Junqueira_s Basic Histology, Text and Atlas, 14th Edition.pdf

C H A P T E R 5 CELLS OF CONNECTIVE TISSUE Connective Tissue 96 Reticular Fibers Elastic Fibers...

C H A P T E R 5 CELLS OF CONNECTIVE TISSUE Connective Tissue 96 Reticular Fibers Elastic Fibers 106 109 Fibroblasts 97 Adipocytes 97 GROUND SUBSTANCE 111 Macrophages & the Mononuclear TYPES OF CONNECTIVE TISSUE 114 Phagocyte System 97 Connective Tissue Proper 114 Mast Cells 99 Reticular Tissue 116 Plasma Cells 101 Leukocytes 102 Mucoid Tissue 119 SUMMARY OF KEY POINTS 119 FIBERS 103 Collagen 103 ASSESS YOUR KNOWLEDGE 120 C onnective tissue provides a matrix that supports and migrate from their site of origin in the embryo, surrounding physically connects other tissues and cells together and penetrating developing organs. In addition to producing all to form the organs of the body. The interstitial fluid types of connective tissue proper and the specialized connective of connective tissue gives metabolic support to cells as the tissues bone and cartilage, the embryonic mesenchyme includes medium for diffusion of nutrients and waste products. stem cells for other tissues such as blood, the vascular endothe- Unlike the other tissue types (epithelium, muscle, and lium, and muscle. This chapter describes the features of so%, nerve), which consist mainly of cells, the major constituent of supportive connective tissue proper. connective tissue is the extracellular matrix (ECM). Extra- cellular matrices consist of different combinations of protein fibers (collagen and elastic fibers) and ground substance. › › MEDICAL APPLICATION Some cells in mesenchyme are multipotent stem cells Ground substance is a complex of anionic, hydrophilic pro- potentially useful in regenerative medicine after grafting teoglycans, glycosaminoglycans (GAGs), and multiadhesive to replace damaged tissue in certain patients. Mesenchyme- glycoproteins (laminin, fibronectin, and others). As described like cells remain present in some adult connective tissues, briefly in Chapter 4 with the basal lamina, such glycoproteins including that of tooth pulp and some adipose tissue, and help stabilize the ECM by binding to other matrix compo- are being investigated as possible sources of stem cells for nents and to integrins in cell membranes. Water within this therapeutic repair and organ regeneration. ground substance allows the exchange of nutrients and meta- bolic wastes between cells and the blood supply. The variety of connective tissue types in the body reflects differences in composition and amount of the cells, fibers, and ground substance which together are responsible for the › CELLS OF CONNECTIVE TISSUE remarkable structural, functional, and pathologic diversity of Fibroblasts are the key cells in connective tissue proper connective tissue. (Figure 5–2 and Table 5–1). Fibroblasts originate locally from All connective tissues originate from embryonic mesen- mesenchymal cells and are permanent residents of connective chyme, a tissue developing mainly from the middle layer of tissue. Other cells found here, such as macrophages, plasma the embryo, the mesoderm. Mesenchyme consists largely of cells, and mast cells, originate from hematopoietic stem cells viscous ground substance with few collagen fibers (Figure 5–1). in bone marrow, circulate in the blood, and then move into Mesenchymal cells are undifferentiated and have large nuclei, connective tissue where they function. These and other white with prominent nucleoli and fine chromatin. They are o%en said blood cells (leukocytes) are transient cells of most connec- to be “spindle-shaped,” with their scant cytoplasm extended as tive tissues, where they perform various functions for a short two or more thin cytoplasmic processes. Mesodermal cells period as needed and then die by apoptosis. 96  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, especially in tissues whose cells divide poorly or not at all (eg, cardiac muscle), are filled by connective tissue, forming dense irregu- lar scar tissue. The healing of surgical incisions and other wounds depends on the reparative capacity of connective tissue, particularly on activity and growth of fibroblasts. Mesenchyme consists of a population of undifferentiated In some rapidly closing wounds, a cell called the myo- cells, generally elongated but with many shapes, having large 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 substance increased amounts of actin microfilaments and myosin rich in hyaluronan (hyaluronic acid), but with very little collagen. and behave much like smooth muscle cells. Their activity (X200; Mallory trichrome) 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 his- tologically (Figure 5–3b). Cells with intense synthetic activity are morphologically different from the quiescent fibroblasts Macrophages & the Mononuclear Phagocyte that are scattered within the matrix they have already synthe- System sized. Some histologists reserve the term “fibroblast” to denote Macrophages have highly developed phagocytic ability and the active cell and “fibrocyte” to denote the quiescent cell. The specialize in turnover of protein fibers and removal of dead active fibroblast has more abundant and irregularly branched cells, tissue debris, or other particulate material, being especially cytoplasm, containing much rough endoplasmic reticulum abundant at sites of inflammation. Size and shape vary consid- (RER) and a well-developed Golgi apparatus, with a large, erably, corresponding to their state of functional activity. A typi- ovoid, euchromatic nucleus and a prominent nucleolus. The cal macrophage measures between 10 and 30 &m in diameter quiescent cell is smaller than the active fibroblast, is usually and has an eccentrically located, oval or kidney-shaped nucleus. spindle-shaped with fewer processes, much less RER, and a Macrophages are present in the connective tissue of most organs