Endochondral Ossification

Endochondral Ossification

Endochondral ossification is a complex process compared to intramembranous ossificationBone formation from mesenchymal tissue, producing flat bones..  Endochondral ossification introduces us to a lot of new vocabulary.  Similar to intramembranous ossification, we have to start out with a model of the bone.  instead of starting with a sheet of dense connective tissue, the model is a hyaline cartilageThe most abundant cartilage type, found in joints, ribs, and the nose. model. It is made by chondrocytes, rather than the mesenchymal cellsThe basic structural and functional units of life. that create the model in intramembranous ossification.  Because cartilageA flexible connective tissue found in joints, the ear, nose, and rib cage. Cartilage can be of three is not a vascularizedTo invade a tissue with blood capillaries. tissue, oxygen and glucoseA simple sugar that is the main source of energy for cells. cannot reach the chondrocytes at the center of the model. These chondrocytes are cut off.   They begin to die, leaving their lacunae empty.  This results in a multitude of tiny holes in the cartilage model.  Osteoblasts Bone-forming cells that secrete osteoid. at the outer edges of the hyaline cartilage model begin to secrete bone. 

This forms the bone collar. It is a layer of compact boneDense, strong bone tissue forming the outer layer of bones.. This layer is along the outer edges of almost all bones in your body.  This collar provides support to the elongating bone. It functions much like a collar of a shirt, which can be starched to support a specific shape.  The osteoblasts at the outer edges begin the formation of the inner cellular layer of the periosteum. This separates the hyaline cartilage model from the surrounding tissues.  As all this happens, blood vessels are coming close periosteum and even perforating it in certain places.  When the blood vessels finally vascularize the center of the diaphysisThe shaft of a long bone., they form our first ossification center. It is called the primary ossification centerAn area where bone formation begins in the diaphysis of a long bone..  At the ossification center, osteoblasts are secreting osteoid and the matrix of the bone is mineralized with calcium. 

Slowly, the osteoblasts in the primary ossification center replace the cartilage of the diaphysis with bone.  This primary ossification center will ultimately turn into the medullary cavityThe hollow space inside bones that contains bone marrow.. This cavity contains the adipose connective tissue of the bone marrow.

Endochondral Ossification

As osteoclasts begin forming the medullary cavity, the bone becomes more vascularized. Perforating canalsCanals that connect central canals, allowing blood flow between osteons. run transverse, and the central canalsA hollow canal in an osteon containing blood vessels and nerves. of each osteon contribute to this vascularization.  An area referred to as the metaphysis forms.  This is the area where chondrocytes still remain and are creating cartilage via appositional growthThe process of bone widening and thickening by adding layers to the surface., or growthAn increase in size and number of cells. at the edge.  As they create more cartilage on the border of the diaphysis, the developing epiphysisThe expanded ends of a long bone. moves away from the diaphysis. This movementA fundamental property of life involving motion of the body or its parts. elongates the bone.  SpiculesSmall, needle-like pieces of bone found in spongy bone., something like the stalactites and stalagmites of caverns and caves, stick out into the medullary cavity of the diaphysis.  Here, osteoblasts are turning that newly formed cartilage into bone tissue. 

As the chondrocytes move away from the diaphysis, the osteoblasts actively chase them. They lay down bone to replace the hyaline cartilage.  The final steps of ossification occur with even more vascularization of the bone.  This time, these are called secondary ossification centers and are located in both the proximalCloser to the point of attachment or origin. and distal epiphyses. I feel like these last two diagram skip a bunch of steps between them.  To form these centers, chondrocytes have to die. Osteoblasts have to replace the cartilage with bone. Finally, osteoclasts will create the spongy boneA porous bone tissue found inside bones, providing lightweight support. and ossification centers. 

The Epiphyseal Plate

Activity at the metaphysis continues after birth.  This ever-expanding region is called the epiphyseal plateA growth plate in developing bones made of cartilage. or growth plates.  While the chondrocytes are still active, they push themselves away from the diaphysis to elongate the bone. During this time, these plates are referred to as “open” or “active.”  This means that the bones are still growing, or elongating. 

This histological inset here is of the epiphyseal plate.  At the top of the slide, we can see the hyaline cartilage. It has its characteristic look of chondrocytes in lacunae. The matrix has a glassy background.  That blue swath there is a bunch of chondrocytes doing mitosis and replicating themselves.  Mitosis is happening to the chondrocytes in the lacunae and new cartilage is being built in this zone.  But, the blue swath never really gets bigger.  It’s because the chondrocytes at the bottom of the blue swath are dying.  This white area here is the dying chondrocytes being calcified.

As this happens, osteoblasts are moving upward and replacing the newly formed and newly dead cartilage with bone.  I once heard someone say, “The osteoblasts chase the chondrocytes.” This pushes the epiphysis away from the diaphysis.  I like that because those chondrocytes are doing appositional growth only on this inferiorBelow or toward the lower part of the body. edge.  Every time they replicate themselves, they push themselves more and more superiorAbove or toward the upper part of the body..  Then, the osteoblasts ossify the dead remains of the cartilage and follow the chondrocytes that are moving superiorly.

Closing the Epiphyseal Plate

The activity of the chondrocytes has ceased. The osteoblasts are no longer replacing the newly formed cartilage with bone tissue. As a result, the epiphyseal plate becomes an epiphyseal lineA remnant of the epiphyseal plate, marking where growth stopped..  This means that growth has ceased and there is no longer any activity to elongate the bone. 

Closing of the plate to a line is influenced by estrogen and testosterone in puberty, among other things.

You can see epiphyseal plates of long bones in x-rays.  When we talk about long bones, we tend to think of the femurThigh bone; longest and strongest bone in the body; has a large round head and prominent trochanters and the humerusLong bone of the upper arm; articulates with scapula at shoulder and radius/ulna at elbow.. However, the metacarpalsPalm bones numbered thumb (1) to pinky (5); heads form knuckles., metatarsalsLong bones of the foot; numbered big toe (1) to pinky toe (5)., and all the phalangesFinger bones; each finger has three (proximal, intermediate, distal), thumb has two. are long bones too.  The open epiphyseal plate almost looks like an open space on an x-ray. It seems as though the epiphyses of the bone are not connected.  Once the plate closes, the epiphyseal line is made of all bone and there is no open space.  Thus, why we call them “open” plates to denote growth.