Chapter Four: Epithelial Tissue

Epithelium

A sheet of cells that cover a body surface or lines a body cavity

Covering and Lining Epithelium

Covers the outer and inner surfaces of most body organs

Glandular Epithelium

Forms most of the body glands

Functions of Epithelia

Protection of the underlying tissues, secretion, absorption, diffusion, filtration, sensory reception

Cellularity

Epithelia are composed almost entirely of cells. They are separated by a minimal amount of extracellular material, mainly projections of their integral membrane proteins into the narrow spaces between the cells

Specialized Cell Junctions

Adjacent epithelial cells are directly joined at many points

Polarity

All epithelia have a free apical surface and an attached basal surface

Apical Surface

Abuts the open space of a cavity, tubule, gland, or hollow organ

Basal Surface

Lies on a thin supporting sheet (basal lamina) which is part of the basement membrane

Support by Connective Tissue

All epithelial sheets in the body are supported by an underlying layer of this tissue

Avascular but Innervated

Epithelial cells lack blood vessels and receive nutrients from capillaries in the underlying connective tissue. Nerve endings do penetrate epithelial sheets.

Regeneration

Epithelial tissue has a high capacity of this due to some exposure to friction and the surface cells rub off

Simple and Stratified

Describes the number of cell layers in an epithelium

Simple Epithelia

Contain a single layer of cells, which each cell attached to the basement membrane

Stratified Epithelia

Contain more than one layer of cells. The cells on the basal surface are attached to the basement membrane; those on the apical surface border an open surface. Cell shape of this type is named according to the shape of the cells in the apical layer

Squamous, Cuboidal, or Columnar

Describes cell shape and refers to the appearance of the cells in section

Squamous Cells

Flat cells with flat, disc-shaped nuclei

Cuboidal Cells

Cube-shaped cells with spherical, centrally located nuclei

Columnar Cells

Taller than they are wide, like columns. The nuclei are located near the basal surface and are commonly oval in shape, elongated from top to bottom

Simple Squamous Epithelium Description

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

Simple Squamous Epithelium Function

Allows materials to pass by diffusion and filtration in sites where protection is not important; secretes lubricating substances in serosae (lining of ventral body cavity)

Simple Squamous Epithelium Location

Kidney glomeruli; air sacs of lungs; lining of heart; blood vessels; and lymphatic vessels; serosae

Simple Squamous Epithelium Photomicrograph

Simple Cuboidal Epithelium Description

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

Simple Cuboidal Epithelium Function

Secretion and absorption

Simple Cuboidal Epithelium Location

Kidney tubules; ducts and secretory portions of small glands; ovary surface

Simple Cuboidal Epithelium Photomicrograph

Simple Columnar Epithelium Description

Single layer of tall cells with round to oval nuclei; many cells bear microvilli, some bear cilia; layer may contain mucus-secreting unicellular glands (goblet cells)

Simple Columnar Epithelium Function

Absorption; secretion of mucus, enzymes, and other substances; ciliated type propels mucus (or reproductive cells) by ciliary action

Simple Columnar Epithelium Location

Nonciliated type lines most of the digestive tract (stomach to rectum), gallbladder, and excretory ducts of some glands; ciliated variety lines small bronchi, uterine tubes, and some regions of the uterus

Simple Columnar Epithelium Photomicrograph

Pseudostratified Columnar Epithelium Description

Single layer of cells of differing heights, some not reaching the free surface; nuclei seen at different levels; may contain mucus-secreting cells and bear cilia

Pseudostratified Columnar Epithelium Function

Secrete substances, particularly mucus; propulsion of mucus by ciliary action

Pseudostratified Columnar Epithelium Location

Ciliated variety lines the trachea and most of the upper respiratory tract; no ciliated type in males’ sperm-carrying ducts and ducts of large glands

Pseudostratified Columnar Epithelium Photomicrograph

Stratified Squamous Epithelium Description

Thick epithelium composed of several cell layers; basal cells are cuboidal or columnar and metabolically active; surface cells are squamous; in the keratinized type, the surface cells are full of keratin and dead; basal cells are active in mitosis and produce the cells of the more superficial layers

Stratified Squamous Epithelium Function

Protects underlying tissues in areas subjected to abrasion

Stratified Squamous Epithelium Location

Nonkeratinized type forms the moist linings of the esophagus, mouth, and vagina; keratinized variety forms the epidermis of the skin, a dry epithelium

