Human Reproduction and Development
Human Reproduction
Humans reproduce sexually. Women produce female gametes (ova), and men produce male gametes (sperm). Fertilization occurs when these gametes meet to form a zygote. These gametes are produced in the gonads: ovaries (female) and testicles (male). Fertilization is internal, meaning the egg and sperm unite within the female’s reproductive tract. Humans are viviparous, as the development of the new being occurs inside the mother.
Reproduction includes the following processes:
- Production of gametes in specialized reproductive organs.
- Fertilization – Union of the nuclei of the gametes. It can be external (e.g., fish) or internal, requiring a copulatory organ. This forms the zygote, which contains the genetic information.
- Zygote development: A process that occurs from the zygote until the baby is born. It has two parts:
- Embryonic (inside – viviparous; or outside – oviparous)
- Post-embryonic (childhood, puberty)
- Birth or delivery: When the individual is fully formed.
Male Reproductive System
- Testicles: Glands located in the scrotum where sperm is produced.
- Prostate: A walnut-sized gland enveloping the urethra. It produces prostatic fluid that protects sperm from the acidity of the urethra and vagina.
- Urethra: The tube that connects to the exterior. Its walls contract during ejaculation, boosting sperm.
- Epididymis: A long, very thin tube coiling over each testicle where sperm matures.
- Vas deferens: A continuation of the epididymis that flows into the urethra.
- Seminal vesicle: Produces fluid in the seminal duct. It flows into the vas deferens.
- Penis: A cylindrical organ whose end, the glans, is enlarged and covered by the foreskin.
- Scrotum: A pocket formed by skin surrounding the testicles. It has sebaceous glands.
Male Gametes
Spermatozoa (sperm) are produced continuously in the seminiferous tubules. They move to the epididymis, where they complete their maturation and develop a flagellum for swimming. Sperm are much smaller and more numerous than eggs; hundreds of millions mature daily. Their output is performed by ejaculation, in which semen is expelled. Semen is a fluid formed by sperm and lubricating seminal and prostatic fluid. Sperm remain alive for 3 to 5 days in the vagina, during which time fertilization can occur.
Fertilization and Early Pregnancy
Ovulation occurs 14 days before menstruation. The egg released from the Graafian follicle is driven along the fallopian tube to the uterus. This takes more than a week. An egg can only be fertilized during the 24 hours after ovulation. Intercourse is the introduction of the penis into the vagina. During ejaculation, sperm are released into the vagina. The genetic material of the sperm and egg fuse, and fertilization occurs. The resulting cell is the zygote or egg cell.
The beginning of pregnancy: When a zygote forms, it produces a substance that prevents the Graafian follicle from degenerating, due to progesterone. Nidation occurs when the zygote sticks to the endometrium. From that moment, it is called an embryo and develops:
- Placenta: An organ that develops in the endometrium and serves as a connection between mother and embryo.
- Umbilical cord: Connects the embryo with the placenta. It consists of a tube through which two arteries pass, carrying nutrients to the embryo, and a vein that carries waste from the embryo to the mother.
- Amniotic sac: Contains the amniotic fluid where the embryo floats.