Histology

Histology is the branch of biology that studies the structure and arrangement of tissues. Recall that a tissue is a group of cells that share a common embryonic origin and are similar in structure, and which work together to carry out a shared function. This is the level of organisation that sits between the cell and the organ: individual cells combine in an orderly way to build tissues, several tissues combine to build a functional organ, and organs in turn combine into an organ-system — summarised as Cells → Tissues → Organs → Organ systems → Body.

All of an animal's body cells fall into two categories: somatic cells (from the Greek soma, 'body' — every body cell except the reproductive cells) and germ cells (the sperm and ova, which belong to the reproductive system and are not part of the four tissue types described in this chapter). Animals are built from four basic tissue types, each taken up in turn in this chapter: epithelial tissue, connective tissue, muscular tissue and nervous tissue.