Fertilization and Implantation
- 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 the Ampullary region of the fallopian tube.
- Fertilization happens only if ovum & sperms are transported simultaneously. So all copulations do not lead to fertilization & pregnancy.
- A sperm contacts with the 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 a blastocyst.
- In the blastocyst, blastomeres are arranged into a trophoblast (outer layer) and an inner cell mass attached to the trophoblast.
- The trophoblast layer gives nourishment to the inner cell mass. Also, it gets attached to the 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.
Sperms
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vagina
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cervical canal
↓
uterus
↓
isthmus
↓
Ampullary region (Fertilization)
↑
infundibulum
↑
fimbriae
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Ovum (from ovary)
