At the age of 45 up to 50 years, the ovaries become inactive i.e. the hormonal secretion is decreased, and the uterine lining is wrinkled. This is known as menopause.

Study of a T.S. in the ovary:

The ovary contains a group of cells in different stages, and ovum inside the Graafian follicle. This follicle is transformed into the Corpus luteum after the release of the ovum.

Stages of Oogenesis:

1. Multiplication phase:

In which, the Primary germ cells (2N) divide mitotically forming the Oogonia (2N). This phase occurs during the development of the female embryo.

2. The growth phase:

The Oogonia (2N) store an amount of food, increase in size, and are transformed into Primary Oocytes (2N). This phase occurs during the development of the female embryo.

3. Maturation phase:

Each Primary Oocyte (2N) divides 1^st. meiotic division to give two haploid cells Secondary Oocyte (N) and 1^st. polar body (N). (the Oocyte is larger than the polar body). Then 2^nd. meiotic division takes places in which the Secondary Oocyte (N) divides giving an ovum and 2^nd. polar body, while the 1^st. polar body divides giving 2 polar bodies. So the resultants are 3 polar bodies.

The ovum contains cytoplasm and a nucleus. It is enveloped with a thin cellular coat, its cells are held together by Hyaluronic acid, the enzymes secreted by the acrosomes of sperms dissolved this coat at the penetration position. Therefore millions of sperms are required to penetrate the ovum.

Breeding cycle

In the life of placental mammals generally, there are periods where the ovary becomes regularly active in the adult female. These cycles are periodic, and coincide with mating, and production of young. Each is known as breeding cycle. The period of such cycles differs in various mammals:

- In case of lion and tiger, the cycle is annual.

- In case of cats and dogs, there are two breeding cycles per year.

- In case of rabbits and cats, the breeding season occurs frequently per month.

- In Human being, breeding cycle is known as the menstrual cycle which lasts for 28 days. The two ovaries alternate with each other to produce the matured ova.

The menstrual cycle:

This cycle is classified into three phases:

1. Phase of proliferation:

The pituitary gland releases the Follicle stimulating hormone (FSH). This stimulates few follicles to develop in the ovary, to form a matured one which is known as Graafian follicle, inside which the ovum grows and matures within 10 days. Graafian follicle secretes Estrogen hormone which stimulates the growth of the Endometrium.

2. Phase of ovulation:

The anterior lobe of the pituitary gland secretes the Luteinizing hormone (LH) in the 14^th.day from the beginning of the previous menstruation. This stimulates the ovum to liberate from the Graafian follicle. So, the Graafian follicle is transformed into the Corpus luteum, which produces Progesterone and Estrogen. These hormones act to increase the thickness of the Endometrium, and its blood supply. This phase lasts for 14 days.

3. Phase of menstruation:

- If the ovum is not fertilized:

The Corpus luteum degenerates gradually, and its secretion of Progesterone stops, so the Endometrium degenerates and the blood vessels tear due to the successive contractions of the uterus. Thus menstrual bleeding takes place, which lasts for 3 – 5 days. And new cycle for the other ovary begins.

- If the ovum is fertilized:

The Corpus luteum remains to secrete the Progesterone and Estrogen, these inhibit ovulation, and thus the menstrual cycle stops till after birth. The Corpus luteum reaches to its maximum growth at the end of the 3^rd. month of pregnancy. Then it starts to degenerate during the 4^th. month, when the Placenta that grows in the uterus takes over its function in secreting Progesterone, that preserves the endometrium, and stimulates the maternal mammary glands to develop gradually. So, if the Corpus luteum is removed before the 4^th. Month, abortion occurs.

Fertilization

It’s the fusion of the male gamete (sperm) with the female gamete (ovum) to form the zygote, which divides forming the embryo.

Events of fertilization:

1. After the ovum is released in the 14^th. day from the beginning of menstruation. It will be ready for fertilization through 1-2 days. Fertilization takes place in the 1^st. third part of the Fallopian tube.

2. The number of sperms ranges between 300 – 500 million per ejaculation, many of them are lost during their journey to the ovum, therefore the Human male is considered infertile if the number of sperms is less than 20 million.

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