Cytology 244 First Term of year 1432-1433 Nuha AL-Abdulhadi lab 4

Modifications of plasma membrane mm Modifications of plasma membrane Specializations of cell surface Specializations of contact surfaces between cells

Specializations of cell surface

-A cell may have modifications that make it function more efficiently -A cell may have modifications that make it function more efficiently. - Movement is an important property of cells; so many motile cells, like: A- the Euglenia and the sperm cell, have a flagellum (tail) that propels them through the extracellular fluid. Some cells have more modest means of movement.

B- The Paramecium is covered with tiny hairs, called cilia, which help it move. Cilia may also appear on cells that move something along a pathway. Cells lining the human respiratory pathway also bear cilia, to assist in moving contaminants and mucous out of the lungs.

C-Another specialized cell function that requires modification is absorption. Cells that absorb nutrients in the small intestines, for example, have extensive foldings of their surfaces, called microvilli, which increase the surface area of the cell that’s available for absorption

p.c.tu.in kidney

Specializations of contact surfaces between cells

A cell junction (or intercellular bridge) is a type of structure that exists within the tissue of a some multicellular organism -Cell junctions consist of: protein complexes and provide contact between neighbouring cells or between a cell and the extracellular matrix. They also build up the paracellular barrier of epithelia and control the paracellular transport. Cell junctions are especially abundant in epithelial tissues.

1- Adherens junctions (or zonula adherens) intermediate junction, or "belt desmosome) :are protein complexes that occur at cell–cell junctions in epithelial tissues, An adherens junction is defined as a cell junction whose cytoplasmic face is linked to the actin cytoskeleton. They can appear as bands encircling the cell (zonula adherens) or as spots of attachment to the extracellular matrix (adhesion plaques). A similar cell junction in non-epithelial cells is the fascia adherens. It is structurally the same, but appears in ribbon like patterns that do not completely encircle the cells. One example is in cardiomyocytes

Electron microscopy of the intercalated disc in control mice. Electron microscopy of the intercalated disc in a control mouse heart (A) shows a gap junction (arrow) in close proximity to adherens junctions (white arrowheads) and desmosomes (black arrowheads), while a representative intercalated disc in heart

2-A gap junction or nexus is a specialized intercellular connection between a multitude of animal cell-types. It directly connects the cytoplasm of two cells, which allows various molecules and ions to pass freely between cells. One gap junction channel is composed of two connexons) which connect across the intercellular space. A notable use of gap junctions is in the electrical synapse found in some neurons.

3-Tight junctions, or zonula occludens, are the closely associated areas of two cells whose membranes join together forming a virtually impermeable barrier to fluid. It is a type of junctional complex present only in vertebrates.

4-Desmosome (Greek: desmos, band, soma, body), also known as macula adherens : is a cell structure specialized for cell-to-cell adhesion. A type of junctional complex, they are localized spot-like adhesions randomly arranged on the lateral sides of plasma membranes. Desmosomes help to resist shearing forces and are found in simple and stratified squamous epithelium. The intercellular space is very wide (about 30 nm). Desmosomes are also found in muscle tissue where they bind muscle cells to one another .

Tight junctions Desmosome

Tight junctions

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