The mechanisms and mediators of tooth eruption--models for developmental biologists

Tooth eruption is a localized process in the jaws which exhibits precise timing and bilateral symmetry. It involves resorption and formation of bone on opposite sides of the erupting tooth and these activities depend on the dental follicle, a thin connective tissue investment of the developing and erupting tooth. Biochemical studies have shown that during eruption cells, proteins and enzymes change in the dental follicle and several growth factors and proteins known to accelerate or retard eruption have been identified. This review discusses these aspects of tooth eruption and proposes testable hypotheses and strategies that can make studies of tooth eruption new experimental opportunities for developmental biologists

Colony-stimulating factor-1 (CSF-1) rescues osteoblast attachment, survival and sorting of beta-actin mRNA in the toothless (tl-osteopetrotic) mutation in the rat

We have shown that in the osteopetrotic rat mutation toothless (tl) osteoblasts are absent from older bone surfaces in mutants and that mutant osteoblasts in vivo lack the prominent stress fiber bundles polarized along bone surfaces in osteoblasts from normal littermates. Our recent data demonstrate that in normal osteoblasts in vitro beta- and gamma-actin mRNAs have different, characteristic intracellular distributions and that tl (mutant) osteoblasts fail to differentially sort these mRNAs. Because bone resorption and formation are highly interdependent and injections of CSF-1, a growth factor, increase bone resorption and growth in tl rats, we examined the effects of CSF-1 treatment on osteoblast survival and ultrastructure in vivo and ability to sort actin mRNAs in vitro. Neonatal CSF-1 treatment of mutants restores osteoblasts on older bone surfaces, normalizes the intracellular distribution of stress fibers in osteoblasts in vivo and promotes normal sorting of beta-actin mRNA in mutant osteoblasts in vitro without normalizing gamma-actin distribution. These data suggest the beta- and gamma-actin mRNAs in osteoblasts are sorted by different mechanisms and that the differential sorting of beta-actin mRNA is related to the characteristic polarization of stress fibers in osteoblasts and their survival on bone surfaces. This experimental system can be used to explore the relationships and regulation of these aspects of cell and tissue biology

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

Endochondral bone formation in toothless (osteopetrotic) rats: failures of chondrocyte patterning and type X collagen expression

The pacemaker of endochondral bone growth is cell division and hypertrophy of chondrocytes. The developmental stages of chondrocytes, characterized by the expression of collagen types II and X, are arranged in arrays across the growth zone. Mutations in collagen II and X genes as well as the absence of their gene products lead to different, altered patterns of chondrocyte stages which remain aligned across the growth plate (GP). Here we analyze GP of rats bearing the mutation toothless (tl) which, apart from bone defects, develop a progressive, severe chondrodystrophy during postnatal weeks 3 to 6. Mutant GP exhibited disorganized, non-aligned chondrocytes and mineralized metaphyseal bone but without cartilage mineralization or cartilaginous extensions into the metaphysis. Expression of mRNA coding for collagen types II (Col II) and X (Col X) was examined in the tibial GP by in situ hybridization. Mutant rats at 2 weeks exhibited Col II RNA expression and some hypertrophied chondrocytes (HC) but no Col X RNA was detected. By 3rd week, HC had largely disappeared from the central part of the mutant GP and Col II RNA expression was present but weak and in 2 separate bands. Peripherally the GP contained HC but without Col X RNA expression. This abnormal pattern was exacerbated by the fourth week. Bone mineralized but cartilage in the GP did not. These data suggest that the tl mutation involves a regulatory function for chondrocyte maturation, including Col X RNA synthesis and mineralization, and that the GP abnormalities are related to the Col X deficiency. The differences in patterning in the tl rat GP compared to direct Col X mutations may be explained by compensatory effects

Evidence that the rat osteopetrotic mutation toothless (tl) is not in the TNFSF11 (TRANCE, RANKL, ODF, OPGL) gene

