119 Tissues are collections of specialized cells and cell products organized to perform one or more select functions Histology = study of tissues All cells in the body are classified into one of the four tissue types: Epithelial (Epithelium and Glands) Connective Nervous Muscular TISSUES

120 Epithelial Tissue: our first tissue type Types of Epithelial Tissue 1. Epithelial sheets (“epithelium”) 2. Glands

Epithelial Tissue Lecture Outline 1.Characteristics of Epithelial Sheets 2.Cell-Cell Junctions : Gap Junctions, Tight Junctions, Desmosomes, and Lateral Interdigitations 3.Naming Epithelial Sheets by Thickness and Cell shape 4.Glands : Endocrine and Exocrine 121

122 Locations: covers body surface (outermost part of skin) lines all hollow structures in body Functions: Physical protection Control permeability Produce specialized secretions Epithelium: a layer of tightly adjoining related by their embryology and function

123 Characteristics of Epithelial sheets: Cells are polarized Little space between cells Cells tightly held together by junctions Avascular (no blood vessels between cells) Specialized for absorption or protection Continuous rate of cell division

124 Epithelial Cells have Polarity (sided-ness) Apical (free or exposed surface) Basal (surface anchored to connective tissue) Lateral (neighboring epithelial cells)

125 Epithelial cells are attached at the basal surface to a basement membrane The epithelial cells are attached to connective tissue through the glue-like proteins of the basement membrane. We sometimes use the phrases “basal lamina” and “basement membrane” interchangeably

126 The avascular concept Little space between adjacent cells for blood vessels. Epithelial cells are usually kept alive by diffusion of oxygen and nutrients from the blood vessels in the adjacent connective tissue below the basal lamina Epithelial layers are limited in thickness – diffusion is poor over long distances or dense areas Epithelium one layer in thickness Epithelium multiple layers in thickness

127 Some Epithelial Cells may have specializations on their apical surfaces MICROVILLI Small fingerlike projections of the apical or basal surface of an epithelial cell (may be hundreds per cell) Microvilli increase surface area for the cell

128 Some Epithelial Cells have specializations on their apical surfaces CILIA Long slender extensions of the apical cell surface Cilia have a core of microtubules Cilia exhibit rhythmical movement

129 cilia

130 Epithelial Cells are tightly held to their neighboring epithelial cells Cells attach via cell adhesion molecules Cells attach at specialized cell junctions: Gap junctions, Desmosomes, Tight junctions, Lateral Interdigitations

131 Connections between Epithelial Cells : Gap Junctions Cells are connected by membrane proteins that form pores or channels between cells Cytoplasm of one cell is continuous with cytoplasm of adjacent cell allowing ions to move between cells Important in electrical signaling and organs where cells work in close synchrony

132 Connections between Epithelial Cells : Tight Junctions The plasma membranes of adjacent cells are “fused” together by membrane proteins

133 Connections between Epithelial Cells : Tight Junctions Tight junctions form an occluded zone just under the apical surface where the neighboring cells have fused their membranes together Extends all the way around the cell, with different neighbors Water and solutes cannot pass between cells Apical surface

134 Connections between Epithelial Cells : Desmosomes Adjacent cells joined by membrane proteins and a glue-like material

135 Review: Tight Junctions, Gap Junctions, and Desmosomes and their relative locations

136 Lateral interdigitations Adjacent cells have intertwined membranes Tight Junction Desmosome Lateral Interdigitation

137 Basement membrane joins epithelial layer to underlying connective tissue layer Epithelial stem cells replace short-lived epithelial cells –Because epithelial sheets are in vulnerable body positions, they have a high rate of cell division to constantly replace damaged cells –Remember that carcinomas, tumors of epithelial cells, account for 90% of all cancers. Other features of typical epithelium

138 Number of cell layers –Simple –Stratified Shape of apical surface cells –Squamous –Cuboidal –Columnar Epithelial sheets are described with 2 adjectives One cell thick More than one cell thick Flat, broad cells Cube-shaped Tall, narrow cells

139 Simple Squamous Epithelium Single layer of flat cells; little protection, allows for fast diffusion Locations: lines all the cardiovascular organs (heart, vessels), air spaces in lungs, etc.

140 Simple Cuboidal Epithelium One layer thick; cells block–like in shape (as tall as they are wide) Function: allows for regulated exchange; cytoplasm typically holds many mitochondria to drive active transport processes; may have microvilli Locations: kidneys, ducts

141 Simple Columnar Epithelium Cells taller than they are wide, one layer thick; cells often have microvilli or cilia on apical surface Functions: Some protection; active transport; often have mucous cells interspersed Locations: intestines

Stratified Squamous Epithelium Many layers of cells thick apical surface is flattened grows from basal surface Functions: Mainly as a barrier - denies access to connective tissue below; does not allow for exchange through epithelium Locations: surface of skin, lining of mouth, anus, vagina, etc. 142

143 Transitional Epithelium Unique to urinary system

Ciliated Pseudostratified Columnar Epithelium the special epithelium of the respiratory system The ciliated surface of the respiratory tract 144

145 Endocrine glands Release hormones into surrounding fluid to be picked up and carried away by the blood Exocrine glands Secrete through ducts onto the surface of the organ Glandular epithelia cells of epithelial line which are specialized to produce and secrete materials outside the cell Merocrine (product released through exocytosis) Apocrine (involves the loss of both product and cytoplasm) Holocrine (destroys the cell)

146 The Structure of Endocrine Glands Product is released from cell into extracellular fluid Product is picked up and carried away by bloodstream where it may affect distant tissues

147 Mechanisms of Exocrine Gland Secretion: Merocrine Product is packaged into secretory granules Released from cell into duct by exocytosis Example: salivary glands, pancreas

148 Mechanisms of Exocrine Gland Secretion Apocrine Apical portion of cell is shed, taking product with it into duct Example: mammary glands

149 Mechanisms of Exocrine Gland Secretion: Holocrine The entire cell bursts, releasing product into duct. Cell dies and must be replaced Example: oil glands associated with hair follicles

150 Individual secretory cells are embedded within an epithelium (example: Goblet cells) Structure of Unicellular Glands

151 Structural Classification of Multicellular Glands Don’t memorize these terms, just appreciate the different levels of complexity