Question

There are five stages of bone remodeling. They are the resting stage, bone resorption, transition, bone formation/deposition and mineralization. Define each stage to create a timeline of bone remodeling. Include the terms osteoblasts, osteoclasts and osteocytes.

Answer 1

1. Resting stage/Quiescence : Following mineralization, mature osteoblasts undergo apoptosis, revert back to a bone lining phenotype or become embedded in the mineralized matrix and differentiate into osteocytes. The resting bone surface environment is re-established and maintained until the next wave of remodelling is initiated.

2. Bone resorption : Bone remodeling (or bone metabolism) is a lifelong process where mature bone tissue is removed from the skeleton (a process called bone resorption) and new bone tissue is formed (a process called ossification or new bone formation).

3. Transition stage : Bone remodeling is a cycle consisting of three phases: “initiation” of bone resorption by osteoclasts, the “transition” from resorption to formation (or “reversal”), and the “bone formation.” To maintain bone quality and strength for the normal bone homeostasis.

4. Bone formation : Bone modeling is defined as either the formation of bone by osteoblasts or resorption of bone by osteoclasts on a given surface. This contrasts with bone remodeling (discussed below), in which osteoblast and osteoclast activity occur sequentially in a coupled manner on the same bone surface.

5. Bone mineralization, the process by which the organic bone matrix becomes filled with calcium phosphate nanocrystals, occurs in a specific, highly ordered process. The process is mediated by osteoblasts and confined to the organic osteoid matrix produced by osteoblasts.

OSTEOBLASTS are the cells that form new bone. They also come from the bone marrow and are related to structural cells. They have only one nucleus. Osteoblasts work in teams to build bone. They produce new bone called "osteoid" which is made of bone collagen and other protein.

Osteoclast: A cell that nibbles at and breaks down bone and is responsible for bone resorption. Osteoclasts are large multinucleate cells (cells with more than one nucleus) that differentiate from another type of cell called a macrophage.

Osteocytes are simply osteoblasts trapped in the matrix that they secrete. They are networked to each other via long cytoplasmic extensions that occupy tiny canals called canaliculi, which are used for exchange of nutrients and waste through gap junctions.