Figure 422

Demonstration of basement membrane material in splenic vessels, a. Photomicrograph of a silver preparation revealing two longitudinally sectioned venous sinuses in the spleen. These blood vessels are surrounded by a modified basement membrane, which takes the form of a ring-like structure, much like the hoops of a barrel, rather than a continuous layer or lamina. The rings are blackened by the silver and appear as bands where the walls of the vessel have been tangentially sectioned (arrows). To the right, the cut has penetrated deeper into the vessel and shows the lumen (L). Here, the cut edges of the rings are seen on both sides of the vessel. In the lower vessel, the cut rings have been sectioned in a virtually perpendicular plane, and the rings appear as a series of dots. x400. b. Electron micrograph of the wall of a venous sinus, showing a longitudinally sectioned endothelial cell (ErtC). The nucleus (N) of the cell is protruding into the lumen. The basal lamina material (asterisks) has the same homogeneous appearance as seen by electron microscopy in other sites except that it is aggregated into ringlike structures rather than into a flat layer or lamina. Moreover, its location and plane of section correspond to the silver-reactive, dot-like material in the panel above. x25,000.

growing processes of a cell use the basal lamina that remains after cell loss, thus helping to maintain the original tissue architecture. For example, when nerves are damaged, new neuromuscular junctions from a growing axon will be established only if the external lamina remains intact after injury.

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