Morphogenesis and bone integration of the mouse mandibular third molar Ivana Chlastakova 1,2 , Vlasta Lungova 2,3 , Kirsty Wells 4 , Abigail S. Tucker 4 , Ralf J. Radlanski 5 , Ivan Misek 1,2 , Eva Matalova 1,2 1 Laboratory of Animal Embryology, IAPG v.v.i., Academy of Sciences, Brno, Czech Republic; 2 Department of Physiology, University of Veterinary and Pharmaceutical Sciences, Brno, Czech Republic; 3 Faculty of Science, Palacky University, Olomouc, Czech Republic; 4 Department of Craniofacial Development and Orthodontics, KingÕs College, GuyÕs Hospital, London, UK; 5 Department of Craniofacial Developmental Biology, Freie Universitat, Berlin, Germany All teeth, regardless of shape and identity, pass through the same developmental stages and consist of the same tissues (1). From the point of view of the functional dentition, dynamic integration of the tooth into the surrounding bone is essential. Alveolar bone osteoblasts differentiate directly from mesenchymal cells with physi- cal contribution of the tooth germ mesenchymal cells. Thus, the alveolar bone, unlike the majority of bones, undergoes intramembranous ossification, which starts in regions with shearing detraction originating from the growth of different tissues at different speeds. Establish- ment and maintenance of the tooth–bone complex must be tightly balanced because these two components influ- ence each other reciprocally. Decreased bone minerali- zation in the jaw and interdental bones, such as that observed in osteoporosis or osteolysis, may result in pre- mature tooth loss. Whereas alveolar bone homeostasis, may be affected by hypodontia, surgical interventions, trauma or unhealed tooth disease such as periodontitis. Alveolar and jaw bone development, along with forma- tion of the tooth–bone interface and related disorders, have been recently reviewed (2). Importantly, because of the different timing of initialization of individual teeth in the jaw, different teeth develop under different minerali- zation conditions. Nowadays, a wealth of information has been gleaned on the morphogenesis of the developing mouse dentition, based particularly on research in the mouse first molar (M1) (3–6). Development of the mouse M1 starts in the jaw before mineralization occurs in the embryonic head (4). The basic space for the tooth germ is established before jaw mineralization and is later maintained by several signalling molecules, such as receptor activator of nuclear factor-jB (RANK), receptor activator of nuclear factor-jB ligand (RANKL), and osteoprotegerin (OPG) (7). The mouse second molar (M2) develops shortly after the mouse M1; however, the mouse third molar (M3) tooth bud only becomes apparent perinatally and mor- phogenesis continues postnatally. The mouse M3 may resemble the secondary human dentition, which also integrates into mineralized bone. Similarly, potential stem-cell-based tooth-replacement treatments in molec- ular dentistry require anchorage of an engineered soft tooth germ into hard bone tissue (7–9). These facts indicate that the mouse M3 represents a suitable model for studying the basic principles of tooth and bone integration and these may be compared with the M1 at the same stage of tooth development, but with a different mineralization status of the surrounding bone. The mouse M3 is also interesting to study as it is one of the most frequent teeth to be anomalous or absent in both mice and humans. Several studies have focused on the mouse M3, especially on its absence or abnormalities in the jaw, in different mouse strains (10–15). The findings, among others, revealed the existence of a competitive relationship between neighbouring teeth, where the size of the mouse M1 is a limiting factor for the growth of the M2 and the M3 (12, 13). This relationship was recently formulated into a mathematical model, where the size of the M1 predicts the size of the M2 and the M3 (16, 17). Chlastakova I, Lungova V, Wells K, Tucker AS, Radlanski RJ, Misek I, Matalova E. Morphogenesis and bone integration of the mouse mandibular third molar. Eur J Oral Sci 2011; 119: 265–274. Ó 2011 Eur J Oral Sci The mouse third molar (M3) develops postnatally and is thus a unique model for studying the integration of a non-mineralized tooth with mineralized bone. This study assessed the morphogenesis of the mouse M3, related to the alveolar bone, comparing M3 development with that of the first molar (M1), the most common model in odontogenesis. The mandibular M3 was evaluated from initiation to eruption by morphology and by assessing patterns of proliferation, apoptosis, osteoclast distri- bution, and gene expression. Three-dimensional reconstruction and explant cultures were also used. Initiation of M3 occurred perinatally, as an extension of the second molar (M2) which grew into a region of soft mesenchymal tissue above the M2, still far away from the alveolar bone. The bone-free M3 bud gradually became encapsulated by bone at the cap stage at postnatal day 3. Osteoclasts were first visible at postnatal day 4 when the M3 came into close contact with the bone. The number of osteoclasts increased from postnatal day 8 to postnatal day 12 to form a space for the growing tooth. The M3 had erupted by postnatal day 26. The M3, although smaller than the M1, passed through the same developmental stages over a similar time span but showed differences in initiation and in the timing of bone encapsulation. Eva Matalova, Laboratory of Animal Embryology, IAPG, Academy of Sciences, Veveri 97, 602 00 Brno, Czech Republic Telefax: +42–3–15639510 E-mail: matalova@iach.cz Key words: 3D models; apoptosis; osteoclast activity; proliferation; tooth development Accepted for publication May 2011 Eur J Oral Sci 2011; 119: 265–274 DOI: 10.1111/j.1600-0722.2011.00838.x Printed in Singapore. All rights reserved Ó 2011 Eur J Oral Sci European Journal of Oral Sciences