1. ROSTAMI ESTABRAGH,SAMIRA
DMD1J
The basal lamina
The basal lamina (lamina - layers), also known as the basement membrane, is a
specialised form of extracellular matrix. The basal lamina can be organised in three
ways:
1. it can surround cells (for example muscle fibres have a layer of basal lamina
around them);
2. it lies underneath sheets of epithelial cells
3. it separates two sheets of cells, such as the endothelial cells of blood vessels and
epithelial cells of another tissue. This type of arrangement is found in the kidney
glomerulus, where the basal lamina acts as a permeability barrier or sieve.
Functions of the basal lamina.
The exact composition of the basal lamina varies between different types of cell. In
the kidney, the basal lamina acts as a molecular filter. At the neuromuscular
junction, the basal lamina that surrounds the muscle cells, separates the nerve cell
from the muscle cell at the synapse, and helps to regenerate the synapse after
injury, and helps to localise acetylcholine receptors.
The basal lamina provides support to the overlying epithelium, limits contact
between epithelial cells and the other cell types in the tissue and acts as a filter
allowing only water and small molecules to pass through. If the epithelial cells
become transformed (cancerous) and become 'malignant', they are able to break
through the basement membrane and invade the tissues beneath. This characteristic
is one used in the diagnosis of malignant epithelial tumors.

2. Components of the
basal lamina.
The basal
lamina consists of a
mixture of collagens,
laminin
(glycoprotein),
perlecan (heparan
sulphate
glycoprotein),
entactin
(glycoprotein).
These proteins can
bind to each other to
make a highly
crosslinked
extracellular matrix
as shown in this
diagram.
All epithelia have
a basal lamina which
lies between the cells
and the underlying
connective tissue.
This layer is so thin
that it is often
difficult to see with
conventional light
microscopy and is
usually only clearly
defined under the
electron microscope.

3. The basal
lamina helps to
attach and anchor the
cells to the
underlying
connective tissue.
Proteins (integrins
and proteglycans) in
the cell membranes
attach to proteins in
the basal lamina,
which in turn is
linked to
theextracellular
matrix of connective
tissue.
Viewed with the
electron microscope,
three distinct layers
of the basal lamina
can be described:
lamina lucida -
electron lucent (very
little staining in the
EM). This picture shows the arrangement of these three layers of
the basal lamina lies underneath an epithelial cell.
lamina densa -
electron dense. The integrins and proteoglycans in the cell membrane that
attach to proteins in the extracellular matrix/basal lamina
lamina reticularis - communicate with and signal to each other. Cells can
can be associated organise their own extracellular matrix, and the
with reticular fibres extracellular matrix in turn helps to organise cells. Changes
of the underlying in the integrins can mean that cells stop being adhesive, and
connective tissue. staying put, to moving away, or vice versa.

4. Disease:
Dystrophin is a glycoprotein in the plasma membrane of muscle cells, that binds to
laminin in the extracellular matrix. In muscular dystrophy, this protein is defective
or missing, and reduces the attachment of muscle cells to their basal lamina. This
reduced attachment results in muscle degeneration and muscle weakness.
This diagram shows part of the cuboidal epithelium in the photographs
opposite together with its basal lamina.
This picture shows a duct from the kidney, stained with alcian blue, where the
basement membrane can be seen more clearly, underlying the cuboidal epithelium.
See if you can identify the lumen of the duct, the simple cuboidal epithelium, and
the basement membrane