7.1. Cartilage

Fibrocartilage, intervertebral disc, human, Mallory's trichrome x700.

Th is fi gure shows the area circumscribed by the rectangle in the micrograph above at higher magnifi cation. Th e chondro- cytes are contained within lacunae (arrows), and their cytoplasm stains deeply. Th e surrounding cartilage matrix material is scant and blends into the dense connective tissue. Cartilage matrix mate- rial can be detected best by observing the larger group of chondrocytes at the left of this fi gure and then observing this same area in the fi gure above. Note the light homogeneous area around the cell nest in the lower power view. Th is is the region of cartilage matrix. At the greater magni- fi cation of this fi gure, it is possible to see that some of the collagen fi bers are incorporated in the matrix, where they appear as wispy bundles.

Hyaline cartilage, trachea, human, H&E x160.

The hyaline cartilage in this micrograph is from a specimen obtained shortly after death and kept cool during fixation. The procedure reduces the loss of its negatively charged sulfate groups; thus, the matrix is stained more heavily with hematoxylin. Also, note the very distinct and deeply stained capsules (arrows) surrounding the chondrocytes. The capsule represents the site where the sulfated glycosaminoglycans are most concentrated. In contrast to the basophilia of the cartilage matrix, the perichondrium (P) is stained with eosin. The lightly stained region between the perichondrium and the deeply stained matrix is matrix that has not yet matured. It has fewer sulfate groups.

Hyaline cartilage, trachea, human, H&E x850.

This higher magnification micrograph reveals the area within the rectangle in the lower left figure. The chondrocytes (Ch) in the upper part of the micrograph represent an isogenous group and are producing matrix material for interstitial growth. A prominent capsule is not yet evident. The lightly stained basophilic area reveals immature chondrocytes (arrows) within the perichondrium (P). Closest to the cartilage matrix, within the perichondrium (P), are several chondrocytes that exhibit just barely detectable cytoplasm and elongate nuclei (FCh). These cells are formative chondrocytes that are just beginning to, or will shortly, produce matrix material. In contrast, the nuclei near the bottom edge of the micrograph are fibroblast nuclei (Fib); they belong to the outer layer of the perichondrium. Note how attenuated their nuclei are compared with the formative chondroblast nuclei of the inner perichondrial layer.

Fibrocartilage, intervertebral disc, human, Mallory's trichrome x160.

This is a low-magnification view of fibrocartilage. The Mallory method stains collagen light blue. The tissue has a fibrous appearance, and at this low magnification, the nuclei of the fibroblasts (F) appear as small, elongate, or spindle-shaped bodies. There are relatively few fibroblasts present, as is characteristic of dense connective tissue. The chondrocytes (Ch) are more numerous and exhibit close spatial groupings, that is, isogenous groups. Some of the chondrocytes appear as elongate clusters of cells, whereas others appear in single-fi le rows. Th e matrix material immediately surround- ing the chondrocytes has a homogeneous appearance and is, thereby, distinguishable from the fi brous connective tissue.

Hyaline cartilage, trachea, human, H&E x450

This micrograph reveals hyaline cartilage from the trachea as seen in a routinely prepared specimen. The cartilage appears as an avascular expanse of matrix material and a population of cells called chondrocytes (Ch). The chondrocytes produce the matrix; the space each chondrocyte occupies is called a lacuna (L). Surrounding the cartilage and in immediate apposition to it is a cover of connective tissue, the perichondrium (P). The perichondrium serves as a source of new chondrocytes during appositional growth of the cartilage. Often, the perichondrium reveals two distinctive layers: an outer, more fibrous layer and an inner, more cellular layer. The inner, more cellular layer, containing chondroblasts and chondroprogenitor cells, provides for external growth. Cartilage matrix contains collagenous fibrils masked by a ground substance in which they are embedded; thus, the fibrils are not evident. The matrix also contains, among other components, sulfated glycosaminoglycans that exhibit basophilia with hematoxylin or other basic dyes. Also, the matrix material immediately surrounding a lacuna tends to stain more intensely with basic dyes. This region is referred to as a capsule (Cap). Not uncommonly, the matrix may appear to stain more intensely in localized areas (asterisks) that look much like the capsule matrix. This results from inclusion of a capsule within the thickness of the section but not the lacuna it surrounds. Frequently, two or more chondrocytes are located extremely close to one another, separated by only a thin partition of matrix. These are isogenous cell clusters that arise from a single predecessor cell. The proliferation of new chondrocytes by this means with the consequent addition of matrix results in interstitial growth of the cartilage.

Developing skeleton, fetal finger, human, thionine-picric acid x30.

