Sunday, July 26, 2020

Human Reproduction - Notes | Class 12 | Part 3: Spermatogenesis

3. HUMAN REPRODUCTION


GAMETOGENESIS


-    It is the formation of gametes in the gonads.

-    It is 2 types: Spermatogenesis and Oogenesis.


1. Spermatogenesis

It is the process of formation of sperms (spermatozoa) in seminiferous tubules of testis. It has 2 stages:

a.    Formation of spermatids: In this, Sperm mother cells (Spermatogonia or male germ cells) produce spermatids.

b.    Spermiogenesis: Spermatids transform into sperm.


Schematic representation of spermatogenesis

-    4 spermatids are formed from each primary spermatocyte.

-    After spermiogenesis, sperm heads are embedded in Sertoli cells to get nourishment. Then they are released to lumen of seminiferous tubules. It is called spermiation.

Diagrammatic sectional view of a seminiferous tubule


Role of Hormones in Spermatogenesis


-    Hypothalamus releases Gonadotropin releasing hormone (GnRH).

-    GnRH stimulates the anterior pituitary gland to secrete 2 gonadotropins such as Luteinizing hormone (LH) and

follicle stimulating hormone (FSH).

-    LH acts on the Leydig cells and stimulates secretion of androgens. Androgens stimulate the spermatogenesis.

-    FSH acts on the Sertoli cells and stimulates secretion of some factors for the spermiogenesis.


Structure of spermatozoa (Sperm)


-    A mature sperm is about 60 µ (0.06 mm) long.

-    A plasma membrane envelops the whole body of sperm.

-    A sperm has 3 regions:


a.  Head: Oval shaped. Formed of nucleus and acrosome. Acrosome is formed from Golgi complex. It contains lytic enzymes. Behind the head is a neck.

b.  Middle piece: Composed of axial filament surrounded by mitochondria & cytoplasm. Mitochondria produce energy for the sperm motility.

c.  Tail: Consists of a central axial filament. The sperm moves in fluid medium and female genital tract by the undulating movement of the tail.


-   Man ejaculates 200-300 million sperms during a coitus.

-   For normal fertility, at least 60% sperms must have normal shape and size. 40% of them must show vigorous motility.

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