Understanding Epithelial Tissue: Structure and Functions

1. 4 PART 1 Tissues

2. Tissues Cells work together in functionally related groups called tissues Tissue A group of closely associated cells that perform related functions and are similar in structure

3. Four Basic Tissue Types and Basic Functions Epithelial tissue—covering (Chapters 4 and 5) Connective tissue—support (Chapters 4, 5, 6, and 9) Muscle tissue—movement (Chapters 10 and 11) Nervous tissue—control (Chapters 12–16 and 25)

4. Epithelial Tissue Covers a body surface or lines a body cavity Forms parts of most glands Functions of epithelia Protection Secretion Absorption Diffusion Filtration Sensory reception

5. Special Characteristics of Epithelia Cellularity Cells separated by minimal extracellular material Specialized contacts Cells joined by special junctions Polarity Cell regions of the apical surface differ from the basal surface

6. Special Characteristics of Epithelia Support by connective tissue Avascular butinnervated Epithelia receive nutrients from underlying connective tissue Regeneration Lost cells are quickly replaced by cell division

7. Figure 4.1 Special characteristics of epithelium. Cilia Narrowextracellularspace Microvilli Apical region ofan epithelial cell Cell junctions Tight junction Epithelium Adhesive belt Desmosome Gap junction Basal region Basallamina Basementmembrane Reticularfibers Nerve ending Connectivetissue Capillary

8. Classifications of Epithelia First name of tissue indicates number of cell layers Simpleepithelia Single layer of cells attached to basement membrane Stratified epithelia Multiple layers of cells Basal layer of cells attached to basement membrane

9. Classifications of Epithelia Last name of tissue describes shape of cells Squamous—cells are wider than tall (plate-like) Cuboidal—cells are as wide as tall, like cubes Columnar—cells are taller than they are wide, like columns

10. Figure 4.2 Classification of epithelia. Squamous Apical surface Cuboidal Basal surface Simple Apical surface Basal surface Stratified Columnar Classification based on numberof cell layers Classification based on cell shape

11. Table 4.1 Function of Epithelial Tissue Related to Tissue Type

12. Simple Squamous Epithelium Description—single layer; flat cells with disc-shaped nuclei Function Passage of materials by passive diffusion and filtration Secretes lubricating substances in serosae Location Renal corpuscles Alveoli of lungs Lining of heart, blood, and lymphatic vessels Lining of ventral body cavity (serosae)

13. Figure 4.3a Epithelial tissues. Simple squamous epithelium Description: Single layer offlattened cells with disc-shapedcentral nuclei and sparsecytoplasm; the simplest of the epithelia. Air sacsof lungtissue Nuclei ofsquamousepithelialcells Function: Allows passage ofmaterials by diffusion andfiltration in sites where protectionis not important; produceslubricating fluid in serosae. Location: Kidney glomeruli; airsacs of lungs; lining of heart,blood vessels, and lymphaticvessels; lining of ventral bodycavity (serosae). Photomicrograph: Simple squamous epitheliumforming part of the alveolar (air sac) walls (140).

14. Simple Cuboidal Epithelium Description Single layer of cubelike cells with large, spherical central nuclei Function Secretion and absorption Location Kidney tubules, secretory portions of small glands, ovary surface

15. Figure 4.3b Epithelial tissues. Simple cuboidal epithelium Description: Single layer ofcubelike cells with large,spherical central nuclei. Simplecuboidalepithelialcells Function: Secretion andabsorption. Basementmembrane Location: Kidney tubules;ducts and secretory portions ofsmall glands; ovary surface. Connectivetissue Photomicrograph: Simple cuboidal epitheliumin kidney tubules (430).

16. Simple Columnar Epithelium Description—single layer of column-shaped (rectangular) cells with oval nuclei Some bear cilia at their apical surface May contain goblet cells Function Absorption; secretion of mucus, enzymes, and other substances Ciliated type propels mucus or reproductive cells by ciliary action

17. Simple Columnar Epithelium Location Nonciliated form Lines digestive tract, gallbladder, ducts of some glands Ciliated form Lines small bronchi, uterine tubes, and uterus

18. Figure 4.3c Epithelial tissues. Simple columnar epithelium Description: Single layer of tallcells with round to oval nuclei;some cells bear cilia; layer maycontain mucus-secretingunicellular glands (goblet cells). Microvilli Gobletcell Simplecolumnarepithelialcell Function: Absorption; secretionof mucus, enzymes, and othersubstances; ciliated type propelsmucus (or reproductive cells) byciliary action. Location: Nonciliated type linesmost of the digestive tract(stomach to anal canal),gallbladder, and excretory ductsof some glands; ciliated varietylines small bronchi,uterine tubes,and someregions ofthe uterus. Basementmembrane Photomicrograph: Simple columnar epitheliumof the small intestine (650).

