five layers of cells. There is a well-marked external squam- 

 ous layer (Figure 8-16) and a thick basement membrane. In 

 the epidermis there are large mucous cells (Becker's cells) 

 which move outward as they develop. They shed their con- 

 tents from a narrow surface tip, and then are moved out of 

 the epidermis. Multicellular glands like those of the amphib- 



pigment 



mucous ce 



scales 



fibroblast 



heavy 

 collagenous 

 fibers 



muscle 



Figure 8-16. Section through the skin of the lungfish Profopterus 

 showing the overlapping scales deep in the dermis. 



ian, but smaller, have been described but are not observed 

 in most preparations. Sensory organs, much like those of 

 Squalus (Figure 8-19 C), are scattered over the body. Pig- 

 ment cells are found in the epidermis and throughout the 

 dermis. 



The dermis has many fibroblasts in it and is permeated 

 superficially by a network of capillaries. The outer layer of 

 the dermis, which contains the scales, is a loose areolar (re- 

 ferring to the scattered spots or nuclei) tissue which may 

 have fat cells scattered through it. A dense layer of collag- 

 enous fibers lies next to the body muscles. In cross sections 

 of the dermis, one sees parts of several scales, which lie in 

 dermal pockets. 



Actinopterygian fishes 



The skin of the actinopterygian fish is like that of the 

 choanate (Figure 8-17). The epidermis is dotted by pearl 

 (or sensory) organs and overlies the scales (Figure 8-18). 

 The pearl organs are perhaps related to the prototrichs of 

 higher forms. In addition to the mucous cells of the surface, 

 two types of secretory cells are observed. These are the 

 granular secretory cells, or serous bladders, and the club 

 cells. The former begins as a large Leydig's cell, then be- 

 comes filled with a granular secretion which is expelled to 

 the surface by a tubular extension of the cell. The club cells 

 produce the intercellular matrix of the epidermis. Tubular, 

 multicellular glands are observed on the snout and with de- 

 creasing frequency further back on the body. Poison glands, 

 formed by clumps of holocrine cells, are associated with 

 opercular or dorsal spines in some fishes. The epidermis is 

 occasionally invaded by blood capillaries as in the upper lip 

 of Perwphlhalmus. 



As in the choanates, the dermis encloses the scales lying 

 in sacs. Sharpey's fibers penetrate the ganoid type of scales, 

 tying them together as well as into the fibrous network of 

 the dermis (Figure 8-36). Sharpey's fibers are evident only 

 in teleosts with thick bony scales (siluroids. Figure 8-39 C). 

 The dermis contains several colors of chromatocytes and 



mucous cells 



mucous cell 



mucous cell 



cuticle 



granular secretory cell 



stratum germmotivum 



B C 



Figure 8-17. Sections through the epidermis of three teleosts, A. Co//ichfhys punctotus, an 

 armored catfish; Anguilla vulgaris, the common eel; C, Perco flavescens, the yellow perch. 



SKIN • 225 



