Page 45 - Anatomy and Physiology of Farm Animals, 8th Edition

30 / Anatomy and Physiology of Farm Animals

Table 2-1. Metric Linear Measurements
VetBooks.ir Unit Abbreviation Relationships 3 6 9 10

m
2
1 m = 10 cm, 10 mm, 10 μm, 10 nm, 10 Å
Meter
7
−2
8
Centimeter cm 1 cm = 10 m, 10 mm, 10 μm, 10 nm, 10 Å
4
1
−4
4
Micrometer (micron) μm 1 μm = 10 m, 10 cm, 10 mm, 10 nm, 10 Å
−6
3
−3
Nanometer nm 1 nm = 10 m, 10 cm, 10 mm, 10 μm, 10 Å
−6
−7
−9
−3
−7
−1
−4
−8
Angstrom Å 1 Å = 10 m, 10 cm, 10 mm, 10 μm, 10 nm
−10
among metric units of measurement used few other tissues. In these instances, living
for microscopy. (For example, 1 μm is one‐ cells can be observed directly. This tech­
thousandth of a millimeter, and there are nique is useful for the study of fluid samples,
about 25 mm in 1 inch; thus, approximately including blood, urine, and milk. Specific
1000 cells, each 25 μm in diameter, could cells or tissues may also be taken from a
be lined up between the 1‐ and 2‐inch living animal and grown on artificial
marks of a ruler.) Sizes of cells vary consid­ medium by tissue culture. These cells may
erably from one type of cell to another, then be studied, even under high magnifi­
but with the exception of the yolks of birds’ cation, in a living state. This process often
eggs (which are considered single cells), requires separation (enzymatic dissocia­
the distance from the center of the cell tion) of the tissue into individual cells, thus
to some portion of the cell membrane these cells are not in their “natural” state.
(surface of the cell) is seldom more than a Replication and generation of new cells
few micrometers. The outer cell membrane is an important process in vivo (in the
is also thin (7 to 10 nm). Regardless of its animal). This proliferative capacity is often
composition, a membrane of this dimen­ lost in dissociated cells in vitro (in a test
sion can have little tensile strength; this is tube or dish) and these primary cells will
another reason cells must be small. have a finite period of time for which they
The uniformly small size of cells and the will “live” in culture, whereas some cell
much smaller sizes of structures within types may be immortal and proliferate
the cell have made effective study of cells indefinitely in culture. The immortal
challenging. As noted earlier, the existence nature of cancer cells has allowed scien­
of cells was not confirmed before the micro­ tists the opportunity to make great strides
scope was invented. Details of the actual in human and animal health that would
structure of the various parts of cells were otherwise not have been possible.
not known with any degree of certainty As with the study of cells in tissue culture,
until after the development of the electron the study of cells from tissue samples
microscope. The study of gross anatomy usually requires some degree of manipula­
goes back several centuries, but understand­ tion, so that what is actually seen with the
ing of the finer structure of the animal microscope may bear little resemblance to
body awaited more recent technological the living cell. A typical treatment of tissue
developments. before it can be examined with a light
microscope includes the following:
Light Microscopy 1. Fixation with an agent, such as an
aldehyde, that will cross‐link the tissue
Some cells are in tissues that are thin enough proteins and prevent further changes in
to be illuminated from one side and observed the tissue, such as autolysis and bacterial
with a microscope from the opposite side. degradation. Alternatively, tissue may
This is true of the web of the foot of the be frozen in liquid nitrogen to prevent
frog, the mesentery of the intestine, and a such degenerative changes.