| This transmission electron micrograph demonstrates the cellular composition of the blood/water barrier. The water space is indicated by No. 1. The entire gill is lined by an outer epithelial layer with small microvilli or plicae (listed as No. 2). In some places there is a second layer of epithelium, the inner epithelial cells. Note how many cells separate the vascular compartment with red blood cells (No. 8) from the air space. The capillaries are lined by pillar cells (No. 7). In addition to outer epithelial cells, occasional cuboidal cells are found in the epithelial lining. These cells are called chloride cells. The nuclei of two chloride cells are indicated by the number 3. No. 5 points to the mucous lining of the chloride cells. Chloride cells have a large number of mitochondria (No. 4). |
