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Surface Specializations of Mechanically Laden Epithelia

1968; Elsevier BV; Volume: 9; Issue: 5 Linguagem: Inglês

10.1016/s0034-5288(18)34522-3

1532-2661

Richard Tucker,

Cellular Mechanics and Interactions

Fine specializations in the mechanically laden epithelia of the ox, the finch, the skink and the carpet snake, which are spread over solid bases, moveable diaphragms and motile folds, as well as in those epithelia which are present in tubular and epithelio-parenchymatous organs, were investigated. It was found that in the oral mucosa the structure of skink and carpet snake epithelium is differentiated least. This epithelium, however, possesses all the essential features of the finch and bovine epithelia. In the skink and the carpet snake the intercellular fibrillous system is weak, and the peripheral fibrillous system (desmosomes) only moderately developed. The enlargements of the intercellular spaces are usually simple, their extensions into the cytoplasm barely indicated, and intercellular slits are adapted to the formation of transient cavities (pressure chambers). The finch oral mucosa develops towards the elaboration of pressure chambers, while the bovine mucosa differentiates extensions of intercellular spaces into the cytoplasm as well as the pronounced enlargements of the intercellular spaces. It seems probable that the energy transfer between intercellular (hydraulic) and intracellular fibrillous systems in the finch, as expressed by penetration of intercellular spaces into the cytoplasm, is limited to a much smaller area of the mucosal cell than in the ox, in which it involves the whole cell. The mechanically-laden epithelia are always connected with fibrocytes and with collagenous fibres. In the epithelium of the contracted gall bladder the collagenous fibres can form intra-epithelial lamellae, which may stabilize the epithelial layer during volume changes of the whole organ. In the nictitating membrane dense and loose collagenous areas can be distinguished. Differentiation of fibroblasts into collageno- and elasto-cytes appears to be determined by the kind of mechanical stimulus in the given area. The organs which periodically alter their volumes extensively have cells with pronounced membranous foldings. The magnitude and the character of stresses present in epithelia depend as much on the forces at their surfaces as on the kind of tissues which are present at their bases. Epithelia which are subjected to the highest mechanical demand are those sandwiched between strong primary forces and strong reactive forces from their stable and inert background. The mechanical adaptations in epithelia involve the intracellular fibrillous system, the intercellular hydraulic system, and the interconnections between those systems. In epithelia which are compressed strongly and frequently a marked expansion of mechanical organelles can be observed. In such epithelia the electric resistance of the cells is increased by the division of the cytoplasm into multiple fibrous compartments. This may be followed by the complete replacement of the cytoplasm by fibrils or by keratin. In epithelia which are compressed frequently there seems to be little fluctuation in the fluids of the intercellular spaces. In such epithelia the transmission of stresses from one mechanical system to another is particularly efficient. An increase in frequency or in magnitude of the shearing stresses in epithelia appears to be paralleled by an increased number and strength of desmosomes. In epithelia in which the shearing and compressive stresses are weaker the intracellular fibrils become restricted to the periphery of the cell. In such epithelia the number of fibrils is reduced, and enlargements of the intercellular spaces are less frequent and less regular. In tubular and hollow organs the intracellular fibrillous system develops only weakly, if at all, and the intercellular hydraulic system may undergo considerable changes related to the amount of fluid present in them.