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Lesson: Chapter 1 — Living Tissues

1.2.1. Epithelial tissue 1 of 3

What epithelial tissue is

If a surface exists anywhere in a vertebrate — the outside of the body, the inside of the gut, the inside of a blood vessel — there is epithelial tissue on it.

Epithelial tissue

All the surfaces of the vertebrate body, external and internal, are lined by epithelial tissue. Some epithelial tissues consist of a single layer of cells; some consist of several layers.

Places to find it

  • The wall of a blood capillary.
  • The wall of the thyroid gland.
  • The wall of the alimentary canal.
  • The wall of the urinary bladder.
  • The epidermis of the skin.

Inside counts as a surface

The lining of your gut is on the outside of you, in the sense that matters here — it is a boundary between the body and something that is not the body. So is the inside of a blood vessel and the inside of the bladder. Epithelium is the tissue of boundaries, and a body has far more of them on the inside than on the outside.

How epithelia are classified

Epithelial tissues are classified according to the shape of the cells they contain and the number of cell layers. A single layer of flat cells makes a lining thin enough for substances to pass straight through, which is what a capillary wall needs; many layers of tough cells make a barrier, which is what skin needs.