Connective Tissues PDF

 Cells of Connective Tissue 97 Fibroblasts are targets of many families of proteins called FIGURE 5–1 Em...

 Cells of Connective Tissue 97 Fibroblasts are targets of many families of proteins called FIGURE 5–1 Embryonic mesenchyme. growth factors that influence cell growth and differentiation. C H A P T E R In adults, connective tissue fibroblasts rarely undergo division. However, stimulated by locally released growth factors, cell cycling and mitotic activity resume when the tissue requires additional fibroblasts, for example, to repair a damaged organ. Fibroblasts involved in wound healing, sometimes called myofibroblasts, have a well-developed contractile function 5 and are enriched with a form of actin also found in smooth Connective Tissue Cells of Connective Tissue muscle cells. › › MEDICAL APPLICATION The regenerative capacity of connective tissue is clearly observed in organs damaged by ischemia, inflammation, or traumatic injury. Spaces left after such injuries, espe- cially in tissues whose cells divide poorly or not at all (eg, cardiac muscle), are filled by connective tissue, forming dense irregular scar tissue. The healing of surgical incisions and other wounds depends on the reparative capacity of connective tissue, particularly on activity and growth of Mesenchyme consists of a population of undifferentiated fibroblasts. cells, generally elongated but with many shapes, having large In some rapidly closing wounds, a cell called the myo- euchromatic nuclei and prominent nucleoli that indicate high fibroblast, with features of both fibroblasts and smooth levels of synthetic activity. These cells are called mesenchymal muscle cells, is also observed. These cells have most of cells. Mesenchymal cells are surrounded by an ECM that they the morphologic characteristics of fibroblasts but contain produced and that consists largely of a simple ground sub- stance rich in hyaluronan (hyaluronic acid), but with very little increased amounts of actin microfilaments and myosin collagen. (X200; Mallory trichrome) and behave much like smooth muscle cells. Their activity is important for the phase of tissue repair called wound contraction. Fibroblasts Adipocytes Fibroblasts (Figure 5–3), the most common cells in connec- Adipocytes (L. adeps, fat + Gr. kytos, cell), or fat cells, are tive tissue proper, produce and maintain most of the tissue’s found in the connective tissue of many organs. These large, extracellular components. Fibroblasts synthesize and secrete mesenchymally derived cells are specialized for cytoplasmic collagen (the most abundant protein of the body) and elas- storage of lipid as neutral fats, or less commonly for the pro- tin, which both form large fibers, as well as the GAGs, pro- duction of heat. Tissue with a large population of adipocytes, teoglycans, and multiadhesive glycoproteins that comprise called adipose connective tissue, serves to cushion and insu- the ground substance. As described later, most of the secreted late the skin and other organs. Adipocytes have major meta- ECM components undergo further modification outside the bolic significance with considerable medical importance and cell before assembling as a matrix. are described and discussed separately in Chapter 6. Distinct levels of fibroblast activity can be observed histologically (Figure 5–3b). Cells with intense synthetic activity are morphologically different from the quiescent Macrophages & the Mononuclear fibroblasts that are scattered within the matrix they have Phagocyte System already synthesized. Some histologists reserve the term Macrophages have highly developed phagocytic ability and “fibroblast” to denote the active cell and “fibrocyte” to specialize in turnover of protein fibers and removal of apop- denote the quiescent cell. The active fibroblast has more totic cells, tissue debris, or other particulate material, being abundant and irregularly branched cytoplasm, contain- especially abundant at sites of inflammation. Size and shape ing much rough endoplasmic reticulum (RER) and a well- vary considerably, corresponding to their state of functional developed Golgi apparatus, with a large, ovoid, euchromatic activity. A typical macrophage measures between 10 and 30 μm nucleus and a prominent nucleolus. The quiescent cell is in diameter and has an eccentrically located, oval or kidney- smaller than the active fibroblast, is usually spindle-shaped shaped nucleus. Macrophages are present in the connective with fewer processes, much less RER, and a darker, more tissue of most organs and are sometimes referred to by pathol- heterochromatic nucleus. ogists as “histiocytes.” 98 CHAPTER 5 Connective Tissue FIGURE 5–2 Cellular and extracellular components of connective tissue. Blood vessel Ground substance Extracellular matrix Protein fibers Elastic fiber Collagen fiber Reticular fiber Resident cells Mesenchymal cell Macrophage Adipocyte Fibroblast Connective tissue is composed of fibroblasts and other cells and watery ground substance. In all types of connective tissue, the an ECM of various protein fibers, all of which are surrounded by extracellular volume exceeds that of the cells. TABLE 5–1 Functions of cells in connective › › MEDICAL APPLICATION tissue proper. Besides their function in turnover of ECM fibers, macro- Cell Type Major Product or Activity phages are key components of an organism’s innate immune defense system, removing cell debris, neoplastic cells, bac- Fibroblasts (fibrocytes) Extracellular fibers and ground teria, and other invaders. Macrophages are also important substance antigen-presenting cells required for the activation and Plasma cells Antibodies specification of lymphocytes. Lymphocytes (several types) Various immune/defense When macrophages are stimulated (by injection of functions foreign substances or by infection), they change their Eosinophilic leukocytes Modulate allergic/vasoactive morphologic characteristics and properties, becoming acti- reactions and defense against vated macrophages. In addition to showing an increase in parasites their capacity for phagocytosis and intracellular digestion, Neutrophilic leukocytes Phagocytosis of bacteria activated macrophages exhibit enhanced metabolic and lysosomal enzyme activity. Macrophages are also secretory Macrophages Phagocytosis of ECM cells producing an array of substances, including various components and debris; antigen processing and presentation enzymes for ECM breakdown and various growth factors or to immune cells; secretion of cytokines that help regulate immune cells and reparative growth factors, cytokines, and functions. other agents When adequately stimulated, macrophages may Mast cells and basophilic Pharmacologically active increase in size and fuse to form multinuclear giant cells, leukocytes molecules (eg, histamine) usually found only in pathologic conditions. Adipocytes Storage of neutral fats  Cells of Connective Tissue 99 FIGURE 5–3 Fibroblasts. 