11 min readPathway sheetA 3 or 5-mark answer on oogenesis with a diagram of the ovary, spermatogenesis vs oogenesis as a 3-mark question, and spotting the T.S. of ovary in the practical; also a NEET favourite

Oogenesis and the T.S. of ovary

Oogenesis begins before birth, when primary oocytes pause in prophase I inside follicles. From puberty, a follicle matures into a Graafian follicle that releases a secondary oocyte.

OogoniaA couple of million form in each foetal ovary; no more are formed or added after birth

At pubertyOnly 60,000 to 80,000 primary follicles are left in each ovary

First meiotic divisionUnequal: a large haploid secondary oocyte and a tiny first polar body

Completion of meiosis IIOnly after a sperm enters; it gives the ovum (ootid) and a second polar body

Stages of oogenesis

1
Before birth: a couple of million oogonia (egg mother cells) form in each foetal ovary. They enter prophase I of meiosis and pause there. These are the primary oocytes.
2
Each primary oocyte is wrapped in a layer of granulosa cells: a primary follicle. Many follicles degenerate between birth and puberty, leaving only 60,000 to 80,000 in each ovary.
3
From puberty, more layers of granulosa cells and a new theca form around some follicles: secondary follicles.
4
A fluid-filled cavity, the antrum, appears: a tertiary follicle. Its theca forms an inner theca interna and an outer theca externa.
5
The primary oocyte grows and completes meiosis I. The division is unequal: a large haploid secondary oocyte keeps most of the food-rich cytoplasm, and a tiny first polar body forms.
6
The follicle becomes a mature Graafian follicle. The secondary oocyte forms a new membrane around it, the zona pellucida.
7★
The Graafian follicle ruptures and releases the secondary oocyte (ovum) from the ovary: ovulation.

★ key step

Draw it: schematic of oogenesis

Spermatogenesis vs oogenesis

SpermatogenesisStarts at puberty and continues even in old men. Happens in the seminiferous tubules. Divisions are equal; one primary spermatocyte gives four sperms. No polar bodies.

OogenesisStarts before birth, pauses, resumes at puberty, and stops around 50 years (menopause). Happens in ovarian follicles. Divisions are unequal; one primary oocyte gives one ovum and polar bodies.

The unequal divisions keep most of the food-rich cytoplasm in the one egg.

Microscopic anatomy: T.S. of ovary (spotting)

A thin outer epithelium covers the stroma, which has an outer cortex and an inner medulla.
Follicles at different stages lie in the cortex: small primary follicles, secondary follicles with more granulosa layers, and tertiary follicles with an antrum.
A large Graafian follicle shows the secondary oocyte, the zona pellucida, a big antrum and the theca layers.
After ovulation, a corpus luteum forms from the remains of the ruptured follicle.
Spotting comment: identify it as the T.S. of a mammalian ovary; name two follicle stages and one feature of each.

How to draw a sectional view of the ovary

Draw a large oval for the ovary with a thin outer layer; show blood vessels entering at one side.
Going round the edge, draw follicles in order: primary follicle (oocyte with one layer of granulosa cells), secondary follicle, and tertiary follicle with a crescent-shaped antrum.
Draw a large Graafian follicle with the secondary oocyte inside the zona pellucida, a big antrum and the theca.
Next to it, show a follicle rupturing with the ovum being released, and then a corpus luteum.
Title: Diagrammatic sectional view of the ovary.

In real life

A girl is born with all the egg-forming cells she will ever have, and their number keeps falling. This is why doctors say female fertility falls with age and ends at menopause, around 50.

Exam angle

3 marks: “Give three differences between spermatogenesis and oogenesis.” Start: puberty vs before birth. Division: equal vs unequal (polar bodies). Product per mother cell: four sperms vs one ovum. Duration: lifelong vs ends at menopause.

5 marks: “Describe oogenesis in humans. Draw a labelled sectional view of the ovary.” Oogonia in foetal ovary; primary oocytes paused in prophase I; primary → secondary → tertiary (antrum) → Graafian follicle; unequal meiosis I gives secondary oocyte and first polar body; zona pellucida; ovulation; meiosis II after sperm entry; labelled diagram.

1 marks: “At which stage is the primary oocyte paused from foetal life until puberty?” Prophase I of meiosis.

Remember it

Follicles grow P-S-T-G: Primary, Secondary, Tertiary (antrum), Graafian, like a Plant Stem Turning Green.