Page 50 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Cell / 35

called stereocilia, are irregular branched
extensions of the cell cytoplasm that are
VetBooks.ir not motile.

Motile cilia (kinocilia) are complex
elongated, fingerlike projections from cell
surfaces found in areas where material is
moved past the surface, as in the linings
of the trachea and the uterine tubes.
Each cilium is associated with a basal body
that resembles a centriole normally seen in
the cytoplasm of all cells.

Intercellular Contact and Adhesion

All vertebrates develop from division of a
single cell, the fertilized egg. Unicellular
organisms also develop by division of a
single parent cell. When the parent cell of a
unicellular organism divides, the resulting
daughter cells each go their own separate
way. In contrast, the daughter cells of the
fertilized ovum of a multicellular animal
stay together and eventually differentiate Figure 2-9. Transmission electron micrograph
of two desmosomes connecting the membranes of
into cells making up different tissues, and two cells. Source: Dellmann and Eurell, 1998.
eventually another animal. Reproduced with permission of John Wiley &
The ability of multiple individual cells Sons, Inc.
to remain together and function as a
tissue or organ depends on local modi­
fications of the outer cell membrane. In or as a more extensive belt‐like circle
some cases, these modifications simply surrounding cells.
physically connect one cell to another. In Tight junctions appear as an area or
other cases, these modifications connect zone where the plasma membranes of two
cells and form a passageway for exchanges adjacent cells immediately adhere to each
between them. The cell membrane modi­ other. These are often found just below
fications entail focal accumulations of the free surface of epithelial cells. Each
specific membrane proteins termed tight junction passes completely around
cell adhesion molecules. The areas of the periphery of the cell at the same level.
cell membranes involved in intercellular Tight junctions restrict the movement of
contact and adhesion were named when water or dissolved materials into the space
they were initially examined by light and between adjacent cells.
electron microscopy. Gap junctions are formed by membrane
In electron micrographs, desmosomes proteins that extend between adjacent cells
appear as local thickenings of adjacent to form a passage for exchange of small
plasma membranes, with tiny fibrils radi­ molecules and ions (Fig. 2‐10). The exchange
ating from the thickening into the cyto­ of ions permits one cell to affect the elec­
plasm of the cell (Fig. 2‐9). Desmosomes trical activity of the adjacent cell. These
tightly bind adjacent cells by the inter­ types of exchanges have special functional
actions between membrane proteins importance in cardiac muscle and certain
extending out from the surface of the cell types of smooth muscle in the gastrointes­
membranes. They are seen as a single site tinal tract.