5 C H A P T E R Connective Tissue Cells of Connective Tissue C a b (a) Fibroblasts typically have large active nuclei and eosinophilic (b) Both active and quiescent fibroblasts may sometimes be dis- cytoplasm that tapers off in both directions along the axis of tinguished, as in this section of dermis. Active fibroblasts have the nucleus, a morphology often referred to as “spindle-shaped.” large, euchromatic nuclei and basophilic cytoplasm, while inactive Nuclei (arrows) are clearly seen, but the eosinophilic cytoplasmic fibroblasts (or fibrocytes) are smaller with more heterochromatic processes resemble the collagen bundles (C) that fill the ECM and nuclei (arrows). The round, very basophilic round cells are in leu- are difficult to distinguish in H&E-stained sections. kocytes. (Both X400; H&E) In the TEM, macrophages are shown to have a characteris- marrow (see Chapter 13). The transformation from mono- tic irregular surface with pleats, protrusions, and indentations, cytes to macrophages in connective tissue involves increase in features related to their active pinocytotic and phagocytic cell size, increased protein synthesis, and increase in the num- activities (Figure 5–4). They generally have well-developed ber of Golgi complexes and lysosomes. In addition to debris Golgi complexes and many lysosomes. removal, macrophages secrete growth factors important for Macrophages derive from precursor cells called mono- tissue repair and also function in the uptake, processing, and cytes circulating in the blood (see Chapter 12). Monocytes presentation of antigens for lymphocyte activation, a role dis- cross the epithelial wall of small venules to enter connective cussed later with the immune system. tissue, where they differentiate, mature, and acquire the mor- phologic features of macrophages. Monocytes formed in the yolk sac during early embryonic development circulate and Mast Cells become residents in developing organs throughout the body, Mast cells are oval or irregularly shaped cells of connective comprising a group of related cells called the mononuclear tissue, between 7 and 20 μm in diameter, filled with basophilic phagocyte system. Many of these macrophage-like cells secretory granules that often obscure the central nucleus with prominent functions in various organs have special- (Figure 5–5). These granules are electron dense and of vari- ized names (Table 5–2). All are long-living cells, surviving able size, ranging from 0.3 to 2.0 μm in diameter. Because of with relative inactivity in tissues for months or years. During the high content of acidic radicals in their sulfated GAGs, mast inflammation and tissue repair which follow organ damage, cell granules display metachromasia, which means that they macrophages become activated and play a very important can change the color of some basic dyes (eg, toluidine blue) role. Under such conditions, these cells increase in number, from blue to purple or red. The granules are poorly preserved mainly in the connective tissue stroma, both by proliferation by common fixatives, so mast cells may be difficult to identify and by recruiting additional monocytes formed in the bone in routinely prepared slides. 100 CHAPTER 5 Connective Tissue FIGURE 5–4 Macrophage ultrastructure. L L L N Nu Characteristic features of macrophages seen in this TEM of one phagocytic vacuoles near the protrusions and indentations of the such cell are the prominent nucleus (N) and the nucleolus (Nu) cell surface. (X10,000) and the numerous secondary lysosomes (L). The arrows indicate Mast cells function in the localized release of many bioactive Histamine, which promotes increased vascular perme- substances important in the local inflammatory response, innate ability and smooth muscle contraction immunity, and tissue repair. A partial list of molecules released Serine proteases, which activate various mediators of from these cells’ secretory granules includes the following: inflammation Heparin, a sulfated GAG that acts locally as an Eosinophil and neutrophil chemotactic factors, which attract those leukocytes anticoagulant TABLE 5–2 Distribution and main functions of the cells of the mononuclear phagocyte system. Cell Type Major Location Main Function Monocyte Blood Precursor of macrophages Macrophage Connective tissue, lymphoid organs, Production of cytokines, chemotactic factors, and lungs, bone marrow, pleural and several other molecules that participate in inflammation peritoneal cavities (defense), antigen processing, and presentation Kupffer cell Liver (perisinusoidal) Same as macrophages Microglial cell Central nervous system Same as macrophages Langerhans cell Epidermis of skin Antigen processing and presentation Dendritic cell Lymph nodes, spleen Antigen processing and presentation Osteoclast (from fusion of several Bone Localized digestion of bone matrix macrophages) Multinuclear giant cell (several fused In connective tissue under various Segregation and digestion of foreign bodies macrophages) pathological conditions  Cells of Connective Tissue 101 FIGURE 5–5 Mast cells. C H A P T E R E G 5 BV Connective Tissue Cells of Connective Tissue M N C a b Mast cells are components of loose connective tissues, often mitochondria (M). The granule staining in the TEM is heteroge- located near small blood vessels (BV). neous and variable in mast cells from different tissues; at higher (a) They are typically oval shaped, with cytoplasm filled with magnifications some granules may show a characteristic scroll-like strongly basophilic granules. (X400; PT) substructure (inset) that contains preformed mediators such as histamine and proteoglycans. The ECM near this mast cell includes (b) Ultrastructurally mast cells show little else around the nucleus elastic fibers (E) and bundles of collagen fibers (C). (N) besides these cytoplasmic granules (G), except for occasional Cytokines, polypeptides directing activities of leuko- cells, triggering rapid release of histamine, leukotrienes, che- cytes and other cells of the immune system mokines, and heparin from the mast cell granules that can Phospholipid precursors, which are converted to produce the sudden onset of the allergic reaction. Degranu- prostaglandins, leukotrienes, and other important lipid lation of mast cells also occurs as a result of the action of the mediators of the inflammatory response. complement molecules that participate in the immunologic reactions described in Chapter 14. Occurring in connective tissue of many organs, mast cells Like macrophages, mast cells originate from progenitor are especially numerous near small blood vessels in skin and cells in the bone marrow, which circulate in the blood, cross mesenteries (perivascular mast cells) and in the tissue that the wall of small vessels called venules, and enter connective lines digestive and respiratory tracts (mucosal mast cells); tissues, where they differentiate. Although mast cells are in the granule content of the two populations differs somewhat. many respects similar to basophilic leukocytes, they appear to These major locations suggest that mast cells place themselves have a different lineage at least in humans. strategically to function as sentinels detecting invasion by microorganisms. Release of certain chemical mediators stored in mast cells promotes the allergic reactions known as immediate Plasma Cells hypersensitivity reactions because they occur within a Plasma cells are lymphocyte-derived, antibody-producing few minutes after the appearance of an antigen in an indi- cells. These relatively large, ovoid cells have basophilic cyto- vidual previously sensitized to that antigen. There are many plasm rich in RER and a large Golgi apparatus near the examples of immediate hypersensitivity reaction; a dra- nucleus that may appear pale in routine histologic preparations matic one is anaphylactic shock, a potentially fatal condi- (Figure 5–7). tion. Anaphylaxis consists of the following sequential events The nucleus of the plasma cell is generally spherical (Figure 5–6). The first exposure to an antigen (allergen), but eccentrically placed. Many of these nuclei contain com- such as bee venom, causes antibody-producing cells to pro- pact, peripheral regions of heterochromatin alternating with duce an immunoglobulin of the IgE class that binds avidly lighter areas of euchromatin. At least a few plasma cells are to receptors on the surface of mast cells. Upon a second present in most connective tissues. Their average life span is exposure to the antigen, it reacts with the IgE on the mast only 10-20 days. 102 CHAPTER 5 Connective Tissue FIGURE 5–6 Mast cell secretion. Antigens 2 IgE IgE receptor 3 Adenylate