A summary of the various kinds of connective tissues is given in Figure 1 and Table 1.
Figure 1. General characteristics of connective tissues.
The following information identifies a few select features of connective tissue.
Nerve supply. Most connective tissues have a nerve supply (as does epithelial tissue).
Blood supply. There is a wide range of vascularity among connective tissues, although most are well vascularized (unlike epithelial tissues, which are all avascular).
Structure. Connective tissue consists of scattered cells immersed in an intercellular material called the matrix. The matrix consists of fibers and ground substance. The kinds and amounts of fiber and ground substance determine the character of the matrix, which in turn defines the kind of connective tissue.
Cell types. Fundamental cell types, characteristic of each kind of connective tissue, are responsible for producing the matrix. Immature forms of these cells (whose names end in blast) secrete the fibers and ground substance of the matrix. Cells that have matured, or differentiated (whose names often end in cyte), function mostly to maintain the matrix:
- Fibroblasts are common in both loose and dense connective tissues.
- Adipocytes, cells that contain molecules of fat, occur in loose connective tissue, as does adipose tissue.
- Reticular cells resemble fibroblasts, but have long, cellular processes (extensions). They occur in loose connective tissue.
- Chondroblasts and chondrocytes occur in cartilage.
- Osteoblasts and osteocytes occur in bone.
- Hemocytoblasts occur in the bone marrow and produce erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (formerly called thrombocytes).
- In addition to the fundamental cell types, various leukocytes migrate from the bone marrow to connective tissues and provide various body defense activities:
- Macrophages engulf foreign and dead cells.
- Mast cells secrete histamine, which stimulates immune responses.
- Plasma cells produce antibodies.
Fibers. Matrix fibers are proteins that provide support for the connective tissue. There are three types:
- Collagen fibers, made of the protein collagen, are both tough and flexible.
- Elastic fibers, made of the protein elastin, are strong and stretchable.
- Reticular fibers, made of thin collagen fibers with a glycoprotein coating, branch frequently to form a netlike (reticulate) pattern.
Ground substance. Ground substance may be fluid, gel, or solid, and, except for blood, is secreted by the cells of the connective tissue:
- Cell adhesion proteins hold the connective tissue together.
- Proteoglycans provide the firmness of the ground substance. Hyaluronic sulfate and chondroitin sulfate are two examples.
Classification. There are five general categories of mature connective tissue:
- Loose connective tissue has abundant cells among few or loosely arranged fibers and a sparse to abundant gelatinous ground substance.
- Dense connective tissue has few cells among a dense network of fibers with little ground substance.
- Cartilage has cells distributed among fibers in a firm gellike ground substance. Cartilage is tough but flexible, avascular, and without nerves.
- Bone has cells distributed among abundant fibers in a solid ground substance containing minerals, mostly calcium phosphate. Bone is organized in units, called osteons (formerly known as the Haversian system). Eachosteon consists of a central canal, which contains blood vessels and nerves, surrounded by concentric rings (lamellae) of hard matrix and collagen fibers. Branching off the central canal at right angles are perforating canals. These canals consist of blood vessels that branch off the central vessels. Between the lamellae are cavities (lacunae) that contain bone cells (osteocytes). Canals (canaliculi) radiate from the central canal and allow nutrient and waste exchange with the osteocytes.
- Blood is composed of various blood cells and cell fragments (platelets) distributed in a fluid matrix called blood plasma.
Tissue origin. All mature connective tissues originate from embryonic connective tissue. There are two kinds of embryonic connective tissues:
- Mesenchyme is the origin of all mature connective tissues.
- Mucous connective tissue is a temporary tissue formed during embryonic development.
An epithelial membrane is a combination of epithelial and connective tissues working together to perform a specific function. As such, it acts as an organ. There are four principle types of epithelial membranes:
Serous membranes line interior organs and cavities. The serous membranes that line the heart, lungs, and abdominal cavities and organs are called the pericardium, pleura, and peritoneum, respectively.
Mucous membranes line body cavities that open to the outside of the body. These include the nasal cavity and the digestive, respiratory, and urogenital tracts.
Synovial membranes line the cavities at bone joints.
- The cutaneous membrane is the skin.