Histogenesis and Organogenesis
The process of how the primary germ layers develop into many different kinds of tissues is called histogenesis. The way in which those tissues arrange themselves into organs is called organogenesis.
The fascinating story of histogenesis and organogenesis in human development is long and complicated—its telling belongs to the science of embryology. But for the beginning student of anatomy and physiology, it seems sufficient to appreciate that human development begins when two sex cells unite to form a single-celled zygote. It is also necessary to understand that the offspring’s body evolves by a series of processes consisting of cell differentiation, multiplication, growth, and rearrangement, all of which take place in a definite, orderly sequence (Figure 24-7).
FIGURE 24-7Critical periods of neonatal development. The red areas show when teratogens are most likely to cause major birth defects, and the yellow areas show when minor defects are more likely to arise. Numbers refer to weeks of gestation.
Development of structure and function go hand in hand, and from 4 months of gestation, when every organ system is complete and in place, until term (about 280 days), fetal development is mainly a matter of growth. Figure 24-8, step 1, shows the normal intrauterine placement of a fetus just before birth in a full-term pregnancy.
FIGURE 24-8 Parturition.
QUICK CHECK
- What is the postnatal period? The prenatal period?
- What is a zygote? How is it different from a morula or blastocyst?
- Name and describe the three primary germ layers.
- What are stem cells?
- What is meant by the term organogenesis?