Page 60 - The Cell in 40 Topics

hat is it that helps a newborn baby weighing 3 kilograms (6
pounds) and only 50 centimeters (19 inches) tall to turn into
an adult weighing around 80 kilograms (176 pounds) and
some 1.80 meters (6 feet) tall over 20 to 25 years?
The answer to that question lies hidden in growth hormone, a miracu-
lous molecule secreted by the pituitary gland.
Growth in the body takes place in two different ways. Some cells sim-
ply increase their volume, while others divide and multiply, creating more
of themselves. But growth hormone directs and brings about both of these
processes.
Growth hormone affects all the body's cells. Every cell knows the
meaning of the message secreted by the pituitary gland. If it needs to grow,
it does so, and if commanded to divide and multiply, it does so.
For example, the heart of a newborn baby is about 1/16 the size of an
adult's. Yet it contains the same number of cells as an adult heart. Growth
hormone affects the heart cells one by one during their developmental
stage. Every cell develops to the extent commanded by the growth hor-
mone. Thus it is that the heart grows and eventually reaches an adult size
(Figure 48).

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