Epithelium definition



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Histology SSN October 12, 2004

Tresha - tae2101 Anand - apd2104

EPITHELIUM

Definition: Avascular tissue made of cells that cover exterior surfaces and line both internal closed cavities and body tubes that communicate with the exterior. Epithelium also forms the secretory portion of glands and ducts.

Features:

  • POLARITY: distinct apical, basal, and lateral surfaces

  • Basal Surface: attached to basal lamina (collagen Type IV, made by epithelial cells) which is part of basement membrane

  • Cells adhere to each other via specialized junctions (explained below)

Functions

EXAMPLE

Protective Layer

Epidermis (stratified squamous)

Absorption of water and solutes

Intestinal Epithelium (simple columnar)

Secretion

Glands: salivary, pancreatic (cuboidal)

Containment

Bladder (transitional)

Excretion

Kidney tubules (simple cuboidal)




Types OF ePITHELIA

Characteristics

Simple

  • One cell layer thick

  • Absorption, secretion, diffusion

  • Ex.: simple columnar in small intestine,

simple squamous in capillaries

Pseudostratified

  • A simple epithelium

  • All cells rest on basement membrane, but not all reach apical surface

  • Ex.: Lining of trachea(ciliated) and

epididymis (stereociliated)

Stratified

  • Two or more cell layers thick

  • Classified based on cell type of surface cells

  • Protection, barrier

  • Ex: Stratified squamous in epidermis (keratinized) and esophagus (non-keratinized)

Transitional

  • A stratified epithelium

  • Apical surface may appear “half-domed”

  • Accomodates distension by flattening

  • Ex.: lining of the bladder, ureters, urethra




TYPES OF CELLS

CHARACTERISTICS

Squamous

(stratified is usually protective, and simple for diffusion)



  • Cells are flattened and irregularly shaped

  • Appear “scale-like” or “squashed”

  • Ex.: endothelium of vasculature, alveoli

Cuboidal

(often absorptive, but sometimes secretory)



  • Round, central nucleus

  • Width = height (ice cube shaped)

  • Ex.: pancreas-secretory, kidney-absorptive

Columnar

(usually absorptive, but sometimes secretory)



  • Elongated nucleus

  • Width < height (cells long and tall)

  • Ex.: lining of small and large intestines



SPECIALIZATIONS

DESCRIPTION

Cilia

  • Insert into basal bodies (1 cilium per 1 body)

  • Motile processes of microtubules move synchronously

  • 9 +2 microtubule arrangement

  • Ex.: trachea and oviduct

Microvilli

  • insert into terminal web (stains eosinophilic – pink)

  • actin skeleton above intermediate filaments

  • increase surface area for absorption

  • Ex.: small intestine

Stereocilia

  • long microvilli – actin (NOT cilia!)

  • non-motile

  • Ex.: epididymis (pseudostratified)

Keratin

  • Formed from dead layer of squamous cells

  • Protects against desiccation and abrasion

  • Ex.: epidermis (stains strongly eosinophilic)



Basement membrane = basal lamina & reticular lamina

Basal Lamina

  • Separates epithelia from connective tissue

  • Collagen type IV, proteoglycans, glycoproteins

  • Synthesized by epithelial cells

Reticular Lamina

  • Connective tissue below epithelium

  • Collagen type III


CELL-CELL CONTACTS:

Zonula Occludens (apical end)

Terminal bar = Junctional Complex = Zonula Adherens

Macula Adherens

  • Stains dark with Bodian silver

CELL CONTACT

DESCRIPTION

Zonula Occludens (tight junction)

Zonula Adherens

  • Forms band around cell at lateral surfaces

  • Adds to integrity of epithelial surface

Macula Adherens (desmosome)

Hemidesmosome

  • Link cell to basement membrane at basal surface

IMPORTANT: Don’t confuse terminal bar (junctional complex) with terminal web

(network of actin and intermediate filaments microvilli insert into)

Questions


  1. What kind of epithelium lines the secretory alveoli of this exocrine gland?

    1. Simple columnar

    2. Simple cuboidal

    3. Squamous

    4. Transitional

Questions 2-3: Figure A (Lab 3, slide 35); Figure B (Lab 3, slide 25)




  1. Select the one correct statement regarding the surface epithelium:

    1. In both figures all of the cells reach the lumen.

    2. In both figures the superficial cells are keratinized

    3. In both figures all of the cells rest on a basal lamina

    4. Only in Figure B do all the cells rest on a basal lamina




  1. The tissue or tissues that are specialized to provide a barrier to luminal absorption are shown in:

    1. Figure A only

    2. Figures A and B

    3. Figure B only

    4. Neither figure A or B




  1. This cell type is typically found in the:

    1. Bladder

    2. Kidney tubules

    3. Intestinal epithelium

    4. Epidermis

Answers:


1. B; The small ducts of exocrine glands are lined by simple cuboidal cells.

2. D; In the trachea pseudostratified epithelium, all cells rest on the basal lamina. Bladder transitional epithelium is stratified and therefore not all cells touch the basal lamina.

3. A; Another function of stratified epithelia is to serve as a barrier.

4. C; Simple columnar epithelium lines the small intestine and colon. The stomach lining, gastric glands, and lining of the gall bladder are all lined by simple columnar epithelium.

5. D; This is the desmosome, also known as the macula adherens. Note the characteristic desmosomal shape.

6. A; The arrow is pointing to a brush border, composed of microvilli. Microvilli are made of actin filaments.



7. C; Basal lamina is made of collagen IV (non-fibrillar). The reticular lamina contains collagen III, whereas the anchoring filaments connecting the basal lamina to the reticular lamina are made of collagen VII.

8. D; This is a silver stain which stains the juncitonal complex, also known as the terminal bar. The terminal web is the structure into which the actin filaments of microvilli insert.


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