Question

How is blastocoel formed?

Answer 1

Hint: The fertilisation of male and female gametes results in the formation of a diploid zygote further develops into an embryo. The development of embryo from fertilised ovum and its development into an organism is called embryogenesis.

Complete answer:
Embryonic development or embryogenesis has three major stages- cleavage, gastrulation and organogenesis. Cleavage divisions produce a solid ball of cells called morula. It has 8-16 cells, sometimes 32 cells. Further, blastocyst formation takes place called blastulation. Endometrium secretes a nutrient fluid called uterine milk. As morula enters the uterus, it obtains enriched supply of nutrients which results in increase in growth. Outer peripheral cells enlarge and flatten further. They form trophoblast or trophectoderm, which secretes a fluid into the interior. It creates a cavity called blastocoel. Now the inner cell mass comes to lie on one side as an embryonal knob. Morula is converted into blastula with the formation of blastocoel, and is called blastocyst in mammals because of the different nature of surface layer and eccentric inner cell mass.

Until now blastocyst has been covered by zona pellucida. However, the size of blastocyst soon becomes almost three times the size of a morula. The growing blastocyst exerts pressure due to which a slit is produced in zona pellucida. Blastocyst may also corrode zona pellucida by trypsin-like enzymes. The growing blastocyst comes out of a slit of zona pellucida in the form of letter 8.

Note: The growing blastocyst in the form of letter 8 gets broken into two parts sometimes, which then give rise to identical twins (monozygotic twins). The blastocysts get embedded into the endometrium of the uterus known as implantation.