Epithelium & Glands - Histology PDF

Epithelium & Glands Dr Despina Moissidou Histology Lead QMUL Malta [email protected] Learning Objectives • Know the histological features of the epithelial tissue types. • Know the types of epithelium and discuss the function, structural characteristics and cellular specialisations of each....

Epithelium & Glands Dr Despina Moissidou Histology Lead QMUL Malta [email protected] Learning Objectives • Know the histological features of the epithelial tissue types. • Know the types of epithelium and discuss the function, structural characteristics and cellular specialisations of each. • Name an example of a body site where it would be located, and describe • Know the structure and role of cell basement membrane • Describe the various types of glands, relate their structure to their function, and give an example of where each might be found in the body Reading material • Part 2 “Basic tissue types”, Ch. 5 Epithelia: Wheater's Functional Histology, 6th Edition (E-Book available from the library) • Chapter 5: Epithelial tissue: Histology: A Text and Atlas, with Correlated Cell and Molecular Biology, 8th Edition, 2018, W. Pawlina, Michael H. Ross • Chapter 5: Epithelial tissue: Junqueira's Basic Histology: Text and Atlas, 15th Edition, 2018 Overview of Tissues • Tissues are aggregates of cells organized to perform specific functions, through a distinctive pattern of organization. • Tissues in the human body can be classified into four major types: – Epithelium (epithelial tissue) • Covers body surfaces, cavities and makes up glands – Connective tissue • Supports – Muscle tissue • Provides movement – Nerve tissue • Receives, transmits and integrates information. • Classification of epithelial/connective tissue is based primarily on morphology. • Classification of muscle/nerve tissue is based primarily on function. Epithelium and its function • Derive from the ectoderm, mesoderm and endoderm (early embryonic layers) • Line and cover all body surfaces, except the articular cartilage, tooth enamel and anterior of the iris. • Protection (skin), detection, sensation and regenerative capacity (response to damage, metaplasia). • Absorption (small and large intestine). • Material transport at the surface (by cilia). • Gas exchange (lung alveolus). • Excretion (kidneys tubules), secretion (glands) Epithelium and its function • Characterized by sheets of tightly bound contiguous cells. Cell are joined by specialized cell-to-cell junctions. Renewed by mitosis • Gliding between surfaces (mesothelium). • They have a free surface, such as the cover body surfaces and cavities. • Avascular (no blood vessels). Lack a direct blood/lymphatic supply. Nourishsment and oxygen derive from underlying connective tissue • Supported by a Basement Membrane and get separated from connective tissue: (basal lamina + connective tissue = basement membrane), a non-cellular, protein-polysaccharide-rich layer Epithelial cell structure Epithelium: structural classification • Sub-classification of epithelium are based on: – Number of cell layers (Simple, Stratified) – Cell Shape (Squamous, cuboidal, columnar) Epithelium: structural classification Epithelium: structural classification LAYERS OF CELLS: • Simple: one layer thick • Stratified: Multiple layers of cells SHAPE OF THE CELLS: • Squamous: Flat cells, width greater than height • Cuboidal: Cubes, width equal with height • Columnar: Height greater than width Sometimes the classification of the apical surface will be added to the name ex. Ciliated, keratinized, non-keratinized etc. Epithelium: structural classification Transitional lower urinary tract, stratified epithelium with morphological characteristics that allow it to distend. Pseudostratified Appears stratified but they are actually simple due to the location of the nucleus Simple Squamous Simple Cuboidal Simple Columnar Classification of Epithelium and its location Endothelium/mesothelium: simple squamous epithelia lining the vascular system and body cavities. • Endothelium: epithelial lining of blood vessels • Endocardium: epithelial lining of ventricles and atria of the heart • Mesothelium: epithelial lining of body cavities Classification of Epithelium and its location NOTE: • In special situations, epithelial cells lack a free surface. • Closely packed cells, sitting on a BM they lack free surface. These cells are called epithelioid cells. Leydig cells in the testis Lutein cells of the ovary Langerhan islets of the pancreas Epithelial structure: Cell Polarity Exhibit polarity: apical, lateral and basal domains • Apical domain: exposed to the lumen or external environment. • Lateral domain: adjacent epithelial cells, by cell adhesion molecules and junctional complexes. • Basal domain: rests on the basal membrane (basal lamina). Lateral Domain and Cell-to-cell adhesions • Tight Junctions: seals adjacent cells together ex. Zonula occludens. • Anchoring Junctions: Structural support of adjacent cells. Ex. Zonula adherens, hemidesmosomes. • Gap junctions: Create communication contacts between adjacent cells. Epithelial cell structure: apical domain & structures • Microvilli: – fingerlike projections on the apical surface, with passive motility – rough appearance under microscopy called striated border (intestines) or brush border (kidney) ---- indicates the absorptive function of the cell. – Internal structure of the microvilli contains actin filaments. Epithelial cell structure: apical domain & structures • Stereocilia: – Long immotile microvilli. – Not widely distributed among epithelial- limited to epididymis (contribute to sperm maturation) and inner ear. – Supported by actin filaments that are crosslinked by fibrin. Epithelial cell structure: apical domain & structures • Cilia: in nearly all cells in the body, hair-like extensions of the apical plasma membrane, supported by microtubules (develop from basal bodies). Classified into: • Motile cilia: Can move • Primary cilia: Immotile cilia found in almost every cell. No active movement but can passively bend • Nodal cilia: found in the embryo Basement Membrane structure and function • Laminin and fibronectin: distinct proteins of the ECM, associate with collagens, proteoglycans + other proteins organisation of the BM • Two components: - Basal lamina: sheet-like ECM in direct contact with epithelial cells (self assembly of laminin with Collagen IV, entactin and proteoglycans). - Reticular lamina: supports the BL and is continuous with connective tissue (collagen fibres). • Functions: Structural attachment, filtration, compartmentalization, tissue scaffolding during regeneration, regulation and signaling Basement Membrane structure and function • An important component of the glomerular filtration barrier in the kidneys. • Covers the surface of muscle cells and contributes to maintaining the integrity of the skeletal muscle fibre during contraction. • Disruption of basal lamina-cell muscle relationship gives rise to muscular dystrophies. H&E Glands or glandular epithelium • Epithelial tissue structures originated from invaginated epithelial cells • Secretory units/ducts make up the parenchyma • Supported by connective tissue (stroma) • Secrete mucus, enzymes, hormones, etc. • Their products are manufactured intracellularly Glands (glandular epithelium) and their location Glands morphology Glandular epithelium Glands: structural function • Exocrine: product secretion onto the surface directly or through epithelial ducts that are connected to the surface. • Endocrine: Lack a duct system. Products (hormones) are secreted into the connective tissue from where they enter the bloodstream. Cells of exocrine glands exhibit different mechanisms of secretion: • Merocrine/Eccrine secretion: product is delivered through exocytosis • Apocrine secretion: product is secreted via partial loss of the cell apical portion • Holocrine secretion: product is secreted through apoptosis. Glands: structural function • Merocrine/Eccrine secretion: product is delivered through exocytosis • Apocrine secretion: product is secreted via partial loss of the cell apical portion • Holocrine secretion: product is secreted through apoptosis. Glands: structural function • Exocrine glands are classified as: – Unicellular glands: secretion by a single cell, i.e. goblet cell (mucus secreting cells). – Multicellular glands: more than one cells and can assume different structural organization. NOTE: Mucus (viscous secretions) and serous glands (watery) named because of the type of secretion Mucus-secreting gastric cells Thank you Any questions??