Lecture 3.1: Connective Tissues Flashcards
What are connective tissues?
They are tissues that provide structure, support, strength and space filling
Connective Tissue Components (4)
1) Cells
2) Extracellular Matrix (ECM)
3) Ground Substance
4) Fibrillar proteins: Collagen Fibres, Elastin, Fibronectin, Fibrillin
Ground Substance Properties
Highly Hydrated
Inflexible
Highly polar, hence attract water (90% of ECM is water)
Resistant to compression (useful in cartilage)
Ground Substance Structure
Comprised of highly-glycosylated proteins called Proteoglycans
Attached to a high molecular weight hyaluronic acid (HA) backbone.
HA and the sugar chains of proteoglycans are long, unbranched polysaccharides called glycosaminoglycans (GAGs).
Together, these form hyaluronate proteoglycan aggregates.
Where does connective tissue come from?
Cells of the middle embryonic germ layer (mesoderm), proliferate and
migrate to form mesenchyme in the early embryo
Mesenchyme gives rise to the various connective tissues, as well as serous
membranes, the vascular and urogenital systems and muscle
Mesenchyme persists in adults as stromal stem cells.
Mesenchymal Tissue in Developing Foetus’
Mesenchymal cells are morphologically similar BUT will give rise to cells that differentiate into a variety of different cell types
Mesenchymal cells persist in the adult and facilitate healing and renewal
Connective Tissue Classification
1) Embryonic connective tissue
2) Connective tissue proper
3) Specialised connective tissue
Embryonic Connective Tissue (2)
Mesenchyme (mesoderm of early embryo)
Mucous connective tissue (foetal umbilical cord)
Connective Tissue Proper (2)
Areolar (loose) Connective Tissue
Dense connective tissue (‘irregular’ or ‘regular’)
Types of Specialised Connective Tissue (4)
Adipose Tissue
Blood (haemopoietic) and Lymphatic Tissue
Cartilage (type II collagen)
Bone (type I collagen)
Collagen
Collagen is the most abundant human protein, making up ~30% of our whole-body protein content (90% of this is type I)
Collagens forms fibrils and fibres, sheets (e.g. Type IV) or anchors.
Collagen Synthesis
RER: Synthesis of pre-procollagen α-chains, Vitamin C-dependent hydroxylation of prolyl and lysyl residues – stabilises and strengthens collagen cross-links, Assembly of triple helix to form procollagen
Golgi: Packing into secretory vesicles
Cell membrane: Constitutive exocytosis
Extracellular environment: Non-helical terminal peptides cleaved, Collagen molecules assemble into fibrils
Type I Collagen Assembly
Each fibril is composed of staggered collagen molecules with a periodic banding
Fibrils assemble into fibres (visible under light microscopy), and fibres can assemble into larger bundles
Types of Collagen
Type I
Type II
Type III
Type IV
Type VII
Type I Collagen
90% of all collagen
Fibrils aggregate into fibres and fibre bundles
Found in tendons, capsules of organs and dermis
Organic component of bone
Type II Collagen
Fibrils do not form fibres
Instead forming a very fine mesh
Present in hyaline and elastic cartilage
Type III Collagen
Fibrils form branching fibres (not bundles)
Found around muscle and nerve cells, within lymphatic tissues and organs, where they are called reticulin.
Also found in skin (particularly papillary dermis)
Type IV Collagen
Unique (sheet) form present in basal lamina of basement membrane
Type VII Collagen
Anchors basal lamina to underlying reticular lamina
Disorders of Collagen Synthesis: Ehlers-Danlos
Can result in failures in various stages of collagen biosynthesis
In type IV ED (vascular type)
A failure in type III collagen production can lead to aortic rupture
Disorders of Collagen Synthesis: Scurvy
A lack of vitamin C reduces prolyl hydroxylase activity
This leads to gum ulceration and haemorrhage
Disorders of Collagen Synthesis: Osteogenesis imperfecta
Aka brittle bone disease
In type I OI, loss of function of COL1A1 (collagen 1A gene) can result in
spontaneous bone fractures
What are elastic fibres? Where are they found? What is their purpose?
