LECTURE - Epithelial Tissue

Question 1

Epithelia mediate a wide range of activities

Answer 1

selective diffusion
absorption
secretion
physical protection
containment

Question 2

all epithelia are supported by a ..

Answer 2

basement membrane

• separate epithelia from underlying supporting tissues and are never penetrated by blood vessels
• epithelia depend on diffusion of oxygen and metabolite from supporting tissues

Question 3

classification of epithelia (3)

Answer 3

1. number of cell layers
2. shape of component cells
3. presence of surface specializations

Question 4

simple epithelia

Answer 4

• selective diffusion, absorption, secretion

- not found on surfaces subject to stress bc little protection against mechanical forces

Question 5

simple epithelia

Answer 5

• selective diffusion, absorption, secretion

- not found on surfaces subject to stress bc little protection against mechanical forces

Question 6

simple squamous epithelium

Answer 6

• flattened, irregularly shaped cells forming a continuous surface = pavement epithelium
• line surfaces involved in passive transport (diffusion) of gases or fluids
• lining of of pleural (lung), pericardial (heart), and peritoneal (abdominal) cavities

Question 7

simple cuboidal epithelium

Answer 7

• in sections perpendicular to basement membrane = square
• nucleus usually round; centre of cell
• lines small duct and tubules which may have excretory, secretory, or absorptive functions

Question 8

simple columnar epithelium

Answer 8

• similar to cuboidal except cells are taller
• nuclei elongated; may be located towards base, center, or apex of cytoplasm
• found mostly on highly absorptive surfaces such as small intestine

Question 9

simple columnar ciliated epithelium

Answer 9

• presence of cilia makes it different from simple columnar epithelium
• among ciliated cells are scattered non-ciliated cells which usually have a secretory function

Question 10

pseudostratified columnar ciliated epithelium

Answer 10

• appearance of more than one layer of cells; but because all cells rest on basement membrane, this is true simple epithelium
• nuclei at different levels (illusion of stratification)
• almost exclusive to larger airways of resp. tract = respiratory epithelium

Question 11

how do we distinguish pseudostratified from true stratified epithelia

Answer 11

• nuclei are mainly confined to basal two-thirds of epithelium
• clia are never present on stratified epithelia

Question 12

stratified epithelia

Answer 12

• two or more layers of cells
• mainly a protective function; poorly suited for secretion/absorption due to thickness
• classification based on shape + structure of surface cells (basal cells usually cuboidal)

Question 13

stratified squamous epithelium

Answer 13

• cuboidal basal layer to a flattened surface layer
• basal cell divide continuously to replenish surface cells that are lost
• lines oral cavity, pharynx, esophagus, anal canal, uterine cervix, vagina

Question 14

keratinized stratified squamous epithelium

Answer 14

• specialized form of stratified squamous epithelium that contributes to epithelial surface of skin epidermis
• maturation = epithelial cells accumulate cross-linked cytoskeletal proteins = keratinization
• keratin allows epithelium to withstand the constant abrasion and dessication to which it is exposed

Question 15

stratified cuboidal epithelium

Answer 15

• thin, stratified epithelium = only two or three layers of cuboidal or low columnar cells
• larger excretory ducts

Question 16

transitional epithelium

Answer 16

• urinary tract
• features of both stratified cuboidal and squamous = transitional
  relaxed (contracted) state = 4-5 cell layers thick ; stretched = 2-3 cells thick (intermediate and surface layers extremely flattened)
• basal cells cuboidal for relaxed state, intermediate is polygonal, and surface cells are large and rounded and may contain two nuclei = binucleate cells; umbrella cells
• urothelium

Question 17

intercellular surfaces

Answer 17

adjacent cells have cell junctions that permit the epithelia to form a continuous cohesive layer in which all of the cells ‘communicate’ and cooperate

Question 18

three functional types of cell junctions:

Answer 18

• occluding junctions (tight)
• adhering junctions
• communicating junctions (gap or nexus junctions)

Question 19

occluding junctions

Answer 19

• tight junctions
• immediately beneath luminal surface of simple columnar epithelium where they seal intercellular spaces and may prevent migration of floating membrane proteins from their appropriate domains on the cell surface
• each tight junction forms continuous circumferential band around the cell

