Gametogenesis produces haploid gametes in gonads; fusion at fertilisation forms a diploid zygote. (p.288)
Spermatogenesis occurs in seminiferous tubules, stimulated by FSH, testosterone and LH. (p.288)
Spermatogonium (2n) → primary spermatocyte → meiosis I → two secondary spermatocytes → meiosis II → four spermatids → sperms. (p.289)
Oogenesis begins before birth; primary oocytes stop at prophase I. (p.290)
At puberty the primary oocyte completes meiosis I to give a secondary oocyte and first polar body; meiosis II stops at metaphase II. (p.291)
Meiosis II finishes only when a sperm penetrates the secondary oocyte. (p.291)
Secondary follicle develops into the Graafian follicle, which releases oestrogen; ovulation releases the secondary oocyte into the Fallopian tube. (p.291)
After ovulation the remaining follicle forms the corpus luteum, which secretes oestrogen and progesterone. (p.291)
Sperm has head, midpiece (mitochondria) and tail. (p.292)
Sperm head contains the nucleus; the midpiece is packed with mitochondria for energy to swim. (p.292)
Graafian follicle: secondary oocyte surrounded by follicular cells and a gel-like substance. (p.292)
Spermatids differentiate into sperm; secondary oocytes do not differentiate. (p.292)
Spermatogenesis gives four sperms and is continuous from puberty; oogenesis gives one oocyte and three polar bodies per cycle. (p.292)
15.3 Menstrual Cycle
FSH stimulates follicle growth; LH stimulates ovulation and corpus luteum formation. (p.293)
Oestrogen repairs and thickens the endometrium; progesterone thickens it further and inhibits FSH and LH. (p.293)
Days 6–14: oestrogen peaks day 12, LH peaks day 13, ovulation day 14. (p.295)
Days 15–21: corpus luteum secretes oestrogen and progesterone; endometrium thickens. (p.295)
Days 22–28: without fertilisation the corpus luteum degenerates and menstruation begins. (p.295)
The corpus luteum works for 3–4 months of pregnancy; then the placenta takes over. (p.296)
Progesterone drop can cause miscarriage; menopause occurs at 46–50 years. (p.296)
Progesterone inhibits FSH and LH, so no menstrual cycle or ovulation occurs during pregnancy. (p.296)
Premenstrual syndrome appears 7–14 days before menstruation due to oestrogen and progesterone imbalance. (p.296)
Menopause: ovulation and menstruation stop naturally because progesterone and oestrogen secretion fall. (p.296)
15.4 Development of a Human Foetus
One sperm penetrates the secondary oocyte in the Fallopian tube; nuclei fuse to form a diploid zygote. (p.297)
Zygote divides by mitosis: two-cell, morula, blastocyst, implantation in the endometrium. (p.297)
Each ejaculation has 300–500 million sperms, but only about 300–500 reach the secondary oocyte. (p.297)
After one sperm enters, the oocyte membrane changes to block other sperms. (p.297)
HCG keeps the corpus luteum secreting oestrogen and progesterone in early pregnancy. (p.298)
Umbilical vein carries oxygen and nutrients to the foetus; umbilical arteries carry CO2 and urea to the placenta. (p.298)
Placenta exchanges substances and secretes hormones; maternal and foetal bloods are separated by a thin membrane. (p.299)
Placenta absorbs glucose, amino acids, hormones, antibodies and oxygen into foetal blood, and removes CO2 and urea. (p.299)
Placenta replaces the corpus luteum from month four to make progesterone and oestrogen. (p.299)
Blood separation protects the foetus from toxins and bacteria, prevents foetal vessels bursting under maternal pressure and avoids agglutination from blood-group mismatch. (p.299)
Drugs, nicotine, alcohol, HIV and rubella virus can still cross the placenta. (p.299)
15.5 Formation of Twins
Identical twins: one zygote from one sperm and one ovum divides into two embryos; same genetic makeup and sex; one placenta shared. (p.301)
Fraternal twins: two ova fertilised by two sperms; separate placentas. (p.301)
Conjoined twins: incomplete division of the embryo in identical twins. (p.301)
Fraternal twins have different genetic makeup and may be of different sexes. (p.301)
Conjoined twins may share organs; separation surgery depends on the joined part. (p.301)
15.6 Health Issues Related to the Human Reproductive System
Impotency is inability of a couple to conceive; treatments include hormone therapy, surgery and IVF. (p.302)
Male causes: no sperm produced, erectile dysfunction, hormonal imbalance, low or abnormal sperm. (p.302)
Female causes: no secondary oocyte produced, abnormal uterus or growth, blocked Fallopian tubes, hormonal imbalance. (p.302)
15.7 Growth in Humans and Animals
Growth is an irreversible increase in cell number, size, volume and weight. (p.303)
Growth is measured by size, fresh weight or dry weight. (p.303)
Dry weight (dried at 100°C to constant mass) is accurate but kills the organism; fresh weight is less accurate because of fluid content. (p.303)