Most of the organ systems that present to the surgical pathologist consist of cellular tissues. In contrast, the connective tissues are made up principally of an extracellular matrix, which serves a mechanical function (1). In general, most of the symptoms of connective tissue disease are a result of structural failure of the extracellular matrix (e.g., fracture).

Connective tissues (e.g., ligaments, tendons, bone, cartilage) are subjected principally to three types of applied stress: (a) compression, (b) tension, and (c) shear. The constituents of the extracellular matrix are ideally suited to resisting one or another of these types of stress. Whereas collagen resists tension, hydroxyapatite in the bone and the large globular proteoglycan aggregates in the cartilage are particularly well suited to resisting compression. Thus, the combination of matrix constituents in each type of tissue reflects the types of stresses that are applied to it.

The extracellular matrix is synthesized and, to a greater or lesser extent, broken down by its intrinsic cells (i.e., fibroblasts, osteoblasts, and chondroblasts) (2). The matrix has a very definite microarchitecture designed to resist the forces that act on the anatomic structure (3,4). For example, collagen fibers run more or less parallel to the length of the ligaments and tendons. Similarly, within the cancellous bone, the trabeculae are arranged along the principal lines of stress. The cortex is thicker in the middle of the shaft of a long bone to resist bending forces, whereas the cancellous bone is concentrated at the ends of the long bones to resist the mainly compressive forces that are common at that site. It is important for the surgical pathologist to understand these physiologic and anatomic principles because without them, it is not easy to interpret the histologic findings in diseased connective tissues.

The quality and quantity of the extracellular matrix of the bone reflect cellular activity (7,8 and 9). Osteoblasts synthesize the collagenous matrix of the bone and regulate its mineralization. Osteoclasts, which may be multinucleated, are fewer in number than osteoblasts and are responsible for removing bone. For the amount of bone in the skeleton to remain stable, the rate of production must equal the rate of resorption; this equilibrium is often referred to as cell coupling or linkage. The rate of matrix production is reflected in the cytologic appearance of the osteoblasts, which range from flat (inactive) to plump (active) (Fig. 7.2). As bone formation proceeds, some of the osteoblasts are buried within the matrix and become osteocytes. The osteocytes maintain contact with each other and with the surface osteoblasts through cellular processes that run through a network of canaliculi within the bone matrix to form a functional syncytium, which is thought to play a major role in both mineral and structural homeostasis (Fig. 7.3). Cortical bone in adults is composed of closely applied, densely compact cylindrical units called osteons (Fig. 7.4). Primary osteons are formed in juveniles by the ingrowth of periosteal blood vessels that follow a “cutting cone” of osteoclasts, which tunnel through the existing cortex deposited by the periosteum. The tunnel,
or haversian canal, formed by the osteoclasts and containing the vessels is subsequently partially filled in by osteoblasts, which deposit concentric layers of bone matrix. Secondary and subsequent osteons are formed during the process of remodeling by the outgrowth from existing haversian systems of new vessels; these are also preceded by a cluster of osteoclasts. Cortical bone has a predominantly structural role, whereas cancellous bone prevails in day to day metabolic activity. Osteoblastic activity is probably positively influenced by increased physical activity, parathyroid hormone, and certain growth factors. Activity of the osteoblasts is suppressed by inactivity and steroid hormones (10,11 and 12). Throughout life, maintenance of normal healthy bone is dependent on adequate load.

When the osteoblasts synthesize collagen, it is deposited in parallel layers to produce the lamellar pattern that characterizes normal adult bone (Fig. 7.5). Time is required to achieve this high degree of organization, and when bone is formed and deposited rapidly, as in the fetus, in fracture callus, or in certain neoplastic and hypermetabolic states, it is less obviously organized, and the collagen has a basket-like weave. This type of bone is referred to as woven or immature bone (Fig. 7.6). This is always abnormal after the age of 3 years and should alert the pathologist to look for underlying pathology. (The microscopic examination of collagen is greatly aided by the use of polarized light, which demonstrates not only the collagen fibers but also their organization.)

The distribution of calcium in bone cannot be studied by examining decalcified sections. However, it is possible to study the mineralization of the matrix if undecalcified sections are prepared. For the study of those diseases in which disturbed mineralization is suspected, this examination is essential (13). When the organic matrix is first deposited by the osteoblast, it is not mineralized; therefore, a layer of unmineralized matrix, termed osteoid, is always present at the formative surface of bone (Fig. 7.7). Normally, this layer is very thin because the time between matrix deposition and subsequent mineralization is short; however, when the rates of deposition and mineralization are unbalanced, the amount and extent of the osteoid increase, and hyperosteoidosis may develop. Hyperosteoidosis occurs basically for three reasons. First, in any condition in which bone formation is increased, the osteoid surfaces are both more extensive and thicker (e.g., fracture callus, Paget disease, hyperparathyroidism). Second, the mineralization of bone depends on the presence of many substances, including
calcium, phosphorus, vitamin D, and alkaline phosphatase. If the amount of any of these substances is inadequate, mineralization is delayed, perhaps markedly. Third, the presence in bone of certain inhibitory or toxic substances, such as aluminum, iron, and fluoride, may block mineralization, even though the amounts of calcium, alkaline phosphatase, and vitamin D in the tissues are adequate.

Tetracycline binds to actively mineralizing surfaces and exhibits fluorescence in ultraviolet light on microscopic examination; these features have made it a valuable aid in the study of bone and bone diseases (14). Pulsed doses of tetracycline can be administered to a patient before biopsy to determine the extent of mineralization and the amount of bone formed over a given time (Fig. 7.8). Additionally, the morphology of the tetracycline labels reflects the cause of the disease process. When mineralization is blocked, especially in an aluminum-associated disease, the label is not taken up. On the other hand, in patients with vitamin D deficiency, the labels are often extensive and smudged, whereas they are sharp in persons with normal bone formation (Fig. 7.9). (It should be noted that tetracycline may be passively absorbed onto a recently resorbed surface and thus confuse interpretation.)

The resorption of bone by osteoclasts, rather than reflecting physiologic calcium homeostasis, is primarily associated with maintenance of the structural integrity of the skeleton. Morphologically, osteoclasts, which are large cells containing approximately two to four nuclei, adhere to the bone surface and are seen in depressions referred to as Howship lacunae or resorption bays (Fig. 7.10). In cancellous bone, resorption is generally limited to the bone surface, and the portions of the surface that have been resorbed can be recognized by an irregular, scalloped appearance, which contrasts with the smooth surface of formative or resting bone. Eventually, the resorptive surfaces are again covered by osteoblasts, and new bone forms on them. The junction between the original resorbed surface and the new bone is marked on sections stained with hematoxylin and eosin by a sharp basophilic line referred to as a reversal front or cement line (Fig. 7.11) (15).

In diseases characterized by increased remodeling, such as Paget disease and osteoporosis, the osteoclastic activity, although often markedly increased, is generally confined to the surface. However, in hyperparathyroidism, whether primary or secondary, the osteoclasts characteristically tunnel into the mineralized bone matrix, creating a “wormhole” appearance referred to as dissecting resorption/tunnel resorption (Fig. 7.12). However, regardless of the cause or patterns of resorption, the architecture of the bone is distorted in all pathologic resorptive states.