Question 20

adhering junctions

Answer 20

• tightly bind constituent cells of epithelium together
• act as anchorage sites for cytoskeleton each cell so cytoskeleton of all cells are effectively linked into a single functional unit
  two types:
• zonula adherens: deep to tight junctions of columnar epithelial cells, forms a continuous band around cell providing structural; reinforcement to occluding junction
• desmosomes (macula adherens): small circular patches or pots that are circumferentially arranged around columnar cells deep to the continuous zonula adherens

Question 21

communicating junctions (gap/nexus)

Answer 21

circular intercellular contact areas containing hundreds of tiny pores which permit the passage of small molecules between adjacent cells

Question 22

junctional complex

Answer 22

tight junctions, zonula adherens, desmosomes

Question 23

the luminal surface may incorporate three main types of specialization:

Answer 23

• cilia
• microvilli
• stereocilia

Question 24

cilia

Answer 24

• relatively long
• motile structures which project in parallel rows from certain epithelial surfaces (resp and female reproductive tract)
• beat with a wave-like synchronous rhythm propelling surface films of mucus or fluid in a consistent direction over the epithelial surface
• axoneme = central core; 20 microtubules arranged as a central pair surrounded by nin peripheral doublets; base = inserts into basal body
• each axoneme doublet = one tubule which is c-shaped in cross-section
• dynein extends towards incomplete tubule of adjacent doublet and towards central microtubule pair

Question 25

microvilli

Answer 25

- minute finger-linked projections of luminal PM - can be seen as brush border on surface of cells - usually for absorption; can increase SA available for absorption by up to 30x - cytoplasmic core = fine actin filaments which insert into terminal web - terminal web anchored to zonula adherens at periphery of cell

Question 26

stereocilia

Answer 26

- extremely long microvilli - found in small number in parts of male reproductive tract - filamentous skeleton inside - may facilitate absorption

Question 27

basal surface

Answer 27

- hemidesmosomes are present on the inner aspect of basal plasma membrane adjacent to basement membrane and provides a means of anchorage of cell via cytoskeleton to basement membrane and underlying supporting tissues

Question 28

gland that discharge their secretory product via a duct onto an epithelial surface

Answer 28

exocrine gland

Question 29

exocrine glands classified according to two major characteristics

Answer 29

MORPHOLOGY - simple: single, unbranched duct; secretory portions of simple glands have two main forms = tubular or acinar (which may be coiled and/or branched) - compound: branched duct system and their secretory portions have similar morphological forms as simple glands MEANS of discharge of secretory products from cells: - eccrine secretion: involves process of exocytosis; proteins are major secretory product - apocrine: discharge of free, unbroken membrane-bound vesicles containing secretory product; applies to lipid secretory products in beast and some sweat glands - holocrine: disintegration of secretory cell to release secretory product; sebaceous glands

Question 30

secretory products of some exocrine glands are embraced by contractile cells called

Answer 30

myoepithelial cells

Question 31

ductless glands

Answer 31

endocrine | - secretory products diffuse directly into bloodstream

Question 32

secretory products of endocrine glands

Answer 32

hormones - control activity of cell and tissues far removed from the site of secretion - most consist of groups or cords of secretory cells surrounded by a rich network of blood vessels; each group of cells surrounded by basement membrane reflecting its epithelial origin - release hormones into intercellular space and then diffuse rapidly into surrounding blood vessels

Question 33

epithelial cells are closely bound to one another by a variety of membrane specialization called ...

Answer 33

cell junctions - provide physical strength and mediate exchange of 'information' and metabolites

Question 34

adhering junctions

Answer 34

- tightly bind constituent cells of epithelium together - act as anchorage sites for cytoskeleton each cell so cytoskeleton of all cells are effectively linked into a single functional unit two types: - zonula adherens: deep to tight junctions of columnar epithelial cells, forms a continuous band around cell providing structural; reinforcement to occluding junction - desmosomes (macula adherens): small circular patches or pots that are circumferentially arranged around columnar cells deep to the continuous zonula adherens

Question 35

simple epithelia

Answer 35

- selective diffusion, absorption, secretion | - not found on surfaces subject to stress bc little protection against mechanical forces