Human Reproduction

The whole story in 7 steps

1. Gametogenesis: making sperms and eggs.
2. Insemination: sperms are transferred into the female.
3. Fertilisation: sperm + egg → zygote.
4. Cleavage and implantation: the embryo attaches to the uterus wall.
5. Gestation (pregnancy): the baby grows for about 9 months.
6. Parturition: birth of the baby.
7. Lactation: the mother feeds milk.

Male reproductive system

• Testes (2) are the sperm factories. They sit outside the body in a skin pouch called the scrotum.
• The scrotum keeps the testes 2–2.5 °C cooler than the body, because sperms need a cooler temperature to form.
• Each testis has about 250 compartments called testicular lobules. Each lobule has 1–3 seminiferous tubules, where the sperms are made.

The sperm's journey out

Seminiferous tubules → rete testis → vasa efferentia → epididymis (sperms stored and mature) → vas deferens → ejaculatory duct → urethra → outside.

• Accessory glands: 2 seminal vesicles, 1 prostate and 2 bulbourethral glands.
• Their fluid is seminal plasma. It is rich in fructose (energy for sperms), calcium and enzymes.
• Semen = sperms + seminal plasma.

Female reproductive system

• Ovaries (2) make the eggs and the hormones estrogen and progesterone.
• Oviduct (fallopian tube) has 3 parts: infundibulum (funnel with finger-like fimbriae that catch the egg) → ampulla (wide part) → isthmus (narrow part).
• Fertilisation happens at the ampullary–isthmic junction.
• Uterus (womb): where the baby grows. Its wall has 3 layers:

• Mammary glands (breasts) make milk. Each has 15–20 lobes full of milk-making cells (alveoli).

Making gametes (gametogenesis)

Spermatogenesis: making sperms

Spermatogonium (2n) → grows → primary spermatocyte (2n) → meiosis I → 2 secondary spermatocytes (n) → meiosis II → 4 spermatids (n) → spermiogenesis → 4 sperms (n).

• Spermiogenesis means spermatids changing into sperms (they get a tail).
• Spermiation means the release of sperms from the seminiferous tubules.

Hormones control it all

• The hypothalamus releases GnRH, which acts on the anterior pituitary.
• The anterior pituitary releases:
  • LH → acts on Leydig cells → they make testosterone.
  • FSH → acts on Sertoli cells → they help spermiogenesis.

Parts of a sperm

• One ejaculation releases 200–300 million sperms. For normal fertility, at least 60% must have normal shape and at least 40% must swim strongly.

Oogenesis: making eggs

• It starts before birth. Millions of egg mother cells (oogonia) form in each ovary of the unborn baby girl, and no new ones are made after birth.
• Many follicles die between birth and puberty. At puberty, only 60,000–80,000 primary follicles are left in each ovary.
• Follicles grow: primary → secondary → tertiary (with a fluid space called the antrum) → Graafian follicle (mature).
• The secondary oocyte is released at ovulation. It is covered by the zona pellucida layer.

The menstrual cycle (about 28 days)

• If no fertilisation: the corpus luteum dies → progesterone falls → the endometrium breaks down → the next period starts.
• Menarche is the first period (at puberty). Menopause is when periods stop, around 50 years.

Fertilisation and implantation

1. The sperm meets the egg at the ampullary–isthmic junction.
2. The sperm enters; changes in the zona pellucida block other sperms.
3. The egg (secondary oocyte) finishes meiosis II.
4. The zygote (2n) forms.
5. Cleavage (mitosis) → morula (8–16 cells) → blastocyst.
6. Implantation: the blastocyst embeds in the endometrium.

• Blastocyst has 2 parts:
  • Trophoblast (outer layer): sticks to the uterus and helps form the placenta.
  • Inner cell mass: becomes the baby. It has stem cells that can form any tissue.

Boy or girl?

• The mother's eggs all carry X.
• The father's sperms are 50% X and 50% Y.
• X egg + X sperm = XX → girl. X egg + Y sperm = XY → boy.

Pregnancy: the baby grows

• Placenta is the link between mother and baby. It gives food and oxygen and removes waste. It is joined to the baby by the umbilical cord.
• Placenta also makes hormones: hCG, hPL, estrogens, progestogens and relaxin.

Three germ layers

Month-by-month milestones

Birth and breastfeeding

• Parturition (birth) is started by signals from the fully grown baby and the placenta. This is called the foetal ejection reflex.
• It causes the release of oxytocin, which makes the uterus muscles squeeze. Squeezing causes more oxytocin, and so on, until the baby is born.
• Colostrum is the first milk after birth. It is full of IgA antibodies, which protect the newborn.

30-second revision

• Testes are in the scrotum, 2–2.5 °C cooler than the body.
• Sertoli = nurse cells; Leydig = testosterone.
• Fertilisation happens at the ampullary–isthmic junction.
• LH surge → ovulation (day 14); corpus luteum → progesterone.
• Zygote → morula → blastocyst → implantation.
• The father decides the baby's sex (X or Y sperm).
• Oxytocin → birth; colostrum → IgA antibodies.