Module 21: Bone Tissue and the Skeletal System

Lesson 3: Bone Structure

Cấu Trúc Xương

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Medical Terminology: Bone Tissue and the Skeletal System

articular cartilage
thin layer of cartilage covering an epiphysis; reduces friction and acts as a shock absorber
articulation
where two bone surfaces meet
bone
hard, dense connective tissue that forms the structural elements of the skeleton
canaliculi
(singular = canaliculus) channels within the bone matrix that house one of an osteocyte’s many cytoplasmic extensions that it uses to communicate and receive nutrients
cartilage
semi-rigid connective tissue found on the skeleton in areas where flexibility and smooth surfaces support movement
central canal
longitudinal channel in the center of each osteon; contains blood vessels, nerves, and lymphatic vessels; also known as the Haversian canal
closed reduction
manual manipulation of a broken bone to set it into its natural position without surgery
compact bone
dense osseous tissue that can withstand compressive forces
diaphysis
tubular shaft that runs between the proximal and distal ends of a long bone
diploë
layer of spongy bone, that is sandwiched between two the layers of compact bone found in flat bones
endochondral ossification
process in which bone forms by replacing hyaline cartilage
endosteum
delicate membranous lining of a bone’s medullary cavity
epiphyseal line
completely ossified remnant of the epiphyseal plate
epiphyseal plate
(also, growth plate) sheet of hyaline cartilage in the metaphysis of an immature bone; replaced by bone tissue as the organ grows in length
epiphysis
wide section at each end of a long bone; filled with spongy bone and red marrow
external callus
collar of hyaline cartilage and bone that forms around the outside of a fracture
flat bone
thin and curved bone; serves as a point of attachment for muscles and protects internal organs
fracture
broken bone
fracture hematoma
blood clot that forms at the site of a broken bone
hematopoiesis
production of blood cells, which occurs in the red marrow of the bones
hole
opening or depression in a bone
hypercalcemia
condition characterized by abnormally high levels of calcium
hypocalcemia
condition characterized by abnormally low levels of calcium
internal callus
fibrocartilaginous matrix, in the endosteal region, between the two ends of a broken bone
intramembranous ossification
process by which bone forms directly from mesenchymal tissue
irregular bone
bone of complex shape; protects internal organs from compressive forces
lacunae
(singular = lacuna) spaces in a bone that house an osteocyte
long bone
cylinder-shaped bone that is longer than it is wide; functions as a lever
medullary cavity
hollow region of the diaphysis; filled with yellow marrow
modeling
process, during bone growth, by which bone is resorbed on one surface of a bone and deposited on another
nutrient foramen
small opening in the middle of the external surface of the diaphysis, through which an artery enters the bone to provide nourishment
open reduction
surgical exposure of a bone to reset a fracture
orthopedist
doctor who specializes in diagnosing and treating musculoskeletal disorders and injuries
osseous tissue
bone tissue; a hard, dense connective tissue that forms the structural elements of the skeleton
ossification
(also, osteogenesis) bone formation
ossification center
cluster of osteoblasts found in the early stages of intramembranous ossification
osteoblast
cell responsible for forming new bone
osteoclast
cell responsible for resorbing bone
osteocyte
primary cell in mature bone; responsible for maintaining the matrix
osteogenic cell
undifferentiated cell with high mitotic activity; the only bone cells that divide; they differentiate and develop into osteoblasts
osteoid
uncalcified bone matrix secreted by osteoblasts
osteon
(also, Haversian system) basic structural unit of compact bone; made of concentric layers of calcified matrix
osteoporosis
disease characterized by a decrease in bone mass; occurs when the rate of bone resorption exceeds the rate of bone formation, a common occurrence as the body ages
perforating canal
(also, Volkmann’s canal) channel that branches off from the central canal and houses vessels and nerves that extend to the periosteum and endosteum
perichondrium
membrane that covers cartilage
periosteum
fibrous membrane covering the outer surface of bone and continuous with ligaments
primary ossification center
region, deep in the periosteal collar, where bone development starts during endochondral ossification
projection
bone markings where part of the surface sticks out above the rest of the surface, where tendons and ligaments attach
proliferative zone
region of the epiphyseal plate that makes new chondrocytes to replace those that die at the diaphyseal end of the plate and contributes to longitudinal growth of the epiphyseal plate
red marrow
connective tissue in the interior cavity of a bone where hematopoiesis takes place
remodeling
process by which osteoclasts resorb old or damaged bone at the same time as and on the same surface where osteoblasts form new bone to replace that which is resorbed
reserve zone
region of the epiphyseal plate that anchors the plate to the osseous tissue of the epiphysis
secondary ossification center
region of bone development in the epiphyses
sesamoid bone
small, round bone embedded in a tendon; protects the tendon from compressive forces
short bone
cube-shaped bone that is approximately equal in length, width, and thickness; provides limited motion
skeletal system
organ system composed of bones and cartilage that provides for movement, support, and protection
spongy bone
(also, cancellous bone) trabeculated osseous tissue that supports shifts in weight distribution
trabeculae
(singular = trabecula) spikes or sections of the lattice-like matrix in spongy bone
yellow marrow
connective tissue in the interior cavity of a bone where fat is stored
zone of calcified matrix
region of the epiphyseal plate closest to the diaphyseal end; functions to connect the epiphyseal plate to the diaphysis
zone of maturation and hypertrophy
region of the epiphyseal plate where chondrocytes from the proliferative zone grow and mature and contribute to the longitudinal growth of the epiphyseal plate
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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 1). 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 yellow marrow. The walls of the diaphysis are composed of dense and hard compact bone.