This photomicrograph shows a developing long bone of the finger and its articulation with the distal and proximal bones. Before the stage shown here, each bone consisted entirely of a hyaline cartilaginous structure similar to the cartilages seen in the figure above but shaped like the long bones into which they would develop. Here, only the ends, or epiphyses, of the bone remain as cartilage, the epiphyseal cartilage (C). The shaft, or diaphysis, has become a cylinder of bone tissue (B) surrounding the marrow cavity (MC). The dark region at the ends of the marrow cavity is calcified cartilage (arrowhead) that is being replaced by bone. The bone at the ends of the marrow cavity constitutes the metaphysis. With this staining method, the calcified cartilage appears dark brown. The newly formed metaphyseal bone, which is admixed with this degenerating calcified cartilage and is difficult to define at this low magnification, has the same yellow-brown color as the diaphyseal bone. By the continued proliferation of cartilage, the bone grows in length. Later, the cartilage becomes calcified; bone is then produced and occupies the site of the resorbed cartilage. With the cessation of cartilage proliferation and its replacement by bone, growth of the bone stops, and only the cartilage at the articular surface remains. The details of this process are explained under endochondral bone formation (Plates 13 and 14).

Elastic cartilage, epiglottis, human, H&E and orcein stains x80.

This section of the epiglottis contains elastic cartilage (EC) as the centrally located, purple-stained tissue. The essential components of the cartilage, namely, the matrix containing elastic fibers, which stains purple, and the light, unstained lacunae surrounded by matrix, are evident in this low-magnification micrograph. The perimeter of the cartilage is covered by perichondrium (PC); its fi brous character is just barely visible in this figure. Epiglottis contains many small perforations (epiglottic foramina); note the presence of adipose tissue (AT) within these openings. Adipose tissue in this micrograph is visible within the boundaries of the elastic cartilage. Above and below the elastic cartilage is connective tissue, and each surface of the epiglottis is formed by stratifi ed squamous nonkeratinized epithelium (SE). Mucous glands (MG) are in the connective tissue in the bottom of this figure.

Developing skeleton, fetal foot, rat, H&E x85

This section shows the cartilages that will ultimately become the bones of the foot. In several places, developing ligaments (L) can be seen where they join the cartilages. The nuclei of the fibroblasts within the ligaments are just barely perceptible. They are aligned in rows and are separated from other rows of fibroblasts by collagenous material. The hue and intensity of the color of the cartilage matrix, except at the periphery, are due to the combined uptake of the H&E. The collagen of the matrix stains with eosin; however, the presence of sulfated glycosaminoglycans results in staining by hematoxylin. The matrix of cartilage that is about to be replaced by bone, such as that shown here, becomes impregnated with calcium salts, and the calcium is also receptive to staining with hematoxylin. The many enlarged lacunae (seen as light spaces within the matrix where the chondrocytes have fallen out of the lacunae) are due to hypertrophy of the chondrocytes, an event associated with calcification of the matrix. Thus, where these large lacunae are present, that is, in the center region of the cartilage, the matrix is heavily stained. This figure also shows that the cartilage is surrounded by perichondrium, except where it faces a joint cavity (JC). Here, the bare cartilage forms a surface. Note that the joint cavity is a space between the cartilages whose boundaries are completed by connective tissue (CT). The connective tissue at the surface of the cavity is special. It will constitute the synovial membrane in the adult and contribute to the formation of a lubricating fluid (synovial fluid) that is present in the joint cavity. Therefore, all the surfaces that will enclose the adult joint cavity are derived originally from the mesenchyme. Synovial fluid is a viscous substance containing, among other things, hyaluronan and glycosaminoglycans; it can be considered an exudate of interstitial fluid. The synovial fluid could be considered an extension of the extracellular matrix, as the joint cavity is not lined by an epithelium.

Elastic cartilage, epiglottis, human, H&E and orcein stains x250; inset x400.

This shows an area of the elastic cartilage at higher magnification. The elastic fibers appear as the purple, elongate profiles within the matrix. They are most evident at the edges of the cartilage, but they are obscured in some deeper parts of the matrix, where they blend with the elastic material that forms a honeycomb about the lacunae. Elastic fibers (E) are also apparent in the adipose tissue (AT), between the adipocytes. Some of the lacunae in the cartilage are arranged in pairs separated by a thin plate of matrix. The plate of matrix appears as a bar between the adjacent lacunae. This is a reflection of interstitial growth by the cartilage, in that the adjacent cartilage cells are derived from the same parent cell. They have moved away from each other and secreted a plate of cartilage matrix between them to form two lacunae. Most chondrocytes (Ch) shown in this figure occupy only part of the lacuna. Th is is, in part, due to shrinkage, but it is also due to the fact that older chondrocytes contain large lipid droplets that are lost during tissue preparation. The shrinkage of chondrocytes within the lacunae or their loss due to dropping out of the section during preparation causes the lacunae to stand out as light, unstained areas against the darkly stained matrix. The inset shows the elastic cartilage at still higher magnifi cation. Here, the elastic fibers (E) are again evident as elongate profiles, chiefly at the edges of the cartilage. Most chondrocytes in this part of the specimen show little shrinkage. Many of the cells display a typically rounded nucleus, and the cytoplasm is evident. Note, again, that some lacunae contain two chondrocytes, indicating interstitial growth.