The toothless (tl) osteopetrotic mutation in the rat affects an osteoblast-derived factor that is required for normal osteoclast differentiation. Although the genetic locus remains unknown, the phenotypic impact of the tl mutation on multiple systems has been well characterized. Some of its actions are similar to tumornecrosis factor superfamily member 11(TNFSF11; also called TRANCE, RANKL, ODF and OPGL) null mice. TNFSF11 is a recently described member of the tumor necrosis factor superfamily which, when expressed by activated T cells, enhances the survival of antigen-presenting dendritic cells, and when expressed by osteoblasts, promotes the differentiation and activation of osteoclasts. The skeletal similarities between tl rats and TNFSF11(-/-) mice include 1) profound osteoclastopenia (TNFSF11-null mice, 0% and tl rats 0-1% of normal); 2) persistent, non-resolving osteopetrosis that results from 3) a defect not in the osteoclast lineage itself, but in an osteoblast-derived, osteoclastogenic signal; and 4) a severe chondrodysplasia of the growth plates of long bones not seen in other osteopetrotic mutations. The latter includes thickening of the growth plate with age, disorganization of chondrocyte columns, and disturbances of chondrocyte maturation. These striking similarities prompted us to undertake studies to rule in or out a TNFSF11 mutation in the tl rat. We looked for expression of TNFSF11 mRNA in tl long bones and found it to be over-expressed and of the correct size. We also tested TNFSF11 protein function in the tl rat. This was shown to be normal by flow cytometry experiments in which activated, spleen-derived T-cells from tl rats exhibited normal receptor binding competence, as measured by a recombinant receptor assay. We also found that tl rats develop histologically normal mesenteric and peripheral lymph nodes, which are absent from TNFSF11-null mice. Next, we found that injections of recombinant TNFSF11, which restores bone resorption in null mice, had no therapeutic effect in tl rats. Finally, gene mapping studies using co-segregation of polymorphic markers excluded the chromosomal region containing the TNFSF11 gene as harboring the mutation responsible for the tl phenotype. We conclude that, despite substantial phenotypic similarities to TNFSF11(-/-) mice, the tl rat mutation is not in the TNFSF11 locus, and that its identification must await the results of further studies

A regional approach to the classic metaphyseal lesion in abused infants: the distal femur

OBJECTIVE: The purpose of this study was to analyze the spectrum of morphologic alterations of the classic metaphyseal lesion involving the distal femur of abused infants and to identify features that aid in radiologic diagnosis and assessment of healing. MATERIALS AND METHODS: Thirty-one infants who died with evidence of inflicted skeletal injury were studied with high-detail skeletal surveys, resected specimen radiography, and histologic analysis. We recorded the number of fractures, the portions of the distal femoral metaphyses involved, and the age of the lesions. RESULTS: Fifteen classic metaphyseal lesions of the distal femur were seen in 11 infants. The lesions were bilateral in four infants and unilateral in seven. Nine healing and three acute classic metaphyseal lesions were found. In the remaining three lesions, the age of the lesions was indeterminate. Fractures always involved the posteromedial aspect of the femur; anterior and lateral extension occurred in more diffuse lesions. Fractures tended to be less conspicuous when they were acute and were more easily recognized with healing, especially with specimen radiography. Histologically, the fracture line consistently undercut the medial subperiosteal bone collar. Extension of hypertrophic chondrocytes from the growth plate into the region of fracture was found in all nine healing lesions. CONCLUSION: The classic metaphyseal lesion of the distal femur has distinctive radiologic and histopathologic characteristics that relate to the anatomy of the region. Because the distal femur is a common site for this strong indicator of infant abuse, the region should be carefully evaluated with well-collimated, high-detail skeletal radiographs in all cases of suspected infant abuse

A regional approach to classic metaphyseal lesions in abused infants: the distal tibia

