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What is epithelial tissue?

A sheet of cells that:

• covers a body surface, or

• lines a body cavity

What are the two types of epithelial tissue?

1. Covering and lining epithelium

• Forms the outer layer of skin

• Dips into and lines the open cavities of the:

  • urogenital system

  • digestive system

  • respiratory system

• Covers the walls and organs of the closed ventral body cavity

2. Glandular epithelium

• Forms the glands of the body

What are the functions of epithelium?

• Protection

• Absorption

• Filtration

• Excretion

• Secretion

• Sensory reception

What does polarity mean in epithelial tissue?

Epithelia exhibit apical-basal polarity because they have two different surfaces:

• Apical surface

  • Not attached to surrounding tissue

  • Exposed to either:

    • the outside of the body, or

    • the cavity of an internal organ

• Basal surface

  • Attached to the underlying connective tissue

For this reason, we say that epithelia exhibit apical-basal polarity.

What are microvilli, and what is their significance in epithelial tissue?

• Increase the exposed surface area

• In epithelia that absorb or secrete substances, microvilli are often so dense that the cell apices have a fuzzy appearance

• This fuzzy appearance is called a brush border

What are cilia, and what do they do in epithelial tissue?

• Some epithelia, such as the epithelium lining the trachea, have motile cilia

• These cilia propel substances along the free surface of the epithelium

What is the basal lamina, and what are its composition and functions?

• A thin, noncellular supporting and adhesive sheet

• Consists largely of:

  • Glycoproteins secreted by epithelial cells

  • Some fine collagen fibers

• Acts as a selective filter, determining which molecules diffusing from the underlying connective tissue are allowed to enter the epithelium

• Acts as scaffolding along which epithelial cells can migrate to repair a wound

What are the junctions in epithelial cells, and what do they do?

With the exception of glandular epithelia, epithelial cells fit closely together to form continuous sheets.

Adjacent epithelial cells are tied together by:

• Tight junctions → prevent substances from leaking through the spaces between cells

• Desmosomes → keep cells from pulling apart

What is the basement membrane, and what are its functions/components?

• Located between epithelial and connective tissues

• Reinforces the epithelial sheet

• Helps the epithelium resist stretching and tearing

• Includes:

  • the epithelial basal lamina

  • the reticular lamina

What is the reticular lamina, and what is it made of?

• Located deep to the basal lamina

• Consists of a layer of extracellular material

• Contains a fine network of collagen protein fibers

• These fibers belong to the underlying connective tissue

What does avascular mean in epithelial tissue?

Avascular = contains no blood vessels

• Epithelium is avascular

What does innervated mean in epithelial tissue, and how are epithelial cells nourished?

• Innervated = supplied by nerve fibers

• Epithelial cells are nourished by substances that diffuse from blood vessels in the underlying connective tissue

What is epithelial regeneration, and why is it important?

Epithelium has a high regenerative capacity.

• Some epithelia are exposed to friction, causing their surface cells to rub off

• Other epithelia are damaged by hostile substances in the external environment, such as:

  • bacteria

  • acids

  • smoke

• If and when apical-basal polarity and lateral contacts are destroyed, epithelial cells begin to reproduce rapidly

• As long as epithelial cells receive adequate nutrition, they can replace lost cells by cell division

What are simple epithelia, and what are their main functions?

• Consist of a single cell layer

• Typically found where a thin epithelial barrier is desirable

• Mainly involved in:

  • Diffusion or filtration → simple squamous

  • Absorption and secretion → simple cuboidal and simple columnar

• Protection is not one of their specialties

What are stratified epithelia, and what are their key characteristics?

• Composed of 2 or more cell layers stacked on top of each other

• Common in high-abrasion areas where protection is important, such as:

  • skin surface

  • lining of the mouth

• Named according to the shape of the cells in the apical layer

• Regenerate from below:

  • basal cells divide

  • new cells are pushed apically

  • they replace older surface cells

What are the characteristics of squamous cells?

