Page 55 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

The cell (cellula) 37

Basal surface specialisations type III collagen of the basement membrane. This con-
VetBooks.ir The basal cell domain exhibits several modifications: nection is supported by transmembranous collagen type
XVII, via laminin and collagen type IV in the basal lamina.
· infolding of the basal cell membrane to increase sur-
face area, Basement membrane (membrana basalis)
· junctions that anchor epithelial cells to the underly- All epithelial cells rest upon a layer that can be demon-
ing extracellular matrix and strated with the light microscope using periodic-acid
· the basement membrane. Schiff staining or silver salt impregnation. In the context
of light microscopy, this layer is referred to as the basement
The basal surface of most fluid-transporting epithelial cells membrane.
is characterised by deep, irregular infoldings. These are Electron microscopic examination of the region occu-
particularly pronounced in the metabolically active cells pied by the basement membrane of epithelia reveals three
of the proximal and distal renal tubules and in the stri- layers:
ated (secretory) ducts of the salivary glands. Mitochondria
are usually associated with the folds, providing a source · lamina lucida (lamina rara interna),
of energy. · lamina basalis (basal lamina, lamina densa) and
Connections between the base of the cell and the extra- · lamina fibroreticularis.
cellular matrix strengthen the association between the
epithelium and the underlying connective tissue. These In some sources, the terms basement membrane and basal
include: lamina are used interchangeably. Throughout this text, the
material present between epithelial cells and underlying
· focal adhesions – anchor long cytoskeletal actin fila- connective tissue is referred to as the basal lamina, except
ment bundles to the basement membrane and in specific circumstances.
· hemidesmosomes – anchor intermediate filaments
of the cytoskeleton to the basement membrane; LAMINA LUCIDA
include transmembrane adhesion proteins of the The lamina lucida appears as a relatively light zone, up
integrin class. to 40 nm in width, between the base of the epithelium
and the lamina basalis. This zone contains the extracellu-
lar portions of cell adhesion proteins of the integrin class
Focal adhesions (fibronectin receptors). These transmembrane proteins are
By virtue of their actin filament component, focal adhe- connected to the cytoskeleton of the epithelial cell and,
sions play an important role in the dynamic processes via collagens and laminins, to the lamina fibroreticularis.
associated with epithelial cells (e.g. cell migration dur- Some evidence suggests that the lamina lucida may not be
ing wound healing). Within transmembrane connecting a true layer, instead representing an artifact of fixation in
zones, an attachment forms on the cytoplasmic side which epithelial cells have shrunk away from underlying
between transmembrane proteins (mainly integrins) macromolecules.
and actin-binding proteins (α-actinin, vinculin, talin, pax-
illin), and with regulatory proteins (e.g. tyrosine kinase). LAMINA BASALIS
Extracellularly, integrins are anchored to the basement The lamina basalis, or basal lamina, is a thin (50–150
membrane via glycoproteins (laminin and fibronectin), nm) filamentous layer of extracellular matrix between
thus binding the epithelial cell to the underlying connec- the base of the epithelium and the underlying connec-
tive tissue. tive tissue stroma (Figure 1.50). The filaments are woven
into a network, forming an electron dense layer (hence its
Hemidesmosomes alternative term lamina densa). At least four groups of
Hemidesmosomes are similar in structure to desmosomes. structural proteins are found in the lamina basalis includ-
They are found at the base of epithelia that are subjected ing collagens, proteoglycans, laminins and glycoproteins
to mechanical forces, serving to anchor the epithelial cells (entactin/nidogen, fibronectin).
to the basement membrane. These include the epithelium Type IV collagen is the predominant fibre type in the
of the skin, cornea, oral cavity, oesophagus, forestomachs basal lamina. In contrast to other forms of collagen, which
and vagina. are synthesised by fibrocytes, type IV collagen of the basal
The plaque found on the cytoplasmic side of hemides- lamina is produced by epithelial cells. An additional type
mosomes incorporates desmoplakin-like proteins of collagen (VII) serves to anchor the basal lamina to the
connected to cytoplasmic intermediate filaments. On the lamina fibroreticularis, with reinforcement from fibronec-
extracellular face, anchoring fibrils connect integrins with tin and other glycoproteins.

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