CH.YBW.CTS.C06.Final.q 6/21/03 1:07 PM Page 79
Tissues
Figure 6.1 This figure is a diagrammatic view of the basement membrane, an extracellular layer that defines an epithelial boundary and also helps reinforce epithelial sheets, resisting stretching and tearing. The basement membrane is actually two layers. The outer layer, the basal lamina, is secreted by epithelial cells, whereas the reticular lamina is formed by underlying connective tissue (indicated by the two arrows). Mesenchymal cells are found in an embryo and are the source of all connective tissues, which secrete an interstitial (extracellular) matrix that contains ground substance and protein fibers.
of flattened, scale-like cells, which allow substances to easily pass through. That is why this tissue type is a common site for diffusion and filtration. For instance, simple squamous epithelia lines the air sacs (alveoli) of the lungs and also the inside walls of blood capillaries. Simple cuboidal epithelium forms a single layer of cube-shaped cells. It covers structures in the ovaries and lines tubules within the kidneys and many glands, such as salivary glands, the thyroid gland, and the pancreas. Simple columnar epithelium is a single layer of elongated cells and is found in the uterus and most organs of the digestive tract, including the stomach and small intestine.
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