cyclase Ca 2 + Fusion of granules ATP Phosphorylated cAMP proteins Active Microfilaments ATP Heparin protein kinase Histamine Inactive 4 protein kinase Proteoglycans 1 ECF-A Exocytosis Membrane 5 Phospholipases phospholipids Leukotrienes IgE receptors Mast cell secretion is triggered by reexposure to certain antigens exocytosis of some granules (4). In addition, phospholipases act and allergens. Molecules of IgE antibody produced in an initial on specific membrane phospholipids, leading to production and response to an allergen such as pollen or bee venom are bound release of leukotrienes (5). to surface receptors for IgE (1), of which 300,000 are present per The components released from granules, as well as the leu- mast cell. kotrienes, are immediately active in the local microenvironment When a second exposure to the allergen occurs, IgE molecules and promote a variety of controlled local reactions that together bind this antigen and a few IgE receptors very rapidly become normally comprise part of the inflammatory process called the cross-linked (2). This activates adenylate cyclase, leading to immediate hypersensitivity reaction. “ECF-A” is the eosinophil phosphorylation of specific proteins (3), entry of Ca2+ and rapid chemotactic factor of anaphylaxis. › › MEDICAL APPLICATION Leukocytes Plasma cells are derived from B lymphocytes and are respon- Other white blood cells, or leukocytes, besides macrophages sible for the synthesis of immunoglobulin antibodies. Each and plasma cells normally comprise a population of wandering antibody is specific for the one antigen that stimulated cells in connective tissue. Derived from circulating blood cells, the clone of B cells and reacts only with that antigen or they leave blood by migrating between the endothelial cells molecules resembling it (see Chapter 14). The results of the of venules to enter connective tissue. This process increases antibody-antigen reaction are variable, but they usually neu- greatly during inflammation, which is a vascular and cellular tralize harmful effects caused by antigens. An antigen that defensive response to injury or foreign substances, including is a toxin (eg, tetanus, diphtheria) may lose its capacity to do pathogenic bacteria or irritating chemical substances. harm when it is bound by a specific antibody. Bound antigen- Inflammation begins with the local release of chemical antibody complexes are quickly removed from tissues by mediators from various cells, the ECM and blood plasma pro- phagocytosis. teins. These substances act on local blood vessels, mast cells, macrophages, and other cells to induce events characteristic of  Fibers 103 FIGURE 5–7 Plasma cells. 5 C H A P T E R Connective Tissue Fibers a b Antibody-secreting plasma cells are present in variable numbers (b) Plasma are often more abundant in infected tissues, as in the in the connective tissue of many organs. inflamed lamina propria shown here. A large pale Golgi apparatus (a) Plasma cells are large, ovoid cells, with basophilic cyto- (arrows) at a juxtanuclear site in each cell is actively involved in the plasm. The round nuclei frequently show peripheral clumps of terminal glycosylation of the antibodies (glycoproteins). Plasma heterochromatin, giving the structure a “clock-face” appear- cells leave their sites of origin in lymphoid tissues, move to con- ance. (X640; H&E) nective tissue, and produce antibodies that mediate immunity. (X400; PT) inflammation, for example, increased blood flow and vascular permeability, entry and migration of leukocytes, and activa- › FIBERS tion of macrophages for phagocytosis. The fibrous components of connective tissue are elongated Most leukocytes function in connective tissue only for a structures formed from proteins that polymerize after secre- few hours or days and then undergo apoptosis. However, as tion from fibroblasts (Figure 5–2). The three main types of discussed with the immune system, some lymphocytes and fibers include collagen, reticular, and elastic fibers. Col- phagocytic antigen-presenting cells normally leave the inter- lagen and reticular fibers are both formed by proteins of the stitial fluid of connective tissue, enter blood or lymph, and collagen family, and elastic fibers are composed mainly of the move to selected lymphoid organs. protein elastin. These fibers are distributed unequally among the different types of connective tissue, with the predominant fiber type conferring most specific tissue properties. › › MEDICAL APPLICATION Increased vascular permeability is caused by the action of Collagen vasoactive substances such as histamine released from mast The collagens constitute a family of proteins selected dur- cells during inflammation. Classically, the major signs of ing evolution for their ability to form various extracellular inflamed tissues include “redness and swelling with heat fibers, sheets, and networks, all of which extremely strong and pain” (rubor et tumor cum calore et dolore). Increased and resistant to normal shearing and tearing forces. Collagen blood flow and vascular permeability produce local tissue is a key element of all connective tissues, as well as epithelial swelling (edema), with increased redness and warmth. basement membranes and the external laminae of muscle Pain is due mainly to the action of the chemical mediators and nerve cells. on local sensory nerve endings. All these activities help Collagen is the most abundant protein in the human body, protect and repair the inflamed tissue. Chemotaxis representing 30% of its dry weight. A major product of fibro- (Gr. chemeia, alchemy + taxis, orderly arrangement), the blasts, collagens are also secreted by several other cell types phenomenon by which specific cell types are attracted by and are distinguishable by their molecular compositions, mor- specific molecules, draws much larger numbers of leuko- phologic characteristics, distribution, functions, and patholo- cytes into inflamed tissues. gies. A family of 28 collagens exists in vertebrates, numbered 104 CHAPTER 5 Connective Tissue in the order they were identified, and the most important are Linking/anchoring collagens are short collagens that listed in Table 5–3. They can be categorized according to the link fibrillar collagens to one another (forming larger structures formed by their interacting α-chains subunits: fibers) and to other components of the ECM. Type VII collagen binds type IV collagen and anchors the basal Fibrillar collagens, notably collagen types I, II, and lamina to the underlying reticular lamina in basement III, have polypeptide subunits that aggregate to form membranes (see Figure 4–3). large fibrils clearly visible in the electron or light micro- scope (Figure 5–8). Collagen type I, the most abundant Collagen synthesis occurs in many cell types but is a and widely distributed collagen, forms large, eosinophilic specialty of fibroblasts. The initial procollagen α chains are bundles usually called collagen fibers. These often polypeptides made in the RER. Several different α chains of densely fill the connective tissue, forming structures variable lengths and sequences can be synthesized from the such as tendons, organ capsules, and dermis. related collagen genes. In the ER three α chains are selected, Network or sheet-forming collagens such as type IV aligned, and stabilized by disulfide bonds at their carboxyl collagen have subunits produced by epithelial cells and terminals, and folded as a triple helix, another defining fea- are major structural proteins of external laminae and all ture of collagens. The triple helix undergoes exocytosis and epithelial basal laminae. is cleaved to a rodlike procollagen molecule (Figure 5–9) TABLE 5–3 Collagen types. α-Chain Type Composition Structure Optical Microscopy Major Location Main Function Fibril-Forming