3.2 Hyaline Cartilage

Hyaline cartilage is the most common form of cartilage in the body. It is found in the joints covering the ends of long bones (called articular cartilage), and it forms the costal cartilages, connecting the sternum to the ventral ends of the ribs. Hyaline cartilage also forms portions of the nose, larynx, trachea, bronchial tubes, and makes up a large portion of the embryonic skeleton.

The fibers and matrix of cartilage are formed by cells called chondroblasts. Each chondroblast becomes enveloped by the fibers and matrix that it produces. As a result, these chondroblasts eventually occupy small spaces called lacunae. When cartilage formation is finished, these chondroblasts produce only enough matrix to maintain the cartilage and then they are called chondrocytes.

The overall organization of a segment of hyaline cartilage from the trachea is shown in image 3.2a. Note the numerous cartilage cells, or chondrocytes (labeled "Ch" in image 3.2a) which are embedded in a cartilage matrix (labeled "CM" in image 3.2a). Also, note at this time that the cartilage is covered by a layer of dense, irregular connective tissue called the perichondrium (labeled "Pe" in image 3.2a). The perichondrium is organized into an outer fibrous layer that consists primarily of collagenic fibers and an inner more cellular chondrogenic layer which can synthesize and secrete new cartilage matrix. Therefore, new layers of cells and matrix can be added to the cartilage surface by process known as appositional growth.

The nature of the lacunae is shown in image 3.2b. The lacunae (labeled "a" in image 3.2b) are the "places" that the chondrocytes occupy, and they tend to occur in clusters. These clusters form because the single chondrocytes aggregate and then undergo mitosis, producing the grouping pattern that is seen on the screen. Between these clusters is the plentiful matrix (labeled "b" and "c" in image 3.2b) which consists of fine collagen fibers and a ground substance of glycosamines and proteoglycans, repeating organic molecules that give hyaline cartilage a physical property like soft plastic.

Image 3.2c is a close-up of a cluster of lacunae. The arrow points to an area where two cartilage cells are separating due to the deposition of cartilage matrix in between them. This deposition, when completed, will leave the cluster with six lacunae in it.

Image 3.2d is a magnified view of the perichondrium. As stated above, the perichondrium consists of an outer fibrous layer (the layer above "a" in image 3.2d) and an inner more cellular chondrogenic layer (below "a" in image 3.2d). The flattened cells seen in the chondrogenic area are capable of differentiating into active chondroblasts (labeled "b" in image 3.2d) which then begin to secrete matrix, completing the production of hyaline cartilage.

3.3 Elastic Cartilage

Elastic cartilage maintains the shape of certain body parts such as the larynx, the external part of the ear (the pinna), and the auditory tubes (the internal ducts between the middle ear cavity and the throat). It provides a rubbery flexibility to these parts, giving them an important spring-back quality. In elastic cartilage, the large chondrocytes are located in a thick threadlike network of elastic fibers. Note the darkly stained elastic fibers and the numerous chondrocytes in image 3.3a.

Image 3.3b is a closer view of elastic cartilage. Note the numerous dark red elastic fibers (seen more clearly at the bottom of the slide) and the white lacunae which contain the chondrocytes. The lighter pink material is the matrix which has the rubbery, sponge-like nature.

3.4 Fibrocartilage

Fibrocartilage chondrocytes (arrows in image 3.4a) scattered through a light purple matrix are found in this type of cartilage. The design of fibrocartilage differs from the hyaline cartilage in that its collagenous fibers are arranged in thick, parallel bundles that give the matrix a coarse, ribbon-like appearance. Actually, fibrocartilage resembles regular connective tissue (because of its string-like organization) more than hyaline cartilage.

Image 3.4b shows a histological sample of fibrocartilage. The superabundance of tough collagen fibers makes fibrocartilage capable of withstanding strong impact. This shows a magnified view of the chondrocytes (labeled "a" in image 3.4b) found in fibrocartilage. Between these chondrocytes (red) the light-pink stained matrix can be seen. This matrix has dense layers of collagen fibers surrounded by a ground substance with a quality of tough plastic.

3.5 Chondrocyte

Image 3.5a shows a scanning electron micrograph of a cluster of chondroblasts (labeled "Ch" in image 3.5a) imbedded in their cartilage matrix (labeled "CM" in image 3.5a) in a section of hyaline cartilage. In addition to appositional growth from the perichondrium layer, cartilage can grow by the division of pre-existing cartilage cells within the matrix. This type of "interstitial" growth occurs primarily during embryonic development and can occur in the adult only as long as the cartilage matrix remains pliable enough to allow cell division. As a progenitor cell divides by mitotic division, it generates a clone of closely aggregated daughter cells which is called a "cell nest" (labeled "CN" in image 3.5a)- a cluster of four chondroblasts.

