Epithelium 2008

TISSUE • An association of cells and intercellular materials morphologically and physiologically integrated for the performance of specific functions

EPITHELIUM A tissue consisting almost totally y of an aggregation of cells in apposition, specialized i li d for f absorptive, secretory, secretory excretory, or p protective functions, and resting on a basement membrane.

An example of epithelium

CHARACTERISTICS OF EPITHELIA: SEPARATION • True epithelia i h li are separated from the underlying CT – “Epithelioid Epithelioid” ” tissue lacks the BL but is similar – BL usually PAS+

The periodic acid/Schiff (PAS) reaction

BASEMENT MEMBRANE • Sometimes also called “basal lamina” – Not exactly the same but close enough

• A definitive characteristic – Isolates epithelium from underlying CT – All true epithelia h have it

BASEMENT MEMBRANE • Sometimes also called “basal lamina” – Not exactly the same but close enough

• A definitive characteristic – Isolates epithelium from underlying CT – All true epithelia h have it

CATEGORIES OF EPITHELIA: “COVERING & LINING”

• Neatly y classifiable • Lines hollow organs and forms sheets: Epidermis of skin, Lining of glandular ducts, Lining of blood vessels

CATEGORIES OF EPITHELIA: “GLANDULAR” GLANDULAR OR “SECRETORY” SECRETORY

• Forms solid masses – Usually secretory

• Not neatly classified – Exocrine & endocrine glands

Pancreas: an example of glandular epithelium

CHARACTERISTICS OF EPITHELIA: PROLIFERATION & HIGH TURNOVER Cells C ll short shorth -lived li lived: d: typically days to weeks. M t epithelia Most ith li also have significant regeneration capability. Right: g Proliferating g cells in intestine, BRDU stain

CHARACTERISTICS OF EPITHELIA: POLARITY

• Cells have ha e a “top” and “bottom” i.e., different activities take place at different places. • Most obvious in epithelial i h li l sheets h – True of most epithelia

CHARACTERISTICS OF EPITHELIA: POLARITY APICAL

• Cells have ha e a “top” and “bottom” i.e., different activities take place at different places. • Most obvious in epithelial i h li l sheets h – True of most epithelia

LATERAL

BASAL

CHARACTERISTICS OF EPITHELIA: VASCULARITY

– Epithelial sheets (“C&L”) are AVASCULAR • Nourished N i h db by diff diffusion i only l – The reason for rapid loss & replacement

CHARACTERISTICS OF EPITHELIA: VASCULARITY

• EPITHELIAL MASSES (Secretory epithelium) ARE VASCULAR – All cells near a BV!

CHARACTERISTICS OF EPITHELIA: VASCULARITY

• EPITHELIAL MASSES (Secretory epithelium) ARE VASCULAR – All cells near a BV!

FUNCTIONS OF EPITHELIA • Secretion: S ti Mainly M i l glandular l d l but b t C&L forms f can be b secretory – Secretory sheets found in reproductive tract, conjunctiva & GI tract

GOBLET CELLS

Common in many epithelial sheets! Secretes mucinous material

FUNCTIONS OF EPITHELIA

• Interchange with the environment : Gas exchange – Nutrient absorption – Skin functions

FUNCTIONS OF EPITHELIA • Protection: – UV barrier – Physical barrier to infection – Pigmentation/camouflage

FUNCTIONS OF EPITHELIA

• Excretion: – Disposal of nitrogenous waste by sweating

FUNCTIONS OF EPITHELIA • Stimulus Reception: – Chemotactic sensations: olfaction and gustation

OLFACTORY EPITHELIUM

TASTE BUDS IN TONGUE

CLASSIFICATION OF C&L EPITHELIA

• ONLY TWO CRITERIA – SHAPE of cells in the TOP layer – NUMBER of layers 1 = “simple” >1 = “stratified”

CLASSIFICATION OF C&L EPITHELIA BY SHAPE

SQUAMOUS • Cells have a “Fried Fried Egg Egg” shape; irregular outlines • EXAMPLES: Lining of blood vessels Mesothelium covering i internal i t l organs

