Capillary histology

A capillary under the microscope barely looks like a vessel. It has the thinnest wall in the whole system: the endothelium, the layer of cells that lines the inside, surrounded by connective tissue, sitting on a basement membrane where cells called pericytes live. Nothing more.

And they are not all alike. The continuous capillary has unbroken endothelium and a continuous basal lamina, the supporting sheet the cells rest on: this is the sealed wall. The fenestrated capillary has fenestrations, circular pores in the wall, with the basal lamina still continuous: this is the wall with little windows. And the discontinuous capillary has endothelium broken by large gaps between cells, and the basal lamina too: this is the wall with outright holes. Three walls built from the same brick with different mortar.

When the section is stained with hematoxylin and eosin, the routine stains in pathology, what you can make out is a thin endothelial layer with the nuclei stained blue, basophilic, and the cytoplasm pink, eosinophilic. Sometimes you can even see an erythrocyte, a red blood cell, stuck there filling the capillary lumen.