328 S. N. BHASKAR 



eruption is illustrated by numerous reports. Dramatic illustration of 

 this interdependence is seen in recessive mutations in the rat (Greep, 

 1941; Schour et al, 1949; Bhaskar et al, 1950, 1952), mouse (Grune- 

 berg, 1935, 1936, 1937, 1938), and rabbit (Pearce and Brown, 1948; 

 Pearce, 1948, 1950rt, 19505). 



Although the changes in these mutations are similar in nature, 

 only those of the ia rat will be described. 



Dentition and the Bone Tissue of the ia Rat 



The ia mutation in the albino rat is characterized by a retardation 

 of resoi'ption of bone tissue in the entire skeleton ( hereditary hypo- 

 osteoclasis) {Bhaskar et al, 1950; Bhaskar, 1953; Mohammed, 1957). 

 As a consequence, the developing and growing bones show general- 

 ized osteosclerosis. In the jaws, the lack of physiologic modeling 

 resorption of the alveolar process leads to the distortion and anky- 

 losis of developing teeth (Figs. 7 and 8) (Schour et al., 1949). The 

 proliferation and differentiation of the odontogenic epithelium is 

 normal. However, because of the failure of remodeling of the bony 

 crypt, the growing epithelial dental organ approaches the wall of 

 the crypt and then extends into the surrounding marrow spaces. 



Although the odontogenic epithelium continues to proliferate at 

 a normal rate, the normal growth pattern of the tooth is disturbed 

 owing to the persistence of the adjacent bone trabeculae. The grow- 

 ing odontogenic epithelium follows the course of least resistance 

 and invades the bone marrow spaces in a haphazard and disorderly 

 fashion. The odontogenic epithelium maintains its physiologic poten- 

 tialities, that is, it differentiates into pulp tissue and odontoblasts, 

 and this leads consequently to the formation and calcification of 

 dentin and enamel. The irregular invasion of the odontogenic epi- 

 thelium into the bone marrow spaces leads to the presence of strands 

 and islands of dental tissues in different stages of development, on 

 and between the bone trabeculae. This has two consequences. First, 

 the teeth are locked to the surrounding bone, causing ankylosis, 

 which prevents first any movement of the tooth germ and later the 

 eruption of the tooth. Secondly, the continuous growth and the 

 differentiation of the odontogenic epithelium leads to the accumula- 