• Flattened and scale-like

• Have a flattened, disc-shaped nucleus

What are the characteristics of cuboidal cells?

• Boxlike

• Approximately as tall as they are wide

• Have a spherical nucleus

What are the characteristics of columnar cells?

• Tall and column-shaped

• Have a nucleus that is elongated from top to bottom

• The nucleus is usually located closer to the cell base

Simple squamous epithelium

Description

• Single layer of flattened cells

• Disc-shaped central nuclei

• Sparse cytoplasm

• Simplest of epithelia

Function

• Allows materials to pass by diffusion and filtration in sites where protection is not important

• Secretes lubricating substances in serosae (linings of the ventral body cavity)

Location

• Kidney glomeruli

• Air sacs of the lungs

• Lining of the heart

• Blood vessels

• Lymphatic vessels

• Serosae

Simple cuboidal epithelium

Description

• Single layer of cubelike cells

• Have large, spherical, centrally located nuclei

Function

• Secretion

• Absorption

Location

• Kidney tubules

• Ducts and secretory portions of small glands

• Ovary surface

Simple columnar epithelium

Description

• Single layer of tall cells

• Nuclei are round to oval

• Many cells have microvilli

• Some cells have cilia

• May contain mucus-secreting unicellular glands (goblet cells)

Function

• Absorption

• Secretion of:

  • mucus

  • enzymes

  • other substances

• The ciliated type propels mucus or reproductive cells by ciliary action

Location

• Nonciliated type:

  • lines most of the digestive tract (stomach to rectum)

  • gallbladder

  • excretory ducts of some glands

• Ciliated type:

  • small bronchi

  • uterine tubes

  • some regions of the uterus

Pseudostratified columnar epithelium

Description

• Single layer of cells of differing heights

• Some cells do not reach the free surface

• Nuclei are located at different levels

• May contain mucus-secreting cells

• May bear cilia

Function

• Secretes substances, especially mucus

• Propels mucus by ciliary action

Location

• Ciliated variety:

  • trachea

  • most of the upper respiratory tract

• Nonciliated types:

  • sperm-carrying ducts in males

  • ducts of large glands

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, surface cells are full of keratin and dead

• Basal cells are active in mitosis and produce the cells of the more superficial layers

• It is the most widespread type of stratified epithelium

Function

• Protects underlying tissues in areas subject to abrasion

Location

• Nonkeratinized type forms the moist linings of the:

  • esophagus

  • mouth

  • vagina

• Keratinized type forms the epidermis of the skin, which is a dry epithelium

• Forms the external surface of the skin and extends a short distance into every body opening that is directly continuous with the skin

Transitional epithelium

Description

• Resembles both stratified squamous and stratified cuboidal epithelium

• Basal cells are cuboidal or columnar

• Surface cells are:

  • dome-shaped when less stretched

  • squamous-like depending on the degree of organ stretch

Function

• Stretches readily

• Allows stored urine to distend urinary organs

Location

• Lines the:

  • ureters

  • bladder

  • part of the urethra

What is endothelium?

• A type of simple squamous epithelium

• Provides a slick, friction-reducing lining in:

  • lymphatic vessels

  • all hollow organs of the cardiovascular system, including blood vessels and the heart

• Capillaries consist exclusively of endothelium

• Its exceptional thinness promotes efficient exchange of nutrients and wastes between the blood and surrounding cells

What is mesothelium?

• A type of simple squamous epithelium

• Found in the serous membranes

• These membranes:

  • line the ventral body cavity

  • cover the organs within it

What modifications of columnar cells make the digestive tract ideal for secretion and absorption?

• Columnar cells

• Dense microvilli on the apical surface of absorptive cells

• Tubular glands made primarily of cells that secrete mucus-containing intestinal juice

Together, these modifications make the digestive tract ideal for secretion and absorption.

