Osteocyte with recent informations

Osteocyte :

It is the mature bone cell enclosed by bone matrix that was previously secreted as an osteoblast features of osteocyte:

·         The process of the transformation from osteoblast to osteocyte takes approximately 3 days

·         It reduces 70% of its volume of the original osteoblast

·         They are metabolically active and multifunctional cells that respond to mechanical forces applied to the bone

·         Similar to other cells the osteocyte also has primary cilium that may be involved in mechanosensation and molecular signalling

·         They also are long-lived, surviving as long as the bone they occupy exists.

·         Although each osteocyte is physically isolated within the bone matrix, it communicates with nearby osteocytes, osteoblasts, osteoclasts, and even distant tissues and organs through multiple canaliculi that penetrate the surrounding bone. Like an octopus, each osteocyte extends tentacle-like dendrites into its canaliculi, and these dendrites allow the osteocyte to maintain direct contact with its neighbors.

Histological features of osteoblast :

It is typically smaller than the osteoprogenitor cells

Cytoplasm less basophilic than the osteoblast

Life span of osteocytes : 10-20 years

Number:  Osteocytes comprise 95% of the living cells in adult bone.

 The adult human body has about 42 billion osteocytes

Fate of osteocyte

Apoptosis

Degeneration /necrosis

Senesence (old age )

Bone remodelling activity of the osteoclasts

Percentage of death osteocyte:

1% at birth and 75% in the eighth decade of life

Types of osteocyte:

1.       Quiescent osteocytes

Less amount of rER and golgi complex

Osmiophilic lamina (Mature calcify matrix ) seen near the cell membrane

2.       Formative osteocytes

Amount of rER and golgi complex are more

Deposite bone matrix

Exhibit certain characters similar to osteoblast

3.       Resorptive osteocytes

Amount of rER and golgi complex are abundant

Lysosomes are conspicuous

Degradation of bone is occcurred by resorptive osteocytes 

Formation of osteocyte from osteoblast

The process of transformation from osteoblast to osteocyte takes 3 days

At first osteoblast secrete large amount of extra cellular matrix (nearly three times its own cellular volume)

Osteoblast lose 70% of its volume

Osteoblast reduces its cell organelle

It develops long processes that radiate from its cell body

Each osteocyte contain average 50 cell processes

Each osteocyte occupy a space called lacuna that conforms the shape of the cell

Osteocytes cytoplasmic processes  are enclosed by  canaliculi

Osteocytes also has primary cilium that may involve in mechanosensation and molecular signalling

Cytoplasmic processes of osteocytes communicate with

1.       Other osteocytes

2.       Distant osteoblast

3.       Endothelial cells of bone marrow vasculature

4.       Pericytes of blood vessels  

Recent discovery show

The osteocytes are metabolically active and multifunctional, response to mechanical forces apply to the bone 

It also respond to reduced mechanical stress like immobilization , muscles weakness

Weightlessness in space

Clinical anatomy : Osteocyte cell death can occur in association with pathologic conditions such as osteoporosis and osteoarthritis, which leads to increased skeletal fragility, linked to the loss of ability to sense microdamage and/or signal repair. Oxygen deprivation that occurs as the result of immobilization (bed rest), glucocorticoid treatment, and withdrawal of oxygen have all been shown to promote osteocyte apoptosis

Role of osteocytes in multiple myeloma bone disease:

Multiple myeloma bone disease is characterized by exacerbated bone resorption and the presence of osteolytic lesions that do not heal because of a concomitant reduction in bone formation. Osteocytes produce molecules that regulate both bone formation and resorption. Recent findings suggest that the life span of osteocytes is compromised in multiple myeloma patients with bone lesions. In addition, multiple myeloma cells affect the transcriptional profile of osteocytes by upregulating the production of pro-osteoclastogenic cytokines, stimulating osteoclast formation and activity. Further, patients with active multiple myeloma have elevated circulating levels of sclerostin, a potent inhibitor of bone formation which is specifically expressed by osteocytes in bone.

How osteocytes respond to mechanical stimuli

Mechanical force of bone ( during walking ) cause flow of interstitial fluid out of the canaliculi and lacunae on the compressed side of the bone .

Movement of interstitial fluid generates a transient electrical potential . this transient electrical potential opens voltage gated calcium channels in the membrane of osteocytes over which tissue fluid flow

Resulting increase in intracellular calcium , ATP , nitric oxide concentration  and prostaglandin E 2 synthesis alter expression of genes responsible for bone formation.

 the stess region of bone will have the largest deposition of new bone

When  mechanical stresses are reduce, osteocytes secrete matrix metalloproteinases (MMPs) which degrade bone matrix

Only recently was the complexity of the osteocyte network even described. These cells grow out in the bone space, interacting with ion signals through long dendritic processes, creating a fascinating system of intercellular connections. These cells interact not only with each other but also with vasculature, osteoblasts, osteoclasts, and marrow cells. It has long been argued such a network simply must have a function, and while this is now almost undeniably true the full extent of its purpose remains an area of much investigation

Dr Bonewald’s 2011 review in the Journal of Bone and Mineral Research, “The Amazing Osteocyte”, describes just a handful of the proven roles osteocytes play not just in bone health and homeostasis, but also their role in a much broader signaling network.

• Osteocytes not only possess potential to activate osteoblasts, but also osteoclasts, essentially regulating both sides of the bone anabolism/catabolism balance.\
• Osteocyte death leads to inflammation and bone fragility.
• Wnt/b-catenin regulate osteocyte function (whereas previously osteocytes hadn’t been considered important in bone regulation).
• Osteocytes are a source of minerals and factors, like Calcium, FGF-23, and Dmp1, which signal not just to other bone cells but in an endocrine fashion to many other organs.
• Osteocytes play a mechanosensory role. The dendritic network of osteocytes allows communication across large areas of bone, and these cells are subject to all the stress and strains of the bone matrix itself.