494 



Special Vertebrate Organogenesis 



odontogenic. Experiments on the induction 

 of tooth development have been conducted 

 in amphibians. Hohfreter ('35) transplanted 

 indifferent ectoderm of the tree-frog, Hyla, 

 to the ventral head region of the newt, 

 Triton. Horny teeth developed which were 

 typical of the donor species, but did not 

 occur in the host species. 



Sellman ('46), on the basis of extirpation 

 and transplantation experiments in the uro- 

 deles which are equipped with dentin teeth 

 even as larvae, concluded that tooth forma- 

 tion requires three components: (1) neural 

 crest, (2) presumptive movxth ectoderm and 

 (3) oral entoderm (the oral plate of the fore- 

 gut). 



Andres ('46) transplanted undetermined 

 ventral ectoderm of a gastrula of a toad to 

 the lateral head region of a salamander neu- 

 rula. Though this ectoderm is induced by 

 the host to form a variety of tissues, such as 

 cartilage, teeth did not develop. 



PROLIFERATION 



The odontogenic cells tmdergo rapid mi- 

 totic multiplication leading to changes in the 

 form and size of the enamel organ (the bud, 

 cap, and bell stages. Fig. 191). Adjacent 

 connective tissue cells also proliferate to form 

 the dental papilla (the pulpal organ) and 

 the dental follicle (the periodontal organ). 

 The enamel organ, the dental papilla, and the 

 dental follicle together form the tooth germ. 



The enamel organ does much more than 

 its name implies. In addition to supplying 

 the inner enamel epithelium which gives 

 rise to the ameloblasts it exerts an organizing 

 influence on the adjacent mesenchymal cells 

 and outlines the future dentino-enamel and 

 dentino-cemental junctions. It would, there- 

 fore, be more appropriate to call it the odon- 

 togenic organ. 



It appears that even at this early stage the 

 tooth germ contains the entire growth po- 

 tential of the future tooth. Explants of this 

 stage of development continue to develop in 

 tissue culture through the subsequent stages 

 of histodifferentiation and appositional 

 growth (Glasstone, '36). 



Proliferation of a given cell normally 

 ceases with the assumption of the next stage 

 of histodifferentiation. The inverse relation 

 between proliferation and differentiation and 

 specific activity is well illustrated in the 

 ameloblasts. If they do not attain differentia- 

 tion they continue to proliferate excessively 

 with resulting cyst or tumor formation 



(ameloblastoma) (Thoma and Goldman, 



'46). 



HISTODIFFERENTIATION 



The formative cells vmdergo strvictural as 

 well as chemical changes. They give up their 

 proliferative activity. The cells of the inner 

 enamel epithelium differentiate into tall col- 

 umnar ameloblasts and exert an organizing 

 influence vipon the subjacent mesenchymal 

 cells of the dental papilla, which then dif- 

 ferentiate into odontoblasts. 



Chemical Interaction and Interdependence 

 of Epithelium and Odontoblasts; Transplanta- 

 tion Experiments. Von Brunn (1887) and 

 others have shown that the presence of the 

 inner epithelium is essential to the differen- 

 tiation of the odontoblasts and the initiation 

 of dentin formation. However, once differen- 

 tiation has reached a certain stage, the odon- 

 toblasts can proceed with dentin formation 

 without the further presence of the epithe- 

 lium. 



Transplantation experiments with tooth 

 germs of higher vertebrates beginning with 

 those of Legros and Magitot (1874) have 

 yielded further evidence on the interdepend- 

 encies in tooth development. Hviggins et al. 

 ('34) observed dentin formation in 14 days 

 following transplantation of odontoblasts to 

 the abdominal wall. Enamel, on the other 

 hand, will not form in the absence of odonto- 

 blasts or dentin (Huggins et al., '34; Glass- 

 tone, '36). Transplanted ameloblasts lose 

 their cylindrical character, change to strati- 

 fied squamous epithelium and fail to form 

 enamel tmless the odontoblasts accompany 

 the transplant (Hahn, '41). Dentin forma- 

 tion, therefore, precedes and is essential to 

 enamel formation, although the presence of 

 the epithelial cells and their chemical inter- 

 action precede and are essential to the differ- 

 entiation of the dentin-forming cells and the 

 initiation of dentin formation. 



The influence of dentin on enamel forma- 

 tion may be indirect by reversing the stream 

 of tissue fluid (Wassermann, '44). The chem- 

 ical influence of epithelial cells upon ad- 

 jacent mesenchymal cells is not only evi- 

 denced in the tooth but also has been 

 demonstrated in bone growth. Transplanta- 

 tion of epithelium of the gallbladder or of 

 the urinary tract distal to the kidney causes 

 the differentiation of the adjacent mesenchy- 

 mal cells into osteoblasts and initiates the 

 formation of bone (Huggins, '31). 



Concomitant with their morphological dif- 