Stratified cuboidal epithelium

• Rare in the body

• Typically has 2 layers of cuboidal cells

• Found mainly in the ducts of larger glands, including:

  • sweat glands

  • mammary glands

Stratified columnar epithelium

• Has a limited distribution in the body

• Found in small amounts in:

  • the pharynx (throat)

  • the male urethra

  • some glandular ducts

• Often occurs at transition areas or junctions between two other types of epithelia

• Only the apical layer is columnar

What is a gland, and what does it secrete?

• Consists of one or more cells that make and secrete a particular product

• This product is called a secretion

• A secretion is usually an aqueous fluid containing proteins

• Some glands instead release lipid-rich or steroid-rich secretions

What do endocrine and exocrine mean?

• Endocrine = internally secreting

• Exocrine = externally secreting

What are unicellular and multicellular epithelial glands?

Unicellular glands

• Are scattered within epithelial sheets

Multicellular epithelial glands

• Most form by invagination (inward growth) of an epithelial sheet

• The epithelial sheet grows into the underlying connective tissue

Endocrine glands

• Lose their ducts during development → often called ductless glands

• Produce hormones, which are chemical messengers

• Hormones are secreted by exocytosis directly into the extracellular space

  • From there, they enter the blood or lymphatic fluid

  • Then travel to specific target organs

• Are structurally diverse:

  • Most are compact multicellular organs

  • Some are individual hormone-producing cells scattered throughout the body, such as in the digestive tract lining (mucosa)

  • These scattered cells are collectively called the diffuse endocrine system

• Secretions vary and include:

  • modified amino acids

  • peptides

  • glycoproteins

  • steroids

• Not all endocrine glands arise from epithelial tissue

Exocrine glands

• Secrete products onto body surfaces such as the skin, or into body cavities

• Unicellular exocrine glands secrete by exocytosis

• Multicellular exocrine glands release secretions through an epithelium-walled duct

  • The duct transports the secretion to the epithelial surface

• Exocrine glands are diverse

Unicellular exocrine glands

• Include mucous cells and goblet cells

• All such glands in humans produce mucin

  • Mucin is a complex glycoprotein

  • It dissolves in water when secreted

  • Once dissolved, mucin forms mucus → a slimy coating that protects and lubricates surfaces

Goblet cells

• Accumulating mucin distends the top of the cell, making it look like a glass with a stem

• Are sprinkled among columnar cells in the epithelial linings of the:

  • intestinal tract

  • respiratory tract

Mucous cells

• Are not interspersed with other cells

• Are not goblet-shaped

Multicellular exocrine glands

Two basic parts

• Epithelium-derived duct

• Secretory unit (acinus) made of secretory cells

Supportive connective tissue

• In all but the simplest glands, supportive connective tissue:

  • Surrounds the secretory unit

  • Supplies it with blood vessels and nerve fibers

  • Forms a fibrous capsule that extends into the gland and divides it into lobes

Structural classification

• Based on the duct:

  • Simple → unbranched duct

  • Compound → branched duct

• Further classified by the shape of the secretory units:

  • Tubular → secretory cells form tubes

  • Alveolar → secretory cells form small flask-like sacs

  • Tubuloalveolar → contains both tubular and alveolar secretory units

Merocine glands

• A type of multicellular exocrine gland

• Secrete products by exocytosis

• The secretory cells are not altered in any way during secretion

• Examples include:

  • pancreas

  • most sweat glands

  • salivary glands

Holocrine gland

• A type of multicellular exocrine gland

• Secretory cells accumulate products inside themselves until they rupture

• The ruptured cells are replaced by division of underlying cells

• Holocrine secretions contain:

  • the synthesized product

  • dead cell fragments

• The only example in humans is the sebaceous (oil) glands of the skin

Apocrine gland

• There is controversy over whether humans have this gland type

• Secretory cells accumulate products just beneath the free surface

• Eventually, the apex pinches off, releasing:

  • secretory granules

  • a small amount of cytoplasm

• The cell then repairs the damage, and the process repeats