On 4th and 5th day morula in uterine cavity covered by film of mucus.

Fluid pass through canaliculi of zona pellucida; separate cells of morula called blastocyst.

Due to blastocyst enlargement zona pellucida become stretched, thinned and gradually disappears.

Lysis of zona pellucida and escape of embryo called zona hatching.

Outer side of morula become trophectoderm (adhesion of blastocyst to uterine epithelialium.

Inner side of morula become inner cell mass.

Changes in Blastocyst

Trophoblast cell adjacent to inner cell mass primarily involved in adhesion to endometrial cell.

Responsible for blastocyst attachment are;

Selectins, Integrins, trophinin, tasin, EGF (epidermal growth factor), proteoglycan, Heparin sulphate.