504 



Special Vertebrate Organogenesis 



what by morphogenetic movements and by 

 growth movements of their tissue substrate 

 should not be overlooked. 



The direction and paths of movement of 

 the melanoblasts concerned with skin pig- 

 mentation do not appear to be at random. 

 Experimental evidence indicates that they 

 follow more or less predetermined routes, 

 proceeding dorsoventrally in the mesen- 

 chymal tissues subjacent to the skin ecto- 

 derm. This would seem to imply that their 

 movement is somehow directed by proper- 

 ties intrinsic to the skin and also by the 

 interface between the ectoderm and the 

 prospective dermis. The association of cells 

 and tissues of distinctly different types pre- 

 supposes some kind of surface compatibility 

 or "affinity." The migratory movement of 

 the precursor pigment cells, therefore, could 

 be dependent upon their specific interaction 

 or contact relationship with cells of the der- 

 mis. On this supposition, movement would 

 continue imtil the contact relationship be- 

 tween the combining elements reaches some 

 sort of equilibrium (Holtfreter, '39; Tyler, 

 '47; Weiss, '47). 



Interesting experimental evidence in favor 

 of selective association of cells and tissues of 

 specific types has been obtained recently by 

 Weiss and Andres ('52). They injected disso- 

 ciated embryonic cells of the chick, includ- 

 ing melanoblasts which served as markers, 

 into the blood stream of host chick embryos. 

 Since these injected cells became scattered 

 at random throughout the body of the host, 

 it was possible to determine whether or not 

 any "selectivity" governed the definitive lo- 

 cations of the pigment cells. The results 

 demonstrated conclusively that donor mel- 

 anoblasts proliferate profvisely and synthe- 

 size melanin granules of the color and shape 

 characteristic of their genotype, only in lo- 

 cations in the host identical to those in which 

 they would normally have developed pig- 

 ment in the donor individual. Never were 

 they found in unusual cell and tissvie associa- 

 tions. 



Certain results obtained from grafting 

 skin in fowl indicate that the invasion of 

 melanoblasts is controlled by the skin and 

 feather germs. When, for example, an area 

 of skin, experimentally deprived of its nor- 

 mal source of pigment cells, is grafted at 

 hatching to a chick host of similar age, 

 melanoblasts from the surrounding regions 

 of the host skin migrate freely into the 

 graft and establish themselves permanently 

 (Rawles, '44). Such an invasion of melano- 

 blasts does not take place when an area 



of normal skin containing its full comple- 

 ment of melanoblasts is grafted similarly 

 (Danforth and Foster, '29). It would appear, 

 then, that invasion does not take place if a 

 state of equilibrium has already been at- 

 tained between the tissues of the skin and 

 the melanoblasts. This phenomenon has been 

 interpreted by Willier ('48) to mean that a 

 constant ratio has been established between 

 the number of melanoblasts and the cells of 

 the skin. The number of melanoblasts, ac- 

 cording to this view, is limited not by a 

 self-limitation of their capacity for multi- 

 plication, but rather by the cell community 

 (skin). Such a constant ratio may be tem- 

 porarily thrown off balance by an active 

 regenerating feather papilla in which special 

 conditions are set up favoring the invasion 

 of some of the melanoblasts from the dermis 

 or its specialized unit, the dermal papilla, 

 into the epidermal region (collar) which 

 gives rise to the feather parts. As this in- 

 vasion of melanoblasts into the regenerat- 

 ing feather parts takes place, other melano- 

 blasts of the dermal regions multiply to 

 restore again the constant ratio. Thus a 

 mechanism is provided for maintaining this 

 constant relationship between the pigment 

 cells and the feather cells throughout the 

 life span of a bird (Willier, '52). 



REGIONAL specialization OF THE 

 SKIN 



In the higher vertebrates the epidermis 

 enters into a closer and more intimate rela- 

 tionship with the dermis or corium than in 

 the lower vertebrates. The early smooth con- 

 tour between the two, such as exists perma- 

 nently in the lower forms, is lost owing 

 principally to the formation of folds or 

 papillae which project into the epidermis 

 and alternate with similar downward pro- 

 jections from the epidermis. The extent of 

 development of the papillae varies greatly in 

 the different body regions. On the palms 

 and soles they reach their greatest height. 



Structural DifFerences. Studies on the de- 

 velopment of the skin in the various types 

 of vertebrates show that it is the ectodermal 

 portion which undergoes characteristic struc- 

 tural modifications to fit it for the carrying 

 out of special fvmctions. Increase in epider- 

 mal surface is brought about by both evagi- 

 nations and invaginations which develop in 

 many different ways. Arising as external 

 processes are the so-called epidermal ap- 

 pendages, scales, feathers, hairs, nails, and 

 teeth; arising as invaginations are a va- 



