CHAPTER 4: The epithelial tissue - 4 basic types of Tissue 1. Epithelial Tissue 2. Connective Tissue 3. Muscular Tissue 4. Nervous Tissue Each tissue is an assemblage of similarly specialized cells united in performing a specific function. The basic tissues all contain an Extracellular matrix (ECM) as well as cells associated with one another in the variable of proportions and morphologies of each organ. Extracellular Matrix - a complex network of protein and polysaccharide outside of the cell but within the tissue. Parenchyma - is the functional cells of an organ E.g. Hepatocyte of the liver Stroma - supporting role in the organ E.g. CT in the liver Epithelial Tissue - Is composed of CLOSELY aggregated polyhedral cells adhering strongly to one another. - Closely packed - multidimensional - THIN layer of ECM - Lines all internal and external surfaces of the body Function of the Epithelial Tissue - Covering, lining, and protecting surfaces (e.g. Epidermis) - Absorption (e.g. intestinal lining - Secretion (e.g. parenchymal cells of glands) Contractile i.e. Myoepithelial cells Sensory cells (e.g. taste buds, or the olfactory epithelium) Characteristic Features of Epithelial cells ● Shape andDimension ○ Tall Columnar ■ Elongated nuclei ○ Cuboidal ■ Spherical nuclei ○ Squamous ■ Flattened nuclei Due to the Lipid-rich membrane, EC are frequently indistinguishable by light microscope, the number of and shape of the nuclei help in identifying the cell shape and density. The nuclei also help in determining the number of layers of the epithelium. - Primary morphologic criteria of classifying epithelia. Most epithelia are adjacent to the Connective tissues from which they get nutrients and O2 from. Lamina propria - the CT under the epithelial lining of the digestive, respiratory, and urinary system Papillae - small evaginations between the area of contact of two tissues. Epithelial cells are polarized according to its purpose. Basal Pole - region contacting the ECM and CT Apical Pole - facing the tissue space Lateral Surfaces - regions of cuboidal or columnar cells that are adjoined by neighboring cells BASEMENT MEMBRANE - Thin extracellular felt-like sheet of macromolecules - Made up of collagen, glycoproteins, and proteoglycans - Provides structural support, acts as a barrier and filter, and regulates cell behavior. - Vital in tissue integrity and function Component of the Basement Membrane 1. Type IV collagen (Basal Lamina) - Two dimensional mesh-like network and provides strength and support 2. Laminin (BL) - Large glycoproteins that attach to the transmembrane protein integrin. - Plays a role in cell differentiation, adhesion, and migration. 3. Nidogen (Entactin) (BL) - Connects Laminin and Type IV collagen networks, stabilizing the overall structure of the basement membrane 4. Perlecan (BL) - Proteoglycan that contributes to the filtration properties of the BM and binds to other components like Laminin and collagen. 5. Type III Collagen (Reticular Lamina) - Bound to BL by anchoring fibrils of Type VII collagen Basal Lamina components provide structural support for epithelial cells and attach EC to underlying CT. The basement membrane acts like a scaffold or framework. If the epithelial layer gets damaged, this scaffold helps the cells repair and regenerate quickly, ensuring that the tissue functions properly again. Intercellular Adhesion and Other Junctions - Cell communication and adhesion 1. Tight or Occluding junctions - Forms a seal between adjacent cells - Tight interaction by claudin and occludin 2. Adherent or anchoring junctions - Site of strong cell adhesion - Below tightjunction - Actin filaments linked to AJ forms part of the terminal web (cytoskeletal feature at the apical pole. 3. Desmosomes - Spot-weld - disc -shaped structures 4. Gap Junction - Channels for cell communication Specialization of the Apical Cell Surface structures play crucial roles in absorption, secretion, and sensory processes. 1. Microvilli - small, finger-like projections on the apical surface of epithelial cells. - They significantly increase the surface area for absorption, especially in the small intestine, where they form a dense brush border. This allows for more efficient absorption of nutrients. - Each microvillus contains bundled actin filaments that are linked to the cell membrane by actin-binding proteins, allowing for dynamic interactions that support the absorption processes. 2. Stereocilia - similar to microvilli but are typically longer and less motile. - primarily found in the male reproductive system, where they also enhance absorption. stereocilia in the inner ear have a sensory function, detecting mechanical vibrations, which are essential for hearing. 3. Cilia - larger than microvilli and stereocilia and are highly motile. internal structure of microtubules rather than microfilaments. - - move rhythmically to propel substances across the epithelial surface, such as mucus in the respiratory tract. This motility is critical for clearing debris and pathogens from the airways. with nuclei at different levels. Usually ciliated and involved in secretion and movement of mucus, found in the respiratory tract. 