Microvilli : minute, finger-like projections which increase the surface area of the membrane. Found on the surface of absorptive cells such as kidney tubules and intestinal cells. Membrane Junctions: two types of junctions formed between cells that make up tissues. (1.) junctions that fasten cells together (2.) junctions that permit transfer of ions and other molecules between cells. In tight junctions protein molecules in adjacent membranes fuse together preventing free passage of molecules through intercellular space between cells. In the digestive tract they form a barrier to enzymes and microorganisms preventing them from entering the blood. Desmosomes are spotlike patches that act as mechanical couplings between adjoining cells. The membranes do not actually touch, but are held together by fine glycoprotein filaments stretched between button-like thickening of the inner membranes. They are abundant in tissues subject to mechanical stress (skin, heart, muscles). Gap Junctions
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