Histology 



305 



Stratified Epithelium. On Amphioxus, a slender marine animal 

 only 4 or 5 cm. long, an epidermis one cell thick affords adequate pro- 

 tection. On an elephant it would not. Surfaces of large, heavy animals 

 are exposed to excessive mechanical friction and impact. Loss of ma- 

 terial at the surface is best compensated for by a stratified epithelium 

 whose lower layers persistently grow to replace the loss. 



A stratified epithelium may be two or several or many cells in 

 thickness (Fig. 244D-F). In all vertebrates the epidermis is stratified 

 (Fig. 246). Its thickness varies with the size and habits of the animal, 

 and, in a particular animal, it varies locally depending upon the degree 

 of exposure to mechanical wear. 



In a thick stratified epithelium the cells of the bottom layer are 

 usually columnar, their deeper ends resting on a thin nonprotoplasmic 

 basement membrane, and those of the outer layers are more or less 

 flattened. The intermediate cells have a form such as would result from 

 crowding tightly together a mass of compressible spheres — that is, 

 polyhedral. Yet the cells are not actually packed tightly together. 

 They are separated by excessively thin intercellular lymph spaces 

 through which seeps lymph derived from underlying blood-vessels and 

 serving to provide for the metabolic needs of the individual cells. Cells 

 on opposite sides of the intercellular space are connected by delicate 

 strands of solid, or at least dense, substance. Presumably protoplasmic, 

 the strands are called protoplasmic bridges or plasmodesms. 



Epithelia, even thick stratified, rarely contain blood-vessels. Cer- 

 tain salamanders (amphibians) possessing neither gills nor lungs carry 



Fig. 244. Types of epithelia. (B) Simple squamous. (C) Simple columnar. 

 (D) Stratified columnar, ciliated at (E). (F) Stratified polyhedral, upper cells 

 squamous. (Courtesy, Kingsley: "Comparative Anatomy of Vertebrates." Phila- 

 delphia, The Blakiston Company.) 



