The most abundant tissue in the body
Connective tissues are the most abundant and most widely distributed tissues in complex animals. The name states the job: they link and support the other tissues and organs of the body, holding the whole structure together rather than lining it, contracting it or wiring it.
They are not one tissue but a family that ranges across a spectrum of firmness. At the soft end sits areolar tissue; further along come the dense connective tissues; and at the far end sit the three specialised ones, cartilage, bone and blood. A cartilage plate, a thigh bone and a drop of blood therefore belong to the same tissue category, which is exactly why blood is described as a specialised connective tissue rather than a fluid of its own kind.
Fibres and matrix
With the single exception of blood, connective tissues secrete fibres of structural proteins - either collagen or elastin. These fibres give the tissue its strength, elasticity and flexibility.
The same tissues also secrete modified polysaccharides, which accumulate between the cells and the fibres and act as the matrix, or ground substance. Whether a connective tissue turns out soft, dense, pliable or rock-hard depends almost entirely on what this matrix is made of and how the fibres are packed within it.
Loose connective tissue
In loose connective tissue, cells and fibres are loosely arranged in a semi-fluid ground substance. Areolar tissue is the classic example. It lies beneath the skin and frequently serves as a support framework for epithelium, filling the space between organs and binding them loosely to one another. Its residents include fibroblasts, which manufacture the fibres and matrix, macrophages, and mast cells.
Adipose tissue is the other loose connective tissue, located mainly beneath the skin. Its cells are specialised to store fat, and excess nutrients that are not used immediately are converted into fats and deposited here.
Dense connective tissue
In dense connective tissue, the fibres and the fibroblasts are compactly packed instead of loosely scattered. The subdivision turns on whether that packing is orderly.
Dense regular connective tissue has parallel bundles of collagen fibres, with fibroblasts arranged in rows between them; this alignment is built to resist pulling in one direction. Tendons are of this type, and they attach skeletal muscles to bones. Ligaments, also dense regular tissue, attach one bone to another.
Dense irregular connective tissue contains fibroblasts and many fibres, mostly collagen, that are oriented differently rather than in parallel. This tissue is present in the skin, where stress can arrive from any direction and a criss-crossed fibre arrangement resists all of them.
Cartilage
Cartilage is a specialised connective tissue in which the intercellular material is solid and pliable, so it resists compression without shattering. The cells of cartilage, the chondrocytes, are enclosed in small cavities within the matrix that they themselves secrete; these cavities are called lacunae.
Cartilage is found at the tip of the nose and in the pinna of the outer ear, at articular surfaces between adjacent limb bones, and between adjacent vertebrae. Most of the cartilage laid down in vertebrate embryos is later replaced by bone in the adult, so the material is as much a scaffold for development as a permanent tissue.
Bone
Bone has a hard and non-pliable ground substance rich in calcium salts and collagen fibres, and it is this combination that gives bone its strength. Bone provides the structural frame of the body, supporting and protecting the softer tissues and organs around it.
Like chondrocytes, the bone cells, the osteocytes, sit in spaces called lacunae. Some bones additionally house bone marrow, which is the site of production of blood cells. The limb bones, such as the long bones of the legs, bear the weight of the body and interact with skeletal muscles to bring about movement, so a bone is not merely a passive girder.
Blood
Blood is a fluid connective tissue, and it is the one connective tissue without fibres. It contains:
- Plasma, the fluid portion
- Red blood cells
- White blood cells
- Platelets
Blood is the main circulating fluid of the body, and its role is transport - it carries substances from where they are made or absorbed to where they are needed, which is how a tissue with no solid matrix still counts as one that connects everything.
Connective-Tissue Identification Grid
| Tissue | Diagnostic arrangement | Key cells or matrix | Typical role/site |
|---|---|---|---|
| Areolar | Loose fibres in semi-fluid ground substance | Fibroblasts, macrophages and mast cells | Packing and support beneath epithelia |
| Adipose | Closely packed fat-storing cells | Adipocytes | Energy storage and insulation beneath skin |
| Dense regular | Parallel collagen bundles | Fibroblasts in rows | Tendons and ligaments; one-directional pull |
| Dense irregular | Collagen fibres running in different directions | Fibroblasts | Dermis; stress from several directions |
| Cartilage | Solid but pliable matrix | Chondrocytes in lacunae | Nose, pinna, joints and between vertebrae |
| Bone | Hard matrix rich in calcium salts and collagen | Osteocytes in lacunae | Support, protection and movement |
| Blood | Fluid matrix called plasma | Formed elements | Transport |
[NEET Important] Mammalian platelets are anucleate cell fragments derived from megakaryocytes. RBCs, WBCs and platelets are grouped as the formed elements of blood; do not call all three complete cells.