darker, more heterochromatic nucleus. and are sometimes referred to by pathologists 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 which are surrounded by watery ground substance. In all types of an extracellular matrix (ECM) of various protein fibers, all of connective tissue the 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. When macrophages are stimulated (by injection of Lymphocytes Various immune/defense (several types) functions foreign substances or by infection), they change their morphologic characteristics and properties, becoming acti- Eosinophilic leukocytes Modulate allergic/vasoactive vated macrophages. In addition to showing an increase in reactions and defense against parasites their capacity for phagocytosis and intracellular digestion, activated macrophages exhibit enhanced metabolic and Neutrophilic leukocytes Phagocytosis of bacteria lysosomal enzyme activity. Macrophages are also secretory Macrophages Phagocytosis of ECM components cells producing an array of substances, including various and debris; antigen processing enzymes for ECM breakdown and various growth factors or and presentation to immune cells; secretion of growth factors, cytokines that help regulate immune cells and reparative cytokines, and other agents functions. When adequately stimulated, macrophages may Mast cells and basophilic Pharmacologically active leukocytes molecules (eg, histamine) increase in size and fuse to form multinuclear giant cells, 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 distin- cytoplasm that tapers off in both directions along the axis of the guished, as in this section of dermis. Active fibroblasts have large, nucleus, a morphology often referred to as “spindle-shaped.” Nuclei euchromatic nuclei and basophilic cytoplasm, while inactive fibro- (arrows) are clearly seen, but the eosinophilic cytoplasmic pro- blasts (or fibrocytes) are smaller with more heterochromatic nuclei cesses resemble the collagen bundles (C) that fill the ECM and are (arrows). The round, very basophilic round cells are in leukocytes. difficult to distinguish in H&E-stained sections. (Both X400; H&E) In the TEM, macrophages are shown to have a characteris- Langerhans cells in the skin, and osteoclasts in bone. All are tic irregular surface with pleats, protrusions, and indentations, long-living cells and may survive in the tissues for months. features related to their active pinocytotic and phagocytic In addition to debris removal, these cells are highly impor- activities (Figure 5–4). They generally have well-developed tant for the uptake, processing, and presentation of antigens Golgi complexes and many lysosomes. for lymphocyte activation, a function discussed later with Macrophages derive from bone marrow precursor cells the immune system. The transformation from monocytes to called monocytes that circulate in the blood. These cells macrophages in connective tissue involves increases in cell cross the epithelial wall of small venules to enter connec- size, increased protein synthesis, and increases in the num- tive tissue, where they differentiate, mature, and acquire the ber of Golgi complexes and lysosomes. morphologic features of phagocytic cells. Therefore, mono- cytes and macrophages are the same cell at different stages of maturation. Macrophages play a very important role in the Mast Cells early stages of repair and inflammation a%er tissue damage. Mast cells are oval or irregularly shaped cells of connective Under such conditions these cells accumulate in connective tissue, between 7 and 20 &m in diameter, filled with basophilic tissue by local proliferation of macrophages and recruit- secretory granules which o%en obscure the central nucleus ment of more monocytes from the blood. Macrophages are (Figure 5–5). These granules are electron-dense and of vari- distributed throughout the body and are normally present able size, ranging from 0.3 to 2.0 &m in diameter. Because of in the stroma of most organs. Along with other monocyte- the high content of acidic radicals in their sulfated GAGs, mast derived cells, they comprise a family of cells called the cell granules display metachromasia, which means that they mononuclear phagocyte system (Table 5–2). All of these can change the color of some basic dyes (eg, toluidine blue) macrophage-like cells are derived from monocytes, but have dif- from blue to purple or red. The granules are poorly preserved ferent names in various organs, for example, Kupffer cells by common fixatives, so that mast cells may be di(cult to in the liver, microglial cells in the central nervous system, identify 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 M Connective Tissue Cells of Connective Tissue 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). (a) They are typically oval neous and variable in mast cells from different tissues; at higher shaped, with cytoplasm filled with strongly basophilic granules. magnifications some granules may show a characteristic scroll-like (X400; PT) substructure (inset) that contains preformed mediators such as (b) Ultrastructurally mast cells show little else around the nucleus histamine and proteoglycans. The ECM near this mast cell includes (N) besides these cytoplasmic granules (G), except for occasional elastic fibers (E) and bundles of collagen fibers (C). Cytokines, polypeptides directing activities of leuko- the antigen it reacts with the IgE on the mast cells, trigger- cytes and other cells of the immune system ing rapid release of histamine, leukotrienes, chemokines, and Phospholipid precursors, which are converted to heparin from the mast cell granules which can produce the prostaglandins, leukotrienes, and other important lipid sudden onset of the allergic reaction. Degranulation of mast mediators of the inflammatory response. cells also occurs as a result of the action of the complement molecules that participate in the immunologic reactions Occurring in connective tissue of many organs, mast cells described in Chapter 14. are especially numerous near small blood vessels in skin and Like macrophages, mast cells originate from progenitor mesenteries (perivascular mast cells) and in the tissue that cells in the bone marrow, which circulate in the blood, cross lines digestive and respiratory tracts (mucosal mast cells); the wall of small vessels called venules, and enter connective the granule content of the two populations differs somewhat. tissues, where they differentiate. Although mast cells are in These major locations suggest that mast cells place themselves many respects similar to basophilic leukocytes, they appear to strategically to function as sentinels detecting invasion by have a different lineage at least in humans. 