size the amnion covers the embryo from all sides. After about eight weeks of fertilization, amnion is completely incorporated into connecting stalk, which finally forms the umbilical cord. Embryo, in this stage, is called as foetus remains hanging in amniotic fluid.
Fig: Formation of extraembryonic membrane in human2. Chorion- It is formed by the extra embryonic parietal layer of mesoderm and the cell of trophoblast.
After implantation of blastocyst, the trophoblast gives out several finger like processes, the chorionic villi which get embedded into uterine endometrium Mesoderm also contributes in the formation of these villi. After a period of four month these villi disappear from all parts except the connecting stalk where they grow rapidly and participate in the formation of placenta.
3. Yolk sac- Yolk sac is formed by the cells of extra embryonic visceral mesoderm and endoderm. Initially the size of yolk sac is larger as compared to that of the embryo. About eight weeks after fertilization, the yolk is reduced in size and changes into a tubular structure. Ultimately a placenta is developed with the incorporation of yolk sac and mesodermal connecting stalk with the amnion and chorion.
4. Allantois- It is a solid and cylindrical mass formed by embryonic mesoderm. A small cavity lined by endodermal cells develops in it. The mesoderm of allantois forms many small blood vessels in this region. These vessels connect the embryo with placenta and ensure nutritional and respiratory supply to embryo. In human, allantois does not function to store the excretory wastes as it does in reptiles and birds.
The eggs of viviparous animals are unable to develop into their embryos outside the uterus independently. This is because of the very little or negligible amount of yolk present in these eggs, which can not fulfill the nutritional and other physiolgical demands of a developing embryo. Here the embryo depends upon maternal tissues for shelter, nutrition, respiration etc. These animals therefore, have developed adaptation, respiratory and other physiological requirements from mother's body.
Placenta is found in all viviparous (except sub-class-prototheria; oviparous) animals.
Structure of Placenta
Placenta is not a simple membrane.It is made up of the tissues from two different sources
Maternal tissue - These include uterine epithelium, connective tissues and blood capillaries.
Embryonic tissue - These include extra embryonic membranes (mainly chorion). Yolk sac and allantois may also take part in placenta formation. Embryonic connective tissues and blood capillaries are also constituents of it.
Fig: The placental villi embryology
On the basis of extra embryonic membranes, the placenta is of three types.
1. Yolk sac placenta- It is formed by yolk sac and uterine epithelium.For example, Elasmobranchs (Sharks), Mustelus etc.
2. Chorio-vitelline placenta- It is formed by chorion and yolk sac combinedly. Hence it is called as choriovitelline placenta. For example, Didelphis, Macropus and other metatherian mammals.
3. Chorio-allantoic placenta- This type of placenta is formed by embryonic chorion and allantoic membranes.
It is also referred to as a true placenta. It is found in eutherian mammals.
Chorio-allantoic placenta in mammals.
1. In this type of placenta, allantoic mesodern and the mesoderm of umbilical cord jointly form the blood vessels of umbilical cord. The endodermal part of the allantois remains as a very small cavity.
2. To obtain nutrition from maternal blood several finger like processes or villi are formed by chorion which penetrate deeply into the crypts of uterus.Initially the villi are scattered over the whole surface of chorion but later they become restricted in the decidua besalis region . The chorionic villi on the remaining surface disappear shortly. The part of chorion, which helps in placenta formation is known as chorionic frondosum.
Classification of Placenta
On the basis of different characters, the placenta are classified in following manner:-
1. On the basis of intimacy After implantation, the wall of uterus is called as decidua, instead of endometrium. The part of decidua, where placenta is formed is called decidua basalis whereas, the part separating the embryo from lumen of uterus is called decidua capsularis. The remaining part of lumen of uterus is called decidua parietalis. Decidua also comes out from uterus at the time of parturition. On the basis of intimacy between embryo and uterine wall the placenta is classified into three classes
(i) Non-deciduate or Semi placenta- In this type of placenta, there is no close and rigid association between embryo and the wall of uterus. Hence, at the time of parturition, there is no bleeding as the chorionic villi are easily pulled out from the crypts of uterus. For example, cow, buffalo, horse, pig.
(ii) Contra-deciduate placenta- There is a close association between embryonic and maternal tissues. However at parturition, the damaged maternal and embryonic tissues along with the part of placenta remain inside the uterus which are absorbed in situ by leukocytes.
For example- Parameles, Talpa etc.
(iii) Deciduate placenta- This type of placenta is found in human, dog, hare etc.It is characterized with a very close association between chorionic villi and uterine wall. At the time of birth, the mucosal covering of the uterus is also damaged and discarded outside. This results in an extensive bleeding at child birth. This placenta is known as a true placenta.
2. On the basis of implantation
Three types of placenta are found on the basis of implantaion.
(i) Superficial- When the placenta is situated in the lumen of uterus. For example, Parameles, pig, cow, cat etc.
