CONNECTIVE TISSUE PROPER (CTP)
1. (LUMEN) Connective Tissue Proper.
Connective tissue proper (CTP) is an abundant tissue that consists of three components,
each of which is found in many different varieties depending on the location and function
of the CTP. The three components are cells, fibers and ground substance.
Cells: in CTP the only cell type that’s ‘indigenous’ to the tissue is the fibroblast. It’s the
cell that makes the components from which the entire tissue is constructed. There are
many other kinds of cells in CTP such as lymphocytes, macrophages, neutrophils,
basophils, etc, but these are all blood cells and wander into the CTP from capillaries.
Fibers: the main fiber-forming protein of CTP is collagen type I. Reticular collagen,
type III, is found in some CTPs and is made by fibroblasts. Type IV is the basal lamina
collagen but it is made by epithelial cells. There can be fibers made from fibronectin and
a few other ECM proteins as well.
Ground substance: Glycosaminoglycan polysaccharides are built up on protein cores
during O-linked glycosylation. The final product is proteoglycan and it’s the major
component of ground substance. PG domains are large, very polar, and relatively
incompressible, giving CTP much of its physical properties.
In CTP matrix (fibers and GS) the fibers are X-linked to PGs by fibronectin primarily
and PGs linked to other PGs, fibers to fibers, and to cells as well via especially integrin
IMPs linked to ECM components. Any cell that makes the right kinds of integrin IMPs
can be potentially be located in CTP.
The various kinds of CTPs (loose: areolar, reticular, adipose, etc; and dense: regular,
irregular) are different from each other primarily in the kinds of molecules making up
their matrices, the particular fibers and their X-linkers. The proportions of the 3
components also varies a lot from one type of CTP to another; eg, loose CTP has lots of
cells and ground substance but relatively few and thin fibers.
LM of loose CTP, dense irregular (DICTP), and dense regular (DRCTP)
2. (LUMEN) Connective Tissue Proper Fibroblasts
Fibroblasts are solitary cells – no cell-cell junctions. Unlike epithelia, there is a very
large intercellular space in CTP. CTP fibroblasts are intermittently active in adults so
their ultrastructure can vary from quiescent to active in protein synthesis and secretion.
CTP components are turned over constantly: old matrix components are broken down and
new ones made and released. In their active phase, fibroblasts have RER to make
collagen and other matrix proteins.
In the TEM the RER lumens are expanded, implying
active synthesis of export proteins.
3. (Diagram from Karp Cell Bio) Collagen (Cross-striated; types I, II, III).