They are composites of elastin deposited on a fibrillin microfibril scaffold
Varying amounts of elastic fibres are found in most connective tissues
The random coils and cross-links allow tissues to distend and return to their original shape
Disorders of Elastic Fibres: Marfan Syndrome
An autosomal dominant disorder resulting from a mutation in fibrillin-1
Symptoms:
- Abnormally tall
- Exhibit arachnodactyly
- Frequent joint dislocation
- Can be at risk of catastrophic aortic rupture
Disorders of Elastic Fibres: Williams Syndrome
Results from the spontaneous deletion of a region of chromosome 7 (which includes ELN-elastin)
Is associated with learning and cardiovascular problems
Elastic Fibres and Ageing: Intrinsic vs Extrinsic
Fine wrinkles of intrinsic ageing result from the gradual fragmentation of the elastic fibre network
Coarse wrinkles of extrinsic (photoageing) results from elastic fibre hyperplasia and accumulation of amorphous elastic material (solar elastosis)
Tunica Intima
Thin layer of connective tissue
Superficial
Tunica Media
Thick region
Many elastic lamellae between layers of smooth muscle
Tunica Media of the Aorta
Smooth muscle cells produce the elastin, collagen and matrix (rather than fibroblasts)
Elastic lamellae are stained deep purple with trichrome stains
Collagen and extracellular matrix is stained turquoise
Smooth muscle is stained red
Areolar (loose) Connective Tissue: Structure
- Branching elastic fibres
- Collagen fibres (non-branching)
- Reticular fibres
- Ground Substance
- Small blood vessels
- Fibroblasts
- Mast cells (immune system) + Macrophages
- Mesenchymal cells
- Adipocyte
Dermis of the Skin
Contains layers of loose (papillary dermis) and dense (reticular dermis) irregular connective tissue
In reticular dermis the bundles of collagen are densely packed but orientated in multiple planes
The skin can thus resist forces in multiple directions to prevent shearing
Connective Tissues of Glands
Connective tissue encapsulates glands and divides them into lobules
A capsule of connective tissue (which can be loose or dense) surrounds the gland
Trabeculae of connective tissue divide the gland into lobules
Reticular Fibres in a Lymph Node
Consist of type III collagen
Lymphocytes are densely packed in the spaces between the fibres
Reticular fibres form an irregular anastomosing (connective, branching) network throughout the node
Connective Tissue of Tendon
Dense-regular connective tissue
Type I collagen bundles in tendons lie in a parallel, densely-packed formation in line with the tensile force exerted by the muscle on bone
Rows of elongated flattened fibroblasts lie between the collagen bundles
Myotendinous Junctions
Mechanical force is transmitted from muscle to tendons at myotendinous junctions
Tendinous collagen fibres associate directly with complex infoldings of the sarcoalemma at myotendinous junctions
Epimysium (the connective tissue surrounding muscle fibre bundles [fascicles]), is also continuous with the collagen of tendons
Short Ligament (bone to bone)
Collagen bundles are densely packed in parallel arrangement
But they undulate and are arranged in fascicles, separated by loose connective tissue
White Adipose Tissue
Most adipose tissue in the body is white fat
In typical H&E-stains, white adipose cells look empty and stretched as the
chemicals used in preparation dissolve away the lipid
Fat is better preserved in cryosections
Adipose tissue is a fuel reserve (triglyceride), but also has a role in thermal insulation and in shock absorption
Brown Adipose Tissue
Brown fat cells (multilocular adipose cells) each contain many lipid droplets and a central nucleus
Found close to the scapula, sternum and axillae, especially in newborns, Non-shivering thermogenesis’ is important for babies
Also appear to be present in the upper chest and neck of adults
Brown colour is due to the rich vascular supply and abundant mitochondria, thus high respiratory capacity for heat production
Hyaline Cartilage: Structure
Perichondrium (dense irregular connective tissue composed mainly of type I collagen containing chondroblasts)- at top
Matrix (type II collagen fibres embedded in ground substance)
Chondrocytes in lacunae (unfilled space)
Hyaline Cartilage: Function
Support and reinforcement
Cushioning
Resistant to compression
Hyaline Cartilage: Locations
Comprises most of the embryonic
skeleton
Articular surfaces of joints
Covers ends of long bones
Costal cartilages of ribs
Cartilage of nose, trachea and larynx
Decalcified Bone: Structure
Haversian canals (surround blood vessels and nerve fibres throughout the bone)
Osteocytes in lacunae
Lamellae
Hard, calcified matrix with many type I collagen fibres
Well vascularised
Decalcified Bone: Function
Support and protection
Levers for muscle to act on
Site of haematopoiesis
Storage of calcium and other minerals.
