MP Board · Class 12 · Biology · Human ReproductionDescribe the events of fertilization and implantation in human reproduction, starting from the entry of sperm into the female reproductive tract up to the establishment of pregnancy.

Introduction to Fertilization\nFertilization is the process of fusion of a sperm with an ovum. During coitus (copulation), semen is released by the penis into the vagina (insemination). The motile sperms swim rapidly, pass through the cervix, and reach the ampullary-isthmic junction of the fallopian tube, where fertilization takes place. Meanwhile, the ovum released by the ovary is also transported to the ampullary-isthmic junction.

Mechanism of Fertilization

1. Capacitation: Before fertilization, sperms undergo functional maturation in the female reproductive tract known as capacitation, which makes them capable of fertilizing the ovum.
2. Acrosomal Reaction: The sperm makes contact with the zona pellucida layer of the ovum and induces the acrosome to release hydrolytic enzymes (sperm lysins like hyaluronidase and acrosin). These enzymes digest the egg's outer layers, allowing the sperm to reach the plasma membrane.
3. Cortical Reaction: As a sperm enters the cytoplasm of the ovum through the plasma membrane, it induces changes in the zona pellucida (cortical reaction) that block the entry of additional sperms, thus preventing polyspermy.
4. Completion of Meiosis II: The entry of the sperm triggers the completion of the second meiotic division of the secondary oocyte, forming a second polar body and a mature haploid ovum (ootid).
5. Syngamy: The haploid nucleus of the sperm and that of the ovum fuse together to form a diploid zygote (2n = 46).

Cleavage and Implantation

1. Cleavage: As the zygote moves down the isthmus of the oviduct toward the uterus, it undergoes mitotic divisions (cleavage) to form 2, 4, 8, 16 daughter cells called blastomeres.
2. Morula Formation: The embryo with 8 to 16 blastomeres is called a morula. The morula continues to divide and transforms into a blastocyst.
3. Blastocyst Structure: The blastocyst consists of an outer layer of cells called trophoblast and an inner group of cells attached to trophoblast known as the inner cell mass.
4. Implantation: The trophoblast layer then gets attached to the endometrium of the uterus, and the inner cell mass gets differentiated as the embryo. The uterine cells divide rapidly and cover the blastocyst. Consequently, the blastocyst becomes embedded in the endometrium of the uterus, a process called implantation, which leads to pregnancy.