19. Pseudostratified Columnar Epithelium Description All cells originate at basement membrane Only tall cells reach the apical surface May contain goblet cells and bear cilia Nuclei lie at varying heights within cells Gives false impression of stratification

20. Pseudostratified Columnar Epithelium Function—secretion of mucus; propulsion of mucus by cilia Locations Nonciliated type Ducts of male reproductive tubes Ducts of large glands Ciliated variety Lines trachea and most of upper respiratory tract

21. Figure 4.3d Epithelial tissues. Pseudostratified columnar epithelium Description: Single layer ofcells of different heights, somenot reaching the free surface;nuclei seen at different levels;may contain mucus-secretinggoblet cells and bear cilia. Cilia Gobletcell Pseudo-stratifiedepitheliallayer Function: Secretion, particularlyof mucus; propulsion of mucusby ciliary action. Location: Nonciliated type inmale’s sperm-carrying ducts andducts of large glands; ciliatedvariety lines the trachea,most of the upper respiratorytract. Basementmembrane Photomicrograph: Pseudostratified ciliatedcolumnar epithelium lining the human trachea(780). Trachea

22. Stratified Epithelia Properties Contain two or more layers of cells Regenerate from below (basal layer) Major role is protection Named according to shape of cells at apical layer

23. Stratified Squamous Epithelium Description Many layers of cells are squamous in shape Deeper layers of cells appear cuboidal or columnar Thickest epithelial tissue Adapted for protection from abrasion

24. Stratified Squamous Epithelium Two types—keratinized and nonkeratinized Keratinized Location—epidermis Contains the protective protein keratin Waterproof Surface cells are dead and full of keratin Nonkeratinized Forms moist lining of body openings

25. Stratified Squamous Epithelium Function—Protects underlying tissues in areas subject to abrasion Location Keratinized—forms epidermis Nonkeratinized—forms lining of mucous membranes Esophagus Mouth Anus Vagina Urethra

26. Figure 4.3e Epithelial tissues. Stratified squamous epithelium Description: Thick membranecomposed of several cell layers;basal cells are cuboidal orcolumnar and metabolicallyactive; surface cells are flattened(squamous); in the keratinizedtype, the surface cells are full ofkeratin and dead; basal cells areactive in mitosis and produce thecells of the more superficiallayers. Stratifiedsquamousepithelium Nuclei Function: Protects underlyingtissues in areas subjected toabrasion. Basementmembrane Connectivetissue Location: Nonkeratinized typeforms the moist linings of theesophagus, mouth, and vagina;keratinizedvarietyforms theepidermis ofthe skin, adry membrane. Photomicrograph: Stratified squamousepithelium lining the esophagus (280).

27. Stratified Cuboidal Epithelium Description—generally two layers of cube-shaped cells Function—protection Location Forms ducts of Mammary glands Salivary glands Largest sweat glands

28. Figure 4.3f Epithelial tissues. Stratified cuboidal epithelium Description: Generally twolayers of cubelike cells. Basementmembrane Cuboidalepithelialcells Function: Protection. Location: Largest ducts ofsweat glands, mammary glands,and salivary glands. Duct lumen Photomicrograph: Stratified cuboidal epitheliumforming a salivary gland duct (290).

29. Stratified Columnar Epithelium Description—several layers; basal cells usually cuboidal; superficial cells elongated Function—protection and secretion Location Rare tissue type Found in male urethra and large ducts of some glands

30. Figure 4.3g Epithelial tissues. Stratified columnar epithelium Description: Several cell layers;basal cells usually cuboidal;superficial cells elongatedand columnar. Stratifiedcolumnarepithelium Basementmembrane Function: Protection; secretion. Underlyingconnectivetissue Location: Rare in the body;small amounts in male urethraand in large ducts of someglands. Photomicrograph: Stratified columnar epitheliumlining the male urethra (360). Urethra

31. Transitional Epithelium Description Has characteristics of stratified cuboidal and stratified squamous Superficial cells dome-shaped when bladder is relaxed, squamous when full Function—permits distension of urinary organs when they are filled with urine Location—epithelium of urinary bladder, ureters, proximal urethra

32. Figure 4.3h Epithelial tissues. Transitional epithelium Description: Resembles bothstratified squamous and stratifiedcuboidal; basal cells cuboidal orcolumnar; surface cells domeshaped or squamous-like,depending ondegree oforganstretch. Transitionalepithelium Function: Stretches readily andpermits distension of urinaryorgan by contained urine. Basementmembrane Location: Lines the ureters,bladder, and part of the urethra. Connectivetissue Photomicrograph: Transitional epitheliumlining the bladder, relaxed state (365); note thebulbous, or rounded, appearance of the cells atthe surface; these cells flatten and becomeelongated when the bladder is filled with urine.

