Unit 1: Epithelial Tissue Flashcards
What is a cell?
An aggregate of cells similar in structure and function
- About 200 types, differentiated in embryonic stage
- Their specialization leads them to lose their ability to survive on their own, they depend on functions of different cell types in the organism
4 cell type categories
- Epithelial: covers surfaces of body, lines cavities, secrete
- Connective: binds organs together, protection/support
- Nervous: coordinates body functions via transmission of nerve impulses
- Muscle: contract to enable movement
Epithelial tissue composition
Sheets of closely packed cells (single/multiple layer) that cover other tissues
- Sides that attach to neighboring cells by junctional complexes -> forms sheets
- Rapidly regenerate by mitotic division, destroyed and replaced daily
Epithelial tissue functions
- Protects, covers
- Filters biochemical substances
- Absorbs nutrients
- Provides sensory input (some! others lack nerves)
- Makes secretions (inside - mucus, hormones, enzymes) and excretions (outside - sweat, milk)
2 classifications of epithelial tissue
Covering and lining
Glandular
Glandular epithelial tissue
Produce secretions in exocrine and endocrine glands
The 2 ends of epithelial tissue
POLAR
Apical surface: surface of cell exposed to body cavity, lines internal organ (facing lumen), exposed to exterior
Basal surface: surface of the cell attached to basement membrane (faces underlying tissue)
Do epithelial cells have a blood supply?
NO, avascular. Rely on underlying tissue for nutrients and waste exchange
What are the ways cells are held together?
- Sides fit interlocking
- Matrix-filled channels running between cells
- Membranes joined w/junctional complexes (3 types)
3 Intercellular junctions?
Tight, Anchoring, Communicating
Tight junctions
- Fluid-tight seal (no penetration of substances. eg. bladder leakage, digestive tract stops enzymes from getting out into the body)
- The outer layers of plasma membrane between 2 cells are fused
- Found around circumference of cell
Desmosomes/Anchoring junctions
Fastens cells to one another OR to extracellular materials (basement membrane)
- Plaque of interlocking filaments (some filaments penetrate into cytoplasm to stabilize)
- Area with stretching (skin, heart, uterus)
Gap/Communicating junctions
Fluid-filled tunnels between cells
- Connexons: tubular channel proteins, join cytoplasm of adjoining cells together
- Exchange of nutrients, ions, electrical impulses
- Help coordinate action of cells
- Intestine, heart, smooth muscle
Basement membrane
Attaches epithelium to underlying connective tissue
- Function: resists rubbing
- Not a true membrane, so often referred to as ‘basal lamina’
- Acellular
- Helps with movement of oxygen, nutrients, and waste from blood vessels in connective tissue below -> basement membrane of epithelium
Surface specialization
- According to location and function
- Blood vessel epithelia = very smooth
- Microvili = fingerlike projections (increase surface area up to 20x, found in intestine and urinary tract, help absorb/secrete)
- Cilia = tiny hairlike projections (help move stuff like mucus/debris)
- Keratin = waterproofing that fills mature epithelial cells
How are epithelial tissues classified?
- Cell shape at exposed surface (squamous = scale, cudoidal = cube, columnar = elongated)
- Presence of surface specialization
- # of layers (simple = 1, stratified = multiple)
Simple epithelium
- 1 layer of cells
- Squamous, cuboidal, or columnar (->ciliated or non-ciliated)
- Occur where diffusion/secretion/filtration happens OR where smooth surface is required
Simple squamous epithelium
Single flat layer of scalelike cells
Irregular outlines
Central nucelus
Function: nutrient exchange, reduce friction, weak
Location: air sacs of lungs, inner surface of vessels/heart walls (‘endothelium’)
- Line peritoneal, pleural, pericardial cavities (‘mesothelium’)
- Also called serous membranes because they secrete
Simple cuboidal epithelium
Single-layer cube shaped w/central nucelus
Location: lining of kidney tubules, smaller ducts of exocrine glands (salivary, pancreas), secretory part of endocrine gland (thyroid), surface of ovary, lens, retina
Function: lining, sectretion (thyroid), absorption (kidney)
Simple columnar epithelium - Non-ciliated
Single layer of elongated cells
Smooth apical surface
Location: lines GIT, ducts of many glands
Functions:
- Cells with microvilli: absorption and secretion
- Goblet cells: secret mucin, lubricative barrier
Simple columnar epithelium - Ciliated
