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 layers of cells covering internal or external surfaces Glands: structures that produce secretions

Characteristics of Epithelia Cellularity: composed of cells bound by cell junctions Polarity: apical and basal surfaces Attachment: via basal lamina to underlying connective tissue Avascularity: no blood vessels (but richly innervated) Regeneration: germinative cell division

Free Surface and Attached Surface Polarity: apical and basolateral surfaces Figure 4–1

Repairing and Replacing Epithelia Epithelia are replaced by division of germinative cells (stem cells) Near basal lamina

Functions of Epithelial Tissue Provide physical protection Control permeability Move fluids over the surface Provide sensation (e.g. neuroepithelia) 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

Simple Squamous Epithelium Figure 4–3a

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 have 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 Urinary bladder Figure 4.2f

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 Exocrine More numerous than endocrine glands 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 most notable 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 accumulate 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