a-textbook-of-histology-fawcett-don-w-don-wayne- 8.pdf

CONNECTIVE TISSUE The connective tissues are a diverse group of These transients are concerned with the de¬ tissues that share a common origin from the fense of the tissue and are normally present mesenchyme of the embryo. Cartilage and...

CONNECTIVE TISSUE The connective tissues are a diverse group of These transients are concerned with the de¬ tissues that share a common origin from the fense of the tissue and are normally present mesenchyme of the embryo. Cartilage and in limited numbers but may increase dramati¬ bone are connective tissues with a firm extra¬ cally in mounting an inflammatory reaction cellular matrix specialized for support of the against invading bacteria. body as a whole. Adipose tissue is specialized The principal function of loose connective for storage of lipid as an energy source. These tissue is to bind together and support the pa¬ will be the subject of separate chapters. This renchyma of the organs in the body, but it chapter is concerned with the more general¬ has recently become apparent that it has other ized connective tissue that is found through¬ important roles. The polarity of epithelial out the body and provides for cohesion of cells, the stabilization of their basal surface, other structural elements and serves as the the organization of their cytoskeleton, and medium through which blood vessels are dis¬ some of their metabolic functions are depen¬ tributed to nourish the organs and to elimi¬ dent on the interaction of the epithelial cells nate the waste products of their metabolism. with matrix components of the underlying Its cells are widely scattered in an abundant connective tissue. The nature of these interac¬ extracellular matrix consisting of fibrous and tions is now a subject of intensive investi¬ amorphous components. gation. The relative abundance of cells, fibers, and ground substance in this kind of connective tissue varies greatly from region to region. THE GROUND SUBSTANCE Different terms are used to facilitate descrip¬ tion of some of these variations. Loose connec¬ The translucent material in which the cells tive tissue and dense connective tissue differ ac¬ and fibers of connective tissue are embedded cording to whether the fibers are moderately is a highly hydrated gel commonly referred abundant and loosely interwoven or very to as the ground substance. Its aqueous phase abundant and densely packed. Modifiers are is the medium through which all nutrients and added to these terms to indicate the G organiza¬ waste products must pass in transit between tion of the fibers. Where fibers are closely in¬ the blood and the parenchymal cells of the terwoven in seemingly random orientation, organs. The ground substance is poorly pre¬ 60 the tissue is described as dense irregular connec¬ served in routine histological preparations, tive tissue, and where the fibers are closely but its residues can be detected by use of cer¬ packed in parallel bundles, as in tendon, or tain dyes that undergo a change in color on in flat sheets, as in aponeuroses, the tissue is binding to it, a staining property called met- called dense regular connective tissue. - achromasia. The dye toluidine blue, for exam¬ The several cell types of· loose connective ple, takes on a purple color when bound to tissue are categorized as fixed cells or free cells. the ground substance. It can also be stained The fixed cells are a stable population of long- with the periodic-acid-Schiff reaction, owing lived, relatively immobile fibroblasts, so named to the numerous polysaccharide chains on because they produce and maintain the sur¬ some of its molecules. rounding fibers. They also secrete the amor¬ The stainable components of the ground phous ground substance of the extracellular ma¬ substance were formerly classified as acid mu¬ trix. The free cells are a heterogeneous copolysaccharides, but as more has been learned population of motile cells of limited life span about their chemical nature, this term has that emigrate from the blood and wander fallen into disuse. The major polysaccharides through the interstices among the fibers. of the ground substance are now identified as 133 134 CONNECTIVE TISSUE Proteoglycan subunits Figure 5-1. Drawing of the organization of the extracellular matrix of cartilage in which the interstices between collagen fibers are believed to be occupied by entwining long proteoglycan molecules with hundreds of polysaccharide side-chains. glycosaminoglycans, a class of macromolecules be partially avoided by fixation of frozen sec¬ that are long, linear polymers of disaccharide tions in ether-formaldehyde vapor. The pro¬ subunits. The major glycosaminoglycans of teoglycans, in such preparations, are polyani¬ connective tissue are chondroitin sulfate, keratan ons due to the sulfate and hydroxyl groups sulfate, heparan sulfate, and hyaluronic acid. Hya¬ on the disaccharide groups of the glycosami¬ luronic acid, which is abundant in loose con¬ noglycans, and they can be stained by cationic nective tissue in joint fluid and in the vitreous dyes such as Alcian-blue or by colloidal iron, humor of the eye, is a very large molecule which has a high affinity for anionic groups. made up of some 5000 disaccharides in a For electron microscopy, incorporation of the chain that would be nearly 2.5 p.m in length polycationic dye ruthenium-red in the fixative (Fig. 