Image 3.5b is an example of chondrocytes sitting in their lacunae (at the tip of the arrow). Note how scattered these chondrocytes are compared to those in image 3.5a. This is a slide of fibrocartilage, and it illustrates well the dense concentration of thick layers of collagen fibers (stained in green and blue) that separates the chondrocytes. It is these fibers which give fibrocartilage its tough and resilient quality.

Image 3.5c is an example of chondrosarcoma, a tumor composed of malignant chondrocytes. Note the excessive number of chondrocytes, and observe their bizarre shape with densely-staining, irregular nuclei.

3.6 Perichondrium

Image 3.6a shows a scanning electron micrograph of a section of perichondrium. Most cartilage structures, with the exception of fibrocartilage and articular hyaline cartilage, are surrounded by a layer of dense, irregular connective tissue-the perichondrium. It is typically organized into an outer fibrous layer (labeled "FL" in image 3.6a) and an inner more cellular chondrogenic layer (labeled "CL" in image 3.6a). The fibrous layer consists primarily of collagen fibers (labeled "CF" in image 3.6a), while the chondrogenic layer consists of cells (arrows in image 3.6a) capable of differentiating into chondroblasts which synthesize and secrete new cartilage matrix. Note the conspicuous lack of blood vessels in this layer, so that the perichondrium, like cartilage itself, is considered a non-vascularized tissue.

Image 3.6b shows an example of a tumor of the upper femur, called a chondroblastoma. It generally forms in the epiphyses of large bones, such as the femur, the tibia, or the humerus. The tumor itself consists of primitive chondroblasts arranged in cellular masses with numerous calcific granules and reticular fibers lying in between the cell aggregates. The tumor typically expands by stimulating osteoclastic reabsorption and may perforate the outer cortex of the bone (as seen in image 3.6b), but generally remains confined within the periosteum.

3.7 Bone

Bone forms the major portion of the adult skeleton. Image 3.7a shows a longitudinal section of the femur which is the longest and heaviest bone in the human body. Its proximal end (far right of image 3.7a) articulates with the coxal bone, and the distal end (far left of image 3.7a) articulates with the tibia. Note the enlarged region located close to the distal end is a form of bone cancer called an osteogenic sarcoma.

A typical long bone (image 3.7b) may be divided into a shaft (diaphysis) and a knob-like expansion (epiphysis) at each end. The diaphysis is formed of a hollow cylinder of compact and spongy bone that surrounds a medullary cavity. This cavity, which is used as a fat storage site in the adult, is also called the bone marrow cavity. It is lined by a thin connective tissue layer called endosteum. The outer surfaces of the epiphyses are formed of compact bone and spongy (cancellous) bone fills the central region. The cavities of spongy bone are lined with endosteum and in the epiphyses of certain bones these cavities can contain active red bone marrow. In growing humans, there is a separation of the epiphysis and diaphysis called the epiphyseal plate, which consists of proliferating cartilage. It is here where the growth in length of long bones takes place. Growth in the diameter of a bone, by comparison, is due to the action of the periosteum layer which surrounds both the epiphyses and diaphysis.

3.8 Haversian System (Osteon)

Image 3.8a shows a cortical cross section of bone, illustrating Haversian systems. Note the centrally located Haversian canals (labeled "a" in image 3.8a), blood vessels which radiate from them (labeled "b" in image 3.8a), and the thick lamellar bony matrix between them (labeled "c" in image 3.8a). Identify the small red osteocytes (tip of the arrow in image 3.8a) which maintain this osteon (Haversian) structure.

A specimen of compact bone, whose cells and other organic constituents have been removed, is also useful for studying the organization of Haversian systems. In image 3.8b, a Haversian canal, formerly containing blood vessels, nerves, and a thin endosteal lining, can be observed. In this transverse section, the lamellar sheets of calcified bone matrix appear as concentric rings (black arrows in image 3.8b) surrounding each Haversian canal. Interposed among the lamellae are numerous elliptical depressions, or lacunae (labeled "La" in image 3.8b), which were the former locations of single osteocytes. Radiating from each lacuna are numerous minute canals, called canaliculi (labeled "Ca" in image 3.8b). During growth, and even in adulthood, there is a continuous process of reabsorption and rebuilding of the preexisting Haversian systems.