Stratified Squamous Epithelium Photomicrograph

Stratified Cuboidal Epithelium Description

Generally two layers of cube like cells

Stratified Cuboidal Epithelium Function

Protection

Stratified Cuboidal Epithelium Location

Largest ducts of sweat glands, mammary glands, and salivary glands

Stratified Cuboidal Epithelium Photomicrograph

Stratified Columnar Epithelium Description

Several cell layers; basal cells usually cuboidal; superficial cells elongated and columnar

Stratified Columnar Epithelium Function

Protection and Secretion

Stratified Columnar Epithelium Location

Small amounts in male urethra and in large ducts of some glands

Stratified Columnar Epithelium Photomicrograph

Transitional Epithelium Description

Resembles both stratified squamous and stratified cuboidal; basal cells cuboidal or columnar; surface cells doomed shaped or squamous-like, depending on degree of organ stretch

Transitional Epithelium Function

Stretches readily, permits stored urine to distend urinary organ

Transitional Epithelium Location

Lines the uterus, bladder, and part of the urethra

Transitional Epithelium Photomicrograph

Glands

A structure whose cells are specialized for secretion. They secrete aqueous (water-based) fluids that usually contain proteins

Secretion

The process whereby gland cells obtain needed substances from the blood and transform them chemically into a product that is then discharged from the cell

Endocrine Glands

Lack ducts and are often referred to as ductless glands. They secrete directly into the tissue fluid that surrounds them. They produce hormones which they release into the extracellular space

Hormones

Messenger molecules that are released by endocrine glands and travel in the blood to regulate specific body functions. They enter capillaries and travel through the bloodstream to specific target organs, which are far removed from the endocrine gland that produces them.

Exocrine Glands

Numerous and many of their products are familiar ones. They secrete their products onto body surfaces (skin) or into body cavities. Multicellular glands have ducts that carry product to the epithelial surfaces. Any activity Is local and is a diverse group. Includes sweat glands, oil glands, salivary glands, the liver, pancreas, mammary glands, etc.

Goblet Cell

The only major one-celled exocrine gland. They are scattered within the epithelial lining of the intestines and respiratory tubes. They produce mucin and mucus.

Globet Cell Diagram

Multicellular Exocrine Glands

Have two basic parts consisting of an epitehlium-walled duct and a secretory unit consisting of the secretory epithelium

Simple Tubular Gland

Duct does not branch and secretory structure is tubular in nature

Simple Branched Tubular

Duct does not branch and secretory structure is tubular in nature

Compound Tubular

Duct branches and tubular secretory structure

Simple Alveolar

Duct does not branch and alveolar secretory strucure

Simple Branched Alveolar

Duct does not branch and alveolar secretory structure

Compound Alveolar

Duct branches and alveolar secretory structure

Compound Tubuloalveolar

Duct branches and both tubular and alveolar secretory structures

Factors that act to bind epithelial cells to one another

Adhesion proteins in the plasma membranes of the adjacent cells link together in the narrow extracellular space

The wavy contours of the membranes of adjacent cells join in a tongue-and-groove fashion

Special Cell Junctions

Cell Junctions

Crucial structures that bind epithelial cells together, ensuring the integrity and function of tissues. Has three main types.

Tight Junction

Impermeable junctions prevent molecules from passing through the intercellular space

Desmosomes

Also known as anchoring junctions. Bind adjacent cells together and help form an internal tension-reducing network of fibers. Linker glycoproteins (cadherins) just out from each cell membrane and interdigitize like a zipper so that if one cell gets pulled, it won’t easily detach.

Gap Junctions

Communicating junctions allow ions and small molecules to pass from one cell to the next for intercellular communication

Basal Lamina Structure

At the border between the epithelium and the connective tissue. It is a thin, noncellular sheet consisted of proteins secreted by the epithelial cells

Basal Lamina Function

Acts as a selective filter determining which molecules from capillaries in the underlying connective tissue are allowed to enter the epithelium. Acts as scaffolding along which regenerating epithelial cells can migrate

Basal Membrane

Made up of reticular fibers directly deep to the bassal lamina and the basal lamina itself

Microvilli

Fingerlike extensions of the plasma membrane of apical epithelial cells. Contain a core of actin filaments that extend into the network of actin microfilaments of the cytoskeleton which stiffen the extensions. Maximize the surface area on the surface of the cell

Cilia

Whiplike, highly motile extensions of the apical surface membranes of certain epithelial cells. Each one contains a core of microtubules held together by cross-linking and radical proteins. Movement is generated when adjacent doublets grip one another with side arms made of dynein and the arms start to oscillate

Basal Body

Centriole at the base of each cilium