Collagens I [α1 (I)]2[α2 (I)] 300-nm molecule, Thick, highly picrosirius Skin, tendon, bone, Resistance to tension 67-nm-banded fibrils birefringent, fibers dentin II [α1 (II)]3 300-nm molecule, Loose aggregates of fibrils, Cartilage, vitreous Resistance to pressure 67-nm-banded fibrils birefringent body III [α1 (III)]3 67-nm-banded fibrils Thin, weakly birefringent, Skin, muscle, blood Structural maintenance in argyrophilic (silver-binding) vessels, frequently expansible organs fibers together with type I V [α1 (V)]3 390-nm molecule, Frequently forms fiber Fetal tissues, skin, Participates in type I N-terminal globular together with type I bone, placenta, most collagen function domain interstitial tissues XI [α1 (XI)] [α2 (XI)] 300-nm molecule Small fibers Cartilage Participates in type II [α3 (XI)] collagen function Network-Forming Collagens IV [α1 (IV)]2 [α2 (IV)] Two-dimensional Detected by All basal and Support of epithelial cells; cross-linked network immunocytochemistry external laminae filtration X [α1(X)]3 Hexagonal lattices Detected by Hypertrophic Increases density of the immunocytochemistry cartilage involved in matrix endochondral bone formation Linking/Anchoring Collagens VII [α1 (VII)]3 450 nm, globular Detected by Epithelial basement Anchors basal laminae domain at each end immunocytochemistry membranes to underlying reticular lamina IX [α1 (IX)] [α2 (IX)] 200-nm molecule Detected by Cartilage, vitreous Binds various [α3 (IX)] immunocytochemistry body proteoglycans; associated with type II collagen XII [α1 (XII)]3 Large N-terminal Detected by Placenta, skin, Interacts with type I domain immunocytochemistry tendons collagen XIV [α1 (XIV)]3 Large N-terminal Detected by Placenta, bone Binds type I collagen domain; cross-shaped immunocytochemistry fibrils, with types V and molecule XII, strengthening fiber formation  Fibers 105 FIGURE 5–8 Type I collagen. 5 C H A P T E R Connective Tissue Fibers C a Subunits of type I collagen, the most abundant collagen, assem- ble to form extremely strong fibrils, which are then bundled together further by other collagens into much larger structures called collagen fibers. C C (a) TEM shows fibrils cut longitudinally and transversely. In longi- tudinal sections, fibrils display alternating dark and light bands; b in cross section, the cut ends of individual collagen molecules appear as dots. Ground substance completely surrounds the fibrils. (X100,000) may fill the extracellular space. Subunits for these fibers were (b) The large bundles of type I collagen fibrils (C) appear as secreted by the fibroblasts (arrows) associated with them. acidophilic collagen fibers in connective tissues, where they (X400; H&E) that is the basic subunit from which the fibers or sheets are assembled. These subunits may be homotrimeric, with all › › MEDICAL APPLICATION three chains identical, or heterotrimeric, with two or all A keloid is a local swelling caused by abnormally large three chains having different sequences. Different com- amounts of collagen that form in scars of the skin. Keloids binations of procollagen α chains produce the various occur most often in individuals of African descent and can types of collagen with different structures and functional be a troublesome clinical problem to manage. Not only can properties. they be disfiguring, but excision is almost always followed by recurrence. FIGURE 5–9 The collagen subunit. 8.6 nm In the most abundant form of collagen, type I, each procol- hydrogen bonds and hydrophobic interactions. The length of lagen molecule or subunit has two α1- and one α2-peptide each molecule (sometimes called tropocollagen) is 300 nm, chains, each with a molecular mass of approximately 100 kDa, and its width is 1.5 nm. Each complete turn of the helix spans a intertwined in a right-handed helix and held together by distance of 8.6 nm. 106 CHAPTER 5 Connective Tissue An unusually large number of posttranslational pro- assemble further to form large, extremely strong collagen cessing steps are required to prepare collagen for its final fibers that may be further bundled by linking collagens and assembly in the ECM. These steps have been studied most proteoglycans. Collagen type II (present in cartilage) occurs thoroughly for type I collagen, which accounts for 90% of as fibrils but does not form fibers or bundles. Sheet-forming all the body’s collagen. The most important parts of this collagen type IV subunits assemble as a latticelike network in process are summarized in Figure 5–10 and described epithelial basal laminae. briefly here: When they fill the ECM (eg, in tendons or the sclera of the eye), bundles of collagen appear white. The highly regu- 1. The procollagen α chains are produced on polyribosomes lar orientation of subunits makes collagen fibers birefringent of the RER and translocated into the cisternae. These with polarizing microscopy (see Figure 1–7). In routine light typically have long central domains rich in proline and microscopy, collagen fibers are acidophilic, staining pink with lysine; in type I collagen every third amino acid is glycine. eosin, blue with Mallory trichrome stain, and red with Sirius 2. Hydroxylase enzymes in the ER cisternae add hydroxyl red. Because collagen bundles are long and tortuous, their (-OH) groups to some prolines and lysines in reactions length and diameter are better studied in spread preparations that require O2, Fe2+, and ascorbic acid (vitamin C) as rather than sections, as shown in Figure 1–7a. A very small cofactors. mesentery is frequently used for this purpose; when spread on 3. Glycosylation of some hydroxylysine residues also occurs, a slide, this structure is sufficiently thin to let the light pass to different degrees in various collagen types. through; it can be stained and examined directly under the 4. Both the amino- and carboxyl-terminal sequences of microscope. α chains have globular structures that lack the Gly-X-Y Collagen turnover and renewal in normal connective repeats. In the RER the C-terminal regions of three tissue is generally a very slow but ongoing process. In some selected α chains (α1, α2) are stabilized by cysteine disul- organs, such as tendons and ligaments, the collagen is very fide bonds, which align the three polypeptides and facili- stable, whereas in others, as in the periodontal ligament tates their central domains folding as the triple helix. surrounding teeth, the collagen turnover rate is high. To be With its globular terminal sequences intact, the trimeric renewed, the collagen must first be degraded. Degradation is procollagen molecule is transported through the Golgi initiated by specific enzymes called collagenases, which are apparatus, packaged in vesicles and secreted. members of an enzyme class called matrix metalloprotein- ases (MMPs), which clip collagen fibrils or sheets in such a 5. Outside the cell, specific proteases called procollagen way that they are then susceptible to further degradation by peptidases remove the terminal globular peptides, con- nonspecific proteases. Various MMPs are secreted by mac- verting the procollagen molecules to collagen molecules. rophages and play an important role in remodeling the ECM These now self-assemble (an entropy-driven process) into during tissue repair. polymeric collagen fibrils, usually in specialized niches near the cell surface. 