THE EVENTS IN MAMMALIAN REPRODUCTION

FERTILISATION

Ovum is released in the secondary oocyte stage (arrested in metaphase-II). Due to ciliary current produced by fimbriae of oviduct, ovum is drawn in through ostium.

It reaches ampulla, the site of fertilisation, by the ciliary action of ciliated columnar epithelium lining of oviduct.

A human sperm can live for many weeks in male genitial duct. Once ejaculated sperm can live alive only for 24 to 48 hours outside the body. Sperms move in the liquid medium secreted by female genital tract (1.5-3.0 mm/minute).

Prostaglandins of semen help in movement of spermatozoa and finally reach ampulla portion of the oviduct.

Capacitation of sperm occurs in the female genital system due to

(1) Removal of membrane cholesterol present over acrosome, weakening the membrane cover.

(2) Dilution of decapacitation factors.

(3) Entry of Ca2+ into sperms causing rapid whiplash motion of the tail.

Fusion of gametes/Syngamy: The various steps involve :

Acrosomal reaction: Number of sperms adhare to the surface of egg covers (Agglutination). The acrosome starts releasing its hydrolytic enzymes (sperm lysins).

It includes,

(a) Hyaluronidase: Dissolves the hyaluronic acid responsible for cementing of follicle cells or granulosa cells.

(b) Corona digesting enzyme (CDE) : Dissolves corona radiata.

(c) Zona lysin/Acrosin : Digests the zona pellucida. It involves zona pellucida compatibility reaction determined by 'fertilizin' protein over zona pellucida and 'antifertilizin' in case of sperm.

Contact of acrosome stimulates development of an outgrowth by the oocyte called fertilisation cone or cone of reception.

As the sperm head gets in contact with the fertilization cone, it cause opening up of Na+ channel to cause depolarisation of membrane (fast block to check polyspermy) and Ca2+ wave inside the egg.

Sperm and egg membranes dissolve. Male pronucleus & proximal centriole of sperm enter the cytoplasm of egg & rest part is left out.

Ca2+ wave causes extrusion of cortical granules (cortical reaction) and zona reaction makes the zona pellucida impervious to second sperm by destroying sperm receptors.

Cortical reaction and zona reaction constitute slow block to check polyspermy.

Entry of sperm causing breakdown of metaphase promoting factor (MPF) and turning on anaphase promoting complex (APC). This results in oocyte completing its meiosis -II.

Male and female pronuclei approach each other and finally mixing up of paternal and maternal chromosomes (Amphimixis) occur resulting in the formation of zygote/ synkaryon.