M. Y.
MANGRULKAR
183
arrived at by different authors would be difficult
to reconcile.
From the standpoint of histology, melanoblast
are suggested to be ' highly differentiated connect-
tive tissue cells which arise early in the ontogenic
development from mesenchymal cells' [Maximow
and Bloom, 1935]. Two types of melanoblast
are recognized in the skin, distinct from the dermal
melanophores which are situated in the superfi-
cial layers of the dermis, in large numbers at places
(e.g. around the anus), and which contain coarse
uneven pigment-granules not formed by them,
but perhaps received from the epidermis [Post,
1894; Ewing, 1934]. Of the true melanoblasts,
elaborating and containing small and uniform
pigment granules and giving a positive 'dopa'
reaction, one set consists of branched cells—epi-
dermal melanoblasts*—situated just under the
basal epidermal cells between which their out-
growths project, while the second set consists
of dermal melanoblasts and is known to occur in
the skin of apes and in the scaral region of the new-
born Mongol ('Mongolian Spots', 'Blue Naevi').
The melanin in the basal layer of the epidermis
would appear to be derived from epidermal
melanoblasts, for the former do not give a posi-
tive 'dopa' reaction. Yet, a large number of
workers on the subject holds that they are pig-
ment-producing cells and hence true melano-
blasts. Such an opinion was held as early as
1894 by Post and subsequently by a long line.
of observers including Cowdry [1932, 1934].
The conciliatory suggestion that the epidermal
'dopa'—positive melanoblasts are derivatives of
the basal cells seems unlikely in view of Bloch's
observations on foetal epidermis which indicate
that the former are migrated mesoblastic chromato-
phores and that they give rise to the tactile cells
of Markel-Ranvier's corpuscles and probably
constitute the melanin-producing component of
pigmented naevi which, according to some
authors, develop from the end-organs of the sen-
sory nerves of the skin (corpuscles of Meissner
and of Markel-Ranvier).
Pigmented cells ['Melanophores' Maximow and
Bloom, 1935; pigment-producing true melano-
blasts, Ewing, 1934 ; and Arey, 1932] are also
present in the choroid of the eye—one of the sites
of melanomata in man—and in certain positions
in association with the nervous system. Accord-
ing to Ewing [1934], 'the mesoblastic origin of
the choroidal chromatophores must be conceded',
and Arey [1932] is of opinion that these cells are
' treumelanoblasts and can revive this potentiality
when stimulated pathologically '. Recorded cases
in human beings, however, tend to indicate that
melanomata of the eye may arise not only from
the choroidal elements but also from the pig-
mented retinal epithelium [Ewing, 1934].
Ewing [1934] declares that melanomata are
derived 'from a specific mesoblastic cell, the chro-
matophore, probably also from tactile cells lying
in the epidermis and probably also from nerve
cells in the derma' and thus summarizes the posi-
tion, obviously uncertain, regarding the histogene-
sis of melanomata. In determining the origin of
the melanoblast various workers have concen-
trated upon the congenital naevus a potential
fore-runner of melanoma in man. This has,
naturally, not been possible in animals wherein
naevi are not so common and their behaviour as
melanoma-producers is not so well understood.
Jaeger [1909] in 63 autopsies on grey horses
failed to discover pigmented naevi of the
skin.
The histology of the naevus often suggests an
epidermal rather than a mesodermal derivation.
Unna [1893] suggested, on strong evidence,
the epidermal origin of naevus cells by
metaplasia of groups of epidermal cells a view which
has attracted several followers. Among them,
Dawson [1925] holds that the basal epidermal cells
are true melanoblasts which may migrate into the
dermis to form naevus cells, since bridging strands
of cells between the naevus cells and the. epidermis
have been repeatedly demonstrated. That ecto-
dermal cells are capable of producing mealnin is
further supported by the pigmented cells of the
retina and by the occurrence of undoubted mela-
notic epitheliomata in man [melanotic whitlow,
Hutchinson 1886] and animals [M'Fadyean, 1890;
McGowan, 1928 ; Jackson, 1936]. It must be
admitted, however, that such tumours are rare.
While Post [1894] claims pigment formation in
mammals as the almost exclusive function of
epithelial cells, and while as histological studies
of naevi and early melanomata point towards the
conclusion that 'epidermal pigment arises in the
epithelial cells' [Ewing 1934], there are others
[Ehrmann, 1885 ; Aeby, 1885] who consider that
epithelial tissue can form no pigment but may
acquire it from wandering pigmented connective
tissue cells.
Others [Pini, 1902; Rieke, 1903; Jaeger, 1909]
look upon naevus cells as modified fibroblasts,
i.e., derived from mesodermal elements.
Pigment formation, at least in the lower mam-
mals, is recognized by many to be essentially a
function of specific mesoblastic cells, and it seems
certain that both in man and animals most, though
not all, melanomata are of mesodermal origin.
M'Fadyean [1933] interprets the characteristic
*These are often referred to as Langerhans' stellate or
dendritic cells. Maximow and Bloom [1935] apply the
term to altogether different structures in the epidermis
and doubt whether these are cells at all.