182
Melanomata in Domesticated Animals
warts growing usually on the sides of the trunk
or on the feet or prepuce. Recorded cases show
that severe generalized melanomatosis occasional-
ly occurs in the dog, involving the skin and in-
ternal organs, but at times excluding the liver.
Rarely, the central nervous system may also be
involved [Feldman, 1932]. Melanosis is seen in
the subcutis, lungs, liver, kidneys, etc., the pig-
ment being present in the tissue spaces or in
phagocytes.
Whether melanosis in the pig and dog is capable
of stimulating the growth of melanomata is not
clear, but such a possibility would appear to exist.
Jackson's cases [1936]—five canine and two por-
cine—"had much the same morphology as the
human melanotic skin tumours ranging from
naevi' to definite anaplastic neoplasms". It was
not possible, however, to classify them as melano-
carcinoma or melanosarcoma. Jackson's por-
cine cases were distinctly different from epithelio-
matoid melanomata of Duroc Jersey pigs, en-
countered by Caylor and Schlotthauer [1926].
Melanomata have been described in white goats
in South Africa and are known to affect mainly,
but not exclusively, the Angora goats. Their
incidence may be gauged by the fact that they
form approximately 40 per cent of all caprine
tumours at Onderstepoort [Jackson, 1936].
While regarding the caprine material as 'an un-
rivalled wealth of specimens of the study of what
are usually regarded as typical melanotic epithe-
liomas', Jackson refrains from definitely attri-
buting an epiblastic origin to them. Thomas
[1929] has classified these tumours as basal-cell
melanotic epitheliomata. In the list of melano-
mata at Onderstepoort are also included one
acanthoma of the horse and a basal-cell
epithelioma of the cat.
Melanomata have also been described from the
rabbit [Brown and Pearce, 1926], cat [Mulvey,
1906], and chicken [Goldberg, 1919; McGowan,
1928]. McGowan's cases, in the fowl, included
one affecting the ovary, which, unlike the rest
(mesoblastic) of his specimens, was considered by
him to be of epithelial origin.
Histogenesis. It is generally agreed that melano-
mata result from the unchecked proliferation of a
specific pigment-producing cell, the melanoblast,
which is responsible under normal conditions for
supplying pigment for physiological purposes. The
neoplastic (malignant) melanoblast is capable of
metastasizing in the same way as cells of other
malignant tumours.
The pigment, melanin, is not derived from the
blood but is a special metabolite of the cell and is
contained in or outside the nucleus. It is probably
formed by the action of an oxidase contained in
the melanoblast upon a melanogenic protein
substance resembling, or related to, tyrosin.
Bloch's 'dopa' reaction involving the use of an
extract of broad bean containing dioxyphenyl-
alanin is well known for demonstrating the fer-
ment. The substance is related to tyrosin,
tryptophane and adrenalin, and is similarly oxi-
dized to a dark brown pigment resembling mela-
nins. The reaction is given by certain cells of the
epidermis, blood and bonemarrow and by the cells
of non-pigmented melanomata. According to
MacCallum [1932], the ferment is not demonstrable
in the cells of the cutis, some of which (melano-
phores) may contain melanin, not elaborated by
them, but merely derived from the epidermal
melanoblasts. This, by the way, indicates the
passage of melanin from the epidermis to the
dermis.
The origin of melanins by the action, as suggest-
ed by the 'dopa' phenomenon, of an oxidase upon
an oxidizable chromogen, perhaps allied to tyro-
sin, is helpful in elucidating the bronzing of the
skin in Addison's disease wherein the precursor
(tyrosin) of adrenalin is said to be oxidized
in the skin to form a brown pigment [MacCallum,
1932]. Gessard [1902] detected the presence of
tyrosin and tyrosinase in equine melanoma.
Even so, the usual presence of sulphur in natural
melanins remains unexplained.
Oxidase is entirely absent from the skin and hair
of albinos or in leucodermic patches or white
hair; but when present in less amount may be
increased by the use of actinic light ('tanning').
The terms 'melanocarcinoma' and 'melanosar-
coma' are often used to indicate the histological
resemblance of the tumour to epithelioma and
sarcoma, respectively. The terms should, how-
ever, be discontinued as they are suggestive of
the histogenesis of the neoplasm, which is any-
thing but settled even today. Further even, from
the point of view of structure, the existence of
demonstrable intermediate stages convinces one at
once that the expressions are artificial. Some
melanomata of the human skin and nervous
system may resemble peritheliomata in their
microscopical structure. Feldman [1932] prefers
to classify melanomata simply as benign and
malignant.
The origin of the melanoblast is undetermined,
and a large number of hypotheses has been put
forward by various workers. The observations
made have been mainly based on factors such as
the development of colour function in the animal
kingdom, the structure of the naevus, the nature,
morphology and situation of pigmented cells and
their association with blood vessels and nerves.
It may be said at once that, unless the melano
blast has a multiple origin and perhaps an in-
constant morphology, the diverse conclusions