30. Reproductive tissue. Spermatogenesis. Spermatozoa.

It consists of gonads which are the ovaries and testes which produce gametes. The gametes in females and males are eggs and sperm, respectively.

The formation of gametes is via meiosis. Reproductive tissue includes highly differentiated germ cells with progenitor and satellite cells. Satellite cells are split into 2 forms; follicular cells, present female reproductive systems and sertolli cells present male reproductive systems. 

Satellite cells are derived from primordial germ cells. 

Primordial germ cells develop in the 3rd embryonal week in the wall of the yolk sac. They are large cells with light cytoplasm and are PAS reaction positive. These cells proliferate and migrate to the primitive sex cords which gives rise to satellite cells. At this stage the gonad is indifferent as one cannot morphologically distinguish it being male or female.

The formation of sperm occurs via meiosis and is called spermatogenesis. It can be divided into 2 processes; spermatocytogenesis and spermiogenesis.

Spermatogenesis occurs in the male testis, starts at the beginning of puberty and is a continuous process. During puberty, primitive germ cells called spermatogonia, enlarge and become mitotically active. These cells undergo spermatogenesis, in which diploid spermatogonia gives rise to haploid spermatozoa. 

Seminiferous tubules are coated by a germinal epithelium that contains Sertoli cells and germ cells. 

During differentiation, spermatozoa move towards the lumen of these tubules where they undergo mitosis, meiosis, and maturation. This process takes 64-74 days and is coordinated by Sertoli cells, and continues throughout life.

The formation of sperm comprises of 4 phases; proliferative phase where germ cells develop into spermatogonia, growing phase in which primary and secondary spermatocytes form, maturation phase in which spermatid forms and finally the transformation phase in which spermatids transform into spermatozoa. 

The first process is spermatocytogenesis which involves meiosis and is outlined below:

DNA replication occurs before meiosis begins. This is where spermatogonia gives rise to primary spermatocyte.  Meiosis has 2 divisions. The primary spermatocyte enters the first division. The first stage is prophase 1, metaphase 1, anaphase 1 and telophase 1. At the end of the first division we have 2 diploid cells and the primary spermatocyte gives rise to the secondary spermatocyte, which enters the second division of meiosis. At the end of meiosis each cell has a haploid set of chromosomes and each chromosome has only one chromatid. The secondary spermatocytes gives rise to spermatids. 

Next we have spermiogenesis. Here the spermatids differentiate into sperm cells. 

Spermiogenesis is the final part of spermatogenesis and involves 4 parts: 

1. Nuclear condensation: reduction and condensation of the nuclear contents.

2. Acrosome formation: containing enzymes that penetration through the zona pellucida of the oocyte during fertilisation. 

3. Flagellum formation: is the tail of the sperm cells.

4. Cytoplasm reduction: elimination of all unnecessary cytoplasm.

At the end of spermatogenesis we are left with a sperm cell that consists of a head (nucleus and acrosome), a tail called flagellum made of microtubules and a mid-piece which is full of mitochondria to provide energy mobility. 

The control of spermatogenesis is regulated by 3 hormones: FSH and LH, secreted by the pituitary glands and testosterone, released by leydig cells.