The wider section at each end of the bone is called the epiphysis (plural = epiphyses), which is filled with spongy bone. Red marrow fills the spaces in the spongy bone. Each epiphysis meets the diaphysis at the metaphysis, the narrow area that contains the epiphyseal plate (growth plate), a layer of hyaline (transparent) 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 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”). 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 meet other bones to form joints (Figure 2). In this region, the epiphyses are covered with articular cartilage, a thin layer of 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), lined on either side by a layer of compact bone (Figure 3). 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.
The surface features of bones vary considerably, depending on the function and location in the body. Table 1 describes the bone markings, which are illustrated in Figure 4. There are three general classes of bone markings: (1) articulations, (2) projections, and (3) holes. As the name implies, an articulation 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 markings, their size and shape reflect the size of the vessels and nerves that penetrate the bone at these points.
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 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 5).

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. They can 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 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 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 2 reviews the bone cells, their functions, and locations.
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. Compact bone is the denser, stronger of the two types of bone tissue (Figue 6). It can be found under 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). 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 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 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 be removed from them.

Like compact bone, spongy bone, also 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 (singular = trabecula) (Figure 7). The trabeculae may appear to be a random network, but each trabecula 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.
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 openings in the diaphysis (Figure 8). 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 then pass out of the bone through the foramina.

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.

OpenStax. (2022). Anatomy and Physiology 2e. Rice University. Retrieved June 15, 2023. ISBN-13: 978-1-711494-06-7 (Hardcover) ISBN-13: 978-1-711494-05-0 (Paperback) ISBN-13: 978-1-951693-42-8 (Digital). License: Attribution 4.0 International (CC BY 4.0). Access for free at openstax.org.

A typical long bone shows the gross anatomical characteristics of bone.

The periosteum forms the outer surface of bone, and the endosteum lines the medullary cavity.

This cross-section of a flat bone shows the spongy bone (diploë) lined on either side by a layer of compact bone.

MarkingDescriptionExample
ArticulationsWhere two bones meetKnee joint
HeadProminent rounded surfaceHead of femur
FacetFlat surfaceVertebrae
CondyleRounded surfaceOccipital condyles
ProjectionsRaised markingsSpinous process of the vertebrae
ProtuberanceProtrudingChin
ProcessProminence featureTransverse process of vertebra
SpineSharp processIschial spine
TubercleSmall, rounded processTubercle of humerus
TuberosityRough surfaceDeltoid tuberosity
LineSlight, elongated ridgeTemporal lines of the parietal bones
CrestRidgeIliac crest
HolesHoles and depressionsForamen (holes through which blood vessels can pass through)
FossaElongated basinMandibular fossa
FoveaSmall pitFovea capitis on the head of the femur
SulcusGrooveSigmoid sulcus of the temporal bones
CanalPassage in boneAuditory canal
FissureSlit through boneAuricular fissure
ForamenHole through boneForamen magnum in the occipital bone
MeatusOpening into canalExternal auditory meatus
SinusAir-filled space in boneNasal sinus

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

Four types of cells are found within bone tissue. Osteogenic cells are undifferentiated and develop into osteoblasts. When osteoblasts get trapped within the calcified matrix, their structure and function changes, and they become osteocytes. Osteoclasts develop from monocytes and macrophages and differ in appearance from other bone cells.

Cell typeFunctionLocation
Osteogenic cellsDevelop into osteoblastsDeep layers of the periosteum and the marrow
OsteoblastsBone formationGrowing portions of bone, including periosteum and endosteum
OsteocytesMaintain mineral concentration of matrixEntrapped in matrix
OsteoclastsBone resorptionBone surfaces and at sites of old, injured, or unneeded bone

(a) This cross-sectional view of compact bone shows the basic structural unit, the osteon. (b) In this micrograph of the osteon, you can clearly see the concentric lamellae and central canals. LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Spongy bone is composed of trabeculae that contain the osteocytes. Red marrow fills the spaces in some bones.

Blood vessels and nerves enter the bone through the nutrient foramen.

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Script:
  1. A long bone has a hollow medullary cavity running through the length of its diaphysis, filled with yellow marrow.
  2. The diaphysis walls are composed of compact bone.
  3. The wider sections at each end, known as epiphyses, contain spongy bone and red marrow.
  4. The epiphyseal plate, a layer of hyaline cartilage, is replaced by osseous tissue as the bone grows in length.
  5. The medullary cavity is lined with the delicate endosteum.
  6. The outer surface, except where covered by articular cartilage, is protected by the fibrous periosteum.
  7. Flat bones consist of two layers of compact bone surrounding a layer of spongy bone.
  8. Bone markings, such as articulations, projections for tendon and ligament attachments, and holes, depend on bone function and location.
  9. The bone matrix is composed of collagen fibers and organic ground substance, primarily hydroxyapatite formed from calcium salts.
  10. Osteogenic cells transform into osteoblasts, responsible for creating new bone.
  11. Osteoblasts mature into osteocytes when trapped in the matrix.
  12. Osteoclasts engage in bone resorption.
  13. Compact bone, dense and structured in osteons, contrasts with spongy bone, less dense and made up of trabeculae.
  14. Blood vessels and nerves enter bones through nutrient foramina to nourish and innervate them.
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