Structure Of Ovum

Embryonic Development of Class 12

Structure Of Ovum of Human Reproduction 

An ovum is generally spherical, non motile gamete with volky cytoplasm and enclosed in one or more egg envelopes. Size of ovum varies in different animals and depends upon the amount of yolk. Size of ovum varies from 10 to a few cm. Largest sized egg is of ostrich and is about 170 × 135 mm. Egg size and yolk amount are interdependent. It is about 50 micron in many polychaete worms, 150 micron in tunicates but very large sized in birds and reptiles. In mammals, it is generally microlecithal and about 100 micron.

Human ovum is microlecithal with large amount of cytoplasm. Cytoplasm is differentiated into outer, smaller and transparent exoplasm or egg cortex and inner, larger and opaque endoplasm or ooplasm.

Egg cortex is with some cytoskeletal structures like microtubules and microfilaments (Balinsky, 1981), pigment granules and cortical granules of mucopolysaccharides. Endoplasm is with cell-organelles, informosomes tRNAs, histones, enzymes etc. Nucleus of ovum is large, bloated with nucleoplasm and is called germinal vesicle. Nucleus is eccentric in position so human ovum has a polarity. The side of ovum with nucleus and polar body is called animal pole, while the opposite side is called vegetal pole.

Egg membranes are of three categories in different eggs :

  • Primary membrane : Secreted by the egg itself, e.g. zona pellucida, (mammal); vitelline membrane in mammals (mollusca, amphibia, birds, echinoderms etc.)
  • Secondary membrane : Contributed by the ovary or follicular cells e.g., corona radiata (granulosa layer) in mammal, chorion of insects egg.
  • Tertiary membrane : Secreted by the reproductive tract e.g. albumen, shell membranes and shell of hen’s egg, jelly around frog’s egg etc.

Yolk (nutritive material)

Synthesized in egg by the process of vitellogenesis, main constituents as proteins and phospholipids.

Hen’s egg contains 48.7% water, 16.6% protein, 32.6% phospholipids and 1% carbohydrates.

Proximation of gametes

In external fertilization both gametes are released in very high number.

Sperm reaches ovum by lashing movement of its tail at the rate of 1.5 to 3.0 mm/minute. It is also aided by the contraction of uterine wall and the wall of fallopian tube.

Ovum (after ovulation) reaches upto ampulla of fallopian tube through ostium helped by the fimbriae.

Sometimes sperms come out into coelomic cavity through ostium and fertilizes egg in coelomic cavity. This condition of ectopic pregnancy is dangerous.

Recognition of gametes

It is based upon the principle of antigen - antibody reaction i.e. the fertilizin - antifertilizin reaction, the former released from ovum and the latter from sperm (F.R. Lillie - 1919).

Capacitation

Enables sperm to fertilize ovum. In female genital tract the layer of glycoprotein acquired in epididymis is removed from sperm’s surface.

Acrosomal reaction

Acrosomal membrane ruptures releasing, sperm lysin. This dissolves corona radiata, zona pellucida (or vitelline membrane) and the viscous cytoplasmic material of egg to help sperm penetrate.

Cortical Reaction of Ovum

In some (not human) fertilization cone is formed at the site of contact with sperm.

Depolarization of egg membrane by the touch of first sperm renders it insensitive for other sperms.

Fertilization membrane is formed around egg which inhibits the entry of other sperm.

Penetration of Sperm

Dissolution of plasma membrane of sperm and ovum along the side of fertilization cone facilitates entry of nucleus (head) and proximal centriole into egg cytoplasm. Middle piece and tail remain outside or absorbed by the egg later on.

Merger of male pronucleus with female pronucleus formed by the dissolution of nuclear envelope takes place along a particular fertilization path.

Proximal centriole of the sperm initiates spindle formation and thus cell division (cleavage).

Significance of Fertilization

Activation of egg to start cell division and embryonic development.

Restoration of the diploidy and recombination of two different sets of genes.

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