Epithelial Polarity and Cell Surface Specializations PDF

## Tissues ### Epithelial Polarity (Cell Surface Specialization) - Epithelial cells have an apical, basal, and lateral surface. - Each surface has specific structural modifications. - **Apical Specializations** are exposed to the body exterior or internal organ cavities. - **Microvilli** are no...

## Tissues ### Epithelial Polarity (Cell Surface Specialization) - Epithelial cells have an apical, basal, and lateral surface. - Each surface has specific structural modifications. - **Apical Specializations** are exposed to the body exterior or internal organ cavities. - **Microvilli** are non-motile, finger-like evaginations that contain a core of actin filaments. - Increase surface area for fluid absorption. - Present in the intestines and kidney tubules. - **Stereocilia** are long, branching microvilli. - Found mainly in the non-ciliated pseudostratified columnar epithelium of male genital ducts (epididymis). - Increase surface area for fluid absorption. - **Cilia and Flagella** are generally-motile cytoplasmic projections that extend from the cell surface. - **Cilia** are hair-like processes longer than microvilli. - **Flagella** resemble cilia in structure, but are much longer and are single per cell (e.g. flagellum of the spermatozoon). - **Histological structure of cilia:** - A **basal body**, **shaft**, and **rootlets**. - The **basal body** is a replicate of the cell centriole with 9 triplets of microtubules (9x27=3 microtubules). - The **shaft (axoneme)** extends from the cell surface. - Contains 9 peripheral doublets of microtubules together with a central pair of singlet microtubules (9x20=2+2 microtubules). - **Rootlets** extend from underneath the basal body and anchor the cilium into the cytoplasm. ### Basal Specializations - Structural modifications in the basal domain include basal infoldings, the basement membrane, and cell-to-matrix junction. - **Basal infoldings** increase the surface area where ions transport occurs, such as in kidney tubules. ### Cell Adhesion Molecules (CAMs) - Transmembrane glycoproteins with three domains: - **Extracellular (EC) domain:** Binds with other CAMs on adjacent cells or matrix proteins. - **Intramembranous (transmembrane) domain:** Embedded in the cell membrane. - **Cytoplasmic domain:** Attached to the cytoskeleton via linker proteins. ### Cell Adhesion - A dynamic process where non-adhesive cells become adhesive, such as leukocytes during inflammation and platelets in blood clotting. - Temporal loss of adhesion occurs during migration of skin cells during wound healing, or the spread of cancer cells and occurrence of metastasis. ### Types of Connecting Cell Junctions - **Occluding Junction (Tight Junction or Zonula Occludens):** - Found mainly where no substance should pass into the intercellular space (simple columnar epithelium lining the intestine). - Restricts the passage of water, electrolytes, and small molecules between epithelial cells. - The outer leaflets of adjacent cell membranes are fused together around the whole perimeter of each cell, forming a continuous belt-like junction. - Two transmembrane proteins, **occludin** and **claudin**, are embedded in each of the two adjacent cell membranes, join together to create a tight seal and occlude the intercellular space. - **Anchoring Junctions:** - Provide strong attachment and act as a link between cytoskeletons of adjacent cells. - Found in tissues like cardiac muscles and epidermis of skin. - **Zonula Adherens:** - Encircles the apical part of two adjoining cells. - Contains cadherins, whose extracellular domains are linked in the presence of calcium. - The intercellular space between the opposing cell membranes is 20 nm. - **Macula Adherens (Desmosomes):** - Contains cadherins which interact to connect the two cells. - Consists of 2 disk-shaped attachment plaques, dense vertical filamentous material in the intracellular space, and keratin intermediate filaments. - Removal of calcium separates the desmosomes into two halves. - **Hemidesmosomes (Basal cell-to-matrix adhesion):** - Half of a desmosome. - Found at the base of epithelial cells to connect them with the underlying basement membrane. - Contains integrins; the extracellular domains bind to proteins of the basal lamina, and the intracellular domains bind to cytokeratin intermediate filaments. - **Communicating Junction (Gap Junction, Nexus):** - Mediate communication rather than adhesion. - Found in cardiac and smooth muscle cells. - Permit the exchange of molecules (ions, amino acids). - Involve connexons, which are formed of 6 closely packed transmembrane proteins. - When two connexons of opposing cell membranes are in register, they form a channel connecting the cytoplasm of adjacent cells. ## Connective Tissue - Most abundant and widely distributed tissue. - Arises from the mesenchyme (mesoderm). - Contains cells and an extracellular matrix. - The composition and arrangement of extracellular matrix elements varies greatly in different types of connective tissue. ### Connective Tissue Ground Substance - Composed of: - **Interstitial tissue fluid:** Formed of plasma proteins of low molecular weight that escape through the capillary wall as a result of hydrostatic pressure. - **Adhesive glycoproteins:** Fibronectin and laminin serve as connective tissue glue, allowing connective tissue cells to bind themselves to matrix elements. - **Proteoglycans:** Consist of a protein core to which glycosaminoglycans (GAGs) are attached. - GAGs are large, negatively charged polysaccharides that extend from the core protein. - Examples include chondroitin sulfate and keratan sulfate. - Form huge proteoglycan aggregates with hyaluronic acid. - Trap water and form substances that vary from a fluid to a viscous gel. - Functions: - Holds large amounts of fluid and functions as a medium for diffusion between blood capillaries and cells. - Resists compression and acts as a lubricant. - Acts