Epithelium

1.General feature: 1) contain more cells and less extracellular ground substance 2) Polarization: ---free outer surface: face air or other things ---basal surface: face underlying CT, have basement membrane 3) No vasa, but nerve: ---no blood vessels ---rich in nerve terminals 4) Functions of protection, secretion, absorption excretion and sensory reception

2.Classification 1) Covering epithelium: the epithelium which cover body surface or line the inner surface of body cavities, tubes and sac. 2) Glandular epithelium: the epithelium which main function is secretion. 3) Sensory epithelium: the epithelium which has special sensory function.

Covering epithelium

Where to find Outer surface of the human body Inner surface of the canal, cavity or duct Eg: epidermis of skin, cornea of eye Eg: respiratory tract, blood vessel ， urinary duct

According to the number of layer and shape of cells Simple epi.: ---simple squamous epi. ---simple cuboidal epi. ---simple columnar epi. ---pseudostratified ciliated columnar epi. Stratified epi.: ---stratified squamous epi. ---stratified columnar epi. ---transitional epi.

1) simple squamous epi: ---structural feature: /one layer flattened cells, unclear cell border /with flattened or ellipsoid nucleus /cell width > cell height (flattened)

---distribution: mesothelium: the simple squamous epi. which line the inner surface of body cavities such as thoracic, pericardiac and abdominal cavities. endothelium: the simple squamous epi.which line the inner surface of cardiovascular and lymphatic system. other place: alveoli, parietal layers of renal capsule. ---function: a) transport of materials b) facilitates movement of viscera

2) simple cuboidal epi.: ---structural feature: cell width = cell height (square) round centrally-located nucleus

---distribution: /the renal tubule /thyroid /the some ducts of male glands ---main function: covering and secretion

3) simple columnar epi.: ---structural features: cell width < cell height (rectangle) basally located ovoid nucleus goblet cell: scattered, secreting granules-mucinogen granules-mucus

---distribution: /gastrointestinal tract /gall bladder /uterus ---function: secretion and absorption

4) pseudostratified ciliated columnar epi.: ---structural feature: four types of cells /columnar cell:ciliated /fusiform cell /basal cell:pyramid-shaped /goblet cell

---distribution: inner surface of large duct of respiratory /trachea /bronchi /nasal

5) stratified squamous epi.: ---structural features: deepest(basal)cells: one layer of cuboidal cells intermediate regions: several layers of polygonal –shaped cells to the surface: more and more flattened

---distributon: non-karatinised:mouth,pharynx, oesophagus, urethra and vagina karatinised: the surface of body, make up the skin

6) transitional epi.: ---structural feature: flexible-including the number of layers and shape of cells in the distended bladder: there are two to three layers of cells. The cells become flattened. in the contracted bladder : there are six to seven layers of cells. The surface cells are very large and cuboidal in shape, covering several deep cells. ---distribution: bladder

Epithelial specializations

1) Specialisations of free surface

① cell coat: ---defination: a thick layer of extracellular glycoprotein ---function: adherence, supporting, protection, exchange of material and recognize

② microvilli: ---defination: delicate finger-liked projections of cell-membrane and cytoplasm protruding from the free surface

---structure: 0.1um in diameter, with different longth. surface: cell membrane with cell coat core: longitudinal microfilament-actin filament fixed on terminal web terminal web: made up of transverse-arranged filament at the apical side of cells

---function: increase the surface areas ---distribution: /striated border: intestinal epi.cell /brush border: proximal renal tubule

③ cilia: ---defination: elongated, mobile projections of cell membrane and cytoplasm protruding from free surface

---structure: 5-10um long, nm in diameter surface: cell membrane core: microtubules, 9X2+2, dynein basal body: centrioles-connected with microtubules

---function: swing to produce a forward-moving wave ---distribution: epithelial cells of respiratory tract

2) specializations of the lateral surface

---intercellular connection of adjacent cells: non-special: the minute space and cadherin-cell adherent molecules special: junctional structures

① Tight junction( zonula occludens): ---structure: apical part point-liked fused between adjacent cells arranged in 2-4 thread-liked structures form anastomosing network ---function: seal the space between cells

② intermediate junction(zonula adherens): ---structure: below the tight junction a gap of 15-20nm in width with medium electron-density filament material plaque of electron-dense materials, with attached microfilament-make up of terminal web ---function: /adherens /keep the cell shape /transfer cell contract force

③ desmosome(macula adherens): ---structure: plate or spot-shaped a gap of nm, with low electron-density filaments interdigitate to form intermediate line attachment plaque: with attached tonofilament(10nm)- intermediate filament(karatin) ---function: firmly connection

④ gap junction(communicating junction): ---structure: the smallest gap of 2-3 nm connexons: -consist of protein -7~9nm in diameter -composed of 6-subunits of proteins- connexin -2nm channel: hydrophilic channel ---function: provide a pathway between cells

*junctional complex: four types of junctional structures(at least two types) get together.

3) specialization of basal surface

① basement membrane: ---defination: a sheet of membrane-liked amorphous material interposed between epi.cells and underlying CT. ---structure: HE: pink colour, hard to see

EM: three layers --lamina lucida: nm, electron-lucent --basal lamina: nm, electron-dense, thread-liked and amorphous ground substance, produced by epi. Cell --reticular lamina: RT+ground substance, produced by CT glycoprotein: laminin, type IV collagen, heparin sulfate proteoglycan and fibronectin

---function: support, connection, fixaton semi-premeable membrane induce the movement, proliferation and differentiation of epi.cell

② plasma membrane infolding(basal longitudinal striation): ---defination: the infolding of cell-membrane with many mitochondria at the basal surface of epi.cell

---function: increase the basal surface areas facilitate the passage of water and ions ---distribution: mainly in proximal and distal renal tubule.

③ hemidesmosomes ---is half of desmosome.

Gland

glandular epi.: epi are specialized for secretion gland: organs composed mainly of glandular epi.

Classification of gland According to the number of cell number: Unicellular glands multicellular glands

This is unicellular gland. There are some goblet cells interspersed between the columnar cells.

This is multicellular gland. There are many special glandular cells in groups.

According to the presence or absence of ducts: exocrine gland: discharge the secretion through a duct system endocrine gland: release the secretion directly into blood steam

1) structure of exocrine gland: ① acinus(secreting unit): according the nature of secretion a. serous acinus: serous secretory cells

---structure: pyramid-shaped cell basally-located round nucleus acidophilic cytoplasm-eosinophilic zymogen granules-contain enzymes EM: RER,Golgi complex ---function: produce a serous secretion

1 2 2

b. mucous acinus: mucous secreting cells ---structure: pyramid-shaped cell flattened dark nucleus against the basal cell membrane slightly basophilic cytoplasm-large mucigen granules EM: some RER, Golgi complex ---function: secretes mucus

c. mixed acinus: two types of cells ---structure: mucous acinus with several serous cells attach on one side- serous demilune

demilune

② ducts: ---from simple squamous epi to simple columnar or stratified epi. ---carry out the secretions ---secrete or absorb water and ions

2) structure of endocrine gland: Ductless gland rich in network of blood capillaries or sinusoid arranged in cords, in clumps or follicles e.g. thyroid gland (see other chapters)