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Cancers are named according to the organ in which they originate.
Even if a cancer metastasizes to another part of the body,
it keeps its original name. Cancer names such as breast cancer,
brain cancer, lung cancer, skin cancer are examples. However,
cancer names may also be based on the type of tissue affected.
This section will introduce you to some basics regarding the
derivation of tissues, in the context of embryology, which
is the study of the development of an organism.
Derivation of Cells
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Human beings begin life as a single,
newly fertilized cell. Like every cell that contains
a nucleus, the fertilized cell holds all the instructions
for its growth and development. The characteristics
common to all living cells include the ability to reproduce,
exchange gases, move, react to external stimuli, and
create or utilize energy to perform their tasks.
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Shortly after the ovum or egg is fertilized, it divides to
form two cells. These two cells then divide to form a total
of four, these again divide to form eight, and so on. This
group of cells continues dividing; after nine days it attaches
to the wall of the uterus and becomes an embryo.
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About two weeks after conception, the
cells of the embryo continue to divide, changing their
shape and structure. This process is known as differentiation.
The cells arrange into distinct layers called germ layers:
an outer ectoderm and inner endoderm (entoderm). A third
embryonic layer, the mesoderm, develops between the
ectoderm and the
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endoderm. All the organs of the body develop or differentiate
in an orderly fashion from these three primary germ layers.
Derivation of Tissue
Cells that are similar in structure tend to group themselves
together and form tissues. A tissue, then, is composed of
a group of cells that are similar in structure and perform
one or more common functions. Some tissues contain intercellular
material which is very important in the performance of a particular
function belonging to that tissue.
The body tissues and organs develop from the three primary
germ layers that form during the growth process of the human
embryo.
The tissues derived from the ectoderm are: some epithelial
tissue (epidermis or outer layer of the skin, the lining for
all hollow organs which have cavities open to a surface covered
by epidermis), modified epidermal tissue (fingernails and
toenails, hair, glands of the skin), all nerve tissue, salivary
glands, and mucous glands of the nose and mouth.
In fact, epithelial tissue can be derived from either the
ectoderm or endoderm. The epithelial tissue derived from the
endoderm includes the epithelial lining of the digestive tract,
except at the open ends, and the epithelial lining of all
hollow structures formed as outpockets in the digestive tract.
This includes:
- The parenchyma of the liver including communicating or
connecting ducts.
- The lining of the pharynx and respiratory tract (except
the nose). This includes the lungs and the passageways leading
from the pharynx to the lungs.
- The epithelium of the bladder and urethra.
- Glands that form secretions in the digestive tract.
Epithelial tissue derived from ectoderm is generally squamous
epithelium; epithelial tissue derived from endoderm is essentially
glandular epithelium.
There are a variety of body tissues derived from the third
or middle primary germ layer known as the mesoderm. These
body tissues include:
- Muscles
- Fibrous tissue
- Bone and cartilage
- Fat or adipose tissue
- Blood and lymph vessels
- Blood cells
In the early embryo the first cavity that develops is the
coelomic cavity; this is derived from mesoderm. Parts of the
urinary and genital systems are derived as outpouchings of
the coelomic cavity. Later this coelomic cavity divides into
the pleural cavity and the pericardial cavity. The linings
of these cavities are composed of a single layer of cells
called mesothelium. A few epithelial cells are of mesodermal
origin, e.g. endometrium of the uterus, vaginal epithelium,
and mucosa of the bladder.
Endothelium derived from mesoderm lines the blood and lymphatic
vessels and the walls of the heart. In the capillaries where
the endothelium is covered only by a basement membrane, diffusion
takes place. It is surrounded elsewhere by supportive layers
of connective tissue and smooth muscle. This is necessary
because the endothelium is so thin that diffusion would occur
otherwise. Many authorities classify this endothelium as connective
tissue.
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