3. HUMAN REPRODUCTION
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 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.
