The reproductive events
Humans reproduce sexually and are viviparous (they give birth to young). The events are: gametogenesis (forming gametes), insemination, fertilisation, cleavage and implantation, gestation (embryonic development), and parturition (birth).
The male reproductive system
Part | Role |
|---|---|
Testes | in the scrotum, which keeps them about 2–2.5 °C below body temperature, needed for sperm production. Each testis has about 250 lobules containing seminiferous tubules |
Seminiferous tubules | lined by spermatogonia (which form sperm) and Sertoli cells (which nourish developing sperm) |
Leydig (interstitial) cells | between the tubules; secrete testosterone (androgens) |
Ducts | rete testis → vasa efferentia → epididymis (stores sperm) → vas deferens → ejaculatory duct → urethra |
Accessory glands | seminal vesicles, prostate and bulbourethral glands; their secretions form seminal plasma, rich in fructose, calcium and enzymes |
The female reproductive system
Part | Role |
|---|---|
Ovaries | produce ova and the hormones oestrogen and progesterone |
Oviducts (fallopian tubes) | the funnel-shaped infundibulum with finger-like fimbriae collects the ovum; the ampulla is where fertilisation usually occurs; the isthmus joins the uterus |
Uterus | pear-shaped; its wall has three layers: perimetrium, muscular myometrium (contracts strongly at birth) and glandular endometrium (changes during the menstrual cycle) |
Cervix and vagina | the birth canal |
Mammary glands | produce milk; each has 15–20 mammary lobes of alveoli |
Gametogenesis
Spermatogenesis begins at puberty in the seminiferous tubules:
Spermatogonia (2n, 46 chromosomes) multiply by mitosis; some grow into primary spermatocytes (2n).
Each primary spermatocyte undergoes meiosis I, giving two secondary spermatocytes (n, 23 chromosomes).
Meiosis II gives four haploid spermatids.
Spermatids change into spermatozoa (sperm) by spermiogenesis. Sperm are released from the tubules by spermiation.
It is controlled by GnRH from the hypothalamus, which stimulates the pituitary to release LH (acting on Leydig cells to make androgens) and FSH (acting on Sertoli cells).
A sperm has a head (haploid nucleus capped by the acrosome, with enzymes for fertilisation), a middle piece packed with mitochondria (energy for swimming), and a tail. About 200–300 million sperm are released in one ejaculation; for normal fertility at least 60% should have normal shape and 40% vigorous motility.
Oogenesis begins before birth:
Oogonia form in the fetal ovary; no more are formed after birth. They start meiosis and stop at prophase I as primary oocytes, each in a primary follicle. Many degenerate; at puberty only 60,000–80,000 remain in each ovary.
Follicles mature into secondary, tertiary and finally Graafian follicles.
The primary oocyte completes meiosis I unequally, giving a large secondary oocyte and a tiny first polar body.
The secondary oocyte is released at ovulation. It completes meiosis II only if a sperm enters, forming the ovum and a second polar body.
So one primary spermatocyte gives four sperm, but one primary oocyte gives just one ovum.
The menstrual cycle
The reproductive cycle of human females lasts about 28–29 days. The first menstruation, at puberty, is menarche; cycles stop around age 50 at menopause.
Phase | Days (approx.) | Events | Hormones |
|---|---|---|---|
Menstrual | 1–5 | the endometrium breaks down and is shed with blood, because no fertilisation occurred | progesterone falls |
Follicular (proliferative) | 6–13 | a follicle matures; the endometrium regrows | FSH rises; the follicle secretes oestrogen |
Ovulation | about day 14 | the Graafian follicle releases the ovum | an LH surge (with FSH) triggers it |
Luteal (secretory) | 15–28 | the empty follicle becomes the corpus luteum, which secretes progesterone, keeping the endometrium ready for implantation | progesterone high |
If there is no fertilisation, the corpus luteum degenerates, progesterone falls and menstruation begins again. If pregnancy occurs, the corpus luteum persists and menstrual cycles stop.
Fertilisation and implantation
Sperm deposited in the vagina swim to the ampulla of the oviduct. Only one sperm fertilises the ovum: its entry changes the zona pellucida layer, blocking other sperm. The ovum then completes meiosis II, and the haploid nuclei fuse to form a diploid zygote.
Sex of the baby: the ovum always carries an X chromosome; the sperm carries X or Y. XX gives a girl and XY a boy, so the father's sperm decides the sex.
The zygote divides by cleavage as it moves to the uterus, forming a morula (8–16 cells) and then a blastocyst. The blastocyst's outer layer, the trophoblast, attaches to the endometrium, and its inner cell mass forms the embryo. Embedding in the endometrium is implantation, which leads to pregnancy.
Pregnancy and development
After implantation, the placenta forms from fingers of the trophoblast (chorionic villi) interlocking with uterine tissue. Connected to the embryo by the umbilical cord, it:
supplies oxygen and nutrients and removes CO₂ and wastes,
acts as an endocrine gland, producing hCG (human chorionic gonadotropin, detected by pregnancy tests), hPL, oestrogens and progestogens.
The inner cell mass forms the ectoderm, mesoderm and endoderm, from which all tissues develop. The inner cell mass contains stem cells that can give rise to all tissues and organs.
Milestones: the heart forms after one month; limbs and digits by the end of the second month; most major organs by the end of the first trimester (12 weeks); the first fetal movements and hair on the head around the fifth month; a fine body hair and eyelids by the end of the second trimester (24 weeks). Human pregnancy lasts about 9 months.
Parturition and lactation
At the end of pregnancy, signals from the fully developed fetus and placenta start mild uterine contractions (the foetal ejection reflex). These cause the release of oxytocin from the pituitary, which makes the uterine muscles contract more strongly, and the cycle continues until the baby is born. The placenta is expelled afterwards.
The mammary glands produce milk after birth (lactation). The first milk, colostrum, is rich in antibodies (IgA) and is vital for protecting the newborn.
Summary
Testes make sperm (Sertoli cells nourish them; Leydig cells make testosterone); ovaries make ova and hormones.
Spermatogenesis gives four sperm per primary spermatocyte; oogenesis gives one ovum per primary oocyte.
Menstrual cycle: menstrual, follicular, ovulation (LH surge, day 14), luteal (progesterone from the corpus luteum).
Fertilisation occurs in the ampulla; the blastocyst implants; the placenta nourishes the fetus and makes hCG.
Oxytocin drives birth; colostrum gives the newborn antibodies.