Gross Anatomy of the Typical Long Bone Review Sheet 7
vi.3 Os Structure
Learning Objectives
By the cease of this section, you volition be able to:
Describe the microscopic and gross anatomical structures of bones
- Place the gross anatomical features of a bone
- Describe the histology of bone tissue, including the function of bone cells and matrix
- Compare and contrast compact and spongy bone
- Identify the structures that compose compact and spongy bone
- Describe how basic are nourished and innervated
- function?
Os tissue (osseous tissue) differs profoundly 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 likewise dynamic in that its shape adjusts to conform stresses. This section volition examine the gross anatomy of bone first and then move on to its histology.
Gross Beefcake of Bones
A long bone has two main regions: the diaphysis and the epiphysis (Effigy six.3.1). The diaphysis is the hollow, tubular shaft that runs betwixt the proximal and distal ends of the bone. Inside the diaphysis is the medullary cavity, which is filled with yellowish bone marrow in an adult. The outer walls of the diaphysis ( cortex, cortical bone)are composed of dumbo and difficult meaty bone, a form of osseous tissue.
The wider section at each end of the bone is called the epiphysis (plural = epiphyses), which is filled internally with spongy bone, some other blazon of osseous tissue. Carmine bone marrow fills the spaces between the spongy bone in some long bones. Each epiphysis meets the diaphysis at the metaphysis.During growth, the metaphysis contains the epiphyseal plate, the site of long bone elongation described later in the chapter. When the bone stops growing in early adulthood (approximately eighteen–21 years), the epiphyseal plate becomes an epiphyseal line seen in the figure.
Lining the inside of the os adjacent to the medullary cavity is a layer of bone cells called the endosteum (endo- = "within"; osteo- = "bone"). These bone cells (described later) cause the bone to grow, repair, and remodel throughout life. On the outside of bones in that location is another layer of cells that grow, repair and remodel bone as well. These cells are function of the outer double layered structure called the periosteum (peri– = "around" or "surrounding"). The cellular layer is side by side to the cortical bone and is covered by an outer fibrous layer of dense irregular connective tissue (encounter Figure 6.3.4a). The periosteum also contains claret vessels, nerves, and lymphatic vessels that nourish compact bone. Tendons and ligaments attach to bones at the periosteum. The periosteum covers the unabridged outer surface except where the epiphyses see other bones to form joints (Figure half dozen.3.2). 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 basic, like those of the cranium, consist of a layer of diploë (spongy bone), covered on either side by a layer of compact bone (Effigy 6.3.three). The two layers of compact bone and the interior spongy bone piece of work together to protect the internal organs. If the outer layer of a cranial bone fractures, the encephalon is still protected by the intact inner layer.
Osseous Tissue: Os Matrix and Cells
Os Cells
Although bone cells compose less than 2% of the os mass, they are crucial to the part of basic. Four types of cells are plant inside bone tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts (Figure 6.3.5).
The osteoblast is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the endosteum and the cellular layer of the periosteum. Osteoblasts, which exercise non separate, synthesize and secrete the collagen matrix and other proteins. Every bit the secreted matrix surrounding the osteoblast calcifies, the osteoblast go trapped inside it; as a result, information technology changes in construction and becomes an osteocyte, the main cell of mature bone and the about common blazon of os cell. Each osteocyte is located in a small cavity in the bone tissue called a lacuna (lacunae for plural). Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes. Like osteoblasts, osteocytes lack mitotic activity. They can communicate with each other and receive nutrients via long cytoplasmic processes that extend through canaliculi (atypical = canaliculus), channels within the os matrix. Osteocytes are connected to one some other within the canaliculi via gap junctions.
