Human development begins with fertilization of the egg and continues until death. It proceeds through specific stages throughout life: gestation, the preembryonic stage, the embryonic stage, infancy, childhood, adolescence, and adulthood. Fertilization marks the start of gestation. A fertilized egg becomes a zygote, and the zygote enters the preembryonic stage. The embryonic stage follows. The fetal development stage takes place over 40 weeks, at the end of which an infant is born. Development is complex and rapid from fertilization through infancy. Children undergo another rapid developmental phase with adolescence. Adulthood marks the developmental peak and the start of physical degeneration. Adulthood continues as the body undergoes the aging process and finally death.
At A Glance
- The male and female reproductive systems have similar features and functions.
Human gestation has three stages: the germinal stage, the embryonic stage, and the fetal stage.
- The preembryonic stage involves the zygote undergoing cell division to develop into blastomeres, a morula, and then a blastocyst, leading to implantation into the uterine lining.
- The embryonic stage involves further development of the embryo and organ systems.
- The fetal stage, which occurs from week nine of gestation to birth, is characterized by growth and further differentiation of organ systems, including distinct development of genitalia in males and females.
Infancy follows the neonatal period, which lasts just a few weeks. Infancy lasts until age two and is marked by more dramatic development.
Adolescence is a time of rapid physical change during which sexual maturation, rapid growth, and many physiological and hormonal changes occur, some of which are different for males and females.
- Physical changes that occur during adulthood include changes in body shape, muscular development, sexual activity, and, eventually, the loss of body functions because of aging.
Aging affects many systems in the body, including the skeletal system, muscular system, and nervous system.
Diseases such as type 2 diabetes, heart failure, and stroke are associated with aging and reduce the body's ability to maintain homeostasis.
- Physiological changes that occur before death, the cessation of respiratory or circulatory function or electrical activity in the brain, involve all body systems at different rates.