Causes of dyspnoea

Epithelia

Assoc. Prof. Angela Barbour Department of Pathology

angela.barbour@unimelb.edu.au

Unless otherwise specified, images are the author’s own

Foundations of Biomedical Science 2015

Kerr, Atlas of Histology Mosby. Fig 14.10b 0

Introduction

• Epithelia are an important part of most organs in the body

• Some epithelia line surfaces, others are the major cell type of organs e.g. liver, thyroid, pancreas

• Many diseases involve epithelia • The most common cancers arise in epithelia

e.g. in breast, prostate, colon, lung, skin

Lecture outline

• Types and characteristics of epithelia • Surface epithelia

– Classification – Functions – Types – Polarity and cell adhesion – Basement membrane – Mucosa and serosa

• Glandular epithelia – Endocrine, exocrine – Morphological types – Types of exocrine secretion

• Skin

Objectives Following this lecture and after further reading and study students should be able to:

• Name the different types of surface epithelia and their locations

• Relate the structure of the different types of surface epithelia to their function

• Recognise the different types of surface epithelia

• Name and explain different surface specialisations of epithelia including cell junctions and cell adhesion molecules

• Explain the structure and function of basement membranes

• Explain the terms mucosa and serosa

• Name and explain the main types of glandular epithelia

• Appreciate that exocrine glandular epithelia have varying morphologies and secrete a variety of different substances

• Explain the basic nature and role of myoepithelial cells

• Understand and explain relevant terminology

Epithelial tissue

• Surface epithelium: lines surfaces and lumina. Everything that enters or leaves the body must pass across an epithelium

• Skin • Gastrointestinal tract • Respiratory system • Kidney • Reproductive tract

• Glandular epithelium: predominantly involved in secretion. Include – Single cells – Invaginations of multiple cells forming glands – Solid organs e.g. pancreas, thyroid

• Surface epithelia may also be components of glands e.g. ducts

Functions

• Protection • Barrier, selective diffusion • Absorption • Secretion • Receptors e.g. smell, taste

Characteristics of epithelial cells

• Exhibit polarity: apical, lateral and basal domains

• Connected by cell junctions

• Supported by a basement membrane

• Avascular

Classification of surface epithelium

• Based on – Number of cell layers

• Simple • Stratified

– Shape of cells (of top-most layer for stratified epithelium) • Squamous • Cuboidal • Columnar

– Surface specialisations e.g. • Cilia • Keratinisation

Functions of surface epithelium

• Protection • Absorption • Secretion • Barrier, selective diffusion

Simple squamous

e.g. mesothelium, endothelium, lining of alveoli, glomeruli

Simple cuboidal

e.g. thyroid follicles, renal tubules

Simple columnar

• Non-ciliated e.g. stomach, small and large intestines, gallbladder and bile ducts, endocervix

• Ciliated: Fallopian tubes, bronchioles

Pseudostratified ciliated columnar

e.g. respiratory tract (Non-ciliated: epididymis and vas deferens)

Stratified squamous

• Keratinising: skin • Non-keratinising: oral cavity, oesophagus, anus, vagina, ectocervix

Stratified cuboidal

e.g. some ducts

Other stratified

• Surface columnar layer overlying myoepithelial layer – Breast – Sweat glands – Salivary glands

• Surface columnar layer overlying basal layer – Prostate

Transitional/urothelium

Renal pelvis, ureters, bladder

Polarisation

• Outer (apical) surface – Microvilli, stereocilia, cilia – Biochemical modifications

• Inner (basal) surface attaches to basement membrane

• Lateral surfaces attach to adjacent epithelial cells • Each surface functionally specialised

Apical surface: microvilli

• 0.5 - 1μm in length • Most epithelia have only a

few, some have numerous: striated or brush border, just seen on LM

• Increase surface area by a factor of 20 e.g. in small intestine and proximal renal tubules

• Contain cytoskeletal elements

Kerr, Atlas of Histology Mosby. Fig 13.18

Kerr, Atlas of Histology Mosby. Fig 13.3c

Apical surface: cilia

• Longer finger-like projections

• 2 - 10 μm long • Organised core of

microtubules allowing movement in co-ordinated waves

• E.g. in respiratory tract, fallopian tubes

Kerr, JB, Atlas of Functional Histology, Mosby, 0723430721, Fig. II.2b

Cilia

Junqueira et al, Basic Histology 9th ed Lange. Fig 17-2 Junqueira et al, Basic Histology 9th ed Lange. Fig 17-3

Intercellular junctions

http://www.columbia.edu/itc/hs/medical/sbpm_histology_old/lab/lab02_micrograph.html

Tight junction http://163.178.103.176/Fisiologia/general/celulas/FG03_14a.jpg

Seal intercellular spaces to block passage of substances between cells, form a continuous circumferential band around the apex of cells

Adherens junction http://163.178.103.176/Fisiologia/general/celulas/FG03_14b.jpg

Adhering junctions and desmosomes: mechanically strong attachments between cells, link cytoskeletons

