Lining Epithelia Flashcards
composition and characteristics of lining epithelia
-mainly cells with little ECM
-cells apposed and connected with tight junctions
-lie on a basement membrane which lays on CT
-have a free surface on apical domain (usually facing lumen)
General functions of ET (5)
- secretion (stomach)
- absorption (intestine)
- transportation (using motile cilia OR bulk transport to and from CT)
- mechanical protection (epidermis and bladder)
- receptor function (taste buds/retina/nasal mucosa)
why do epithelial cells always have an underlying portion of CT?
ET is avascular but CT is vascularised. ET gets its nutrients from diffusion from blood vessels of CT
different apical specialisations that can be possessed by ET
- microvili (1 micrometer)
- cilia (10 micrometers)
- sterocilia (100 micrometers)
3 LOCATION BASED definitions of epithelium
- ENDOTHELIUM: ET lining on blood and lympathic vessels
- MESOTHELIUM: ET lining walls of closed cavities (eg, abdominal and pleural)
- ENDOCARDIUM: ET lining ventricles and atria in the heart
!! ALL: simple squamous cells (with 2 exceptions)
exceptions where endothelium is NOT simple squamous (2)
- HEV: high endothelial venules - postcapillary venules of certain lymphatic tissues, ET is cuboidal
- SPLEEN: venous sinuses contain ROD shaped ET.
Cell polarity definition
The specific characteristics associated with the apical/lateral/basal domains of cells that produce an intrinsic asymmetry of the cell needed for important functions (eg. barrier creation)
Structure of microvilli
-make up the brush border with the addition of the glycocalyx (glycoproetins and glycosaminoglycans)
-core is made of 20-30 actin filaments connected at their tip with villin (actin-bundling protein)
-horizontal network of actin extends into apical cytoplasm forming the terminal web
-proteins that allow crosslinking of actin at 10nm intervals: fascin/ espin/fimbrin
-core is associated with myosin 1
-spectrin stabilises terminal web
-myosin 2 and tropomyosin gives terminal web a contractile ability
microvilli: function and location
-number and shape corresponds with the cells absorbance capacity
-increases SA to increase diffusion via absorption
-visible in LM as striated brush border but an individuall microvillus is below the resolution power of the microscope
Sterocilia structure
-actin internal bundles (positive ends at the tips and negative ends at the base)
- ezrin proteins connect actin to plasma membrane
-absence of villin at the tip
-espin and fimbrin are actin bundling proteins that stabilise filaments
-actin filaments form cross bridges in the apical cytoplasm via interaction with alpha actinin
-connected via cytoplasmic bridges of alpha actinin
!! Derived from microvilli by lateral addition and elongation of actin
stereocilia function and location
-only present in the epidydymis, proximal ductus deferens, sensory hair cells of the inner ear
IN GENITAL DUCTS: act as larger microvilli to faciliate absorption
IN SENSORY HAIR CELLS: fucntion as mechanoreceptors and are capable of regeneration
!! BOTH CASES: IMMOTILE
Special characteristics of stereocilia in the sensory ET of the inner ear (3):
- STAIRCASE PATTERN: uniform in diamter and organised into bundles of increasing heights
- HIGH ACTIN DENSITY: high amounts of actin, extensively cross linked by espin
- LACK OF EZRIN AND ALPHA ACTININ
mechanism by which sterocilia maintain their structure
TREADMILLING: actin monomers are continuously added to the tips (+ve) and removed from the base (-ve)
- REASON: overstimmulation of stereocilia can lead to a loss of structure
Types of cilia (3)
- MOTILE: typical 9+2 axoneme organisation
- PRIMARY: solitary projections found on almost all euk cells. IMMOTILE (due to lack of motor proteins). Act as chemo/osmo/mechano receptors
- NODAL: found in bilaminar embryonic disc of embryo during gestation, and perform rotational movement, play a role in embryonic development
Motile cilia function and locations
-movement of material
-trachea, bronchi, oviducts
structure of MOTILE cilia
-9 doublets of peripheral MTs, one having a full 13 protofilament shell and the other having 11 and being completed by the first one
-outer filaments link to the basal bodies (modified centrioles)
-2 central MTs surrounded by a central sheath
-MTs linked by:
1. dynein arms (MAP)
2. nexin linking proteins (connects peripheral MTs with eachother)
3. radial spokes (connects peripheral to central MTs)
4. kinesin motor proteins
Process of ciliary movement (MOTILE CILIA)
- EFFECTIVE STROKE: dynein arms originating in A MT forms temporary cross bridges with B MT of adjacent doublet. This is ATP dependent (hydrolysis), and causes a sliding movement via shear stress
- RECOVERY STROKE: passive elastic connections provided by nexin and radial spokes accummulate energy to bring cilium to upright position
!! creates a synchronised wave spreading throughout the ET: called metachronal rhythm and is controlled mainly by the direction the basal feet are facing
primary cilia structure
-contains 9+0 arrangement because only the peripheral MTs exist
primary cilia main characteristics (6)
-one present in each cell
- nonmotile and bend passively with fluid flow
-lack MT proteins (dynein)
-lack of radial spokes and central MT pair
-axoneme originates from a BB positioned orthogonally
-formation is synchronised with cell cycle progression and centrosome duplication events
primary cilia function
-cellular sinaling devices receiving chemical, osmotic, light and mechanical stimuli from extracellular environment
-cilia then send signals to cells to modufy cellular processes (eg, cell division and cell elongation)
-also modulates Ca2+ release
How is machinery for ciliar growth transported within the cilia?