If osteoblasts and osteocytes are incapable of mitosis, and then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cells—the osteogenic (osteoprogenitor) cell. These osteogenic cells are undifferentiated with loftier mitotic activity and they are the only bone cells that dissever. Young osteogenic cells are found in the cellular layer 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 erstwhile, injured, or unnecessary bone is dissolved for repair or for calcium release. The cells responsible for bone resorption, or breakdown, are the osteoclasts. These multinucleated cells originate from monocytes and macrophages, ii types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking downwardly erstwhile bone while osteoblasts are continually forming new bone. The ongoing balance betwixt osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of os. Table 6.3 reviews the os cells, their functions, and locations.
| Bone Cells (Table 6.three) | ||
|---|---|---|
| Jail cell type | Role | Location |
| Osteogenic cells | Develop into osteoblasts | Endosteum, cellular layer of the periosteum |
| Osteoblasts | Bone formation | Endosteum, cellular layer of the periosteum, growing portions of bone |
| Osteocytes | Maintain mineral concentration of matrix | Entrapped in matrix |
| Osteoclasts | Bone resorption | Endosteum, cellular layer of the periosteum, at sites of old, injured, or unneeded bone |
Compact and Spongy Bone
Nigh bones incorporate meaty and spongy osseous tissue, just their distribution and concentration vary based on the bone's overall function. Although compact and spongy os are fabricated of the same matrix materials and cells, they are different in how they are organized. Compact bone is dense and so that information technology can withstand compressive forces, while spongy bone (likewise called cancellous bone) has open spaces and is supportive, but also lightweight and can be readily remodeled to accommodate changing trunk needs.
Compact Os
Compact bone is the denser, stronger of the two types of osseous tissue (Figure half dozen.iii.6). It makes upwardly the outer cortex of all bones and is in firsthand contact with the periosteum. In long basic, as you lot move from the outer cortical compact bone to the inner medullary cavity, the os transitions to spongy bone.
If you look at compact os under the microscope, you will observe a highly organized arrangement of concentric circles that look like tree trunks. Each group of concentric circles (each "tree") makes upwards the microscopic structural unit of measurement of compact bone chosen an osteon (this is also called a Haversian system). Each ring of the osteon is made of collagen and calcified matrix and is called a lamella (plural = lamellae). The collagen fibers of next lamallae run at perpendicular angles to each other, allowing osteons to resist twisting forces in multiple directions (see effigy six.34a). Running down the center of each osteon is the cardinal canal, or Haversian canal, which contains claret vessels, nerves, and lymphatic vessels. These vessels and nerves branch off at right angles through a perforating canal, also known every bit Volkmann'south canals, to extend to the periosteum and endosteum. The endosteum too lines each central canal, assuasive osteons to exist removed, remodeled and rebuilt over time.
The osteocytes are trapped within their lacuane, constitute at the borders of next lamellae. As described earlier, canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This organization allows nutrients to exist transported to the osteocytes and wastes to be removed from them despite the impervious calcified matrix.
Spongy (Cancellous) Bone
Like compact bone, spongy bone, too known every bit cancellous bone, contains osteocytes housed in lacunae, just they are not arranged in concentric circles. Instead, the lacunae and osteocytes are found in a lattice-similar network of matrix spikes called trabeculae (singular = trabecula) (Figure half dozen.3.8). The trabeculae are covered past the endosteum, which can readily remodel them. The trabeculae may announced to be a random network, simply each trabecula forms along lines of stress to straight forces out to the more solid compact os providing force to the os. Spongy bone provides remainder to the dumbo and heavy meaty bone by making bones lighter so that muscles tin can motility them more than easily. In addition, the spaces in some spongy bones contain red os marrow, protected past the trabeculae, where hematopoiesis occurs.
Aging and the…Skeletal Arrangement: Paget'due south Illness
Paget's disease usually occurs in adults over age 40. It is a disorder of the os remodeling process that begins with overactive osteoclasts. This ways more bone is resorbed than is laid downwardly. The osteoblasts try to compensate just the new bone they lay down is weak and brittle and therefore decumbent to fracture.
While some people with Paget'due south disease have no symptoms, others feel pain, bone fractures, and bone deformities (Figure 6.iii.nine). Bones of the pelvis, skull, spine, and legs are the nearly unremarkably affected. When occurring in the skull, Paget's affliction tin crusade headaches and hearing loss.