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 a%er 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 prepara- matic one is anaphylactic shock, a potentially fatal condi- tions (Figure 5–7). tion. Anaphylaxis consists of the following sequential events The nucleus of the plasma cell is generally spherical but (Figure 5–6). The first exposure to an antigen (allergen), such eccentrically placed. Many of these nuclei contain compact, as bee venom, causes antibody-producing cells to produce an peripheral regions of heterochromatin alternating with lighter immunoglobulin of the IgE class which binds avidly to recep- areas of euchromatin. At least a few plasma cells are present in tors on the surface of mast cells. Upon a second exposure to most connective tissues. Their average lifespan is 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 to release of leukotrienes (5). surface receptors for IgE (1), of which 300,000 are present per mast The components released from granules, as well as the leu- 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 the cells in connective tissue. Derived from circulating blood cells, clone of B cells and reacts only with that antigen or mol- they leave blood by migrating between the endothelial cells ecules 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 greatly during inflammation, which is a vascular and cellular neutralize harmful effects caused by antigens. An antigen defensive response to injury or foreign substances, including that is a toxin (eg, tetanus, diphtheria) may lose its capacity pathogenic bacteria or irritating chemical substances. to do harm when it is bound by a specific antibody. Bound Inflammation begins with the local release of chemical antigen-antibody complexes are quickly removed from tis- mediators from various cells, the ECM, and blood plasma pro- sues by 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 in (b) Plasma are often more abundant in infected tissues, as in the the connective tissue of many organs. inflamed lamina propria shown here. A large pale Golgi appara- (a) Plasma cells are large, ovoid cells, with basophilic cytoplasm. tus (arrows) at a juxtanuclear site in each cell is actively involved The round nuclei frequently show peripheral clumps of hetero- in the terminal glycosylation of the antibodies (glycoproteins). chromatin, giving the structure a “clock-face” appearance. (X640; Plasma cells leave their sites of origin in lymphoid tissues, move to H&E) connective 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 few hours or days and then undergo apoptosis. However, as secretion from fibroblasts (Figure 5–2). The three main discussed with the immune system, some lymphocytes and types of fibers include collagen, reticular, and elastic phagocytic antigen-presenting cells normally leave the inter- fibers. Collagen and reticular fibers are both formed by stitial fluid of connective tissue, enter blood or lymph, and proteins of the collagen family, and elastic fibers are com- move to selected lymphoid organs. posed mainly of the protein elastin. These fibers are dis- tributed unequally among the different types of connective tissue, with the predominant fiber type conferring most › › MEDICAL APPLICATION specific tissue properties. Increased vascular permeability is caused by the action of vasoactive substances such as histamine released from mast Collagen cells during inflammation. Classically, the major signs of The collagens constitute a family of proteins selected dur- inflamed tissues include “redness and swelling with heat and ing evolution for their ability to form various extracellular pain” (rubor et tumor cum calore et dolore). Increased blood fibers, sheets, and networks, all of which extremely strong flow and vascular permeability produce local tissue swell- and resistant to normal shearing and tearing forces. Collagen ing (edema), with increased redness and warmth. Pain is is a key element of all connective tissues, as well as epithelial due mainly to the action of the chemical mediators on local basement membranes and the external laminae of muscle sensory nerve endings. All these activities help protect and and nerve cells. repair the inflamed tissue. Chemotaxis (Gr. chemeia, alchemy Collagen is the most abundant protein in the human + taxis, orderly arrangement), the phenomenon by which body, representing 30% of its dry weight. A major prod- specific cell types are attracted by specific molecules, draws uct of fibroblasts, collagens are also secreted by several much larger numbers of leukocytes into inflamed tissues. other cell types and are distinguishable by their molecular compositions, morphologic characteristics, distribution, 104 CHAPTER 5 Connective Tissue functions, and pathologies. A family of 28 collagens exists densely fill the connective tissue, forming structures such in vertebrates, numbered in the order they were identified, as tendons, organ capsules, and dermis. and the most important are listed in Table 5–3. They can Network or sheet-forming collagens such as type IV be categorized according