Menstrual Phase (1-5 days)

Endrometrium breaks down the cell of endometrium secretions unfertilised ovum constitute menstrual flow. Progesteron production is reduced

Follicular Phese (6-13 days)

Endometrium rebuilds, FSH and oestrogen secretion is increased.

Ovulatory Phase (14-16 days)

Both LH and FSH attain peak level. Estrogen level is also high. It leads to ovulation.

Luteal Phase (16-28 days)

In absence of fertilisation corpus luteum secretes progesterone. Endometrium Thickens and uterine glands become secretary.

The menstrual cycle starts with the menstrual phase, when menstrual flow occurs and it lasts for 3-5 days. It results due to breakdown of endometrial lining of the uterus and its blood vessels.

Follicular Phase The menstrual phase is followed by the follicular phase.

During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation. These changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones.

The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase and stimulates follicular development as well as secretion of estrogens by the growing follicles. Both LH and FSH attain a peak level in the middle of cycle (about 14th day).Rapid secretion of LH leading to its maximum level during the mid-cycle called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation). The ovulation (ovulatory phase) is followed by the luteal phase during which the remaining parts of the Graafian follicle transform as the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium.

Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy. In the absence of fertilisation, the corpus luteum degenerates.

This causes disintegration of the endometrium leading to menstruation.

$$ \text { Oogenesis is different from that of spermatogenesis in the following aspects } $$

Spermatogenesis	Oogenesis
Sperm generation starts at puberty.	Oocytes generated before birth.
Many millions generated at a time.	Only one matures at a time, every month.
After two complete meiotic divisions, four equal sized cells produced.	Meiosis-I get arrested at prophase-I and when completed at later stage, one big cell with almost all the cytoplasm and three very small sized cells produced.
They mature into flagellated and motile cell.	Mature ovum is non-flagellated and non-motile.

Reasons

(i) Unequal cell division makes the ovum much larger than the other three polar bodies. Because ovum has more cytoplasm and more organelles, it has a better chance of surviving.

(ii) The male makes millions of tiny sperms while, the female makes only one egg per month that also waits for second meiotic division, until just before fertilisation. This is a way of conserving energy.

(iii) Sperm is smaller and motile as it has to move out of male system to female reproductive system. Larger egg has abundant reserve food so that embryo starts developing right after fertilisation.

The zygote passes through the following stages till implantations

(i) The embryo with 8-16 blastomeres is called a morula.

(ii) The morula continues to divide and transforms into blastocyst as it moves further into uterus.

(iii) The blastomeres in the blastocyst are arranged into a surface layer called trophoblast and a cluster of interior cells attached to trophoblast are called the inner cell mass.

(iv) The trophoblast layer then gets attached to the endometrium and inner mass cells get differentiated as embryo.

(v) After attachment, the uterine cells divide rapidly to cover the blastocyst.

(vi) The blastocyst becomes embedded in the uterine endometrium. This is called implantation.

$$ \text { Transport of ovum, fertilisation and passage of growing embryo through Fallopian tube } $$

The female reproductive system consists of a pair of ovaries along with a pair of oviducts, uterus, cervix, vagina and the external genitalia located in pelvic region.

These parts are integrated structurally and functionally to support the processes of ovulation, fertilisation, pregnancy and birth.

Diagrammatic sectional view of the female reproductive system

Labels

(i) Ovary (production of gamete)

(ii) Isthmus-ampullary junction(site of fertilisation)

(iii) Uterine endometrium (site of implantation)

(iv) Cervix and vagina (birth canal)

The description of mammary gland is as follows

(i) The mammary glands are paired structures (breasts) that contain undeveloped duct system and adipose tissue (fat containing cells).

(ii) During pregnancy, in response to oestrogen and progesterone, a glandular system develops for milk production.

(iii) Glandular tissue of each breast develops mammary lobes containing clusters of cells called alveoli.

(iv) The cells of alveoli secrete milk, which is stored in the cavities (lumens) of alveoli

(v) The alveoli open into mammary tubules. The tubules of each lobe join to form a mammary duct.

(vi) Several mammary ducts join to form a wider mammary ampulla which is connected to lactiferous duct through which milk is sucked out by the baby.

$$ \text { A diagrammatic sectional view of mammary gland } $$