lesson 5 learning objectives

Tissue

A group of similar cells and cell products working together to perform a specific role in an organ.

Histology

The study of tissues and how they are arranged into organs.

Four primary tissue classes

Epithelial, connective, nervous, and muscular tissue.

Tissue matrix

Extracellular material surrounding cells that consists of fibrous proteins and ground substance.

Ground substance

A clear gel also known as tissue fluid, extracellular fluid, or interstitial fluid.

Ground substance contents

Water, gases, minerals, nutrients, wastes, and hormones.

Organ

A structure with discrete boundaries composed of two or more tissue types.

Epithelial tissue

A sheet of closely adhering cells, one or more cells thick, with an upper surface exposed to the environment or an internal space.

Where epithelial tissue is found

It covers body surfaces, lines body cavities, and constitutes most glands.

Epithelial tissue vascularity

Epithelial tissue is avascular and is usually nourished by blood vessels in underlying connective tissue.

Epithelial tissue functions

Protection, secretion, excretion, absorption, filtration, and sensation.

Epithelial protection

Protects deeper tissues from injury and infection.

Epithelial secretion

Produces and releases substances such as mucus, sweat, enzymes, hormones, and other substances.

Epithelial excretion

Voids wastes from tissues, such as CO2 and bile.

Epithelial absorption

Absorbs chemicals such as nutrients.

Epithelial filtration

Selectively filters substances leaving the body.

Epithelial sensation

Nerve endings in epithelia detect stimuli.

Cellularity

The characteristic of epithelial tissue in which cells are interconnected by cell junctions.

Polarity

The characteristic of epithelial cells having an orientation with apical and basal surfaces.

Attachment

The characteristic of epithelial tissue being attached to a basement membrane.

Avascularity

The characteristic of epithelial tissue lacking blood vessels.

Regeneration

The continuous replacement of epithelial cells by mitosis of epithelial stem cells.

Basal surface

The surface of an epithelial cell facing the basement membrane.

Apical surface

The surface of an epithelial cell facing away from the basement membrane.

Lateral surface

The side surface of an epithelial cell between the basal and apical surfaces.

Basement membrane

A structure beneath epithelial tissue consisting of a superficial basal lamina and a deeper reticular lamina.

Basal lamina

The clear layer secreted by the basal surface of the epithelium that helps prevent proteins and large molecules from moving from connective tissue into epithelium.

Reticular lamina

The dense layer secreted by underlying connective tissue that provides strength and filtration of materials.

Simple epithelium

Epithelium consisting of one cell layer in which every cell contacts the basement membrane.

Stratified epithelium

Epithelium consisting of multiple cell layers in which not all cells contact the basement membrane.

Squamous cell

An epithelial cell that is flat and thin.

Cuboidal cell

An epithelial cell that is approximately as tall as it is wide.

Columnar cell

An epithelial cell that is taller than it is wide.

Eight types of epithelium

Simple squamous, simple cuboidal, simple columnar, pseudostratified columnar, stratified squamous, stratified cuboidal, stratified columnar, and transitional epithelium.

Simple squamous epithelium

A single layer of thin, flat cells specialized for rapid diffusion, filtration, and exchange.

Simple squamous structure

One thin layer of flat cells with flattened nuclei.

Simple squamous location

Found in alveoli, blood vessels, lymphatic vessels, serous membranes, and other areas requiring rapid exchange.

Simple squamous function

Allows rapid diffusion, filtration, and exchange while providing a smooth surface.

Endothelium

Simple squamous epithelium lining the inner surface of the heart and blood vessels.

Mesothelium

Simple squamous epithelium lining body cavities such as the pleura, pericardium, and peritoneum.

Simple cuboidal epithelium

A single layer of cube-shaped cells specialized for secretion and absorption.

Simple cuboidal structure

One layer of cells that are approximately as tall as they are wide, with round central nuclei.

Simple cuboidal location

Found in kidney tubules, small gland ducts, thyroid follicles, and other areas.

Simple cuboidal function

Secretion and absorption.

Simple columnar epithelium

A single layer of tall cells specialized for absorption and secretion.

Simple columnar structure

One layer of tall cells with elongated nuclei usually located toward the basal region.

Simple columnar location

Found in much of the digestive tract and other areas involved in absorption and secretion.

Simple columnar function

Absorption and secretion.

Pseudostratified columnar epithelium

A single layer of cells in which all cells contact the basement membrane but the nuclei are at different levels, making the tissue appear stratified.

Pseudostratified columnar structure

All cells contact the basement membrane, but not all cells reach the apical surface.

