Chapter 2 Epithelium

What is epithelium? Conception: Epithelium is a tissue composed of closely aggregated polyhedral cells and very little extracellular matrix. Distribution: Cells cover body surfaces, line body cavities, and constitute glands.

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

2.Classification of Epithelium 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.

According to the number of layer and shape of cells 3. Classification of covering epithelium 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, cell border are interdigitate /with flattened ellipsoid nucleus

---distribution: Three types: 1.mesothelium: the simple squamous epi. which line the inner surface of body cavities such as thoracic, pericardiac and abdominal cavities. 2. endothelium: the simple squamous epi. which line the inner surface of cardiovascular and lymphatic system. 3. other place: alveoli, parietal layers of renal capsule.

Mesothelium on abdominal cavity

Vascular endothelium

2)      Simple cuboidal epi.: ---structural feature: one layer of cells, with same height and width , hexagonal outline in surface view. spherical centrally-located nucleus

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

3)      Simple columnar epi.: ---structural features: one layer of columnar cells, with basally located ovoid nucleus

---distribution: gastrointestinal tract gallbladder uterus ---function: secretion and absorption goblet cell: scattered, secreting granules-mucinogen granules-mucus simple columnar epi goblet cell striated border

4) Pseudostratified ciliated columnar epi.: ---Structural feature: 1, Four types of cells columnar cell (ciliated); goblet cell fusiform cell; basal cell: pyramid-shaped 2, Every cell locate on basement membrane : Simple epi. four types of cells

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

5) Stratified squamous epi.: Three layers ---structural features: 1. Deepest (basal) cells: one layer of cuboidal cells 2. The cells in intermediate regions: several layers of polygonal –shaped cells 3. To the surface: more and more flattened cells

non-keratinised: mouth, pharynx, esophagus, and vagina ---distributon: non-keratinised: mouth, pharynx, esophagus, and vagina keratinised: the surface of body, make up the skin non-keratinised keratinised

keratinised non-keratinised

6) Transitional epi.: Two condition: flexible-including the number of layers and shape of cells Two condition: 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.

in the contracted bladder in the distended bladder

---distribution: Urinary tract (ureters, bladder, urethra) The surface cells are very large and cuboidal in shape, covering several deep cells. Superficial cells

Summary 1 General feature of ep.: 1) contain more cells and less extracellular ground substance 2) Polarisation: ---free outer surface: face air or other things ---basal surface: have basement membrane, to face underlying CT, 3)  ---no blood vessels ---rich in nerve terminals 4) Having many functions

According to the number of layer and shape of cells Classification of covering epithelium 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.

4. Epithelial specializations

Specialisations of free surface specializations of the lateral surface specialization of basal surface

1) Specialisations of free surface Microvilli Cell coat Cilia

     ①    microvilli: defination: Microvilli are finger-like cytoplasmic projection on the free surface of most epithelial cells. Distribution: striated border: intestinal epi. cell brush border: proximal renal tubule

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

② cell coat: ---defination: a thick layer of extracellular glycoprotein ---function: adherence, supporting, protection, exchange of material and recognize

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

---structure: 5-10um long, 0.2um in diameter surface: cell membrane core: microtubules, 9X2+2 basal body: centrioles-connected with microtubules

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

junctional structures Tight junction (zonula occludens) Intermediate junction (zonula adherens) Gap junction (communication junction) Desmosome (macula adherens)

① Tight junction (zonula occludens): ---structure: apical part point-liked fused between adjacent cells 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 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

---structure: ④desmosome(macula adherens): plate or spot-shaped a gap of 20-30 nm, with low electron-density filaments interdigitate attachment plaque: with attached tonofilament-intermediate filament (karatin) ---function: firmly connection

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

---function: support, connection, fixation semi-permeable 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

increase the basal surface areas ---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.

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

1)classification: exocrine gland: discharge the secretion through a duct system endocrine gland: release the secretion directly into blood steam, no duct system

2) structure of exocrine gland: ①acinus (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

flattened dark nucleus against the basal cell membrane 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

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

Questions 1. Describe the characteristics of epithelial tissue 2. Compare the structure of microvilli with cilia

The composition of final score: 1. Lecture and Lab attendence: 10 % 2. Quiz: 15% （3 quizes，each quiz 5%） 3. Lab test: 25% 4. Final written examination：50% Histology： 35-40% Embryology：10-15% *. If you want to pass this course: final written examination score must be more than 50 and final score>60 Preparation of laboratory work Have the tools ready: Pencils (red-blue pencil) rubber, ruler and so on

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