Histology 303 



for the functional requirements of the particular surface. Consequently, 

 with very rare exceptions, every free surface of an animal, external or 

 internal, is the surface of a more or less specialized cellular layer, an 

 epithelium. 



Epithelial Tissues 



Epithelia are tissues of primary importance. They are, in double 

 sense, the most primitive of tissues. The smaller, simpler coelenterate 

 consists merely of an outer and an inner epithelium. The gastrula of 

 animal embryos consists of two epithelia. Therefore all adult tissues 

 are derived from epithelia. It is evident, then, that epithelium pro- 

 vides for all animal needs. All-epithelial animals may and do exist. 



Not only are epithelia the immediate agencies in the necessary 

 interaction between animal and environment, but they tend to pre- 

 vent invasion by disease-producing organisms, and they seal surfaces 

 against leakage of the omnipresent intercellular fluid (lymph). If 

 human skin is abraded so slightly as not to cause bleeding, a clear, 

 watery lymph exudes. The outer layer of the intact epidermis prevents 

 such leakage. 



The outer layer of the vertebrate gastrula, while it is the source of 

 various structures which attain a deeper position, otherwise persists as 

 the epidermis, which is the external epithelium of the adult body. 

 The inner layer of the gastrula, giving rise to various organs such as the 

 liver, pancreas, and lungs, which grow outward from the enteron, other- 

 wise persists as the lining of the digestive tube, the digestive epithe- 

 lium, which is the innermost epithelium of the adult body. By far the 

 greater part of the massive adult has been inserted between the two 

 primary layers. 



The term endothelium is commonly applied to the lining layer of 

 blood-vessels and lymphatics. Lymph-glands are exceptional in that 

 their irregular lymph-filled spaces have no definite endothelial cover- 

 ing. An important function of the deeper tissue of these glands is the 

 liberation of single cells which drift away in the lymph stream as new 

 white blood-cells. An endothelium would hamper this process of shed- 

 ding cells. In "blood-glands" such as the spleen, the blood-filled spaces 

 are more or less deficient in endothelial lining. The term mesothelium 

 may be used for the peritoneal epithelium. Cells may form a layer 

 resembling an epithelium but not abutting upon a cavity. The tissues 

 of some endocrinal glands are of this nature. To such tissues is applied 

 the adjective epithelioid. 



Epithelia carry on functions of most diverse kinds. The diversity is 

 reflected in the structure of epithelia. Only a few of the more general 

 features of structure can be mentioned here. 