OBJECTIVE: The purpose of this study was to analyze systematically the spectrum of morphologic alterations of classic metaphyseal lesions (CML) involving the distal tibia of abused infants and to identify features that assist in the radiologic diagnosis and assessment of healing. MATERIALS AND METHODS: Thirty-one infants who died with evidence of inflicted injury were studied with high-detailed skeletal surveys, resected-specimen radiography, and histopathologic analysis. The number of fractures identified, the portions of the distal tibial metaphyses involved, and the age of the lesion were assessed. RESULTS: A total of 16 CML of the distal tibia were noted. Eight infants had unilateral injury that always involved the left side, and four had bilateral lesions. When a CML was visible radiographically, it was always seen along the medial aspect of the metaphysis; lateral metaphyseal involvement was evident with more extensive injuries and always accompanied medial involvement. Fractures tended to be less conspicuous when acute and were more easily recognized with healing, especially with specimen radiography. Radiographically, the typical fracture separated a medial fragment that was tall and triangular; this appearance was histologically related to the undercutting of the long subperiosteal bone collar. In healing lesions, extension of hypertrophic chondrocytes from the growth plate into the region of the fracture was a consistent finding. CONCLUSION: CML of the distal tibia have distinctive radiologic and histopathologic characteristics that relate to the anatomy of the region. Visualization of these fractures depends on the quality of the radiographs obtained. An understanding of the radiologic and histopathologic features of CML should aid in the recognition of this strong indicator of infant abuse

A regional approach to the classic metaphyseal lesion in abused infants: the proximal humerus

OBJECTIVE: The purpose of this study was to systematically analyze the spectrum of morphologic alterations of the classic metaphyseal lesion (CML) involving the proximal humerus of abused infants and to identify radiologic features that aid in the diagnosis and assessment of healing. MATERIALS AND METHODS: Thirty-one infants who died with evidence of inflicted skeletal injury were studied with high-detail skeletal surveys, resected specimen radiography, and histologic analysis. In each infant, the number of fractures, the involved portions of the proximal humeral metaphyses, and the age of the lesion were assessed. RESULTS: Seven proximal humeral CMLs were seen in four infants. Radiography of the only infant with an acute injury revealed no fracture; however, evidence of trabecular disruption was found histologically. Radiographic patterns in the six healing injuries included focal corner-fracture appearance, transmetaphyseal radiolucency, irregularity of the metaphyseal margin, and flame-shaped mineralized extensions into the physeal cartilage. Histologically, all CMLs undercut the lateral subperiosteal bone collar, explaining the corner-fracture appearance. Focal and diffuse thickening of the zone of hypertrophic chondrocytes was seen, and peninsulas of primary spongiosa surrounded by hypertrophied cartilage corresponded to the flame-shaped densities revealed radiographically. The fracture morphology was better seen on specimen radiography than on skeletal survey images. CONCLUSION: CML of the proximal humerus has distinctive radiologic and histopathologic characteristics that relate to the anatomy of the region. In our study, changes at the chondroosseous junction revealed by radiography correlated well with histologic evidence of healing. Although the proximal humerus is a less common location for CML than the lower extremity, the proximal humerus should be carefully scrutinized on high-detail radiographs in cases of suspected infant abuse

The clinical anatomy of laparoscopic inguinal hernia repair

Laparoscopic approaches for abdominal surgery are being used with increasing frequency. Their advantages are sometimes negated by the disturbing incidence of postoperative sequelae. In the case of inguinal hernia repair, these are often the result of failing to understand that the anatomy of the anterior approach to the abdominal wall cannot necessarily be directly applied to laparoscopy. The inguinal ligament, easily identified in an anterior approach, is only seen laparoscopically after removal of the iliopubic tract, a key structure which lies in the plane of the original defect of most groin hernias. Thus, an understanding of the incompletely trilaminar anterior abdominal wall, including the iliopubic tract, is the foundation for effective inguinal hernia repair using any approach (anterior or posterior) or technique (sutures, mesh or staples). Laparoscopic inguinal hernia repair has produced an increase in the frequency of debilitating neuropathies, most notably of the lateral femoral cutaneous nerve (LFCN). This is directly related to the variable intrapelvic course of this nerve or its branches. In more than 13% of the 114 pelves we examined, the LFCN was within 0.5 cm of the iliopubic tract or in the vertical plane of the anterior superior iliac spine, key lateral landmarks and anchoring sites for mesh in laparoscopic hernia repairs. Medial landmarks also have variable features. These data indicate that the identity of anatomical landmarks and the variability of other structures will continue to be important in the successful development of new laparoscopic techniques