Epithelia Flashcards
Characteristics of epithelia
Avascular
Closely attached to each other (with many specialised junctions)
Functionally distinct surfaces
What does the epithelial group consist of
Cells that cover the external body surfaces, line the internal closed cavities, and body tubes that communicate with the exterior
They also form secretory units &a ducts of glands
Epithelial fxs
Protection Absorption Secretion/excretion Contractility Line/cover surfaces of body Interfaces between diff bio compartment Form glands and ducts
Types of epithelia
Simple
Stratified
What is simple epithelia
One cell layer thick
Ideal for secretion diffusion absorption
Range in shape
What are stratified epithelia
2+ cell layers thick
Mostly protective fx
Classified according to morphology of surface cells
Types of simple epithelia
Squamous Cuboidal Columnar Columnar ciliated Paeudostratified columnar ciliated
Types of stratified epithelia
Squamous Squamous keratinised Cuboidal Columnar Transitional
Eg junctional complexes
Hemidesmosome
Desmosome
Tight junction
Zona adherens (adherens jctn)
Simple squamous
Found lining surfaces suitable for diffusion eg alveoli capillary walls lining of pleural pericardial peritoneal cavities
One cell layer thick
Look like fried eggs/flat
Simple cuboidal
Found lining ducts and tubules with excretory secretory or absorptive fx eg kidney tubule salivary gland pancreas
Round centrally located nucleus
Simple columnar
Line highly absorptive or secretory surfaces eg small intestines stomach gall bladder often interspersed with goblet cells
Nucleus oval shaped and basal border
Simple columnar ciliated
Found lining the oviduct cilia beat synchronised
Pseudostratified columnar ciliated aka respiratory epithelia
Found in upper respiratory tract Always ciliated Interspersed with goblet cells Nuclei not all situated at basal border Give appearance of more than one layer
Stratified squamous
Found in oral cavity, pharynx, parts of larynx, oesophagus, anal canal, uterine cervix, vagina
Protective fx withstand mech abrasion but no desiccation must be kept moist
Basal layer always cuboidal
Continual new production of cells from basal layer
Variable no of cell layers which change morphology as they move towards the surface and nuclei also become flattened toward the surface
Stratified squamous keratinised
In the skin/epidermis
Protective fx withstand mech abrasion and dessication
Process of keratinisation
As they push towards surface they mature then degenerate.
Nuclei become flattened and pyknotic so cell is dead
Stratified cuboidal
Found lining ducts eg salivary glands
Usually 2-3 layers
Minimal secretory absorptive fx
Provides more substantial lining compared to simple
Stratified cuboidal
Both layers tall - conjunctiva
Basal layers cuboidal - distal urethra, lacrimal sac & duct, salivary gland ducts
Protective fx
Transitional epithelia
Urinary tract, bladder
Highly specialised to accommodate stretch
Outer/luminal cells become flattened in a stretched state
Relaxed state 4-5 layers
Stretched state 2-3 layers
Other BM names
Basal lamina
External lamina
Features of the BM
Secreted by the epithelia
Never penetrated by blood vessels
What are the layers of the BM
Lamina lucida
Lamina densa
Lamina fibroreticularis
Lamina lucida
Electron Lucent layer next to the epithelial cells
Lamina densa
Intermediate electron dense layer (visible by light microscopy)
Lamina fibroreticularis
Relatively electron Lucent layer which merged with the underlying CT
BM composition
Anchoring filaments
Structural glycoproteins
Proteoglycans
Types of anchoring filament in the BM
Collagen 4 (exclusive to BM) Some C 3 (reticulin) found in the lamina fibroreticularis
Types of structural glycoproteins in BM
Laminin (bind to C 4 and other BM constituents and laminin receptors (shaped glycoproteins) in basal membrane of epithelial cells)
Entactin (mediates binding of laminin to C4)
Fibronectin (produced by fibroblasts of underlying CT. binds membrane spanning receptor proteins (integrins) and extra cellular components eg collagen, heparin sulphate so play important cell adhesion role)
Types of proteoglycans in the BM
Heparan sulphate (extensively hydrated and forming most of bulk of BM)
BM fxs
Structural (intermediate in attachment of CT to cells Compartmentalises (isolate CT and cells) Filters (regulate movement of substances into the kidneys urinary space within renal corpuscle) Polarity induction ( defines in vivo shape of epithelia (along with mesenchyme below) Tissue scaffolding (guide during regeneration of damaged epithelial tissue)
Specialisations
Cilia
Micro villi
Stereo cilia
Keratin
What are Cilia
Long motile cytoplasmic structures projecting from surface in parallel rows
7-10microm long
Seen with light microscope
Internal structure made of several micro tubule pairs
Cilia fx
Beat wave like to move surface film of mucus or fluid in consistent direction
What are microvilli
Relatively short static cytoplasmic projections extending from cell surface forming striated/brush border
0.5-1microm long
Not seen individually with light microscope
At high electron mag can see the core containing several thin actin filaments
Microvilli fx
Size And density related to cells absorptive capacity
They increase the cell surface area
What are stereocilia
Microvilli of unusual length
Named before electron microscopy showed they have a different internal structure to cilia
No micro tubules just filament pus actin skeleton like microvilli
Found in epididymis
Keratin fx
Prevents desiccation and withstand abrasion
Origins of epithelia
3 germ layers of early embryo
Ectoderm
Endoderm
Mesoderm
Ectoderm
Give rise to corneal and lens epithelium of eye, epidermis of the skin and its derivatives.
Lining of oral and anal canal cavities and cells that produce tooth enamel
Endoderm
Give rise to epithelia lining the alimentary tract (- mouth pharynx anus)
Respiratory system
Intestinal glands (liver gall bladder pancreas)
Thyroid, anterior pituitary gland
Parathyroid, thymus
Mesoderm
Epithelium of urogenital system
Provide continuous layer lining blood and lymph vessels (endothelium) and peritoneal and other serous cavities (mesothelium)
