POST FERTILIZATION CHANGES (In Humans):

 

POST FERTILIZATION CHANGES: Fusion of male and female gamete leads to the formation of zygote. Zygote divides rapidly by mitotic division as it moves through the isthmus of oviduct towards the uterus. This is called cleavage and the cells that are formed are called blastomeres. From single cell zygote there is the formation of 2 cells, 2 to 4 cells, and then 8 cells. 8 cells then leads to the formation of 16 cells. This condition of 16 to 32 solid ball of cells is called Morula.

Morula – It is the solid mass of cell comprising of 16 – 32 cells formed from the zygote by mitotic divisions (after 5^th cleavage). It takes approximately 3 days. A morula can be differentiated from blastula in the absence of cavity. It looks like a mulberry shaped ball of cells. The morula reaches the uterus about 4-6 days after fertilization.

In mammals due to the presence of insignificant amount of yolk, cleavage division cut the ovum into complete daughter cells. This type of cleavage in mammalian egg is termed as holoblastic.  Cleavage in the mammalian egg takes place during its passage through the fallopian tube to the uterus.

The outer layer of cells become flattened and is termed as Trophoblast, which absorbs fluids from the uterus. These fluid absorbed by the trophoblast gets collected in a cavity called blastocoels, or blastocyst cavity. As the quantity of fluid increases , the morula increases rapidly and assumes the shape of a cyst, which passes further into the uterus.

The blastocytes has over 100 cells. The portion of the blastocytes where the embryonal knob located lies against endometrium.

Implementation is the attachment of the blastocysts in the uterine wall. It takes place about seven days after fertilization.

Role of Zona Pellucida : The function of zona pellucida is to prevent the implementation of the blastocyst at an abnormal site.

The trophoblast of the invading blastocyst develops the rudiment of chorion, an extra embryonic membrane, which later forms placenta. The chorionic cells secretes a hormone named human chorionic gonadotrophin ( hCG), which resembles and takes over the job of pituitary LH during pregnancy.

Finally transformation of the blastula into gastrula takes place. Gastrula has three primary germ layers. In this stage morphogenetic movements takes place.

          

                                      ****************************