Epithelial tissue

Characteristics

1. Covers outside of the body, lines organs cavities

2. Closely packed

3. Avascular (no blood supply)

4. Free surface exposed to fluid/air, bottom surface attached to basement membrane (thin non cellular region, separates epithelium from underlying tissue, structural support for epithelium and binds it to neighboring structures)

5. Composed of cells that form continous sheet

6. High apoptotic and regeneration rate: rate of damage is high and so they are also replaced rapidly

7. Cells are polar

8. Selectively permeable

Embryonic tissue

1. Ectoderm: outer layer of skin

2. Mesoderm: muscle, connective, skeleton kidney

3. Endoderm: lining of gut, respiratory tract, bladder

Functions

1. Protection: protect underlying tissue from mechanical injury harmful chemical and pathogens

2. Sensation: changes in environment detected by specialized cells found in skin, nose ear

3. Secretion: glandular epithelium secretes specific chemical substances

4. Excretion: cells in kidney excrete waste and sweat glands excrete sweat

5. Diffusion: simple epithelium promotes the diffusion of gases, liquids and nutrients. Thin layer allows for diffusion gases

6. Absorption: cells lining intestines absorb nutrients from digested food

Squamous

Cells fit closely together, oval nuclei, thin basement membrane, provides smooth low friction surface

Alveolar epithelium = lungs

Mesothelium= heart

Endothelium= blood vessels

Cubodial

Cubodial shaped, spherical nuclei, excretory secretory or absorptive function

Secretion or absorption in different glands l, ducts and tubules, endo/exocrine glands, pancreas, kidneys, ovary and testes

Columnar

Cells elongated and column shaped, nuclei also elongated and located near base, found in highly absorptive surfaces (small intestines) and secretory surfaces (stomach)

Modified to secrete mucus, made in goblet cells, lubricates to minimize wear and tear and neutralize stomach acids

Some specialized for sensory reception, nose ears

Simple epithelium

Once cell thick, all in contact with basement membrane, lines cavities ducts and tube’s

Stratified epithelium

Multilayered , only 1st layer touches basement membrane, found where body lining have to withstand mechanical/chemical abrasion, layers can be lost

Poorly suited for absorption and Secretion due to thickness serves protective function

Classified as keratinised and transitional

Pseudostratified

Appears layered but each cell touches basement membrane, some elongated others smaller looks stratified, nuclei appear to form multiple layers

Keratinised

Apical layers dead, no nucleus or cytoplasm

Covered in keratin (tough resistant and waterproof protein)

Layers of squamous cells flattened towards surface

Cells constantly sloughed from surface and replaced by basal layer, 4-5 layers

Transitional

Found in tissue that can stretch

Appear stratified cubodial when tissue not stretched or stratified squamous when organ is swollen and tissue is stretched

Urothelium (found in bladder uterus and urethra)

Ciliated epithelia

Cilia- fine hair like extensions of apical membrane

Energy dependent beating in certain direction + mucus (secreted by goblet cells) lubricates/traps pathogens/partials- removes from body

Found in airway, uterus and fallopian tube’s

Pseudostratified collated columnar epithelium lines respiratory tract

Microvilli

Mostly found in simple columnar epithelium

Apical surface may have tiny projections- increase surface area available for absorption in intestines

Whole border is referred to as striated border