Agriculture Reference
In-Depth Information
5
Heredity
. Some strains of animals are prone to develop
tumours, being inherited in the germ line.
6
Parasites
, for example,
Gongylonema neoplasticum
,
can produce gastric carcinoma in the rat and
Cysticercus fasciolaris
(cystic stage of
Taenia taeniae-
formis
) sarcoma in rat livers.
7
Ageing
. Tumours occur more commonly in older
animals. Some, however, like malignant lymphoma,
tend to affect young animals more often. Since most
food animals are slaughtered at a young age, tumour
formation is encountered relatively infrequently in
them. The increased incidence with age may be a
reflection of the longer period of exposure to carcino-
gens or to innate changes in metabolism or both.
death, and some mesotheliomas are flattened in appear-
ance. Much depends on the invasive properties (which
vary in degree) and location, type of tissue involved, type
of tumour, etc.
Structure
. All forms of tumours (benign and malig-
nant) consist of a parenchyma of neoplastic cells and a
supporting stroma or matrix of connective tissue, blood
vessels and sometimes lymphatics. The parenchyma is
responsible for the functioning of the tumour and deter-
mines its classification.
Classes of tumours
Benign tumours
usually grow slowly by expansion and by
compressing or displacing adjacent tissues. They remain
localised (although several may appear in an area) and
are often spherical in shape with a fibrous capsule. They
are thus capable of excision. There is a tendency for
benign tumours to resemble the original cells more than
malignant tumours.
Malignant tumours
(cancers) in contrast grow more
rapidly by invasion and destruction of the tissues they
invade. Although the resemblance to original cells is usual,
some types differ in cell structure. Pleomorphism (wide
variety in cell morphology and staining) is more common
with malignant tumours which contain more abnormal
products of mitosis (cell division) such as broken chromo-
somes, huge, dark staining nucleoli and various protein
and polypeptide products. They have a great tendency to
metastasise, reaching distant sites in the body via the
bloodstream, lymphatics and across tissue spaces.
Benign and malignant tumours occurring in endocrine
glands may elaborate hormones, for example, insulin in
the pancreas and corticosteroids in the adrenal gland.
Gene mutation (genetic damage) is the basic charac-
teristic in all cases of neoplasia and is associated with
the activity of proto-oncogenes and oncogenes, the for-
mer non-transforming genes being converted into trans-
forming cellular oncogenes under special circumstances
to initiate the neoplastic events. Fundamental to all
tumour formation is the total lack of response to normal
growth controls.
Effect on host
Malignant tumours are more life threatening than benign
tumours. The latter can occur in animals without any
untoward effects, for example, papillomata. In both types,
much depends on their location and proximity to vital
structures: obstruction of blood vessels, intestine, tra-
chea, oesophagus, bile ducts, etc. may result in addition
to ulceration of natural surfaces with haemorrhage and
secondary infection. Tumours themselves, being well
vascularised, can bleed profusely, even causing anaemia.
Hormone production can result in excess insulin from
the pancreas with hypoglycaemia and overproduction of
corticosteroids from the adrenal gland to cause sodium
retention and hypertension.
The usual accompaniment of
malignant tumours
with
their metastatic effect is severe loss of condition with
anaemia, anorexia, weakness and prostration. In some
cases, however, for example, lymphosarcoma, malig-
nancy does not appear to cause undue loss of weight.
Causes of tumours
Although the exact cause of many neoplasms is unknown,
several specific factors, environmental and genetic, are
involved:
1
Physical carcinogens
, for example, ionising and ultra-
violet radiation, chronic irritation, etc., cause DNA
damage.
2
Chemical carcinogens
such as benzene, vinyl chloride,
arsenic, chromium, β-naphthylamine, etc. also lead to
DNA damage.
3
Viruses
. Tumour-inducing (oncogenic) viruses which
cause tumours in animals include the DNA viruses of
the adenovirus, herpesvirus, papovavirus and poxvi-
rus groups and the RNA retroviruses.
4
Hormones
. Disordered hormone metabolism (pitui-
tary, ovary, thyroid, adrenal gland, parathyroid,
pancreas) may produce neoplasia (adenomas and
carcinomas), especially in man.
Nomenclature of neoplasms
In general, the parenchyma (the essential cellular ele-
ments) of the neoplasm determines its behaviour and its
name. However, there are serious inconsistencies in the
nomenclature of tumours.
Benign
tumours usually have the suffix -
oma
attached
to the cell type from which they originate, for example,
fibroma from fibrous tissue, osteoma from bone,
adenoma (Greek,
aden
, gland) from glandular tissue,