Single layer of ciliated elongated cells
Nucelus towards basal surface
Goblet cells in some spots
Location: uterus, respiratory, spinal cord
Function: move fluid/particles along through ciliary action
Stratified epithelium
More than 1 layer of cells
More resistant to abrasion
Only the inner layers produce new cells (mitosis), pushed to the surface where cells die/are lost to abrasion
Named according to shape of cells at surface
Function: protect underlying tissue
‘Weird’ stratified epithelium? (2)
Pseudostratified columnar
Transitional
Stratified squamous epithelium
Several layers of cells
Basal layer: cudboidal -> columnar
- actively growing/multiplying
- start cuboidal, become more flattened as they die and move
to surface
Middle: irregular cells
Surface: squamous
2 types: keratinized/non
- Keratinized: skin
- Non: Upper GIT, epiglottis, vagina, cornea
Function: protection, thickest epithelia
Keratinization
Keratin = waterproofing protein incorporated into the outer layer of stratified squamous epithelium, protects against water loss to external environment
- AKA cornification
Usually at sites with lots of friction, become callouses
Stratified cuboidal epithelium
2+ layers of cells, surface cell cube shaped
Location: ducts (sweat, mammary, salivary glands)
Function: protection
Stratified columnar epithelium
Rare: male urethra, large excretory ducts, pharynx, anal mucous membranes, conjunctiva of eye
Function: protection and secretion (mucus in goblet cells)
Pseudostratified Columnar Epithelium
All cells attached to the basement membrane, but not all reach su rface
- Plain or ciliated
- Plain: male urethra, lining of large glandular ducts
- Ciliated: upper respiratory, auditory tubes
- Often have goblet cells for mucus production.
- Function: protection, secretion, movement of mucus via ciliary action
- Mucus protects host from disease
Transitional epithelium
Changes shape according to degree of stretching (can stretch to a single layer of cells)
- Superficial cells: large dome when relaxed, flat squamous when stretched
- Deep layers: cuboidal, columnar, polyhedral. Not as closely packed as other epithelium
Location: bladder, kidneys, ureters/urethra (so urine can accumulate)
Function: distension without leakage
Glandular epithelia
Gland: cells that make and secrete
- Usually make protein in RER, package in Golgi Apparatus, and release from cell
- Originally an inward folding during embryo development
- Some cut off from contact with environment (endocrine glands)
Endocrine glands
Lack ducts - secretions released into blood/lymph to be carried
Function: produce hormones to be carried around blood and impose humoral control on body’s activities
Location: pituitary, thyroid, ovaries, GIT, pancreas, testes, thymus, kidneys
Exocrine glands
Have ducts (small tubes) that carry secretions to specific areas, usually an epithelial surface
Location: salivary, sweat, liver, pancreas, sebaceous, ceruminous (ear wax) glands
Classified by:
- Structure (uni/multicellular. tubular/alveolar. simple/compound)
- Manner of secretion (holocrine/merocrine/apocrine)
- Type of secretion (serous, mucous, mixed)
Unicellular exocrine glands
Only one is goblet cell
Location: columnar cells of respiratory and GIT and conjunctiva of eyes
Function: Secrete mucin (sticky protein/carb mixture that becomes mucus when H2O added -> traps bacteria)
Multicellular exocrine gland
Divided into: secretory unit (secretions made) and duct (secretions carried out)
- May be surrounded by myoepthilial cells to help squeeze the secretion
Shape: tubular or alveolar (secretory unit - either looks like a tube or a ball… or tubuloalveolar if combo)
- Simple or compound (duct branches or not)
- Compound subdivided into: lobes or lobules (ducts draining the glands also interlobar/interlobular ducts)
Holocrine gland (exocrine)
Cells fill with secretory material and discharge the entire cell contents into the gland lumen, destroying cell (eg. sebaceous)
Merocrine gland (exocrine)
Creates vesicles which pass secretory products through the membrane without major loss of cytoplasm (eg. pancreas, sweat, salivary glands)
- Most common
Apocrine gland (exocrine)
Intermediate to holocrine/merocrine (some cytoplasm and membrane lost, but cell not destroyed)
- Mammary tissue, prostate, some sweat glands
Serous secretions (exocrine)
Watery, lots of enzymes
Mucous secretions (exocrine)
Thick, viscous, glycoproteins
Common in digestive and respiratory tracts
Mixed secretions (exocrine)
Produce both serous and mucous
- Mandibular salivary glands (some cells make mucous, others enzymes)
