Epithelial Tissue

BIOL241

Epithelial Tissue

• Overview:– Characteristics and functions of epithelia– Cell junctions– Classification of epithelia– Exocrine glands

Four tissue types in the body

• Groups of cells similar in structure and function

• The four types:– Epithelial– Connective– Muscle– Nerve

Remember the levels of organization

What is an Epithelium?

• Epi = “on” or “around”• Thele = “nipple”

• Covers the external body surface (epidermis), lines cavities and tubules, and generally marks off our insides from our outsides

• Other examples?

Epithelial Tissues – two types

• Epithelia: – layers of cells covering internal or external

surfaces

• Glands: – structures that produce secretions

Characteristics of Epithelia

1. Cellularity: composed of cells bound by cell junctions

2. Polarity: apical and basal surfaces

3. Attachment: via basal lamina to underlying connective tissue

4. Avascularity: no blood vessels (but richly innervated)

5. Regeneration: germinative cell division

Figure 4–1

Free Surface and Attached Surface

• Polarity: – apical and basolateral surfaces

Repairing and Replacing Epithelia

• Epithelia are replaced by division of germinative cells (stem cells)

• Near basal lamina

Functions of Epithelial Tissue

1. Provide physical protection

2. Control permeability

3. Move fluids over the surface

4. Provide sensation (e.g. neuroepithelia)

5. Produce specialized secretions (glandular epithelium)

Specializations

• Microvilli increase absorption or secretion• Cilia (ciliated epithelium) move fluids

Indentations & Protrusions Increase Surface Area

More Examples

Effective Barriers

• Physical integrity is maintained by:– intercellular connections– attachment to basal lamina– maintenance and repair

Cell Junctions

Cell junctions

• Tight Junctions – surround cells, waterproof– Isolates wastes in the lumen

• Gap junctions – allow rapid communication

• Desmosomes – tie cells together with great strength (like rivets)– Hemidesmosomes attachment

Figure 4–2b

Desmosomes

• CAMs, dense areas, and intercellular cement

Figure 4–2d

Attachment to Basal Lamina

• Hemidesmosomes

Figure 4–2e

Classification of epithelia

• Cell shape– Squamous: flat “square”– Cuboidal: cubes (2-D: “square”)– Columnar: tall

• Layers of cells– Simple: one layer of cells (what is a function?)– Stratified: many layers of cells (what is a

function?)

Classes of Epithelia• Based on shape and layers

Table 4–1

Classification of Epithelia

• Simple or stratified

Figure 4.1a

Classification of Epithelia

• Squamous, cuboidal, or columnar

Figure 4.1b

Simple Squamous Epithelia

• Single layer of flattened cells with disc-shaped nuclei and sparse cytoplasm

Look like a fried egg from the top• most delicate• Diffusion, friction reduction• Special names

Mesothelium:• lines body cavities (e.g. peritoneum, pleura)

Endothelium:• lines heart and blood vessels

Epithelia: Simple Squamous

Figure 4.2a

Figure 4–3a

Simple Squamous Epithelium

Epithelia: Simple Cuboidal

• Single layer of cube-like cells with large, spherical central nuclei

• Function in secretion and absorption• Present in kidney tubules, ducts and

secretory portions of small glands, and ovary surface

Epithelia: Simple Cuboidal

Figure 4.2b

Simple Cuboidal Epithelium

• Kidney tubules

Figure 4–4a

Epithelia: Simple Columnar

• Single layer of tall cells with oval nuclei• May contain microvilli• Goblet cells are often found in this layer• Function in absorption and secretion• Line digestive tract and gallbladder, small

bronchi, uterine tubes, and some regions of the uterus

Epithelia: Simple Columnar

Figure 4.2c

Epithelia: Simple Columnar

• Intestinal lining

Epithelia: Pseudostratified Columnar

• Single layer of cells with different heights; all touch the basal lamina but some do not reach the free surface

• Nuclei are seen at different layers• Function in secretion and propulsion of

mucus• Present in the male sperm-carrying ducts

(nonciliated) and trachea (ciliated)

Epithelia: Pseudostratified Columnar

Figure 4.2d

Pseudostratified Columnar Epithelium

• Trachea

Figure 4–5b

Epithelia: Transitional

• Several cell layers, basal cells are cuboidal, surface cells are dome shaped (or flat)

• Stretches to permit the distension of the urinary bladder

• Lines the urinary bladder, ureters, and part of the urethra

Epithelia: Transitional

Figure 4.2f

Epithelia: Transitional

Figure 4.2f

• Urinary bladder

Stratified epithelia

Epithelia: Stratified Squamous

• Thick membrane composed of several layers of cells (the only one with more than 2 or 3 true layers)

• Functions in protection of underlying areas subjected to abrasion

• Forms the external part of the skin’s epidermis (keratinized cells), and linings of the esophagus, mouth, and vagina (nonkeratinized cells)

Epithelia: Stratified Squamous

Figure 4.2e

Epithelia: Stratified Cuboidal

• Quite rare in the body• Found in some sweat and mammary

glands• Typically two cell layers thick• Only top layer is cuboidal

Stratified Cuboidal Epithelium

• Sweat gland ducts

Figure 4–4b

Epithelia: Stratified Columnar

• Limited distribution in the body• Found in the pharynx, male urethra, and

lining some glandular ducts• Also occurs at transition areas between

two other types of epithelia

Stratified Columnar Epithelium

• Rare• Salivary gland duct

Figure 4–5c

Epithelia: Glandular

• A gland is one or more cells that makes and secretes an aqueous fluid

• Classified by:– Site of product release – endocrine or

exocrine– Relative number of cells forming the gland –

unicellular or multicellular

Glandular Epithelia

• Endocrine and exocrine glands

Figure 4–6

Glands• Endocrine

– Ductless glands that produce hormones– Secretions include amino acids, proteins, glycoproteins, and

steroids• Exocrine• More numerous than endocrine glands• Secrete their products onto body surfaces (skin) or into

body cavities via ducts• Examples include mucous, sweat, oil, digestive, and

salivary glands• The only important unicellular gland is the goblet cell• EXAMPLES?

Goblet Cell

Figure 4.3b

Glands are classified based on mode of secretion - 3 types

Modes of Secretion

• Merocrine – products are secreted by exocytosis (e.g., pancreas, sweat, and salivary glands)

• Holocrine – products are secreted by the rupture of gland cells (e.g., sebaceous glands)

• Apocrine – products acumulate in the top of the cell and then it breaks down

Summary

• Epithelial tissue structures and functions• Cell junctions• Classification by cell shape and layers• Glands