Sandy C. Marks

Tooth eruption: Theories and facts

The mechanisms of tooth eruption (i.e., the answer to the question of how and why teeth erupt) has been a matter of long historical debate. This review focuses on human and other mammalian teeth with a time‐ and spacewise limited period of eruption and analyzes recent observations and experimental data on dogs, rats, primates, and humans in a framework of basic biological parameters to formulate a guiding theory of tooth eruption. Acknowledging basic parameters (i.e., that teeth move in three‐dimensional space, erupt with varying speed, and arrive at a functional position that in inheritable) eliminates a number of previously held theories and favors those that accommodate basic parameters, such as alveolar bone remodeling in association with root elongation, with possible correction factors in the form of cementum apposition and periodontal ligament formation. We have critically analyzed, summarized, and integrated recent findings associated with preeruptive movements of developing teeth, the intraosseous stage of premolar eruption in dogs, molar eruption in rodents, and premolar and molar eruption in primates. The variable speeds of eruption are particularly important. We conclude with basic principles of tooth eruption—that is, the type of signals generated by the dental follicle proper, the conditions under which teeth are moved and the clinical understanding to be derived from this knowledge.

Expression of connective tissue growth factor in bone: its role in osteoblast proliferation and differentiation in vitro and bone formation in vivo.

Connective tissue growth factor (CTGF) is a secreted, extracellular matrix‐associated signaling protein that regulates diverse cellular functions. In vivo, CTGF is expressed in many tissues with highest levels in the kidney and brain. The purpose of this study was twofold; first, to localize CTGF in normal bone in vivo during growth and repair, and second, to examine CTGF expression and function in primary osteoblast cultures in vitro and test its effect on bone formation in vivo. Northern and Western blot analyses confirmed that CTGF is expressed in normal long bones during the period of growth or modeling. In situ hybridization and immunohistochemical analysis demonstrated intense staining for CTGF mRNA and protein in osteoblasts lining metaphyseal trabeculae. Examination of CTGF expression in the fracture callus demonstrated that it was primarily localized in osteoblasts lining active, osteogenic surfaces. In primary osteoblast cultures, CTGF mRNA levels demonstrated a bimodal pattern of expression, being high during the peak of the proliferative period, abating as the cells became confluent, and increasing to peak levels and remaining high during mineralization. This pattern suggests that CTGF may play a role in osteoblast proliferation and differentiation as previously demonstrated for fibroblasts and chondrocytes. Treatment of primary osteoblast cultures with anti‐CTGF neutralizing antibody caused a dose‐dependent inhibition of nodule formation and mineralization. Treatment of primary osteoblast cultures with recombinant CTGF (rCTGF) caused an increase in cell proliferation, alkaline phosphatase activity, and calcium deposition, thereby establishing a functional connection between CTGF and osteoblast differentiation. In vivo delivery of rCTGF into the femoral marrow cavity induced osteogenesis that was associated with increased angiogenesis. This study clearly shows that CTGF is important for osteoblast development and function both in vitro and in vivo.

Experimental study in the dog of the non-active role of the tooth in the eruptive process

The role of the tooth in eruption was studied radiographically and histologically after experimental manipulations of the crowns of permanent mandibular premolars in dogs. Crowns were removed and dead crown shells or metal or silicone replicas were substituted into dental follicles just prior to scheduled eruption. These replacements erupted on schedule after formation of the usual eruption pathways and formation of trabecular bone from the base of the bony crypt. Removal of crowns, but without adding replacements, also exhibited these same hallmarks of eruption. We conclude that tooth eruption is a series of metabolic events in alveolar bone characterized by bone resorption and formation on opposite sides of the dental follicle and the tooth does not contribute to this process.

Ultrasound attenuation of the calcaneus: a sensitive and specific discriminator of osteopenia in postmenopausal women.

SummaryRecent studies have evaluated techniques for estimating bone mass without radiation. The present study compares broadband ultrasound attenuation of the calcaneus and bone densities of the femoral neck and the lumbar spine in 17 normal women and 41 women with osteoporosis. Twenty of the osteoporotic women had spine (n=16) or femoral neck (n=4) fractures. There was a significant decrease in the broadband ultrasound attenuation (P<0.001) in women with osteoporosis compared with normal women. The osteoporotic women also showed a decrease in vertebral (P<0.0001) and femoral neck (P<0.0001) densities compared with normal women. At 63 dB/MHz, the sensitivity and specificity of broadband ultrasound attenuation for decreased bone mineral density with or without fractures were 76%. All women with fractures had a broadband ultrasound attenuation less than 72 dB/MHz. This corresponded to a specificity of 41%. To determine whether broadband ultrasound attenuation correlated with trabecular bone volume, samples of cadaver calcaneus were analyzed. The histologic determination showed a significant correlation between broadband ultrasound attenuation and trabecular bone volume (r=0.992,P=0.008). These results suggest broadband ultrasound attenuation of the calcaneus reflects bone mass and can be used as a safe and sensitive indicator for decreased axial bone density.

