enamel 



pulp cavity 



epidermis^ 



stratum laxum — 

 Sharpey's fibers 



fibers of stratum compactum -' 



Figure 8-22. Section of placoid scale of Squo/us. 



stronger than that of the normal dentine, the collagen has 

 almost disappeared, and the lime-impregnation has in- 

 creased. These hard substances do not differ much from true 

 enamel and may represent enamel at an early phylogenetic 

 stage. 



One of the most controversial enamels is ganoin, which 

 occurs in many of the actinopterygian fishes. It has been 

 considered as bone, as dentine, and as enamel. Many have 

 regarded ganoin as a special type of hard tissue not because 

 of its structure, which has been fairly little known in recent 

 forms, but because of its development — its mesodermal ori- 

 gin. The growth of ganoin takes place as the result of periodic 

 overgrowth of the external face of the scale by mesodermal 

 tissue. A salt-impregnated layer is laid down in this tissue 

 under the influence of the overlying ectoderm. This periodic 

 growth in thickness by apposition of new layers is quite dif- 

 ferent from the development of mammalian enamel. The 

 position of the ganoin films between layers of bone or den- 

 tine in the fully developed spines and dermal bones of 

 Polyplerus does not prove anything with regard to the ori- 

 ginal place of formation and the nature of ganoin. 



Dentine Definition of dentine presents similar problems. 

 There are three principle types: osteodentine, tubulodentine, 

 and orthodentine. Orthodentine consists of an outer layer 

 of pallial dentine and inner layers of circumpular dentine 

 (Figure 8-23). In these the dentinal tubules radiate out from 

 a central pulp canal. Vasodentine is a modification lacking 

 the dentinal tubules but with radiating capillary channels 

 which anastomose (join) through numerous cross channels. 

 Plicidentine, as observed in the labyrinthodont tooth (Fig- 

 ure 8-61), is a variety of orthodentine. 



Tubulodentine is made up of parallel orthodentine tubules 

 joined by an interstitial material: an enamel-like petroden- 

 tine or bone (Figure 8-55). This type appears to have 

 originated by the fusion of a large number of teeth into a 

 plate, as in the lungfishes, and some rays (Figure 8-55). 



Osteodentine (Figure 8-23) is composed of a reticulum 



pulp cavity 

 circumpulpar dentine 



, , . , pulp cavity 



trabecular bone ' 



between osteones of dentine 



B 



Figure 8-23. Types of dentine observed in fish teeth. A, orthoden- 

 tine outer layer of pallial dentine enclosing circumpulpar lominoe; B, 

 osteodentine inside pallial orthodentine; C, vasodentine. (After 0rvig, 

 1951) 



(net) of dentinal osteons, similar to the Haversian systems 

 of bone, and an interstitial bony substance which contains 

 or is devoid of cell spaces. This bony substance arises first 

 as trabecles, in the pulp tissue. The dentinal osteons are then 

 deposited within the trabecles without any prior resorption 

 of hard tissue. The periodic deposition of the osteodentine 

 produces the increasingly smaller tubular laminae making 

 up the osteon. 



Typical dentine has very fine, sometimes branching, 

 nearly parallel tubules extending outward from the pulp 

 cavity. In semidentine (Figure 8-27) the tubules are still 

 quite parallel one to another, but they have fine side 

 branches extending out almost at right angles. The unipolar 

 odontoblasts, the cells producing dentine, are embedded in 

 the dentine next to the pulp or vascular cavity and the single 

 large protoplasmic extension (which may branch) of each 

 extends into the dentinal tubule for some distance. The term 

 unipolar refers to this process. In mesodentine, a variant 

 of semidentine (Figure 8-26), the embedded cells are of the 

 unipolar type; there are more of them, and their lacunae 

 (the spaces in which the cell bodies lie) are interconnected. 



Summarizing, enamel is produced by the epidermis or in 

 the most superficial part of the dermis under the influence 

 of the epidermis; it may consist of modified dentine, calci- 

 fied basement membrane of the epidermis, a modified layer 

 of mesodermal tissue, or a secreted layer of epidermal ori- 

 gin. Dentine is presumed to be a product of the outer region 

 of the dermis. The deeper this material is formed in the co- 

 rium, the more bone-like it is. Dentine may enclose the cells 

 forming it or be acellular. 



Bone Thin sections of mammalian bone show Haversian 

 systems of cell lacunae around vascular canals (Figure 

 8-24). These systems, or osteons, can be laid down or 

 resorbed by the cells of the bone. In a typical bone, pri- 

 mary osteon systems are sometimes partly resorbed and the 



228 



THE SKIN AND ITS DERIVATIVES 