Image 3.8c is compact bone illustrating a Haversian canal (labeled "b" in image 3.8c) surrounded by concentric lamellae in which small black osteocytes (arrow in image 3.8c) are imbedded. In this slide fracture lines ( labeled "a" and "c" in image 3.8a) help demarcate the central osteon (with a haverisan canal, "b", at its center).

3.9 Osteocyte

When osteoblasts have finished their active bone formation and they retire to the much slower process of maintaining bone, they are called osteocytes. In image 3.9a, an osteocyte (labeled "Os" in image 3.9a) is seen in its lacuna (labeled "La" in image 3.9a), surrounded by bony matrix (labled "BM" in image 3.9a). From the osteocyte a number of long, slender osteocyte processes (labeled "OP" in image 3.9a) extend into the bone matrix through tunnels or canaliculi (labled "Ca" in image 3.9a). Many of these processes (arrows in image 3.9a) were broken during the fracture preparation of the specimen. Osteocyte processes are commonly in contact (via junctional complexes) with those extending from neighboring cells to form an interconnecting network. In some cases, the branching of osteocyte processes can occur. Nutrients and metabolites are thought to circulate within the narrow space that surrounds each osteocyte process, thereby facilitating the exchange between the blood vessels and the cells in bone.

Image 3.9b is an electron micrograph of an osteocyte sitting in its lacuna. The outer black zone is the mineralized matrix, and the gray-colored osteocyte is seen surrounded by a white, unmineralized zone of matrix which contains fine collagen fibers. Note the large nucleus of the osteocyte with its dark chromatin condensed under the nuclear membrane. Since this osteocyte has reduced its biosynthetic activity and is just in the business of matrix maintenance, its cytoplasm shows a reduction in the presence of organelles-Golgi, rough ER, and mitochondria.

Image 3.9c has an osteoblast cell, which is the much more active, younger version of the osteocyte. During the active deposition of new matrix osteoblasts arrange themselves in an epithelial layer of low columnar cells which are connected to one another by short, slender cytoplasmic processes. Similar to the prior slide on the osteocyte, the dark area to the left of the osteoblasts is the mineralized matrix and the white border area between the dark matrix and the gray-colored cell is the lighter-stained zone of unmineralized matrix, loaded with collagen fibers. This osteoblast is a very active biosynthetic cell, which shows extensive Golgi complexes and rough endoplasmic reticulum in its cytoplasm (note the wavy dark lines).

3.10 Osteoclast

During adulthood, and especially during growth, remodeling of the trabeculae within spongy bone occurs. This remodeling process involves the reabsorption of bone matrix from the trabecular surface by the activity of multinucleate cells called osteoclasts. Osteoclasts migrate over bony surfaces and secrete hydrolytic enzymes and acids which solubilize and reabsorb the bone matrix. This destruction of bone by osteoclasts is matched by the synthesis of bone by osteoblastic cells in the adult human. Thus, the human skeleton is a dynamic structure that is always in the process of renovation to meet the demands of the environment.

Image 3.10a depicts two osteoclasts (labeled "Oc" in image 3.10a) are seen sitting in their eroded cavities, known as Howship's lacunae (labeled "HL" in image 3.10a). On the inner surface of the two Howship's lacunae, collagen fibers (labeled "CF" in image 3.10a) have been revealed by the solubilization and removal of inorganic calcium phosphate crystals by the osteoclasts. Many of the crystals are in the form of small spherules (arrows in image 3.10a) because they are undergoing solubilization.

Image 3.10b is a light microscope slide of an osteoclast. This gigantic cell is located in the center of the slide and is typically characterized by its large size and multinucleated nature. First, note the immense size of the cell itself (compare its size to the very much smaller osteoblasts at the bottom of the slide). Osteoblasts have been measured as big as 100 microns in size (red blood cells are typically only 7-8 microns in diameter). Second, note the three large, light-purple nuclei in the cell, each with a very dark dense nucleolus inside. Osteoclasts have been observed with as many as 50 nuclei in a single cell.

Image 3.10c is another light microscope picture of osteoclasts (labeled "d" in image 3.10c) sitting in a Howship's lacunae (labeled "a" in image 3.10c) located just below some trabecular bone (labeled "b" and "c" in image 3.10c). Two hormones, parathormone and calcitonin, appear to have a strong effect on the activity of osteoclasts. Parathormone stimulates the osteoclasts to dissolve bone and release the calcium and phosphate ions into the blood. The hormone directly causes osteoclasts to release lysosomal acid hydrolases which digest collagen fibers and acids (citric and lactic) which dissolve the mineral matrix. Calcitonin has an opposite effect: it inhibits the action of osteoclasts-reducing their RNA synthesis, eliminating their ruffled membrane border which is required for active reabsorption, and finally separating their attachments to the bony surface.

