PLACENTA

PLACENTA:

STRUCTURE:

Placenta is a structure that establishes firm connection between the foetus and the mother.

From the outer surface of the chorion a number of finger like projections known as chorionic villi grow into the tissue of the uterus. These villi penetrate the tissue of the uterine wall of the mother and form placenta.

The placenta is a connection between foetal membrane and the inner uterine wall. Thus, placenta is partly maternal and partly embryonic. By means of placenta the developing embryo obtains nutrients and oxygen from the mother and gives off carbon dioxide and nitrogenous waste.

In the placenta, the foetal blood comes very close to the maternal blood, and this permits the exchange of materials between the two. Food (glucose, amino acids, lipids), water, mineral salts, vitamins, hormones, antibodies and oxygen pass from the maternal blood into the foetal blood, and foetal metabolic wastes, such as carbon dioxide, urea and warn pass into the maternal blood.

The placenta, thus, serves as the nutritive, respiratory and excretory organ of the foetus. The blood of the mother and foetus do not mix at all in the placenta or at any other place. The blood of the foetus in the capillaries of the chorionic villi comes in close contact with the mother’s blood in the tissue between the villi, Inn they are always separated by a membrane, through which substances must diffuse or lie transported by some active, energy requiring process.

The type of placenta in man is of described as deciduate (intimate contact between loetal and maternal tissue), discoidal (villi occur in the form of disc), haemo-chorial (chorionic epithelium in direct contact with maternal blood).

TYPES OF PLACENTA:

The Placenta is divided into several types:

A. Depending on the Involvement of Embryo­nic Tissue:

(i) Yolk-Sac Placenta:

When the midgut extension of the splanchnopleure enclosing the yolk fuses with the extraembryonic somatopleure to make embryonic contact with the uterine wall. Examples: Mustelus.

(ii) Chorio-Allantok Placenta:

The allan­toic evagination of the hindgut unites with the extraembryonic somatopleure to make contact with the uterine tissue. Examples: Eutherian mammals and a lizard, Chalcides.

B. Depending on the Distribution of Villi:

(i) Diffused Placenta:

The villi are nume­rous and distributed uniformly over the whole of chorion. Examples: Ungulates, Cetacea.

(ii) Cotyledonary placenta:

The villi become aggregated in special regions to form small tufts. Examples: Ruminants.

(iii) Zonary Placenta:

The villi are confined to an annular zone on the chorion. Exam­ples: Carnivora (Pinnipedia).

(iv) Discoidal Placenta:

The villi become restricted to a discoidal area as seen iruro- dents and insectivores. In apes and man, the placenta is of metadiscoidal type.

C. Based on the Relationship of Villi with the Uterine Wall:

(i) Deciduate Placenta:

The villi become intimately connected with the mucous membrane of the uterine wall which comes out with the embryo at the time of birth.

(ii) Indeciduate or Adeciduate Placenta:

The villi are loosely united with the uterine walls which separate from the uterus at birth.

D. Based on the Degree of Involvement of Foetal and Maternal Tissues:

(i) Epitheliochortal Placenta:

The epithe­lium of uterus remains in simple apposition with the chorion of the embryo. Examples: Pig and Horse.

(ii) Syndesmochorial Placenta:

The epithe­lium of the uterus disappears and the chorion comes in direct contact either with the glandular epithelium or endometrium of the uterus. Example: Sheep.

(iii) Vasochorial or Endotheliochorial Placenta:

Both the glandular epithelium and the. endometrium disappear and the chorion comes in close contact with the endothe­lium of the uterine capillaries. Examples: Dogs and Cats.

(iv) Haemochorial Placenta:

The glandular epithelium, endometrium and endothelium of the capillaries disappear and the cho­rion is bathed with circulating maternal blood. 

Example: Man.

(v) Haemoendothelial Placenta:

Like that of haemochorial type of placenta, the glan­dular epithelium, endometrium and the endothelium of the maternal blood capil­laries disappear. With the disappearance of these maternal structures, the tropoblastic epithelium (outer layer of the blas­tocyst) of the foetus also disappears, as a result the foetal endothelium separates the maternal and foetal circulating blood stream.

Examples: Many rodents.

Organogenesis of Human Placenta:

In human females, implantation of the developing embryo occurs in the early luteal phase when the endometrium of the uterus remains in optimum condition. The deve­loping egg reaches the uterus in blastocyst condition with a greatly enlarged blastocoelic space.

The placental organogenesis is described under two broad aspects:

a. Previllous period (6th-13th day).

b. Villous period (14th day to term).

a.Pre Villous Period:

The implantation of human embryo takes place about 6th to 9th days after fer­tilization (Figs. 5.44, 5.45).

