|  | Home
| School of Anatomy and Human Biology - The University of Western Australia
| Blue Histology - Skeletal Tissues - Bone

Topics | Lab Guides and Images
Histological Organisation of Bone | Compact Bone - diaphysis of a long bone - ground, unstained and Schmorl stained
Bone Cells and Matrix | Trabecular Bone - epiphysis of a long bone, H&E
Formation of Bone Intramembranous Ossification Endochondral Ossification | Intramembranous Ossification - developing human mandible, H&E
Endochondral Ossification and Epiphyseal Disc growing long bone and developing vertebral column, H&E
Reorganisation and Restoration of Bone

#### Additional Resources

These links will open a new browser window.

Large Images
  Search the Large Images page with these keywords: bone, trabecular bone, compact bone, Haversian system, osteon, canaliculi, lacunae, circumferential lamellae, interstitial lamellae, intramembranous ossification, endochondral ossification or epiphyseal plate.
VScope
  Magnification & Stage Simulation: bone formation, H&E - endochondral ossification
  Focus & Stage Simulation: compact bone, ground
  Iris & Light Simulation: compact bone, ground
Self Assessment
  Choose subject area "skeletal tissues" on the Quiz page. This subject area covers cartilage and bone.

### BONE

Bone is the main component of the skeleton in the adult human. Like cartilage, bone is a specialised form of dense connective tissue. Bone gives the skeleton the necessary rigidity to function as attachment and lever for muscles and supports the body against gravity.

Two types of bone can be distinguished macroscopically:

Trabecular bone (also called cancellous or spongy bone) consists of delicate bars and sheets of bone, trabeculae, which branch and intersect to form a sponge like network. The ends of long bones (or epiphyses) consist mainly of trabecular bone. Compact bone does not have any spaces or hollows in the bone matrix that are visible to the eye. Compact bone forms the thick-walled tube of the shaft (or diaphysis) of long bones, which surrounds the marrow cavity (or medullary cavity). A thin layer of compact bone also covers the epiphyses of long bones. |

Bone is, again like cartilage, surrounded by a layer of dense connective tissue, the periosteum. A thin layer of cell-rich connective tissue, the endosteum, lines the surface of the bone facing the marrow cavity. Both the periosteum and the endosteum possess osteogenic potency. Following injury, cells in these layers may differentiate into osteoblasts (bone forming cells) which become involved in the repair of damage to the bone. |

### Histological Organisation of Bone

#### Compact Bone

Compact bone consists almost entirely of extracellular substance, the matrix. Osteoblasts deposit the matrix in the form of thin sheets which are called lamellae. Lamellae are microscopical structures. Collagen fibres within each lamella run parallel to each other. Collagen fibres which belong to adjacent lamellae run at oblique angles to each other. Fibre density seems lower at the border between adjacent lamellae, which gives rise to the lamellar appearance of the tissue. Bone which is composed by lamellae when viewed under the microscope is also called lamellar bone.

In the process of the deposition of the matrix, osteoblasts become encased in small hollows within the matrix, the lacunae. Unlike chondrocytes, osteocytes have several thin processes, which extend from the lacunae into small channels within the bone matrix , the canaliculi. Canaliculi arising from one lacuna may anastomose with those of other lacunae and, eventually, with larger, vessel-containing canals within the bone. Canaliculi provide the means for the osteocytes to communicate with each other and to exchange substances by diffusion.

In mature compact bone most of the individual lamellae form concentric rings around larger longitudinal canals (approx. 50 µm in diameter) within the bone tissue. These canals are called Haversian canals. Haversian canals typically run parallel to the surface and along the long axis of the bone. The canals and the surrounding lamellae (8-15) are called a Haversian system or an osteon. A Haversian canal generally contains one or two capillaries and nerve fibres.

Irregular areas of interstitial lamellae, which apparently do not belong to any Haversian system, are found in between the Haversian systems. Immediately beneath the periosteum and endosteum a few lamella are found which run parallel to the inner and outer surfaces of the bone. They are the circumferential lamellae and endosteal lamellae.

A second system of canals, called Volkmann's canals, penetrates the bone more or less perpendicular to its surface. These canals establish connections of the Haversian canals with the inner and outer surfaces of the bone. Vessels in Volkmann's canals communicate with vessels in the Haversian canals on the one hand and vessels in the endosteum on the other. A few communications also exist with vessels in the periosteum.

