BASEMENT MEMBRANE ZONE, HEMIDESMOSOME, AND DESMOSOME
The basement membrane zone (BMZ) of the epidermis is a beautiful and complex structure and a marvel of biological engineering. The zone acts to attach the overlying epidermis to the underlying stromal tissue, in this case the papillary dermis, which is made predominantly of collagen bundles. A plethora of unique and specialized proteins play critical roles in the proper functioning of the BMZ. Any defect or abnormal antibody that can cause disruption of the normal architecture can result in fracturing of the BMZ and blister formation.
The BMZ can
be appreciated on routine hematoxylin and eosin (H&E) staining as an
eosinophilic band below the basilar keratinocytes. The components of the BMZ
are produced in two locations: the epidermal keratinocyte and the dermal
fibroblast. These cells act to produce the required proteins in the correct
ratio to maintain a functional basement membrane. The basement membrane’s most
important function is to keep the epidermis firmly attached to the underlying
dermis. This is necessary for life. This specialized structure also acts to
encourage migration of cells and repair of the epidermal-dermal barrier after
trauma. Many other critical processes and physiological roles depend on the
proper functioning of the BMZ, including permeability of water and other
chemical substrates, proteins, and cellular elements. The BMZ is a highly
organized structure that is consistent from person to person.
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| BASEMENT MEMBRANE ZONE AND HEMIDESMOSOME |
The
structure of the BMZ can be subdivided into individual compartments for study,
with the under standing that the entire unit functions as one. These are the
epidermal basilar cell cytoskeleton, hemidesmosome, lamina lucida, lamina
densa, and sublamina papillary dermis. Each of these components is made up of
unique proteins that act in harmony to preserve the functional role of the BMZ.
The basilar keratinocytes contain intracellular cytoskeleton components made of
keratin intermediate filaments, predominantly keratin 5 and keratin 14. The
keratin intermediate filaments are interwoven into the hemidesmosomal plaque to
firmly adhere the basilar cell to the hemidesmosome.
The keratin
intermediate filaments interact with bullous pemphigoid antigen 1 (BP230) and
plectin. These two proteins are the main components of the hemidesmosomal
plaque. Plectin and BP230 are bound tightly together. Plectin and bullous
pemphigoid antibody 1 also bind to the integrin class of proteins and to
bullous pemphigoid antigen 2 (BP180). Integrins and BP180 are transcellular
proteins that bind to the intra-cellular molecules, plectin and BP230; they
also extend out from the basilar keratinocyte and interact with the laminin 5
and collagen IV molecules in the lamina lucida and lamina densa.
The lamina
lucida is so named because of its trans- lucent appearance on electron
microscopy. In comparison, the lamina densa is an electron-dense region that
lies just below the lamina lucida. The lamina lucida is composed of the
transversing parts of the integrin and BP180 proteins. These two molecules
attach to the laminin class of proteins in the lamina densa. The lamina lucida
is considered to be the weakest part of the BMZ, and it is the blister plane in
suction blisters, junctional epidermolysis bullosa, and salt-split skin. The
lamina densa is composed of a latticework of type IV collagen. Type IV collagen
is found only in the lamina
densa. It is unique in that it retains its globular regions on either end.
These form attachments to other type IV collagen molecules to create the
lattice. Collagen type IV binds strongly to a dumbbell-shaped protein named nidogen.
This nidogen protein is critical in attaching to the laminin proteins in the
lamina densa. Nidogen locks the type IV collagen to the laminins, which are
bound to the overlying integrin and BP180.
The laminin proteins appear as inverted
crosses and serve to attach the aforementioned proteins to the papillary dermis
that underlies the lamina densa by interacting with type VII collagen. Type VII
collagen, which is made up of three identical alpha chains, is also known as
the anchoring fibril. These fibrils interweave among the type I and type II
collagens of the papillary dermis and attach either end to the laminin proteins
in the lamina densa, thus firmly anchoring the entire over- lying epidermis and
BMZ to the papillary dermal collagen.
Many
blistering diseases are caused by genetic abnormalities in the BMZ proteins;
these are classified as the epidermolysis bullosa group of blistering diseases.
Each of these diseases is unique due to different protein defects that lead to
the various phenotypes. Autoimmune blistering diseases of the pemphigoid class
target the BMZ and its components, including the hemidesmosome. Autoimmune
diseases in the pemphigus class of diseases target the desmosome.
HEMIDESMOSOME
The
hemidesmosome is one of the main components of the BMZ. Its purpose is to
attach the basilar layer keratinocytes to the underlying stroma that is, the papillary
dermis. The hemidesmosome is made up of many unique and highly integrated
groups of protein- to-protein connections. The main proteins in the
hemidesmosomal plaque are the bullous pemphigoid antigens BP180 and BP230,
integrin, plectin, and laminin. Their interactions and how they connect the
keratinocyte cytoskeleton to the underlying collagen have already been
described. Antibodies directed against the components of the hemidesmosome can
be seen in the pemphigoid group of disease states.
DESMOSOME
The
desmosome provides the major connection between one keratinocyte and another.
It is the most complex of the keratinocyte connection points, which also
include tight junctions, adherens junctions, and gap junctions. Desmosomes are
present on all keratinocytes from the stratum basalis through the stratum
granulosum. Once they reach the stratum corneum, the desmosomes start to
degrade and break apart as the corneocytes are desquamated off the surface of
the skin. The main purpose of desmosomes is to connect the actin cytoskeleton
of one keratinocyte to that of the adjacent keratinocyte. They achieve this
goal through a series of highly coordinated protein connections. The main
proteins that allow for the connection between adjacent cells and the strength
of the connection are the cadherin proteins, desmoglein and desmocollin. These
are calcium-dependent adhesion molecules. Desmoglein and desmocollin are
transmembrane proteins.
A
desmocollin protein from one keratinocyte interacts with a desmoglein protein
from the adjacent keratinocyte in a one-to-one ratio. There is more than one
type of desmogleins and desmocollins, but they all interact similarly. Some of
the subtypes are expressed at slightly different rates in various locations
such as mucous membranes and the different levels of the epidermis. Each
desmoglein or desmocollin molecule is anchored within the keratinocyte to
plakoglobin, which in turn is bound to a group of proteins named desmoplakins. The desmoplakin
proteins ultimately connect with the intercellular actin cytoskeleton.
The
pemphigus group of diseases are autoimmune blistering diseases caused by the
formation of auto-antibodies against desmoglein and, in some cases, also
against desmocollin. These autoantibodies interrupt the cell-to-cell adhesion
process, resulting in su erficial blistering of the skin and mucous membranes.
