Biology today

Gametogenesis in females

• The process of development and maturation of ovum is
known as oogenesis.

• It consists of three phases : multiplication, growth and
maturation phase.

multiplication phase

• During multiplication phase, the cells of germinal epithelium
divide and detach to produce oogonia.

• The oogonia multiply by mitotic divisions and project into
the stroma as a cord, the egg tube of Pfluger, which later
becomes a round mass, the egg nest. One cell in the egg
nest grows and becomes the primary oocyte.

• Primary oocytes are diploid cells containing the same number
of chromosomes as in the parent somatic cells. The primary
oocytes cease to divide and enter the growth phase.

Growth phase

• This phase of the primary oocyte is very long. It may extend
over many years.

• There is accumulation of food materials and other resources
for nourishment of the oocyte.

• meiosis begins in the primary oocytes soon after their
formation. however, the oocytes are arrested in the early

part of meiotic prophase I (diplotene stage). This is the first

resting stage. They undergo a round of DNa synthesis,

and chromosome pairing takes place, but meiosis does not

proceed further until years later.

maturation phase

•    after primary oocyte has finished its growth, there is two

specialised nuclear divisions, first one of which is the

reductional division.

•    Beginning of puberty, a small number of primary oocytes are

activated each month. however, only one continues meiosis

I, producing two haploid cells of dissimilar size.

•    The smaller cell is called first polar body and larger cell is

called secondary oocyte.

•    The secondary oocyte proceeds with meiosis II but the division

gets arrested in metaphase II stage. This is the second

resting stage. This is due to an activity called cytostatic

factor which maintains arrest through preventing loss of

maturation Promoting Factor (mPF).

•    MPF is a protein in cell cycle which stimulates m-phase of

cell cycle.

•    It is in this stage of oocyte that the ovum is shed during

ovulation. It passes into oviduct, where in the ampulla part,

cell cycle will resume only after the entry of sperm.

Fig.:Summary of hormonal control of female reproductive system

LH stimulates

Anterior

pituitary gland

GnRH stimulates

release of FSH and LH

GnRH Hypothalamus

FSH stimulates

Growing Ovary Ovulation

follicles

Initial development of

ovarian follicles

Further development of ovarian

follicles and their secretion of

estrogens and inhibin

Secretion of progesterone, estrogen

and inhibin by corpus luteum.

Estrogens Progesterone Inhibin

Inhibitsreleaseof

FSH and, to a lesser

extent, LH.

FSH LH

 Promote developmentand

maintenance of female reproductive

structures,femininesecondary sex

characteristics and breasts.

 Increase protein anabolism.

 Moderate levels inhibit release of

GnRH, FSH, and LH.

 Workswithestrogensto

prepareendometriumfor

implantation.

 Prepares mammary glands to

secrete milk.

 Inhibits release of GnRH and

LH.

Corpus luteum