In simple words, an osteoblast builds the bone, whereas an osteoclast eats up the bone so that it can be reshaped into a stronger and resilient load-bearing structure. Other differences are mentioned below.
Osteoblasts or lining cells: Osteoblasts are considered the main type of bone cells. They regulate the passage of calcium in and out of the bone, and they express a special type of protein on their membranes by responding to various hormones. These specialized proteins activate another type of cells called osteoclasts:
- The main function of osteoblasts in bone formation and maintaining bone tissue integrity and shape.
- The cells of osteoblasts are small and have only one nucleus. Their cell structure is comparatively less complicated (contains a lesser number of cell organelles).
- These cells help in making a protein called osteoid, which helps in forming and maintaining the structure of the bone.
- The cells in each osteoblast unit are linked to each other by tight junctions and gap junctions. Gap junctions are tiny pores that connect multiple cells such that they become one functional compartment.
- Osteoblasts also produce chemicals called prostaglandins and an enzyme called alkaline phosphatase enzyme. Osteoblasts also produce a compound called hydroxyapatite that makes the bone harder. The matrix material in which they deposit hydroxyapatite is collagen, a much softer material that is synthesized and secreted by osteoblasts.
- Osteoblasts operate in groups. Each group oversees building an osteon unit of the bone.
- The osteoblast group synthesizes densely cross-linked collagen fibrils plus several other proteins needed in the bone matrix.
- The cells also have receptors that express parathyroid hormone.
- Osteoblasts produce anti-swelling or anti-inflammatory cytokines that help in the bone healing process.
Osteoclasts: They are large cells produced by the fusion (joining) of several smaller ones. Osteoclasts travel over the surface of the bone matrix and secrete acids and enzymes to disintegrate it, forming a little bit on the surface of the bone. They are found on the surface of the bone mineral next to the dissolving bone. This increases the surface area for the absorption of minerals. The minerals (in their ionic form) are absorbed into the osteoclasts, which later releases them into the tissue fluid located between the cells. From there, the ions enter the blood. The process of bone breakdown and mineral uptake by the osteoclasts is known as resorption:
- The main function of osteoclasts is reabsorbing the bone.
- The cells of osteoblasts are large and have many nuclei within them. Their cell structure is comparatively more complex (contains more number of cell organelles).
- They produce more phosphatase enzyme and tartrate-resistant acid.
- They come from the bone marrow and are related to white blood cells. They are formed from two or more cells that fuse together. The cells of osteoclasts are equipped with engulfs bone fragments mechanism.
- Osteoclasts usually dissolve collagen enzymes. They secrete enzymes and acid that break apart the bone matrix.
What are osteocytes?
Osteocytes are cells inside the bone. As osteoblasts mature, they become osteocytes. Osteoblasts turn into osteocytes while the new bone is being formed, and the osteocytes then get surrounded by the new bone. Once osteoblasts turn into osteocytes, they express different proteins and settle themselves into life as active bone regulatory cells. Osteocytes are the most abundant cell type in the bone, and they live about 25 years. Osteocytes function as part of the regulatory network that controls the body’s calcium and phosphate homeostasis. Osteocytes affect bone remodeling by producing regulatory factors to influence the activity of osteoblasts and osteoclasts in response to endocrine signals including the blood level of vitamin D. Osteocytes can sense pressures or cracks in the bone and help to direct where osteoclasts will dissolve the bone.