Category : 12th Class

Definition : Placenta is defined as a temporary intimate mechanical and physiological connection between foetal and maternal tissues for the nutrition, respiration and excretion of the foetus.

Structure : Human placenta consist of chorion only. Hence, it is called a chorionic placenta. Allantois remains small. The allantoic blood vessels, however, extend to vascularize it. A large number of branching villi from the vascular chorion penetrate the corresponding pits, the crypts, formed in the uterine wall. The latter becomes very thick and highly vascular to receive the villi. The intimate connection established between the foetal membrane and the uterine wall is known as the placenta.



The placenta is fully formed by the end of the third month and it lasts throughout pregnancy. When complete, it is a reddish - brown disc. 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, simple proteins, 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 and urea, also water and hormones, pass into the maternal blood. The placenta, thus, serves as the nutritive, respiratory and excretory organ of the foetus. The continuous uptake of oxygen by foetal blood is ensured by the difference in affinity for oxygen between foetal and maternal haemoglobin.

The maternal and foetal blood are not in direct contact in the placenta, because (i) the two may be incompatible; (ii) the pressure of maternal blood is far too high for the foetal blood vessels; and (iii) there must be a check on the passage of harmful materials (blood proteins, germs) into the foetal blood.

(iii) Functions

(1) Placenta helps in the nutrition of the embryo as the nutrients like amino acids, monosugars, vitamins, etc. pass from the maternal blood into foetal blood through placenta.

(2) It also helps in respiration of the embryo as \[{{O}_{2}}\] of the maternal blood and \[C{{O}_{2}}\] of the foetal blood diffuse through placenta into the foetal blood and maternal blood respectively.

(3) It also helps excretion of the embryo as nitrogenous wastes of foetal blood like urea pass into maternal blood through placenta.

(4) Though the placenta acts as an effective barrier for certain toxic chemicals like histamine but certain germs like AIDS virus, syphilis bacteria, viruses of German measles, etc, intoxicants like nicotine of cigarette smoke; and addictive drugs like heroin and cocaine can pass through the placenta and cause the developmental defects.

Classification of placenta

(1) According to the foetal membrane involved in the formation of placenta.

(i) Yolk sac placenta : In metatheria or marsupials, such as kangaroo (macropus) and opossum (Didelphys), placenta is derived from yolk sac and chorion.


(ii) Allantoic placenta : In the majority of Eutherian, the chief organ of embryonic nutrition is the allantoic placenta consist of allantois and chorion and also called allantochorionic placenta. Outside Eutheria, a primitive allantoic placenta occurs only in perameles (bandicoot) which is a metatherian.



(iii) Chorionic placenta : It occurs in primates (man and apes) and is formed only by chorion. Allantois remains small, burrows into body stalk (umbilical cord) and does not reach chorion. However, its mesoderm and blood vessels grow upto chorion whose villi enter the uterine crypts forming chorionic placenta.



(2) On the presence or absence of above barriers histologically placenta is divided into following types

(i) Epithelio-chorial : Most primitive and simplest type with all six placental barriers.

Examples : Odd hoofed mammals such as horse, ass, pig and lemurs.

(ii) Syndesmo-chorial : Uterine epithelium absent, with five placental barriers.

Examples : Even hoofed mammals such as cow, Buffalow, sheep, goat, camel, Girraffe etc.

(iii) Endothelio-chorial : Uterine epithelium and uterine connective tissues are absent, with four placental barriers.

Examples : Carnivores (dog, cat, lion, tiger etc.), Tree shrew and mole.

(iv) Haemo-chorial : Uterine epithelium, uterine connective tissue and endothelium of maternal blood vessel absent, with 3 foetal layers.

Examples : Primates (man, apes and monkey).

(v) Haemo-endothelial : Foetal capillaries indirect contact with maternal blood, only one placental barrier.

Examples : Rat, guinea pig and rabbit.



According to shape and distribution of villi : Depending on the shape of placenta, manner of distribution of villi, degree of connection between foetal and maternal tissues and behaviour of placenta at the time of birth, the following types and subtypes of allantoic placenta can be recognized.

(i) Non deciduous placenta : In most mammals villi are simple, unbranched and merely opposed without intimate contact between foetus and uterine wall. At the time of birth or parturition, villi are easily withdrawn from maternal crypts without causing any tissue damage. Thus no part of uterine tissue comes out and no bleeding occurs. Non deciduous or non-deciduate placenta has following subtypes according to the manner of distribution of villi.

(a) Diffuse : Villi remain scattered all over the surface of allantochorion. e.g., pig, horse, lemur.

(b) Cotyledonary : Villi are arranged in separate tufts or patches called cotyledons. e.g., goat, sheep, cow, deer.

(iii) Intermediate : Villi are arranged in cotyledons as well as scattered. e.g., camel, giraffe.

(ii) Deciduous placenta : Villi are complicated, branched and intimately connected. At birth, a variable amount of maternal tissue is pulled out with the shedding of blood. Deciduous or deciduate placenta is also differentiated in the following subtypes

(a) Zonary : Villi form an incomplete (e.g., racoon) or complete girdle encircling the blastocyst. e.g., cat, dog, seal, Lion, Tiger, Elephant etc.

(b) Discoidal : Villi are restricted to a circular disc or plate on the dorsal surface of blastocyst. e.g., insectivores, bats, rodents (rat, mouse), rabbit, bear.

(c) Metadiscoidal : Villi are at first scattered but later become restricted to one or two discs. It is monodiscoidal in man and bidiscoidal in monkeys and apes.



(iii) Contra-deciduous : Foetal villi and uterine crypts are so intimately connected that even most of foetal placenta is left behind at birth to be broken and absorbed by maternal leucocytes e.g., bandicoot (perameles), mole (Talpa).


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