Download Connective Tissue I - Wk 1-2

Connective Tissue I
1. Know the basic composition of Connective tissue (CT)
2. Be able to demonstrate an understanding of the importance of
Glycosaminoglycans (GAGs) in the function of the extracellular matrix
3. Identify key cells and components of CT and describe their function
4. Know the basics structure and function of the basement membrane in CT
All information can be found in the ‘Night Starvation’ lecture notes, as well as
Marieb and Hoehn “Human Anatomy and Physiology” Seventh Edition.
Connective tissue
CT is the most abundant and widely distributed of the bodies primary tissues,
however its amount varies in particular organs (ie. skin consists primarily of CT,
while brain has very little)
There are four main classes of CT; Connective Tissue Proper, Cartilage, Bone Tissue,
and Blood. The functions of CT are supportive, for strength, roles in differentiation,
communication between cells, protection and insulation to name a few.
CT arises from mesenchyme (embryonic tissue) and is largely non living Extracellular
Matrix (ECM), widely separating the living cells of the tissue. This kind of matrix
allows it to bear great weight, withstand tension and endure physical trauma.
Connective Tissue has three main components,
- Ground Substance
- Fibers
- Cells
1. Ground Substance
Ground Substance is composed of interstitial (tissue) fluid, Glycosaminoglycans
(GAGs) and Glycoproteins (GPs).
GAGs are linear polymers of repeating disaccharide units (most commonly
hexosamine and uronic acid). Common GAGs include hyaluronic acid, chondroitin,
dermatan, keratin and heparin sulphates. GAGs are covalently linked to a protein core
(becoming Proteoglycans) and intertwine to trap water, forming a substance that
varies from a fluid to a viscous gel (ie increased GAG means a more viscous gel)
GPs are structural proteins with one or more saccharide (sugar) units attached. They
are also called cell adhesion proteins and important examples are fibronectin and
laminin. GPs allow connective tissue cells to attach themselves to matrix elements.
Both fibronectin and Laminin have multiple binding sites to allow matrix components
to connect.
2. Fibres
The function of fibres in CT is to provide support – tensile strength and deformability.
There are 3 types and all are proteins, composed of long chains of amino acids joined
via peptide linkages. The 3 types include;
Collagen Fibres
- extremely tough ie found in tendons
- many different types of collagen (28), and the type of collagen depends on location
and the functional need
- composed of 3 cross linked collagen fibrils helically twisted together into collagen
fibres
- this cross linking allows collagen fibres to be extremely tough.
Elastic Fibres
- found in loose CT (skin) and blood vessels
- long thin fibres that form a branching network in the ECM
- contain a rubber-like protein, ELASTIN (composed mainly of the amino acids
desmosine and isodesmosine), which allows stretch and recoil
- found in areas where greater elasticity is needed – ie blood vessels
Reticular Fibres
- short, fine collagenous fibres (Type III)
- branch extensively and forms a fine network of supportive fibres known as
RETICULIN
- found in basement membranes, small blood vessels, in lung and liver sinusoids
3. Cells
Undifferentiated, mitotically active cells are termed ‘BLASTS’ and secrete ground
substance and fibres characteristic of their particular matrix.
Once they have synthesized the matrix components, they assume their less active
mature mode, termed ‘CYTES’. These cells maintain the health of the matrix.
However, if the matrix is injured, they can revert back to their active state to repair
and regenerate the matrix.
Types of CT cells include;
Fibrocytes (Fibroblasts)
- synthesize ECM and fibres within it
- cigar shaped (fusiform) cells
- important function in healing and disease as they can develop components (ie
myofibrils to contract wound) or transform into other cells (osteocyte, chondrocyte,
smooth muschle cells) when old structures are damaged or broken.
- hence are Very Versatile
Fat Cells (Adipocytes)
- syntesise and store triglycerides
- can be unilocular (one vacuole) or multilocular (many vacuoles)
Mobile Cells
- Migrate into CT matrix from blood stream. Include;
* White Blood Cells – Neutrophils, Eosinophils, Lymphocytes
* Mast Cells
* Macrophages
Mast Cells
- detect Foreign substance and initiate local inflammatory response against them.
- Mast cell cytoplasm secretory granules contain several chemicals that mediate
inflammation
* heparin – anti-coagulant – binds to other mast cells and regulates their activity
* histamine – makes capillaries leaky – dilates
* proteases – protein degrading enzymes
Macrophages
- phagocytic and surveillance role in tissues
- contains hydrolytic enzymes in lysozomes that are capable of digesting and
ingesting
- named differently according to region found within the body
Basement Membrane
The Basement Membrane (BM) is a specialized structure located next to the basal
domain of epithelial cells and the underlying connective tissue.
The Basal Lamina is the structural attachment site for the overlying epithelial cells
and underlying CT. Electron Microscopy shows that there are 3 zones that form the
basal lamina;
Lamina rara – Composed of laminins
Lamina densa – Composed of Type IV collagen coated with perlacan
Lamina reticualris – Composed of reticulin (and fibronectin)
- this provides the anchoring properties
The interwoven network of proteins provides the basis for a variety of basal lamina
functions including* Structural attachment – epithelium to CT
* Compartmentalization – epithelium isolated and separate from CT
* Filtration – movement of substances to and from CT regulated by basal lamina
* Tissue Scaffolding – guide or scaffold during regeneration – newly formed or
growing cells use basal lamina to maintain original architecture
* Signaling – molecules residing in Basal Lamina can interact with cell surface
receptors, influencing various functions such as epithelial cell behavior, proliferation,
differentiation, motility etc