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What Two Type Of Cells Remove Excess Bone Tissue After Fracture Repair

Written By Tuesday, April 5, 2022 Add Comment Edit

Past the end of this section, you will be able to:

  • Identify the anatomical features of a bone
  • Define and listing examples of bone markings
  • Describe the histology of os tissue
  • Compare and contrast compact and spongy bone
  • Identify the structures that compose compact and spongy bone
  • Describe how bones are nourished and innervated

Bone tissue (osseous tissue) differs profoundly from other tissues in the body. Os is difficult and many of its functions depend on that characteristic hardness. Subsequently discussions in this affiliate volition show that bone is also dynamic in that its shape adjusts to suit stresses. This section volition examine the gross anatomy of bone first and and so move on to its histology.

Gross Anatomy of Os

The structure of a long bone allows for the all-time visualization of all of the parts of a bone (Effigy six.7). A long bone has two parts: the diaphysis and the epiphysis. The diaphysis is the tubular shaft that runs between the proximal and distal ends of the bone. The hollow region in the diaphysis is called the medullary cavity, which is filled with xanthous marrow. The walls of the diaphysis are composed of dense and hard compact bone.

This illustration depicts an anterior view of the right femur, or thigh bone. The inferior end that connects to the knee is at the bottom of the diagram and the superior end that connects to the hip is at the top of the diagram. The bottom end of the bone contains a smaller lateral bulge and a larger medial bulge. A blue articular cartilage covers the inner half of each bulge as well as the small trench that runs between the bulges. This area of the inferior end of the bone is labeled the distal epiphysis. Above the distal epiphysis is the metaphysis, where the bone tapers from the wide epiphysis into the relatively thin shaft. The entire length of the shaft is the diaphysis. The superior half of the femur is cut away to show its internal contents. The bone is covered with an outer translucent sheet called the periosteum. At the midpoint of the diaphysis, a nutrient artery travels through the periosteum and into the inner layers of the bone. The periosteum surrounds a white cylinder of solid bone labeled compact bone. The cavity at the center of the compact bone is called the medullary cavity. The inner layer of the compact bone that lines the medullary cavity is called the endosteum. Within the diaphysis, the medullary cavity contains a cylinder of yellow bone marrow that is penetrated by the nutrient artery. The superior end of the femur is also connected to the diaphysis by a metaphysis. In this upper metaphysis, the bone gradually widens between the diaphysis and the proximal epiphysis. The proximal epiphysis of the femur is roughly hexagonal in shape. However, the upper right side of the hexagon has a large, protruding knob. The femur connects and rotates within the hip socket at this knob. The knob is covered with a blue colored articular cartilage. The internal anatomy of the upper metaphysis and proximal epiphysis are revealed. The medullary cavity in these regions is filled with the mesh like spongy bone. Red bone marrow occupies the many cavities within the spongy bone. There is a clear, white line separating the spongy bone of the upper metaphysis with that of the proximal epiphysis. This line is labeled the epiphyseal line.

Figure 6.7 Beefcake of a Long Bone A typical long os shows the gross anatomical characteristics of bone.

The wider section at each end of the bone is called the epiphysis (plural = epiphyses), which is filled with spongy bone. Cherry marrow fills the spaces in the spongy bone. Each epiphysis meets the diaphysis at the metaphysis, the narrow expanse that contains the epiphyseal plate (growth plate), a layer of hyaline (transparent) cartilage in a growing os. When the bone stops growing in early on adulthood (approximately 18–21 years), the cartilage is replaced by osseous tissue and the epiphyseal plate becomes an epiphyseal line.

The medullary crenel has a delicate membranous lining called the endosteum (end- = "inside"; oste- = "bone"), where os growth, repair, and remodeling occur. The outer surface of the bone is covered with a gristly membrane called the periosteum(peri- = "around" or "surrounding"). The periosteum contains blood vessels, nerves, and lymphatic vessels that nourish compact bone. Tendons and ligaments also attach to bones at the periosteum. The periosteum covers the entire outer surface except where the epiphyses run across other basic to course joints (Figure 6.8). In this region, the epiphyses are covered with articular cartilage, a sparse layer of cartilage that reduces friction and acts as a stupor absorber.

