What is spermatogenesis? Write a short note about this.

Spermatogenesis: the process by which stem cells develop into  mature spermatozoa. There are three phases: (1) Spermatocytogenesis (Mitosis), (2) Meiosis, and (3) Spermiogenesis.

1. Spermatocytogenesis (also called Mitosis): Stem cells (Type A spermatogonia; singular = spermatogonium) divide mitotically to replace themselves and to produce cells that begin differentiation (Type B spermatogonia).  Spermatogonia have spherical or oval nuclei, and rest on the basement membrane.

2. Meiosis: Cells in prophase of the first meiotic division are primary spermatocytes.  They are characterized by highly condensed chromosomes giving the nucleus a coarse chromatin pattern and an intermediate position in the seminiferous epithelium. This is a long stage, so many primary spermatocytes can be seen.  Primary spermatocytes go through the first meiotic division and become secondary spermatocytes.  The cells quickly proceed through this stage and complete the second meiotic division. Because this stage is short there are few secondary spermatocytes to be seen in sections. You are not responsible for identifying secondary spermatocytes in lab.  Meiosis is the process by which the diploid number of chromosomes present in spermatogonia (the stem cells) is reduced to the haploid number present in mature spermatozoa.

The products of the second meiotic division are called spermatids. They are spherical cells with interphase nuclei, positioned high in the epithelium.  Since spermatids go through a metamorphosis into spermatozoa, they occur in early through late stages.

3. Spermiogenesis:

This is the metamorphosis of spherical spermatids into elongated spermatozoa. No further mitosis or meiosis occurs.  During spermiogenesis,  the acrosome forms, the flagellar apparatus forms, and most excess cytoplasm (the residual body) is separated and left in the Sertoli cell. Spermatozoa are released into the lumen of the seminiferous tubule. A small amount of excess cytoplasm (the cytoplasmic droplet) is shed later in the epididymis.

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