to the structures formed by their collagen have subunits produced by epithelial cells and interacting )-chains subunits: are major structural proteins of external laminae and all epithelial basal laminae. Fibrillar collagens, notably collagen types I, II, Linking/anchoring collagens are short collagens that and III, have polypeptide subunits that aggregate to form link fibrillar collagens to one another (forming larger large fibrils clearly visible in the electron or light micro- fibers) and to other components of the ECM. Type VII scope (Figure 5–8). Collagen type I, the most abundant collagen binds type IV collagen and anchors the basal and widely distributed collagen, forms large, eosinophilic lamina to the underlying reticular lamina in basement bundles usually called collagen fibers. These o%en membranes (see Figure 4–3). TABLE 5–3 Collagen types. α-Chain Type Composition Structure Optical Microscopy Major Location Main Function Fibril-Forming Collagens I [)1 (I)]2[)2 (I)] 300-nm molecule, Thick, highly picrosirius Skin, tendon, bone, Resistance to tension 67-nm banded fibrils birefringent, fibers dentin II [)1 (II)]3 300-nm molecule, Loose aggregates of fibrils, Cartilage, vitreous Resistance to pressure 67-nm banded fibrils birefringent body III [)1 (III)]3 67-nm banded fibrils Thin, weakly birefringent, Skin, muscle, blood Structural maintenance argyrophilic (silver- vessels, frequently in expansible organs binding) fibers together with type I V [)1 (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 [)1 (XI)] [)2 (XI)] 300-nm molecule Small fibers Cartilage Participates in type II [)3 (XI)] collagen function Network-Forming Collagens IV [)1 (IV)]2 [)2 (IV)] 2-dimensional cross- Detected by All basal and external Support of epithelial linked network immunocytochemistry laminae cells; filtration X [)1(X)]3 Hexagonal lattices Detected by Hypertrophic Increases density of the immunocytochemistry cartilage involved in matrix endochondral bone formation Linking/Anchoring Collagens VII [)1 (VII)]3 450 nm, globular Detected by Epithelial basement Anchors basal laminae domain at each end immunocytochemistry membranes to underlying reticular lamina IX [)1 (IX)] [)2 (IX)] 200-nm molecule Detected by Cartilage, vitreous Binds various [)3 (IX)] immunocytochemistry body proteoglycans; associated with type II collagen XII [)1 (XII)]3 Large N-terminal Detected by Placenta, skin, tendons Interacts with type I domain immunocytochemistry collagen XIV [)1 (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 lon- b gitudinal sections fibrils display alternating dark and light bands; 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) Collagen synthesis occurs in many cell types but undergoes exocytosis and is cleaved to a rodlike procolla- is a specialty of fibroblasts. The initial procollagen α gen molecule (Figure 5–9) that is the basic subunit from chains are polypeptides made in the RER. Several differ- which the fibers or sheets are assembled. These subunits ent ) chains of variable lengths and sequences can be syn- may be homotrimeric, with all three chains identical, or thesized from the related collagen genes. In the ER three heterotrimeric, with two or all three chains having different ) chains are selected, aligned, and stabilized by disulfide sequences. Different combinations of procollagen ) chains bonds at their carboxyl terminals, and folded as a triple produce the various types of collagen with different struc- helix, another defining feature of collagens. The triple helix tures and functional properties. FIGURE 5–9 The collagen subunit. 8.6 nm In the most abundant form of collagen, type I, each procollagen hydrophobic interactions. The length of each molecule (sometimes molecule or subunit has two )1- and one )2-peptide chains, each called tropocollagen) is 300 nm, and its width is 1.5 nm. Each com- with a molecular mass of approximately 100 kDa, intertwined in plete turn of the helix spans a distance of 8.6 nm. a right-handed helix and held together by hydrogen bonds and 106 CHAPTER 5 Connective Tissue › › MEDICAL APPLICATION many steps in collagen biosynthesis, there are many points at which the process can be interrupted or changed by defective A keloid is a local swelling caused by abnormally large enzymes or by disease processes (Table 5–4). amounts of collagen that form in scars of the skin. Keloids Type I collagen fibrils have diameters ranging from 20 to occur most often in individuals of African descent and can 90 nm and can be several micrometers in length. Adjacent rod- be a troublesome clinical problem to manage. Not only can like collagen subunits of the fibrils are staggered by 67 nm, with they be disfiguring, but excision is almost always followed by small gaps (lacunar regions) between their ends (Figure 5–11). recurrence. This structure produces a characteristic feature of type I colla- gen visible by EM: transverse striations with a regular period- icity (Figure 5–11). Type I collagen fibrils assemble further to form large, extremely strong collagen fibers that may be further bundled by linking collagens and proteoglycans. Collagen type An unusually large number of posttranslational process- II (present in cartilage) occurs as fibrils but does not form fibers ing steps are required to prepare collagen for its final assembly or bundles. Sheet-forming collagen type IV subunits assemble in the ECM. These steps have been studied most thoroughly as a lattice-like network in epithelial basal laminae. for type I collagen, which accounts for 90% of all the body’s When they fill the ECM (eg, in tendons or the sclera of collagen. The most important parts of this process are sum- the eye), bundles of collagen appear white. The highly regu- marized in Figure 5–10 and described briefly here: lar orientation of subunits makes collagen fibers birefringent 1. The procollagen ) chains are produced on polyribosomes