Gametogenesis - Spermatogenesis

Spermatogenesis:

• Spermatogenesis is the sequence of events in the seminiferous tubules of the testes that produce the male gametes, the sperms.
• Spermatogenesis begins at puberty. It occurs in testis.
  

  Spermatogenesis

1. During development, the primordial germ cells migrate into the testes and become immature germ cells called sperm mother cells or spermatogonia (sing. spermatogonium) in the inner surfaces of the seminiferous tubules as shown below figure.
   

   Cross-sectional view of seminiferous tubule

2. The spermatogonia begin to undergo mitotic division at puberty and continues throughout life.
3. In the first stage of spermatogenesis, the spermatogonia migrate among sertoli cells towards the central lumen of the seminiferous tubule and become modified and enlarged to form primary spermatocytes which are diploid with 23 pairs i.e., 46 chromosomes.
4. Some of the primary spermatocytes undergo first meiotic division to form two secondary spermatocytes which are haploid with 23 chromosomes each.
5. The secondary spermatocytes undergo second meiotic division to produce four haploid spermatids.
6. The spermatids are transformed into mature spermatozoa (sperms) by the process called spermiogenesis.
7. After spermiogenesis, sperm heads become embedded in the Sertoli cells and are finally released from the seminiferous tubules by the process called spermiation.
   

   Seminiferous tubules (Enlarged)

8. Spermatogenesis starts at the age of puberty and is initiated due to the increase in the release of Gonadotropin Releasing Hormone (GnRH) by the hypothalamus.
9. GnRH acts on the anterior pituitary gland and stimulates the secretion of two gonadotropins namely Follicle Stimulating Hormone (FSH) and Lutenizing Hormone (LH).
10. FSH stimulates testicular growth and enhances the production of Androgen Binding Protein (ABP) by the sertoli cells and helps in the process of spermiogenesis.
11. LH acts on the Leydig cells and stimulates the synthesis of testosterone which in turn stimulates the process of spermatogenesis.

The whole process of spermatogenesis takes about 64 days. At any given time, different regions of the seminiferous tubules contain spermatocytes in different stages of development. The sperm production remains nearly constant at a rate of about 200 million sperm per day.

Structure of human spermatozoan:

• The human sperm is a haploid, elongated and motile male gamete.
• The human sperm is a microscopic structure and flagellated.
• The human male ejaculates about 200 to 300 million sperms during coitus. It is estimated that around 60 percent of sperms must have the normal shape of which at least 40 percent must show vigorous motility for normal fertility.
• The whole body of the sperm is enveloped by a plasma membrane and is composed of a head, neck, and tail.

1. Head: The head comprises of two parts namely the acrosome and nucleus. The acrosome is a small cap-like pointed structure present at the tip of the nucleus and is formed mainly from the Golgi body of the spermatid. It contains hyaluronidase, a proteolytic enzyme, popularly known as sperm lysin which helps to penetrate the ovum during Fertilization. The nucleus is flat and oval.
2. Neck: The neck is very short and is present between the head and the middle piece. It contains the proximal centriole towards the nucleus which plays a role in the first division of the zygote and the distal centriole gives rise to the axial filament of the sperm.
3. Middle piece: The middle piece possesses mitochondria spirally twisted around the axial filament called mitochondrial spiral or nebenkern. It produces energy in the form of ATP molecules for the movement of sperms.
4. Tail: The tail is the longest part of the sperm and is slender and tapering. It is formed of a central axial filament or axoneme and an outer protoplasmic sheath. The lashing movements of the tail push the sperm forward.