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Aina Meducci 2012


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Bone formation

Alas, I knew I have to come to this part. You see, studying histology isn't my interest. However, to start Pathology next sem needs a new refreshment on this inter-linking subject with histology. So I randomly choose this topic from my old lecture notes.


Bone formation

Bone is formed either by direct ossification of embryonic connective tissue (intramembranous ossification) or by replacement of hyaline cartilage (intracartilaginous or endochondral ossification). Intramembranous ossification takes place in the so-called membrane bones of the skull, while endochondral ossification is characteristic of the bones of the trunk and extremities.

Intramembranous Ossification

Look at the slide above. This is the histological section of intramembranous ossification. This type of bone formation allows for the building of flat bones like the scapula, skull, and pelvic bones.

As seen under a microscope, membranous bones first appear as flat, membrane-like layers of early connective tissue (mesenchyme). These layers are provided with a constant flow of nutrient blood supply by networks of blood vessels (Bv) formed in between the layers. Early connective tissue cells first arrange themselves among the layers and then differentiate into bone-forming cells called osteoblasts (OB). The osteoblasts then remove calcium from the blood and deposit it among the bone matrix (the cartilage). As a result, layers of spongy bone are formed around the original cartilage. Later in development, spaces among the spongy bone are filled with bone matrix and become compact bone.

Osteoblasts continue to deposit calcium supplements into the matrix until it is totally surrounded by it. Once this occurs the osteoblasts are considered to be encased in a lacunae and now called osteocytes (Os). The original connective cells first formed around the network of blood vessels are now called the periosteum. Osteoblasts still not isolated in a lacunae can emerge from beneath the layer of compact bone and form layers of spongy bone over compact bone.

Two dense connective tissue "membranes", "P", separated by a thin space form early periosteum. Special fibroblasts within the dense CT differentiate into osteoblasts,"Ob", which begin to lay down bone martix and fibers, but not on a cartilage model. Bone matrix is built into trabeculae, "BS", which are shaped by dissolving osteoclasts, not shown. Within the trabeculae are osteocytes, "OC", in lacuna. When the spongy bone (trabeculae) is mature, the two CT "membranes" build a two layers of compact bone, covering and connecting the spongy bone on both sides with a smooth surface. Most of compact bone are derived from IO.

Endochondral Bone Formation

Endochondral ossification forms bone by replacing a cartilaginous model (hyaline cartilage), or precursor, that appeared there earlier in embryonic development. The cartilaginous models first undergo quick changes as the connective tissue cells enlarge which in turn destroys the surrounding matrix. Soon after, the connective tissue cells die. While the cells disintegrate, a periosteum is formed on the outside of the developing structure (a membrane with many blood vessels). Next blood vessels and undifferentiated cells raid on into the disintegrating tissue. Certain connective tissue cells differentiate and form spongy bone around the previous template of cartilage.

There are 5 stages os EO

  1. Resting zone
  2. Zone of proliferation (multiplication of cell)
  3. Zone of hypertrophy (cell hypertrophy and blood vessels embedded)
  4. Zone of degeneration (calcification and erosion)
  5. Zone of ossification (contain large no of osteoclast, bone absorbing cells)

The hyaline cartilage is the top half of the photo. "RC" is the label for the Reserve Zone. The chondrocytes are evenly spaced in the matrix (gel). The "PC" is the Proliferation Zone, where the chondrocytes divide so rapidly that they stack up from too little time between divisions to secrete much matrix and fibers (collagen). The "HC" is the Hypertrophic or Maturation Zone where the lacunae are enlarging and the chondrocytes are dying. The line between light and dark just below the "HC" is the Calcification Zone, where dissolved minerals are seeping into the hyaline cartilage matrix andcalcifying it. From here and below we call the matrix calcified hyaline cartilage matrix. Everything below the calcification zone is the Ossification or Resorption Zone. Osteoclasts, "Ocl", (blood cells) dissolve some calcified hayline cartilage matrix and leave spikes leftover called trabeculae (purple spikes). Osteoblasts, "Ob", (from the osteo-progenitor cells in the endosteum) sit on the surface of the calcified hyaline cartilage matrix and begin to secrete bone matrix (solid) and fibers (collagen), which stains pink around the edges of the original purple spike. When the osteoblasts have secreted enough matrix and fibers to surround themselves in lacunae, they are called osteocytes, "Oc".

Summary of bone formation


  • Transformed into compact bone
  • Bone development occurs within connective tissue membrane (mesenchyme)
  • Formed most flat bones

  • Transformed Hyaline to spongy or bone marrow (during embryonic formation)
  • Formed most skeletal element from neck down to certain cranial bones
  • Growth of log bones depends upon the no pf cartilaginous growth plates found at tissue end

Classes of bone

1. Compact Bone

  • Derived from IO
  • Supportive and weight-bearing role
  • Shaft long bones and cranial bones

2. Cancellous Bone

  • Derived from EO
  • Form marrow cavities at the end of long bones

Compact and Spongy bone

Types of bone

1. Long bones

These bones typically have an elongated shaft and two expanded ends one on either side of the shaft. The shaft is known as diaphysis and the ends are called epiphyses. Normally the epiphyses are smooth and articular. The shaft has a central medullary cavity where lies the bone marrow. Example of long bones; humerus, ulna, tibia,fibula

2. Short bones

These bones are short in posture and can be of any shape. Most of them are named according to their shape. Examples of this class of bones include cuboid, cuneiform, scaphoid, trapezoid etc. In fact all the carpal and tarsal bones are included in this category.

3. Irregular bone

The shape of these bones is completely irregular and they do not fit into any category of shape. Examples of this type of bones are vertebrae, hip bone and bones in the base of skull.

4. Flat bone


These bones are flat in appearance and have two prominent surfaces. They resemble shallow plates and form boundaries of certain body cavities. Examples include scapula, ribs, sternum etc

5. Sesamoid bone

These are not like the other types of bones because they are in the form of nodules embedded in tendons and joint capsules. They do not possess any periosteum and their ossification also takes place after birth. Examples of this type of bones are patella, pisiform and fabella.

Sources: projectskeleton.tripod.com, types of bone;A man anatomy,

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