What Are Bone Cells?

Bone cells are the main cells in mature bone tissue, which is equivalent to human adulthood and transformed from osteoblasts.

Bone cells are the main cells in mature bone tissue, which is equivalent to human adulthood and transformed from osteoblasts.

Chinese name

Bone cells

Foreign name

osteocyte; osteocytes; bone cell;

To source

Osteoblast

Form

Cells with many protrusions, the nucleus is also oblate and stained deeply

Function

Create new matrix

Location

Bone tissue

Bone cell concept

When the new bone matrix is calcified, the cells are embedded in it. At this point, the cell's synthetic activity stops, the cytoplasm decreases, and it becomes a bone cell. Bone cells can generate a new matrix, change the crystalline fluid, and stabilize the calcium and phosphorus deposition and release of bone tissue to maintain blood calcium balance. Bone cells play a role in bone resorption and bone formation, and are the main cells that maintain the metabolism of mature bone.

Bone cell properties

The cellular components of bone tissue include osteoblasts, osteoblasts, bone cells and osteoclasts. Only bone cells exist in the bone tissue, and the other three cells are located at the edges of the bone tissue.

(1) Bone cells (osteocyte): Bone cells are flat elliptical cells with multiple protrusions, and the nucleus is also oblate and deeply stained. The cytoplasm is weakly basophilic. Under the electron microscope, there were a small amount of lysosomes, mitochondria, and rough endoplasmic reticulum in the cytoplasm, and the Golgi complex was not well developed. Bone cells are sandwiched between two adjacent bone plates or are scattered in the bone plates. There are gap connections between the protrusions of adjacent bone cells. In the bone matrix, the oval-shaped cavity occupied by the bone cell body is called bonelacun, and the space where the protrusion is located is called bonecanaliculi. Adjacent bone pits are connected to each other by bone tubules. Bone pits and tubules contain interstitial fluid from which bone cells obtain nutrients.

(2) Osteogenic cells: Osteogenic cells are stem cells in bone tissue. The cells are spindle-shaped, the cell body is small, the nucleus is oval, and the cytoplasm is weakly basophilic. Osteoblasts exist in the inner layer of the periosteum and endosclera and in the central tube, near the surface of the bone matrix. It can divide and proliferate and differentiate into osteoblasts during bone growth and development, or during bone remodeling or bone tissue repair in adulthood.

(3) Osteoblasts (osteoblasts): Osteoblasts are differentiated from osteoblasts, which are larger than osteoblasts, with a short columnar or cubic shape, and with small protrusions. The nucleus is large and round with clear nucleoli. The cytoplasm is basophilic and is rich in alkaline phosphatase. Under the electron microscope, there were a large number of rough endoplasmic reticulum, free ribosomes, and developed Golgi complexes, and there were more mitochondria in the cytoplasm. When bone grows and regenerates, osteoblasts are arranged in a regular layer on the surface of bone tissue, secrete matrix and fibers to the surroundings, and embed itself in them to form osteoid. It becomes bone tissue, and osteoblasts mature into bone cells.

Osteoblasts release membrane-coated vesicles, called matrixvesicles, into the osteoid in the manner of apical secretion, with a diameter of about 0.1 m. There are a large number of alkaline phosphatase and ATPase on the vesicle membrane, and the vesicle contains phospholipids and small calcium salt crystals. Matrix vesicles are generally considered to be important structures for osteoid calcification. Current scientific research suggests that osteoblasts can secrete osteocalcin into the matrix.

(4) Osteoclast: Osteoclast is a large multinucleated cell with a diameter of up to 100 m and 2 to 50 nuclei. It has a strong cytoplasmic eosinophilicity. It is much smaller than osteoblasts. Mostly located in the pit formed by the bone tissue absorption site. Under the electron microscope, there are many high and dense microvilli on the side of the osteoclast near the bone tissue, forming a ruffledborder. The cytoplasm at the base contains a large number of lysosomes and swallowing vesicles. The vesicles contain small Calcium salt crystals and dissolved organic ingredients. There is a circular cytoplasmic area around the fold edge, which contains only microfilaments and few other organelles, known as the clearzone. The cell membrane in the bright area is flat and clings to the surface of the bone tissue, just like a fence around the fold edge, forming a microenvironment at the closed fold edge. Osteoclasts can release a variety of proteases, carbonic anhydrase, and lactic acid into it to dissolve bone tissue. It is believed that osteoclasts are a fusion of multiple monocytes.

Histomorphological characteristics of osteocytes

Osteocyte is the most important cellular component in human bones. In adult bones, bone cells account for 90% to 95% of the total number of cells, about 20 times the number of osteoblasts. However, little is known about bone cells in the past. Until the last 30 years, more and more scholars have begun to focus on the research of bone cells.

Unlike osteoblasts (Osteoblast) and osteoclasts (osteoclasts) located on the bone surface, bone cells grow in the bone matrix inside the bone. The cell body of osteocytes is fusiform or round, and is located in the osteocyticlacuna formed by the matrix. Similar to nerve cells, each bone cell has a large number of outwardly extending synapses (dendrities), and these synapses are located in the canaliculi structure composed of bone matrix. Through these synapses, bone cells can "communicate" with cells on the bone surface and other bone cells around them. It is the existence of this pit-tubule system that has given the anatomical basis for certain specific functions of bone cells. The diameter of the long axis of the human osteocyte cell body is about 20 m, and the diameter of the short axis is about 10 m. The diameter of the synapse is considered to be about half the diameter of the bone tubules, and the diameter of the bone tubules is between 50 nm and 100 nm.

It is generally believed that osteocytes originate from osteoblasts. In the end stage of bone formation, osteoblasts may have three different fate: differentiation into osteocytes, transfer to the bone surface to become temporarily inactive osteoblasts, and entering the process of apoptosis (apoptosis). During bone formation, part of the osteoblasts are left in the mineralized bone matrix, which forms the early bone cells, also known as "osteoidosteocytes." In the process of being "embedded" in the bone matrix, early bone cells were able to emit a large number of synapses to extend to the bone surface and near the blood vessels, making extensive connections with osteoblasts, osteoclasts and endothelial cells. Research by Holmbeck and colleagues shows that the formation of bone cells is an active growth process that requires the removal of collagen fibers and other matrix molecules from the bone matrix. They found that in mice with matrix metalloproteinase (MT1-MMP) deficiency, the number and length of synapses in bone cells were significantly smaller than in normal mice, but the activity and density of bone cells were not significantly affected. The regulatory factors for synaptic growth are not yet clear, and may be related to the location, age, and species of bone cell growth.

Compared with mature bone cells, the newly formed bone cells still retain the characteristics of some osteoblasts and can express osteopontin and integrin 3. These early osteocytes were larger and had a large number of ribosomes, developed endoplasmic reticulum, and Golgi apparatus, which enabled them to synthesize large amounts of proteins and mucopolysaccharides. During the differentiation and maturation of osteocytes, the expression of certain specific markers, such as alkalinephosphatase, boneesialoprotein, and osteocalcin, gradually decreases, while other markers gradually decrease. , Such as osteopontin, integrin 3, can still continue to express. At the same time, new markers such as dentin matrix protein 1 (Dentinmatrix protein DMP1) and extracellular matrix phosphoglycoprotein (MEPE) are gradually emerging. These protein markers are closely related to the stage of differentiation and function of bone cells, and can be used to distinguish them from osteoblasts.