The osteopetrotic mutation toothless (tl) is a loss-of-function frameshift mutation in the rat Csf1 gene: Evidence of a crucial role for CSF-1 in osteoclastogenesis and endochondral ossification

The toothless (tl) mutation in the rat is a naturally occurring, autosomal recessive mutation resulting in a profound deficiency of bone-resorbing osteoclasts and peritoneal macrophages. The failure to resorb bone produces severe, unrelenting osteopetrosis, with a highly sclerotic skeleton, lack of marrow spaces, failure of tooth eruption, and other pathologies. Injections of CSF-1 improve some, but not all, of these. In this report we have used polymorphism mapping, sequencing, and expression studies to identify the genetic lesion in the tl rat. We found a 10-base insertion near the beginning of the open reading of the Csf1 gene that yields a truncated, nonfunctional protein and an early stop codon, thus rendering the tl rat CSF-1null. All mutants were homozygous for the mutation and all carriers were heterozygous. No CSF-1 transcripts were identified in rat mRNA that would avoid the mutation via alternative splicing. The biology and actions of CSF-1 have been elucidated by many studies that use another naturally occurring mutation, the op mouse, in which a single base insertion also disrupts the reading frame. The op mouse has milder osteoclastopenia and osteopetrosis than the tl rat and recovers spontaneously over the first few months of life. Thus, the tl rat provides a second model in which the functions of CSF-1 can be studied. Understanding the similarities and differences in the phenotypes of these two models will be important to advancing our knowledge of the many actions of CSF-1.

The role of three-dimensional information in health care and medical education: The implications for anatomy and dissection

The purposes of medical education can be summarized as learning how to take an effective history, perform a physical examination, and perform diagnostic and therapeutic procedures with minimal risk and maximal benefit to patients. Because patients are three‐dimensional (3‐D) objects, health care and medical education involve learning and applying 3‐D information. The foundation begins in anatomy where students form and confirm or reform their own 3‐D ideas and images of the development and structure of the human body at all levels of organization. Students go on to understand the interdependence of structure and function in health and disease. The basic questions for those teaching anatomy are “How do we learn and use 3‐D information?” and “How is it taught most effectively?” These are not easy questions for teachers and are rarely asked by those who currently defend or reframe curricula. Unfortunately, there is little information on how we learn 3‐D information and no evidence‐based literature on the relative long‐term vocational effectiveness of methods for teaching it. It is clear that we learn in several distinct modalities and that our students represent a spectrum of learning styles. To support the 3‐D learning essential to both medical education and health care, anatomical societies need to provide answers to the following questions: Do the opportunities of dissection (visual, tactile, time, discovery, group process, mentoring) contribute to short‐ and long‐term learning of 3‐D information? If so, how? Does dissection offer significant advantages over other methods for learning, confirming, and using 3‐D information in anatomy? Answers to these questions will provide a rational basis for decisions about curricular changes in anatomy courses (if, where, and when dissection should occur). This, in turn, will link these changes to societys ultimate purposes for medical education and health care rather than to the fiscal concerns of the businesses of health care and medical education, which is the current practice. Clin. Anat. 13:448–452, 2000.

Human anatomy: a foundation for education about death and dying in medicine.

In most medical schools, little curricular time is devoted to the art of medicine, and this is particularly evident with respect to death education. We make a case for including education on death and dying in medical schools, specifically its early introduction in the anatomy course. Studies indicate that whereas dissection of cadavers is an exciting discovery for most students, for many it is traumatic and if not addressed, students may use depersonalization and denial as their approach to suffering.

Administration of colony stimulating factor-1 corrects some macrophage, dental, and skeletal defects in an osteopetrotic mutation (toothless, tl) in the rat

The toothless (tl/tl) mutation in the rat results in a paucity of osteoclasts and osteopetrosis that cannot be corrected by bone marrow transplantation. In the present study we demonstrate that tl/tl rats also have profound deficiencies of femoral, peritoneal, and pleural cavity macrophages. Furthermore, the macrophage colony stimulating activity of post-endotoxin sera from tl/tl rats is substantially reduced, suggesting that, as in the case of the op mutation in mice, the basis of the tl mutation is a deficiency of the macrophage growth factor, colony stimulating factor-1 (CSF-1). Consistent with this suggestion, treatment of tl/tl rats from birth for up to six weeks with CSF-1 reduced the osteopetrosis, increased body weight, and permitted tooth eruption. In addition, CSF-1 treatment induced large numbers of osteoclasts in tl/tl bones and macrophages in the peritoneal cavity and bone marrow. Persistence of metaphyseal sclerosis, however, indicated that the disease was not totally corrected by this treatment. These studies indicate that the basis of the tl mutation is most likely another CSF-1 deficiency, and further emphasize the role of this growth factor in osteoclast differentiation.

Radiologic contributions to the investigation and prosecution of cases of fatal infant abuse.

In 1984 we started a two-year program in Worcester (Mass.) and Boston to provide additional radiologic data for the medical investigation of suspected fatal infant abuse. During that period the investigation of 12 cases of unexplained infant death included the review of complete radiographic skeletal surveys by a pediatric radiologist. Autopsies were supplemented with resection, high-detail radiography, and histologic study of all non-cranial sites of suspected osseous injury. Thirty-four bony injuries were noted, including 12 acute and 16 healing fractures of the long-bone metaphyses and posterior-rib arcs in patterns indicative of infant abuse. The investigations determined that there were eight cases of abuse, two accidental deaths, and two natural deaths (sudden infant death syndrome). At this writing, the radiologic and osseous histologic studies appear to have influenced the determination of the manner of death in six of the eight cases of abuse and the criminal prosecution in four of the five convictions. These findings suggest that a thorough postmortem radiologic evaluation followed by selected histologic studies can have an impact on the investigation and prosecution of cases of fatal infant abuse.

Congenital osteopetrotic mutations as probes of the origin, structure, and function of osteoclasts.

Progress has been made in recent years on the cell biology of the osteoclast and the pathogenesis of congenital osteopetrosis. New information is critically evaluated and summarized with respect to the origin, structure, and function of osteoclasts with special reference to osteopetroses. A broader perspective emerges from which to consider the biology of osteoclasts, the heterogeneities in the osteopetroses, and the value of each in elucidating the other, along with treatment of the disorder.