Patterns of Growth

Two patterns of growth are commonly seen in animals regarding the proportion of the various body parts.

• Isometric growth: In this growth pattern, an organ grows at the same rate as the rest of the body. As the organism growths, the size remains proportional.

Examples: Fish and Locust

• Allometric growth: In this pattern, the organ grows at a different rate from which the body grows. The external form of the organism changes as the body grows.

Example:

Mammals show differential growth of human body parts.

Different parts of the human body, such as the head, limbs and internal organs do not grow uniformly or simultaneously. This can be seen by examining photographs form birth to childhood. The new born baby has an unproportionately large head and comparatively short legs. Due to slow growth of head and fast growth of limbs during the post embryonic phase the body shape and proportion between the different parts of the body changes.

Hormonal Control of Growth Rate in Man

The period extending from birth up to 10 - 13 years is called childhood. During this period, the growth is very slow. Growth during this period is controlled by the hormone thymus, secreted by the thymosin gland.

During puberty (between 14 - 18 years) there is enhanced activity of 2 growth hormones - thyroxin and the somatotropic hormone (STH) secreted by the thyroid gland and anterior pituitary gland respectively. Due to this, growth is also enhanced during this period. Growth is at its peak during puberty. The increase in the amount of the sex hormones - testosterone in male and estrogens and progesterone in females also helps in establishing the secondary sexual characters

At the age of 18 years when puberty comes to an end and a fully grown sexually mature male and female are formed, the physical growth of the body starts declining and by the age of 22 - 23 years ceases completely.