The top of this illustration shows an anterior view of the proximal end of the femur. The top image has two zoom in boxes. The left box is situated on the border between the diaphysis and the metaphysis. Its callout magnifies the periosteum on the right side of the femur. The view shows that the periosteum contains an outer fibrous layer composed of yellow fibers. The inner layer of the periosteum is called the cellular layer, which is composed of irregularly shaped cells. The cellular layer gradually shrinks in width as it transitions from the metaphysis to the diaphysis. A small blood vessel runs through both layers and enters the bone. The right zoom in box magnifies the endosteum on the left side of the bone. The box is situated just inferior to the border between the diaphysis and the metaphysic. It calls out the inner edge of the compact bone layer. The magnified view shows concentric circles of dark colored bone matrix. Between the circles are small cavities containing orange, diamond-shaped cells labeled osteocytes. The left edge of the bone matrix is lined with a single layer of flattened cells called the endosteum. There is a large cell, labeled an osteoclast, between two of the endosteum cells. The osteoclast is cutting a depression into the bony matrix under the endosteum. At another part of the endosteum, three smaller osteoblasts are secreting a blue substance that builds up the outermost layer of the bony matrix.

Figure vi.viii Periosteum and Endosteum The periosteum forms the outer surface of os, and the endosteum lines the medullary crenel.

Apartment basic, like those of the cranium, consist of a layer of diploë (spongy bone), lined on either side past a layer of meaty os (Figure 6.9). The two layers of meaty 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 however protected by the intact inner layer.

This illustration shows a cross section of a cranial bone, constructed somewhat like a sandwich. The topmost and bottommost layers are the thin, translucent, periosteum. The upper and lower periosteum cover an upper and lower layer of compact bone, respectively. The compact bone is solid, with each layer occupying about one tenth of the thickness of the cranial bone. The majority of the cross section is occupied by the spongy bone, or diploe, sandwiched between the upper and lower compact bone. The spongy bone contains many crisscrossing threads of bone. Dark air spaces occur between the threads, giving the bone a porous appearance, much like that of a sponge or Swiss cheese.

Effigy 6.9 Anatomy of a Flat Bone This cross-section of a apartment bone shows the spongy os (diploë) lined on either side by a layer of compact bone.

Bone Markings

The surface features of basic vary considerably, depending on the office and location in the body. Table 6.2 describes the os markings, which are illustrated in (Effigy half dozen.10). There are 3 general classes of os markings: (1) articulations, (2) projections, and (three) holes. Equally the name implies, an articulation is where ii bone surfaces come together (articulus = "joint"). These surfaces tend to conform to one some other, such as one being rounded and the other cupped, to facilitate the function of the joint. A project is an area of a os that projects above the surface of the os. These are the attachment points for tendons and ligaments. In full 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 claret vessels and nerves to enter the bone. Equally with the other markings, their size and shape reverberate the size of the vessels and nerves that penetrate the bone at these points.

Os Markings

Marking Description Example
Articulations Where 2 bones run into Genu joint
Head Prominent rounded surface Caput of femur
Facet Flat surface Vertebrae
Condyle Rounded surface Occipital condyles
Projections Raised markings Spinous process of the vertebrae
Protuberance Protruding Chin
Procedure Prominence characteristic Transverse procedure of vertebra
Spine Sharp procedure Ischial spine
Tubercle Minor, rounded process Tubercle of humerus
Tuberosity Rough 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-scale pit Fovea capitis on the head of the femur
Sulcus Groove Sigmoid sulcus of the temporal bones
Canal Passage in bone Auditory canal
Fissure Slit through bone Auricular fissure
Foramen Hole through os Foramen magnum in the occipital os
Meatus Opening into canal External auditory meatus
Sinus Air-filled space in bone Nasal sinus