2. Stratified Epithelia: ● Types of Epithelia 1. Simple Epithelia: ● ● Definition: A single layer of cells, all of which are in contact with the basement membrane. Types: ○ Simple Squamous Epithelium: Composed of flat, scale-like cells. Found in areas where diffusion or filtration occurs, such as the alveoli of lungs and the lining of blood vessels (endothelium). ○ Simple Cuboidal Epithelium: Made up of cube-shaped cells, often involved in secretion and absorption. Located in glands, ducts, and the kidney tubules. ○ Simple Columnar Epithelium: Tall, column-like cells that often have microvilli or cilia on their surface. Found in the lining of the digestive tract, from the stomach to the rectum, where they absorb nutrients and secrete mucus. ○ Pseudostratified Epithelium: Appears stratified but is a single layer ● Definition: Multiple layers of cells, with only the deepest layer in contact with the basement membrane. Types: ○ Stratified Squamous Epithelium: Has many layers, with the topmost cells being flat. The keratinized form is found in the skin, providing protection, while the non-keratinized type lines moist surfaces like the mouth and esophagus. ○ Stratified Cuboidal Epithelium: Usually two layers of cube-shaped cells, found in the ducts of sweat glands and other large glands. ○ Stratified Columnar Epithelium: Rare, with columnar cells on the surface and cuboidal cells in deeper layers. Found in the conjunctiva of the eye, parts of the pharynx, and the male urethra. 3. Transitional Epithelium: ● ● Definition: A specialized type of stratified epithelium that can stretch. Cells change shape from cuboidal to squamous when stretched. Location: Found in the urinary bladder, ureters, and parts of the urethra, where it allows for expansion and contraction. ○ Key Functions: ● ● ● ● Protection: Especially in stratified epithelia like the skin, which protects underlying tissues from mechanical stress, dehydration, and pathogens. Absorption and Secretion: Simple epithelia are crucial in areas like the intestines, where nutrients are absorbed, and in glands where substances are secreted. Filtration: Simple squamous epithelium, like in the kidneys, plays a key role in filtering blood to form urine. Sensory Reception: Some epithelia, such as in the skin and sensory organs, are involved in detecting changes in the environment. These classifications and functions highlight the diversity of epithelial tissues and their critical roles in maintaining the body's structure and function. ○ ○ 2. Paracellular Transport ● Transport across Epithelia involves the movement of substances through epithelial cells, which are the cells that line surfaces and cavities in the body. According to the document, this transport can occur in several ways: 1. Transcellular Transport ● ● Definition: Movement of substances directly through the epithelial cells, passing through both the apical (top) and basal (bottom) surfaces of the cell. Process: Active Transport: Uses energy (ATP) to move substances against their concentration gradient. For example, the sodium-potassium pump actively transports sodium out of the cell and potassium into the cell. Passive Transport: Moves substances down their concentration gradient without using energy. This can occur through channels or carriers in the cell membrane. Endocytosis and Exocytosis: Large molecules can be transported via vesicles. Endocytosis brings substances into the cell, while exocytosis expels them out of the cell. ● Definition: Movement of substances between epithelial cells, passing through the spaces between them. Characteristics: ○ Tight Junctions: These junctions between epithelial cells regulate the paracellular pathway. The permeability of these junctions can vary, allowing for selective transport of ions and small molecules. ○ Role in Barrier Function: The degree of tightness of these junctions plays a critical role in maintaining the selective barrier function of epithelia, particularly in tissues like the intestines and blood-brain barrier. - 3. Role of Epithelial Cells in Ion Transport ● Electrolyte Balance: Epithelial cells are crucial in maintaining electrolyte balance in the body. For instance, in the kidneys, epithelial cells in the nephron tubules are responsible for the reabsorption of ions, which is essential for fluid balance and blood pressure regulation. 4. Importance in Absorption and Secretion ● ● Absorption: In areas like the intestines, epithelial cells absorb nutrients from digested food into the bloodstream. Secretion: In glands, epithelial cells secrete substances such as enzymes, hormones, and mucus. The transport mechanisms involved in these processes are vital for maintaining homeostasis and proper functioning of the body. These transport mechanisms across epithelia are essential for the proper functioning of organs and tissues, facilitating the controlled movement of nutrients, waste products, and other molecules necessary for physiological processes. Renewal of Epithelial Cells - Continuous renewal by mitosis Rate of renewal varies on the specific type of epithelium - Intestinal epithelium is replaced every week while the glands are slower Stem Cells are located in the basal layer or within specific niche within the epithelium Gives rise to new epithelial cells which then differentiate and replace the older cells that have been shed or lost due to normal physiological processes.
