Question

Oogenesis is the biological process through which an egg forms. Without addressing the details of this process, discuss the different types of follicles that the ovaries contain and allow the development.

Answer 1

Oogenesis refers to the process of of formation of OVA from the primitive germ cells.

PRIMITIVE GERM CELLS: When the bipotential gonads differentiate into ovaries in genetic female embryo, the primitive germ cells increase in number by mitosis to form oogonia. OOGONIA are the stem cells from which were derived. Oogonia undergo mitosis to form PRIMARY OOCYTES. The primary oocytes become enveloped by single layer of flat granulosa cells and then are called PRIMORDIAL FOLLICLES.

Primordial follicles are formed in fetal life. Each primordial follicle consists of the primary oocyte in the prophase of the first meiotic division surrounded by a single layer of flat cells called granulosa cells. Both the granulosa cells and the primary oocyte are enveloped in a thin membrane called basal lamina.

PRIMARY FOLLICLE:- the the primary follicle is formed when primordial follicle undergoes following changes:-

• Granulosa cells become columnar and undergo mitosis to form multi-layered stratum granulosum.
• Suicide enlarges and becomes 20 micrometers in size.
• Homogeneous membrane consisting of glycoproteins appear between the granulosa cells and the oocyte which is called zona pellucida.

SECONDARY FOLLICLE:- it is formed from the primary follicle when:

• Granulosa cells undergo further proliferation.
• Oocyte increases in size upto 100 micrometers.
• A follicular sheath is formed outside the basal lamina from the spindle shaped cells from the stroma of cortex in ovary. It consists of an inner rim of secretory cells called theca interna and an outer rim of spindle shaped cells called theca externa.

TERTIARY FOLLICLE:- the granulosa cells start secreting fluid which causes cavity formation in the stratum granulosum which is called antrum for follicular cavity. The fluid contains oestrogen. Size of follicle further increases.

GRAAFIAN FOLLICLE (ANTRAL FOLLICLE):- after about 7th day of cycle, one of the tertiary follicle increase in size in response to FSH and LH and forms the mature follicle called Graafian follicle. It is also known as antral follicle.

• Size of the follicle increases upto 2-5mm.
• Antrum becomes larger.
• The secretary cells of theca interna becomes more in number and oestrogen secretion is increased.
• Just prior to ovulation, There occurs LH Surge which stimulates the primary oocyte to resume its Meiosis-I and secondary oocyte is formed with a haploid nucleus.

When ovulation occurs there is rupture of Graafian follicle and secondary oocyte is released.

CORPUS HEMORRHAGICUM:- when draw picture of Graafian follicle occurs the outer wall collapses and is filled with blood. This is called corpus hemorrhagicum.

CORPUS LUTEUM:- soon the granulosa cells and theca cells of the follicle lining begin to proliferate and the clotted blood is replaced by lipid rich luteal cells. The total mass of cells is now called corpus luteum. It primarily secretes progesterone .

CORPUS ALBICANS:- when there is no fertilization and pregnancy does not occur, the Corpus luteum starts to progress after 24th day of the cycle and is replaced by a whitish scar tissue called corpus albicans. This involution occurs due to falling levels of FSH and LH . When Corpus luteum gets involuted, the levels of oestrogen and progesterone fall. This removes their negative feedback on the anterior pituitary and FSH and LH are released and next ovarian cycle starts.