Table 6.two

This illustration contains three diagrams. The left diagram is titled examples of processes formed where tendons or ligaments attach. The image shows an anterior view of the femur and an anterior view of the humerus. For the femur, the distal epiphysis contains a smaller lateral bulge and a larger medial bulge. These are examples of condyles. The inner halves of the two condyles as well as the groove between them compose a facet. An oval-shaped ridge on the medial surface of the distal metaphysis is an example of a tubercle. On the proximal epiphysis of the femur, the large knob that attaches to the hip socket is an example of a head. The tip of the head contains a small depression, an example of a fovea called the fovea capitis. On the humerus, the distal epiphysis contains a central depression that is an example of a fossa. Two condyles are located on the right and left sides of the fossa. The diaphysis of the humerus contains a small ridge running up the shaft that is an example of a tuberosity. The proximal epiphysis of the humerus contains a lateral and a medial bulge that are both examples of tubercles. Finally, a narrow groove runs from the center of the proximal metaphysis in between the medial and lateral condyles. This is an example of a sulcus. The middle image is entitled elevations or depressions. It shows an anterior view of the hip bones. The hip bones are shaped like two wings that join at the bottom. The crest along the upper edge of each hip bones, at the tip of each

Figure 6.10 Bone Features The surface features of bones depend on their function, location, attachment of ligaments and tendons, or the penetration of blood vessels and nerves.

Bone Cells and Tissue

Bone contains a relatively small number of cells entrenched in a matrix of collagen fibers that provide a surface for inorganic salt crystals to 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 equally it crystallizes, or calcifies, on the collagen fibers. The hydroxyapatite crystals give basic their hardness and strength, while the collagen fibers give them flexibility so that they are not brittle.

Although bone cells compose a small-scale amount of the bone book, they are crucial to the part of basic. Four types of cells are found within bone tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts (Figure vi.eleven).

The top of this diagram shows the cross section of a generic bone with three zoom in boxes. The first box is on the periosteum. The second box is on the middle of the compact bone layer. The third box is on the inner edge of the compact bone where it transitions into the spongy bone. The callout in the periosteum points to two images. In the first image, four osteoblast cells are sitting end to end on the periosteum. The osteoblasts are roughly square shaped, except for one of the cells which is developing small, finger like projections. The caption says,

Effigy half dozen.xi Bone Cells Four types of cells are establish inside os tissue. Osteogenic cells are undifferentiated and develop into osteoblasts. When osteoblasts get trapped within the calcified matrix, their structure and function changes, and they get osteocytes. Osteoclasts develop from monocytes and macrophages and differ in appearance from other os cells.

The osteoblast is the bone cell responsible for forming new bone and is constitute in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which exercise non divide, synthesize and secrete the collagen matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast become trapped within it; as a result, it changes in structure and becomes an osteocyte, the principal 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 past bone tissue. 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 bone matrix.

If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones dice? 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 merely os cells that divide. Immature osteogenic cells are institute in the deep layers of the periosteum and the marrow. 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 jail 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 former bone while osteoblasts are continually forming new bone. The ongoing residuum betwixt osteoblasts and osteoclasts is responsible for the constant only subtle reshaping of bone. Tabular array 6.3reviews the bone cells, their functions, and locations.

Bone Cells

Cell type Function Location
Osteogenic cells Develop into osteoblasts Deep layers of the periosteum and the marrow
Osteoblasts Os germination Growing portions of os, including periosteum and endosteum
Osteocytes Maintain mineral concentration of matrix Entrapped in matrix
Osteoclasts Bone resorption Bone surfaces and at sites of old, injured, or unneeded os

Table half dozen.iii

Compact and Spongy Os

The differences betwixt compact and spongy bone are all-time explored via their histology. Most basic contain meaty and spongy osseous tissue, but their distribution and concentration vary based on the os's overall function. Compact os is dumbo so that it can withstand compressive forces, while spongy (cancellous) bone has open spaces and supports shifts in weight distribution.

