- During copulation, semen is released by the penis into the vagina. It is called insemination.
- Fusion of a sperm with ovum is called fertilization. It occurs in Ampullary region of fallopian tube.
Sperms → vagina → cervical canal → uterus → isthmus
Fertilization ← Ampullary region
Ovum (from ovary) → fimbriae → infundibulum
- Fertilization happens only if ovum & sperms are transported simultaneously. So all copulations do not lead to fertilization & pregnancy.
- A sperm contacts with zona pellucida. It induces changes in the membrane that block entry of additional sperms.
- The secretions of the acrosome help sperm to enter the egg cytoplasm via zona pellucida & plasma membrane. This causes second meiotic division of secondary oocyte to form an ovum (ootid) and a second polar body.
- The haploid nuclei of the sperm and ovum fuse together to form a diploid zygote.
- Zygote undergoes mitotic division (cleavage) as it moves through the isthmus towards the uterus and forms 2, 4, 8, 16 daughter cells called blastomeres.
- The embryo with 8-16 blastomeres is called a morula.
- Morula continues to divide and transforms into blastocyst.
- In blastocyst, blastomeres are arranged into trophoblast (outer layer) and an inner cell mass attached to trophoblast.
- The trophoblast layer gives nourishment to inner cell mass. Also, it gets attached to endometrium.
- After attachment, uterine cells divide rapidly and cover the blastocyst. Thus, the blastocyst becomes embedded in the endometrium. This is called implantation.
- The inner cell mass gets differentiated to 3 germ layers (outer ectoderm, middle mesoderm & inner endoderm). This 3-layered structure (gastrula) forms the embryo.