The osteocytes are a type of cells found in the bone, a specialized connective tissue. They derive from other cells known as osteoblasts and are found to a large extent within places called "gaps", within the bone matrix.
Bone is mainly made up of three types of cells: osteoblasts, osteoclasts, and osteocytes. In addition to the extracellular fluid, it has a complex calcified extracellular matrix, which is responsible for the hardness of these tissues that serve as structural support for the entire body.
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Osteocyte
Osteocytes are one of the most abundant cells in bone. These account for more than 90% of the total cellular content in said tissue, while osteoblasts represent about 5% and osteoclasts are around 1%. It is said that in the bone of an adult human there are 10 times more osteocytes than osteoblasts.
Its functions are diverse, but among the most prominent is its participation in the signaling processes for both the formation and resorption of bone, a fact that is also implicated in some known clinical pathologies.
Training
Osteocytes are derived from osteoblasts, their progenitor cells, through a process that occurs thanks to the recruitment of osteoblasts towards the bone surface, where certain signals trigger the initiation of differentiation.
This differentiation brings with it a series of drastic changes both in cell form and function, since osteoblasts go from being "cuboidal" cells specialized in the secretion of extracellular matrix, to being elongated cells with small bodies that are connected to neighboring cells through long cytoplasmic projections.
The new differentiated cells (osteocytes), connected to cells that are embedded in bone, are later encapsulated in osteoid, a non-mineralized organic material composed mainly of collagen fibers and other fibrous proteins.
When the osteoid around the osteoid-osteocyte complex (transitional stage) hardens by mineralization, the cells become confined and immobilized within "gaps" in the extracellular matrix, where differentiation culminates. This process is seen as the reclusion of cells in their own extracellular matrix.
The formation and extension of the dendrites or cytoplasmic projections of the osteocytes is controlled by various genetic, molecular and hormonal factors, among which it has been shown that some matrix metalloproteinases stand out.
Signs for differentiation
Many authors agree that these processes are genetically determined; that is, in the different stages of the differentiation of osteoblasts to osteocytes, different and heterogeneous patterns of genetic expression are observed.
From a morphological point of view, the transformation or differentiation of osteoblasts into osteocytes occurs during bone formation. In this process the projections of some osteocytes grow to maintain contact with the underlying osteoblast layer to control their activity.
When growth stops and communication between osteocytes and active osteoblasts is disrupted, signals are produced that induce recruitment of osteoblasts to the surface, and that is when their cell fate is compromised.
At present, from the molecular point of view, some effectors of this transition have already been identified. Among them are transcription factors that activate the production of proteins such as type I collagen, osteopontin, bone sialoprotein, and oteocalcin.
characteristics
Osteocytes are cells with flattened nuclei and few internal organelles. They have a greatly reduced endoplasmic reticulum and Golgi apparatus, and their cell body is small in size compared to other cells in related tissues.
Despite this, they are very active and dynamic cells, since they synthesize many non-collagenic matrix proteins such as osteopontin and osteocalcin, and also hyaluronic acid and some proteoglycans, all important factors for the preservation of bones.
The nutrition of these cells depends on transport through what is known as the peri-cellular space (that between the wall of the cavity or lagoon and the plasma membrane of the osteocyte), which constitutes a critical site for the exchange of nutrients and metabolites, information and some metabolic waste.
One of the most prominent characteristics in these cells is the formation of long "dendrite-like" processes of cytoplasmic origin that are capable of traveling through small tunnels in the matrix known as "canaliculi", in order to connect each osteocyte with its neighboring cells and those on the bone surface.
These processes or projections join each other through “gap junctions”, which allow them to facilitate the exchange of molecules and the conduction of hormones to distant sites in the bone tissue.
The communication of osteocytes with other cells depends on these projections that emerge from the cell body and come into direct contact with other cells, although it is also known that they depend on the secretion of some hormones for this purpose.
Osteocytes are very long-lived cells, and can last for years and even decades. The half-life of an osteocyte is believed to be around 25 years, a very long time especially compared to osteoblasts and osteoclasts that last only a couple of weeks and even a few days.
Features
In addition to being important structural components of bone tissue, one of the main functions of osteocytes consists in the integration of the mechanical and chemical signals that govern all the processes of starting bone remodeling.
These cells appear to act as "drivers" that direct the activity of osteoclasts and osteoblasts.
Recent studies have shown that osteocytes exert regulatory functions that go far beyond bone boundaries, since they participate, through some endocrine pathways, in the phosphate metabolite.
These cells have also been considered to have functions in the systemic metabolism of minerals and their regulation. This fact is based on the mineral exchange potential of the fluid peri-cellular spaces (around the cells) of the osteocytes.
Since these cells have the ability to respond to parathyroid hormone (PTH), they also contribute to the regulation of calcium in the blood and the permanent secretion of the new extracellular bone matrix.
References
- Aarden, EM, Burger, EH, Nijweide, PJ, Biology, C., & Leiden, AA (1994). Function of Osteocytes in Bone. Journal of Cellular Biochemistry, 55, 287-299.
- Bonewald, L. (2007). Osteocytes as Dynamic Multifunctional. Ann. NY Acad. Sci., 1116, 281-290.
- Cheung, MBSW, Majeska, R., & Kennedy, O. (2014). Osteocytes: Master Orchestrators of Bone. Calcif Tissue Int, 94, 5–24.
- Franz-odendaal, TA, Hall, BK, & Witten, PE (2006). Buried Alive: How Osteoblasts Become Osteocytes. Developmental Dynamics, 235, 176–190.
- Gartner, L., & Hiatt, J. (2002). Text Atlas of Histology (2nd ed.). Mexico DF: McGraw-Hill Interamericana Editores.
- Johnson, K. (1991). Histology and Cell Biology (2nd ed.). Baltimore, Marylnand: The National medical series for independent study.
- Kuehnel, W. (2003). Color Atlas of Cytology, Histology, and Microscopic Anatomy (4th ed.). New York: Thieme.