Medicine and Health related Information

1) The trophoblast is at first made up of a single layer of cells. As these cells multiply two distinct layers are formed. The cells that are nearest to the decidua loose their cell boundaries. Thus one continous sheet of cytoplasm containing many nuclei is formed. Such a tissue is called “SYNCYTIUM”.

Hence this layer of trophoblast is called the syncitio trophoblast or “PLASMODIO TROPHOBLAST”. Deep to the syncitium the cells of the trophoblast retain their cell walls and form second layer called the “CYTOTROPHOBLAST” also called Langhan’s Layer.

2) The syncitiotrophoblast grows rapidly and become thick small cavities called “lacunae” appear in this layer, gradually the lacunae increase in size. At first they are irregularly arranged but gradually they come to lie radially around the blastocyst. The lacunae are seperated from one another by partitions of syncitium which are called “TRABECULAE”. The lacunae gradually communicate with each other so thet eventually one large space is formed. Each trabeculus is now surrounded all round by this lacunar space.

3) The syncitiotrophoblast grows into the endometrium. As the endometrium is eroded some of its blood vessels are opened up and blood from them fills the lacunar space.

4) Each trabeculae is initially made up entirely of the syncitiotrophoblast. Now the cells of the cytrophoblast begin to multiply and grow into each trabeculus. The trabeculus thus comes to have a central core of cytotrophoblast covered by an outer layer of syncitium. It is surrounded by the maternal blood filling the lacunar space. The trabeculus is now called “PRIMARY VILLUS” and the lacunar space is now called “INTERVILLOUS SPACE”.

5) The extraembryonic mesoderm lining the inner side of the trophoblast, now invades the centre of each primary villus. The villus thus comes to have a core mesoderm. Outside this there is a layer of cytrophoblast which is inturn surrounded by syncitium. This structure is called “SECONDARY VILLUS”.

6) Blood vessels are seen in the mesoderm forming the core of each villus, with their appearance the villus is fully formed and is called “TERTIARY VILLUS”. The blood vessels of the villus establish connections with the circulatory system of the embryo, fetal blood now circulates through the villi, while maternal blood curculates through intervillous space.

7) The cytotrophoblast, that grows into the trabeculus does not penetrate the entire thckness of syncitium and therefore does not come in contact with the decidua. At a later stage however the cytotrophoblast emerges through the syncitium,of each villus the cells of trophoblast now spread out to form a layer that completely cuts off the syncitium from the decidua. This layer of cells is called as “CYTOTROPHOBLASTIC SHELL”. The cells of this shell multiply rapidly and placenta increases in size.

The villi that are first formed are attached on the fetal side to the embryonic mesoderm and on thematernal side to the cytotrophoblastic shell. They are therefore called “ANCHORING VILLI”. Each anchoring villi consists of a stem (truncus chorii) this divides into a number of branches (rami chorii) which inturn divides into finer branches (ramuli chorii). The ramuli are attached to the cytotrophoblastic shell, the anchoring villi gives off numerous branches which grow into intervillous space as free villi. New villi also spread from the chorionic side of intervillous space. Ultimately almost the whole intervillous space becomes filled with villi. As a result the surface area available for the exchange between maternal and fetal circulations becomes enormous.

These newly formed villi at fist consists only of the syncitiotrophoblast, they are subsequently invaded by the cytotrophoblast, mesoderm and the blood vessels and pass through the stages of primary, secondarty and tertiary villi.