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
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.