AN INTRODUCTION TO CELLS
Domestic animals, like all others, are made up
of an amazing aggregation of simple units - the
living cells. Each cell exhibits a degree of
independence from all others, meaning that it is
a living unit in its own right. The real beauty of
function of the complex animal, however,
results from the cells coexisting as a
community. Individual cells support each other's
needs, communicate with and protect each
other. In some situations, specialized
populations of cells serve to protect the whole
animal by seeking out and destroying other,
abnormal cells. By cooperating with each other,
a mixture of cells in a tissue can perform tasks
that would be impossible for individual cells in
These functions of the basic units of living
matter must be understood to appreciate how the
whole animal functions, both in health and in
disease. At this time, some descriptive
information about cells is both useful and
important; much greater detail will follow in
At the simplest level, each cell consists of a
mass of jelly-like material called
which is made up of an inner
. At the outer surface, the
cytoplasm is contained within a cell membrane,
. There is no cell wall in animal
cells. In most cells, numerous specialized
are found within the
cytoplasm (Figure 2-1). In most cases the
powerful resolving power of the electron
microscope is needed to observe intracellular
structures smaller than the nucleus. The
organelles are also usually bounded by
membranes, and their location within the three-
dimensional space of the cell interior is
determined by their attachments to a delicate
intracellular latticework called the
Cells are alive and dynamic but are usually
killed in preparation for microscopic
The generalized cell.
The cell is bounded by a
plasma membrane (PM), which may be specialized at the apical
(ap) surface and possess microvilli (Mv).
At the base of the
microvilli may be pinocytotic vesicles (pv).
On the lateral
margins, the membrane may form junctions (j) with adjacent
cells. The cell may be associated at its basal surface with a
basement membrane (bm), an extracellular supporting material.
The nucleus (N), containing a nucleolus (nu) and chromatin
(ch), is bounded by a nuclear membrane (nm) with characteristic
nuclear pores (np).
Membranous organelles extending into the
cytoplasm include smooth endoplasmic reticulum (ER) and
rough endoplasmic reticulum (RER).
The latter is decorated
with ribosomes (R) that may also be free in the cytoplasm.
Golgi (G), a layered vesicular membrane system, gives rise to
membrane-enclosed granules (gr).
These are usually secretory
granules that can accumulate in the cytoplasm awaiting a
Mitochondria (M) vary in number depending
upon the metabolic activity level of the cell; they are involved in
2007 version – page 4