Bone tissue (osseous tissue) differs greatly from other tissues in the body. Bone is hard and many of its functions depend on that characteristic hardness. Later discussions in this chapter will show that bone is also dynamic in that its shape adjusts to accommodate stresses. This section will examine the gross anatomy of bone first and then move on to its histology.
The structure of a long bone allows for the best visualization of all of the parts of a bone (Figure \(\PageIndex\)). A long bone has five zones: the diaphysis , two metaphyses, and two epiphyses . The diaphysis is the narrow, tubular shaft that runs between the two bulbous ends of the bone. These bulbous ends are the epiphyses (proximal and distal) and metaphyses (proximal and distal). Within the diaphysis is a hollow region called the medullary cavity , which is filled with yellow marrow (in adults). The walls of the diaphysis are composed of dense, hard compact bone . The epiphyses contain the bone parts that make up the joint structures, so they are covered with articular cartilage. The epiphysis extends from the end of the bone to the epiphyseal plate (growth plate in children) or epiphyseal line (in adults). The metaphyses extend from the epiphyseal plate/line to the diaphysis. Both the epiphyses and metaphyses have a thin cortical layer of compact bone that is filled with a porous bone arrangement called spongy bone. The spaces within the spongy bone are filled with red bone marrow containing the stem cells for blood cell production (the process of hematopoesis).
Figure \(\PageIndex<1>\): Anatomy of a Long Bone. Structures characteristic of a long bone, no matter how long or short it is, are the hollow diaphysis and the two metaphyses and epiphyses filled with compact bone. (Image credits: "Anatomy of a Long Bone" by Justin Greene is licensed under CC BY-NC-SA 4.0.)
The epiphyseal plate (growth plate) is a layer of hyaline cartilage in a growing bone. When the bone stops growing in early adulthood (approximately 18–21 years), the cartilage is replaced by osseous tissue and the epiphyseal plate becomes an epiphyseal line. The process of longitudinal bone growth is covered in more detail in section 5.4.
The medullary cavity has a delicate membranous lining called the endosteum (end- = “inside”; oste- = “bone”), where bone growth, repair, and remodeling occur. The outer surface of the bone is covered with a fibrous membrane called the periosteum (peri- = “around” or “surrounding”) (Figure \(\PageIndex\)). The periosteum consists of two layers; a superficial fibrous layer and a deeper cellular layer. Like the endosteum, the periosteum plays a role in bone growth, repair, and remodeling. It contains blood vessels, nerves, and lymphatic vessels that nourish compact bone, and also serves as a point of attachment for tendons and ligaments. The periosteum covers the entire outer surface except where the epiphyses meet other bones to form joints (Figure \(\PageIndex\)). In this region, the epiphyses are covered with articular cartilage , a thin layer of hyaline cartilage that reduces friction and acts as a shock absorber.
Flat bones, like those of the cranium, consist of a layer of diploë (spongy bone specifically found in skull bones), lined on either side by a layer of compact bone (Figure \(\PageIndex\)). The two layers of compact bone and the interior spongy bone work together to protect the internal organs. If the outer layer of a cranial bone fractures, the brain is still protected by the intact inner layer.
Bone contains a relatively small number of cells entrenched in a matrix of collagen fibers that provide a framework to which inorganic salts adhere. These salt crystals form when calcium phosphate and calcium carbonate combine to create hydroxyapatite, which incorporates other inorganic salts like magnesium hydroxide, fluoride, and sulfate as it crystallizes, or calcifies, on the collagen fibers. The hydroxyapatite crystals give bones their hardness and strength, while the collagen fibers give them flexibility so that they are not brittle.
Although bone cells compose a small amount of the bone volume, they are crucial to the function of bones. Four types of cells are found within bone tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts (Figure \(\PageIndex\)).