The end of the blastocyst containing the developing ger­minal disc attaches itself to the uterine wall (Fig. 5.46A). The uterine epithelium is eroded at the region of contact. The trophoblast tissue increases in thickness in this contact area due to the division of epithelial cells of the trophoblast layer.

Prelacunar Stage:

The implanted embryo, consisting of a bilaminar disc, is protruded into a cavity (lecithocoel). This cavity is enclosed by the trophoblast. Profound cytological changes occur in the tropho­blast layer. The inner trophoblast cells remain cellular and are designated as the cytotrophoblast while the outer cells fuse together to form a syncytium called the syncytiotrophoblast.

The syncytiotrophoblast serves as the invading tissue of the embryo into the uterine wall. The growth of this syncytiotrophoblast is caused by differen­tiation of the cytotrophoblast and by ami­totic division of the syncytial nuclei.

Lacunar stage (10th to 13th days of Pla­cental development):

As the syncytiotro­phoblast invades and increases in quantity, irregular spaces are produced in the syn­cytiotrophoblast. These spaces are called trophoblastic lacunae. (Fig. 5.46B).

b.Villous Period:

During the period between 14 and 18 days, the trophoblastic lacunae merge with one another to form large cavities bor­dered by syncytiotrophoblast. Such a cavity is named as the intervillous space and the primary villi are formed through the proliferation of the cytotrophoblastic ele­ments into the syncytial trabeculae.

The primary villi, when first formed, lack meso­dermal core. The mesoderm of the somatopleure invade into them to form the secondary villi (Fig. 5.46D).

With the forma­tion of the blood islands and appearance of blood vessels, the secondary villi transform into the definitive tertiary villi. At this time, some endometrial tissue including blood vessels near to the invading chorionic vesicle break down to produce liquefied areas called the embryotroph. The liquefied

material from the embryotroph is assi­milated by the syncytiotrophoblast for the growth of the embryo. This particular type of. nutrition is called the histotrophic nutrition.

From the physiological point of view the foetal villi developing from chorionic plate can be divided into three categories:

(i) Chorionic villi, through which phy­siological exchange of materials taking place between the foetus and the mother

(ii) Anchoring villi for mechanical anchor­age of foetus and

(iii) Free villi.

The developing chorionic vesicle grows and invades the endometrium of the uterus. The uterine mucosa extends over the invading vesicle. The endometrial tissue covering the chorionic vesicle is called the decidua capsularis while the endo­metrial portion which is not concerned with the closure of such vesicle is called the decidua parietalis or decidua vera.

The endo­metrial portion lying between the mus­culature of the uterine wall and the invad­ing villi is called decidua basalts.

Although the chorionic villi are formed over the entire chorionic vesicle, only the villi in relation to decidua basalis are re­tained and those grown to decidua parie­talis are reabsorbed to form a smooth area named as the chorion leave. The villi within decidua basalis become greatly enlarged to serve the main role of physiological interchange (Fig. 5.46E).

This area of chorionic vesicle with the villi is the chorion frondosum which together with the tissue of decidua basalis forms the actual placenta. Thus the placenta is composed of decidua basalis (maternal placenta) and chorion frondosum (foetal placenta) (Fig. 5.46F).

The villi, at the early phase of develop­ment, consist of blood capillaries within mesodermal core covered over by cytotrophoblast and syncytiotrophoblast on the outer side. As development goes on the blood capillaries grow enormously and the cytotrophoblast layer is extremely reduced to few scattered cells below syncytiotro­phoblast.

The villi are aggregated into groups call­ed the cotyledons which are separated by incomplete placental septa. The villi in each cotyledon remain surrounded by a pool of maternal blood and by this way a hemochorial type of placenta is established.

Functions of Placenta:

The placenta performs the following functions:

1. Nutrition:

Food materials pass from the mother’s blood into the foetal blood through the placenta.

2. Digestion:

The trophoblast ol the placenta digest protein before passing them into foetal blood.

3. Respiration:

Through the placenta oxygen passes from the maternal blood to the foetal blood, and carbon dioxide passes from foetal blood to maternal blood.

4. Excretion:

Nitrogenous wastes such as urea pass from foetal blood into maternal blood through placenta and are filtered out by the kidneys of the mother.

5. Storage:

The placenta stores glycogen, fat etc. for the foetus before liver is formed.

6. Barrier:

Placenta functions as an efficient barrier (defensive wall) and allows useful: aerials to pass into the Social blood. Harmful substances such as nicotine from cigarette and addictive drugs such as heroin can pass through placenta. Therefore, pregnant women should avoid cigarette and drugs. Viruses and bacteria can pass through placenta.

7. Endocrine function:

Placenta functions as an endocrine gland it secretes hormones such as oestrogen, progesterone and human chorionic gonadotropin (HCG).

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