Histology                                                                                                                     Stanley Shostak
BioSc 1450                                                                                                                  Spring 05

Lecture 2. Epithelium: Surface and Glandular

maximum cell-to-cell contact; minimum extracellular material; cell junctions; supported by basement membrane (basal lamella); not penetrated by blood vessels; derived from all embryonic germ layers, including mesoderm (endothelia and mesothelia)

Polarity: basal vs apical (luminal); lateral; specialized plasmalemma, appendages and junctional complexes

glycocalyx (apical) vs basement membrane (bm: basal lamina secreted by epithelium + lamina reticularis secreted by connective tissue)

Epithelia: thelia = nipple; epithelia = upon the nipple (convex [surface layer: epidermis, intestinal lining]); endothelia = within the nipple (concave [lining blood and lymph vessels and heart; cornea]); mesothelia = epithelia derived from splanchnic and somatic mesoderm lining body cavity (peripheral serosa) and covering organs (visceral serosa) within peritoneal, plural- and cardiac cavities.

Classification: surface versus glandular

Surface epithelia: continuous sheet of one or more cellular layers
number cell layers

simple: all cells touch basal lamina simple proper: all cells also reach free surface
pseudostratified: basal cells fail to reach free surface stratified (layered): surfaces subject to wear keratinized
nonkeratinized (mucous membranes)
parakeratinized (papillae of tongue)
transitional: in urinary tract; dome-shaped cells at free surface shape of cells
outermost layer of stratified epithelium: looked at section perpendicular to surface; generally reflected in shape of nuclei
squamous (flat; scale-like): endothelium, mesothelium and peritoneum (pleural and pericardial cavity), lining alveoli
cuboidal: spherical nuclei; typically located in center of cell but displaced basally in exocrine cells; frequently described as low (verging of squamous), medium, or high cuboidal (verging on columnar).
columnar: elongate; cigar-shaped nuclei surface specializations surface (luminal) specializations: cilia and flagella (7 to 10 micro-meters): expecially pseudostratified and columnar epithelia
microvilli (about 1micro meters = brush border (proximal convoluted tubule) also called striated border (small intestine); 0.5 to 1.0 micro meters; as many as 3000 per apical surface; microfilaments in core of microvilli attached to electron-dense material at tip
stereocilia (microvilli >10 micro-meters); appear branched due to fixation artifact; in male reproductive tract (e.g., epididymis); facilitate absorptive processes lateral specializations (junctions and patterns of folding): intercellular surfaces specializations junctional complex (always paired on opposing surfaces): combination zonula occludens, zonula adherens and desmosomes
occluding junctions = tight junctions: collar; sealing strands (fine matching ridges); stitch membranes together
fascia occludens: discontinuous strips of tight junctions between endothelial cells (except in brain)
zonula occludens: tight junctions form continuous circumferential band
adhering junctions: anchorage of cytoskeleton and binding cite between cells (also in cardiac and visceral muscle)
zonula adherens = belt desmosomes: circumferential; beneath zonula occludens; terminal web: fine meshwork of microfilaments joined to zonula adherens desmosomes (circumferential and spot adhering junctions) also called macula adherens: patches; beneath zonula adherens; transmembrane linkers: cadherins (calcium dependent cellular adhering proteins); (midway across intercellular space); : unique desmoplakin (desmogleins) between cells; tonofilaments (intermediate filaments) coalesce at plaque within cell intermediate filaments inserted into electron-dense plaque = prickles of spiny layer of epidermis communicating junctions = gap or nexus junctions: contain hundreds of tiny pores (< 2 nanometers) through connexon complex (of 6 transmembrane proteins); permitting passage chemical messages (cations; nutrients); pore closes in presence of increased calcium ion (also in cardiac and visceral muscle)

 basal specialization folding: ion transport
hemidesmosome: resemble desmosome but do not occur in pairs; integrins: receptors for laminin and collagen type IV.
  types and location simple squamous: selective diffusion, absorption or secretion; pavement epithelium

simple cuboidal: lines small ducts and tubules; excretory, secretory or absorptive; ; nuclear chromatin dispersed; nucleoli prominent; bm.

