8.4 Human Reproduction

Asexual reproduction uses one parent and ordinarily produces descendants genetically similar to that parent except for mutation. It is rapid and preserves a successful genotype. Sexual reproduction combines haploid gametes produced through meiosis, creating new allele combinations by independent assortment, crossing over and random fertilization. Its costs include finding a mate and transmitting only part of an individual's genome, but the resulting variation supplies material for selection in changing environments.

The male reproductive system includes testes that produce sperm and testosterone, the epididymis for sperm maturation and storage, the vas deferens, accessory glands and the urethra. A sperm has a haploid nucleus, an acrosome containing enzymes, a midpiece rich in mitochondria and a flagellum. These structures support production, transport and delivery of the male gamete.

The female reproductive system includes ovaries that produce oocytes and hormones, oviducts that receive an ovulated oocyte and are the usual site of fertilization, and a uterus whose endometrium supports implantation. The cervix opens from the uterus into the vagina. Structure and timing together bring gametes into contact and prepare a site for development.

At the beginning of a cycle, low estrogen and progesterone reduce inhibition of hypothalamic GnRH and pituitary FSH and LH. FSH supports follicle development; the growing follicle secretes estrogen, which repairs and thickens the endometrium. For much of this phase, estrogen exerts negative feedback that limits FSH and helps one dominant follicle outcompete the others.

Sustained high estrogen near mid-cycle changes from negative to positive feedback, producing a sharp LH surge. LH triggers ovulation and transforms the emptied follicle into the corpus luteum. The corpus luteum secretes progesterone and estrogen, maintaining a vascular secretory endometrium and inhibiting GnRH, FSH and LH. If implantation does not occur, the corpus luteum degenerates, steroid concentrations fall and the endometrium is shed.

The ovarian and uterine cycles are related but distinct: ovarian events produce the steroid changes that organize endometrial events. Hormone graphs must be read as continuous feedback, not as four independent curves. A peak is meaningful because of what preceded it and which receptor-bearing tissue responds. Ovulation occurs after the LH surge begins, not whenever any one hormone reaches an arbitrary high value.

Fertilization usually occurs in an oviduct. Sperm bind molecules around the oocyte, and enzymes released from the acrosome help one sperm reach and fuse with the oocyte membrane. The haploid nuclei from the two gametes then unite, restoring the diploid chromosome number and producing a zygote.

In vitro fertilization commonly uses hormone treatment to develop several follicles, timed final maturation, oocyte retrieval, laboratory fertilization and embryo culture, followed by transfer to the uterus. Intracytoplasmic sperm injection places one sperm into an oocyte when fertilization is unlikely by conventional mixing. Success is constrained by age, embryo quality, implantation and maternal health; transferring many embryos can raise multiple-pregnancy risk.

Infertility treatment creates ethical decisions about access, cost, unused embryos, donor identity, genetic testing and the welfare of patients and future children. A biological account should separate evidence about safety and success from value judgments. Hormones used in treatment do not invent a new reproductive process; they alter timing and number within pathways already regulated by FSH, LH, estrogen, progesterone and hCG.