3.11 Compact Bone

Image 3.11a shows a low-magnification view of the diaphyseal region of a femur which illustrates the clear difference between the outer compact bone layer, the cortex (labeled "CB" in image 3.11a) and the internal cancellous bone (labeled "CaB" in image 3.11a). In this slide there is also a cast of the internal vascular system. Note the numerous blood vessels (labeled "BV" in image 3.11a) which enter the marrow cavity through numerous openings called "Volkmann canals" (black arrow in image 3.11a) in the endosteal surface. The microscopic nature of this compact bone is typified by the Haversian organization seen in a previous (Osteon) card. Although cortical bone is usually thin compared to the overall diameter of a bone, it provides the major support structure and is the densest form of osseous tissue.

Image 3.11b is an example of Paget's disease, a relatively (4%) common bone disorder of the elderly (over 65 years). The diagnostic feature of this disease is an abnormal arrangement of lamellar (Haversian) bone, in which islands of irregular bone formation produce a mosaic pattern in the bone. In the cortex the osteons are destroyed and the concentric lamellae are incomplete. Typically, the bone thickens dramatically (as you can readily see in this cross section of the human skull). The major thickening occurs in the outer and inner cortical tables, which then narrows the central "diploe" (hemopoetic marrow) of the skull.

3.12 Spongy Bone

Image 3.12a is a view down the central diaphysis cavity of a developing femur. This is a preparation in which the organic component has been removed, so as to clearly illustrate the early development of internal cancellous, or spongy bone (labeled "CaB" in image 3.12a). Note that in the center of the shaft the bony spicules, called trabeculae (labeled "Tr" in image 3.12a), become more sparsely distributed, and, as this bone matures, these central trabeculae are the first to be reabsorbed to allow for the expansion of the marrow cavity. Note also the dense compact bone (labeled "CB" in image 3.12a) of the external shaft and also the numerous openings of the Volkmann canals (black arrows in image 3.12a) along the inner endosteal surface.

Image 3.12b is an example of a rather common metabolic bone disease, called osteoporosis. It is estimated that 15% of white women over 65 years of age have significant osteoporosis and 30% of those women will suffer bone fractures once they pass their 75th birthday. Numerous factors contribute to this disease, but the fractures usually occur in the humerus, forearm, hip, spine, and pelvis. This slide shows a specimen of spine, in which the vertebrae are shaped like purple-red hourglasses, separated by light-colored intervertebral discs. Note the thin outer cortex layer of the vertebrae and the poorly trabeculated cancellous bone inside of the vertebral body.

Image 3.12c is a histological example of eroded spongy bone due to osteoporosis. Note the distinctive lack of trabeculae (purple bony spicules). In normal bone these trabeculae would be more numerous, larger, and much more highly interconnected. This illustrates that the basic problem in this disease is that the rate of bone reabsorption exceeds that of bone formation.

3.13 Bone Marrow

Image 3.13a is a scanning electron micrograph of active (red) bone marrow as it sits in the marrow cavity. Note the dense packing of the developing bone marrow cells (labeled "Ma" in image 3.13a) and the extensive network of channels, called sinusoids, or venous sinuses (labeled "VS" in image 3.13a), through which blood flows. Most of the cellular portion of bone marrow is composed of developing blood cells, but reticular cells and adipose cells are also present. When these hemopoetic cells have developed sufficiently, they pass out into the sinusoids through tiny pores in the endothelial surface and they become mature functioning blood cells (labeled "BC" in image 3.13a) of the circulatory system.

Image 3.13b is a microscopic view of red bone marrow. Note the numerous developing blood cells (as identified by their purple nuclei in image 3.13b), the numerous sinusoids running between them, and the presence of plasma cells (at the tip of the arrow in image 3.13b).

Image 3.13c to see a histological example of a relatively common neoplastic disorder of bone marrow, acute myelogeneous leukemia. In this disease the bone marrow blast cells fail to mature, and therefore gradually the normal bone marrow cells are replaced by these immature, non-functional blast cells, all of which destroys the normal process of hemopoiesis. The result is progressive anemia, leukopenia, and thrombocytopenia. Histologically, the bone marrow appears diffusely hypercellular (observe the large masses of blue-gray cells in this specimen), consisting mostly of myeloblasts. This disease has been related to genetic defects, chemical exposure (benzene), and exposure to ionizing radiation.