Capillaries are tubes of endothelium, freely permeable to molecules smaller than albumin.

Endothelial cells, in three dimensions, look like fried eggs, sunny-side up. The nucleus would be the yolk. Now think of a long lines of fried eggs, cooked close enough so that their edges meet on the right and left. Roll the line of fried eggs into a tube, and you have a capillary.

This is a section of brain showing capillaries.

With your study partners, find:

• capillaries


• their endothelail cells

Hotshots:

Find the neurons, the brain cells which have easy-to-see pink-staining cytoplasm.

Some of the cells on this capillary are pericytes, a second layer of cells applied on the outside of the first layer. Don't try too hard to figure out which is which. We'll show you how to use a reticulin stain to do this later on, if you're curious.

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