Section 3: Connective Tissue

Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger, Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed

Plates

The connective tissues include a variety of cells, non-living cell products, and blood. A classification and a concise discussion of the various connective tissues follows.

1. Adult Connective Tissue
   1. Connective tissue proper
      1. General
         1. Loose (areolar)
         2. Dense
      2. Special
         1. Elastic
         2. Reticular
         3. Adipose
         4. Pigmented
   2. Cartilage
      1. Hyaline
      2. Fibrous
      3. Elastic
   3. Bone
   4. Blood and lymph
2. Embryonic Connective Tissue
   1. Mesenchyme
   2. Mucoid tissue

Connective Tissue Proper

The loose or areolar connective tissue is made up of many cell types and intercellular materials (matrix), which also comprise other connective tissues but in varying proportions. It is widely distributed in the body and is found most readily beneath the skin and superficial fascia (fatty connective tissue), separating muscles, in all potential spaces, and beneath the epithelial lining in the lamina propria of the digestive system. This web-like tissue binds cells and organs together but permits these cells and organs to move, as necessary, in relation to each other. Because it is composed of a large amount of amorphous ground substance (whose consistency varies from liquid to gel), it allows wandering cells to move around freely and other structures, such as blood vessels and nerve, to pass through it. This connective tissue is important, because of its cellular content, for defense against infection and the repair of damaged tissues.

Important cells found in the loose connective tissue include the following: Fibroblasts, which synthesize collagenous connective tissue fibers that are flexible but of great tensile strength; macrophages (or histiocytes) and monocytes, which ingest, digest, or "store" microscopic particles such as debris of dead cells; certain microorganisms; and other non-biodegradable matter. Capable of ameboid movement, these cells wander throughout the connective tissue and congregate in regions requiring their specialized function. Mast cells synthesize and release substances of physiological importance (e.g., heparin and histamine).

Heparin is a powerful anticoagulant of blood, whereas histamine increases the permeability of blood capillaries. Circulating eosinophils increase in number in parasitic infections and in hypersensitivity reactions, such as in hay fever and asthma. Factors within the specific granules of eosinophils are thought to function as anti-larval agents in helminthic infections; additional factors can be directed against histamine and other inflammatory agents. Lymphocytes and plasma cells also populate loose connective tissue and play a vital role in the defense mechanism by producing antibodies, the immunoglobulins of the blood. Eosinophils, lymphocytes, and plasma cells are particularly abundant in the lamina propria of the digestive system (Plates 29 and 137) and other potentially vulnerable areas of the body.

In the digestive system and elsewhere, the individual is separated from pathogenic organisms of the external environment by only a delicate single cell layer. This cell layer is essential for absorption, excretion, and gaseous exchange; it is therefore vital to maintain defensive cells just below the vulnerable surface. Fat cells may occur singly or in small or large numbers. When fat cells predominate, the tissue is called adipose tissue. One of the special connective tissues, adipose tissue, serves as a reservoir of energy and as a soft packing in potential spaces (e.g., axilla and ischiorectal fossa) and for organs. It also envelopes glands that undergo cyclic or functional variation in size and activity (e.g., mammary glands), surrounds mobile organs (e.g., eye and heart), and protects other organs (e.g., kidney), blood vessels, and nerve fiber bundles.

The more important intercellular components of the loose connective tissue include three kinds of fibers (collagenous, previously mentioned, and elastic and reticular) and amorphous ground substance. Collagenous and reticular fibers belong to the same class of protein, collagen, whereas elastic fibers are formed of elastin. There are many kinds of collagen, and it is the most abundant protein of the human body (about 30 percent of the dry weight). Ground substance is composed primarily of two classes of compounds: glycosaminoglycans and structural glycoproteins. The term glycosaminoglycan is replacing the older, but widely used, term mucopolysaccharide to denote a linear polysaccharide with characteristic repeating disaccharide units. The repeating units are usually a uronic acid and a hexosamine. The uronic acid may be glucuronic or iduronic acid, and the hexosamine may be glucosamine or galactosamine. The structural glycoproteins play an important role in cell interaction and in migration and adhesion of cells. Fibronectin, laminin, and chondronectin are three structural glycoproteins; these cannot, however, be distinguished by routine histological techniques. It might be useful to remember, as you review your microscope slides and the photomicrographs in this book, that collagenous fibers are acidophilic and therefore stain red with eosin, stain blue with Mallory's trichrome, and stain green with Masson's trichrome. Elastic fibers may or may not stain well with eosin (Plate 34) or Masson's stain, and they may or may not stain (red or yellow) with Mallory's stain; however, old elastic fibers will stain better than younger elastic fibers. Elastic fibers do stain well with toluidine blue (often used to stain 1 µm plastic-embedded tissues) (Plate 28), aldehyde fuchsin (Plate 154), orcein (Plate 133), phosphotungstic acid hernatoxylin (Plate 156), and Weigert's elastic tissue stain (Plates 152 and 153). Reticular fibers are argyrophilic and therefore stain with silver stains such as Wilder's method (Plate 37). Ground substance or basement membrane is stained by aldehyde fuchsin (Plate 20), periodic acid-Schiff stain (Plate 223), and toluidine blue (Plate 159).

