Department of Histology & Embryology Chapter 2 Epithelial Tissue Guanhua Huo Department of Histology & Embryology BMU

I. OUTLINE Epithelial Tissue is commonly named Epithelium Epithelium: In Greek ("epi") means "on" or "upon", and ("thēlē") means "nipple“ Epithelium: is made up of cells closely packed and ranged in one or more layers; forms the covering or lining of all internal and external body surfaces, as well as cavitys and lumen of organs.

1. Construction features have more cells(arranged tightly)， only a small amount of intercellular substance, ranged in one or more layers; the cell have polarity basement membrane: provides structural support for the epithelium and also binds it to neighbouring structures. no blood vessels-avascular: nutrients and precursors of products of the epithelial cells diffuse across BM and are taken up through the baso-lateral surface of the epithelial cells through the basement membrane rich in nerve endings: conea, epidermis 2. Distribution features line cavities and surfaces of structures throughout body body surface-covering ep. inner surface of various kinds of tubes, cavities and sacs Free (apical) surface –apical pole Basal surface -basal pole

B. glandular epithelium C. special epithelium 4. Function 3. Classification A. lining epithelium (covering epi.):lines both the outside (skin) and the inside cavities and lumen of bodies. B. glandular epithelium C. special epithelium 4. Function protection: physical / chemical / microorganism /desiccation secretion: mainly for glandular epi. absorption: small intestine / renal tubule excretion: sweat gland / renal tubule others: Sensation/ produce germinal cells, transcellular transport myoepithelium germinal epi. sensory epi.

II. Covering epithelium Principles of classification: Tissues are generally classified by the morphology of their cells, and the number of layers they are composed of.

Simple epi. Stratified epi. Stratified epi. Simple epi.

2. Classification & structure s. squamous epi. s. cuboidal epi. s. columnar epi. pseudostratified ciliated columnar epi. Simple epi. st. squamous epi. st. columnar epi. transitional epi. Stratified epi.

A. simple squamous epithelium Top view: scales-like; have an irregular shape with a serrated border side view: flat

Location: Endothelium: inner lining of blood vessels, heart, and lymphatic vessels Mesothelium: walls of pericardia, pleura, and peritoneal cavities, secrete a lubricating fluid, called serous['sɪrəs] fluid Others: walls of alveoli of the lungs , parietal [pə'raɪətəl] layer of Bowman's capsule some renal tubule

alveoli of the lungs

visceral layer parietal layer Renal Corpuscle: Note the flattened cells of the parietal layer of Bowman's capsule.

B. Simple cuboidal epithelium Top view: hexagonal; side view: square Location: thyroid follicles; renal tubules

C. Simple columnar epithelium Top view: polygonal [pə'lɪɡənl] ; side: column-like，prismatic form Special structure: striated ['straɪeɪtɪd] border, brush-like border Cytoplasmic projections Location: gastrointestinal tract; gall bladder; uterus

D. Pseudostratified ciliated columnar epithelium Taller simple epithelial cells (see columnar, below) are viewed in cross section with several nuclei appearing at different heights, they can be confused with stratified epithelia. This kind of epithelium is therefore described as "pseudostratified" epithelium. 4 types of ell, Nu. at different levels fusiform cell pyramidal cell

c b c f

Location: respiratory passages & epididymis.

E. Stratified squamous epithelium Basal layer: low columnar on B.M.; Middle layers: polygonal and larger; Top layers: flat / largest and scale-like platycyte polygonal cell basal layer cell B.M.

Location: Nonkeratinized: mouth / pharynx/ esophagus/ vagina; Keratinized: epidermis

stratified columnar epithelium Columnar cells Columnar cells + Layers of polygonal cells Location: ocular conjunctiva, large ducts of salivary glands

F. Transitional epithelium Different functional state has different cell layer & cell-shape. surface layer: cap cell Contracted Distended

Location: urinary bladder Full state: 2-3 layers empty state: 5-6 layers

III. Glandular epithelium & gland 1. Definition Glandular epithelia.: The epithelia specialized for secretion Glands: The organs composed mainly of glandular epi. 2. Classification Exocrine gland Endocrine gland

Derived from the localized covering epithelium 3. Development Derived from the localized covering epithelium retain obliterate The agglomerated cells form anastomosing cords interspersed between dilated blood capillaries. The others, cells line a vesicle or follicle filled with noncellular material.

4. Exocrine gland Classification：Depend on number of cells Unicellular glands: goblet cell Multicellular glands secretory unit duct system

Section of large intestine showing goblet cells secreting mucus to the extracellular space. The mucus precursor stored in the cytoplasm of the goblet cells is also stained in a dark color. PAS-PT stain. Medium magnification.