Compact Bone

Meaty bone is the denser, stronger of the two types of bone tissue (Figure 6.12). It tin be plant under the periosteum and in the diaphyses of long bones, where information technology provides support and protection.

A generic long bone is shown at the top of this illustration. The bone is split in half lengthwise to show its internal anatomy. The outer gray covering of the bone is labeled the periosteum. Within the periosteum is a thin layer of compact bone. The compact bone surrounds a central cavity called the medullary cavity. The medullary cavity is filled with spongy bone at the two epiphyses. A callout box shows that the main image is zooming in on the compact bone on the left side of the bone. On the main image, the periosteum is being peeled back to show its two layers. The outer layer of the periosteum is the outer fibrous layer. This layer has a periosteal artery and a periosteal vein running along its outside edge. The inner layer of the periosteum is labeled the inner osteogenic layer. The compact bone lies to the right of the periosteum and occupies the majority of the main image. Two flat layers of compact bone line the inner surface of the ostegenic periosteum. These sheets of compact bone are called the circumferential lamellae. The majority of the compact bone has lamellae running perpendicular to that of the circumferential lamellae. These concentric lamellae are arranged in a series of concentric tubes. There are small cavities between the layers of concentric lamellae called lacunae. The centermost concentric lamella surrounds a hollow central canal. A blue vein, a red artery, a yellow nerve and a green lymph vessel run vertically through the central canal. A set of concentric lamellae, its associated lacunae and the vessels and nerves of the central canal are collectively called an osteon. The front edge of the diagram shows a longitudinal cross section of one of the osteons. The vessels and nerve are visible running through the center of the osteon throughout its length. In addition, blood vessels can run from the periosteum through the sides of the osteons and connect with the vessels of the central canal. The blood vessels travel through the sides of the osteons via a perforating canal. The open areas between neighboring osteons are also filled with compact bone. This

Effigy 6.12 Diagram of Compact Bone (a) This cantankerous-sectional view of meaty bone shows the basic structural unit, the osteon. (b) In this micrograph of the osteon, you tin clearly see the concentric lamellae and cardinal canals. LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2022)

The microscopic structural unit of meaty bone is called an osteon, or Haversian system. Each osteon is composed of concentric rings of calcified matrix chosen lamellae (singular = lamella). Running down the center of each osteon 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 culvert, also known as Volkmann's canals, to extend to the periosteum and endosteum.

The osteocytes are located inside spaces chosen lacunae (singular = lacuna), found at the borders of side by side lamellae. As described earlier, canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This system allows nutrients to be transported to the osteocytes and wastes to exist removed from them.

Spongy (Cancellous) Os

Like compact bone, spongy bone, too known as cancellous bone, contains osteocytes housed in lacunae, but they are not arranged in concentric circles. Instead, the lacunae and osteocytes are found in a lattice-like network of matrix spikes called trabeculae (atypical = trabecula) (Figure 6.13). The trabeculae may appear to be a random network, just each trabecula forms along lines of stress to provide strength to the bone. The spaces of the trabeculated network provide rest to the dumbo and heavy meaty bone by making bones lighter so that muscles can motion them more easily. In improver, the spaces in some spongy bones incorporate red marrow, protected past the trabeculae, where hematopoiesis occurs.

This illustration shows the spongy bone within the proximal epiphysis of the femur in two successively magnified images. The lower-magnification image shows two layers of crisscrossing trabeculae. The surface of each is dotted with small black holes which are the openings of the canaliculi. One of the trabeculae is in a cross section to show its internal layers. The outermost covering of the lamellae is called the endosteum. This endosteum surrounds several layers of concentric lamellae. The higher-magnification image shows the cross section of the trabeculae more clearly. Three concentric lamellae are shown in this view, each possessing perpendicular black lines. These lines are the canaliculi and are oriented on the round lamellae similar to the spokes of a wheel. In between the lamellae are small cavities called lacunae which house cells called osteocytes. In addition, two large osteoclasts are seated on the outer edge of the outermost lamellae. The outermost lamellae are also surrounded by groups of small, white, osteoblasts.