with polarizing microscopy (see Figure 1–7). In routine light of the RER and translocated into the cisternae. These typi- microscopy collagen fibers are acidophilic, staining pink with cally have long central domains rich in proline and lysine; eosin, blue with Mallory trichrome stain, and red with Sirius in type I collagen every third amino acid is glycine. red. Because collagen bundles are long and tortuous, their 2. Hydroxylase enzymes in the ER cisternae add hydroxyl length and diameter are better studied in spread preparations (-OH) groups to some prolines and lysines in reactions rather than sections, as shown in Figure 1–7a. Mesentery is that require O2, Fe2+, and ascorbic acid (vitamin C) as frequently used for this purpose; when spread on a slide, this cofactors. structure is su(ciently thin to let the light pass through; it can be stained and examined directly under the microscope. 3. Glycosylation of some hydroxylysine residues also occurs, Collagen turnover and renewal in normal connective tis- to different degrees in various collagen types. sue is generally a very slow but ongoing process. In some 4. Both the amino- and carboxyl-terminal sequences of organs, such as tendons and ligaments, the collagen is very ) chains have globular structures that lack the gly-X- stable, whereas in others, as in the periodontal ligament sur- Y repeats. In the RER the C-terminal regions of three rounding teeth, the collagen turnover rate is high. To be selected ) chains ()1, )2) are stabilized by cysteine disul- renewed, the collagen must first be degraded. Degradation is fide bonds, which align the three polypeptides and facili- initiated by specific enzymes called collagenases, which are tates their central domains folding as the triple helix. members of an enzyme class called matrix metalloprotein- With its globular terminal sequences intact, the trimeric ases (MMPs), which clip collagen fibrils or sheets in such a procollagen molecule is transported through the Golgi way that they are then susceptible to further degradation by apparatus, packaged in vesicles and secreted. nonspecific proteases. Various MMPs are secreted by macro- 5. Outside the cell, specific proteases called procollagen phages and play an important role in remodeling the ECM peptidases remove the terminal globular peptides, con- during tissue repair. verting the procollagen molecules to collagen molecules. These now self-assemble (an entropy-driven process) into polymeric collagen fibrils, usually in specialized niches › › MEDICAL APPLICATION near the cell surface. Normal collagen function depends on the expression of 6. Certain proteoglycans and other collagens (eg, types V and many different genes and adequate execution of several XII) associate with the new collagen fibrils, stabilize these posttranslational events. It is not surprising, therefore, that assemblies, and promote the formation of larger fibers many pathologic conditions are directly attributable to insuf- from the fibrils. ficient or abnormal collagen synthesis. A few such genetic 7. Fibrillar structure is reinforced and disassembly is disorders or conditions are listed in Table 5–4. prevented by the formation of covalent cross-links between the collagen molecules, a process catalyzed by lysyl oxidase. Reticular Fibers The other fibrillar and sheetlike collagens are formed in Found in delicate connective tissue of many organs, notably in processes similar to that described for collagen type I and sta- the immune system, reticular fibers consist mainly of colla- bilized by linking or anchoring collagens. Because there are so gen type III, which forms an extensive network (reticulum) of  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 ) chains and their ) chains and collagen production depends on several posttrans- assembly into triple helices occur in the RER, and further assem- lational events involving several other enzymes, many diseases bly into fibrils occurs in the ECM after secretion of procollagen. involving defective collagen synthesis have been described. Because there are many slightly different genes for procollagen 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 hydroxylase Ulceration of gums, hemorrhages 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 molecules, 4. Fibrils assemble further and are linked together in larger col- fibrils, fibers, and bundles. lagen fibers visible by light microscopy. 5. Type I fibers often form into still larger aggregates bundled and 1. Rodlike triple-helix collagen molecules, each 300-nm long, self- linked together by other collagens. assemble in a highly organized, lengthwise arrangement of overlapping regions. The photo shows an SEM view of type I collagen fibrils closely 2. The regular, overlapping arrangement of subunits continues as aggregated as part of a collagen fiber. Striations are visible on the 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 node type III collagen that is heavily glycosylated, producing the black (b), networks of delicate, black reticular fibers are prominent. These argyrophilia. Cell nuclei are also dark, but cytoplasm is unstained. fibers serve as a supportive stroma in most lymphoid and hema- (X100) Fibroblasts specialized for reticular fiber production in hema- topoietic organs and many endocrine glands. The fibers consist of topoietic and lymphoid organs are often called reticular cells. thin (diameter 0.5-2 &m) fibers for the support of many dif- in many organs, particularly those subject to regular stretching ferent cells. Reticular fibers are seldom visible in hematoxylin or bending. As the name implies, elastic fibers have rubberlike and eosin (H&E) preparations but are characteristically stained properties that allow tissue containing these fibers, such as the black a%er impregnation with silver salts (Figure 5–12) and are stroma of the lungs, to be stretched or distended and