Skin and Its Derivatives 



501 



the somites between the limb regions of 21/^ 

 day chick embryos, have shown clearly that 

 in normal development the somite material 

 does not migrate into the ventrolateral and 

 ventral regions of the body wall. These cor- 

 roborative results strengthen the conclusion 

 that the dermis arises from mesoderm in 

 contact with and subjacent to the skin ecto- 

 derm. It is highly probable that the develop- 

 ment of the dermis is a result of contact 

 relationships between ectodermal and meso- 

 dermal cells. 



The origin of the dermis in the amphibian 

 has not been so clearly established. Again, 

 the problem centers on whether the mesen- 

 chymal cells subjacent to the skin ectoderm 

 have a localized source, such as the derma- 

 tome or neural crest, or whether their source 

 is more generalized. That the dermatome is 

 not the source of the dermis of lateral and 

 ventral regions is indicated by the results 

 of Detwiler ('37a). When four successive 

 brachial somites were excised on one side 

 of Amblystoma embryos, and I'eplaced by 

 unsegmented, lateral mesoderm and over- 

 lying ectoderm, the dermis of the skin on 

 the operated side appeared to be as com- 

 pletely developed as that on the normal side. 

 The dermatome was thus eliminated as a 

 source of the dermis of the lateral and ven- 

 tral regions at least. Among the amphibians, 

 two possible interpretations are evident. The 

 dermis arises either directly from the ex- 

 ternal layer of the somatic mesoderm of the 

 lateral somatopleure (or ventral myotomic 

 growth) or from the neural crest cells as 

 Raven ('31, '36) first suggested. According 

 to Raven the greater portion if not all of 

 the dermis of the trunk arises from neural 

 crest cells. His interpretation was based on 

 the interchange of prospective neural crest 

 of the trunk region between embryos of dif- 

 ferent species of urodeles {Amblystoma mex- 

 icanum. and Triturus taeniatus or T. al- 

 pestris). Owing to species differences in nu- 

 clear size and quantity of yolk granules he 

 was able to follow the migration of neural 

 crest cells into the dermis, i.e., into a posi- 

 tion between the myotomes and skin ecto- 

 derm. Detwiler ('37b) followed neural crest 

 cells which had been stained with Nile blue 

 sulfate to similar positions suggesting their 

 contribution to the dermis or corium. Holt- 

 freter ('35) and Steamer ('46) have pre- 

 sented further evidence in support of this 

 hypothesis. Although these and other ex- 

 periments leave little doubt that some of 

 the neural crest cells migrate laterally be- 

 tween the somite or myotome and the skin 



ectoderm, it is not altogether clear whether 

 these are precursor pigment cells or mesen- 

 chymal cells of the dermis or a mixture of 

 both. The problem needs reinvestigation in 

 view of the demonstration for the chick and 

 mouse that trunk and limb dermis arises in- 

 dependently of any contribution from the 

 neural crest. At present the most plausible 

 view appears to be that the mesenchyme 

 just beneath the skin ectoderm of the trunk 

 is not restricted to any one site of origin. 

 The mesenchyme of the head is largely if 

 not entirely of neural crest origin; there- 

 fore, it is quite probable that cells derived 

 from the latter give rise to the dermis of 

 the head skin. It is now known that the 

 neural crest exhibits marked regional dif- 

 ferences in its potentialities (Horstadius and 

 Sellman, '46; Niu, '47). 



Time of Localization or Specialization of the 

 Ectoderm. It has been clearly shown by the 

 methods of experimental embryology that 

 the parts of a developing system possess 

 greater developmental potentialities at an 

 early stage than at any later period. The 

 ectodermal cells of an amphibian gastrula, 

 for instance, are potentially capable of dif- 

 ferentiating into almost any cell type. At 

 this stage their prospective fate has not be- 

 come irrevocably established and the course 

 which they follow in differentiation depends 

 upon their reactions to inductive stimuli 

 received from the region in which they hap- 

 pen to be placed. During the process of 

 gastrulation, however, the skin ectoderm ac- 

 quires a certain amount of autonomy or 

 capacity to self-differentiate which enables 

 it to differentiate in a particular direction 

 independent of stimuli from the surround- 

 ing tissues. In other words, specialized prop- 

 erties have now been acquired which govern 

 its further histological differentiation (see 

 Section VI, Chap. 1 for the evidence on 

 change in potency during gastrulation). By 

 the end of the gastrulation stage the ectoderm 

 has become more or less a mosaic of areas or 

 fields with differences in intrinsic organiza- 

 tion. Although regional specialization has 

 begun it is not yet final — changes are still 

 possible. The time when the prospective fate 

 of the areas becomes definitely established 

 varies among the different organ-specific 

 areas, and there are also time differences 

 among species. 



Polar Organizaton of the Skin Ectoderm. 

 The existence of a polarity in the skin ecto- 

 derm of early amphibian embryos is evi 

 denced by the orderly beat of the cilia in 

 a predominantly anteroposterior direction. 



