Herbert Renz

Tooth-bone morphogenesis during postnatal stages of mouse first molar development.

The first mouse molar (M1) is the most common model for odontogenesis, with research particularly focused on prenatal development. However, the functional dentition forms postnatally, when the histogenesis and morphogenesis of the tooth is completed, the roots form and the tooth physically anchors into the jaw. In this work, M1 was studied from birth to eruption, assessing morphogenesis, proliferation and apoptosis, and correlating these with remodeling of the surrounding bony tissue. The M1 completed crown formation between postnatal (P) days 0–2, and the development of the tooth root was initiated at P4. From P2 until P12, cell proliferation in the dental epithelium reduced and shifted downward to the apical region of the forming root. In contrast, proliferation was maintained or increased in the mesenchymal cells of the dental follicle. At later stages, before tooth eruption (P20), cell proliferation suddenly ceased. This withdrawal from the cell cycle correlated with tooth mineralization and mesenchymal differentiation. Apoptosis was observed during all stages of M1 postnatal morphogenesis, playing a role in the removal of cells such as osteoblasts in the mandibular region and working together with osteoclasts to remodel the bone around the developing tooth. At more advanced developmental stages, apoptotic cells and bodies accumulated in the cell layers above the tooth cusps, in the path of eruption. Three‐dimensional reconstruction of the developing postnatal tooth and bone indicates that the alveolar crypts form by resorption underneath the primordia, whereas the ridges form by active bone growth between the teeth and roots to form a functional complex.

Genes, forces, and forms: Mechanical aspects of prenatal craniofacial development

Current knowledge of molecular signaling during craniofacial development is advancing rapidly. We know that cells can respond to mechanical stimuli by biochemical signaling. Thus, the link between mechanical stimuli and gene expression has become a new and important area of the morphological sciences. This field of research seems to be a revival of the old approach of developmental mechanics, which goes back to the embryologists His ( 1874 ), Carey ( 1920 ), and Blechschmidt ( 1948 ). These researchers argued that forces play a fundamental role in tissue differentiation and morphogenesis. They understood morphogenesis as a closed system with living cells as the active part and biological, chemical, and physical laws as the rules. This review reports on linking mechanical aspects of developmental biology with the contemporary knowledge of tissue differentiation. We focus on the formation of cartilage (in relation to pressure), bone (in relation to shearing forces), and muscles (in relation to dilation forces). The cascade of molecules may be triggered by forces, which arise during physical cell and tissue interaction. Detailed morphological knowledge is mandatory to elucidate the exact location and timing of the regions where forces are exerted. Because this finding also holds true for the exact timing and location of signals, more 3D images of the developmental processes are required. Further research is also required to create methods for measuring forces within a tissue. The molecules whose presence and indispensability we are investigating appear to be mediators rather than creators of form. Developmental Dynamics 235:1219–1229, 2006.

Incremental Lines in Root Cementum of Human Teeth: An Approach to their Ultrastructural Nature by Microscopy

In ground sections of human teeth, root cementum shows under the light microscope as alternating, almost concentric, dark and light rings. In paleontology and forensic medicine, the number of these incremental lines or annulations is used to derive the age-at-death of the individual. To find the ultrastructural features underlying these cemental annulations, we used bright-field light microscopy (LM), confocal laser scanning microscopy (CLSM), transmission electron microscopy (TEM), and electron-dispersive x-radiation (EDX) in a scanning electron microscope (SEM). Annulations visible in ground sections of about 100-μm thickness were no longer visible in semi-thin sections (thickness, 1-2 μm) of the same specimen in the same area. The assumption that annulations could be caused by superimposing structures in the depth of field of the light microscopes objective lens was not verified by CLSM. Fiber bundles of higher density than the surrounding matrix in TEM micrographs could not be connected unambiguously with annulations in LM micrographs. After all, the ultrastructural nature of cemental annulations remains an open question.

Prenatal development of the muscles in the floor of the mouth in human embryos and fetuses from 6.9 to 76 mm CRL

The development of the muscles in the floor of the mouth is described in 10 human embryos and fetuses ranging from 6.9 to 76 mm CRL by means of computer-aided graphical 3D-reconstructions. All primordia of the muscles in the floor of the mouth could be identified from the 15.6 mm CRL stage on. The proportions and insertion lines of the early muscles were found to be different from adult anatomy. Each muscle first inserted in the medial surface of Meckels cartilage, but during the developmental period between 19 and 68 mm CRL the insertion lines were gradually transposed to the bony ridges of the mandible which progrediently embraced Meckels cartilage. The fibers of the mylohyoid muscles left the anterior region near the symphysis mentalis free during all stages of this study. The digastric muscle revealed only one belly with a constriction of its continuous fibers where it passed the hyoid bone primordium. There was no attachment of digastric muscle fibers to the hyoid; only geniohyoid and mylohyoid fibers. Geniohyoid and genioglossus muscles basically correspond to their definite arrangement, but they underwent proportional changes. Individual specimens embodied irregularities such as accessory geniohyoid and hyoid portions and muscle fibers separate from the mylohyoide muscle.

