Module 1.2: Epithelial Tissue N. Swailes, Ph.D. Department of Anatomy and Cell Biology Rm: B046A ML Tel: 5-7726 E-mail: [email protected] Recommended Reading Mescher AL, Junqueira’s Basic Histology Text and Atlas, 13th Edition, Chapter 4 (also via AccessMedicine) Learning Objectives 1) Describe the general characteristics of epithelial tissue 2) Determine that epithelial cells have polarity 3) Identify and explain the functions of: a. apical domain modifications (microvilli, cilia, stereocilia) b. lateral domain modifications (tight, adherens, gap and desmosome junctions) c. basal domain modifications (hemidesmosomes and basement membrane) 4) Employ criteria to fully classify the different types of epithelia in the body and, for each, suggest a function for the epithelium based on its morphology 5) Describe the epithelial origins of glands 6) Classify the different types of glands based upon their organization and secretion type Introduction The human body is made up of only four basic tissues: 1. Epithelial tissue 2. Connective tissue 3. Muscle tissue 4. Nervous tissue By adjusting the organization, composition and special features associated with each of these tissues it is possible to impart a wide variety of functions to the region or organ that they form. During this module you will examine the basic histological structure and function of Epithelial Tissue. You will learn to identify and fully classify this tissue and the glands derived from it. 1 | Page: Epithelial Tissue S w a i l e s Ectoderm: skin (epidermis) exterior oral cavity Part A: General Characteristics of Epithelial Tissues surfaces anal canal A1. An introduction to epithelia There are many classes but only two types of epithelia: i. Surface/Covering/Lining epithelium - is composed of cells that cover exterior body surfaces or line closed cavities and body tubes that communicate with an external surface. Mesoderm: - is derived from one of the three endothelium embryonic germ layers. mesothelium Endoderm: digestive closed cavities ii. Glandular epithelium urinary tubes - develops from surface epithelium. respiratory - See Part 3: Glandular Epithelium. A2. Epithelial tissue is avascular & has polarity i. Epithelia are avascular Nutrients and oxygen must therefore diffuse from capillaries in the adjacent connective tissue across a region known as the basal lamina. This organization allows epithelial cells to be easily and regularly shed, or to rupture and release their contents without suffering vascular damage. ii. Epithelial tissue has structural polarity Apical domain - located adjacent to the external environment. Apical domain - site of functional specializations. Lateral domain Lateral domain Lateral domain - located adjacent to neighboring cells - site of intercellular attachments that regulate movement of molecules and allow intercellular communication. Basal domain - located adjacent to the basement membrane - site of junctional attachment (hemidesmosome) Basal domain to underlying connective tissue. 2 | Page: Epithelial Tissue S w a i l e s A3. The Basal Domain: Basement membrane The basement membrane is the interface between an epithelium and the connective tissue below. It is composed of collagen fibers, ground substance and a series of proteins involved in anchoring epithelial hemidesmosomes to the connective tissue. Intermediate filaments Thin (40-120nm) so not usually visible in histological (keratins) sections unless a special stain is used. However electron cell Epithelial micrographs reveal that it is composed of two distinct Integrin regions: Laminin Lucida Cross-linking proteins i. Basal lamina Secreted by the epithelial cells it has two lamina Basal components: Densa Type IV aris Type VII Lamina lucida Basement membrane Basement (anchoring fibrils) - electron lucent layer - contains laminin which is anchored to Type III (reticular fibers) integrin component of a Lamina reticul Lamina hemidesmosome Lamina densa - electron dense layer - composed of Type IV collagen fibers crosslinked to laminin by a number of proteins to form a tightly woven complex ii. Lamina reticularis A layer of Type III collagen fibers (reticular fibers) cell Epithelial that are secreted by fibroblasts in the surrounding Hemidesmosome connective tissue. LL BL The reticular fibers are anchored to the basal LD lamina by looping Type VII collagen fibrils called ‘anchoring fibrils’ Laminin LR Cross-linking proteins and Type IV Basement membrane Basement Type VII Function (anchoring fibrils) The basement membrane is important because it: and Type III (reticular fibers) - attaches the epithelium to the connective tissue - regulates exchange of material - binds factors that control proliferation, differentiation and metabolism of epithelia - orients movement