Human Reproduction: Gametes, Fertilization, and Development
Human Reproduction: From Gametes to Birth
In sexual reproduction, women produce gametes called ova (eggs), and men produce gametes called sperm. These gametes join during fertilization to form a zygote. This process occurs within the gonads: ovaries in females and testicles in males. Fertilization is internal, meaning the egg and sperm unite within the female reproductive tract. Humans are viviparous, meaning the development of the new being occurs inside the mother.
Key Processes in Reproduction
Reproduction involves the following key processes:
- Production of gametes in specialized reproductive organs.
- Union of gametes.
- Fertilization of the gametes. Fertilization can be external (e.g., in fish) or internal, requiring a copulatory organ.
- The zygote receives genetic information.
- Development of the zygote: This process occurs from the zygote stage until birth and includes two phases: embryonic (inside or outside the mother, depending on whether the organism is oviparous or viviparous) and post-embryonic (childhood, puberty, etc.). Mitosis is the process by which the number of cells increases.
- Birth or delivery: Occurs when the individual is fully formed.
Male Reproductive System
- Testicles: Glands located in the scrotum that produce sperm.
- Prostate: A gland about the size of a walnut that surrounds the urethra. It produces prostatic fluid, which protects sperm from the acidity of the urethra and vagina.
- Urethra: The tube that connects to the exterior. Its walls contract during ejaculation to propel sperm.
- Epididymis: A long, coiled tube located on each testicle where sperm mature.
- Vas Deferens: A continuation of the epididymis that connects to the urethra.
- Seminal Vesicles: Produce seminal fluid, which empties into the vas deferens.
- Penis: A cylindrical organ, the end of which is the glans, which is enlarged and covered by the prepuce (foreskin).
- Scrotum: A pouch of skin surrounding the testicles, containing sebaceous glands.
Male Gamete: Sperm
Sperm are produced continuously in the seminiferous tubules. They move to the epididymis, where they complete their maturation and develop a flagellum, which allows them to swim rapidly. Sperm are much smaller and more numerous than ova; millions are produced each day. Sperm are released during ejaculation along with semen, a lubricating fluid composed of sperm and prostatic fluid. Sperm can survive for 3 to 5 days in the vagina, during which time fertilization can occur.
Fertilization and the Beginning of Pregnancy
Ovulation occurs approximately 14 days before menstruation. The egg, released from the Graafian follicle, travels through the fallopian tubes to the uterus over about a week. An egg can be fertilized for approximately 24 hours after ovulation. During intercourse, sperm are released into the vagina. The genetic material of the sperm and egg fuse, resulting in fertilization. The resulting cell is called a zygote or fertilized egg.
Beginning of Pregnancy
The formation of the zygote triggers the production of a substance that prevents the Graafian follicle from degenerating, thus maintaining progesterone production. Implantation occurs when the zygote attaches to the endometrium. From that point forward, it is called an embryo. The placenta, an organ that develops in the endometrium, serves as a link between the mother and the embryo. The umbilical cord connects the embryo to the placenta and consists of a tube through which two arteries carry nutrients to the embryo and a vein carries waste from the embryo back to the mother. The embryo is surrounded by the amniotic sac, which contains amniotic fluid.