EPITHELIUM 73 



Epithelial cells vary in size and shape, the element of i)ressure 

 being a frequent determining factor. They are often extremely 

 elastic, allowing great changes in shape and relation. Thus the 

 epithelium of the collapsed bladder is thick and the cells columnar 

 but as the bladder fills, the cells become flatter until in complete 

 distention the epithelium is thin and consists of flattened cells. 

 Their protoplasm may be clear, finely or coarsely granular, or pig- 

 mented. Each cell usually contains a single well-defined nucleus. 

 Two or more nuclei are sometimes present. Some epithelial cells 

 are, when fully matured, non-nucleated, e.g., respiratory epithelium 

 of lung. 



When epithelium rests upon connective tissue, it is usually sepa- 

 rated from the latter by a thin, apparently homogeneous membrane 

 known as the basal membrane or membrana propria. Authorities 

 differ as to whether this membrane is of connective-tissue or of 

 epithelial origin. 



Surface epithelial cells frequently have thickened free borders or 

 cuticidcB, which unite to form a continuous membrane, the ciiticular 

 membrane. Striations extend from the cytoplasm into the cuticulae. 

 A still greater specialization of the surface of the cell is seen in the 

 ciliated cell. In this cell fine hair-like projections — cilia — extend 

 from the surface of the cell. 



Some epithelial cells show important changes dependent upon 

 their functional activities. An example of this is seen in the mucous 

 cell in which there is a transformation of the greater part of the cyto- 

 plasm into, or its replacement by, mucus. 



Epithelia are devoid, as a rule, of both blood- and lymph-vessels. 

 An exception to this is the stria vascularis of the cochlea. Nerves, 

 on the other hand, are abundant. 



Classification. — EpitheUa may be classified according to shape 

 and arrangement of cells as follows: 



(i) Simple Epithelium. — {a) Squamous; {b) columnar. 



(2) Stratified EpitheUum. — {a) Squamous; {b) columnar. 



(3) Mesothehum and EndotheUum. 



Specializations of the above-mentioned types are known as: {a) 

 Cihated epithelium; {b) pigmented epitheHum: (c) glandular epithe- 

 lium; {d) neuro-epithelium. 



Pigment may occur in any type of epithelium. Ciha are found 

 only in the simple columnar and stratified columnar forms. 



