Integrating Cells Into Tissues And Organs Flashcards
What holds cells together
Cell-Cell adhesion molecules
Extracellular matrix proteins (fibres)
Internal-external scaffolding
Close proximity (pressure effects)
Connective tissue organisation
Connective tissue layers -
Extracellular matrix is plentiful
Cells are sparsely distributed within it
The matrix is rich in fibrous polymers, especially collagen, and it is
the matrix—rather than the cells bears most of the mechanical stress
Direct attachments between one cell and another are relatively rare
The primary cell of the connective tissue is the mesenchymal stem cell (a type of immature fibroblast)
It has the ability to interconvert between several cell types
Produces most of the extracellular fibres that anchor cells into
place or make ‘tissue’
In culture, quickly converts to a mature fibroblast and
produces fibronectin, laminin and collagen that allows cells to
adhere to plastic or glass surfaces
Structure - Made of 3 components:
Cells – mainly mature fibroblasts/fibrocytes, fixed adipocytes, reticular cells (found in lymphatic tissue)
Fibres – collagen, elastin, reticular fibres
Ground substance – glucosaminoglycans, e.g. hyaluronic acid
Function
1) binding and supporting (such as holding skin, gut, lungs, etc. together)
2) protecting (such as bone protecting vital organs)
3) insulating (fat underlying skin)
4) storing reserve fuel and cells (bone marrow and fat tissue)
5) transporting substances within the body (blood and interstitium)
6) separation of tissues (fascia and tendons/cartilage)
Epithelial tissue
Epithelial tissue layers
Cells are tightly bound together into sheets called epithelia
Extracellular matrix is scant, consisting mainly of a thin mat called the basal lamina, which underlies the epithelium.
Cells are attached to each other by cell-cell adhesions, which bear most of the mechanical stresses
For this purpose, strong intracellular protein filaments (components of the cytoskeleton) cross the cytoplasm of each epithelial cell and attach to specialised junctions in the plasma membrane
The junctions, in turn, tie the surfaces of adjacent cells either to each other or to the underlying basal lamina
Epithelial cell sheets line all the cavities and free surfaces of the body
The specialised junctions between epithelial cells help to form tissue barriers (these inhibit movement of water, solutes and cells from one body compartment to the other)
Epithelia almost always rest on a supporting bed of connective tissue
The supporting bed attaches the epithelial layer to other tissues (therefore allowing tissues to join together in caries combinations to form larger functions units called organs
In the lateral surface • Tight junctions • Adherens junctions • Desmosome (adhesion plaque) • Gap Junctions • Cell adhesion molecules
In the basal surface • Hemi-desmosome • Focal adhesions • Integrins • Proteoglycans • Cell adhesion molecules
Tight junctions
Always at the very top of the cell nearest to lumen/apical surface in the lateral border
Relative long cell-to-cell fusion point
Role to prevent movement of larger molecules through the outer layer/lumen into the deeper tissue layers of the organ
In the gut, can transiently open to allow small molecules (sugars, amino acids and water) to cross to the underlying tissues – known as paracellular transport
Adhesion junctions
Almost always found 1/3rd distance from luminal surface
In lateral surface
Found in pairs
Formed from intracellular actin filaments
Linked to E-cadherin proteins that cross the intercellular space
Found throughout this region as ‘a belt of adhesion’
Sometimes called ‘adhesion belt’
Found only in epithelial and endothelial cells
Functions as tissue stabilising factor and additional transport barrier
Desmosomes
The strongest of all the cell-to-cell adhesions
Found ~ ½ way between top and bottom of cells
Random distribution pattern
Found in tissues that experience intense mechanical stress
e.g. cardiac muscle, bladder tissue, gastrointestinal mucosa, epithelia (all types), pregnant uterus, etc.