What causes the osteoclasts to go overactive? The answer is notwithstanding unknown, simply hereditary factors seem to play a part. Some scientists believe Paget'due south disease is due to an as-still-unidentified virus.
Paget's disease is diagnosed via imaging studies and lab tests. 10-rays may evidence bone deformities or areas of os resorption. Bone scans are also useful. In these studies, a dye containing a radioactive ion is injected into the body. Areas of bone resorption take an affinity for the ion, then they volition light upwards on the scan if the ions are absorbed. In addition, blood levels of an enzyme called element of group i 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.
Blood and Nerve Supply
The spongy bone and medullary cavity receive nourishment from arteries that pass through the compact bone. The arteries enter through the nutrient foramen (plural = foramina), small-scale openings in the diaphysis (Figure six.3.10). The osteocytes in spongy bone are nourished by blood vessels of the periosteum that penetrate spongy bone and blood that circulates in the marrow cavities. As the blood passes through the marrow cavities, it is collected by veins, which and then laissez passer out of the bone through the foramina.
In addition to the blood vessels, fretfulness follow the same paths into the bone where they tend to concentrate in the more than metabolically active regions of the bone. The fretfulness sense pain, and it appears the nerves also play roles in regulating claret supplies and in bone growth, hence their concentrations in metabolically active sites of the bone.
External Website
Watch this video to see the microscopic features of a bone.
Chapter Review
A hollow medullary cavity filled with yellowish marrow runs the length of the diaphysis of a long bone. The walls of the diaphysis are meaty bone. The epiphyses, which are wider sections at each stop 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 crenel has a delicate membranous lining called the endosteum. The outer surface of bone, except in regions covered with articular cartilage, is covered with a gristly membrane chosen the periosteum. Flat basic consist of two layers of compact os surrounding a layer of spongy bone. Bone markings depend on the function and location of basic. Articulations are places where 2 bones run across. 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.
Bone matrix consists of collagen fibers and organic footing substance, primarily hydroxyapatite formed from calcium salts. Osteogenic cells develop into osteoblasts. Osteoblasts are cells that make new bone. They get osteocytes, the cells of mature bone, when they get trapped in the matrix. Osteoclasts engage in os resorption. Compact os is dense and composed of osteons, while spongy bone is less dense and made upwardly of trabeculae. Blood vessels and fretfulness enter the bone through the nutrient foramina to nourish and innervate bones.
Review Questions
Critical Thinking Questions
1. If the articular cartilage at the finish of one of your long basic were to degenerate, what symptoms exercise you lot remember you lot would experience? Why?
2. In what means is the structural makeup of compact and spongy bone well suited to their respective functions?
Glossary
- articular cartilage
- thin layer of cartilage covering an epiphysis; reduces friction and acts as a shock absorber
- articulation
- where ii os surfaces meet
- canaliculi
- (atypical = canaliculus) channels within the bone matrix that house one of an osteocyte'due south many cytoplasmic extensions that it uses to communicate and receive nutrients
- central culvert
- longitudinal channel in the eye of each osteon; contains claret vessels, nerves, and lymphatic vessels; likewise known as the Haversian canal
- meaty bone
- dumbo osseous tissue that can withstand compressive forces
- diaphysis
- tubular shaft that runs between the proximal and distal ends of a long os
- diploë
- layer of spongy bone, that is sandwiched between two the layers of compact bone constitute in apartment bones
- endosteum
- delicate membranous lining of a os'due south medullary crenel
- epiphyseal plate
- (likewise, growth plate) sheet of hyaline cartilage in the metaphysis of an immature os; replaced past bone tissue equally the organ grows in length
- epiphysis
- wide section at each end of a long os; filled with spongy bone and red marrow
- hole
- opening or low in a os
- lacunae
- (singular = lacuna) spaces in a bone that firm an osteocyte