IFT: intraflagellar transport
-occurs because cilia lack machinery for protein synthesis
-uses raft platforms assembled from IFT proteins
-cargo molecules are loaded onto the IFT platforms and use motor proteins (dynein/kinesin) for transport
anterograde: towards tip, uses kinesin
retrograde: towards base, using dynein
Lateral ET specialisations
-CAMs: cell adhesion molecules
-make the composition of lateral and apical membranes differ massively
-creation of lateral junctions
(occluding, anchoring and communicating)
-PLICAE: infoldings present on certain ETs to increase SA and transport electrolytes.
Basal specialisations (3)
- basement membrane
- hemidesmosome junctions
- basal labyrynths
Basement membrane def
amorphous dense layer of variable thickness at the basal surfaces of ET
basement membrane morphology
Composed of 3 layers: co produced by ET (by keratinocytes) and CT (by fibroblasts)
- LAMINA LUCIDA: CAMs, fibronectin, laminin receptors (integrins). Electron light
- LAMINA DENSA: laminins, collagen 4, proteoglycans and glycoproteins. Electron dense
- LAMINA RETICULARIS: type 3 collagen reticular fibers
!! 1–> 3 in order from apical to basal. Hemidesmosomes connect 1 with ET
functions of the basement membrane (5)
- links ET with CT
- regulates cell polarity
- selective barrier
- prevents metastasis
- allows ET regeneration and reparation (because it allows migration of cells)
Pathologies arising from a degraded basement membrane
-carcinomas and transformed cells
-this is because BM controls the prevention of metastasis
what ET is derived from each of the three germ layers?
- Ectoderm: epidermis cornea and lens, oral/nasal mucosae, glands of the skin,
mammary glands, tooth enamel - Endoderm: epithelia of liver, pancreas, lining epithelium of the respiratory and gastrointestinal tract
- Mesoderm: kidney tubules, lining of urinary and reproductive tracts, epithelium enveloping the ovary serosa (mesothelium), blood and lymphatic vessels epithelium (endothelium).
Factors affecting the classification of a lining ET (2)
- morphology (squamous, cuboidal, columnar)
- number of cells (simple/stratified)
!!! classification has nothing to do with what germ layer the ET is derived from
simple vs stratified ET
SIMPLE: contains cells on a single layer –> all touch the basement membrane
STRATIFIED/COMPOSED: multiple layers of cell, not all of which lie on basement membrane.
