The biology of growth.

Variability in human growth is not only in the timing of critical periods within the whole pattern of growth but also in the magnitude and rate of change coincident with the period. In addition, for a radical change in, e.g., height to occur there must also be changes in the anatomical parts that make up total height and these changes are themselves variable. Acceleration, for instance in height velocity, may be the result of different changes in the length of the spine, femur, and/or tibia, each of which may contribute differently to the total process. In addition, not only may the process be variable within a single child, it may also be variable between different children of the same or opposite sexes. The mathematical and statistical problems arising from the seemingly simple process of an increase in height are thus complex. In order to review the biology of human growth this contribution will discuss the principles of growth that are fundamental to our ability to interpret the response of the child to factors that might modify the genetically programmed pattern of growth from conception to maturity. In this way the biology of human growth will be described by a set of phenomena that reflect the actions of biological control mechanisms. These mechanisms are subject to genetic and environmental influences and their expression is characterised by variation in timing, magnitude, and duration.