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How to Memorize Endochondral Ossification (Simplified)

There is a any easy way to memorize Endochodral ossification if you can remember the 5 main stages it consists of. If you remember the simplified names you will be able to fill in what these stages do. As an overview of ossification the bone starts as a solid and becomes hollow before you are born.

The 5  stages of endochondral ossification can be memorized with mnemonics. This mnemonic may be inappropriate so I have a clean version and a dirty version:

(clean)Bones Cant Pop During Exercise
(dirty)Boys Cant Pee During Erections

  1. Bone Collar Formation
  2. Cavitation
  3. Periosteal Bud Invasion
  4. Diaphysis Elongation
  5. Epiphiseal Ossification

When we talk about endochondral ossification we are talking about the development of long bones from a hyalin cartilage model.

Hyalin Cartilage Model

Bone Collar Formation

The primary ossification center develops in the center of the bone, and is the source of bone development. Osteoblasts found in the bone will secrete osteoid against the walls of the diaphysis. The diaphysis ,made of hyalin cartilage, is now encased in compact bone. This bone collar gives the developing bone structural support to begin hardening.

Bone Collar

Cavitation

The chondrocytes (cartilage cells) go through hypertrophication or enlargement and signal the hyalin cartilage to harden into bone. This hardening into bone is called calcification. The calcified hyalin cartilage is impermeable to the diffusion of nutrients. Since the chondrocytes cant receive any nutrients they begin to die and leave small cavities. These small cavities leave room in the hardened bone for blood vessels to travel through.

Cavitation

Periosteal Bud Invasion

Periosteal bud invasion is the introduction of a nutrient highway to the bone. Previously nutrients were delivered through the cartilage by simple diffusion. To support the capacity of the hardening bone, the periosteal region is invaded by a bud containing blood vessels and nerves. This allows not only nutrients, but osteoblasts and osteoclasts cells to enter into the cavities that were previously inhabited by chondrocytes. The osteoblasts secrete osteoid into the remaining hyalin cartilage and give rise to early spongy bone. Think of the periosteal bud invasion as a group of grave robbers who invaded the graves (cavities) where fallen chondrocytes used to rest.

Periosteal Bud Invasion

Diaphysis Elongation

After the nutrient source is delivered to the center of the bone, the diaphysis region has the resources to elongate. The elogated region is powered by cells dividing in the primary center of ossification. This elongated region is known as the medullary cavity. The medullary cavity is where the bone marrow is contained.

Diaphysis Elongation

Epiphyseal Ossification

Just before birth the ends of the bone or Epiphysis will develop their own centers of ossification. They cant stay made of cartilage or they would be structurally weak.  These centers are known as the secondary centers of ossification. They go through the same process as the primary center of ossification: hypertrophication, calcification, cavitation, and periosteal bud invasion.

Epiphyseal Ossification

 

Why not just grow completely from the center? Why do we need secondary centers? There is a region left between the primary and secondary centers of ossification known as the Epiphyseal Plate. This is more commonly known as the growth plate. This is how your bones grow after you are born.

Summary

Endochondral ossification can be summed into 5 major steps:

  1. Hypertrophication: Chondrocyte cells grow.
  2. Calcification: Hardening of hyalin cartilage matrix.
  3. Cavitation: Chrondrocytes die and leave cavities in the bone.
  4. Periosteal bud invasion: Nutrients are delivered to the bone via blood vessels, and nerves also enter.
  5. Epiphyseal Ossification: the bone ends develop (secondary) ossification centers.

There is now bone with compact bone on the outside that begun with the bone collar. The inside of the bone is spongy bone because of the cavities made by the chondrocytes. The ends still have hyalin cartilage on them which helps absorb shock.

 

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One comment

  1. only thing that i dint understand is
    if the osteoblasts enter in to the bone from the blood vessels , how can they b present in side the bone during the collar formation of bone…??

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