peg 



growth lines 



ganoin layers 



denticle 



vascular canal 



Shorpey's fibers 



canals of Williamson 



B 



Figure 8-37 Rhomboid ganoid scales of lepisosteus. A, outer view of scole with growth increments 

 indicated by dashed lines; B, longitudinal section through a scale; C, histology of posterior margin 

 of a scale. (After Nickerson, 1893; Kerr, 1952; Rauther, 1927-1940) 



probably nothing more. The processes of the flattened 

 osteoblast of Lepisosteus extend out in a single interlamellar 

 plane, while those of Amia radiate in all directions. 



Teleost The teleost may have laminated scales covered 

 with a generally noncellular bony material called hyoden- 

 tine (Figure 8-39). In Callichthys the hyodentine contains 

 canals for protoplasmic extensions of cells lying in the large 

 vascular channels. The hyodentine is also penetrated by 

 Sharpey's fibers. The scales may be associated with spine- 

 like denticles like those observed in Lepisosteus or Polyplerus. 

 These have an enamel cover and a dentine interior sur- 

 rounding a pulp cavity. The denticles are attached to the 

 scale by ligaments or embedded in the hyodentine. Below 

 the hyodentine is laminated bone or cell-containing, fibrous 

 connective tissue which may be partly calcified. 



The scale of the teleost is of a cycloid type with circum- 

 ferential growth rings (Figure 8-40). In many species, there 

 are tooth-like extensions from the posterior margin; this 

 modification identifies the ctenoid scale. 



EMBRYOLOGiCAL DEVELOPMENT The scales of Polypterus 

 develop in the outer parts of the dermis, where knots of 

 fibroblasts gather to form buds (Figure 8-41). The outer 

 part of this cell aggregation forms the scale sac in which two 

 layers of osteoblasts appear. The two layers become sepa- 

 rated by an imperforate lamina of bone. To this initial 

 lamina, which disappears, spongy bone is added above and 

 below. The outer layer of enamel and dentine is now 

 applied along with a lamina of bone to the under surface. 

 The outer face of the scale lies next to the epidermis or is 

 separated from the epidermis by a mi.xed layer of mesoder- 

 mal and epidermal cells. The first layer of ganoin when 

 completed lies in contact with the epidermis. It is then 

 separated from the epidermis by a mesodermal overgrowth 

 which, under the influence of the epidermis, gives rise to a 

 second layer. A third layer of ganoin forms in the same way 

 after the external face of the scale has become overgrown 



by a new layer of mesodermal tissue. Eventually, the ganoin 

 or enamel consists of several superimposed layers. 



In the teleost, the process is much the same. Scale buds 

 are formed by fibroblast masses in the upper layer of the 

 dermis along the lateral-line canal. From these masses, 

 fibroblasts migrate out along lines extending upward and 

 forward and downward and forward. Successive scale buds 

 are formed along these lines by clumps of fibroblasts. Each 

 scale bud becomes organized into a sac enclosing two layers 

 of osteoblasts. The hyodentine layer is formed between the 

 layers of osteoblasts. As the layer grows in thickness, the 

 lower layer of osteoblasts begins to form laminar bone or 

 fibrous laminae. As the initial scale grows, the posterior 

 margin extends back and outward over the next posterior 



sculptured layer 



calcified layers 



fibrous layers 



Figure 8-38. Section of a skull bone of Amio (A) showing lepisos- 

 teid canals; B, section of a scale showing fine structure. (A offer 

 Goodrich, 1 907; B after Kerr, 1 952). 



SCALES 



237 