of cells (disruption in tumors leads to metastasis) 3 | Page: Epithelial Tissue S w a i l e s A4. The Lateral Domain: Junctions There are a variety of cell junctions that hold epithelial cells together, regulate movement and allow inter cellular communication. Most of these junctions are located in the lateral domain. Their detailed function is beyond the scope of this histology course but knowing their names and a brief overview of their function is useful as their disruption can result in a wide range of predictable clinical presentations. i. Tight junctions (zonula occludens) - form an apical seal around cells ii. Adherens junctions (zonula adherens) - composed of claudins and occludins - also form an apical belt encircling the cell - ensure molecules are transported - composed of cadherins and catenins through cells via basal lamina and not - reinforce tight junctions in between them - are attached to the actin cytoskeleton - fewer tight junctions means cells are - play a role in movement and more permeable to ions and water stability of the plasma membrane - stop cells from drifting apart iii. Desmosomes (macula adherens) - form ‘spot welds’ - hold cells together by interaction between desmogleins and keratin cytoskeleton iv. Gap junctions - composed of connexin proteins that v. Hemidesmosomes form pores between cells - note these are only located at basal domain - permit the movement of molecules, and - attach cells to basal lamina (see previous) therefore communication, between cells 4 | Page: Epithelial Tissue S w a i l e s A5. The Apical Domain: Specializations In many epithelial cells the apical domain exhibits special structural surface modifications to carry out specific functions. The four types of modification are: i. Microvilli Actin filaments Structure - finger-like cytoplasmic processes Myosin I - contain a core of actin filaments - actin filaments are crosslinked by actin binding proteins (e.g. fimbrin, espin) Actin binding - actin filaments are anchored proximally to the proteins terminal web of actin and spectrin molecules - have an outer surface of carbohydrates called a glycocalyx that function in cell recognition Terminal Note: Microvilli are only visible as a ‘striated’ or web ‘brush’ border of closely packed projections in tissue sections Function Increases the surface area across which absorption can occur (in small intestine and renal tubules) ii. Stereocilia Actin filaments Structure Actin - long microvilli found only in the epididymis and binding the inner ear proteins - contain a core of actin filaments - actin filaments are crosslinked by actin binding Cytoplasmic proteins bridges - actin filaments are anchored proximally to the terminal web by alpha actinin - branching (cytoplasmic bridges) - non-motile (even though they are called cilia!) Terminal Function web Increase surface area for absorption (in epididymis) Signal generation (in the inner ear) 5 | Page: Epithelial Tissue S w a i l e s 9 1 dynein 2 8 3 iii. Kinocilia (cilia) 7 4 6 5 Structure 9+2 arrangement - long hairlike projections - form tufts along the apical surface of cells - contain a core of microtubules arranged in a 9 + 2 arrangement (axoneme) - microtubules interact with dynein arms in presence of ATP to generate movement - insert proximally into a region called the basal body (equivalent to a centriole) located at the base of each cilium microtubules Function - beat with a metachronal rhythm - moves fluid and mucus along the luminal surface (e.g in uterine tube or upper respiratory tract) Basal body keratin iv. Keratin Structure - a type of intermediate filament found in cells - in the skin and oral cavity, specialized epithelial cells called keratinocytes synthesize keratin epithelial cells - as the cells get older they die and the (keratinocytes) keratin is released forming a layer of filaments on the surface Function - forms a protective layer that resists abrasive forces 6 | Page: Epithelial Tissue S w a i l e s Part B: How To Classify Epithelial Tissues The traditional criteria for classification of epithelium is descriptive and based on three factors: B1. Cell shape Observing the cells located at the surface of the epithelium (when more than one layer is present) determine if the cells are: i. Squamous - the width of the cell is greater than its height - this cell type is often involved in diffusion Diffusion* ii. Cuboidal *disclaimer: these are - the width of the cell is equal to its height the most common roles, actual roles may differ - this cell type is often involved in secretion Secretion* depending on the tissue or organ involved iii. Columnar - the height of the cell exceeds its width - this cell type is often involved in absorption Absorption* B2. Number of cell layers Observe the epithelium to