return thus termed argyrophilic (Gr. argyros, silver). Reticular fibers to their original shape. In the wall of large blood vessels, espe- are also periodic acid-Schiff (PAS) positive, which, like argyro- cially arteries, elastin also occurs as fenestrated sheets called philia, is due to the high content of sugar chains bound to type elastic lamellae. Elastic fibers and lamellae are not strongly III collagen ) chains. Reticular fibers contain up to 10% carbo- acidophilic and stain poorly with H&E; they are stained more hydrate as opposed to 1% in most other collagen fibers. darkly than collagen with other stains such as orcein and alde- Reticular fibers produced by fibroblasts occur in the hyde fuchsin (Figure 5–13). reticular lamina of basement membranes and typically also Elastic fibers (and lamellae) are a composite of fibrillin surround adipocytes, smooth muscle and nerve fibers, and (350 kDa), which forms a network of microfibrils, embedded small blood vessels. Delicate reticular networks serve as the in a larger mass of cross-linked elastin (60 kDa). Both pro- supportive stroma for the parenchymal secretory cells and rich teins are secreted from fibroblasts (and smooth muscle cells microvasculature of the liver and endocrine glands. Abundant in vascular walls) and give rise to elastic fibers in a stepwise reticular fibers also characterize the stroma of hemopoietic tis- manner are shown in Figure 5–14. Initially, microfibrils with sue (bone marrow), the spleen, and lymph nodes where they diameters of 10 nm form from fibrillin and various glycopro- support rapidly changing populations of proliferating cells and teins. The microfibrils act as scaffolding upon which elastin is phagocytic cells. then deposited. Elastin accumulates around the microfibrils, eventually making up most of the elastic fiber, and is respon- Elastic Fibers sible for the rubberlike property. Elastic fibers are also thinner than the type I collagen fibers The elastic properties of these fibers and lamellae result and form sparse networks interspersed with collagen bundles from the structure of the elastin subunits and the unique 110 CHAPTER 5 Connective Tissue FIGURE 5–13 Elastic fibers. a b c Elastic fibers or lamellae (sheets) add resiliency to connective (b) In sectioned tissue at higher magnification, elastic fibers can tissue. 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 fibroblasts (c) Elastin accumulates and ultimately occupies most of the and smooth muscle cells. electron-dense center of the single elastic fiber shown here. Fibril- (b) Elastin is deposited on the scaffold of microfibrils, forming lin 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 glycoproteins. Filling the space between cells and fibers in connective tissue, ground substance allows diffusion of small molecules and, because it is 5 viscous, acts as both a lubricant and a barrier to the penetration Connective Tissue Ground Substance Relaxed of invaders. Physical properties of ground substance also pro- foundly influence various cellular activities. When adequately fixed for histologic analysis, its components aggregate as fine, poorly resolved material that appears in TEM preparations as Cross-link Single elastin electron-dense filaments or granules (Figure 5–16a). Stretched molecule 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- lar weight from 100s to 1000s of kDa, hyaluronan is a very The diagram shows a small piece of an elastic fiber, in two con- formations. Elastin polypeptides, the major components of elas- long polymer of the disaccharide glucosamine-glucuronate. tic fibers, have multiple random-coil domains that straighten Uniquely among GAGs, hyaluronan is synthesized directly or stretch under force, and then relax. Most of the cross-links into the ECM by an enzyme complex, hyaluronan synthase, between elastin subunits consist of the covalent, cyclic structure located in the cell membrane of many cells. Hyaluronan desmosine, each of which involves four converted lysines in forms a viscous, pericellular network which binds a consider- two elastin molecules. This unusual type of protein cross-link holds the aggregate together with little steric hindrance to able amount of water, giving it an important role in allowing elastin movements. These properties give the entire network its molecular diffusion through connective tissue and in lubricat- elastic quality. ing various organs and joints. All other GAGs are much smaller (10-40 kDa), sulfated, bound to proteins (as parts of proteoglycans), and are syn- cross-links holding them together. Elastin molecules have thesized in Golgi complexes. The four major GAGs found in many lysine-rich regions interspersed with hydrophobic proteoglycans are dermatan sulfate, chondroitin sulfates, domains rich in lysine and proline which are thought to form keratan sulfate, and heparan sulfate, all of which have dif- extensible, random-coil conformations (like natural rubber). ferent disaccharide units modified further with carboxyl and During deposition on the fibrillin microfibrils, lysyl oxidase sulfate groups and different tissue distributions (Table 5–5). converts the lysines’ amino groups to aldehydes and four oxi- Their high negative charge forces GAGs to an extended con- dized lysines on neighboring elastin molecules then condense formation and causes them to sequester cations as well as covalently as a desmosine ring, cross-linking the polypep- water. These features provide GAGs with space-filling, cush- tides. Bound firmly by many desmosine rings, but maintaining ioning, and lubricant functions. the rubberlike properties of their hydrophobic domains, elastic Proteoglycans consist of a core protein to which are cova- fibers stretch reversibly when force is applied (Figure 5–15). lently attached various numbers and combinations of the sul- Elastin resists digestion by most proteases, but it is hydrolyzed