512 



Special Vertebrate Organogenesis 



ridges form in a definite time-space order. 

 On the fingertips, the first regions to show 

 ridge differentiation, folding of the epidermis 

 begins in the central portion of the apical 

 pad. Subsequently other foci arise in the 

 distal, lateral, and proximal regions. From 

 these foci, ridge differentiation progresses 

 in orderly sequence until the systems meet 

 and the final pattern configuration is estab- 

 lished. Less frequently differentiation of the 

 ridges into the definitive pattern is com- 

 pleted by extension from a single focal cen- 

 ter on the apical pad (Bonnevie, '27, '29; 

 Cummins and Midlo, '43). That there is some 

 relationship between pattern type and the de- 

 gree of elevation and the contour of the 

 volar pad is generally agreed upon. By the 

 nineteenth week the pattern is permanently 

 established in the hviman fetus in all of its 

 minute detail. Ridges broaden and lengthen 

 to keep pace with the growth of the hands 

 and feet, but no new formations occur (Hale, 

 '49). In the absence of experimental evi- 

 dence there is no actual proof that the epi- 

 dermal ridge patterns are the products of 

 embryonic induction, bvit the fact that the 

 mesodermal substratum is necessary for the 

 regeneration of normal ridge patterns would 

 appear to strengthen this point of view. 



Mesodermal Substrate Essential for Regen- 

 eration of Epidermis and Cornea. The im- 

 portance of the mesodermal substrate in the 

 regeneration of normal skin epithelium in 

 the adult human has been nicely demon- 

 strated by Bishop ('45). By removing skin 

 from the forearm to various depths it was 

 found that a portion of the papillary layer 

 must remain to insure the regeneration of 

 normal epidermis. Scar formation resulted 

 when removal was sufficiently deep to in- 

 clude the reticular layer and the bases of the 

 hair follicles. 



It has also been pointed out by Cummins 

 and Midlo ('43) that wounds, burns, etc., 

 produce no permanent effect on the epi- 

 dermal ridge patterns of the volar surfaces 

 unless the injury is deep enough to destroy 

 the dermal papillae, in which case scar tis- 

 sue is then formed. Since the epidermis of 

 the palms and palmar surface of the fingers 

 reaches a thickness of about 0.8 mm., it fol- 

 lows that tissue damage to a depth of about 

 1 mm. would be necessary to prevent nor- 

 mal regeneration of the characteristic ridges 

 of the fingers and palm. It is generally 

 known that skin grafted from one region to 

 another of the same individual retains its 

 original characteristics. In this respect the 

 ridged skin of the volar surfaces offers no 



exception. When it is considered that a skin 

 graft normally includes the papillary layer 

 of the dermis, the retention of specificity of 

 the graft is readily understandable. 



Further evidence of a necessary interaction 

 between dermal and epithelial factors in 

 regeneration is shown by the studies of 

 Maumenee and Scholz ('48) on the mam- 

 malian cornea. Epithelial cells from the 

 conjunctiva, which migrate over areas de- 

 nuded of cornea, do not become transformed 

 into typical corneal epithelium unless the 

 underlying stroma is normal. 



INTEGUMENTARY PATTERNS 



Morphological Patterns, The origin of spe- 

 cific skin patterns, as exemplified by hair 

 and feather direction and by the arrange- 

 ment of epidermal ridges, presents many 

 interesting developmental problems which 

 are by no means fully understood. As pointed 

 out by Wright ('49a), the formulation of 

 principles of gene action in relation to 

 morphological pattern is of the greatest im- 

 portance in relating genetics to the physi- 

 ology of development. While certain broad 

 generalizations have long been apparent, 

 there is a definite need for a systematic 

 study of the action and interaction of genes 

 in the formation of specific patterns. 



It is well known that skin patterns are 

 established in their permanent form early 

 in ontogeny. The direction or slope of the 

 hair, for instance, is recognizable imme- 

 diately after the first indication of the hair 

 primordia. Causal factors, therefore, must be 

 looked for early in embryonic life. In order 

 to identify such factors and to analyze their 

 role in the origin, growth, and differentia- 

 tion of the definitive pattern, direct experi- 

 mental evidence is essential. Although nu- 

 merous experiments have been done to ana- 

 lyze the factors concerned with hair di- 

 rection in a variety of mammals, the ma- 

 jority have been carried out after the hair 

 primordia were established and have given 

 negative or inconclusive results. Some recent 

 experiments of Kill ('49) on newborn rats, 

 at a time when the hair primordia are in 

 the processes of development, appear to be 

 the most decisive. By observing tattoo marks 

 that penetrated the upper and lower layers 

 of the skin of the tail and by excising pieces 

 of skin from various regions including the 

 ventral side of the neck where two natural 

 whorls occur, data were obtained which in- 

 dicate that the pattern of organization of 

 the skin is primarily a phenomenon of dif- 



