Mini‐Issue on Primary Cilia in Bone and Cartilage: Preface to Presentations from the Sun Valley Hard Tissue Workshop, August 2007
Year: 2008
Type: editorial
Abstract: The Sun Valley Hard Tissue Workshop is one of the longest, if not the longest, continuously running conference in the United States dedicated to the morphology and function of mineralized tissues. This workshop was originally organized in 1964 by Dr. Webster S.S. Jee, Professor of anatomy at the University of Utah and initially sponsored by the National Institutes of Dental Research (NIDR). The initial purpose of the conference was to provide an opportunity of trainees from various dental schools to engage in a summer research training program and to then present their research to a workshop faculty. The workshop continuously evolved and expanded over the years but has maintained the reputation as one of the premiere “think tanks” in the world of mineralized tissues. The Sun Valley Workshop never really did conform to what might be considered a normal scientific conference. It was intentionally called a “workshop” to emphasize scientific interactions and discussion, although at times it seemed to be more of an exercise in anarchy. Professor Jee, a long time member of the American Association of Anatomists (AAA), organized and hosted the meeting from 1964 to 2002, turning the reigns over to Dr. David Burr, chair of anatomy at Indiana University and current president of the AAA. Information about this meeting may be obtained at www.sunvalleyworkshop.org. For many years, one of the distinguished Sun Valley faculty members was J. Les Matthews from Baylor University. Dr. Matthews was a superb microscopist and morphologist and published some of the landmark studies on the structure of bone cells and the ultrastructural organization of skeletal tissues. During my own graduate training, I became very interested in electron microscopy and had the good fortune to spend some time with Dr. Matthews and his colleague and frequent co-author, Dr. Jim Martin. I learned from them the tricks and secrets of working with mineralized tissue. After all, cutting mineralized bone can be a frustrating endeavor, as well as expensive if diamond knives are destroyed in the process. In the early 1970s, Dr. Matthews had some fascination with primary cilia on osteoblasts and had collected some interesting electron micrographs of these elusive structures. Drs. Matthews and Martin published an electron micrograph of an osteocyte with a solitary cilia as an incidental observation in a manuscript published in 1971 (Mathews and Martin, 1971). The presence of solitary cilia on bone cells was confirmed the following year in a more detailed study by another set of investigators (Tonna and Lampen, 1972). In the early Sun Valley Workshop meetings, I remember Dr. Mathews presenting some of his micrographs of primary cilia on bone cells and speculating as to their function. There was considerable interest at that time about how the “bone fluid compartment” was separated from the interstitial fluid compartments and how ion fluxes between these compartments were regulated. Even today this is still poorly understood. Dr. Matthews suggested at the time that primary cilia on osteocytes might function as ionic and/or fluid flow sensors. Needless to say, the presence or even the existence of this cellular “antenna” evoked a lot of interesting discussion, debate and even some periodic comic relief. Some investigators at the time, including myself, thought that actually being able to observe a single cilia on a bone cell was about as likely as finding a UFO. I must admit, however, that over the years I have my own very small collection of electron micrographs of primary cilia emanating from centrioles in osteoblasts and young osteocytes. Now over 3 decades after the initial ultrastructural description of primary cilia on some bone cells, improved morphological, cellular, and molecular techniques are defining how these cellular structures may function in these cells. I should emphasize that to my knowledge it is still not clear if all types of bone cells contain a primary cilia. Primary cilia have been described in some osteoblasts (and chondrocytes), possibly some bone-lining cells (older osteoblasts that remain on the surface but are not forming bone), and some types of osteocytes. The multinucleated osteoclast appears to consolidate its numerous centrioles into a central cellular structure termed the “centrosome,” also described by Dr. Matthews and his colleagues in the 1960s (Matthews et al., 1967). Whether the bone resorbing osteoclast contains primary cilia has, to my knowledge, yet to be determined. Considering that the early descriptions of primary cilia in bone cells were presented at the Sun Valley Hard Tissue Workshop almost 4 decades ago, it is fitting that this mini-issue on primary cilia on bone cells are from papers presented at this workshop in August, 2007. In the first paper in this issue, Charles Anderson, Alesha Castillo, Christopher Jacobs, and Tim Stearns provide an overview of the formation, growth and structure of primary cilia. They also present a series of important but unanswered questions relative to the possible role(s) of cilia in bone cell functions relative to bone remodeling and the regulation of mineral homeostasis. In the second paper, Eve Donnolly, Rebecca Williams, and Cornelia Farnum describe immunohistochemical and multiphoton microscopy techniques to image primary cilia, cell nuclei, and the extracellular matrix in various connective tissues, including tendon, meniscus, ligament, growth plate (hyaline) cartilage, intervertebral disk cartilage (fibrocartilage), and perichondrium. These investigators provide some of the best morphological and immunohistochemical evidence yet for the presence of primary cilia in connective tissues. In the third paper, Rosa Serra describes the cellular signaling pathways that potentially involve primary cilia and then summarizes the skeletal consequences of the deletion or mutation of genes that are important for the formation and maintenance of primary cilia. Dr. Serra provides compelling evidence for possible roles of primary cilia in limb patterning and early skeletal development and growth. Together, these manuscripts provide an excellent overview of the presence and provocative insights into the possible functions of primary cilia in connective tissues, including skeletal tissues. The editors of The Anatomical Record are grateful that Dr. David Burr suggested this topic for a mini-issue and persuaded the presenters at the Sun Valley Workshop to prepare their articles for submission to The Anatomical Record.
Source: The Anatomical Record
DOI: 10.1002/ar.20732
Author: Scott C. Miller
Language: English
Subfield: Genetics
Domain: Life Sciences
Open Access status: bronze
OpenAlex ID: W2005551315