Cytokeratin fibres intracellularly, E-cadherins intercellularly (spring-like)
Role to provide mechanical strength and prevent tissue destruction
The only cell-to-cell adhesion found in epidermal (skin) cells
Gap junctions
Found close to base of epithelial cells
Distributed throughout cardiac and smooth muscle cells
Role to quickly communicate changes in intercellular
molecular composition e.g. electrolyte and energy
changes
Allows free movement of small molecules from one cell to
another e.g. ions, most sugars, amino acids (<1000 Da)
Important in smooth muscle contraction – allows wave of
electrical impulse
Only spermatozoa, erythrocytes and other motile cells do not have gap junctions
Made of cylinders of proteins (connexins) arranged in a
hexagonal pattern that open and close (ATP)
Switch from connexin 45 to connexin 34 occurs in the
myometrium of the pregnant uterus in preparation for
birth
Hemi-desmosomes
Only found on basal surface of epithelial cells
Attach not to cells but a layer of extracellular matrix
e.g. fibronectin, collagen and laminin fibres
Intracellular intermediate filaments of cytokeratin
attached to laminin through integrins
Basal lamina attached to connective tissue layer through elastin, fibrillin and other collagens
Role to anchor epithelial cells to the basal lamina and prevent loss to external surface
Focal adhesions
Similar function to hemidesmosomes – attachment to basal lamina
Uses Intracellular actin filaments (instead of cytokeratin)
Uses integrins (just like hemidesmosomes)
Binds to fibronectin (instead of laminin)
When bound to fibronectin, conformational change results in binding to collagen fibres
Integrins
Integrins central to cohesive forces holding tissues together
Always work as alpha-beta dimer
Weak binders of extracellular matrix as dimer pair
Phosphorylation by Focal Adhesion Kinase produces heterotetramer that has greater binding capacity - Hence stronger bond
The mucosal membrane
This structure lines all the ‘moist’ hollow internal organs of the body
It is continuous with the skin at various body openings
e.g. the eyes, ears, inside the nose, inside the mouth, lips, vagina, the urethral opening and the anus
(the airways and lungs)
Most mucous membranes secrete mucus, a thick protective fluid Contains mucins (protein), electrolytes, antiseptic enzymes (lysozyme), immunoglobulins
The function of the mucosal membrane is to:
Stop pathogens and ‘dirt’ from entering the body
Prevent bodily tissues from becoming dehydrated
Lubricate the surface
Good examples
• GI tract • Urinary tract • Respiratory tract
GI tract - layers
Mucosa lining the lumen - Epithelial cell lining and supporting mesenchymal layer
Muscularis mucosae - A thin discontinuous smooth muscle layer
Submucosa - A connective tissue layer that contains arteries and veins
Muscularis externa - A smooth muscle layer that has muscle fibres going generally in two different directions
An inner circular muscle
An outer longitudinal muscle
The Serosa - another connective tissue layer
Contains collagen and elastin fibres with some smaller arteries and veins and some nerve fibres
Oesophagus
Oesophageal structure-function relationships:
- Epithelium – stratified squamous non-keratinised (to withstand abrasion)
- Submucosa - subtending layer of connective tissue containing mucus-secreting glands - to protect against exogenous bacteria
- Muscularis externa – smooth muscle layers
(inner – circular; outer – longitudinal) which move a bolus of food by peristalsis
Large intestine
The muscularis mucosae is indistinct at this magnification.
The simple columnar epithelium of the crypts produces mucus and supplies cells to the surface.
The surface epithelial cells absorb water and electrolytes.
GI tract function of mucosa
- To absorb substances from the lumen
- Prevent ingress of pathogens
- Move contents and expel waste
Epithelial cell specialisations aid processes 1 and 2
Folding of mucosa, microvilli, peristaltic actions
Lamina propria contains lymphatic tissue to aid process
Underlying lymphatic tissue
Muscularis mucosae folds mucosa to increase SA (1 and 2)
Critical structure-function stability control
Muscularis externa performs ‘peristalsis’ to aid process 3
Urinary tract structure
Structural unit in the kidney is the nephron
Urinary Tract - Structure
Flat epithelium cells around the edge of the bowman capsule in order to move fluid across into the bowman space
Corpuscle lining is ‘flattened’ (squamous) epithelium
Lining of collecting ducts is ‘square shaped’ (cuboidal) epithelium
Bladder histology
Fat acts as a shock absorber for expanding bladder
Transitional epithelium is present - when the wall is relaxed or contracted then the shapes of the surface cells differ, rounded in the relaxed state and flattened in the distended state - can be binucleate
- Epithelial cells produce mucus
- Protects bladder from damage by acidic urine
- Tight junctions and very well packed – prevents leakage to inner cell layers
Urethra
Structure similar to bladder (except epithelial cells change from transitional epithelium to squamous and the keratinised squamous epithelium at outlet).