(ii) Eccentric- The placenta is situated in the fold or pocket of the cavity of uterus. For example- rat, Squirrel etc.
(iii) Interstitial- This type of placenta is found in man, guinea pig, apes etc. The chorionic sac (placenta) penetrates deep inside the wall of uterus. Hence, the association between embryo and maternal part becomes very close.
3. On the basis of distribution of villi
On this basis, the placenta are of four types.
(i) Diffused placenta- The villi are scattered on the whole surface of placenta. For example pig, horse, lemur etc.
(ii) Cotyledonary placenta- The villi are distributed in small isolated groups on the chorionic surface. These groups of villi are called as cotyledons. For example, cow, buffalo, sheep, deer etc.
(iii) Zonary placenta- This type of placentae have the villi distributed in a belt shaped zone which is large sized and circular.
Zonary placenta is of two types-
(a) Complete Zonary placenta - The belt of villi is complete and ring shaped init. For example- dog, cat, lion etc.
(b) Incomplete Zonary placenta- The belt of villi is incomplete in it. For example- racoon.
(iv) Discoidal placenta- In this type of placenta, whole of the chorionic surface is covered by villi in initial stage, but the villi disappear later from most area except the region of implantation, that is only a disc like region is left with villi.
Discoidal placenta is also of two types–
(a) Mono discoidal placenta– The villi are present only on dorsal surface in a single circular disc like area.
For example- human, hare etc.
(b) Bidiscoidal placenta- If the villi are distributed in two disc like areas, the placenta is called as bidiscoidal,
Fig: Different types of placenta
Types of placenta on the basis of distribution of villi
4. On the basis of histology
The blood of maternal and embryonic do not mix together through placenta. The blood circulations of the two sides are kept separated by one or more layers described below–
The transportation of various materials takes place by diffusion through these six layers. The intimacy between maternal and embryonic tissues in different mammals is determined by the presence or absence of these layers in placenta. Therefore, on the basis of presence of above layers, the placenta is of five types.
1. Epitheliochorial- It is the most primitive type of placenta in which all the six layers, mentioned earlier, remain intact.
For example - pig, horse etc.
2. Syndesmochorial- In this type of placenta the uterine epithelium is eroded by chorionic villi, so only two maternal layers remain functional. Therefore, along with three foetal layers, total five layers are present in this placenta.
For example - sheep, goat, cow etc.
3. Endotheliochorial- Here uterine connective tissue layer is also damaged along with uterine epithelial layer. Therefore only four layers (3 foetal and one maternal) are found in this placenta
For example - dog, cat, etc.
4. Haemochorial- All the three maternal layers are penetrated in this placenta. The chorionic epithelium comes in direct contact with uterine blood sinusoids.
For example- man, monkey, bat etc.
Fig: Classification of placenta
5. Haemo-endotheliochorial/Haemo-endothelial : it is the most typical placenta in which the tropho-blastic epithelium of embryo is also eroded along with all three maternal layers. The foetal capillaries are in direct contact with maternal blood.
For example: rat, guinea pig, rabbit etc.
Hormones of Human Placenta
The placenta of human mainly secretes two steroid hormones like estradiol and progesterone, and two protein hormones like human chorional gonadotropin HCG and human placental somatomammotropin HCS. Large amount of HCG hormone is secreted, during early pregnancy, from the placenta. Because of this reason its quantity increases in the urine of pregnant lady. On the basis of this fact, pregnancy test is performed. The above hormones are also held responsible for keeping the corpus luteum active, protection of embryo, prevention of abortion and growth of mammary glands.
Functions of placenta
1. Exchange of important materials between foetal and maternal blood.
2. The essential materials are exchanged by diffusion, pinocytosis or active transport.
3. The small molecules like O2, CO2, H2O etc. and other inorganic substances like chlorides, phosphates, sodium, potassium, magnesium etc. are also diffused through placenta.
4. Large molecules like lipids, polysaccharides, carbohydrates, proteins etc. are obtained by pinocytosis process.
5. The nutritional substances are supplied to embryo from the mother through placenta.
6. Placenta also serves as a respiratory medium for exchange of O2 and CO2 between embryo and mother.
7. The nitrogenous and metabolic wastes from foetus are released into the blood of mother by diffusion through placenta.
8. The antibodies for measles, chickenpox, polio etc. present in the blood of mother reach the embryo through placenta.
9. Pathogenic viruses may also enter in embryo through placenta.
10. If a female takes some harmful chemicals, liquor, drugs etc.during pregnancy, these may cross the placenta and on reaching into foetus may cause deformity during organo genesis.(eg. Thallidomide)
11. Placenta itself secretes some hormones like progesterone, estrogen, lactogen, HCG, HCS etc.
12. Progesterone, maintains and supports the foetus during the whole pregnancy period . At the time of parturition, relaxin is secreted by placenta which lubricates, and widens the birth canal to facilitate child birth.
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