The osteoblast is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast becomes trapped within it; as a result, it changes in structure and becomes an osteocyte , the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a space called a lacuna and is surrounded by bone tissue. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes. Like osteoblasts, osteocytes lack mitotic activity. As osteoblasts differentiate into osteocytes structural changes occur that increase the surface area of the osteocyte cell membrane. Once trapped in lacunae, this increased surface area enables adjacent osteocytes to communicate with each other and receive nutrients via long cytoplasmic processes that extend through canaliculi (singular = canaliculus), channels within the bone matrix.
If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cells—the osteogenic cell . These osteogenic cells are undifferentiated with high mitotic activity and they are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the endosteum. They differentiate and develop into osteoblasts.
The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the osteoclast . They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone. Table \(\PageIndex\) reviews the bone cells, their functions, and locations.
| Cell type | Function | Location |
|---|---|---|
| Osteogenic cells | Undergo mitosis and develop into osteoblasts | Deep layers of the periosteum and the marrow |
| Osteoblasts | Bone formation | Growing portions of bone, including periosteum and endosteum |
| Osteocytes | Maintain mineral concentration of matrix | Entrapped in matrix within lacunae |
| Osteoclasts | Bone resorption | Bone surfaces and at sites of old, injured, or unneeded bone |
The differences between compact and spongy bone are best explored via their histology. Most bones contain compact and spongy osseous tissue, but their distribution and concentration vary based on the bone’s overall function. Compact bone is dense so that it can withstand compressive forces, while spongy (cancellous) bone has open spaces and supports shifts in weight distribution (Figure \(\PageIndex\)).
Compact bone is the denser, stronger of the two types of bone tissue (Figure \(\PageIndex\)). It can be found deep to the periosteum and in the diaphyses of long bones, where it provides support and protection.
The microscopic structural unit of compact bone is called an osteon , or Haversian system. Each osteon is composed of concentric rings of calcified matrix called lamellae (singular = lamella). The arrangement of collagen fibers within each lamella are aligned in a parallel direction that is diagonal with respect to the osteon for added strength in that direction. Then in adjacent lamellae, the directionality of the collagen fibers is perpendicular to those in the first so that when the lamellae are sandwiched together as concentric rings, the alternating pattern conveys triangulated strength to the osteon. Running down the center of each osteon, mostly parallel to the medullary cavity, is the central canal , or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. These vessels and nerves branch off at right angles through a perforating canal , also known as Volkmann’s canals, to extend to the periosteum and endosteum.
The osteocytes are located inside spaces called lacunae (singular = lacuna), found at the borders of adjacent lamellae. As described earlier, canaliculi radiating out from each lacuna, connect with the canaliculi of other lacunae and eventually with the central canal (Figure \(\PageIndex\) and Figure \(\PageIndex\)). This system allows nutrients to be transported to the osteocytes and wastes to be removed from them.
Other layers of extracellular matrix in compact bone surround and encircle the osteons. The interstitial lamellae are found between the osteons. These are remnants of old osteons that have been remodeled, leaving various shapes and layer numbers essentially filling in the gaps. Located deep to the periosteum are the circumferential lamellae (external and internal) that completely encircle the entire bone and are superficial to the osteons.
Like compact bone, spongy bone , also known as cancellous bone, contains osteocytes housed in lacunae, arranged in concentric lamellae. Unlike the osteons of compact bone, the sets of lamellae do not surround a hollow central canal and they are not arranged in parallel columns alongside each other, but rather in a lattice-like network of matrix spikes called trabeculae (singular = trabecula) (Figure \(\PageIndex\)). Each trabecula is wrapped in a layer of endosteum. The trabeculae may appear to be a random network, but each trabeculae forms along lines of stress to provide strength to the bone. The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where hematopoiesis occurs.
Figure \(\PageIndex<9>\): Spongy Bone Structure. A. Spongy bone is composed of trabeculae that contain the osteocytes. B. Histologically we can see each trabeculae has an exterior cellular layer containing osteoblasts and/or osteoclasts, depending on the need for their functions. (Image credits: "Spongy Bone Structure Illustration" by Justin Greene is licensed under CC BY-NC-SA 4.0 ; "Spongy Bone Histology @ 400x" by Jennifer Lange is licensed under CC BY-NC-SA 4.0; micrograph provided by Virginia Commonwealth University under CC BY-NC-SA 4.0.)