Dense connective tissue contains fewer cells, but, when they are present, they are similar in type to those found in loose connective tissue. Collagenous (Type 1) fibers predominate in this type of connective tissue. Dense connective tissue appears in two forms: dense irregular and dense regular connective tissue. The irregular type is found in the dermis of the skin, deep fascia surrounding and defining muscles, capsules of organs, and nerve sheaths. Dense regular connective tissue is found mainly in ligaments and tendons (Plates 31 and 32), which provide flexible but inelastic unions between bones and between bones and skeletal muscle. At low magnification, a tendon may be confused with striated muscle, because the fibers are axially arranged and the alignment of fibroblast nuclei resembles that found in striated muscle. At higher magnifications, however, the structural differences are easily recognized and a proper identification is readily accomplished. Other examples of dense regular connective tissue include most ligaments (Plate 33), aponeuroses, and the cornea of the eye ( Plate 301).

Connective tissues with special characteristics of structure and function include elastic, reticular, and pigmented types. Adipose tissue, which also belongs to this group, has already been mentioned and will be indelibly remembered by students of gross anatomy as the layer of tissue immediately below the skin. The fatty tissue is also known as superficial fascia or as the panniculus adiposus. Adipose tissue is of two types: white, "signet ring," unilocular fat (Plate 40), which is ubiquitous; and brown, multilocular, fat (Plate 39), which is uncommon in the adult human.

Elastic fibers in dense parallel bands (elastic tissue) can be found associated with the vertebral column (ligamenturn flava), the suspensory ligament of the penis, and vocal cords. The fascia of the lower anterior abdominal wall (Scarpa's fascia) is predominantly elastic tissue. Elastic fibers are obviously functionally important components of skin (Plates 27, 35, and 133) and hollow organs, including elastic arteries of the vascular system (Plates 152, 153, 154 and 156), trachea and bronchi of the respiratory system, and others. Reticular fibers are composed of collagen (Type 3) and are very fine and highly branched. They form supporting networks around blood vessels and cells in some organs. They are in continuity with other collagenous fibers; they are also inelastic. These fibers are only revealed by special histological techniques (Plate 37). Reticular fibers are found in abundance in lymph nodes, blood-forming organs, spleen, liver, and elsewhere, but, as mentioned previously, they cannot be seen unless special methods using silver salts are used to reveal their presence. This type of connective tissue is associated with phagocytic reticular cells, for example, the reticular cells of lymph nodes (Plate 36) and Kupffer's cells of the liver (Plate 219). Pigment tissue is a cellular connective tissue rather than a fibrous non-living connective tissue and has many melanin-containing connective tissue cells. It is found principally in the choroid and iris of the eye (Plates 303, 304 and 305).

Cartilage

Cartilage is a non-vascular tissue containing fibrous connective tissue (collagen Type 2) embedded in an abundant and firm matrix. The cells that produce cartilage are called chondroblasts, and, in mature cartilage where the cells are housed in lacunae, they are termed chondrocytes. In early development, the greater part of the skeleton is cartilaginous, but, during later stages of development, the cartilage is remodeled and replaced by bone. The process is called endochondral ossification. Three types of cartilage are recognized: hyaline, elastic, and fibrocartilage. Hyaline cartilage is found at the ventral ends of ribs and in the nose, larynx, trachea, and articular surfaces of adjacent bones of movable joints. The matrix or ground substance of cartilage is strongly basophilic and stains metachromatically with toluidine blue and other similar basic dyes (Plate 41). It is the acidic sulfate groups of the proteoglycans comprising the ground substance that account for the staining reaction just noted. Metachromasia means that a cell or tissue takes on a color different from the dye solution with which it is stained.

Fibrocartilage has a limited distribution. It is found in the intervertebral discs, pubic symphysis, menisci and ligaments of the knee and other joints, and in the tendons of some muscles where they glide over bones (e.g., tendons of peroneus longus and tibialis posterior; Plate 47). Fibrocartilage is composed predominantly of collagenous (Type 1) fibers arranged in bundles, with cartilage cells surrounded by a sparse cartilage matrix between the fibrous bundles. Fibrocartilage has characteristics similar to both dense connective tissue and hyaline cartilage. It is always associated with dense connective tissue, and, because of its usual paucity of cartilage cells, there appears to be a gradual transition between the two types of connective tissue. Although cartilage cells are not abundant, they are arranged in scattered clusters in parallel arrays, reflecting the direction of stresses placed upon the tissue. Fibrocartilage has no identifiable perichondrium and differs in this regard from hyaline and elastic cartilage. Elastic cartilage is found in the external ear (pinna), auditory tube, epiglottis, and corniculate and cuneiform cartilages of the larynx (Plate 42). It is yellow and is more flexible and elastic than the other cartilage types owing to abundant branching elastic fibers in its matrix. Elastic fibers are often concentrated in the walls of lacunae, which house cartilage cells.

