Intramembranous Ossification

Calcification & Ossification

Let’s clear up a misconception right at the beginning of this lecture: calcificationThe hardening of tissues due to calcium salt deposition. and ossification are not the same things.  Calcification is the depositionThe process of bone matrix formation by osteoblasts. of calcium.  This isn’t restricted to bone. Calcium can be deposited on squishy organs like the aorta. The aorta is the large blood vessel that carries blood from the heart to all your systemic or body tissues.  Calcium deposits can occur through the overconsumption of calcium. The kidney can only clear a certain amount of calcium from the blood per day. 

In contrast ossification is the process of replacing existing tissue with bone tissue. Ectopic ossification is a condition in which squishy organs of the body are converted into bone. This is different than the calcification or hardened calcium deposits that can occur on squishy organs. Ossification is the official word referring to bone formation. Most bones are formed around the 12-week mark of fetus developmentThe process of growth and differentiation.. However, ossification continues into the first 20 years of life. This is because bones keep growing in length and width through childhood and adolescence. Beyond this, breaking a bone will induce ossification. Gaining weight and needing more support from underlying bone will also cause ossification. This ensures that bone is formed appropriately.

There are two types of ossification and these types relate to the types of bones that are formed. Intramembranous ossification is the formation of flat bones whereas endochondral ossificationBone formation replacing a cartilage model, common in long bones is the formation of long bones. Achondroplasia, the most common form of dwarfism, is an inherited disease in which long bones fail to elongate normally after formation.  The name of this disease (a-, without; chondro-, cartilageA flexible connective tissue found in joints, the ear, nose, and rib cage. Cartilage can be of three; -plasia, formation) indicates that the condition is a direct result of poor endochondral ossification at the growthAn increase in size and number of cells. plates.  This is why people with achondroplasia have shorter limbs and fingers. However, the pelvis, scapulae, and bones of the cranium can attain average sizes of growth. The long bones remain compromised in their ability to elongate.  

Intramembranous Ossification

Before we get started, it’s important to define mesenchymal cellsThe basic structural and functional units of life..  Mesenchymal cells could also be referred to as stem cells. These are cells that don’t yet have a specific identity. They could develop into a variety of cells, for example, chondrocytes, osteoblasts, or other types of skeletal-associated cells. Intramembranous ossification starts when osteoblasts gather within dense irregular connective tissue. This tissue is secreted by mesenchymal cells. 

The osteoblast Bone-forming cells that secrete osteoid. party is called an ossification center. The osteoblasts begin by secreting osteoid, which is the organic portion of bones connective tissue. Osteoid is not bone, but only the collagenA structural protein in the dermis that provides strength and elasticity. fibers and ground substance of bone.  Osteoid can be referred to as unmineralized bone or uncalcified bone.  It’s basically bone without the calcium salts.  The osteoblasts lay down this osteoid in sheets called lamella.  We learned from another mini-lecture that lamella come in a few different types. They vary depending on their location within the bone.  As osteoblasts secrete the osteoid they push themselves away from the osteoid in what is called appositional growthThe process of bone widening and thickening by adding layers to the surface..

Appositional is a word that refers to ‘on the edge’. It describes growth that is made from the edges. This growth comes from the outside of an ossification center.  Notice how the picture on the right has a bunch of osteoblasts surrounding the newly formed bone.  These osteoblasts will continue to secrete bone, pushing themselves outward.  They ultimately become the inner, cellular layer of the periosteum.  As the osteoblasts move away from a central point, some of them get surrounded by the osteoid.  The osteoid starts to calcify. This process further solidifies the osteoblasts trapped within the matrix of the newly formed bone tissue.

Ossification is the secretionThe process of moving substances from the blood into the nephron tubule to be excreted in urine. of the osteoid, the collagen fibers and ground substance of the bone tissue.  Recall how osteoid is only one of two parts of the matrix.  Calcification is the deposition of calcium salts into the osteoid.  Osteoid plus calcium salts equals matrix.  Therefore ossification AND calcification are both needed to complete the formation of bone.   

Intramembranous Ossification

The intramembranous ossificationBone formation from mesenchymal tissue, producing flat bones. continues around the blood vessels that vascularizeTo invade a tissue with blood capillaries. the area.  This, in part, forms the trabeculae that will support the spongy boneA porous bone tissue found inside bones, providing lightweight support..  The last steps in intramembranous ossification form the bone collar and the red marrowBone marrow that produces blood cells..  Notice how the picture on the right shows the osteoblasts on the superficialNear the surface of the body. ends of the bone. They form the inner cellular layer of the periosteum.  These osteoblasts continue to secrete osteoid, which is calcified into compact boneDense, strong bone tissue forming the outer layer of bones. to form the bone collar. 

These two layers of compact bone complete the formation of a flat bone or diploe.  Diploe is the orientation of spongy bone sandwiched between two layers of compact bone.  The compact bone is the bread and the spongy bone is whatever lunch meat you prefer.  The formation of the compact bone cuts off the blood vessels that vascularize the forming bone.  The vessels left inside stay in the spaces of the spongy bone, and red marrow forms around them, filling in the spaces of the spongy bone in your flat bones.