Section VII 



CHAPTER 8 



Skin and Its Derivatives'" 



MARY E. RAWLES 



INTRODUCTION 



Although mvich information is available on 

 the detailed histological and physiological 

 characteristics of the adult skin and its 

 numerous derivatives in the vertebrates, com- 

 paratively little attention has been given to 

 the embryological origin of these character- 

 istics. The purpose of this account is, there- 

 fore, to analyze the interrelationship and 

 interaction of the ectoderm and mesoderm, 

 the two primordial tissue components which 

 unite in early development to form the com- 

 posite structure, the skin and its diverse 

 types of derivatives. Special attention will 

 be given to (1) the causal relations and 

 interactions involved in the differentiation 

 of the dermal and epidermal components, 

 (2) regional specialization of the skin with 

 particular reference to the role of the meso- 

 derm in epidermal specialization, and (3) 

 integumentary patterns, structural and pig- 

 mentary. 



ORIGIN OF THE SKIN 



Early Beginnings of an Integrated System. 



Embryologically the skin of all vertebrates 

 is a composite structure formed by the join- 

 ing of two main contributions from separate 

 sources, the ectoderm and the mesoderm. 

 As a result of the formative movements of 

 cells in gastrulation, prospective mesoderm 

 is brought in contact with the inner sur- 

 face of the epidermal portion of the prospec- 

 tive skin ectodei'm. During this process the 

 mesoderm appears to acquire a positive "af- 

 finity" for ectoderm, for prior to gastrula- 

 tion prospective mesoderm resists fusion with 

 ectoderm (Holtfreter, '39). This union be- 

 tween the two is probably the resultant of 

 progressive changes in both ectoderm and 

 mesoderm and is not to be regarded simply 



* The writer is indebted to her husband. Mr. 

 John S. Spurbeck, for the preparation of the illustra- 

 tions. 



as an operation of chance. The embryonic 

 layers are presumably held together at first 

 by complementary forces residing directly 

 in the naked contact surfaces of their con- 

 stituent cells. Sooner or later these primary 

 affinities are superseded by more permanent 

 vmions in the form of "cementing tissues" 

 or basement membranes (see Weiss, '47). 

 Thus the embryonic components enter into 

 an intimate union at the surface of the body 

 and behave afterwards as an integrated 

 system. 



Source Material of the Epidermis and Der- 

 mis. The entire surface ectoderm between 

 the union of the neural folds in the median 

 dorsal line and the median ventral line of 

 the embryo may be regarded roughly as 

 prospective skin epidermis. By means of 

 vital staining and other techniques its ori- 

 gin has been traced to a surface area in the 

 early gastrula in the amphibians and in 

 equivalent stages in the chick (Vogt, '25, '29; 

 Pasteels, '37). Through growth and move- 

 ments of the skin ectoderm and the accom- 

 panying gastrulation processes, the entire 

 embryo is covered by ectoderm at the late 

 gastrula stage (amphibian) or later stages 

 (chick). Strictly speaking, the prospective 

 fate of the skin ectoderm is not entirely epi- 

 dermal, for in certain loci its development 

 is modified by induction in specific direc- 

 tions, as for instance into lens and con- 

 junctiva over the primordium of the eye- 

 cup, oral epithelium and parts of teeth in 

 the mouth, inner ear in the region of the 

 myelencephalon, etc. Nevertheless, the ecto- 

 derm in such loci is capable of forming epi- 

 dermis when grafted to atypical positions, 

 e.g., mouth ectoderm grafted to trunk forms 

 epidermis (Stroer, '33). In a balanced physio- 

 logical salt solution explanted ectodermal 

 cells merely undergo proliferation and form 

 only epidermis (Holtfreter, '31); and the 

 ectoderm of exo-gastrulae remains a wrin- 

 kled mass of epidermal cells (Holtfreter, 

 '33a, b). Furthermore, it is well known that 



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