Structure and Functions of Hemidesmosomes

Cellular junctions are the connections that exist between cells. They are multiprotein complexes present in animal cells’ cell membranes. These complexes aid in the attachment of animal cells to one another as well as the extracellular matrix. There are several types of cell junctions that provide various functions between cells. Intercellular connections are another name for cell junctions.

Cell junctions are classified into three types

• Occluding Junction-These junctions act as a barrier between the 2 cells, they will not let solutes and particles pass through in between the cell.
• Anchoring Junction-These junctions help in binding the nearby cell together, these junctions are present in both the basal and lateral surface of the cell.
• Communicating Junction-These junctions help in the transportation of particle between the cells, they act as a passage between the cell.

Hemidesmosome

Hemidesmosomes are asymmetric structures that link epithelial cells. The cell’s basal domains are connected to the underlying basal lamina. They are especially crucial in tissues that are constantly under mechanical strain.

These epithelial junctions are responsible for strengthening the overall stability of epithelial tissues through the cooperation of the cytoskeleton’s intermediate filaments and the various components of the basal lamina. In other words, they induce stable adhesions in connective tissue.

The phrase “hemidesmosome” might be deceptive. Although a hemidesmosome resembles a “middle” desmosome (another type of structure linked with cell adhesion), few molecular components are shared by both structures, therefore the resemblance is only cosmetic. Hemidesmosomes are regarded as anchor junctions in cell junction categorization and are associated alongside tight junctions, belt desmosomes, and point desmosomes. Anchor junctions are essential for maintaining cells together, whereas gap junctions are responsible for communication between nearby cells.

Description of Hemidesmosome

• Cells are the fundamental building components of all living things. However, the parallel with a brick or structural block falls short in various ways. Adjacent cells, unlike construction bricks, contain a network of connections and interact with one another.
• There are many structures that link cells and allow for contact and communication. Desmosomes are one of these anchoring structures.
• Hemidesmosomes are cell connections found in many epithelia that are constantly subjected to abrasion and mechanical stresses.
• Mechanical stress may cause a separation of the epithelial cells from the underlying connective tissue in certain areas. The word hemidesmosome is derived from the apparent resemblance to half desmosomes.
• They are found in the skin, the cornea (a structure in the eye), and other mucous membranes of the mouth cavity, esophagus, and vagina.
• They are found on the basal cell surface and aid in the adherence of the basal lamina.

Structure 

A desmosome is an asymmetric linking structure composed of two major components

• A cytoplasmic lamina is seen in connection with intermediate filaments, which are also known as keratins or tonofilaments.
• The outer membrane plate, which connects the hemidesmosome to the basal lamina, is the second component of hemidesmosomes. This interaction is made up of anchor filaments (made up of laminin 5) and integrin.

Function

Hemidesmosomes are adhesion sites between cells and the extracellular matrix that link to the keratin cytoskeleton. Research on individuals with abnormal expression of hemidesmosome components, as well as studies utilizing targeted inactivation of mice genes encoding these components, have substantially enhanced our understanding of their function. In vitro cell transfections, blot overlay, and yeast two-hybrid experiments have recently provided insight into the creation of hemidesmosomes as well as protein-protein interactions that occur in these junctional complexes. as well as According to recent findings, the alpha6 beta4 integrin is implicated in the transmission of extracellular matrix-induced signals that control processes such as cell proliferation, differentiation, apoptosis, migration, and tissue morphogenesis. Thus, it appears that hemidesmosomes are involved in intracellular signaling as well as dermo-epidermal adhesion and tissue integrity. 

Features 

• Hemidesmosomes are responsible for cell attachment to the basal lamina. The latter is a thin layer of the extracellular matrix that serves as a barrier between epithelial tissue and cells. The extracellular matrix, as the name indicates, is made up of exterior protein molecules rather than cells.
• In layman’s terms, hemidesmosomes are molecular structures that keep our skin together and function like a screw.
• They are found in areas (such as the mucosa and the eyes) that are frequently subjected to mechanical stress, and their existence aids in the maintenance of the cell-lamina union.

Proteins involve in Hemidesmosomes 

• Type 1 hemidesmosomes are seen in the stratified and pseudo-stratified epithelium.
• P1a, tetraspanin protein CD151, BPAG1e, or bullous pemphigoid antigen isoform e, and BPAG2 are the five key constituents of type 1 hemidesmosomes (also known as BP180 or type 17 collagen).

	Hemidesmosomes	Desmosomes
1	Hemidesmosomes are structures that help cells and basement membranes stick together.	Desmosomes are cell-to-cell adhesion structures.
2	Desmocollin and desmoglein	Integrin and plectin
3	gastrointestinal epithelium in cardiac muscle tissue	Keratinocytes are predominantly
4	Cell-to-cell adhesion and cell stiffness	Adhesion to the basal membrane
5	Autoimmune disorders and arrhythmogenic cardiomyopathy	Muscular dystrophy with epidermolysis bullosa

FAQs on Hemidesmosomes

Question 1: What exactly is a hemidesmosome?

Answer: 

A structural unit that aids in cell attachment to produce tissue. Hemidesmosomes are very tiny stud-like structures found in skin keratinocytes that adhere to the extracellular matrix. When seen under an electron microscope, they have a similar shape to desmosomes; however, desmosomes adhere to neighboring cells.

Question 2: What do desmosomes and hemidesmosomes look like under a microscope?

Answer:

Under a microscope, they both seem extremely similar. Desmosomes connect cells. Hemidesmosomes connect the intermediate filament of a cell to the extracellular matrix. Desmosomes link two cells, whereas hemidesmosomes connect basal keratinocytes to the basement membrane.

Question 3: What exactly is the distinction between Type 1 and Type 2 hemidesmosomes?

Answer:

Type 1 hemidesmosomes are seen in epithelial tissue that is stratified or pseudostratified. Integrin 64 and plectin are present on type 2 hemidesmosomes but lack the BP antigens. Integrin 64 and Plectin 1a are membrane-spanning components of hemidesmosomes.

Question 4: In a mouse, where are the hemidesmosomes located?

Answer:

There are well-defined, ordered hemidesmosomes in a typical mouse (a), with darkish patches in the lamina densa bordering the hemidesmosome (arrows).