Cellular Structures and Intercellular Junctions Explained

Cellular Structures and Functions

Microvilli

Microvilli are finger-shaped projections, most frequently found in cells of the kidney and small intestine. Their primary function is to increase the absorption of nutrients.

Interdigitations

Interdigitations are located on the sides of cells. They increase the membrane surface area and promote functional integration.

Pits

Pits are found on the basal (bottom) face of the cell. They increase its surface area, provide resistance to cellular movements, and determine protein binding at the base of the cell.

Cilia

Cilia are short, mobile appendages developed by lipid-transporting cells or mucous-secreting cells. They exhibit a whipping movement. Their function is to help cells move through the human respiratory tract. There are approximately 100 cilia per cell, measuring 0.2 micrometers in diameter and between 7 and 10 micrometers long.

Internal Structure of Cilia

Internally, the axoneme consists of two central microtubules and nine peripheral pairs.

Flagella

Flagella share the internal organization of cilia but differ in number and length; they are longer and less numerous than cilia. They are found in bacteria, typically one or more per cell. Flagella arise from centrioles, and their movement differs from that of sperm tails. Their primary function is to allow the movement of cells.

Cell Adhesion and Intercellular Junctions

Cell adhesion forms strong tissues, allowing for the specialized distribution of functions in epithelial cells. Epithelial cells are located on the surface tissue of organs and are crucial for their protection.

Types of Intercellular Junctions

Tight Junctions (Occluding Junctions)

These junctions are located below the apical cell surface and act as a selectively permeable barrier. Transmembrane proteins, such as occludins and claudins, link neighboring cells. Their function is to prevent the passage of substances from the lumen into the underlying tissue, connecting neighboring cells to prevent the passage of soluble molecules.

Anchoring Junctions

These junctions develop in tissues subjected to mechanical stress, such as the skin, tendons, muscles, and other organs.

Specific Adhesion Junctions:

• Desmosomes: Structures that keep cells attached to neighboring cells. Internally, they act as rope-like anchors, forming a network that provides rigidity. They have two types of filaments: keratin in epithelial cells and desmin in cardiac muscle fibers.
• Hemidesmosomes: Similar to desmosomes, they are composed of plasma membrane components. They are responsible for anchoring cells to the connective tissue.

Gap Junctions

These junctions form a pore through which molecules pass into the cytoplasm. They are composed of connexins, which open or close as needed in response to extracellular signals. Gap junctions are regulated by pH, allowing molecules such as ions to pass through their pores.