6. Certain proteoglycans and other collagens (eg, types V and XII) associate with the new collagen fibrils, stabilize › › MEDICAL APPLICATION these assemblies, and promote the formation of larger Normal collagen function depends on the expression of fibers from the fibrils. many different genes and adequate execution of several posttranslational events. It is not surprising; therefore, many 7. Fibrillar structure is reinforced and disassembly is pre- pathologic conditions are directly attributable to insufficient vented by the formation of covalent cross-links between or abnormal collagen synthesis. A few such genetic disorders the collagen molecules, a process catalyzed by lysyl or conditions are listed in Table 5–4. oxidase. The other fibrillar and sheetlike collagens are formed in processes similar to that described for collagen type I and sta- Reticular Fibers bilized by linking or anchoring collagens. Because there are so Found in delicate connective tissue of many organs, notably many steps in collagen biosynthesis, there are many points at in the immune system, reticular fibers consist mainly of col- which the process can be interrupted or changed by defective lagen type III, which forms an extensive network (reticulum) enzymes or by disease processes (Table 5–4). of thin (diameter 0.5-2 μm) fibers for the support of many dif- Type I collagen fibrils have diameters ranging from 20 ferent cells. Reticular fibers are seldom visible in hematoxy- to 90 nm and can be several micrometers in length. Adja- lin and eosin (H&E) preparations but are characteristically cent rodlike collagen subunits of the fibrils are staggered by stained black after impregnation with silver salts (Figure 5–12) 67 nm, with small gaps (lacunar regions) between their ends and are thus termed argyrophilic (Gr. argyros, silver). Reticu- (Figure 5–11). This structure produces a characteristic feature lar fibers are also periodic acid–Schiff (PAS) positive, which, of type I collagen visible by EM: transverse striations with like argyrophilia, is due to the high content of sugar chains a regular periodicity (Figure 5–11). Type I collagen fibrils bound to type III collagen α chains. Reticular fibers contain  Fibers 107 FIGURE 5–10 Collagen synthesis. C H A P T E R Intracellular environment Nucleus Formation of mRNA for each type of α chain. 5 RER Synthesis of procollagen α chains with propeptides Connective Tissue Fibers at both ends. Clipping of signal peptide. OH OH Hydroxylation of specific prolyl and lysyl residues OH OH in the endoplasmic reticulum. Vitamin C dependent. Gal-Glu OH Attachment of soluble galactosyl and glucosyl sugars to specific hydroxylysyl residues. OH Gal-Glu Assembly of procollagen molecules (triple helix). Nonhelical propeptides. Transfer vesicles Transport of soluble procollagen to Golgi complex. Packaging of soluble procollagen in secretory Golgi vesicles. Centrioles Secretory Secretory vesicles assisted by microtubules and vesicles microfilaments transport soluble procollagen molecules to cell surface. Extracellular environment Exocytosis of procollagen molecules to extracellular space. Procollagen peptidases cleave most of the Procollagen Procollagen nonhelical terminal peptides, transforming peptidases peptidases procollagen into insoluble collagen molecules, which aggregate to form collagen fibrils. Collagen molecules Microtubule arrays Fibrillar structure is reinforced by the formation of covalent cross-links between collagen molecules catalyzed by the enzyme lysyl oxidase. Hydroxylation and glycosylation of procollagen α chains and procollagen α chains and collagen production depends on sev- their assembly into triple helices occur in the RER, and further eral posttranslational events involving several other enzymes, assembly into fibrils occurs in the ECM after secretion of pro- many diseases involving defective collagen synthesis have been collagen. Because there are many slightly different genes for described. 108 CHAPTER 5 Connective Tissue TABLE 5–4 Examples of clinical disorders resulting from defects in collagen synthesis. Disorder Defect Symptoms Ehlers–Danlos type IV Faulty transcription or translation of collagen type III Aortic and/or intestinal rupture Ehlers–Danlos type VI Faulty lysine hydroxylation Increased skin elasticity, rupture of eyeball Ehlers–Danlos type VII Decrease in procollagen peptidase activity Increased articular mobility, frequent luxation Scurvy Lack of vitamin C, a required cofactor for prolyl Ulceration of gums, hemorrhages hydroxylase Osteogenesis imperfecta Change of 1 nucleotide in genes for collagen type I Spontaneous fractures, cardiac insufficiency FIGURE 5–11 Assembly of type I collagen. Gap region Overlapping region Procollagen subunit 1 300 nm 2 300 nm 3 Collagen fibril Gap Overlapping region (about 10% Bundle of region of a procollagen subunit’s length) collagen fibers 67 nm 5 Collagen fiber 4 Shown here are the relationships among type I collagen mol- 4. Fibrils assemble further and are linked together in larger ecules, fibrils, fibers, and bundles. collagen fibers visible by light microscopy. 1. Rodlike triple-helix collagen molecules, each 300-nm long, 5. Type I fibers often form into still larger aggregates bundled self-assemble in a highly organized, lengthwise arrange- and linked together by other collagens. ment of overlapping regions. The photo shows an SEM view of type I collagen fibrils closely 2. The regular, overlapping arrangement of subunits continues aggregated as part of a collagen fiber. Striations are visible on the as large collagen fibrils are assembled. surface of the fibrils. 3. This structure causes fibrils to have characteristic cross stria- tions with alternating dark and light bands when observed in the EM.  Fibers 109 FIGURE 5–12 Reticular fibers. 5 C H A P T E R Connective Tissue Fibers a b In these silver-stained sections of adrenal cortex (a) and lymph the black argyrophilia. Cell nuclei are also dark, but cytoplasm is node (b), networks of delicate, black reticular fibers are promi- unstained. (X100) Fibroblasts specialized for reticular fiber pro- nent. These fibers serve as a supportive stroma in most lymphoid duction in hematopoietic and lymphoid organs are often called and hematopoietic organs and many endocrine glands. The fibers reticular cells. consist of type III collagen that is heavily glycosylated, producing up to 10% carbohydrate as opposed to 1% in most other col- to their original shape. In the wall of large blood vessels, espe- lagen fibers. cially arteries, elastin also occurs as fenestrated sheets called Reticular fibers produced by fibroblasts occur in the elastic lamellae. Elastic fibers and lamellae are not strongly reticular lamina of basement membranes and typically also acidophilic and stain poorly with H&E; they are stained more surround adipocytes, smooth muscle and nerve fibers, and darkly than collagen with other stains such as orcein and alde- small blood vessels. Delicate reticular networks serve as the hyde fuchsin (Figure 5–13). supportive stroma for the parenchymal secretory cells and rich Elastic fibers (and lamellae) are a composite of fibrillin microvasculature of the liver and endocrine glands. Abundant (350 kDa), which forms a network of microfibrils, embedded reticular fibers also characterize the stroma of hemopoietic in a larger mass of cross-linked elastin (60 kDa). Both pro- tissue (bone marrow), the spleen, and lymph nodes where they teins are secreted from fibroblasts (and smooth muscle cells support rapidly changing populations of proliferating cells and in vascular walls) and give rise to elastic fibers in a stepwise phagocytic cells. manner are shown in Figure 5–14. Initially, microfibrils with diameters of 10 nm form from fibrillin and various glycopro- teins. The microfibrils act as scaffolding upon which elastin is Elastic Fibers then deposited. Elastin accumulates around the microfibrils, Elastic fibers are also thinner than the type I collagen fibers eventually making up most of the elastic fiber, and is respon- and form sparse networks interspersed with collagen bundles sible for the rubberlike property. in many organs, particularly those subject to regular stretching The elastic properties of these fibers and lamellae result or