#### Trabecular Bone

The matrix of trabecular bone is also deposited in the form of lamellae. In mature bones, trabecular bone will also be lamellar bone. However, lamellae in trabecular bone do not form Haversian systems. Lamellae of trabecular bone are deposited on preexisting trabeculae depending on the local demands on bone rigidity.

Osteocytes, lacunae and canaliculi in trabecular bone resemble those in compact bone.

? Note the distinction between macroscopic (visible to the eye) and microscopic (only visible under the microscope) appearance when the bone is named. Lamellar bone forms both trabecular bone and compact bone, which are the two macroscopically recognizable bone forms.

Suitable Slides
  sections of compact bone (usually part of the diaphysis of a long bone) - ground (unstained), Schmorl stained or H&E

Compact bone, human - Schmorl stain
Lamellae which run parallel to the surface of the bone are visible both on the outer, convex surface of the bone (circumferential lamellae) and on the inner, concave surface of the bone facing the marrow cavity (endosteal lamellae). The surface formed by the endosteal lamellae is often more irregular than the surface formed by the circumferential lamellae. The space between these two sets of lamellae is filled by Haversian systems and interstitial lamellae. Only few of the Haversian systems are "textbook" circular. Osteocyte lacunae are visible between the lamellae. Canaliculi become visible at high magnification (illustrated in the ground section below).
You should be able to see, draw and identify Haversian systems, interstitial and circumferential lamellae and/or endosteal lamellae.

Compact bone, human - ground (unstained)
The osteocyte lacunae are the main feature of the ground section. They are visible as elongated black spots in the bone matrix. Canaliculi, radiate from the lacunae into the surrounding bone matrix. Some lamellae are visible in the ground section. There is actually no distinct border between most lamellae, but our brain can use the elongated osteocyte lacunae and their orientation to "reconstruct" the lamellae. Volkman's canals connect to a few of the Haversian canals.
This is one of the cases where it may pay off to close down the iris diaphragm. The canaliculi should stand out more clearly if you do so. Remember to open the diaphragm afterwards!
Draw a small section of the a Haversian system at high magnification and identify in your drawing lacunae, canaliculi and (if visible) lamellae.

Suitable Slides
  sections of part of a vertebra or an epiphysis of a long bone - H&E, van Gieson
  Sections prepared to show articular cartilage will often also contain trabecular bone in the epiphysis below the articular cartilage.

Articular cartilage, bovine - H&E
Thin sheets and bars of bone, trabeculae, are visible at low magnification. Although they may appear as individual pieces in sections, they form an interconnected meshwork in the living bone. The spaces between the trabeculae, the marrow cavity of the epiphysis, is filled by either red bone marrow or yellow bone marrow. At high magnification, elongated osteocyte lacunae, which in well preserved tissue still contain osteocytes, are visible in the matrix. If the H&E stain also turned out well, it should be visible that the matrix of the trabecular bone is formed by lamellae. Canaliculi are present but hard to identify in most H&E stained sections. Haversian systems are not present in the trabeculae.
In trabecular bone obtained from young individuals, in which the bone is still growing, small areas of calcified cartilage are occasionally seen in the bone trabeculae. They are remnants of the cartilage scaffold on which bone matrix was deposited during the formation of the trabeculae. With the reorganisation of bone such areas will eventually be lost.
Draw trabecular bone at high and low magnification. Make sure that it is visible that the trabecular bone is also lamellar bone. Include enough detail of the marrow as you think you will need to identify this type of preparation.

### Bone Matrix and Bone Cells

#### Bone Matrix

Bone matrix consists of collagen fibres (about 90% of the organic substance) and ground substance. Collagen type I is the dominant collagen form in bone. The hardness of the matrix is due to its content of inorganic salts (hydroxyapatite; about 75% of the dry weight of bone), which become deposited between collagen fibres.

Calcification begins a few days after the deposition of organic bone substance (or osteoid) by the osteoblasts. Osteoblasts are capable of producing high local concentration of calcium phosphate in the extracellular space, which precipitates on the collagen molecules. About 75% of the hydroxyapatite is deposited in the first few days of the process, but complete calcification may take several months.

#### Bone Cells

Osteoprogenitor cells (or stem cells of bone)

  are located in the periosteum and endosteum. They are very difficult to distinguish from the surrounding connective tissue cells. They differentiate into

Osteoblasts (or bone forming cells).

  Osteoblasts may form a low columnar "epitheloid layer" at sites of bone deposition. They contain plenty of rough endoplasmatic reticulum (collagen synthesis) and a large Golgi apparatus. As they become trapped in the forming bone they differentiate into

Osteocytes.