33. Glands Endocrine glands Ductless glands that secrete directly into surrounding tissue fluid Produce messenger molecules called hormones Covered in detail in Chapter 17

34. Glands Exocrine glands Ducts carry products of exocrine glands to epithelial surface Include the following diverse glands: Mucus-secreting glands Sweat and oil glands Salivary glands Liver and pancreas

35. Unicellular Exocrine Glands (The Goblet Cell) Goblet cells produce mucin Mucin  water  mucus Protects and lubricates many internal body surfaces Goblet cells are a unicellular exocrine gland

36. Figure 4.4 Goblet cell (unicellular exocrine gland). Microvilli Secretoryvesiclescontainingmucin Golgiapparatus Rough ER Nucleus

37. Lateral Surface Features—Cell Junctions Factors binding epithelial cells together Adhesion proteins link plasma membranes of adjacent cells Contours of adjacent cell membranes Special cell junctions

38. Lateral Surface Features—Cell Junctions Tight junctions (zona occludens)—close off intercellular space Found at apical region of most epithelial tissues types Some proteins in plasma membrane of adjacent cells are fused Prevent certain molecules from passing between cells of epithelial tissue

39. Figure 4.6a Cell junctions. Interlockingjunctionalproteins Intercellularspace Tight junctions: Impermeablejunctions prevent molecules frompassing through the intercellularspace.

40. Lateral Surface Features—Cell Junctions Adhesive belt junctions (zonula adherens)—anchoring junction Transmembrane linker proteins attach to actin microfilaments of the cytoskeleton and bind adjacent cells With tight junctions, these linker proteins form the tight junctional complex around apical lateral borders of epithelial tissues

41. Lateral Surface Features Desmosomes—main junctions for binding cells together Scattered along abutting sides of adjacent cells Cytoplasmic side of each plasma membrane has a plaque Plaques are joined by linker proteins

42. Lateral Surface Features Desmosomes (continued) Intermediate filaments extend across the cytoplasm and anchor at desmosomes on opposite side of the cell Are common in cardiac muscle and epithelial tissue

43. Figure 4.6b Cell junctions. Intercellularspace Plaque Linkerglycoproteins(cadherins) Intermediatefilament(keratin) Desmosomes: Anchoring junctionsbind adjacent cells together and helpform an internal tension-reducingnetwork of fibers.

44. Lateral Surface Features—Cell Junctions Gap junctions—passageway between two adjacent cells These let small molecules move directly between neighboring cells Cells are connected by hollow cylinders of protein Function in intercellular communication

45. Figure 4.6c Cell junctions. Intercellularspace Channelbetween cells(connexon) Gap junctions: Communicatingjunctions allow ions and smallmolecules to pass from one cell to thenext for intercellular communication.

46. Basal Feature: The Basal Lamina Located at the boundary between the epithelium and connective tissue Noncellular supporting sheet between the epithelial tissue and the connective tissue deep to it Consists of proteins secreted by epithelial cells

47. Basal Feature: The Basal Lamina Functions Acts as a selective filter, determining which molecules from capillaries enter the epithelium Acts as scaffolding along which regenerating epithelial tissue cells can migrate Basal lamina and reticular layers of the underlying connective tissue deep to it form the basement membrane

48. Epithelial Surface Features Apical surface features Microvilli—fingerlike extensions of plasma membrane Have a core of actin filaments that stiffen the microvillus Abundant in kidney tubules and small intestine Maximize surface across which small molecules enter or leave cells

49. Figure 4.7 Microvilli. Microvillus Actinfilaments

50. Epithelial Surface Features Apical surface features Cilia—whiplike, highly motile extensions of apical surface membranes Contain a core of microtubules held together by cross-linking and radial proteins Microtubules arranged in pairs called doublets Movement is generated when adjacent doublets grip each other with the motor protein dynein Cilia originate as microtubules assemble around centrioles