Page 64 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Cell / 49

responsible for degrading lipids, alcohols, they are called basal bodies. A pair of
centrioles, the centrosome, also occurs in
and a variety of potentially toxic substances.
VetBooks.ir Hydrogen peroxide is produced within the all cells near the nucleus and organizes the
microtubules, which form the mitotic
peroxisome as a result of this enzymatic
action. Other enzymes within the peroxi­ spindle during cell division.
some rapidly degrade the potentially toxic
hydrogen peroxide to protect the cell.
Microtubules, intermediate filaments, Nucleus
and microfilaments are rod‐like organelles
that make up the cytoskeleton, which Structure of the Nucleus
primarily functions to determine the shape
of the cell and assist with cell movement. The nucleus contains the genetic material
Microtubules are scattered throughout the of the cell encoded in molecules of DNA.
cytoplasm in most cells and are the largest With light microscopy, DNA and its asso­
and most rigid of the three cytoskeletal ciated proteins are seen as a more diffusely
components (Fig. 2‐20). Microtubules are staining chromatin in the nondividing
spindle fibers in cell division, motile ele­ cell and as chromosomes in the dividing
ments in cilia, and assisters of transport of cell. The nuclei of somatic cells contain the
molecules within some cells, such as in information necessary for determining
the processes of neurons (nerve cells). the form and structure of new cells, and the
Intermediate filaments are primarily found nuclei of sex cells contain the information
in association with specialized cell‐to‐cell necessary to determine the characteristics
junctions, such as desmosomes (Fig. 2‐9). of a new individual. The nucleoli consist
Microfilaments are thinner than micro­ largely of clustered DNA for ribosomal RNA
tubules, but they make up most of the transcription, processing and ribosome
cytoskeleton. Microfilaments are composed assembly; the nucleoli are seen as densely
of actin, a protein involved in cell move­ staining spherical bodies in the nucleus
ment and muscle contraction. (Figs. 1‐2 and 2‐7).
The centriole is a short cylinder about The nuclear envelope (Figs. 2‐7 and
0.2 μm wide and 0.4 μm long. Centrioles, 2‐19), which surrounds the cell nucleus, is
composed of nine triplets of microtubules, composed of two distinct membranes
usually occur at the bases of cilia, where separated by about 20 nm. The outer mem­
brane is continuous with the endoplasmic
reticulum. Pores (small gaps or interrup­
tions) in the nuclear envelope permit
exchange between the protoplasm of the
nucleus (nucleoplasm) and the cytoplasm
outside the nucleus, including the move­
ment of RNA synthesized in the nucleus
out into the cytoplasm.
The functional activity and the contin­
ued life of the cell depend on the presence
and functional integrity of a nucleus. A cell
from which the nucleus has been removed
(enucleated) gradually ceases activity,
atrophies, and finally dies. However, if the
nucleus is replaced with a nucleus from a
Figure 2-20. Microtubules in axons of neuro­ cell from the same species prior to irre­
secretory neurons. Source: Dellmann and Eurell, versible atrophy, function of the cell can be
1998. Reproduced with permission of John Wiley restored. The only cells in higher animals
& Sons, Inc. that do not have nuclei are mature red