Human Reproduction

Oogenesis is the process of formation of a mature female gamete (ovum). During oogenesis, each diploid primary oocyte (formed by mitotic divisions of oogonia or egg mother cells) undergoes two maturation divisions.? In the first meiotic division, the primary oocyte divides into two very unequal haploid daughter cells - a large secondary oocyte and a very small first polar body or polocyte. In the second maturation division the first polar body may divide to form two second polar bodies. The secondary oocyte again divides into unequal daughter cells, a large ootid and a very small second polar body. The ootid grows into a functional haploid ovum. Thus from one primary oocyte, one ovum and three polar bodies are formed. The ovum is the actual female gamete.

Somatic ectoderm forms epidermis of skin, epidermal modifications e.g., scales of snakes, feathers of birds, nails, enamel of teeth, skin glands, eye lens, cornea, internal ear, anterior pituitary, oral and anal epithelium etc.

Cleidoic eggs are found in birds and reptiles. They are covered by a protective shell, which may be flexible or calcareous and contains all the food the embryo needs to develop and for hatching.

Central nervous sytem consists of brain and spinal cord. The ectoderm generates the outer layer of the embryo, and it is formed from the embryo's epibalst. The ectoderm develops into the surface ectoderm, neural crest, and the neural tube. The neural tube of the ectoderm develops into brain and, spinal cord (central nervous system), posterior pituitary, motor neurons and retina.

Just after the fusion of sperm and plasma membranes of oocyte, the secondary oocyte shows a cortical reaction, The cortical granules are present beneath the .plasma membrane of the secondary oocyte, These granules fuse with the plasma membrane of athe oocyte and release their contents including cortical enzymes between? the plasma membrane and the zona pellucida. These enzymes harden the zona pellucida to form fertilization membrane which prevents entry of additional sperms (polysperrny),

The process of sperm formation is called spermatogenesis and occurs in seminiferous tubules of the testis. The germ cells or spermatogonia divide mitotically to form primary spermatocytes (2n) which through meiosis form secondary spermatocytes (n) which further undergo meiosis II to form spermatids (n). Spermatids transform into mature sperms or spermatozoa. Hence, primary spermatocytes which are diploid have the same number of chromosomes as any somatic cells i.e., 22 pairs of autosomes and XY chromosomes