simple columnar: highly absorptive surfaces (sm. intestine); secretory surfaces (stomach); lining gall bladder (absorbs water); nuclei may be basal; microvilli

simple columnar ciliated: 200 to 300 cilia per cell; female reproductive tract; nuclei apically distributed; includes secretory, nonciliated cells (with basal nuclei)

pseudostratified columnar ciliated: nuclei disposed at different levels; basal cells do not extend to surface; larger airways of respiratory system = respiratory epithelium (mammals); mucociliary escalator.

stratified squamous (mucous variety): resists abrasion; moistened by glandular secretions; but poorly adapted to withstand desiccation; oral cavity, pharynx, esophagus, anal canal, uterine cervix, vagina

stratified squamous keratinizing: surface of skin (epidermis); cells keratinize; accumulate keratin

stratified cuboidal: only 2 to 3 cell layers; lining large excretory ducts of exocrine glands (salivary gland); not absorptive (?)

transitional: urinary tract (mammals); accommodates stretching and toxicity of urine; surface cells larger, pale-staining, scalloped surface outline; luminal surface appears thickened; may be binucleate; large, round, prominent nucleoli

 Parenchymous epithelia = glandular epithelia (primarily secretory); separated from connective tissue by basal lamina; Parenchyma: historically, tissue poured into a capsule (= stroma; bed and network); word also used for cells in plant tissue.

Generally formed by down growth of surface epithelium into underlying connective tissue.

exocrine: retain connection with surface single cell glands goblet cells: PAS (periodic acid Schiff: carbohydrates stain red) positive; secrete mucus; scattered among cells of simple epithelium (respiratory and gastrointestinal tracts); mucigen granules (mixture of neutral and acidic proteoglycans; form viscid mucus. multicellular glands serous glands: secretion watery and rich in protein
mucous glands: secretion mucus; poor in protein
seromucous glands: intermediate 8 types distinguished by morphology of secretory portion (tubular [coiled versus straight] versus acinar [simple versus compound]) and branching versus straight excretory portions (which may also secrete bicarbonate) simple tubular: large intestine: single, straight; lined by goblet (mucous) cells

simple coiled tubular: sweat glands; terminal secretory portion lined by simple cuboidal epithelium; followed by nonsecretory (excretory) duct lined by stratified cuboidal epithelium

simple branched tubular: stomach; secretory portions converge into unbranched duct (wider diameter); lined by mucus-secreting cells.

simple acinar: rounded secretory unit; pockets in epithelial surface; secretory cells (e.g., mucus-secreting glands of penile urethra)

simple branched acinar: several acini emptying into single excretory duct (stratified epithelium); sebaceous gland

compound branched tubular: = duct branched; secretory portions tubular and branched; duodenal (Brunner's) glands

compound acinar: secretory units are acinar and drain into branched ducts; pancreas; branched excretory ducts of increasing diameter lined by simple cuboidal epithelium

compound tubulo-acinar: 3 types secretory units: branched tubular; branched acinar (serous cells); branched tubular (mucous cells) with acinar end pieces ([serous] demilunes); submandibular salivary gland

myoepithelial cells: sometimes embedded in basement membrane; may aid secretion of acinus glands

 striated ducts: striations due to mitochondria lined up along folds of basal membrane; transport Na and bicarbonate; cells high cuboidal to columnar

intercalated ducts come between acini or acini and striated ducts; cells low cuboidal
mechanism of secretion merocrine (eccrine): exocytosis (e.g., proteins)

apocrine: discharge free, unbroken, membrane-bound vesicles; lipid secretory products (breast) and some sweat glands

holocrine: discharge who cell; sebaceous glands (sebum)
endocrine or ductless (hormones) lose connection with surface or formed without duct; surrounded by bm; secretions enter circulatory system
clumps
cords
follicular: thyroid
Basement membranes (bm; epithelia) = external lamina (of muscle & nerve)
3 layers in em
first two produced by epithelia: contain collagen type IV, laminin (binds CAM); entactin (binds laminin to type IV collagen) lamina lucid 10 to 50 micro-meters; abuts parenchymal cells; lamina densa 20 to 300 micro-meters; intermediate lamina fibroreticularis merges with CT type III; binds fibronectin. glycosaminoglycans heparan sulphate: PAS positive collagen type IV glycoproteins fibronectin (produced by fibroblasts) not produced by fibroblasts laminin
entactin last revised 01-07-05