|Index to this page| The development of a fertilized egg into a
newborn child requires an average of 41 rounds of mitosis (241 =
2.2 x 1012). During this period, the cells produced by mitosis
enter different pathways of differentiation; some becoming blood
cells, some muscle cells, and so on. There are more than 100
visibly-distinguishable kinds of differentiated cells in the
vertebrate animal. These are organized into tissues; the tissues
into organs. Groups of organs make up the various systems —
digestive, excretory, etc. — of the body. |The actual number of
differentiated cell types is surely much larger than 100. This
page will give a brief introduction to the major types of animal
tissues. The links along the left side of the figure will take
you directly to the individual paragraphs indicated. Epithelial
tissue is made of closely-packed cells arranged in flat sheets.
Epithelia form the surface of the skin, line the various
cavities and tubes of the body, and cover the internal organs.
The basolateral s