Bone tissue is made up of four different types of bone cells; Osteoblasts, osteocytes, osteoclasts and osteoprogenitor cells. Together, these cells form the compact and spongy layers of bone and work together to maintain the mineral composition and structure of the bones.

Bone cells are found in bone tissue
There are four types of bone cells in bone tissue

What are bone cells?

Bone cells are the cells that make up bone tissue. Bone is a highly specialized connective tissue with three main functions; to protect the internal organs, to create a rigid framework for muscle movement and to store minerals such as calcium and phosphorus.

Bone tissue can be classified as compact or spongy, depending on its density and function.

Types of bone tissue

Compact bone tissue

Compact bone (AKA cortical Bone tissue) is the hard material that makes up the outer layer of bone. Its main function is to strengthen the bones and protect the spongy tissue underneath.

Spongy bone tissue

Spongy Bone (AKA spongy Bone tissue) forms the inner layer of all bones. It is lighter and less dense than compact bone tissue. Its main functions are to give strength and flexibility to the bones and to act as a shock absorber in the event of trauma. Since it is lighter than compact bone tissue, the spongy layer also reduces the overall weight of the skeleton.

There are two types of bone tissue;  compact and spongy
Bone tissue consists of compact and spongy layers

Types of bone cells

Bone tissue is made up of four types of cells: osteoblasts, osteoclasts, osteocytes, and osteoprogenitor cells. Each of these cells has a unique function and together they maintain the homeostasis of bone tissue Bone reconstruction (the replacement of old bone tissue with new bone tissue). In doing so, they regulate the structure and function of the bone tissue.

Bone tissue contains four types of cells
There are four different types of bone cells

Functions of bone cells

Osteoblasts

Osteoblasts are cuboid cells that are arranged in a tightly packed layer along the surface of the bone. They make up 4-6% of all bone cells and their main function is to create new bone tissue. Osteoblasts are specially adapted for this role, with abundant rough endoplasmic reticulum and a large Golgi apparatus for abundant protein production.

These cells secrete a wide variety of substances, including collagen, various proteins, and calcium salts. Together these materials form the Bone matrix, A complex framework that supports the structure of the bones. The bone matrix also plays a key role in bone homeostasis as it releases molecules that affect the activity of bone cells and thus the remodeling of bone tissue.

Osteoblasts do not divide. Over time, the bone matrix grows and calcifies around them, trapping the cells in the center. This causes the osteoblast to change structure and become a mature bone cell known as Osteocytes.

Steoblasts play a key role in bone remodeling
Osteoblasts are responsible for the formation of new bone tissue

Osteocytes

The majority of bone cells are osteocytes, which are essentially osteoblasts that have been surrounded and trapped by the substances they secrete. Each osteocyte is in a small space (called a) gap), which is surrounded by bone tissue.

Osteocytes make up 90-95% of the cells in bone tissue. Like osteoblasts, they do not divide but have a long lifespan of up to 25 years.

Osteocytes have several important functions in maintaining the mineral composition of bone tissue. They are able to secrete and reabsorb bone, and signal to other osteocytes even if the bones are only slightly damaged. They regulate the activity of osteoblasts and osteoclasts and manage the calcium reservoir of the bone. Osteocytes therefore play a central role in bone remodeling.

Osteocytes communicate with one another via long channels within the aforementioned bone matrix Canaliculi, which they also use for the transfer of nutrients and waste products.

Osteocytes make up up to 95% of all bone cells
Osteocytes are the most common type of bone cell

Osteoclasts

Osteoclasts are large cells whose main function is to break down and reabsorb bone tissue. They are found on the surface of bone tissue and come from white blood cells (monocytes and macrophages) rather than other bone cells.

Osteoclasts are constantly breaking down and absorbing old bone tissue, while osteoblasts form new bone tissue. Together, these two cells control the gradual remodeling of the bones.

Osteoclasts are found on the surface of bone tissue
Osteoclasts break down and take up bone tissue again

Osteoprogenitor cells

Osteoprogenitor cells (AKA osteogenic cells) are the stem cells in bone tissue. Specialized bone cells (i.e., osteoblasts and osteocytes) come from osteoprogenitor cells in the bone marrow. Osteoclasts do not develop from osteogenic cells; instead, they come from blood stem cells in the bone marrow.

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