Skeletal System: Paget’s Disease
Paget’s disease affects approximately 4% of adults over age 40 that are of European descent. It is a chronic metabolic disorder of characterized by excessive abnormal bone remodeling that begins with overactive osteoclasts. This means more bone is resorbed than is laid down. The osteoblasts try to compensate but the new bone they lay down is weak and brittle and therefore prone to fracture.
While most people with Paget’s disease have no symptoms upon diagnosis (discovery is incidental finding on imaging for something unrelated), others experience pain, bone fractures, and bone deformities (Figure \(\PageIndex\)). The pelvis, spine, skull, and proximal long bones are most frequently affected. When occurring in the skull, Paget’s disease can cause headaches and hearing loss. The classic radiological appearances are expanded bone with a coarsened trabecular pattern, but appearance varies depending upon which bones are affected and the stage of the disease.
Figure \(\PageIndex<10>\): Paget's Disease. Normal bones are relatively straight, but those affected by Paget’s disease are porous and curved. These structural changes can be seen using both x-ray and radiographic tracing imaging. (Image credits: Illustration - "Feature Pagets Disease" by OpenStax is licensed under CC BY 4.0. Imaging - "Paget_Ulna_69jw_-_Roe_Unterarm_2_Eb_und_Szinti_frueh_spaet_-_001", CC BY-SA 3.0, via Wikimedia Commons)
What causes the osteoclasts to become overactive? The answer is still unknown, but hereditary factors seem to play a role. Some scientists believe Paget’s disease is due to an as-yet-unidentified virus.
Paget’s disease is diagnosed via imaging studies and lab tests. X-rays may show bone deformities or areas of bone resorption. Bone scans are also useful. In these studies, a dye containing a radioactive ion is injected into the body. Areas of bone resorption have an affinity for the ion, so they will light up on the scan if the ions are absorbed. In addition, blood levels of an enzyme called alkaline phosphatase are typically elevated in people with Paget’s disease.
Bisphosphonates, drugs that decrease the activity of osteoclasts, are often used in the treatment of Paget’s disease. However, in a small percentage of cases, bisphosphonates themselves have been linked to an increased risk of fractures because the old bone that is left after bisphosphonates are administered becomes worn out and brittle. Still, most doctors feel that the benefits of bisphosphonates more than outweigh the risk; the medical professional has to weigh the benefits and risks on a case-by-case basis. Bisphosphonate treatment can reduce the overall risk of deformities or fractures, which in turn reduces the risk of surgical repair and its associated complications.
The spongy bone and medullary cavity receive nourishment from three major groups of arteries that pass through the periosteum and compact bone. The nutrient artery enters through the nutrient foramen , a small opening in the diaphysis (Figure \(\PageIndex\)), where it branches to provide blood supply to the entire diaphysis. At the metaphyseal regions, the metaphyseal arteries enter the bone, while epiphyseal arteries enter and provide blood flow to the epiphyseal regions. As the blood passes through the marrow cavities, it is collected by veins, which then pass out of the bone through the same foramina that the arteries entered through.
In addition to the blood vessels, nerves follow the same paths into the bone where they tend to concentrate in the more metabolically active regions of the bone. The nerves sense pain, and it appears the nerves also play roles in regulating blood supplies and in bone growth, hence their concentrations in metabolically active sites of the bone.