Mucus glands produce large amounts of ‘sticky’ mucus and so prevent ingress of pathogens
Urinary tract - summary - Functionally very similar to the GI Tract 1. Absorption of essential nutrients in the kidney
- Prevention of pathogen entry (especially in lower urinary tract)
- Removal of waste products
Respiratory structure
Gaseous transport and exchange
Divided into 2 parts
• Conducting portion
• Respiratory portion
Conducting portion of respiratory tract = nasal cavity to bronchioles.
Respiratory portion of respiratory tract = respiratory bronchioles to alveoli.
Trachea
The TRACHEA (10 cm long; 2.5 cm wide) divides into two primary bronchi in the mid-thorax. - has pseudostratified ciliated columnar epithelium
Primary bronchi have a histology similar to that of the trachea, but their cartilage rings and spiral muscle completely encircle the lumen.
Path of right bronchus more vertical than left so foreign objects more likely to lodge in the right bronchus.
Don’t need muscle here as there is cartilage keeping the trachea open
Trachea and primary bronchi structure
Mucosa: The epithelial layer is several cells deep and the surface is covered in cilia (moves debris, dust, bacteria, etc. towards mouth)
The lamina propria is very thin, no longitudinal muscularis mucosa layer
Submucosa: The connective tissue layer contains mainly collagen and elastin fibres and many fibroblasts. Also contains seromucous glands – produce a watery mucus that thickens during infection
The C-shaped Hyaline cartilage can be palpated externally and is made of two layers:
• perichondrium that has fibroblasts that lay down collagen fibres
• chrodrogenic layer, from which cartilage is formed.
The cells present can interconvert from chrondroblasts to chondrocytes and so can make both hyaline and elastic cartilage
Note: no outer layer of smooth muscle
Tracheal and bronchial secretions
Secretions from the epithelium (E) and submucosal glands (SM) of the trachea and bronchi contain:
Mucins and water - make sticky mucus
Serum proteins - lubricates the surfaces
Lysozyme - destroys bacteria
Anti-proteases - inactivate bacterial enzymes
Together with ‘a cilia wave’ mucus moves materials to the oral cavity where the material can be swallowed – known as ‘mucocilliary escalator’
Mucocillary escalator in trachea
Tracheal mucosa: approx. 250 cilia/cell; ciliary basal bodies evident as thin line.
Cilia beat at 12 Hz beneath a moveable, viscoelastic mucus blanket (5μm deep).
Mucus travels up the trachea to expel any dust/pathogen that has been caught in the mucus
Secondary and tertiary bronchi
Histology similar to primary bronchi except
that the cartilage is no longer present as full
circle of rings
Epithelium (E) pseudostratified and ciliated
Bounded by smooth muscle (M)
Supported by seromucous glands in the submucosa (G)
Airway kept open with crescent shaped cartilage Artery
Again, no outer layer of smooth muscle
Alveolus - structure
In the alveoli , the capillaries are lined with flattened specialised epithelial cells (endothelium) that are attached to a fused basal lamina with even thinner epithelial cells of the air sac on the opposite side
The folds in the basal lamina allow for expansion of the air sacs when air is drawn into the lungs
At the junction are small amounts of collagen that add rigidity to the mucosa surrounded by many layers of elastin fibres, whose role is to provide elastic recoil to return the sac to the empty state on exhaling gases
The connective tissue muscle layers in this mucosa are created by the collagen and elastin fibres
