Human Reproductive Biology: Glands, Hormones, and Fertilization
Glands: Classification and Function
Glands are organs composed of specialized cells that manufacture and secrete substances. Glands can be classified into two main types:
Endocrine Glands
Such as the pituitary or thyroid, these glands produce substances called hormones that are released directly into the blood.
Exocrine Glands
Such as the salivary glands and liver, these glands discharge their secretions outside the body or into the digestive tract and are therefore not part of the endocrine system.
Hormones: Chemical Messengers
Hormones are substances, which are lipid or protein in nature, that are carried in the blood and regulate certain body functions. Hormones have the following key characteristics:
- Specificity: Each hormone acts on certain cells called target cells, which are specially programmed to respond to its presence.
- High Effectiveness: A minimal amount of hormone is sufficient to perform its function.
- Controlled Production: An excess or deficit in the amount of a hormone produced can lead to characteristic diseases.
Puberty and Reproductive Organs
Puberty is the stage of life when the reproductive organs mature.
The male and female genital tracts are responsible for producing reproductive cells, enabling their union, and, in the case of the female, hosting the embryo.
Gametes: Specialized Reproductive Cells
Gametes are specialized cells that carry hereditary information from parents to form the first cell of a new individual, the zygote. Male gametes are sperm and female gametes are eggs (ova). Although both play a common function, they differ significantly in structure and size.
The Sperm Cell
Sperm cells are small and highly specialized. The anterior head contains the nucleus, which holds the paternal genetic material. They also have a long thread called a flagellum (or tail) through which they travel.
Formation of Sperm (Spermatogenesis)
Sperm are formed within the testes in very fine tubes called seminiferous tubules. On their walls are located the sperm stem cells that continuously multiply from puberty and throughout life to produce about a thousand sperm per second. Each sperm takes 64 to 72 days to form.
The Egg (Ovum)
Eggs are large cells. Their nucleus contains the hereditary information. In the cytoplasm are reserve substances (the yolk) to nourish the embryo in the early stages of development. Surrounding the egg is a crown of cells that protect it.
Formation of Eggs (Oogenesis)
Female eggs are formed before birth, by the fourth month of gestation. When a girl is born, each of her ovaries already contains a pool of immature eggs in small cavities called follicles. The follicle walls are lined by cells that protect and nourish the egg.
Reproductive Cycles: Ovarian and Menstrual
The Ovarian Cycle and the Menstrual Cycle are cyclical processes that take place, respectively, in the ovary and the lining of the uterus (endometrium).
Fertilization
Fertilization is the process of union of a sperm and an egg. It takes about 36 to 60 hours and typically takes place in the fallopian tubes. There, a single sperm, out of all those that arrive, penetrates the crown of cells protecting the egg and fuses its membrane with the egg’s membrane.
The fusion of an egg and sperm results in the first cell of a new individual: the zygote. Between the first and second day, the zygote begins to divide—first into two cells, then four, and so on—as it moves through the fallopian tube toward the uterus.
Implantation (Nidation)
When the embryo reaches the uterus, it enters the uterine lining (endometrium) where it finally implants. This process is called implantation (or nidation) and extends until day 14 after fertilization. The woman’s pregnancy begins, and from this time, her menstrual periods typically cease until after the baby’s birth.