1 Epithelia and connective tissues Flashcards
Describe epithelia
They are the covering and lining membranes of the body (free surface)
- many types
- separated from the surrounding tissue by the basement membrane
- it is specialised according to function (surface, cell junctions etc.)
Where are epithelium found (different types in different locations)
Epithelium - cells that cover/line surfaces
Endothelium - lining blood vessels
Medothelium - lining body cavities
Name the different types of epithelium
Simple epithelium (1 layer):
- Squamous - flat
- Cuboidal - square/cube-like
- Columnar - taller than wide
Stratified (many layers)
- Can take one of the above types too
> e.g. squamous stratified
- Pseudostratified - cells base on ones on the Basement Membrane, look like different later, but one layer
- Transitional - many layers, can be relaxed or stretched (e.g. urothelium in the bladder)
Describe how and why epithelial are named by the shape
Usually, it is named by looking at the top (apical) layer of cells
But also, helpful to consider their function:
- For diffusion - thin layer needed in blood vessels
- Stretch a lot - transitional
Describe the basement membrane
- and what it is made of
Basement membrane
- Sheets of a matrix at the interface of functional tissue (epithelium) and support tissue (connective tissue)
Composed mainly of:
- Type IV collagen
- Glycoproteins (laminin secreted by epithelial cells. Fibronectin secreted from fibroblasts)
- Glycosaminoglycans (GAG)
Describe the functions of the Basement Membrane
- Adhesion (in the form of hemidesmosome junctions)
- Barrier (to separate environments - but the permeability is important too)
- Organisation of cells (controlling growth and differentiation)
Describe how the Basement membrane can be involved in some pathologies
Disorders:
- can lead to disease like renal disease (when the BM is scarred)
- or cancer, where the melanocytes can invade through the BM
Skin pathology
- another disease is epidermolysis
- separation of epidermis and dermis (top layer peels away) - blisters
Describe secretion and glands (and how this is achieved
When the extra secretory epithelium is needed, the epithelium folds inwards (invaginates) and makes glands
- generally, the secretory portion is columnar
- Whereas the portion of the duct into which it secretes is cuboidal epithelium
Describe different shapes of glands
- Tubular (tube-like)
- Acinar (round, bulbous)
- Mixed
They can also be
- simple
- compound (branched)
Describe exocrine glands
Exocrine glands
- secrete their products onto epithelial surface directly, or via a duct for local action e.g. sweat glands, liver
Describe endocrine glands
Endocrine glands
- secrete their products directly into the bloodstream or act on a different tissue
e. g. pituitary and thyroid glands
Describe connective tissue
It is the support tissue of the body
- originates from the embryonic mesoderm
- it is characterised by an abundance of matrix with few cells
- Contain cells (5%) and main products of cells (extracellular matrix - ECM 95%)
Describe what the ECM is composed of:
- Fibres (collagen and elastin)
- Ground substance (glycoproteins and glycosaminoglycans)
- Cells: the fibroblast secretes ECM for most tissues
Describe the fibres found in connective tissues
Collagen:
- a structural protein
- provides tensile strength and structural support
- 16 types, but mainly Type I, II, III (reticulin) and IV
Elastin:
- Provides elasticity
Describe the synthesis of collagen
- Procollagen polypeptide chains are synthesised on the ribosomes of the rough ER and secreted in the lumen
- where they are modified by the hydroxylation of certain proline and lysine residues and glycosylation before chain association and triple helix formation
- The procollagen molecules are secreted into the extracellular space where the N and C propeptides are cleaved by specific proteases
- The collagen molecules then assemble by the formation of covalent crosslinks
Describe the structure of collagen
- Collagen provides tensile strength, through different layers of fibres
Reduction of this tensile strength from collagen disorders can cause:
- Tissue laxity, hyper joint mobility, proneness to injury
- e.g. Ehlers Danlos syndrome (Type I collagen disorder)
Describe reticulin (type III collagen)
It is a smaller, branched version of collagen
- it surrounds muscles or blood vessels
Describe elastin fibres
Elastin fibres
- Needed to provide stretching and elastic recoil
- Tropoelastin polymerises to elastin
- It requires fibrillin for assembly
- Marans syndrome - genes coding for Fibrillin defective - rupture of Aorta
Elastin core, with microfibrils of fibrillin in and around it
Describe structural glycoproteins
- Fibrillin - microfibrils - 8-12nm - links to elastin
- Fibronectin - deposition and orientation of collagen and its links to cells via integrin
- Laminin - a major component of basement membrane
Describe glycosaminoglycans (GAGs)
GAGs are polysaccharide chains that attract water
4 groups of GAGs:
- Chrondroitin sulfate
- Heparan sulfate
- Hyaluronan
- Keratan Sulfate
e. g.
- GAGs form the centre of the IV disc, forming a compression-resistant core
Describe the organisation and function of connective tissue
Connective tissue forms the structural framework of many body tissues
- Loose: packing material
- Dense: tough physical support: dermis, organ capsule, ligaments, tendons
- Areolar: fatty
- Specialised support: cartilage and bone
- Metabolic: adipose tissue
- Immune: contain immune cells (mast cells, tissue macrophages, WBCs, plasma cells and effect repair
Mechanical and structural role and carry blood and lymph vessels
Mediate exchange of nutrients, metabolites and waste from blood and tissues
Describe the histology of connective tissue
Dermis - appears as pale (ECM is washed out during staining - empty space)
Collagen - pink dashed
Epithelial - purple, not much cytoplasm - nuclei
Stratum corneum - Dark purple, layers of keratinised dead skin cells
