Human Reproduction
Male Reproductive System
- The scrotum keeps the testes about 2–2.5 °C cooler than the abdominal cavity, a temperature difference important for sperm formation. Each testis has about 250 lobules. Seminiferous tubules contain sperm-producing germ cells and Sertoli cells, which support them. Interstitial Leydig cells secrete androgens.
- Sperm leave the seminiferous tubules through the rete testis, vasa efferentia, epididymis and vas deferens. The ejaculatory duct opens into the urethra. The urethra passes through the penis and opens at the urethral meatus.
- The glans penis is covered by a fold of skin called the foreskin. Seminal vesicles, prostate and bulbourethral glands contribute to seminal fluid. It contains fructose, calcium and enzymes. Bulbourethral secretions help lubricate the passage.
Female Reproductive System
- Ovaries produce oocytes and steroid hormones. An outer epithelium covers ovarian stroma, with peripheral cortex and inner medulla.
- The oviduct’s infundibulum has fimbriae near the ovary, followed by ampulla and the narrow isthmus, which joins the uterus.
- The pear-shaped uterus opens through the cervix into the vagina. The cervical canal and vagina form the birth canal. The perimetrium is the outer layer, myometrium the smooth muscle layer, and endometrium the inner lining.
- External genitalia include the mons pubis, labia majora, labia minora and clitoris. The hymen can partially cover the vaginal opening.
Mammary Glands
Each gland contains fat and about 15–20 mammary lobes. The lobes contain milk-secreting alveoli. Milk moves into mammary tubules and ducts, then through ampullae and lactiferous ducts to the nipple.
Gametogenesis
Spermatogenesis
- At puberty hypothalamic GnRH stimulates release of pituitary FSH and LH. LH acts on Leydig cells to produce testosterone, and FSH acts on Sertoli cells.
- Diploid spermatogonia multiply by mitosis. Some accumulate nutrients and enlarge to become primary spermatocytes.
- Meiosis I gives two haploid secondary spermatocytes. Meiosis II produces four haploid spermatids, which differentiate into spermatozoa by spermiogenesis.
Oogenesis
- Oogonia multiply during fetal development. Primary oocytes begin meiosis and arrest in prophase I before birth.
- As a follicle matures after puberty, the primary oocyte completes meiosis I. Unequal division gives a large secondary oocyte and a small first polar body.
- The secondary oocyte is ovulated while arrested in meiosis II. Sperm entry normally triggers completion of meiosis II and formation of the second polar body.
Structure of a Sperm
- The head contains the haploid nucleus and an acrosome with enzymes that aid penetration of egg coverings.
- The neck contains proximal and distal centrioles. A spiral of mitochondria in the middle piece supplies energy for movement.
- The tail contains the flagellar axial structure and propels the sperm.
Menstrual Cycle
| Phase | Main events in a typical 28-day cycle |
|---|---|
| Menstrual | Falling progesterone and oestrogen after corpus luteum regression cause the endometrium to shed. Bleeding generally lasts about 3–5 days. |
| Follicular or proliferative | FSH supports follicle growth. Rising oestrogen promotes rebuilding of the endometrium. |
| Ovulation | A mid-cycle LH surge ruptures the mature Graafian follicle and releases the secondary oocyte. |
| Luteal or secretory | The corpus luteum secretes progesterone. The endometrium thickens and its glands become secretory. Without pregnancy, the corpus luteum regresses and a new cycle begins. |
Luteal Phase
In a typical 28-day cycle the luteal phase is approximately days 15–28. Progesterone from the corpus luteum supports a thick, secretory endometrium. If implantation does not occur, the corpus luteum regresses and progesterone falls.
Hormonal Control in Male Reproduction
Testosterone supports spermatogenesis and the reproductive tract. It feeds back on hypothalamus and pituitary. Sertoli-cell inhibin helps suppress FSH secretion.
Fertilisation and Implantation
- Sperm travel from the vagina through the cervix and uterus to the oviduct. Fertilisation usually occurs near the ampullary–isthmic junction.
- Acrosomal enzymes help sperm penetrate the egg coverings. After sperm entry the zona pellucida changes and helps block additional sperm. Meiosis II completes and the haploid parental nuclei unite to form a diploid zygote.
- Cleavage produces blastomeres without a large increase in overall size. An 8–16-cell morula develops into a blastocyst with an outer trophoblast and an inner cell mass.
- The trophoblast attaches to and embeds in the endometrium. The inner cell mass gives rise to the embryo.
Placenta and Embryonic Development
Chorionic villi and maternal uterine tissue form the placenta. The umbilical cord connects the fetus to it. The placenta transfers nutrients and oxygen and removes wastes. It secretes hCG, human placental lactogen, oestrogens and progesterone. Relaxin also rises in pregnancy.
The inner cell mass gives rise to ectoderm, mesoderm and endoderm and includes cells able to generate many cell types. The fetal heart forms early, limbs and digits form by about the second month, and major organ systems and external genitalia develop during the first trimester. Movements are commonly first felt around the fifth month. By about 24 weeks, fine hair, eyelids and eyelashes are present.
Parturition and Lactation
Pregnancy lasts about nine months. Signals from the fetus and placenta trigger mild uterine contractions, the fetal ejection reflex. Oxytocin from the maternal pituitary strengthens contractions in a positive-feedback loop, leading to delivery and expulsion of the placenta.
Lactation
The mammary glands differentiate during pregnancy and produce milk after birth. Colostrum, the first milk, is rich in antibodies that help protect the newborn.