Embryonic Development in Human

The zygote formed as a result of syngamy is activated to divide.

Cleavage:

• Cleavage is the process of early mitotic division of the zygote into a hollow multicellular blastula. It does not involve the growth of the daughter cells. The cells formed by cleavage are called blastomeres.
• Since there is no growth phase between the cleavages, the size of the blastomeres will be reduced with every successive cleavage. As the size reduces, the metabolic rate increases. Subsequent cleavages are thus faster than the earlier one.
• This requires rapid replication of DNA and high consumption of oxygen.

Process of Cleavage:

• In humans, cleavage is holoblastic, meaning the whole zygote gets divided. The cleavage planes may be longitudinal or meridional, and equatorial or horizontal.
• It is radial and indeterminate, meaning the fate of each blastomere is not predetermined. The first cleavage in the zygote is meridional and occurs about 30 hours after fertilization.
• It divides longitudinally into two blastomeres, one slightly larger than the other. The second cleavage is also longitudinal but at a right angle to the first one and occurs after 30 hours of the first cleavage. The third cleavage is horizontal.
• After the third cleavage, the embryo is in the 8-cell stage. As the cleavages continue, the young embryo is gradually being pushed towards the uterus.
• By the end of the 4th day after fertilization, the embryo is a solid ball of 16-32 cells and externally resembles a mulberry. This stage is thus called the morula.
• The morula shows cells of two types: smaller, clearer cells towards the outer side and an inner cell mass of larger cells. The cells are compactly arranged.
• Until the formation of the morula, the zona pellucida is retained around the embryo, so there is no change in the overall size from the zygote to the morula.
• The morula reaches the isthmus and gains entry into the uterus by the end of day 4.

Process of cleavage and formation of Blastula 

Blastulation:

• Blastulas are formed from morulas. The morula has an 8-16 cell structure, and when the morula receives nutrients, it transforms into a blastula.
• The blastulation then implants in the uterus and further development occurs.
• Numbers of cells in a blastocyst increase (To 100) but the size still remains equal to the ootid. Rearrangement of cells occurs resulting in 2 distinct layers of cells. 

• Trophoblast Layer: Outermost layers surrounding the other inner layers of cells. Made of flat cells. Protects the inner cell mass from the surroundings. It will ultimately form the fetal part of placenta.
• Inner Cell Mass/ Embryonic Disc Proper: Lie clustered at one end, internal, face the blastocoel on one side and attached to the trophoblast on the other. Destined to form the embryo. On about the 5th day after fertilization, the blastocyst/ blastula hatches out by breaking the zona pellucida.

 Depends on the presence/ absence and amount of yolk. 

On the basis of extent of division or Passing of the cleavage Furrow:  

• Cleavage 1 - Holoblastic, equal, completed roughly 30 hours after fertilization. It is meridional. 
• Cleavage 2 - Completed by the end of the 2^nd day. Meridional but at right angles to the first. But at the end of 3^rd day, 8 cells are produced. The progressively smaller cells produced by cleavage are called blastomeres. The solid ball of cells is referred to as morula.