Apical portion: Large, lightly staining granules containing strongly hydrophilic glycoproteins-mucin. mucous acini

Parotid Gland, Human and Sublingual Gland, Human - H&E serous acini

Electron micrograph of a pancreatic cell Electron micrograph of a pancreatic cell. Note the nucleus, mitochondria, Golgi complex, secretory (zymogen) granules in various stages of condensation, and rough endoplasmic reticulum. x13,000.

Depend on shape: Tubular glands Alveolar glands Tubulo-alveolar glands

Depend on function: Serous glands, mucous glands, mixed glands

5. Endocrine gland Consist of endocrine cells; the secretions (hormone) enter bloodstream directly . To be discussed more detail late.

IV. Special structure of epithelium 1. Cellular free surface cell coat microvillus cilium A. Cell coat: extracellular glycoprotein layer

B. Microvilli LM: striated borders brush-like borders EM: finger-like projections (cell-membrane & cytoplasm) (0.1/1.4 μm) microfilaments (actin filaments): interconnect terminal web Function: increase the surface area of absorption

Cell coat Electron micrograph of the apical region of an intestinal epithelial cell. Note the terminal web composed of a horizontal network that contains mainly actin microfilaments. The vertical microfilaments that constitute the core of the microvilli are clearly seen. An extracellular cell coat (glycocalyx) ['glaɪkoʊ'kælɪks] is bound to the plasmalemma of the microvilli. x45,000.

C. Cilium LM: elongated & mobile projections(0.2/5-10μm); EM: 9+2 microtubules connected to the basal body (the centrioles)

inner dynein arm microtubule radial spoke central microtubule outer dynein arm radial spoke central microtubule Cilia have a rapid back-and-forth movement- permit a current of fluid or particular matter to be propelled in one direction over the ciliated epithelium--ATP

Ciliary ultrastructure, Left, Normal cilium from a healthy individual in which both inner and outer dynein arms can clearly identified. Right, the absence of outer and inner dynein arms in a patient with primary ciliary dyskinesia [dɪskɪ'ni:ʒə] .

2. Lateral surfaces A. Tight junction B. Intermediate junction C. Desmosome D. Gap junction

A. Tight junction (zonula occludens ) Location: around the apex of cells Structure: belt-shaped anastomosing network of ridges formed by fused protein in adjacent cellular membrane Function: form a seal & a barrier to prevention. Zonula:form a band completely encircling occludens: membrane fusion that close off the intercellular space

B. Intermediate junction, zonula adherens Location: around adjacent cells, just below T.J. Structure: a gap: 15-20nm, amorphous material (glycoprotein) electron-dense material: on the cytoplasmic faces filaments: numerous, being continuous terminal web Function: hold adjacent cells Firmly, maintain the cell shape, and pass cellular contract forces

Desmosome, macula adherens Location: along the intercellular faces, widely distributed, plate-shaped Structure: a gap: 20-30nm, filled with extracellular material & an intermediate line attechment plaque: electron dense, on inner surface of cell-membrane tonofillaments: a great number, inserted into the plaque, hairpin loop-like Function: “spot weld [weld] ” type attachment, provide a firm adhesion

D. gap junction Location: deep part Structure: a narrow gap: 2-3nm, spanned by connexons in opposing cell membranes connexons（D=7-8nm): 6 subunits of protein , a central channel (2nm)； Function: communication (transfer the impulses; passion & molecules) ; rich in cardiac muscles and neurons.

E. Junction complex 2 or more types of junction present collectively in same area

3. Basal face A. Basement membrane B. Plasma membrane infolding C. Hemi-desmosome

A. Basement membrane Location: between epi. & underlying C.T. 50-300nm thick, PAS +, argyrophilia

Structure: Lamina lucida Basal Lamina densa lamina reticular lamina Basal lamina: glycoprotein; produced by epi. cells. Reticular lamina: reticular fibers + matrix, formed by fibroblasts in C.T. Main chemical components: polysaccharide & glycoprotein (Laminin, type Ⅵ collagen etc.) reticular lamina Basal lamina Lamina lucida Lamina densa

reticular lamina lamina lucida lamina densa basal Function: support, connection, serve as a semi-permeable membrane, facilitate the proliferation and differentiation of cells

B. Hemidesmosome hemidesmosome basal lamina reticular lamina Function: enhance the cell adhesion to the B.M.

Hemidesmosome & Desmosome

C. Plasma membrane infolding Structure: infolding membrane mitochondria, Location: renal tubules Function: increase the basal surface area & enhance the reabsorption of water & ions

Proximal tubule kidney. Basal membrane is highly infolded Proximal tubule kidney. Basal membrane is highly infolded. Note fenestrated capillary at right.