  Osteocytes contain less endoplasmatic reticulum and are somewhat smaller than osteoblasts.

Osteoclasts

  are very large (up to 100 µm), multi-nucleated (about 5-10 visible in a histological section, but up to 50 in the actual cell) bone-resorbing cells. They arise by the fusion of monocytes (macrophage precursors in the blood) or macrophages. Osteoclasts attach themselves to the bone matrix and form a tight seal at the rim of the attachment site. The cell membrane opposite the matrix has deep invaginations forming a ruffled border. Osteoclasts empty the contents of lysosomes into the extracellular space between the ruffled border and the bone matrix. The released enzymes break down the collagen fibres of the matrix. Osteoclasts are stimulated by parathyroid hormone (produced by the parathyroid gland) and inhibited by calcitonin (produced by specialised cells of the thyroid gland). Osteoclasts are often seen within the indentations of the bone matrix that are formed by their activity (resorption bays or Howship's lacunae). |

### Formation of Bone

Bones are formed by two mechanisms: intramembranous ossification (bones of the skull, part of the mandible and clavicle) or endochondral ossification.

#### Intramembranous Ossification

Intramembranous ossification occurs within a membranous, condensed plate of mesenchymal cells. At the initial site of ossification (ossification centre) mesenchymal cells (osteoprogenitor cells) differentiate into osteoblasts. The osteoblasts begin to deposit the organic bone matrix, the osteoid. The matrix separates osteoblasts, which, from now on, are located in lacunae within the matrix. The collagen fibres of the osteoid form a woven network without a preferred orientation, and lamellae are not present at this stage. Because of the lack of a preferred orientation of the collagen fibres in the matrix, this type of bone is also called woven bone. The osteoid calcifies leading to the formation of primitive trabecular bone.

Further deposition and calcification of osteoid at sites where compact bone is needed leads to the formation of primitive compact bone.

? Note the distinction between macroscopic and microscopic appearance when the bone is named. We again have the two macroscopically different forms of bone - trabecular bone and compact bone - but their early developmental ("primitive") forms consist of woven bone.

Through subsequent reorganisation the primitive compact and trabecular bone is converted into mature compact and trabecular bone. During reorganisation and growth, woven bone will, in time, be replaced by lamellar bone.

Intramembranous ossification does not require the existence of a cartilage bone model.

Suitable Slides
  sections of the developing mandible (or some other bones of the skull) or clavicle - H&E, van Gieson
  Sections prepared to show endochondral ossification (see below) may be an alternative if no specifically prepared slides of intramembranous ossification are available. The periosteal collar, i.e. the manchette of bone forming around the diaphysis of the cartilage model of the bone, is formed by a mechanism similar to intramembranous ossification and results in the deposition of woven bone.

Mandible, intramembranous ossification - H&E
The developing bone will in sections usually be associated with a number of other tissues which develop in close association with it. In case of the mandible, there can be developing teeth, the tongue, skin and salivary glands.
The first job - best done at low magnification - is to find the developing bone. It should look like a coarse meshwork (trabecular bone) of pink tissue surrounding patches of much lighter or unstained tissue. Lamellae are not visible (woven bone) and the lacunae are larger than lacunae in mature bone. Ossification centres appear as areas of a gradual transition from connective tissue to bone. Light, pinkish bone matrix is found between the osteoblasts.
Depending on the state of development of the bone, it is occasionally possible to find bone trabeculae which are lined by a layer of osteoblasts. These osteblasts are depositing the first lamellae on the already existing trabeculae. The trabeculae will therefore have a core of woven bone, which is surrounded by lamellar bone. Compare the shapes, sizes and frequencies of lacunae in lamellar and woven bone if both types of bone are present.
Draw a part of trabecular bone consisting of woven bone and an ossification centre. Label the features in your drawings.

#### Endochondral Ossification

Most bones are formed by the transformation of cartilage "bone models", a process called endochondral ossification. A periosteal bud invades the cartilage model and allows osteoprogenitor cells to enter the cartilage. At these sites, the cartilage is in a state of hypertrophy (very large lacunae and chondrocytes) and partial calcification, which eventually leads to the death of the chondrocytes.
Invading osteoprogenitor cells mature into osteoblasts, which use the framework of calcified cartilage to deposit new bone. The bone deposited onto the cartilage scaffold is lamellar bone. The initial site of bone deposition is called a primary ossification centre. Secondary ossification centres occur in the future epiphyses of the bone.
A thin sheet of bone, the periosteal collar, is deposited around the shaft of the cartilage model. The periosteal collar consists of woven bone.

