Epithelium

Epithelium

Consists largely of closely packed cells with little extracellular material.

Arrangement of epithelial cells

Epithelial cells are arranged in continuous sheets of either single (simple) or multiple (stratified) cells.

Epithelia's role in bodily processes

Anything that enters or leaves the body must pass through an epithelium.

Epithelial surfaces

Epithelia have a free surface (exposed) and a basal surface (attached to basal lamina).

Vascularity of epithelia

Epithelia are avascular.

Nerve supply in epithelia

Epithelia have a nerve supply.

Renewal capacity of epithelia

Epithelia have a high capacity for renewal.

Functions of epithelia

Mechanical protection, absorption, secretion, lubrication, surface for diffusion, transportation, excretion, sensory reception, reproduction.

Basement membrane

Formed by basal lamina and reticular lamina.

Basal lamina

Binds epithelial cells to underlying connective tissue.

Composition of basal lamina

Composed of Type IV collagen, glycoproteins (laminin and entactin), proteoglycans.

Role of laminin

Laminin binds Type IV collagen and integrins (hemidesmosomes).

What synthesizes the reticular lamina?

Connective tissue of the lamina propria.

Composition of reticular lamina

Made of Type III collagen and connecting collagen.

Specialized connections on lateral surfaces

Intercellular junctions.

Closest junction to the free surface

Zonula occludens (tight junction).

Proteins involved in tight junctions

Occludin and claudin.

Examples of adhering/anchoring junctions

Zonula adherens, desmosome (macula adherens), hemidesmosomes.

Function of zonula adherens

Anchor cells together and connect actin filaments between cells.

Function of desmosomes

Provide strong spot links between cells.

Function of hemidesmosomes

Join epithelial cells to basal lamina, involve integrins.

Cell adhesion molecules (CAMs) examples

Cadherins, integrins, selectins, immunoglobulin superfamily.

Function of cadherins

Anchor cells together in zonula adherens and suppress epithelial tumor cells.

What do integrins interact with?

Basal lamina (laminin, Type IV collagen) and extracellular matrix.

Function of selectins

Involved in WBC and endothelial cell recognition.

Molecules involved in immune reactions

Immunoglobulin superfamily.

Function of gap junctions

Form small diffusion channels (connexons) between cells.

What are connexons made of?

Six integral proteins called connexins.

Specializations of the free surface of epithelia

Cilia, microvilli, stereocilia.

Function of cilia

Move mucus and other substances.

Location of microvilli

Found in brush border of kidney tubule cells and striated border of intestinal absorptive cells.

Function of microvilli

Increase surface area.

Location of stereocilia

Found in the epididymis and sensory (hair) cells of inner ear.

Main categories of epithelia

Covering and glandular.

Covering epithelium classification by layers

Simple (single layer) and stratified (multiple layers).

Covering epithelium classification by shape

Squamous (flat), cuboidal (cube/hexagonal), columnar (tall).

Function of simple squamous epithelium

Specialized for active and passive movement of substances.

Special names for simple squamous epithelium

Endothelium (lining of heart and blood vessels), mesothelium (lining of ventral body cavities).

Shape of simple cuboidal epithelium

Cube or hexagonal.

Function of simple cuboidal epithelium

Secretion and absorption.

Types of simple columnar epithelium

Ciliated, non-ciliated goblet cells, absorptive cells with microvilli.

Types of stratified epithelium

Squamous, cuboidal, columnar (rare), transitional.

Types of stratified squamous epithelium

Keratinized (dry, cornified) and non-keratinized (wet, non-cornified).

What is pseudostratified columnar epithelium?

Appears multilayered, but all cells touch basement membrane.

Where is transitional epithelium found?

Pelvis of kidney, ureters, urinary bladder, part of urethra.

Characteristic of transitional epithelium

Cells change shape according to degree of organ distension.

Two classifications of glands

Exocrine and endocrine.

Exocrine glands division

Divided into lobes and lobules.

Types of exocrine gland secretions

Serous, mucous, mixed.

Important unicellular gland

Goblet cell.

Types of multicellular glands by duct

Simple (unbranched) and compound (branched).

Types of multicellular glands by secretory part

Tubular and acinar (alveolar).

Example of a simple tubular gland

Crypts of Lieberkühn (colon).

Example of a simple coiled tubular gland

Sweat gland.

Example of a simple branched alveolar gland

Sebaceous gland.

Example of a compound gland

Labial gland.