49.3 Connective tissues contain abundant extracellular material Connective Tissue Proper Connective tissues are derived from embryonic meso- derm and occur in many different forms(table 49.3) These various forms are divided into two major classes connective tissue proper, which is further divided into loose and dense connective tissues; and special connec tive tissues that include cartilage, bone. and blood. At first glance, it may seem odd that such diverse tissues are placed in the same category. Yet all connective tissues do share a common structural feature: they all have abun ant extracellular material because their cells are spaced widely apart. This extracellular material is generically known as the matrix of the tissue. In bone. the extracel lular matrix contains crystals that make the bones hard in blood, the extracellular matrix is plasma, the fluid por- tion of the blood Loose connective tissue consists of cells scattered FIGURE 49.6 within an amorphous mass of proteins that form a ground Collagen fibers. Each fiber is composed of many individual substance. This gelatinous material is stre ed by loose scattering of protein fibers such as collagen(figure 49.6), elastin, which makes the tissue elastic, and reticulin which supports the tissue by forming a collagenous mesh work. The flavored gelatin we eat for dessert consists of the extracellular material from loose connective tissues. The ells that secrete collagen and other fibrous proteins are known as fibroblasts Loose connective tissue contains other cells as well. in- cluding mast cells that produce histamine(a blood vessel lator)and heparin(an anticoagulant) and macrophages, the immune systems first defense against invading organisms as will be described in detail in chapter 57 Adipose cells are found in loose connective tissue, usually in large groups that form what is referred adipose tissue(figure 49.7). Each adipose cell contains a droplet of fat (triglycerides) within a storage vesicle When that fat is needed for energy, the adipose cell hy- drolyzes its stored triglyceride and secretes fatty acids into the blood for oxidation by the cells of the muscles, liver, and other organs. The number of adipose cells in FIGURE 49.7 an adult is generally fixed. When a person gains weight, Adipose tissue. Fat is stored in globules of adipose tissue, a type the cells become larger, and when weight is lost, the cells of loose connective tissue. As a person gains or loses weight, the size of the fat globules increases or decreases. A person cannot Dense connective tissue contains tightly packed colla- decrease the number of fat cells by losing weight. gen fibers, making it stronger than loose connective tis- sue. It consists of two types: regular and irregular. The collagen fibers of dense regular connective tissue are the capsules of the kidneys and adrenal glands. It also cow lined up in parallel, like the strands of a rope. This is the ers muscle as epimysium, nerves as perineurium, and bones structure of tendons, which bind muscle to bone, and liga as periosteum. ments. which bind bone to bone. In contrast the collagen fibers of dense irregular connective tissue have many Connective tissues are characterized by abundant different orientations. This type of connective tissue pro- extracellular materials in the matrix between cells duces the tough coverings that package organs, such as Connective tissue proper may be either loose or dense 990 Part XIlI Animal Form and FunctionConnective Tissue Proper Connective tissues are derived from embryonic meso￾derm and occur in many different forms (table 49.3). These various forms are divided into two major classes: connective tissue proper, which is further divided into loose and dense connective tissues; and special connec￾tive tissues that include cartilage, bone, and blood. At first glance, it may seem odd that such diverse tissues are placed in the same category. Yet all connective tissues do share a common structural feature: they all have abun￾dant extracellular material because their cells are spaced widely apart. This extracellular material is generically known as the matrix of the tissue. In bone, the extracel￾lular matrix contains crystals that make the bones hard; in blood, the extracellular matrix is plasma, the fluid por￾tion of the blood. Loose connective tissue consists of cells scattered within an amorphous mass of proteins that form a ground substance. This gelatinous material is strengthened by a loose scattering of protein fibers such as collagen (figure 49.6), elastin, which makes the tissue elastic, and reticulin, which supports the tissue by forming a collagenous mesh￾work. The flavored gelatin we eat for dessert consists of the extracellular material from loose connective tissues. The cells that secrete collagen and other fibrous proteins are known as fibroblasts. Loose connective tissue contains other cells as well, in￾cluding mast cells that produce histamine (a blood vessel dilator) and heparin (an anticoagulant) and macrophages, the immune system’s first defense against invading organisms, as will be described in detail in chapter 57. Adipose cells are found in loose connective tissue, usually in large groups that form what is referred to as adipose tissue (figure 49.7). Each adipose cell contains a droplet of fat (triglycerides) within a storage vesicle. When that fat is needed for energy, the adipose cell hy￾drolyzes its stored triglyceride and secretes fatty acids into the blood for oxidation by the cells of the muscles, liver, and other organs. The number of adipose cells in an adult is generally fixed. When a person gains weight, the cells become larger, and when weight is lost, the cells shrink. Dense connective tissue contains tightly packed colla￾gen fibers, making it stronger than loose connective tis￾sue. It consists of two types: regular and irregular. The collagen fibers of dense regular connective tissue are lined up in parallel, like the strands of a rope. This is the structure of tendons, which bind muscle to bone, and liga￾ments, which bind bone to bone. In contrast, the collagen fibers of dense irregular connective tissue have many different orientations. This type of connective tissue pro￾duces the tough coverings that package organs, such as the capsules of the kidneys and adrenal glands. It also cov￾ers muscle as epimysium, nerves as perineurium, and bones as periosteum. Connective tissues are characterized by abundant extracellular materials in the matrix between cells. Connective tissue proper may be either loose or dense. 990 Part XIII Animal Form and Function 49.3 Connective tissues contain abundant extracellular material. FIGURE 49.6 Collagen fibers. Each fiber is composed of many individual collagen strands and can be very strong under tension. FIGURE 49.7 Adipose tissue. Fat is stored in globules of adipose tissue, a type of loose connective tissue. As a person gains or loses weight, the size of the fat globules increases or decreases. A person cannot decrease the number of fat cells by losing weight