180
Melanomata in Domesticated Animals
decreasing, particularly after middle age, so that
much melanin remains unutilized. Under the
circumstances, according to van Dorssen [1930],
the cells containing and manufacturing melanin
undergo hypertrophy and hyperplasia, the pro-
cess ending up as an uncontrolled multiplication
of cells as in neoplasms. The frequency with
which primary melanomata arise near the root
of the tail is explained by M'Fadyean [1933] as
possibly due to decline in demand for melanin in
the long hairs of the tail. Whatever the cause
may be, the fact remains that the grey horse is
singularly predisposed to melanomatosis. There is
no such parallel in tumours of man or of animals,
even grey, except apparently in the case of the
grey mule and the white goat [Jackson, 1936]*
This predisposition must be inherited, as certain
records suggest (Virchow), [cited by Ewing 1934,
and Feldman 1932] and according to M'Fadyean
may have originated as a mutation.
The most common sites for primary melanomata
in the horse are the preineum and the ventral sur-
face of the tail (Plate XIV). As a rule, the tumours
are, in these situations, not single but multiple.
Less commonly, melanomata, apparently primary,
may occur elsewhere on the skin, e.g. on the head,
neck, scrotum, udder, sheath and limbs, and per-
haps in some cases in the deeper tissues and in-
ternal organs. One of the specimens in our col-
lection was recorded from beneath the tongue.
The tumours are not always malignant; however,
in sone equine cases, under the influence of some
unknown factor or factors, a benign growth may
suddenly assume malignant characters and
metastasize widely and rapidly, via the lympha-
tics, and later on, perhaps also via the blood,
infiltrating the regional lymph glands, serous
membranes and internal organs, even including
the brain, nerves, blood vessels, bones and heart;
somewhat rapid cachexia follows, causing death
or rendering destruction more economical.
Several workers consider every melanoma as a
potentially malignant tumour and, in medical
pathology, every pigmented mole, naevus or wart
as a potential source of melanoma.
The tumours are more or less pigmented. The
pigment may vary in different tumours, or in
different parts of the same tumour, or within a
group of metastatic growths.
The cells of deeply pigmented tumours are
usually large in size and may occasionally assume,
by fusion or independent nucleus division, the
appearance of giant-cells. The nucleus is small
pale, more or less oval and usually excentrically
situated; altogether, the picture suggests that the
nucleus has undergone hypochromatosis and has
been displaced by an excessive accumulation of
melanin. The aggregation of pigment must
obviously be responsible for the large size of the
cell. In extremely pigmented tumours, the cells,
in depigmented sections (Plate XV, fig. 1), resemble
some what the cells of sebaceous glands and appear
to have lost their staining affinities and perhaps
their normal function. These points are brought
home at once if one compares the picture presented
by melanoblasts of a malignant growth (vide
infra). Excess of pigment must naturally escape
outside the tumour cells to be taken up by phago-
cytes, or deposited in the surrounding connective
tissue.
Different from the above picture is that of a
malignant melanoma. The cells are smaller,
often not so deeply pigmented, or at places even
unpigmented. The nucleus and cytoplasm retain
their normal staining properties. The difference
between these cells and those described above
appears essentially to be one of physiological
differentiation, the cells of the malignant melanoma
exhibiting the tendency towards multiplication
rather than towards pigment-formation. Mitotic
figures are, therefore, easily seen (Plate XV, fig. 2).
The cells tend to be rounded, polyhedral or spindle-
shaped and may or may not show an alveolar
arrangement.
It should be borne in mind, however, that it is
difficult to assess the degree of pigmentation either
by the naked eye or microscopically and that
tumours described as deeply pigmented may pos-
sess malignant propensities. Thus, malignancy de-
pends not on the amount of pigment but on the
presence of a fair proportion of proliferative
rather than melanin-producing melanoblasts. It
is reasonable to suppose that this proportion
bears a direct relationship to the degree of
malignancy.
Local connective tissue response appears to be
relatively delayed in melanomata and a true
stroma may be absent, at least in the early stage
of the growth. Deeply pigmented, clinically
benign tumours of the skin contain at first much of
the local connective tissue. Histologically, such
tumours only show accumulations of melano-
blasts lying in the meshes of a connective tissue
network. The pigmented areas, having at first a
diffuse border, gradually become well separated
from the surrounding tissue as a rounded focus, the
adjacent tissue more or less stimulating a capsule.
In the case of malignant tumours, signs of inva-
sion of the surrounding tissue (Plate XV, fig. 3) and
of metastasis can be observed without much
difficulty also; in such tumours the connective
*One may, however, note that MacCallum [1932] dis-
covered ten melanomata, all in white men, in 12,000
autopsices wherein the proportion of Negroes was high.
Ewing [1934] feels that the rare incidence of the tumour
in Negroes may be due to difficulty in diagnosis.