Pseudostratified columnar nuclei

Nuclei occur at different heights, creating a false appearance of multiple layers.

Pseudostratified columnar location

Commonly found lining the respiratory tract.

Pseudostratified columnar function

Secretion and movement of material along the epithelial surface, especially when cilia are present.

Stratified squamous epithelium

Multiple layers of cells with flattened cells at the apical surface.

Stratified squamous function

Protection from mechanical stress, pathogens, and other environmental damage.

Keratinized stratified squamous epithelium

Stratified squamous epithelium whose apical cells contain keratin and are dead.

Keratinized stratified squamous location

Found in the epidermis of the skin.

Nonkeratinized stratified squamous epithelium

Stratified squamous epithelium whose apical cells remain alive and lack a thick keratin layer.

Nonkeratinized stratified squamous location

Found in areas such as the mouth, esophagus, and vagina.

Stratified cuboidal epithelium

Usually two or more layers of cuboidal cells.

Stratified cuboidal function

Protection and secretion.

Stratified cuboidal location

Found in larger ducts of glands such as sweat glands and mammary glands.

Stratified columnar epithelium

Multiple layers with columnar cells at the apical surface.

Stratified columnar function

Protection and secretion.

Stratified columnar location

Found in relatively rare locations, including portions of the male urethra and some large ducts.

Transitional epithelium

A stratified epithelium specialized to stretch and return to its original shape.

Transitional epithelium structure

Basal cells are cuboidal or columnar, while apical cells become rounded or dome-shaped when relaxed.

Transitional epithelium function

Allows organs to expand and contract while maintaining a protective barrier.

Transitional epithelium location

Lines much of the urinary tract, including the urinary bladder.

Why transitional epithelium is named transitional

Its appearance changes as the tissue stretches and relaxes.

Apical cells in relaxed transitional epithelium

Apical cells appear rounded or dome-shaped.

Apical cells in stretched transitional epithelium

Apical cells become flatter.

Epithelial classification by layers

Epithelium is classified as simple or stratified based on the number of cell layers.

Epithelial classification by shape

Epithelium is classified as squamous, cuboidal, or columnar based on the shape of the cells at the apical surface.

How to identify stratified epithelium

Classify it based on the shape of the cells at the apical surface, not the basal cells.

How to identify pseudostratified epithelium

All cells contact the basement membrane even though nuclei occur at different levels and some cells do not reach the apical surface.

Epithelial structure-function relationship

Thin epithelia are generally specialized for exchange, diffusion, or filtration, while thicker or stratified epithelia provide greater protection.

Why simple squamous epithelium is thin

Its thin structure minimizes the distance substances must travel during diffusion or filtration.

Why stratified epithelium provides protection

Multiple layers provide additional cells that can resist mechanical stress and injury.

Microvilli on epithelial cells

Apical projections that increase surface area for absorption.

Cilia on epithelial cells

Apical projections that move material across the epithelial surface.

Exfoliative cytology

Removal and examination of epithelial cells or fluids produced from epithelial linings for abnormal cellular changes.

Pap test

An example of exfoliative cytology used to examine epithelial cells for abnormal changes.

Cellular junction

A connection between two cells that helps cells communicate, resist mechanical stress, or control movement between cells.

Three major cellular junction types

Tight junctions, gap junctions, and desmosomes/hemidesmosomes.

Tight junction

A zipper-like interlocking linkage between adjacent cells formed by transmembrane cell-adhesion proteins.

Tight junction location

Encircles an epithelial cell near its apical surface.

Tight junction function

Seals the intercellular space and prevents substances from passing between adjacent cells.

Tight junction example

Tight junctions in the stomach and intestines help prevent digestive juices from entering underlying connective tissue.

Gap junction

A communicating channel between cells formed by ring-like proteins.

Connexon

The entire protein structure forming a gap junction channel.

Connexon structure

A ring of six transmembrane proteins arranged around a water-filled channel.

Gap junction function

Allows ions, nutrients, and other small solutes to pass directly between cells.

Gap junction locations

Cardiac muscle, smooth muscle, embryonic tissue, lens, and cornea.

Gap junction importance in cardiac muscle

Allows communication between cardiac muscle cells and helps coordinate contraction.

Desmosome

A strong patch-like junction that holds adjacent cells together and resists mechanical stress.

Desmosome function

Prevents cells from pulling apart.

Desmosome location

Common in the epidermis and other epithelia.

Desmosome cytoskeleton connection

Hook-like, J-shaped proteins arise from the cytoskeleton and attach to the junction.

Intermediate filaments and desmosomes

Intermediate filaments attach to desmosomes and help provide mechanical strength.