Epithelium and Cell Surface Specialization

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Dental Histology

4 basic or fundamental tissue types

Epithelium
Connective tissue
Muscular tissue
Nervous tissue

Epithelial tissue

forms cellular sheets that covers body surfaces (ext/int), lines body cavities, forms glands
has very little extra- or intra-cellular substance (matrix or material)

Connective tissue

connecting and supporting
-surrounds and supports the other three basic tissues both structurally and functionally
-has abundance of extra-cellular substance

Muscular tissue

made up of contractile cells and responsible for movement of body and its parts
specialized function of contraction

Nervous tissue

function in coordination and communication
receives, transmits and generates info from outside and inside the body (nerve impulse) to control the activities of the body and its parts

Main characteristics of ET

Avascular, nourished by diffusion from CT
lots of sensory nerve ending
very little extracellular substance
has basement membrane
renewal, mitosis regeneration, rapid turnover rate
metaplasia - transformed to other type, abnormal, smoking - from PCCE to stratified squamous epithelium, hiatal hernia and bulemic - esophagus - from stratified squamous epithelium to simple columnar

Main functions of ET

covering external surfaces of body
lining body cavities
lining body tubes
absorption
secretion
sensation
contractility
excretion
lubrication
transportation by cilia
reproduction

Ectodermal origin

skin (epidermis only), hair, nail, glands, mouth, nose and anus

Mesodermal origin

endothelial lining of blood vessels
mesothelial lining of body cavities
UG systems

Endodermal origin

lining of respiratory tract
digestive tract and its glands (liver, pancreas)

Epithelial cells

vary in size and shape from flat to tall
Squamous cells (L=fish scales) with flattened nuclei
Cuboidal cells with spherical nuclei
Columnar cells with elongate nuclei

Two types of epithelia

Glandular epithelium
Covering epithelium can be classified according to number of cell layers - Simple Et (one cell layer) and Stratified Et (two or more cell layers)

Epithelia according to cell shape

Squamous
Cuboidal
Columnar

Simple squamous epithelium

endothelium
mesothelium (pleura, pericardium, peritoneum)
lung alveoli
Bowman's capsule of kidney

Simple cuboidal epithelium

collecting tubules of kidney

Simple columnar epithelium

lining of stomach, small and large intestines

Stratified epithelium

classified according to cell shape of its superficial layer

Stratified squamous keratinized epithelium

(skin, dry surface), keratin on top layer, no cell organelles

Stratified squamous non-keratinized epithelium

(mouth, esophagus, vagina, moist surfaces) alive cells with nuclei are at top layer, retain organelles

Stratified cuboidal epithelium

rare (sweat gland and ducts)

Stratified columnar epithelium

rare (large ducts of salivary ducts)

Transitional epithelium

(ureter, urinary bladder, upper urethra) - dome shaped cells on top layer, some may be binucleated

Pseudostratified ciliated columnar epi (PCCE)

(trachea, bronchi) - not true stratified, nuclei at different levels, all cells reach basal lamina, but not all cells reach the surface, with stereocilia in epididymis

Neuroepithelial cells

(tastebuds, retina, inner ear hair cells, olfactory epithelium) - for specialized sensory function

Myoepithelial or basket cells

(acini of salivary, mammary glands) - specialized for contraction

Apical surface specializations

microvilli, stereocilia, cilia, and flagella

Lateral surface specializations

junctional complexes, zonula occludens, zonula adherens, macula adherens (desmosomes), hemidesmosomes, gap junctions (nexus), interdigitation, intercellular canaliculi

Basal surface specializations

hemidesmosomes, basal invaginations/foldings and basement membrane or basal lamina

Microvilli

Actin filaments form core of microvilli
finger-like projections from apical surface
under light microscope microvilli clusters seen as
- striated border in small intestine epi, regular
- brush border in proximal renal tubules, irregular
under electron microscope seen as
- extension of cytoplasm covered by plasma membrane
each covered with glycocalyx which stains +ve with PAS
inside are clusters of actin microfilaments, connects to microfilaments in terminal web at the base of microvilli along apex of the cell

Stereocilia

found in epididymis
not a true cilia
non-motile
long anastomosing microvilli
seen in LM as cone-shaped structures

Cilia

found in trachea, bronchi, oviduct
long motile structures with rapid back and forth movement to propel mucus/egg in one direction
each arises from basal body (centriole)
each has axoneme core composed of 9 pairs of peripheral microtubules (doublets) and 2 single central microtubules (singlets)
doublets have dynein arms with ATPase activity to energize ciliary motion
abnormal respiratory problems

Flagella

only present in spermatozoa in humans
similar to cilia but much longer
only 1 flagellum per sperm cell

Kartagener's Immotile Cilia Syndrome

genetic defect in genes coding for dynein
uncoordinated or absent cilia beating
chest infection
infertility in male due to defected flagella
in females not present because egg moved by smooth muscle contractions

Junctional complex

seen as terminal bar in LM, encircles entire cell at apex, 3 components:
-Zonula occludens (tight or occluding junction)
-Zonula adherens (adhering junction)
-Macula adherens (desmosomes)
ZO and ZA encircle cell while MA does not

ZO or tight junction

most apical, zone-like
completely encircles cell
closes off intercellular space
fusion of outer leaflets of adjacent membranes form a tight seal (greatest resistance) to prevent flow of materials between cells

ZA or adhering junction

band-like
encircle the cell
adhesion of one cell to another
actin filaments attached to dense plague of material

MA or desmosomes

spot adhesions, patches, disk
do not encircle cell membranes are straight and further apart
hairpin loop of actin microfilaments attach to plagues called tonofilaments
only type of junction found or present in epidermis of skin and oral mucosa form firm adhesion between cells

Fascia adherens

intercalated discs of cardiac muscle

Gap junction

for communication, exchange between cells

Hemidesmosomes

half of a desmosome

Basal invaginations

found in proximal/distal tubules of kidney, striated duct of salivary glands, house longitudnally arranged mitochondria, for ion exchange/active transport

Basal membrane

Two components
- basal lamina
- reticular lamina

Basal lamina

epithelial derived, type IV collage (no form), glycoproteins and proteoglycan
has 2 parts
- lamina densa (dense layer, dark color)
- lamina lucida (electron lucent, light color)

Reticular lamina

connective tissue derived
type VII collagen in anchoring fibrils

Functions of Basal Laminae

structural support (epithelium)
filtering functions (in renal glomerulus)
influence cell polarity
regulate cell proliferation, differentiation and migration
influence cell metabolism
epithelial regeneration in wound healing
cell to cell interaction

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