Bone

Bone is a tissue that forms the greatest part of the skeleton and is one of the hardest structures of the body. It is the rack upon which all the soft parts are suspended or attached. Only the dentin and enamel of teeth are harder. The skeleton is tough and slightly elastic, withstanding tension and compression. Bone differs from cartilage by having its collagenous connective tissue matrix impregnated with organic salts (primarily calcium phosphate and lesser amounts of calcium carbonate, calcium fluoride, magnesium phosphate, and sodium chloride). The osteoblasts, which form the osseous tissue (Plate 46), become encapsulated in lacunae but maintain contact with the vascular system via microscopic canaliculi (Plate 48). When they become encapsulated, they are referred to as osteocytes.

A characteristic feature of a cross section of the shaft (diaphysis) of a long bone is its organization in concentric rings around a central canal containing a blood vessel (Plate 49). This is called a Haversian system (osteon). Between neighboring Haversian systems are non-concentric lamellae, devoid of Haversian canals, termed interstitial lamellae. Vascular canals, called Volkmann's canals (Plate 49), traverse the long axis of the bone; they are always at right angles to Haversian canals. Their function is to link vascular canals of adjacent Haversian systems with each other and with the periosteal and endosteal blood vessels of the bone. The outer perimeter of a long bone, beneath the osteogenic connective tissue (called periosteum), is composed of circumferential lamellae, which also lack Haversian canals. This thick-walled hollow shaft of compact bone (the diaphysis) contains bone marrow. At the distal ends of long bones, where Haversian systems are not found, the bone appears spongy and is therefore called cancellous, or spongy, bone. The spongy appearance is misleading, because careful examination of the architecture reveals a highly organized trabecular system providing maximal structural support with minimal density of bony tissue. The epiphyses (Plate 47) at the ends of the diaphysis or shaft contain the spongy bone covered by a thin layer of compact bone. The cavities of the epiphyseal spongy bone are in contact with the bone marrow core of the diaphysis except during growth of long bones in young animals. Interposed between the epiphysis and the diaphysis is the cartilaginous epiphyseal plate. The epiphyseal plate is joined to the diaphysis by columns of cancellous bone; this region is known as the metaphysis.

When bone is formed in and replaces a cartilaginous "model," the process is termed endochondral ossification. Some parts of the skull develop from osteogenic mesenchymal connective tissue, however, without a cartilaginous "model" having been formed first. This is termed intramembranous ossification, and these bones are called membrane bones. In both instances, three types of cells are associated with bone formation, growth, and maintenance: osteoblasts (Plate 46), osteocytes (Plate 48), and osteoclasts (Plate 46). The osteoblasts produce osseous tissue (bone), become embedded in the matrix they manufacture, and are then renamed osteocytes, to reflect their change of status. They remain viable, because they have access to the vascular supply via microscopic canaliculi through which cellular processes extend to receive nutrients and oxygen. Osteoclasts actively resorb and remodel bone as required for growth; these are giant, multinuclear, phagocytic, and osteolytic cells.

Blood and Lymph

This type of connective tissue is peculiar because its matrix is liquid. The blood is carried in blood vessels and is moved throughout the body by the contractile power of the heart. Blood vessels and heart are discussed in Section 8 of this book. Lymph is found in lymph vessels but originates in extracellular spaces as extracellular fluid, which is normally extravasated from blood capillaries. The extracellular fluid, which enters the lymphatic system of vessels, will have mononuclear white blood cells added to it as the fluid is filtered through lymph nodes, which produce such cells. Lymph is returned to the blood stream near the right and left venous angles (junction of the internal jugular and subclavian veins). The lymphatic system is discussed more fully in Section 9.

Mesenchyme

Derived from embryonic mesoderm, mesenchyme (Plate 50) is the first connective tissue formed. The cells are widely spaced, with an abundance of intercellular matrix. The primitive mesenchymal cells differentiate into all the supporting tissues of the body. The cells derived from the mesenchyme include blood cells, megakaryocytes, endothelium, mesothelium, reticular cells, fibroblasts, mast cells, plasma cells, special phagocytic cells of the spleen and liver, cartilage cells, and bone cells as well as smooth muscle.

Mucoid Tissue

Widely distributed in the embryo as a loose connective tissue, mucoid tissue is composed of large stellate fibroblasts in an abundant intercellular substance, which is homogeneous and soft. In the umbilical cord (Plates 51 and 261), it is known as Wharton's jelly.

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