bending. As the name implies, elastic fibers have rubberlike from the structure of the elastin subunits and the unique cross- properties that allow tissue containing these fibers, such as the links holding them together. Elastin molecules have many stroma of the lungs, to be stretched or distended and return lysine-rich regions interspersed with hydrophobic domains 110 CHAPTER 5 Connective Tissue FIGURE 5–13 Elastic fibers. a b c Elastic fibers or lamellae (sheets) add resiliency to connective tis- (b) In sectioned tissue at higher magnification, elastic fibers can sue. Such fibers may be difficult to discern in H&E-stained tissue, be seen among the acidophilic collagen bundles of dermis. (X400; but elastin has a distinct, darker-staining appearance with other Aldehyde fuchsin) staining procedures. (c) Elastic lamellae in the wall of the aorta are more darkly (a) The length, diameter, distribution, and density of dark elastic stained, incomplete sheets of elastin between the layers of eosino- fibers are easily seen in this spread preparation of nonstretched philic smooth muscle. (X80; H&E) connective tissue in a mesentery. (X200; Hematoxylin and orcein) FIGURE 5–14 Formation of elastic fibers. a b c Stages in the formation of elastic fibers can be seen by TEM. are also secreted by the fibroblasts and quickly become covalently (a) Initially, a developing fiber consists of many 10-nm-diameter cross-linked into larger assemblies. microfibrils composed of fibrillin subunits secreted by fibro- (c) Elastin accumulates and ultimately occupies most of the elec- blasts and smooth muscle cells. tron-dense center of the single elastic fiber shown here. Fibrillin (b) Elastin is deposited on the scaffold of microfibrils, forming microfibrils typically remain visible at the fiber surface. Collagen growing, amorphous composite structures. The elastin molecules fibrils, seen in cross section, are also present surrounding the elas- tic fiber. (All X50,000)  Ground Substance 111 FIGURE 5–15 Molecular basis of elastic fiber › GROUND SUBSTANCE C H A P T E R elasticity. The ground substance of the ECM is a highly hydrated (with much bound water), transparent, complex mixture of three major kinds of macromolecules: glycosaminoglycans (GAGs), proteoglycans, and multiadhesive glycopro- teins. Filling the space between cells and fibers in connective tissue, ground substance allows diffusion of small molecules 5 and, because it is viscous, acts as both a lubricant and a barrier Connective Tissue Ground Substance Relaxed to the penetration of invaders. Physical properties of ground substance also profoundly influence various cellular activi- ties. When adequately fixed for histologic analysis, its compo- nents aggregate as fine, poorly resolved material that appears in TEM preparations as electron-dense filaments or granules Single elastin Cross-link Stretched molecule (Figure 5–16a). GAGs (also called mucopolysaccharides) are long poly- mers of repeating disaccharide units, usually a hexosamine and uronic acid. The hexosamine can be glucosamine or galactosamine, and the uronic acid can be glucuronate or idu- ronate. The largest and most ubiquitous GAG is hyaluronan (also called hyaluronate or hyaluronic acid). With a molecu- The diagram shows a small piece of an elastic fiber, in two lar weight from 100s to 1000s of kDa, hyaluronan is a very conformations. Elastin polypeptides, the major components long polymer of the disaccharide glucosamine-glucuronate. of elastic fibers, have multiple random-coil domains that Uniquely among GAGs, hyaluronan is synthesized directly straighten or stretch under force, and then relax. Most of the into the ECM by an enzyme complex, hyaluronan synthase, cross-links between elastin subunits consist of the covalent, cyclic structure desmosine, each of which involves four con- located in the cell membrane of many cells. Hyaluronan verted lysines in two elastin molecules. This unusual type of forms a viscous, pericellular network that binds a consider- protein cross-link holds the aggregate together with little ste- able amount of water, giving it an important role in allowing ric hindrance to elastin movements. These properties give the molecular diffusion through connective tissue and in lubricat- entire network its elastic quality. ing various organs and joints. All other GAGs are much smaller (5-40 kDa), sulfated, bound to proteins (as parts of proteoglycans), and are syn- thesized in Golgi complexes. The four major GAGs found in rich in lysine and proline that are thought to form extensible, proteoglycans are dermatan sulfate, chondroitin sulfates, random-coil conformations (like natural rubber). During keratan sulfate, and heparan sulfate, all of which have dif- deposition on the fibrillin microfibrils, lysyl oxidase converts ferent disaccharide units modified further with carboxyl and the lysines’ amino groups to aldehydes and four oxidized sulfate groups and different tissue distributions (Table 5–5). lysines on neighboring elastin molecules then condense cova- Their high negative charge forces GAGs to an extended con- lently as a desmosine ring, cross-linking the polypeptides. formation and causes them to sequester cations as well as Bound firmly by many desmosine rings, but maintaining the water. These features provide GAGs with space-filling, cush- rubberlike properties of their hydrophobic domains, elastic ioning, and lubricant functions. fibers stretch reversibly when force is applied (Figure 5–15). Proteoglycans consist of a core protein to which are Elastin resists digestion by most proteases, but it is hydrolyzed covalently attached various numbers and combinations of by pancreatic elastase. the sulfated GAGs. Like glycoproteins, they are synthesized on RER, mature in the Golgi apparatus, where the GAG side › › MEDICAL APPLICATION chains are added, and secreted from cells by exocytosis. Unlike glycoproteins, proteoglycans have attached GAGs that often Fibrillins comprise a family of proteins involved in mak- comprise a greater mass than the polypeptide core. As shown ing the scaffolding necessary for the deposition of elastin. in Figure 5–16b, after secretion proteoglycans become bound Mutations in the fibrillin genes result in Marfan syndrome, to hyaluronan polymers by link proteins and their GAG side a disease characterized by a lack of resistance in tissues rich chains associate further with collagen fibers and other ECM in elastic fibers. Because the walls of large arteries are rich in components. elastic components and because the blood pressure is high Proteoglycans are distinguished by their diversity, which in the aorta, patients with this disease often experience aor- is generated in part by enzymatic differences in the Golgi tic swellings called aneurysms, which are life-threatening complexes. A region of ECM may contain several differ- conditions. ent core proteins, each with one or many sulfated GAGs of 112 CHAPTER 5 Connective Tissue FIGURE 5–16 Ground substance of the ECM. Proteoglycan megacomplex Hyaluronan C F Proteoglycan monomer Collagen fibril (type I) C E F Core protein Link protein GAGs Hyaluronan a b (a) TEM of connective tissue ECM reveals ground substance as being more heavily glycosylated and by the addition and sulfation areas containing only fine granular material among the collagen of GAGs, which vary significantly among proteoglycans in their (C) fibers, elastic (E) fibers, and fibroblast processes (F). X100,000. number, length, and the degree to which the sugar polymers are (b) As shown here schematically, connective tissue ground sub- modified. Aggrecan, the most abundant and important proteo- stance contains a vast complex of proteoglycans linked to very glycan in the articular cartilage of joints (see Chapter 8), is a very long hyaluronan molecules. Each proteoglycan