Gametogenesis And Oogenesis ! Process, Mechanism, Stages !! Biological Sciencess !!!

GAMETOGENESIS

The process of formation off gametes in the gonads is called gametogenesis. Gametes are produced by way of meiotic division resulting in the reduction of number of chromosomes to one half. The production of sperms in the seminiferous tubules of testis is called spermatogenesis and production of ova in ovaries is called oogenesis.

1. Spermatogenesis

Spermatogenesis is the process of formation of haploid functional spermatozoa from diploid germinal cells of seminiferous tubules. Lining layer of seminiferous tubules possesses primary germ cells and indifferent cells that mature into nurse cells or Sertoli cells.

(i) Multiplication Phase

Diploid primary germ cells undergo repeated mitosis to form a number of diploid spermatogonia.

(ii) Spermatocytogenesis

Each spermatogonia divided mitotically to from two interconnected primary spermatocyte

(iii) Growth Phase

Primary spermatocytes grow in size.

(iv) Maturation Phase

Each diploid primary spermatocyte undergoes meiosis I to form two haploid secondary spermatocytes. All the secondary spermatocytes derived from a single spermatogonium remain attached to one another. Secondary spermatocytes- divides by meiosis II, each giving rise to two haploid spermatids. The spermatids become partially embedded in Sertoli cells for nourishment and support.

(v) Spermiogenesis

Spermiogenesis is differentiation of a spermatozoon from a spermatid. Golgi apparatus forms acrosome. Centriole divides into two. Distal centriole forms axial filament. Mitochondria collect around upper part of axial filament. Nucleus undergoes condensation. A spermatozoon now separates while the unused cytoplasm degenerates. Heads of spermatozoa remain embedded for some time in Sertoli cells but ultimately the spermatozoa are released into lumen of seminiferous tubule for onward passage.

Sperm

Human sperm is dart-like flagellate structure of 60 µm length. It has four parts—
(i) Head. Knob-like terminal part has two components, acrosome and nucleus. Acrosome contains sperm lysins. Nucleus is compact mass of DNA having some protamines. On the outside is present a double membrane head cap.

(ii) Neck. It is short narrow part between head and middle piece, which contains two
centrioles, unconnected proximal centriole and distal centriole attached to axial
filament (that passes into middle piece).

(iii) Middle Piece. It is cylindrical part. It has axial filament surrounded by 10-14 spiral turns of mitochondria and bearing towards the end a ring centriole or annulus.
Mitochondria provide energy for swimming but food is limited.

(iv) Tail. It is narrow vibratile long part, with two regions, main and end piece. Main
piece of tail is 0.5 mm in diameter near the beginning but gradually narrows behind.
It has an axial filament, small amount of cytoplasm and plasma membrane. In the
end piece, cytoplasm and membrane are absent.

2. Oogenesis

It is the process of formation, development and maturation of haploid ova from diploid germinal cells of ovary. Oogenesis occurs in three phases

(i) Multiplication Phase

Diploid primary germ cells from germinal epithelium of ovary multiply mitotically and form oogonia. The latter produce ovigerous cords or egg tubes of Pfluger in mammals.

(ii) Growth Phase

It is prolonged and slow. Oogonia form rounded masses or egg nests at the tips of egg tubes of Pfluger. An egg nest forms ovarian follicle. One central oogonium grows and functions as primary oocyte. The others form the covering follicular cells. The latter provide nourishment to primary oocyte. Some nourishment also comes from outside. Enlarged primary oocyte secretes mucoprotein membrane or zona pellucida outside its own plasma membrane or vitelline membrane. There is increase in reserve food, size of nucleus, number of mitochondria, functioning of Golgi apparatus and complexity of endoplasmic reticulum.

(iii) Maturation Phase

Meiosis occurs. Nucleus shifts towards animal pole and undergoes meiosis I. A daughter nucleus alongwith small quantity of cytoplasm is extruded as primary polar body or polocyte below zona pellucida. Simultaneously primary oocyte is changed into haploid secondary oocyte. Ovum is generally shed in secondary oocyte stage. Meiosis II is completed in fallopian tube at the time of fertilization. It then produces a small secondary polar body and a large ovum. The primary polar body may also divide.

Ovum

Human egg or ovum is noncleidoic and alecithal (nearly microlecithal), rounded female gamete having a diameter of about 100 mm. The ovum possesses three coverings – inner thickened plasma membrane or vitelline membrane, middle mucoprotein zona pellucida and outer cellular corona radiata with radially elongated scattered cells held in mucopolysaccharide (hyaluronic acid). In between plasma membrane ( = vitelline membrane) and zona pellucida is peri-vitelline space in which are present 1- 3 polar bodies towards animal pole. The opposite end is vegetal pole. Cytoplasm of ovum is called ooplasm. It has a large nucleus or germinal vesicle. Ectoplasm
possesses

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