why do epithelial tissues lack an extracellular matrix
Answers
Epithelial tissues have many characteristics that distinguish
them from other tissue types
1. Cellularity. Epithelia are composed almost entirely of cells.
These cells are separated by a minimal amount of extracel-
lular material, mainly projections of their integral membrane proteins into the narrow spaces between the cells.
2. Specialized contacts. Adjacent epithelial cells are directly
joined at many points by special cell junctions.
3. Polarity. All epithelia have a free apical surface and an
attached basal surface. The structure and function of the
apical and basal surfaces differ, a characteristic called
polarity. The apical surface abuts the open space of a
cavity, tubule, gland, or hollow organ. The basal surface
lies on a thin supporting sheet, the basal lamina, which
is part of the basement membrane
4. Support by connective tissue. All epithelial sheets in
the body are supported by an underlying layer of connec-
tive tissue.
5. Avascular but innervated. Whereas most tissues in the
body are vascular (contain blood vessels), epithelium is
avascular (a-vas′ku-lar), meaning it lacks blood vessels.
Epithelial cells receive their nutrients from capillaries in
the underlying connective tissue. Although blood vessels
do not penetrate epithelial sheets, nerve endings do; that
is, epithelium is innervated.
6. Regeneration. Epithelial tissue has a high regenerative
capacity. Some epithelia are exposed to friction, and
their surface cells rub off. Others are destroyed by hos-
tile substances in the external environment such as bacte-
ria, acids, and smoke. As long as epithelial cells receive
adequate nutrition, they can replace lost cells quickly by
mitosis, cell division.