PCBI100 Lecture Discussion 4. Epithelial Tissue PDF

Epithelial Tissue Prepared by: Benjo R. De Guzman Lecturer, Biological Sciences Department School of Science and Technology Centro Escolar University Manila  Principal Functions of Epithelial Tissue ▪ Covering, lini...

Epithelial Tissue Prepared by: Benjo R. De Guzman Lecturer, Biological Sciences Department School of Science and Technology Centro Escolar University Manila  Principal Functions of Epithelial Tissue ▪ Covering, lining, and protecting surfaces (e.g. epidermis) ▪ Absorption (e.g. intestinal lining) ▪ Secretion (e.g. parenchymal cells of glands) Characteristic Features of Epithelial Cells ▪ Shapes and dimensions ranges from columnar, cuboidal, or squamous cells. ▪ Nuclei also vary in shape: elliptic (oval), spherical, or flattened. ▪ Most epithelia are adjacent to connective tissue containing blood vessels. ▪ Generally show polarity, with organelles and membrane proteins distributed unevenly within the cell. > Basement Membranes ▪ Thin extracellular, felt-like sheet of macromolecules; a semipermeable filter for substances reaching epithelial cells from below. ▪ Basal Lamina – a thin, electron-dense, sheet-like layer of fine fibrils ▪ Reticular Lamina – more diffuse and fibrous layer beneath basal lamina ▪ Basement Membrane (entire structure beneath the epithelial cells visible under light microscope) & Basal Lamina (fine extracellular layer seen ultrastructurally) ▪ Functions of Basement Membrane: - Filters substances reaching epithelial cells from below - Helps provide structural support for epithelial cells - Attach epithelia to underlying connective tissue - Mediate many cell-to-cell interactions involving epithelia - Marks routes for certain cell migrations along epithelia - Serves as scaffold that allows rapid epithelial repair and regeneration ▪ Functions of Basal Lamina: - Help organize integrins and other proteins in the PM of epithelial cells - Maintains cell polarity - Helps localize endocytosis, signal transduction, and other activities > Intercellular Adhesion & Other Junctions ▪ Tight / Occluding Junctions (form a seal between adjacent cells) ▪ Adherent / Anchoring Junctions (sites of strong cell adhesion) ▪ Gap Junctions (channels for communication between adjacent cells) Specializations of the Apical Cell Surface 1. Microvilli ✔ Usually of uniform length, densely packed ✔ Visible as a brush or striated border projecting into the lumen (e.g. small intestine) ✔ 1 µm long, 0.1 µm wide on average; hundreds or thousands present on the end of each absorptive cell; total surface area can be increased by 20-30 fold ✔ Contains bundled actin filaments capped and bound to the surrounding PM by actin-binding proteins 2. Stereocilia ✔ Longer and less motile than microvilli ✔ With specialized mechanosensory function in cells (e.g. inner ear) and for absorption in tissues (e.g. male reproductive tract - epididymis) 3. Cilia ✔ Long, highly motile, larger than microvilli, contains internal arrays of microtubules (not microfilaments) ✔ Most (if not all) epithelial cells have at least one primary cilium (not motile, enriched with receptors and signal transduction complexes for detection of light, odors, motion, and flow of liquid past the cells) ✔ Abundant on cuboidal or columnar cells of many epithelia ✔ 5-10 µm long, 0.2 µm in diameter ✔ Axoneme – consists of 9 peripheral microtubule doublets arrayed around 2 central microtubules (9+2 assembly of microtubules); elastic but relatively stiff; structure is maintained by nexins ✔ Basal Bodies – apical cytoplasmic structures below the cell membrane; structurally very similar to centrioles, consisting of 9 microtubular triplets linked in pinwheel-like arrangement Types of Epithelia Two Divisions: 1. Covering or Lining Epithelia 2. Secretory (Glandular) Epithelia Lining of Stomach Goblet Cells in Trachea Covering (Lining) Epithelia Simple Squamous Epithelium Simple Simple Cuboidal Columnar Epithelium Epithelium Stratified Epithelium Transitional Epithelium Pseudostratified Epithelium (Urothelium) Secretory (Glandular) Epithelia ▪ Epithelial cells that function mainly to produce and secrete various macromolecules through specialized organs called glands. ▪ Proteins (e.g. pancreas) ▪ Lipids (e.g. adrenal, sebaceous glands) ▪ Carbohydrate and Protein Complexes (e.g. salivary glands) ▪ Proteins, Lipids, CHO (e.g. mammary glands) ▪ Water and Electrolytes (e.g. sweat glands) ▪ Scattered secretory cells (unicellular glands) – common in simple cuboidal, simple columnar, and pseudostratified epithelia (e.g. goblet cells in the lining of small intestine and respiratory tract) Formation of glands from covering epithelia Exocrine and Endocrine Glands ▪ Exocrine Glands remain connected with the surface epithelium, the connection forming the tubular ducts lined with epithelium that deliver secreted material where it is used. ▪ Endocrine Glands lose the connection to their original epithelium and therefore lack ducts. Capillaries adjacent to endocrine cells absorb their secreted hormone products for transport in blood to target cells throughout the body. Structural Classes of Exocrine Glands Structural Classes of Exocrine Glands (cont.) Mechanisms of Exocrine Gland Secretion Thank you!