!! cell morphology is based off of the most apical layer seen
characteristics of cells of the 3 ET morphologies
- squamous: flattened cells with oval nuclei that usually protrude from cell
- cuboidal: cuboidal and central nucleus (rounded)
- columnar: elongated cell shape, aligned nuclei on the same plane. usually have apical specialisations (MOST FREQUENT TYPE)
localisations of simple squamous epithelial
-lung alveoli
-endothelium of blood and lymph vessels
-bownmans capsule
-tunica serosa (mesothelium)
localisation of cuboidal ET
-ovary surface
-gland ducts
-terminal bronchioles
localisation of simple columnar ET
-small intestine
-uterine tube
-small bronchioles
-stomach
-colon
-gallbladder
-excretory/secretory ducts
columnar pseudostratified ET description
-all cells lay on the basement membrane but the nuclei are not aligned in a single plane
-not all cells reach the free edge
-this gives the impression that there are multiple clle layers instead of one
-usually contain apical specialisations and intercalated goblet cells
localisation of pseudostratified ET
-mucosa of airways
-urethra
-epididymis
-ear canal
-large excretory ducts
EPIDERMIS morphology
STRATIFIED SQUAMOUS KERATINISED
-superficial layer has cells that do not posess a nucleus
layers from apical to basal:
1. stratum corneum: dead cells containing keratin for mechanical protection
2. stratum lucidum
3. stratum granulosum
4. spinous layer (8-10 layers of cells) - cells have desmosomal connections to keep tissue apposed
5. basal layer - 1 cell thick (stem cells for terminal differentiation into all above layers)
thick vs thin skin
Thin skin: present on all areas of the body, has a thinner stratum corneum and hence the stratum lucidum is not evident
Thick skin: present on palmar and plantar surfaces, has a thicker stratum corneum/lucidum and so the lucidum is distinguishable
epidermal ridges vs dermal papillae and function
EPIDERMAL RIDGES: protrustions of epidermis into dermis
DERMAL PAPILLAE: protrusions made of loose CT rising into dermis.
function: epidermis is avascular so they allow an interface to help expand the surface for nutrient absorption
What is involucrin?
-highly reactive and soluble protein
-found in epidermis
-synthesised in spinous layer and contains cross links in the stratum granulosum
-provides tructural support to the cell
!! increase expression in inflammatory pathologies like psoriasis
what do granules in the stratum granulosum of the skin contain?
- filaggrin → produces cross links between the keratin filaments
- loricrin → reinforces the internal side of the plasma membrane
process of cell production leading to skin regeneration
TERMINAL DIFFERENTIATION:
1. stem cells in basal layer undergo mitosis: one daughter cell remains in basal layer and the other moves to the spinous layer above
2. differentiation starts in spinous layer and continues in the granulosum
3. cornification (cell death exhibited by the keratinocytes) occurs moving into the lucidum and corneum strata –> loss of organelles and retaining of plasma membrane to become sacs of keratin that contribute to the upper skin layer
4. desquamation occuring at the uppermost layer - dettachment/shedding of cells
different cell types found in the epidermis (4)
- keratinocytes
- melanocytes
- langerhans
- merkle cells
cell junctions in the epidermis
-hemidesmosomes between basal layer and basement membrane
-desmosomes between cells in the spinous layer
-tight junctions in stratum granulosum (this seals cells to prevent hydration)
!!! there are differences in the types of proteins contained in desmosomes found in different layers of epidermis:
- Desmoglein and desmocolin 1: higher expression in superior layers
- Desmoglein and desmocolin 2: higher expression in basal layers
Pathology that arises when there is defective expression of desmosomal proteins in diff layers of epidermis
PEMPHIGUS FOLIACEUS:
-autoantibody mediated blistering disease
-production of antibodies against desmoglein 1 causes a loss of adhesion in keratinocytes in superficial skin layers
-causes skin blistering
stratified sqamous ET non keratisied description
-multiple cell layers
-cells possess a nucleus in every layer (so even superficial layers are alive)
-often used as mechanical protection
-moist and is permeable to water (because there in no keratin and so the barrier is less effective)
LOCATION: cornea, oral cavity, pharynx/larynx, oesophagus, vagina
stratified cuboidal ET location
-not very frequent
-present in big ducts and glands (sweat glands, sebaceous glands, and exocrine pancreas)
stratified columnar ET location
-larynx
-male urethra
-conjuctiva
-big glad ducts
morphology and localisation of transitional ET
LOCATION: urinary bladder
-dome shaped cells at rest, which expand to form flattened, elongated cells (called umbrella cells)
-have the possibility to be binucleated
NON DISTENDED: when the bladder is empty - dome shaped
DISTENDED: when the bladder is full - cells flatten, thickenss reduces to about 3 cells thick
melanocyte cells description
-found in epidermis
-very large cells with many prolongments (found between basal layer and spinous stratum)
-produce melanin pigment for UV protection
langerhans cells in epidermis description
-cells with thin and long prolongments
-special macrophages that recognise specific antigens
-move to lymph vessels and are transported to lymph nodes to produce antibodies for immune response
merkle cells description
-present in epidermis
-sensory/tactile receptors
-connected to a sensory nerve fiber which is stimulated upon the release of vesicles containing mediators by the merkle cells
-transfer of info to CNS