Figure six.xiii Diagram of Spongy Bone Spongy bone is composed of trabeculae that contain the osteocytes. Cherry marrow fills the spaces in some basic.

Aging AND THE...

Skeletal System: Paget'southward Illness

Paget's disease ordinarily occurs in adults over age twoscore. It is a disorder of the bone remodeling process that begins with overactive osteoclasts. This means more than bone is resorbed than is laid downwards. The osteoblasts try to compensate but the new bone they lay down is weak and brittle and therefore decumbent to fracture.

While some people with Paget's disease accept no symptoms, others experience pain, os fractures, and bone deformities (Figure 6.14). Bones of the pelvis, skull, spine, and legs are the near commonly affected. When occurring in the skull, Paget'due south disease tin can cause headaches and hearing loss.

This illustration shows the normal skeletal structure of the legs from an anterior view. The flesh of the legs and feet are outlined around the skeleton for reference. A second illustration shows the legs of someone with Paget's disease. The affected person's left femur is curved outward, causing the left leg to be bowed and shorter than the right leg.

Effigy vi.xiv Paget's Affliction Normal leg bones are relatively direct, but those affected past Paget'due south affliction are porous and curved.

What causes the osteoclasts to become overactive? The answer is still unknown, simply hereditary factors seem to play a role. Some scientists believe Paget'due south disease is due to an as-even so-unidentified virus.

Paget'due south disease is diagnosed via imaging studies and lab tests. X-rays may show 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 torso. Areas of bone resorption accept an affinity for the ion, so they volition 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 illness.

Bisphosphonates, drugs that decrease the activeness of osteoclasts, are often used in the treatment of Paget's disease. Nonetheless, in a modest percentage of cases, bisphosphonates themselves have been linked to an increased run a risk of fractures because the old bone that is left after bisphosphonates are administered becomes worn out and brittle. Even so, most doctors feel that the benefits of bisphosphonates more than than outweigh the risk; the medical professional person has to counterbalance the benefits and risks on a case-past-instance basis. Bisphosphonate treatment can reduce the overall adventure of deformities or fractures, which in plow reduces the take chances of surgical repair and its associated risks and complications.

Claret and Nerve Supply

The spongy os and medullary cavity receive nourishment from arteries that pass through the compact bone. The arteries enter through the nutrient foramen (plural = foramina), small openings in the diaphysis (Figure half-dozen.15). The osteocytes in spongy bone are nourished by claret vessels of the periosteum that penetrate spongy bone and blood that circulates in the marrow cavities. As the blood passes through the marrow cavities, information technology is collected by veins, which and so pass out of the bone through the foramina.

In addition to the blood vessels, fretfulness follow the aforementioned paths into the bone where they tend to concentrate in the more metabolically agile regions of the os. 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 os.

This illustration shows an anterior view if the right femur. The femur is split in half lengthwise to show its internal anatomy. The outer covering of the femur is labeled the periosteum. Within it is a thin layer of compact bone that surrounds a central cavity called the medullary or marrow cavity. This cavity is filled with spongy bone at both epiphyses. A nutrient artery and vein travels through the periosteum and compact bone at the center of the diaphysis. After entering the bone, the nutrient arteries and veins spread throughout the marrow cavity in both directions. Some of the arteries and veins in the marrow cavity also spread into the spongy bone within the distal and proximal epiphyses. However, additional blood vessels called the metaphyseal arteries and the metaphyseal veins enter into the metaphysis from outside of the bone.

Effigy vi.fifteen Diagram of Blood and Nerve Supply to Bone Blood vessels and fretfulness enter the bone through the nutrient foramen.

INTERACTIVE LINK

Watch this video to see the microscopic features of a bone.

Source: https://vivaopen.oercommons.org/courseware/lesson/112/overview

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