Why Sperm Come in Fours but Eggs Come Alone

Gametes are made by gametogenesis — spermatogenesis in the testes and oogenesis in the ovaries. Both use meiosis to halve the chromosome number, but they differ sharply in timing and in how many gametes each starting cell yields.

This part covers spermatogenesis and its hormonal control, sperm structure and semen, oogenesis and follicle development, and the menstrual cycle.

From puberty, diploid spermatogonia become primary spermatocytes, whose meiosis gives secondary spermatocytes and then spermatids; spermatids turn into sperm by spermiogenesis and are released by spermiation.

Stages:

- Spermatogonia (2n, $46$ chromosomes) — multiply by mitosis
- Primary spermatocytes (2n) — undergo meiosis I
- Secondary spermatocytes (n, $23$ chromosomes) — undergo meiosis II
- Spermatids (n) — transform into sperm by spermiogenesis
- Spermiation — sperm, whose heads were embedded in Sertoli cells, are released into the tubule

Hormonal control:

- At puberty the hypothalamus increases its secretion of GnRH
- GnRH stimulates the anterior pituitary to release LH and FSH
- LH acts on Leydig cells, stimulating androgen secretion, and androgens stimulate spermatogenesis
- FSH acts on Sertoli cells, stimulating factors that help spermiogenesis

Worked example. From $100$ primary spermatocytes, meiosis I gives $200$ secondary spermatocytes and meiosis II gives $400$ spermatids, which become $400$ sperm.

An everyday example. Puberty in boys brings a deeper voice and facial hair, as rising LH drives Leydig cells to release more testosterone.

The substance. Spermiogenesis is not a cell division — spermatids change shape into sperm with no change in chromosome number.

A human sperm has a head holding the haploid nucleus capped by an enzyme-filled acrosome, a short neck, a middle piece packed with mitochondria that supply energy, and a tail that drives it forward; semen is sperm mixed with seminal plasma from the accessory glands.

Parts of a sperm:

- Head — an elongated haploid nucleus, with a cap-like acrosome at the front filled with enzymes that help it enter the ovum
- Neck — a short link between head and middle piece
- Middle piece — many mitochondria, producing energy for movement
- Tail — lashes to propel the sperm, giving the motility essential for fertilisation

Semen:

- Sperm plus seminal plasma secreted by the seminal vesicles, prostate and bulbourethral glands
- Rich in fructose, calcium and certain enzymes
- For normal fertility, at least $60$ percent of sperm must have normal shape and size, and at least $40$ percent must show vigorous motility

An everyday example. A scooter needs an engine and wheels to move — the sperm's mitochondria-filled middle piece is its engine and the tail its drive.

The substance. The acrosome is essential for fertilisation — its enzymes digest the layers around the ovum so that the sperm can reach it.

Oogenesis begins before birth: oogonia form in the foetal ovary and enter meiosis as primary oocytes, which pause in prophase I; after puberty, follicles mature, and in the Graafian follicle the primary oocyte completes meiosis I to form a large secondary oocyte and a tiny first polar body.

Early stages:

- Oogonia — millions form in each foetal ovary, and no more form after birth
- Primary oocytes — begin meiosis I and pause in prophase I, each surrounded by granulosa cells to form a primary follicle

Follicle stages:

- Primary follicle — primary oocyte with a layer of granulosa cells
- Secondary follicle — more layers of granulosa cells and a new outer theca
- Tertiary follicle — a fluid-filled cavity, the antrum, appears, and the theca forms theca interna and theca externa
- Graafian follicle — the mature follicle, in which the primary oocyte completes meiosis I, giving a haploid secondary oocyte and a small first polar body

Ovulation. The Graafian follicle ruptures and releases the secondary oocyte, surrounded by a new membrane, the zona pellucida.

An everyday example. Every oocyte a woman will ovulate is already present at birth, paused partway through meiosis.

The substance. The cell released at ovulation is a secondary oocyte — meiosis II finishes only when a sperm enters it.

The menstrual cycle, averaging 28 to 29 days, has a menstrual phase when the endometrium breaks down, a follicular phase when FSH and LH grow a follicle and oestrogen rebuilds the lining, an ovulatory phase triggered by an LH surge around day 14, and a luteal phase when the corpus luteum secretes progesterone.

The phases:

- Menstrual phase — lasts about $3$ to $5$ days; the endometrium and its blood vessels break down and flow out, which happens only if the ovum is not fertilised
- Follicular (proliferative) phase — primary follicles grow into a Graafian follicle under FSH and LH; rising oestrogen regenerates the endometrium
- Ovulatory phase — LH and FSH peak in mid-cycle, and the LH surge ruptures the Graafian follicle to release the ovum
- Luteal (secretory) phase — the empty follicle becomes the corpus luteum, which secretes large amounts of progesterone to keep the endometrium ready for implantation

Without fertilisation, the corpus luteum degenerates, progesterone falls and a new cycle begins.

Menarche and menopause. The first menstruation at puberty is menarche; the permanent end of cycles, usually around the age of $50$, is menopause.

An everyday example. A missed period is often the first sign of pregnancy, because the corpus luteum keeps secreting progesterone.

The substance. Ovulation is triggered by LH, not FSH — the LH surge is what ruptures the Graafian follicle.