Physical development in humans is the process whereby cells differentiate into different types and tissues, develop function and ultimately begin to perform these given functions. Development begins at conception, and continues up through puberty, at which point all organs and tissues are fully functional. There are many factors that affect human development.
Hormonal Factors
Much of human development is hormone-mediated. This is as true while an embryo is still in utero as it is during puberty. In utero, for instance, baby boys begin to secrete testosterone. This hormone causes a set of ducts called the Wolffian ducts to develop into the male reproductive system, notes Dr. Lauralee Sherwood in her book, "Human Physiology." Growth hormone from the pituitary encourages both growth and regeneration of tissues. Testosterone at puberty causes existing structures, like the penis and testes, to grow further and become reproductively functional. Properly timed hormonal signals at different stages of development lead to a great variety of developmental changes.
Genetic Factors
Some amount of human physical development is mediated by genetic factors. For instance, when an embryo is about 6 weeks of gestational age, the DNA or genetic material contained in its cells causes the gonads to differentiate, or develop, into either testes or ovaries. This happens regardless of hormonal and other factors. Notes Dr. Gary Thibodeau in his book, "Anatomy and Physiology," the testes or ovaries then go on to exert hormonal control over other developmental processes, but pure genetics determine the tissue type of the gonads themselves. Genetics also determine many other factors, some in isolation, and others through cooperation with hormones and cellular chemical signals.
Location Factors
In the course of human development, genetics and hormones aren't the only factors that influence how cells and tissues develop. In fact, some cells become certain tissue types based purely upon their location relative to other cells. Dr. Thibodeau explains that this is because all cells have identical genetic material, so DNA alone can't account for cells differentiating, or turning into different types of tissue. Early in embryonic development, an embryo folds into three layers. The outer layer becomes the skin and nervous system, the middle layer becomes muscles and bone and the inner layer becomes many of the internal organs. While cells of all three layers have identical DNA, the location of cells relative to other cells--their position in the embryonic layers--determines their developmental fate.
References
- Human Physiology; Lauralee Sherwood, Ph.D.; 2004
- Anatomy and Physiology; Gary Thibodeau, Ph.D.; 2007