monomer has a large macromolecule with a 250 kDa core protein approximately core protein with a few or many side chains of the sulfated GAGs 400 nm long with roughly 100 chondroitin sulfate side chains, listed in Table 5–5. Synthesized in the RER and Golgi apparatus like each 20 kDa, and 30-60 keratan sulfate side chains, each 5-15 kDa. glycoproteins, proteoglycan monomers are distinguished by often different lengths and composition. As mentioned with epi- with very few GAG side chains that bind the surface of type I thelia, perlecan is the key proteoglycan in all basal laminae. collagen fibrils, and syndecan, with an integral membrane One of the best-studied proteoglycans, aggrecan, is very core protein providing an additional attachment of ECM to large, having a 250 kDa core protein heavily bound with cell membranes. chondroitin and keratan sulfate chains. A link protein joins Embryonic mesenchyme (Figure 5–1) is very rich in aggrecan to hyaluronan (Figure 5–16b). Abundant in carti- hyaluronan and water, producing the characteristic wide lage, aggrecan–hyaluronan complexes fill the space between spacing of cells and a matrix ideal for cell migrations and collagen fibers and cells and contribute greatly to the physical growth. In both developing and mature connective tissues, properties of this tissue. Other proteoglycans include decorin, core proteins and GAGs (especially heparan sulfate) of many  Ground Substance 113 Composition and distribution of glycosaminoglycans in connective tissue and their TABLE 5–5 C H A P T E R interactions with collagen fibers. Repeating Disaccharides Electrostatic Interaction Glycosaminoglycan Hexuronic Acid Hexosamine Distribution with Collagen Hyaluronic acid d-glucuronic acid d-glucosamine Umbilical cord, synovial fluid, vitreous humor, 5 cartilage Connective Tissue Ground Substance Chondroitin 4-sulfate d-glucuronic acid d-galactosamine Cartilage, bone, cornea, High levels of interaction, skin, notochord, aorta mainly with collagen type II Chondroitin 6-sulfate d-glucuronic acid d-galactosamine Cartilage, umbilical cord, High levels of interaction, skin, aorta (media) mainly with collagen type II Dermatan sulfate l-iduronic acid or d-galactosamine Skin, tendon, aorta Low levels of interaction, d-glucuronic acid (adventitia) mainly with collagen type I Heparan sulfate d-glucuronic acid or d-galactosamine Aorta, lung, liver, basal Intermediate levels of l-iduronic acid laminae interaction, mainly with collagen types III and IV Keratan sulfate d-galactose d-glucosamine Cartilage, nucleus None pulposus, annulus fibrosus proteoglycans bind and sequester various growth factors and sites for integrins, type IV collagen, and specific proteoglycans, other signaling proteins. Degradation of such proteoglycans providing adhesion for epithelial and other cells. As described during the early phase of tissue repair releases these stored in the previous chapter, all basal and external laminae are rich growth factors, which then help stimulate new cell growth in laminin, which is essential for the assembly and mainte- and ECM synthesis. nance of these structures. Another glycoprotein, fibronectin (L. fibra, fiber + nexus, interconnection), is a 235-270 kDa dimer synthe- › › MEDICAL APPLICATION sized largely by fibroblasts, with binding sites for collagens The degradation of proteoglycans is carried out by several and certain GAGs, and forms insoluble fibrillar networks cell types and depends in part on the presence of several throughout connective tissue (Figure 5–17). The fibronec- lysosomal enzymes. Several disorders have been described, tin substrate provides specific binding sites for integrins and including a deficiency in certain lysosomal enzymes that is important both for cell adhesion and cellular migration degrade specific GAGs, with the subsequent accumulation of through the ECM. these macromolecules in tissues. The lack of specific hydro- As briefly described in Chapter 2, integrins are integral lases in the lysosomes has been found to be the cause of sev- membrane proteins that act as matrix receptors for specific eral disorders, including the Hurler, Hunter, Sanfilippo, and sequences on laminin, fibronectin, some collagens, and certain Morquio syndromes. other ECM proteins. Integrins bind their ECM ligands with Because of their high viscosity, hyaluronan and proteo- relatively low affinity, allowing cells to explore their environ- glycans tend to form a barrier against bacterial penetration of ment without losing attachment to it or becoming glued to it. tissues. Bacteria that produce hyaluronidase, an enzyme that All are heterodimers with two transmembrane polypeptides: hydrolyzes hyaluronan and disassembles proteoglycans com- the α and β chains. Great diversity in the subsets of integrin α plexes, reduce the viscosity of the connective tissue ground and β chains which cells express allows cells to have different substance and have greater invasive power. specific ECM ligands. Integrin-microfilament complexes are clustered in fibro- blasts and other mesenchymal cells to form structures called Making up the third major class of ground substance focal adhesions that can be seen by TEM or immunocyto- macromolecules, multiadhesive glycoproteins all have chemistry. As mentioned in Chapter 4, this type of adhesive multiple binding sites for cell surface integrins and for other junction is typically present at the ends of actin filaments bun- matrix macromolecules. The adhesive glycoproteins are large dled by α-actinin as cytoplasmic stress fibers and focal adhe- molecules with branched oligosaccharide chains and allow sion kinases provide a mechanism by which pulling forces or adhesion of cells to their substrate. An example is the large other physical properties of the ECM can change various cel- (200-400 kDa), trimeric glycoprotein laminin with binding lular activities. 114 CHAPTER 5 Connective Tissue waste products to the detoxifying and excretory organs, the FIGURE 5–17 Fibronectin localization. liver and kidneys. Interstitial fluid is the solvent for these substances. As shown in Figure 5–18, two main forces act on the water in capillaries: The hydrostatic pressure of the blood caused by the pumping action of the heart, which forces water out across the capillary wall The colloid osmotic pressure produced by plasma proteins such as albumin, which draws water back into the capillaries The colloid osmotic pressure exerted by the blood proteins—which are unable to pass through the capillary walls—tends to pull back into the capillary the water forced out by hydrostatic pressure (Figure 5–18). (Because the ions and low-molecular-weight compounds that pass easily through the capillary walls have similar concentrations inside and outside these blood vessels, the osmotic pressures they exert are approximately equal on either side of the capillaries and cancel each other.) The quantity of water drawn back into capillaries is often less than that which was forced out. This excess fluid does not normally accumulate in connective tissue but drains continu- Like laminin of basement membranes, fibronectin is a ously into lymphatic capillaries that eventually return it to the multiadhesive glycoprotein, with binding sites for ECM com- blood. Discussed later with the lymphoid system, lymphatic ponents and for integrins at cell surfaces and has important roles in cell migration and the maintenance of tissue struc- capillaries originate in connective tissue as delicate endothelial ture. As shown here by immunohistochemistry, fibronectin tubes (Figure 5–18). forms a fine network throughout the ECM of connective tis- sue. (X400) › TYPES OF CONNECTIVE TISSUE Different combinations and densities of the cells, fibers, and Water in the ground substance of connective tissue is other ECM components