SIMPLE SQUAMOUS Single Layer of Cells S Squamous shape h Example:: Lining of Example a blood vessel

SIMPLE SQUAMOUS EPITHELIUM

SIMPLE SQUAMOUS EPITHELIUM

CLASSIFICATION OF C& L EPITHELIA BY SHAPE • CUBOIDAL – Cells are APPROXIMATELY as tall as they are wide. – EXAMPLES: – –

Lining of gland ducts Walls of thyroid gland follicles

SIMPLE CUBOIDAL

CLASSIFICATION OF C&L EPITHELIA BY SHAPE

COLUMNAR • Cells are distinctly taller than they t ey are a e wide – EXAMPLES: – Lining of intestine – Some large gland ducts

STRATIFIED SQUAMOUS • More than one l layer! ! – TOP layer is squamous

• Very common – Epidermis of the skin – Lining of bodily orifices

• Often found in regions of heavy wear – M May be b hardened h d d or cornified (keratinized)

STRATIFIED CUBOIDAL CU O OR

COLUMNAR Lining of larger gland ducts Repro tract in some animals

SPECIAL TYPES: PSEUDOSTRATIFIED PSEUDO STRATIFIED • Principally in respiratory tract, tract but also in other locations • Not all cells reach free surface • All cells reach basement membrane • Truly T l a“ “simple” i l ” type • NOT “always y ciliated”!

SPECIAL TYPES: URINARY • Found only in urinary y passages • A “tight” epithelium ith li with many occluding junctions • A truly stratified type

SPECIAL TYPES: CONJUNCTIVA J • A variant form of stratified columnar l – Contains goblet cells

• Found inside eyelids and on surface of eye

Cilia What kind of epithelium is this?

• Frequently associated with epithelial sheets • Associated with transport/protection functions

Microvilli • Found on many cell types; often on epithelial sheets • Associated with secretion or absorption function • “Brush border” or “striated border” in LM border

“EPITHELIALORGANS” • Primarily epithelial tissue – But ALL organs have more than one tissue type!

• 3-dimensional masses of cells – Excludes epithelial sheets • Not normally capable of autonomous t function f ti

– Well vascularized – Set off from CT by a basal lamina – Usually capable of considerable regeneration – Always have some other tissue present (usually CT)

• Examples: – Glands of various types – Liver Li – Kidney

SECRETION

• DEFINITION: The process by which cells take up small molecules from the blood,, and transform them into more complex materials that are then released from the cells. • Secretion is an active process that always requires energy. Excretion is usually a passive process. • Not confined to glands: epithelial sheets can be secretory t

GLANDS GLANDS

•Epithelial organs specialized for secretion •Exocrine and endocrine types: both are classic examples of epithelial organs

CHARACTERISTICS OF SECRETORY CELL TYPES

• Basophilic cytoplasm if secretion is a peptide • Nucleus is “vesicular” – Large amounts of heterochromatin

• Prominent P i t nucleolus l l & Golgi G l i apparatus pp t • May show granularity in cytoplasm

MODES OF SECRETION: ECCRINE CC ((MEROCRINE) OC ) • NO loss of cytoplasm in process • Cells typically cuboidal to columnar • Used by MOST exocrine and ALL endocrine glands

MODES OF SECRETION: APOCRINE

• Loss of SOME cytoplasm in process • Apical region pinches off & degenerates • Used by some sweat glands – No other examples in mammals

MODES OF SECRETION: HOLOCRINE

• One example in mammals: Sebaceous glands • ENTIRE CELL dies, forms secretion by degeneration

STRUCTURAL CLASSIFICATION OF EXOCRINE (DUCTED) GLANDS • BASED ON TWO CRITERIA – SHAPE OF SECRETORY REGION • • • •

Tubular Coiled tubular Acinar/alveolar Tubuloacinar

– DEGREE OF DIVISION OF DUCTWORK • Simple • Compound • “Compound” is NOT = “Branched”

SIMPLE EXOCRINE GLANDS • Duct is NOT divided • Secretory S t R Region i MAY b be di divided id d = “Branched”

COMPOUND EXOCRINE GLANDS • DUCT is divided = “Compound” • Secretory region usually divided