5—1). One of its important properties is appears to improve the preservation of the its high viscosity in aqueous solution, which glycosaminoglycans by interaction with their contributes to the gel-like consistency of the anionic groups. However, the chains collapse ground substance. This consistency is no bar¬ onto the core protein during dehydration and rier to diffusion of metabolites through its the glycosaminoglycans appehr as 10-20-nm aqueous phase, but it is believed to be a sig¬ granules in the interstices of the extracellular nificant barrier to the spread of bacteria that matrix. By high-pressure freezing, freeze- may enter the tissues. In this context, it is substitution, and low-temperature imbed¬ M interesting that the most invasive species of 1 ding, the proteoglycans of cartilage matrix bacteria are those that have acquired the abil¬ have been successfully preserved in a more ity to produce the enzyme hyaluronidase to de- extended state believed to resemble their true polymerize the hyaluronic acid of the ground form. With this method of preparation, they substance. appear as a network of very fine strands There is no entirely satisfactory method for throughout the cartilage matrix, and those in the microscopic study of the ground sub¬ loose connective tissue probably have a similar stance. Its extraction by the aqueous fixatives form. commonly used in specimen preparation can Most of the tissue fluid is held by the hydro-  CONNECTIVE TISSUE 135 philic glycosaminoglycans, but small mole¬ gesting that these are bundles of smaller fi¬ cules are able to diffuse through this bound bers. Under the polarizing microscope, even water. A lesser amount of free fluid, carrying the smallest fibers exhibit birefringence, indi¬ gases and nutrients in solution, also circulates cating that they are made up of submicro- through the ground substance. When the rate scopic subunits oriented parallel to the fiber of exit of fluid from the arterial end of the axis. capillaries exceeds the rate of its uptake at the With the electron microscope, collagen fi¬ venous ends, fluid accumulates in the extracel¬ bers are seen to consist of parallel fibrils 50- lular matrix resulting a swelling of the tissue, 90 nm in diameter. These are the subunits called edema. responsible for the form of birefringence ob¬ served with polarization microscopy. In mi¬ crographs of lead-stained thin sections, these COLLAGEN FIBERS unit fibrils are cross-striated, with denser stain¬ Fibers of collagen are present in all kinds of connective tissue. In unstained preparations, they are colorless strands 0.5—10 p,m in diame¬ G ing transverse bands repeating every 67 nm along their length (Fig. 5—3). The unit fibrils are polymers of collagen molecules, each 300 nm in length and 1.4 nm in diameter. They ter and of indefinite length. In histological are made up of three polypeptide chains, sections, they are acidophilic, staining ② pink called a-chains, each having a molecular ⑧ with eosin, blue with Mallory’s trichrome weight of about 100,000. The chains have a stain, and green with Masson’s trichrome left-handed helical configuration, and the stain. They are unbranched and, in loose con¬ three are entwined to form a right-handed nective tissue, they appear to be randomly ori¬ triple helix, in which each turn spans a dis¬ ented (Fig. 5—2). When not under tension, tance of 8 nm (Fig 5—4). The ct-chains are held they have an undulant course. In larger fibers, together within the triple helix by hydrogen a faint longitudinal striation is evident, sug¬ bonds. Figure 5-2. Photomicrograph of collagen fibers in a thin spread of rat mesentery. Note the variation in fiber diameter and the wavy course of the larger fibers. The preparation was stained by a silver method and the photograph printed as a negative to simulate more closely the appearance of the fibers in unfixed material. (From Fawcett, D.W. In Greep, R.O., ed. 1953. Histology, Blakiston Co. Reproduced by permission of McGraw-Hill Book Co.) 136 CONNECTIVE TISSUE Figure 5-3. Electron micrograph of the unit fibrils of collagen, showing their characteristic pattern of cross-striations. (Micrograph courtesy of D. Friend.) Molecular collagen can be extracted from fibrils. The penetration of the contrast me¬ developing or repairing connective tissue. dium into the gaps results in the dark bands, When such extracts are warmed, in vitro, to and the light bands are regions in which mo¬ body temperature, the collagen molecules 6 > lecular overlapping is complete and no stain spontaneously polymerize to form cross-stri¬ penetrates. ated fibrils with the 67-nm periodicity of na¬ Some banding patterns that do not occur tive collagen. In this process, the molecules in nature can be produced in vitro by varying orient parallel, overlapping one another by a the conditions under which polymerization quarter of their length and leaving a short gap takes place. If a solution of collagen and se¬ between the amino terminus of one molecule rum-glycoprotein is dialyzed against water, and the carboxy terminus of the next (Fig. fibrils are formed that have a periodicity of 5—5). The gaps between molecules and their 240 nm instead of 67 nm. This is called fibrous staggered arrangement are responsible for long-spacing collagen (FLS-collagen) (Fig. 5—6). the cross-striation seen in negatively stained Precipitation of collagen frqm acid solution -280 nm-