CBSE · Class 12 · Biology · Human ReproductionExplain the events of embryonic development starting from fertilization up to the formation and implantation of the blastocyst in humans. Also, briefly describe the role of placenta.

Fertilization and Implantation\nEmbryogenesis in humans is a complex sequence of events following copulation, leading to the formation of a new individual. The step-by-step process is detailed below:

1. Fertilization:

   • During coitus, 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.
   • Simultaneously, an ovum released by the ovary is also transported to the ampullary region where fertilization takes place.
   • Fertilization can only occur if the ovum and sperms reach the ampullary-isthmic junction simultaneously.
   • The sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms (polyspermy block).
   • The secretions of the acrosome help the sperm enter the cytoplasm of the ovum through the zona pellucida and the plasma membrane.
   • This induces the completion of the second meiotic division of the secondary oocyte, forming a second polar body and a haploid ovum, which fuses with the haploid sperm nucleus to form a diploid zygote ($2n = 46$).

2. Cleavage and Morula Formation:

   • As the zygote moves down the isthmus of the oviduct towards the uterus, it undergoes repeated mitotic divisions called cleavage.
   • These divisions produce 2, 4, 8, 16 daughter cells called blastomeres.
   • The embryo with 8 to 16 blastomeres is called a morula.
   • The morula continues to divide and transforms into a blastocyst as it moves further into the uterus.

3. Blastocyst Formation and Implantation:

   • The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass.
   • The trophoblast layer then gets attached to the endometrium, and the inner cell mass gets differentiated as the embryo.
   • The cells of the endometrium divide rapidly and cover the blastocyst.
   • Consequently, the blastocyst becomes embedded in the endometrium of the uterus. This process is known as implantation, which leads to pregnancy.

Role of Placenta\nThe placenta is a structural and functional unit between the developing embryo (foetus) and the maternal body. Its key roles include:

• Nutrition: It supplies oxygen and nutrients from the mother's blood to the foetus.
• Excretion: It helps in the removal of carbon dioxide and waste materials produced by the foetus.
• Endocrine Function: It acts as an endocrine tissue and produces several hormones like Human Chorionic Gonadotropin (hCG), Human Placental Lactogen (hPL), estrogens, and progesterones which are essential to maintain pregnancy.