fated GAGs. Like glycoproteins, they are synthesized on RER, by pancreatic elastase. mature in the Golgi apparatus, where the GAG side-chains are added, and secreted from cells by exocytosis. Unlike glycopro- teins, proteoglycans have attached GAGs which o%en comprise › › MEDICAL APPLICATION a greater mass than the polypeptide core. As shown in Figure Fibrillins comprise a family of proteins involved in making the 5–16b, a%er secretion proteoglycans become bound to the scaffolding necessary for the deposition of elastin. Mutations hyaluronan by link proteins and their GAG side-chains associ- in the fibrillin genes result in Marfan syndrome, a disease ate further with collagen fibers and other ECM components. characterized by a lack of resistance in tissues rich in elastic Proteoglycans are distinguished by their diversity, which fibers. Because the walls of large arteries are rich in elastic is generated in part by enzymatic differences in the Golgi com- components and because the blood pressure is high in the plexes. A region of ECM may contain several different core aorta, patients with this disease often experience aortic swell- proteins, each with one or many sulfated GAGs of different ings called aneurysms, which are life-threatening conditions. lengths and composition. As mentioned with epithelia, per- lecan is the key proteoglycan in all basal laminae. One of the 112 CHAPTER 5 Connective Tissue FIGURE 5–16 Ground substance of the extracellular matrix (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 the RER and Golgi apparatus like glycoproteins, proteoglycan areas containing only fine granular material among the collagen monomers are distinguished by often being more heavily gly- (C) fibers, elastic (E) fibers and fibroblast processes (F). X100,000. cosylated and by the addition and sulfation of GAGs, which vary (b) As shown here schematically, connective tissue ground sub- significantly among proteoglycans in their number, length, and stance contains a vast complex of proteoglycans linked to very the degree to which the sugar polymers are modified. The large long hyaluronan molecules. Each proteoglycan monomer has proteoglycan aggrecan (25,000 kDa) typically has about 50 a core protein with a few or many side chains of the sulfated chains of keratan sulfate chains and twice that number of chon- glycosaminoglycans (GAGs) listed in Table 5–5. Synthesized in droitin sulfate. best studied proteoglycans, aggrecan, is very large (250 kDa), Embryonic mesenchyme (Figure 5–1) is very rich in having a core protein heavily bound with chondroitin and ker- hyaluronan and water, producing the characteristic wide atan sulfate chains. A link protein joins aggrecan to hyaluronan spacing of cells and a matrix ideal for cell migrations and (Figure 5–16b). Abundant in cartilage, aggrecan-hyaluronan growth. In both developing and mature connective tissues, complexes fill the space between collagen fibers and cells and core proteins and GAGs (especially heparan sulfate) of many contribute greatly to the physical properties of this tissue. proteoglycans bind and sequester various growth factors and Other proteoglycans include decorin, with very few GAG side other signaling proteins. Degradation of such proteoglycans chains that binds the surface of type I collagen fibrils, and syn- during the early phase of tissue repair releases these stored decan, with an integral membrane core protein providing an growth factors which then help stimulate new cell growth and additional attachment of ECM to cell membranes. ECM synthesis.  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 5 fluid, vitreous humor, Connective Tissue Ground Substance cartilage Chondroitin 4-sulfate D-glucuronic acid D-galactosamine Cartilage, bone, cornea, skin, High levels of interaction, notochord, aorta mainly with collagen type II Chondroitin 6-sulfate D-glucuronic acid D-galactosamine Cartilage, umbilical cord, skin, High levels of interaction, aorta (media) mainly with collagen type II Dermatan sulfate L-iduronic acid or D-galactosamine Skin, tendon, aorta (adventitia) Low levels of interaction, D-glucuronic acid 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 pulposus, None annulus fibrosus › › MEDICAL APPLICATION Another glycoprotein, fibronectin (L. fibra, fiber + nexus, interconnection), is a 235-270 kDa dimer synthe- The degradation of proteoglycans is carried out by several sized largely by fibroblasts, with binding sites for collagens cell types and depends in part on the presence of several and certain GAGs, and forms insoluble fibrillar networks lysosomal enzymes. Several disorders have been described, throughout connective tissue (Figure 5–17). The fibronectin including a deficiency in certain lysosomal enzymes that substrate provides specific binding sites for integrins and degrade specific GAGs, with the subsequent accumulation of is important both for cell adhesion and cellular migration these macromolecules in tissues. The lack of specific hydro- through the ECM. lases in the lysosomes has been found to be the cause of sev- As briefly described in Chapter 2 integrins are integral eral disorders, including the Hurler, Hunter, Sanfilippo, and membrane proteins that act as matrix receptors for specific Morquio syndromes. sequences on laminin, fibronectin, some collagens, and certain Because of their high viscosity, hyaluronan and proteo- other ECM proteins. Integrins bind their ECM ligands with glycans tend to form a barrier against bacterial penetration of relatively low a(nity, allowing cells to explore their environ- tissues. Bacteria that produce hyaluronidase, an enzyme that ment without losing attachment to it or becoming glued to it. hydrolyzes hyaluronan and disassembles proteoglycans com- All are heterodimers with two transmembrane polypeptides: plexes, reduce the viscosity of the connective tissue ground the ) and β chains. Great