The surface features of bones vary considerably, depending on the function and location in the body. Table \(\PageIndex\) describes the bone landmarks, which are illustrated in (Figure \(\PageIndex\)). There are three general classes of bone landmarks: (1) articular surfaces, (2) projections, and (3) holes. As the name implies, an articular surface is where two bone surfaces come together (articulus = “joint”). These surfaces tend to conform to one another, such as one being rounded and the other cupped, to facilitate the function of the articulation. A projection is an area of a bone that projects above the surface of the bone. These are the attachment points for tendons and ligaments. In general, their size and shape is an indication of the forces exerted through the attachment to the bone. A hole is an opening or groove in the bone that allows blood vessels and nerves to enter the bone. As with the other landmarks, their size and shape reflect the size of the vessels and nerves that penetrate the bone at these points.
| Landmark Type | Description | Examples |
|---|---|---|
| Articular Surfaces | Smooth surfaces where two bone slide past each other | |
| Head | Large, smooth, and rounded prominence typically located at an epiphyseal end | Head of femur |
| Facet | Very slight depression/flat area with smooth surface | Superior articular facet of vertebrae |
| Condyle | Rounded, smooth projection (often described as knuckle-like); in some cases, the condyle is slightly depressed | Occipital condyles; Tibial condyles |
| Projections | Raised markings for tendon/ligament attachment | |
| Epicondyle | Projection adjacent to a condyle | Medial epicondyle of the femur |
| Process | Projection of varying length | Transverse process of vertebra |
| Spine | Sharp, often thin, process | Ischial spine |
| Tubercle | Small, rounded projection | Tubercle of humerus |
| Tuberosity | Large, rough projection | Deltoid tuberosity |
| Trochanter | Massive, rough projection | Greater trochanter of the femur |
| Line | Slight, elongated ridge | Temporal lines of the parietal bones |
| Crest | Narrow, prominent ridge | Iliac crest |
| Holes and depressions | Allow for passage of soft tissue through/ along bone or for the formation of joints | |
| Fossa | Smooth depressions of varying depth | Subscapular |
| Cavity | Depression associated with joint articulation | Glenoid cavity |
| Fovea | Small pit | Fovea capitis on the head of the femur |
| Sulcus | Narrow groove | Sigmoid sulcus of the temporal bones |
| Canal | Passageway through a bone | Vertebral canal |
| Fissure | Slit through bone | Superior orbital fissure |
| Foramen | Round passageway through bone | Foramen magnum in the occipital bone |
| Sinus | Air-filled space in bone | Nasal sinus |
Figure \(\PageIndex<12>\): Examples of Bone Landmarks. The surface features of bones depend on their function, location, attachment of ligaments and tendons, or the penetration of blood vessels and nerves. Green = articular surfaces; Red = projections; Blue = holes and depressions. (Image credits: "Bone Landmarks" by Jennifer Lange are licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)
A hollow medullary cavity filled with yellow marrow runs the length of the diaphysis of a long bone. The walls of the diaphysis are compact bone. The epiphyses, which are wider sections at each end of a long bone, are filled with spongy bone and red marrow. The epiphyseal plate, a layer of hyaline cartilage, is replaced by osseous tissue as the organ grows in length. The medullary cavity has a delicate membranous lining called the endosteum. The outer surface of bone, except in regions covered with articular cartilage, is covered with a fibrous membrane called the periosteum. Flat bones consist of two layers of compact bone surrounding a layer of spongy bone. Bone markings depend on the function and location of bones. Articulations are places where two bones meet. Projections stick out from the surface of the bone and provide attachment points for tendons and ligaments. Holes are openings or depressions in the bones, and can serve as routes for blood vessels and nerves.
Bone matrix consists of collagen fibers and ground substance, primarily hydroxyapatite formed from calcium salts. Osteogenic cells develop into osteoblasts. Osteoblasts are cells that make new bone. They become osteocytes, the cells of mature bone, when they get trapped in the matrix. Osteoclasts engage in bone resorption. Compact bone is dense and composed of osteons, while spongy bone is less dense and made up of trabeculae. Blood vessels and nerves enter the bone through the nutrient foramina to nourish and innervate bones.
Q. Which of the following occurs in the spongy bone of the epiphysis?
B. bone remodeling
D. shock absorption
Answer
