Page 50 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

32 Veterinary Histology of Domestic Mammals and Birds

Apoptosis can be triggered by specific factors (tumour Several epithelia exhibit specialised cell surface modi-
VetBooks.ir necrosis-factor, TNF) and by certain neurotransmitters, fications. These include enzymes such as hydrolases, ion
free radicals, oxidants and UV radiation. In addition to channels and carrier proteins (e.g. for glucose transport).
these external factors, cells contain genes and proteins (e.g. Surface modifications may be temporary or permanent,
Bcl-2 family) that have a regulatory role in apoptosis (e.g. depending on the function of the cell.
Bax [Bcl-associated X] protein is pro-apoptotic). Sac-like invaginations of the plasmalemma occur as
Apoptosis is a physiological process. During embry- impermanent, variable structures. These are associated
onic development of tissues and organs, it serves to with the uptake of material by the cell (phagocytosis or
eliminate cells that are no longer required. Mammary pinocytosis). Cellular evaginations, including micro-
gland involution also occurs through apoptosis. villi (see below) and microplicae, extend in the opposite
direction. These serve to enhance absorption by the cell.
Cell surface specialisations Permanent cell surface modifications (Figures 1.42 to 1.46)
With the exception of free cells (e.g. blood cells), the include:
cells of the body are interconnected to form organs and
tissues. As a result of cell-to-cell connections, most cells · microvilli: generally uniform, mostly non-motile
of the body have a structural and functional relationship cytoplasmic processes,
with neighbouring cells. The existence of several different · cilia (kinocilia): long, motile cytoplasmic processes
types of cellular connections reflects the diversity of cel- and
lular functions. · stereocilia: particularly long, often secretory
In the case of epithelial cells, specialised structures are cytoplasmic process (also described as very long
also present on the apical cell surface. Cell surface speciali- microvilli).
sations are subdivided into:

· free (apical) surface structures, Microvilli
· lateral surface structures and Microvilli are long, finger-like surface projections. They
· basal surface structures. are generally consistent in length (see below) and are
covered by a well-developed glycocalyx (Figures 1.42 and
1.43). Intestinal microvilli are richly endowed with sugar-
Apical surface specialisations cleaving enzymes. Microvilli are involved in absorption;
The free surface of epithelial cells is lined by an electron
microscopically amorphous layer of proteins, glycopro-
teins and sugar residues referred to as the glycocalyx.
These proteins contribute to epithelial cell function by
acting as sites of cell recognition and adhesion.

1.42 Microvilli forming a brush border on the apical 1.43 Apical cell surface with brush border of microvilli
surface of a cell (x40,000). (schematic).

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