60 THE PINEAL ORGAN 



ficial cells, the pigmentation becomes less pronounced, so that only a 

 diffuse colouring of the epidermis is present in the stratum corneum. 

 Besides the pigmented basal cells of the epidermis, large multipolar 

 pigment-cells called melanoblasts are found beneath the epidermis in 

 the superficial layers of the true skin. Their pigmented processes may 

 extend for a considerable distance outwards between the cells of the 

 epidermis, and the cells are characterized by a special staining reaction 

 with the " dopa " reagent — 3.4-dioxyphenylanilin ' — while the pigment 

 cells of the stratum germinativum or Malpighian layer, and the dermal 

 chromatophores do not react. It is generally believed that the melano- 

 blasts are modified epidermal cells, that they elaborate the melanin, and 

 that this passes from them into the cells of the epidermis. 



In addition to the melanoblasts, cells bearing pigment and called 



1 The Origin of Melanin. — Bloch, in 1917, demonstrated that the cells of pig- 

 mented regions contain a specific intracellular oxidase. He isolated from the 

 embryo of the broad bean a substance, 3.4 — dihydroxyphenylanilin, which he 

 called " dopa," and showed that it was readily changed by this oxidase into 

 melanin. When this substance is added to the epidermal cells of skin in frozen 

 formalin-fixed sections granules of melanin are formed — the " dopa " reaction. 

 Bloch concluded that the colourless " mother-substance," or melanogen, is almost 

 certainly closely related or identical with " dopa." 



A large number of groupings in the protein molecule form coloured products 

 on oxidation — tyrosine, tryptophane, phenylaniline — and it seems obvious that 

 melanin is formed as an end-product from one of these chromogen groups. The 

 colourless mother-substance is brought to the cell by the blood-stream ; here it 

 meets the " dopa-oxidase " and is turned into the coloured pigment melanin. 



Melanin is closely allied to adrenaline, and it is probable that the two substances 

 are derived from the same precursors and form alternative end-products in 

 metabolism. When the cells are stimulated to proliferate, so that there is an 

 increase of the oxidative ferment, a pigmented tumour is the result or a general 

 melanomatosis. When owing to cachexia or some dyscrasia of the adrenals these 

 glands fail to utilize the chromogenic substance for the manufacture of adrenaline, 

 an increase of pigment is the result, as is seen in Addison's disease, the effects of 

 which are seen in the skin and even in the epithelial cells of the cornea ; and if 

 there is no enzyme at all present, pigment cannot be formed and the condition of 

 albinism results. 



There is considerable difference of opinion with regard to the kind of cell in 

 which pigment arises, thus : Recklinghausen believed that the pigmented cells 

 of nsevi arise from the endothelium of the vessels. Unna held that pigmented 

 cells were confined to the epithelia, and that the uveal pigment migrates from the 

 retinal epithelium. Also, that malignant pigmented tumours are carcinomatous. 

 Kornfeld showed that in the skin of frog embryos the subepithelial pigment sheet 

 is formed by the migration of epithelial cells through the basement membrane 

 into the corium. Krompecher held that the epithelial cells not only became 

 morphologically similar to connective tissue elements, but also assumed their 

 function, producing collagenous fibrils, a change which he termed " desmoplasia." 

 Ribbert, on the contrary, contended that the pigmentation depended on a 

 specialized connective tissue cell, the chromatophore, which was mesoblastic, 

 and maintained that pigmentation and the formation of pigmented tumours were 

 functions of the mesoderm. Pigment produced artificially by ultra-violet light 

 appears in the deeper germinal layers of the epidermis. (Sir Stewart Duke-Elder, 

 Extract from Recent Advances in Opthalmology , J. and A. Churchill, 1934, London.) 