Close to the zone of ossification, the cartilage can usually be divided into a number of distinct zones :

1. Reserve cartilage, furthest away from the zone of ossification, looks like immature hyaline cartilage.
2. A zone of chondrocyte proliferation contains longitudinal columns of mitotically active chondrocytes, which grow in size in
3. the zone of cartilage maturation and hypertrophy.
4. A zone of cartilage calcification forms the border between cartilage and the zone of bone deposition.

Primary and secondary ossification centres do not merge before adulthood. Between the diaphysis and the epiphyses a thin sheet of cartilage, the epiphyseal plate, is maintained until adulthood. By continuing cartilage production, the epiphyseal plate provides the basis for rapid growth in the length of the bone. Cartilage production gradually ceases in the epiphyseal plate as maturity is approached. The epiphyseal plate is finally removed by the continued production of bone from the diaphyseal side.

Bone formation and bone resorption go hand in hand during the growth of bone. This first deposited trabecular bone is removed (By which cells?) as the zone of ossification moves in the direction of the future epiphyses. This process creates the marrow cavity of the bones. Simultaneously, bone is removed from the endosteal surface and deposited on the periosteal surface of the compact bone which forms the diaphysis. This results in a growth of the diameter of the bone.

Suitable Slides
  sections of bones during the early stages of their development - H&E, van Gieson

Foetal vertebral column, human - H&E
Hold the section against a light background. The cartilage models will stain very light, and the outlines of the skeletal structures they will form can often be identified. If an ossification centre is present, it will appear as a darker area within the cartilage model. The zonation of the cartilage should be visible in all ossification centres. How much bone is present depends on how far ossification has proceeded. The newly formed bone trabeculae will often consist of lamellar bone with a more or less extensive core of darkly staining calcified cartilage. The lamellar organisation of the bone may not be visible.
How the ossification centre exactly will look also depends on the plane of the section in relation to the ossification centre.
Draw the ossification centre/zone, include the adjacent cartilage and identify the zones within it in your drawing.

Suitable Slides
  sections of the epiphyseal disc of growing bones - H&E

| Growing bone, rabbit - H&E The cartilage model has almost entirely been transformed into bone. The only remaining cartilage is found in the epiphyseal disk. Zones of cartilage proliferation, hypertrophy and calcification are visible at high magnification, but only on one side of the epiphyseal disk - towards the diaphysis, which increases in length as the cartilage generated by the epiphyseal disc is transformed into bone. Osteoclasts may be found on the newly formed trabeculae or associated with parts of the cartilage scaffold. Draw the region of the epiphyseal disks. Identify in your drawing the epiphyseal disk and the bone of the epiphysis and diaphysis. Indicate the direction of cartilage proliferation in the epiphyseal disk and the direction of bone growth.

#### Reorganisation and Restoration of Bone | more about .... Bone Dynamics by Lis Mosekilde, M.D., D.Med.Sc.

Changes in the size and shape of bones during the period of growth imply some bone reorganisation. Osteoblast and osteoclast constantly deposit and remove bone to adjust its properties to growth-related demands on size and/or changes of tensile and compressive forces.

Although the reorganisation of bone may not result in macroscopically visible changes of bone structure, it continues throughout life to mend damage to bone (e.g. microfractures) and to counteract the wear and tear occurring in bone. Osteoclasts and osteoblasts remain the key players in this process. Osteoclasts "drill" more or less circular tunnels within existing bone matrix. Osteoblasts deposit new lamellae of bone matrix on the walls of these tunnels resulting in the formation of a new Haversian system within the matrix of compact bone. Parts of older Haversian systems, which may remain between the new ones, represent the interstitial lamellae in mature bone. Capillaries and nerves sprout into new Haversian canals.

Restorative activity continues in aged humans (about 2% of the Haversian systems seen in an 84 year old individual contained lamellae that had been formed within 2 weeks prior to death!). However, the Haversian systems tend to be smaller in older individuals and the canals are larger because of slower bone deposition. If these age-related changes in the appearance of the Haversian systems are pronounced they are termed osteopenia or senile osteoporosis. The reduced strength of bone affected by osteoporosis will increase the likelihood of fractures.

page content and construction: Lutz Slomianka
last updated: 23/05/04