Sexual Reproduction, Development and Growth in Humans and Animals

15.1Reproductive system of humans

15.2Gametogenesis in humans

Spermatogenesis (seminiferous tubules of the testes)

Primordial germ cell → (mitosis) spermatogonium 2n → primary spermatocyte 2n (growth) → (meiosis I) two secondary spermatocytes n → (meiosis II) four spermatids n → (differentiation) four sperms. FSH and testosterone stimulate it; LH stimulates testosterone secretion. Sertoli cells nourish the developing cells.

Oogenesis (ovaries)

• Before birth: oogonium (2n) forms a primary oocyte, which begins meiosis I and stops at prophase I.
• From puberty, each month: one primary oocyte completes meiosis I, giving a secondary oocyte and a first polar body. The secondary oocyte stops at metaphase II.
• Ovulation: the Graafian follicle releases the secondary oocyte. Meiosis II is completed only if a sperm penetrates it, giving the ovum and a polar body.
• The empty follicle becomes the corpus luteum.

Structure of the gametes

• Sperm: head (nucleus), midpiece (many mitochondria for energy) and tail (swimming).
• Graafian follicle: a large secondary oocyte surrounded by a gel-like layer and follicular cells.

15.3Menstrual cycle

Textbook Figure 15.9, p. 294 (hormone graph).

Pregnancy, miscarriage and syndromes

• If fertilisation occurs, the corpus luteum keeps secreting progesterone and oestrogen for three to four months, then the placenta takes over until birth. Progesterone prevents FSH and LH release, so there is no ovulation or menstruation in pregnancy.
• A fall in progesterone can cause miscarriage.
• Premenstrual syndrome: symptoms 7 to 14 days before menstruation, caused by oestrogen and progesterone imbalance.
• Menopause: about 46 to 50 years, ovulation and menstruation stop because oestrogen and progesterone fall.

15.4Development of a human foetus

Textbook Figure 15.10, p. 297.

1. One sperm penetrates the secondary oocyte in the Fallopian tube; the membrane changes so no other sperm enters. The nuclei fuse to form a diploid zygote.
2. The zygote divides by mitosis: two-cell embryo, morula, then blastocyst.
3. The blastocyst implants in the endometrium.

Placenta and umbilical cord

• Umbilical vein: oxygenated blood with nutrients, placenta to foetus.
• Umbilical arteries: deoxygenated blood with carbon dioxide and urea, foetus to placenta.
• Placenta: exchange of glucose, amino acids, antibodies, oxygen in and carbon dioxide, urea out; also secretes progesterone and oestrogen from month 4.

Why foetal and maternal blood do not mix (a thin membrane separates them): it protects the foetus from toxins and bacteria; prevents foetal vessels bursting under the mother's higher blood pressure; prevents agglutination if blood groups differ. Drugs, alcohol, nicotine and viruses such as HIV and rubella can still cross.

15.5Formation of twins

Conjoined twins arise when the embryo of identical twins divides incompletely, so parts such as the chest or abdomen stay fused. Separation by surgery depends on the organs they share.

15.6Impotency

Impotency means a couple cannot conceive. Male causes: erectile dysfunction, testes not producing sperm, low or abnormal sperm, hormonal imbalance. Female causes: ovaries not producing secondary oocytes, blocked Fallopian tubes, abnormal uterus, hormonal imbalance. Treatments: hormone therapy; surgery for blocked tubes or ducts; in vitro fertilisation (IVF) for blocked Fallopian tubes.

15.7Growth in humans and animals

• Parameters: height or length; fresh mass (organism survives, less accurate because of water); dry mass (dried at 100 °C until constant, organism is killed, more accurate).
• Complete metamorphosis (butterfly): egg, larva, pupa, adult.
• Incomplete metamorphosis (grasshopper): egg, nymph (several ecdyses), adult. No pupa.

Intermittent growth in insects: the hard exoskeleton cannot expand, so the insect moults (ecdysis, controlled by hormones). The graph is a staircase: flat parts are instars (no length increase), steep parts are ecdysis with rapid growth.