Gap junction http://163.178.103.176/Fisiologia/general/celulas/FG03_14c.jpg

Gap, nexus or communicating junctions: allow passage of small molecules, communication

Junctional complex

http://www.columbia.edu/itc/hs/medical/sbpm_histology_old/lab/lab02_micrograph.html

Hemidesmosome http://celljunctions.med.nyu.edu/hemidesmosomes/hemiframe.html

Modified desmosomes that link epithelial cells to underlying basement membrane

Cell adhesion molecules (CAMS)

• Transmembrane proteins • Link with CAMS on neighbouring cells or matrix • Involved in cell adhesion, intracellular and extracellular

communication, cell movement and differentiation • Different types on different cells • 4 main groups

– Cadherins: component of adherens junctions – Integrins: interact between actin and extracellular matrix

molecules – Selectins: mediate white blood cell-endothelial cell interactions – Immunoglobulin superfamily: mediate homotypic cell-cell

adhesions e.g. ICAM, CCAM, PECAM

Basement membrane

• Interface between support tissues and parenchymal cells e.g. epithelia

• Mainly produced by the cells being supported i.e. epithelia • 3 layers on EM • Components

– Extracellular matrix: predominantly collagen IV, heparan sulphate and structural glycoproteins: laminins, fibronectin. Laminins and fibronectin are involved in linking integrins of epithelial cells to extracellular matrix

• Underlying collagen VII and reticulin • Collagen VII involved in attachment of BM to underlying

connective tissue

Basement membrane

PAS/haematoxylin

Methanamine silver

http://www.dako.com/au/ar41/p235751/prod_products.htm

Basement membrane

• Muscle cells, adipocytes and Schwann cells surrounded by similar layer (basal lamina)

• Usually too thin to see on LM • Functions

– Structural support – Control of epithelial growth – Links epithelium to underlying tissue – Selective barrier to nutrients

• Underlying connective tissue

Epithelium and underlying connective tissue

http://163.178.103.176/Fisiologia/general/celulas/Membrane%20Structure%20and%20Function.htm

Related terms • Epithelium and its underlying connective tissue are given names in certain

situations • Mucosa or mucous membrane

– Lines body passages that communicate with the exterior e.g. respiratory and alimentary tracts

– Comprises surface epithelium and underlying supportive connective tissue (lamina propria), sometimes also with an underlying layer of smooth muscle (muscularis mucosae)

– Associated glands that secrete mucus or other secretions onto the epithelium

• Serosa, serous membrane – Lines pericardial, pleural, peritoneal cavities and tunica vaginalis – Comprises surface mesothelium and underlying supportive connective

tissue

Glandular epithelium

• Predominantly involved in secretion – Exocrine: secrete contents onto epithelial

surface or duct • Single cell: goblet cell • Invaginations forming secretory structures • Solid organ

– Endocrine: secrete into blood • Single cell: neuroendocrine • Solid organ

Types of exocrine gland structures

Junqueira et al., 1998, Basic Histology, 9ed, 0-8385-0590-2, Lange, Fig 4.16

Simple tubular

e.g. colon

Simple coiled

e.g. sweat glands

Simple branched tubular

e.g. stomach

What do exocrine glands secrete?

• Proteins, lipids or glycoproteins • Serous secretion: protein in aqueous medium

– E.g. pancreatic acini, salivary glands, lacrimal glands

• Mucous secretion: glycoprotein in aqueous medium – E.g. respiratory, GIT, cervix

Goblet cells

• Mucus secreting cells packed with membrane bound mucous droplets

• Nucleus, organelles displaced basally

• Mucus non-staining in H&E • Unicellular glands

Gartner and Hiatt, Colour Textbook of Histology 2nd ed WB Saunders. Fig 5-21

Kerr, Atlas of Histology Mosby. Fig 13.3c

Serous acini

• Serous cells often form a secretory unit called an acinus

• Secrete into common, narrow lumen

• Secretion carried away by a duct

• Basophilic at base, acidophilic at apex

Kerr, Atlas of Histology Mosby. Fig 3.18

Gartner and Hiatt, Colour Textbook of Histology 2nd ed WB Saunders. Fig 18-1

Acinus

Secretory parts connect to duct system

Gartner and Hiatt, Colour Textbook of Histology 2nd ed WB Saunders. Fig 18-5 Ross and Pawlina, 2011 Histology, a Text and Atlas, 6 ed Walters Kluwer. Fig 51

DUCT

ED = excretory duct, SD = Striated duct

Myoepithelial cells

Breast, salivary and sweat glands

Renewal of epithelial cells

• Epithelial cells have finite life span • Are labile or stable • Replaced from stem cells, in basal layer of

stratified epithelia • Cells move upwards – shed at surface • Frequently become malignant: carcinoma • Contain cytokeratin intermediate filaments:

can be used to identify epithelial phenotype