Placenta | Embryology | Anjani Mishra

                                                                                                                  Written by Anjani Mishra

Placenta

The placenta is a temporary physiological organ formed by the close association of the fetal membrane and the uterine lining, which permits the interchange of materials carried in the blood stream of the mother and the fetus.

Functions:

1.      Nutrition- CHO, protein, fat

2.      Respiration- O₂ & CO₂

3.      Excretion- Metabolic waste products

4.      Barrier- Prevent bac. & other antigen

5.      Synthesis –Estrogen, progesteron

Placentation

The term placentation means the events which enable the embryo to get intimate relation with the uterus for its physiological causes or demand.

Classification of placenta

A.    According to the final distribution of the villi of the chorionic sac, the shape of the placenta are classified into:-

1.      Diffuse- Uniform distribution of villi on placenta, i.e the chorionic villi are distributed all over the chorion. Eg; sow, mare

2.      Cotyledonary- If the villi concentrated in some area forming cotyledon, i.e. the villi are grouped in well spaced prominent cotyledon which are separated by smooth chorion. Eg; Cattle, Sheep, Goat

3.      Zonary- Villi concentrated at the centre of the placenta to form a belt like structure, i.e. the villi are distributed and occupy a girdle like band about the middle of the chorionic sac. Eg; Dog, Cat

4.      Disoid- Villi concentrated at one or two end of the placenta to form disc like structure i.e. the villi are limited to one or two disc shaped area of the sac. Eg; Man, Monkey

Chorionic frondosum

The area of the chorion which attach with the maternal uterus is called chorionic frondosum.

Chorionic leave

The area of the chorion which is unattached and smooth part of the chorion is called chorionic leave.

 

B.     On the basis of degree of contact between the chorion and uterus and on the minute microscopic relationship based on the number of tissue layers involved at the zone of junction of the two components, i.e. in between the chorion and the endomethium.

 

1.      Epithelio-chorial- The epithelium of both fetal and maternal part remain intact found in diffuse type of gross placenta. Eg; Mare, Sow

 

2.      Syndesmo-chorial- All the components of the fetus remains intact but the epithelium of the maternal part is destroyed found in cotyledonary type of gross placenta. Eg; Cow, sheep, goat

3.      Endothelio-chorial- All the components of the fetus remains intact, but the epithelial tissue + C.T. of the maternal part is destroyed found in zonary type of gross placenta. Eg; Dog, cat, tigeress

 

4.      Hemo-chorial- All the components of the fetus remains intact, but the epithelial tissue + C.T. + endothelium of  the maternal part is destroyed leaving the blood which comes in contact with the placenta. eg; Man, Bat

 

5.      Hemo-endothelial- The epithelial Tissue + C.T. of fetus is destroyed, where as epithelial Tissue + C.T. + endothelium of maternal part is destroyed leaving the blood which comes in contact with the endothelium of the fetus. eg. Rabbit, guinea pig.

 

Fetal membrane

The peripheral part of the germ layers contributing to the formation of certain membranes, which serves for the purpose of protection, absorption of food, respiration and excretion for the embryo. These membranes are not within the body of the embryo and are discharged at the time of birth. These are also called extra-embryonic membrane. The membranes are:-

1.      Yolk sac

2.      Amnion

3.      Chorion

4.      Allantois

Fetal membranes are formed by the following germ layers, as follows;

1.      Ectoderm

2.      Mesoderm- Somatic and Splanchnic

3.      Endoderm

The somatic membrane is very much close to the ectoderm called somatopleure and the splanchnic membrane is close to the endoderm called splanchnopleure.

1.      Yolk Sac:

It is also part of primitive gut which is not included within the body of the embryo, when the embryo is folded off.

Function: It supplies nutrients to the embryo by the formation of yolk material.

 

2.      Amnion:

It arises as a layer of Somatopleure, which surrounds the developing embryo. In the formation of amnion, the somatopleure thrown into folds, then gradually come upward and after covering the embryo again comes downward and the limbs of the both sides united ventrally.

The inner limbs gradually forms a cavity called amniotic cavity containing amniotic fluid and the outer limb becomes serosa which is gradually converted into chorion.

Function:

i)                   Protects the fetus from mechanical injury

ii)                 Equalizes the pressure around the fetus

iii)               Prevent drying, adhesion and subsequent malformation

3.       Chorion:

It is the outermost layer of fetal membrane which encloses all other fetal membrane including fetus. the outer limb of the somatopleure become serosa first and gradually chorion.

Function:

It forms the placenta and chorionic villi which is the source of nutrition, respiration and excertion for the fetus.

 

4.      Allantois:

It arises as a diverticulum from the hind gut of the embryo and gradually increases in size and the yolk sac is gradually reduces. The wall of the allantois is splanchnopleure. It is a slender endodermal tube which extends from the caudal end of yolk-sac to the mesoderm of body stalk. When the wall of allantois come in contact with the serosa, then the chorion is formed.

Function:

i)                   It helps in maternal circulation

ii)                 It serves for the metabolic interchange between the fetus and mother.

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