Where the Matrix Is the Tissue
Connective tissue inverts the epithelial arrangement. Where epithelium is cells with almost no matrix, connective tissue is mostly matrix with cells scattered through it, and the matrix is what determines the tissue's properties.
The matrix has two components. Fibres: collagen for tensile strength, elastic fibres for recoil, reticular fibres for fine networks. And ground substance, which ranges from liquid to rubbery to solid.
That range produces a strikingly diverse family from one design:
| Tissue | Ground substance | Property |
|---|---|---|
| Loose connective | Viscous gel | Packing, flexible |
| Dense regular | Packed parallel collagen | Tensile strength in one direction |
| Adipose | Cells dominate | Energy store, insulation |
| Cartilage | Firm rubbery gel | Resilient, avascular |
| Bone | Mineralised with calcium phosphate | Rigid, vascularised |
| Blood | Liquid plasma | Transport |
Blood is connective tissue with a liquid matrix, surprising until you notice it fits the definition exactly. Plasma is about 55 % of volume, and formed elements about 45 %, the haematocrit. Red cells carry oxygen and, in mammals, lose their nucleus to make room for haemoglobin. White cells defend. Platelets are cell fragments that clot.
Cartilage is avascular, so its chondrocytes are fed by diffusion through the matrix. This is precisely why cartilage heals so poorly, no blood supply means no rapid delivery of cells or materials, which is why knee cartilage injuries are so persistent.
Bone solves that problem. Its mineralised matrix is rigid, but it is penetrated by blood vessels running in Haversian canals, with osteocytes sitting in small cavities connected by fine channels. Bone therefore heals well and, more remarkably, remodels continuously: osteoblasts build, osteoclasts break down, and the balance responds to mechanical load. Bone is a tissue that redesigns itself according to how it is used, which is why weight-bearing exercise strengthens it and why astronauts lose bone mass.
Common pitfall: treating cartilage as simply "soft bone". They are separate tissues with different matrices, and the functional difference that matters most is vascularity: bone is supplied and heals; cartilage is not and does not.