Cells and Fibers of Loose Connective Tissue
In addition to its function of literally connecting the cells and tissues of the body
to each other, CT provides a compartment, or “highway”, in which cells are many
macromolecules can travel (in principle) throughout the body without having to
cross a basal lamina.
Macrophages, called histiocytes in tissues
Fibroblasts, produce fibers
Endothelial cells/pericytes of capillaries
Connective Tissue Proper
Relatively few cells in large volume of extracellular matrix, ECM
Loose and dense
Loose CT, in particular, houses blood vessels and nerves
The outer covering of blood vessels and nerves consists of a CT sheath
Collagen fibers are inelastic, flexible fibers. Collagen is a three-chain fibrous protein in
which the chains coil around each other (called a coiled-coil structure) like the strands of a rope. This triple-helix molecular organization generates a protein with considerable
A variety of cells produce more than 20 biochemically different types of collagen –
fibroblasts in the connective tissue and their equivalents the osteoblast (bone),
chrondroblast (cartilage), and odontoblast (teeth) synthesize collagen.
A mutation in COL1A1 and COL1A2 genes, encoding the α and α cha1ns of 2ype 1
collagen, respectively, involves cleavage sites for the N-terminal region of the molecule
and interferes with the conversion of precollagen to collagen. This leads to defective
cross-linking and a consequent reduction in the tensile strength of tendons (rich in type I
collagen). This mutation is observed in some clinical forms of Ehlers-Danlos syndrome.
Strickler syndrome is characterized by myopia, hypoplasia of the lower jaw, and arthritis
associated with dysplasia of the epiphyses. Type II collagen is abundant in cartilage and
vitreous humor (eye). The COL2A1 gene is mutated.
Osteogenesis imperfect type 1 is associated with bone fragility. COL1A1 point mutations
determine a reduction in the production of type 1 collagen required for normal
Procollagen → tropocollagen
Tropocollagen → crosslinks → collagen fibrils → fibers → bundle
An inherited defect in the procollagen peptidase – triggered removal of the nonhelical
endings of procollagen results in the formation of defective collagen fibrils.
Another form of the syndrome involves a mutation of the gene encoding the enzyme
lysyl hydroxylase, required for the post-translational modification of lysine into
hydroxylysine. Lysyl oxidase stabilizes the staggered array of tropocollagen molecules
by catalyzing the formation of aldol cross-links between hydroxylysine side chains.
Defective hydroxylation of lysine decreases the strength of the collagen molecule in
This syndrome can be divided into several clinically distinct subtypes, most of them
characterized by joint dislocation (hip and other large joints) and hyperelasticity of the
Quarter Horse stallion: Hyperelastosis cutis (HS), also known as hereditary equine
regional dermal asthenia (HERDA), is a recessive disorder. There is lack of adhesion
within the dermis (the deep layer of the skin) due to a collagen defect. In dramatic
cases, the skin splits along the back and even rolls down the sides. The average
lifespan for an HC horse is two to four years. Elastic fibers (do not contain collagen)
Fibroblasts (in skin and tendons), chondroblasts, chondrocytes (in elastic cartilage of the
auricle of the ear, epiglottis, larynx, and auditory tubes), and smooth muscle cells (in
large blood vessels like the aorta and in the respiratory tree) produce elastic fibers
Elastin contains two characteristic but uncommon amino acids: desmosine and
isodesmosine. These amino acids are responsible for cross-linking mature elastic fibers
and enable their stretching and recoil, like rubber bands.
Proelastin, the precursor of elastin, is secreted as tropoelastin.
In the extracellular space, tropoelastin interacts with fibrillin, a 35 kd glycoprotein, to
organize immature elastic fibers, which aggregate to form ma