502 



Special Vertebrate Organogenesis 



Experimentation has shown that the time 

 in development when this polarity becomes 

 established varies somewhat among the dif- 

 ferent species. In many of the amirans and 

 in the axolotl, polarization appears to be 

 acquired during gastrulation, shortly after 

 the round yolk-plug stage and thus consid- 

 erably in advance of the actual appearance 

 of the cilia (Woerdeman, '25; Tung and 

 Tung, '40). If a rectangular piece of skin 

 ectoderm is excised from a blastula or 

 j'Oung gastrula and reimplanted after a ro- 

 tation of 180 degrees, the cilia arising from 

 this area beat in the normal anteroposterior 

 direction. But if the same rotation operation 

 is performed later in gastrulation, i.e., at 

 the circular blastopore stage and afterwards, 

 the ciliary beat of the particular area is 

 reversed, indicating that polarization had 

 occurred prior to rotation. In Amblystoma 

 punctatum similar experiments by Twitty 

 ('28) have shown that polarity is established 

 in the ectoderm somewhat later in develop- 

 ment, during the closure of the neural folds, 

 hence about the time that the cilia would 

 normally appear. Cilia developing on areas 

 rotated after the closure of the neural folds 

 were found to beat in opposition to those of 

 the surrounding epidermis. About the time 

 that the direction of the effective stroke of 

 the cilia is established in Amblystoma other 

 changes involving polarity are also taking 

 place, such, for example, as the outgrowth 

 of the placodes of the lateral-line system 

 (Stone, '33) and the anteroposterior polari- 

 zation of the ear rudiment (Harrison, '36). 

 At this time the ectoderm can no longer be 

 turned inside out and develop into normal 

 skin (Luther, '34). All of these changes are 

 indicative of some fundamental transfor- 

 mation within the cells of the ectoderm. 

 That polarization is imposed upon the skin 

 ectoderm by the underlying entomesodermal 

 tissue is evidenced by the fact that whenever 

 ectodermic vesicles are formed, e.g., as a re- 

 sult of faulty healing-in of the transplant, 

 the ciliary beat of such areas is uncoordi- 

 nated (Luther, '34). The partial exo-gastru- 

 lae obtained by Holtfreter ('33a, b) afford 

 an even more beautifvd demonstration of this 

 point. He found that the ciliary beat was 

 regular and polarized in the portions of the 

 ectoderm underlaid by entomesodcrm, but 

 was irregular and chaotic in regions not 

 underlaid by this "organizing" tissue. Once 

 ciliary polarity has been induced in the 

 skin ectoderm, it in turn becomes capable 

 of inducing polarity in younger, unpolar- 

 ized ectoderm if brought into direct contact 



(Tung, Tung and Chang, '48). These in- 

 vestigators have also presented evidence that 

 the induction of polarity in the ectoderm 

 involves a chemical interaction. 



Surface-interior differences in cells of the 

 skin ectoderm have been demonstrated in 

 the chick embryo by implanting small iso- 

 lates of head skin ectoderm into the meso- 

 derm of a wing bud (Willier and Rawles, 

 '40). If the isolate becomes completely em- 

 bedded in the svirrounding limb mesoderm, 

 the epithelial character of the ectoderm dis- 

 appears and its constituent cells intermingle 

 with and become indistinguishable from 

 mesodermal cells of the wing bud. If, as 

 sometimes happens, the isolate rolls up to 

 form a vesicle with its original outer sur- 

 face facing the cavity and the inner surface 

 contiguous with wing mesoderm, the ecto- 

 derm will maintain its epithelial character 

 and differentiate into epidermis. In order to 

 develop normally the original outer surface 

 of the skin ectoderm must be free of cellular 

 contact. The free or outer surface of the 

 ectodermal cells appears to be incompatible 

 with and to resist fusion with mesodermal 

 cells. 



Source of the Pigment Cells of the Skin. 

 Pigment cells variously designated as mel- 

 anophores, chromatophores, pigmentophores, 

 dendritic cells, et cetera, are common and 

 distinctive components of the skin of all ver- 

 tebrates, including man. They are found in 

 both epidermal and dermal layers and, also, 

 particularly in the lower vertebrates, in 

 perineural and perivascular layers and in 

 the peritoneal lining of the coelomic wall. 

 Controversy over the site of origin of these 

 highly specialized, branched cells has existed 

 for nearly a century. Although many hy- 

 potheses have been advanced, the most gen- 

 erally accepted view up until 1934 held that 

 the pigment cells were modified connective 

 tissue cells, or at least originated from the 

 mesoderm. The first suggestion that the 

 neural crest might be the source of these 

 cells is traceable to the obs-^rvations of Bor- 

 cea ('09) in teleosts and Weidenreich ('12) 

 in amphibians. Within this same period Har- 

 rison ('10) in his studies on nerve regenera- 

 tion in vitro fovmd pigment cells in cviltures 

 of frog spinal cord and clearly predicted 

 their origin from the neural crest. Many 

 years elapsed before this problem received 

 serious attention and was systematically in- 

 vestigated by experimental methods (Dn- 

 Shane, '35). The proof now firmly estab- 

 lished by numerous workers for many species 

 of amphibians (see reviews by DuShane. 