diversity in the subsets of integrin ) substance and have greater invasive power. and β chains which cells express allows cells to have different specific ECM ligands. Integrin-microfilament complexes are clustered in fibro- Making up the third major class of ground substance blasts and other mesenchymal cells to form structures called macromolecules, multiadhesive glycoproteins all have focal adhesions that can be seen by TEM or immunocyto- multiple binding sites for cell surface integrins and for other chemistry. As mentioned in Chapter 4 this type of adhesive matrix macromolecules. The adhesive glycoproteins are large junction is typically present at the ends of actin filaments bun- molecules with branched oligosaccharide chains and allow dled by )-actinin as cytoplasmic stress fibers and focal adhe- adhesion of cells to their substrate. An example is the large sion kinases provide a mechanism by which pulling forces or (200-400 kDa), trimeric glycoprotein laminin with binding other physical properties of the ECM can change various sites for integrins, type IV collagen, and specific proteoglycans, cellular activities. providing adhesion for epithelial and other cells. As described Water in the ground substance of connective tissue is in the previous chapter, all basal and external laminae are rich referred to as interstitial fluid and has an ion composition in laminin, which is essential for the assembly and mainte- similar to that of blood plasma. Interstitial fluid also contains nance of these structures. plasma proteins of low molecular weight that pass through 114 CHAPTER 5 Connective Tissue As shown in Figure 5–18, two main forces act on the FIGURE 5–17 Fibronectin localization. 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 pro- teins 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 o%en less than that which was forced out. This excess fluid does not normally accumulate in connective tissue but drains continu- ously into lymphatic capillaries that eventually return it to the blood. Discussed later with the lymphoid system, lymphatic Like laminin of basement membranes, fibronectin is a mul- capillaries originate in connective tissue as delicate endothelial tiadhesive glycoprotein, with binding sites for ECM compo- tubes (Figure 5–18). nents and for integrins at cell surfaces, and has important roles in cell migration and the maintenance of tissue structure. › TYPES OF CONNECTIVE TISSUE As shown here by immunohistochemistry, fibronectin forms a fine network throughout the ECM of connective tissue. (X400) Different combinations and densities of the cells, fibers, and other ECM components produce graded variations in histo- logical structure within connective tissue. Descriptive names or classifications used for the various types of connective tis- the thin walls of the smallest blood vessels, the capillaries. sue typically denote either a structural characteristic or a Although only a small proportion of connective tissue pro- major component. Table 5–6 gives a classification commonly teins are plasma proteins, it is estimated that as much as used for the main types of connective tissue. Adipose tissue, one-third of the body’s plasma proteins are normally found in an important specialized connective tissue, and two other sup- the interstitial fluid of connective tissue because of its large porting tissues, cartilage and bone, are covered in Chapters 6, volume and wide distribution. 7, and 8. › › MEDICAL APPLICATION Connective Tissue Proper Edema is the excessive accumulation of interstitial fluid in Connective tissue proper is broadly classified as “loose” or connective tissue. This water comes from the blood, passing “dense,” terms which refer to the amount of collagen present through the capillary walls that become more permeable (Figure 5–19). Loose connective tissue is common, forming during inflammation and normally produces at least slight a layer beneath the epithelial lining of many organs and filling swelling. the spaces between fibers of muscle and nerve (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 waste products to the detoxifying and excretory organs, the found, along with nerves and small blood vessels. Collagen liver and kidneys. Interstitial fluid is the solvent for these fibers predominate, but elastic and reticular fibers are also substances. present. With at least a moderate amount of ground substance,  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 constantly a capillary, because the hydrostatic pressure is greater than mix, nutrients in blood circulate to cells in connective tissue, and the colloid osmotic pressure. However, hydrostatic pressure cellular wastes are removed. decreases toward the venous end of the capillary, as indicated Not all water that leaves capillaries by hydrostatic pressure is at the top of the figure. The fall in hydrostatic pressure parallels drawn back in by osmotic pressure. This excess tissue fluid is nor- a rise in osmotic pressure of the capillary blood because the mally drained by the lymphatic capillaries, open-ended vessels plasma protein concentration increases as water is pushed out that arise in connective tissue and enter the one-way lymphatic across the capillary wall. system that eventually delivers the fluid (now called lymph) back As a result of the increased protein concentration and to veins. decreased hydrostatic pressure, osmotic pressure at the venous loose connective tissue has a delicate consistency; it is flexible into each other and making distinctions between them some- and not very resistant to stress. what arbitrary (Figure 5–19). Dense connective tissue has similar components as loose Dense regular connective tissue consists mostly of connective tissue, but with fewer cells, mostly fibroblasts, and type I collagen bundles and fibroblasts aligned in parallel for a clear predominance of bundled type I collagen fibers over