Abhay kumar

NEPEDUCATION Birgunj, Parsa

GAMETOGENESIS [FROG]

Gametogenesis

The process of formation of gametes is called gametogenesis.

It is occurred in two steps, these are:-

1. Spermatogenesis

2. Oogenesis

1. Spermatogenesis: -

The process of formation of male gamete is called spermatogenesis.

As we know that male gamete is lighter and motile to make fertilization. It can be

studies under following headings: -

A. Formation of Spermatids :- In this step haploid cell is formed. This haploid

cell is called spermatid. Spermatid is karyotypically same as male gamete but

it is non – motile.

B. Spermiogenesis :- In this step non – motile spermatids undergo some

morphological and physiological change to become motile and capable to

penetrate ovum for fertilization. Such motile structure is called spermatozoa.

A. Formation of Spermatids :-

It is occurred in following steps:-

a. Multiplication phase :-

The undifferentiated primordial germ cells undergo repeated mitotic cell

division to form a large number of spermatogonia or, sperm mother cell.

These cells are diploid and enter into next phase of development.

b. Growth Phase :-

During this phase, a limited growth of spermatogonia take place. Fully grown

spermatogonia are called primary spermatocyte which is diploid.

c. Maturation phase :-

In this stage diploid primary spermatocyte is converted into haploid

spermatids.

Primary spermatocyte undergoes meiosis I to form two haploid

secondary spermatocyte. Again haploid secondary spermatocyte undergoes

meiosis II to form haploid spermatids.

B. Spermiogenesis :-

The transformation or, differentiation of the non – motile spermatid into

motile spermatozoon is called spermiogenesis.

To convert non – motile spermatid following morphological and

physiological changes are occurred. Some of them are followings:-

1. Spermatids loose its water from cytoplasm and nucleoplasm to make itself

light.

2. The nucleus gradually diminishes in size and the chromosomes become

closely packed into a small volume.

3. Golgi body is modified into acrosome and gets the position at anterior end.

4. Both pairs of centrioles get the position behind the nucleus or, in the

middle piece of spermatozoon.

5. Mitochondria and other cell organelles are joint continuously and twisted

spirally around the axial filament.

Structure of a matured Spermatozoon :-

A matured spermatozoon is microscopic, elongated, about 0.03 mm

long structure.

It can be differentiated into three distinct parts :-

Head:-

It consists of a small rounded anterior acrosome and a long

cylindrical head containing nucleus.

Middle Piece :-

It is very small middle part containing centrioles and mitochondria.

Tail :-

It is the posterior part much elongated and terminating in a flagellum.

2. Oogenesis:-

The active biological process by which female gamete or, ovum is

formed by female reproductive organ is called oogenesis.

It is occurred in following steps :-

A. Multiplication phase.

B. Growth phase.

C. Maturation Phase.

A. Multiplication phase :-

Primordial germ cells or, egg mother cells of ovary undergo repeated

mitosis to form a large number of oogonia. Oogonium is a diploid cell..

B. Growth phase :-

Oogonium undergoes growth to form primary oocyte. Growth phase of

oogenesis is very important and long stage. In this stage oogonium collects more

nutrient and stored it for further development.

C. Multiplication phase :-

In this stage primary oocyte undergoes meiosis to form haploid ovum.

Primary oocyte undergoes meiosis I to form two cells of different sizes. Larger one is

called secondary oocyte and smaller one is called 1st polar body or, polocyte.

Secondary oocyte further undergoes meiosis II to form two cells of different sizes.

Larger one is matured ovum and smaller one is 2nd polar body.

Polar bodies are degenerated because of less amount of cytoplasm.

Structure of an unfertilized ovum :-

An unfertilized egg of frog is spherical and about 1.75 mm in diameter.

It is mesolecithal and telolecithal.

Egg cell membrane is covered by a vitelline membrane and outermost by

jelly.

Its half part is pigmented black called animal hemisphere. The central part of

animal hemisphere is called animal pole which is metabolically most active part.

The lower part which contains yolk is whitish or, yellowish in colour called

vegetal hemisphere. The central point of vegetal hemisphere is called vegetal pole.

The imaginary line passes through animal pole and vegetal pole is called polar axis.