produce graded variations in histo- referred to as interstitial fluid and has an ion composition logical structure within connective tissue. Descriptive names similar to that of blood plasma. Interstitial fluid also contains or classifications used for the various types of connective plasma proteins of low molecular weight that pass through tissue typically denote either a structural characteristic or a the thin walls of the smallest blood vessels, the capillaries. major component. Table 5–6 gives a classification commonly Although only a small proportion of connective tissue pro- used for the main types of connective tissue. Adipose tissue, teins are plasma proteins, it is estimated that as much as one- an important specialized connective tissue, and two other sup- third of the body’s plasma proteins are normally found in the porting tissues, cartilage and bone, are covered in Chapters 6, interstitial fluid of connective tissue because of its large vol- 7, and 8, respectively. ume and wide distribution. Connective Tissue Proper › › MEDICAL APPLICATION Connective tissue proper is broadly classified as “loose” or Edema is the excessive accumulation of interstitial fluid in “dense,” terms that refer to the amount of collagen pres- connective tissue. This water comes from the blood, passing ent (Figure 5–19). Loose connective tissue is common, through the capillary walls that become more permeable forming a layer beneath the epithelial lining of many organs during inflammation and normally produces at least slight and filling the spaces between fibers of muscle and nerve swelling. (Figure 5–19). Also called areolar tissue, the loose connective tis- sue typically contains cells, fibers, and ground substance in Capillaries in connective tissue also bring the various roughly equal parts. The most numerous cells are fibroblasts, nutrients required by cells and carry away their metabolic but the other types of connective tissue cells are also normally  Types of Connective Tissue 115 FIGURE 5–18 Movement of fluid in connective tissue. C H A P T E R Hydrostatic pressure Osmotic pressure 5 Capillary Connective Tissue Types of Connective Tissue Arteriole Venule Lymphatic capillary Water normally passes through capillary walls into the ECM of end is greater than hydrostatic pressure, and water is drawn back surrounding connective tissues primarily at the arterial end of into the capillary. In this way plasma and interstitial fluid con- a capillary, because the hydrostatic pressure is greater than stantly mix, nutrients in blood circulate to cells in connective tis- the colloid osmotic pressure. However, hydrostatic pressure sue, and cellular wastes are removed. decreases toward the venous end of the capillary, as indicated at Not all water that leaves capillaries by hydrostatic pressure the top of the figure. The fall in hydrostatic pressure parallels a is drawn back in by osmotic pressure. This excess tissue fluid is rise in osmotic pressure of the capillary blood because the plasma normally drained by the lymphatic capillaries, open-ended vessels protein concentration increases as water is pushed out across the that arise in connective tissue and enter the one-way lymphatic capillary wall. system that eventually delivers the fluid (now called lymph) back As a result of the increased protein concentration and to the bloodstream via the thoracic and right lymphatic ducts, as decreased hydrostatic pressure, osmotic pressure at the venous described in Chapter 11. found, along with nerves and small blood vessels. Collagen Dense irregular and loose connective tissues are often fibers predominate, but elastic and reticular fibers are also closely associated, with the two types grading into each present. With at least a moderate amount of ground substance, other and making distinctions between them somewhat loose connective tissue has a delicate consistency; it is flexible arbitrary (Figure 5–19). and not very resistant to stress. Dense regular connective tissue consists mostly of Dense connective tissue has similar components as type I collagen bundles and fibroblasts aligned in parallel for loose connective tissue, but with fewer cells, mostly fibro- great resistance to prolonged or repeated stresses from the blasts, and a clear predominance of bundled type I collagen same direction (Figure 5–20). fibers over ground substance. The abundance of collagen The best examples of dense regular connective tissue here protects organs and strengthens them structurally. are the very strong and flexible tendons (Figure 5–20), In dense irregular connective tissue, bundles of collagen cords connecting muscles to bones; aponeuroses, which fibers appear randomly interwoven, with no definite ori- are sheetlike tendons; and ligaments, bands or sheets that entation. The tough three-dimensional collagen network hold together components of the skeletal system. Consist- provides resistance to stress from all directions. Examples ing almost entirely of densely packed parallel collagen fibers of dense irregular connective tissue include the deep der- separated by very little ground substance and having very mis layer of skin and capsules surrounding most organs. few blood vessels, these inextensible structures are white in 116 CHAPTER 5 Connective Tissue TABLE 5–6 Classification of connective or supporting tissues. General Organization Major Functions Examples Connective Tissue Proper Loose (areolar) connective Much ground substance; many Supports microvasculature, Lamina propria beneath tissue cells and little collagen, randomly nerves, and immune defense epithelial lining of digestive distributed cells tract Dense irregular connective Little ground substance; few Protects and supports organs; Dermis of skin, organ capsules, tissue cells (mostly fibroblasts); much resists tearing submucosa layer of digestive collagen in randomly arranged tract fibers Dense regular connective Almost completely filled with Provide strong connections Ligaments, tendons, tissue parallel bundles of collagen; few within musculoskeletal system; aponeuroses, corneal stroma fibroblasts, aligned with collagen strong resistance to force Embryonic Connective Tissues Mesenchyme Sparse, undifferentiated cells, Contains stem/progenitor cells Mesodermal layer of early uniformly distributed in matrix for all adult connective tissue embryo with sparse collagen fibers cells Mucoid (mucous) connective Random fibroblasts and collagen Supports and cushions large Matrix of the fetal umbilical tissue fibers in viscous matrix blood vessels cord Specialized Connective Tissues Reticular connective tissue Delicate network of reticulin/ Supports blood-forming cells, Bone marrow, liver, pancreas, (see Chapter 14) collagen III with attached many secretory cells, and adrenal glands, all lymphoid fibroblasts (reticular cells) lymphocytes in most lymphoid organs except the thymus organs Adipose Tissue (see Chapter 6) Cartilage (see Chapter 7) Bone (see Chapter 8) Blood (see Chapter 12) the fresh state. Fibrocytes with elongated nuclei lie parallel to the collagen fibers of dense regular connective tissue, with › › MEDICAL APPLICATION cytoplasmic folds enveloping portions of the collagen bundles Overuse of tendon–muscle units can result in tendonitis, (Figure 5–20b). Cytoplasm in these “tendinocytes” is dif- characterized by inflammation of the tendons and their attach- ficult to distinguish in H&E-stained preparations because it ments to muscle. Common locations are the elbow, the Achilles is very sparse and has acidophilia like that of the collagen. tendon of the heel, and the shoulder rotator cuff. The swelling In aponeuroses, the parallel bundles of collagen exist as mul- and pain produced by the localized inflammation restricts the tiple layers alternating at 90° angles to one another. So