Sunday, July 26, 2020

Human Reproduction - Notes | Class 12 | Part 6: Fertilization and Implantation

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.


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