Gap junctions connect osteocytes to one another through long intracanalicular processes that enable cells to exchange nutrients and waste products within neighboring lamellae.
Gap junctions (Illustration A) are a specialized intracellular connection between cells. They are found in a myriad of different cell types including myocytes, osteocytes, and neurons. Gap junctions directly connect the cytoplasm of adjacent cells, allowing for the direct transfer of nutrients, ions, and other substances between cells. Gap junctions are composed of two connexons, which cross and connect the intracellular space, usually at a distance of approximately 4nm. Connexons themselves are composed of 6 smaller connexin proteins (Illustration B).
Xu et al. examine the importance of gap junctions and connexins in osteocyte function and development. They demonstrate that targeted deletion of the connexin 43 gene, responsible for connexon and gap junction creation, in osteoblasts or osteocytes leads to increased osteocyte apoptosis, osteoclast recruitment, and reduced biomechanical properties.
Plotkin summarizes the current knowledge on the presence and function of connexons and gap junctions in osteocytes. He describes the role of gap junction regulation for the maintenance of bone cell viability and, potentially, bone health. Specifically, he explains that this role is controlled by gap junctions opening in response to pharmacological and mechanical stimulation. For example, preservation of the viability of osteoblasts and osteocytes by the anti-osteoporotic bisphosphonates depends on the presence of these gap junctions.
Illustration A is a graphic representation of a gap junction. Illustration B shows how multiple connexins makes up a single connexon.
Answer 2: The plasma membrane is not involved in the primary mode of transportation between osteocytes.
Answer 3: Kinesin is a motor protein which moves along microtubule filaments. It is not involved in cell to cell transportation in osteocytes.
Answer 4: Dynein is a motor protein which causes the sliding of microtubules. It is not involved in cell to cell transportation in osteocytes.
Answer 5: The endoplasmic reticulum is a cellular organelle involved in protein production and packaging. It is not involved in cell to cell transportation.
Xu H, Gu S, Riquelme MA, Burra S, Callaway D, Cheng H, Guda T, Schmitz J, Fajardo RJ, Werner SL, Zhao H, Shang P, Johnson ML, Bonewald LF, Jiang JX. Connexin 43 Channels are Essential for Normal Bone Structure and Osteocyte Viability. J Bone Miner Res. 2014 Oct 1. doi: 10.1002/jbmr.2374. [Epub ahead of print] PubMed PMID: 25270829.
PMID:25270829 (Link to Abstract)
Plotkin LI. Connexin 43 hemichannels and intracellular signaling in bone cells. Front Physiol. 2014 Apr 4;5:131. doi: 10.3389/fphys.2014.00131. eCollection 2014. Review. PubMed PMID: 24772090; PubMed Central PMCID: PMC3983496
PMID:24772090 (Link to Abstract)