Histology I (Epithelial Tissue) Lecture Notes PDF

Lect. 9: Histology I (Epithelial tissue) Assoc. Prof. / Reda M. Mansour & Radwa Lutfy  Types of tissues 1- Epithelial tissue (Epithelium). 2- Connective tissue 3- Muscular tissue 4- Nervous tissue Characters of epithelial tissue: 1. Epithelial tissue cover surface or line cavi...

Lect. 9: Histology I (Epithelial tissue) Assoc. Prof. / Reda M. Mansour & Radwa Lutfy  Types of tissues 1- Epithelial tissue (Epithelium). 2- Connective tissue 3- Muscular tissue 4- Nervous tissue Characters of epithelial tissue: 1. Epithelial tissue cover surface or line cavities all over the body. It consists of crowded sheet of cell with minimal intercellular spaces and rest on basement membrane. 2. It has a high power of regeneration (renewal). 3. Epithelia tissue display polarity i.e. with apical, lateral, and basal surfaces. 4. Lateral & basal surfaces have junctions while apical surface has villi, stereocilia, cilia, flagella. 5. Epithelia are avascular & receive nourishment by diffusion through the basement membrane that separates them from the underlying connective tissue. 6. Epithelia originate from all three embryonic germ layers: 1- Ectodermal, e.g. skin; 2- Endodermal: e.g. gastrointestinal tract (GIT) 3- Mesodermal: serous membrane.  Function of Epithelium Tissue: Epithelia transport, absorb, secrete, protect, and display selective permeability. 1. Transcellular transport of molecules occurs mostly by: a. Diffusion (e.g., oxygen) across the epithelial cells of lung alveoli and capillaries. b. Carrier-mediated transport (e.g., amino acids) across intestinal epithelia c. Vesicle-mediated transport (e.g., immunoglobulin A) by intestinal epithelial cells. 2. Absorption occurs via endocytosis or pinocytosis by cells of various organs. 3. Secretion of various molecules (e.g., hormones) occurs by exocytosis. 4. Protection from abrasion and injury e.g. Epidermis of the skin. 5. Selective permeability restricts what may or may not cross the cell membrane resulting in fluids with different compositions and concentrations to exist on either side of an epithelial layer (e.g., kidney). 1- Covering, lining or surface Epithelium A- Simple Epithelium (one layer of cells) B- Stratified Epithelium (many layer of cells) = Simple epithelium Thyroid follicle = Stratified epithelium  2- Secretory or Glandular Epithelium A- Exocrine gland: It has a duct that leads to another organ or the body surface. B- Endocrine gland: It has no duct that leads to another organ or the body surface. e.g. Endocrine (Hormonal) system. C- Mixed gland: It is formed of exocrine & endocrine parts. e.g. pancreas. Classification of exocrine glands according to:. Number of cells Mechanism of secretion Branching of duct Shape of secretory part a) Unicellular a) Merocrine secretion: a) Simple: a) Tubular: gland: Most common mechanism. The gland has a Secretory part is a tube e.g. Goblet cell single non-branching form. Secretion discharged by duct. b) Multicellular exocytosis. glands: e.g. Salivary glands. b) Compound: b) Alveolar (acinar): e.g. Salivary glands The gland has a Secretory part is & others b) Apocrine secretion: branching duct like a rounded. Secretion discharged by tree. the disintegration of apical c) Tubulo-alveolar: part of cells. Secretory part is flask shape. e.g. Mammary gland. c) Holocrine secretion: Secretion is produced by the disintegration of the secretory cells themselves. e.g. Sebaceous gland. Types of exocrine glands according to branching of ducts Goblet cell Mechanisms of exocrine gland secretion  Lateral & basal epithelial surfaces “Intercellular adhesion & other Junctions” Most epithelial cells have intercellular junctional complexes. 1- At the apical end, tight junctions (zonulae occludens, not very strong) and adherent junctions (zonulae adherens, very strong) are typically close together and each forms a continuous band around the cell. The intercellular seal of tight junctions ensures that molecules crossing an epithelium in either direction by going through the cells (transcellular path) rather than between them (the paracellular pathway). Adhering junctions stabilize and strengthen the circular tight junctions. 2- Both desmosomes and gap junctions are spot-like, not circular. Desmosomes form very strong attachment points & play a major role to maintain the integrity of an epithelium. Gap junctions in form of connexins that form hexameric complexes called connexons with a central hydrophilic pore about 1.5 nm in diameter serve as intercellular channels for flow of molecules. 3- Hemidesmosomes bind epithelial cells to underlying basal lamina.  Apical epithelial surfaces 1- Microvilli Microvilli are 1-um long, glycocalyx-covered, finger-like projections of epithelial cell membrane that extend into a lumen and increase the cell's surface area, constituting the brush border of kidney proximal tubule cells and the striated border of intestinal absorptive cells. A bundle of approximately 25-30 actin filaments runs longitudinally within the core of each microvillus extending from the tip to the terminal web, a zone of intersecting filaments in the apical cytoplasm. Actin-binding proteins bind the actin filaments of the bundle to each other; calmodulin and myosin I secure the peripheral filaments of the actin bundle to the cytoplasmic aspect of the microvillar membrane. Microvilli  2- Stereocilia Stereocilia are very long (15-20 um in length) microvilli (they are not cilia) in the hair cells of the inner ear (sensory), in the epididymis (absorption), and in the vas deferens (absorption) of the male reproductive tract. They are non-motile The core of the stereocilia is composed of actin filaments that are attached to each other by fimbrin and to the plasmalemma of the stereocilia by villin-2.  3- Cilia and Flagella Some eukaryotic cells have flagella (singular, flagellum) and cilia (singular, cilium), cellular extensions that contain microtubules. A specialized arrangement of the microtubules is responsible for the beating of these structures. Motile cilia usually occur in large numbers on the cell surface. Flagella are usually limited to just one or a few per cell, and they are longer than cilia. Flagella and cilia differ in their beating patterns. A flagellum has an undulating motion like the tail of a fish. In contrast, cilia have alternating power and recovery strokes, much like the oars of a racing crew boat. The actively motile cilia are 7- to 10-μm-long processes of the cell that extend from certain epithelia (e.g., tracheobronchial and oviduct epithelium) that propel substances along the epithelial surface.