as a barrier to bacterial penetration. - Hyaluronidase, an enzyme that hydrolyzes the ground substance, can facilitate bacterial invasion. ### Connective Tissue Fibers - Embed in the connective tissue matrix. - Provide support. - Types: - **Collagen fibers:** - Most abundant. - Strongest, providing high tensile strength. - Appear white in fresh state. - In longitudinal section, collagen fibers appear as cylindrical structures that run in wavy bundles. - **Reticular fibers:** - Delicate networks that support the cells of the organs and surround small blood vessels. - Short, fine, and branching fibers forming a network. - Stained brown to black by silver stain. - Consists mainly of type III collagen. - **Elastic fibers:** - Allow the fibers to stretch and recoil. - Appear yellow. - Found in blood vessels, tendons, ligaments, elastic laminae of arteries, and true vocal cords. - **Synthesis of collagen:** - Procollagen is formed inside fibroblasts. - Released exocytosis into the extracellular space. - Cleaved form collagen molecules. - Assemble spontaneously into collagen fibrils. - Bundled together into collagen fibers. ### Connective Tissue Cells - **Fixed (Resident) Cells:** - Develop and remain within the connective tissue. - Stable, long-lived. - Examples include: - **Fibroblasts:** - Spindle-shaped, branching cells with deeply basophilic cytoplasm and a large euchromatic nucleus. - Contain abundant rough endoplasmic reticulum and a well-developed Golgi complex. - Secrete the ground substance and fibers of the matrix. - Become **fibrocytes** after synthesizing the matrix. - **Fat cells (Adipocytes):** - Unilocular: single large lipid droplet that pushes cytoplasm to a thin peripheral rim and a peripheral flattened nucleus. - Multilocular: numerous small lipid droplets and mitochondria with abundant long cristae. - Transformed from undifferentiated mesenchymal cells or hemopoietic stem cells. - **Transient (Wandering) Cells:** - Originate mainly in the bone marrow and circulate in the bloodstream. - Short-lived. - Replaced from a large population of stem cells. - Examples include: - **White blood cells:** - Neutrophils, eosinophils, basophils, monocytes, and lymphocytes. - **Plasma cells:** - Derived from B lymphocytes that enter the connective tissue. - Large, oval cells with basophilic cytoplasm. - Prominent juxtanuclear Golgi apparatus. - **Mast cells:** - Full of basophilic granules. - Nucleus is centrally located and rounded. - Contain numerous secretory granules. - Initiate allergic responses. - Release histamine and heparin. - **Macrophages:** - Derived from monocytes. - Large irregular cells with eccentric kidney-shaped nucleus. - Phagocytic. ## Classification of Connective Tissue - Three categories: - Embryonic Connective Tissue - Mesenchymal CT - Mucoid CT - Connective Tissue Proper - Loose areolar CT - Dense irregular connective tissue - Dense regular connective tissue - Elastic connective tissue - Reticular connective tissue - Adipose connective tissue - Specialized connective tissue: - Cartilage - Bone - Blood ### Embryonic Connective Tissue - **Mesenchymal CT:** - Found in the embryo. - Consists of undifferentiated mesenchymal cells and a gel-like amorphous ground substance. - **Mucoid CT:** - Found in the umbilical cord and pulp of growing teeth. - Consists of abundant ground substance, spindle-shaped UMCs that are widely separated and appear much like fibroblasts, and unapparent fine collagen fibers. ### Connective Tissue Proper - **Loose Areolar CT:** - Most widely distributed. - Binds body parts together while allowing them to move freely over one another. - Contains many small blood vessels. - Found in lamina propria of mucous membranes, papillary layer of dermis, surrounding glands, blood vessels, and nerves. - Consists of all types of fibers, fibroblasts, macrophages, and a good amount of ground substance. - Provides support, holds body fluids, and defends against infection. - **Dense Irregular CT:** - Thick bundles of collagen fibers that are arranged irregularly. - Little ground substance. - Found in the reticular layer of the dermis, capsules of organs, and liver. - Provides tensile strength from many different directions. - **Dense Regular CT (White Fibrous CT):** - Closely-packed wavy bundles of collagen fibers running in the same direction. - Rows of fibroblasts aligned between the collagen bundles. - Little ground substance. - Forms tendons and ligaments. - **Elastic CT:** - Elastic fibers predominate. - Found in elastic laminae of arteries, true vocal cords, and a few ligaments. - **Reticular CT:** - Consists of reticular fibers that form a network. - Found in the stroma of hemopoietic tissue, lymphoid tissue, and hepatocytes. ### Adipose Connective Tissue - **Unilocular adipose tissue:** - White to yellow. - Formed of unilocular adipocytes which are either spherical when single or polyhedral when clustered. - Contains little ground substance and a fine network of reticular fibers surrounding the cells. - Found in subcutaneous tissue, around kidneys, and behind eyeballs. - Function includes storage of energy and shaping the body. - **Multilocular adipose tissue (Brown adipose tissue):** - Large numbers of blood capillaries and mitochondria. - Brown color due to high number of blood capillaries. - Found in the abdomen and neck of the human embryo and the newborn. - Concerned with production of heat (thermogenesis) in the first months of postnatal life. ## The Integumentary System - Consists of: - Skin (integument) - Skin appendages (hair, nails, sweat glands, sebaceous glands, and mammary glands) ### Skin - Largest organ in the body. - Composed of two layers: epidermis and dermis. - **Epidermis:** - Superficial epithelium. - Avascular but contains sensory nerve endings. - Derived from ectoderm. - Composed of four types of cells: - Keratinocytes - Melanocytes - Langerhans cells - Merkel cells - **Dermis:** - Deeper vascular connective tissue layer. - Derived from mesoderm.