- medullary cavity
- hollow region of the diaphysis; filled with xanthous marrow
- nutrient foramen
- small opening in the middle of the external surface of the diaphysis, through which an artery enters the bone to provide nourishment
- osteoblast
- jail cell responsible for forming new bone
- osteoclast
- cell responsible for resorbing bone
- osteocyte
- primary prison cell in mature bone; responsible for maintaining the matrix
- osteogenic cell
- undifferentiated cell with high mitotic action; the only bone cells that separate; they differentiate and develop into osteoblasts
- osteon
- (likewise, Haversian system) bones structural unit of meaty bone; made of concentric layers of calcified matrix
- perforating canal
- (likewise, Volkmann's canal) aqueduct that branches off from the central canal and houses vessels and nerves that extend to the periosteum and endosteum
- periosteum
- fibrous membrane covering the outer surface of bone and continuous with ligaments
- projection
- os markings where part of the surface sticks out above the rest of the surface, where tendons and ligaments attach
- spongy bone
- (also, cancellous bone) trabeculated osseous tissue that supports shifts in weight distribution
- trabeculae
- (atypical = trabecula) spikes or sections of the lattice-like matrix in spongy bone
Solutions
Answers for Critical Thinking Questions
- If the articular cartilage at the finish of one of your long bones were to deteriorate, which is actually what happens in osteoarthritis, you would feel joint pain at the end of that bone and limitation of motility at that joint considering there would be no cartilage to reduce friction between adjacent bones and there would be no cartilage to human activity as a shock absorber.
- The densely packed concentric rings of matrix in compact bone are ideal for resisting compressive forces, which is the function of compact bone. The open spaces of the trabeculated network of spongy bone let spongy bone to support shifts in weight distribution, which is the function of spongy bone.
Bone Markings
Define and listing examples of bone markings
The surface features of bones vary considerably, depending on the role and location in the body. Table 6.2 describes the bone markings, which are illustrated in (Figure six.three.4). There are three general classes of bone markings: (ane) articulations, (2) projections, and (three) holes. As the name implies, an articulation is where ii bone surfaces come together (articulus = "joint"). These surfaces tend to conform to one another, such as ane existence rounded and the other cupped, to facilitate the part of the articulation. A projection is an area of a bone that projects above the surface of the os. These are the attachment points for tendons and ligaments. In general, their size and shape is an indication of the forces exerted through the zipper to the bone. A hole is an opening or groove in the bone that allows blood vessels and nerves to enter the bone. Equally with the other markings, their size and shape reflect the size of the vessels and nerves that penetrate the os at these points.
| Bone Markings (Table 6.two) | ||
|---|---|---|
| Marking | Description | Instance |
| Articulations | Where two basic meet | Genu articulation |
| Caput | Prominent rounded surface | Head of femur |
| Facet | Flat surface | Vertebrae |
| Condyle | Rounded surface | Occipital condyles |
| Projections | Raised markings | Spinous procedure of the vertebrae |
| Protuberance | Protruding | Mentum |
| Process | Prominence characteristic | Transverse process of vertebra |
| Spine | Abrupt process | Ischial spine |
| Tubercle | Small, rounded process | Tubercle of humerus |
| Tuberosity | Crude surface | Deltoid tuberosity |
| Line | Slight, elongated ridge | Temporal lines of the parietal bones |
| Crest | Ridge | Iliac crest |
| Holes | Holes and depressions | Foramen (holes through which blood vessels can pass through) |
| Fossa | Elongated basin | Mandibular fossa |
| Fovea | Small pit | Fovea capitis on the head of the femur |
| Sulcus | Groove | Sigmoid sulcus of the temporal bones |
| Canal | Passage in os | Auditory canal |
| Fissure | Slit through bone | Auricular cleft |
| Foramen | Hole through os | Foramen magnum in the occipital bone |
| Meatus | Opening into canal | External auditory meatus |
| Sinus | Air-filled infinite in bone | Nasal sinus |
Source: https://open.oregonstate.education/aandp/chapter/6-3-bone-structure/
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