Spermatogenesis
Gametogenesis in male happens in testis. In humans, spermatogenesis takes 65 to 75 days. It begins with the spermatogonia, which contain the diploid (2n) number of chromosomes. Spermatogonia which are present in seminiferous tubule keeps dividing, where some remains as precursor stem cells and some differentiate into primary spermatocytes. Primary spermatocytes like sprematogonia are diploid (2n) cells where they have 46 chromosomes.
After each spermatocytes replicates its DNA through mitosis, the meiosis begins and at the end of meiosis I formation of secondary spermatocytes happen where each spermatocyte has haploid (n) 23 chromosomes. Each chromosome within a secondary sprematocyte however is made up of two chromatids (two copies of the DNA) still attached by a centromere. No replication of DNA occurs in the secondary spermatocytes.
In meiosis II, the chromosomes line up in a single line along the metaphase plate, and the two chromatids of each chromosome separates. The four haploid cells resulting from meiosis II are called spermatids. A single primary spermatocyte, therefore produces four spermatids via two rounds of cell division (meiosis I & meiosis II).[2]
The final stage of spermatogenesis is the development of haploid spematids into spermatozoa which is known as spermiogeneis. This process involves an extensive remodelling of the spermatids without further division. Each spermatid gradually differentiates into a minute motile spermatozoan with a head, middle piece and tail. The head consists of haploid nucleus and an acrosome, a cap like vesicle filled with enzymes that help a sperm to penetrate a secondary oocyte to bring about fertilization. The middle piece is packed with mitochondria which provide energy for the movement of the sperm. The tail helps in movement of the sperm. [2]
References:
1. Principles of Anatomy And Physiology 12th edition By Gerard J. Tortora and Bryan Derrickson
2. Langman’s Medical Embryology 12th edition by T.W Sadler
