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Reproduction in Animals

14 min read

Gametogenesis in humans

Gametogenesis is the making of gametes by meiosis. In the testes, spermatogenesis makes four small sperm, each with a head (nucleus and acrosome), a middle piece full of mitochondria, and a tail for swimming, from each spermatogonium. In the ovaries, oogenesis makes one large egg from each primary oocyte, with the other three cells forming small polar bodies that disappear. The egg is large and full of food.

Meiosis: halving the chromosome numberA diploid cell with two pairs of chromosomes copies its DNA, then divides twice. In the first division the paired chromosomes separate, giving two cells each with half the chromosome number. In the second division the two halves of each chromosome separate, giving four genetically different haploid cells, the gametes.diploid cell (2n)copy DNAchromosomes pair updivision 1two cells (n)division 2four gametes (n)Mitosis keeps the chromosome number; meiosis halves it so that fertilisation restores it.
FeatureSpermatogenesisOogenesis
WhereTestesOvaries
Gametes per cell41 (plus polar bodies)
StartAt puberty, continuousBefore birth; one finishes each month
Size of gameteSmall, mobileLarge, not mobile

Fertilisation

A sperm penetrates the egg in the oviduct: the acrosome releases enzymes that digest the egg membrane, the nuclei fuse, and the egg’s membrane changes to block other sperm. The zygote then divides to form a ball of cells, the embryo, which implants in the uterus lining.

The female reproductive systemTwo ovaries make eggs and the hormones oestrogen and progesterone. Each month an egg is released into the oviduct, where it may be fertilised by a sperm. The fertilised egg travels to the uterus, whose wall has thickened, and implants. The cervix at the lower end of the uterus opens into the vagina.ovary (makes eggs)oviduct (egg tube)uterus (womb)cervixvaginaovary

Hormonal control of the menstrual cycle

The pituitary gland releases FSH, which makes a follicle in the ovary grow. The follicle makes oestrogen, which rebuilds the lining of the uterus and stimulates a surge of LH. LH causes ovulation on about day 14 and turns the empty follicle into the corpus luteum, which makes progesterone. Progesterone keeps the uterus lining thick and stops more eggs developing. If there is no pregnancy, the corpus luteum dies, progesterone falls, and the lining is lost in menstruation. If the egg is fertilised, the embryo makes hCG, which keeps the corpus luteum going.

The menstrual cycleA 28-day cycle. Menstruation lasts about the first five days. FSH from the pituitary gland makes a follicle in the ovary grow and release oestrogen, which rebuilds the uterus lining. A surge of LH on about day 14 causes ovulation. The empty follicle makes progesterone, which keeps the lining thick; if there is no pregnancy the hormones fall and the lining is shed.day of the cycle (28 days)hormone leveloestrogenprogesteroneLH surgeovulation (day 14)periodlining rebuildsthick lining (ready for an embryo)
Oestrogen rises before ovulation, and a smaller rise follows when the corpus luteum is active.
Try itDays to ovulation

The egg is released 14 days before the next period. Choose the length of the cycle.

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Key terms

Spermatogenesis
The making of sperm.
Oogenesis
The making of eggs.
Corpus luteum
The structure left in the ovary after ovulation.
Acrosome
The part of the sperm head with enzymes.

Summary

  • Spermatogenesis makes four sperm; oogenesis makes one egg.
  • Fertilisation takes place in the oviduct.
  • FSH, oestrogen, LH and progesterone control the cycle.
  • A fall in progesterone causes menstruation if there is no pregnancy.
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