A scanning electron-microscopic study of developing human deciduous enamel on the dependence of the outline of surface pits on the angle of observation

On the developing enamel surfaces of fetal human deciduous teeth, many of the surface pits were arcade-shaped with the arcade preferentially pointing in a cervical direction. The configuration of the interprism ridges between the pits contributed to this appearance. Surface cracks allowed verification of an incisal inclination of the subsurface prisms. This apparent paradox was solved when the specimens were tilted so that the pits were viewed in the directions of the prisms, giving the pits a compressed arcade-shape with the arcades pointing incisally. It is recommended that care should be exercised and due attention paid to the angle of observation when determining the orientation of pit arcades. Pit entry direction seems to be a more reliable feature for inferring the direction of tangential ameloblast movement.

An atlas of prenatal development of the human orofacial region.

There are several atlases available showing prenatal human development. However, none is focused on prenatal orofacial development during maxillary and mandibular bone formation. These events, together with dental development and formation of the temporomandibular joint, take place during several fetal stages. While photographic atlases are limited to depicting the outer shape, and atlases based on histological sections only show a series of single sections, an atlas based on three-dimensional reconstructions from serial sections can show both the outer skin and the structures underneath, which can be electronically dissected layer by layer. In this Focus article, we present our atlas on prenatal human orofacial development, which is accessible online at the Journals website.

Morphogenesis of the compartmentalizing bone around the molar primordia in the mouse mandible during dental developmental stages between lamina, bell-stage, and root formation (E13–P20)

Despite increasing knowledge of the basic molecular aspects of bone formation and its regulation, the mechanisms of bone morphogenesis leading to a topologically specific shape remain unknown. The formation of the alveolar bone, which houses the dental primordia and later, the dental roots, may serve as a model to understand the formation of bone form in general. Thirty-eight heads of mice (C57 Bl/6J) ranging from stages E13-P20 were used to prepare histological serial sections. For each stage, virtual 3D-reconstructions were made in order to study the morphogenesis of the mandibular molar primordia concomitantly with their surrounding bone. Special focus was given to recording the remodeling pattern. It has been shown that, in early stages (E13, E14), bone formation is characterized by apposition only. In stage E15, the bony crypt around the dental primordia is remodeled mostly by resorption of bone. In stage E18, the bone remodeling pattern shows resorption all along the bony gutter, which houses the molar primordia. The medial and lateral margins are characterized by apposition. At birth (stage P0), a bony septum has begun to form between the primordium m1 and of m2, arising from both sides and characterized by pure apposition of bone. In stage P4, the crypts of m1 and m2, and also that of m3, show bone resorption inside, while the medial and lateral bony margins show apposition of bone throughout. Generally, during development, the bone gradually encapsulates the dental primordia, in such a way that the bone reaches over the dental primordia and leaves only a continuous longish opening of about 200μm width. The opening at the occlusal surface of m1, at the time of eruption, starting at stage P14, appears to have increased in size again. The distance between bone and dental primordium undergoes change during development. In erupted molars, it is around 100μm, during early developmental stages, it may be as less as 20μm. These data show the inevitability of bone remodeling.

Chondral ossification centers next to dental primordia in the human mandible: A study of the prenatal development ranging between 68 to 270 mm CRL

The human mandible is said to arise from desmal ossification, which, however, is not true for the entire body of the mandible: Meckels cartilage itself is prone to ossification, at least its anterior part in the canine and incisor region. Also, within the coronoid and in the condylar processes there are cartilaginous cores, which eventually undergo ossification. Furthermore, there are a number of additional single cartilaginous islets arising in fetuses of 95mm CRL and more. They are located predominantly within the bone at the buccal sides of the brims of the dental compartments, mostly in the gussets between the dental primordia. They become wedge-shaped or elongated with a diameter of around 150-500μm and were also found in older stages up to 225mm CRL, which was the oldest specimen used in this study. This report is intended to visualize these single cartilaginous islets histologically and in 3-D reconstructions in stereoscopic images. Although some singular cartilaginous tissue within the mandible may be remains of the decaying Meckels cartilage, our 3-D reconstructions clearly show that the aforementioned cartilaginous islets are independent thereof, as can be derived from their separate locations within the mandibular bone. The reasons that lead to these cartilaginous formations have remained unknown so far.

The remodeling pattern of human mandibular alveolar bone during prenatal formation from 19 to 270mm CRL.

The underlying mechanisms of human bone morphogenesis leading to a topologically specific shape remain unknown, despite increasing knowledge of the basic molecular aspects of bone formation and its regulation. The formation of the alveolar bone, which houses the dental primordia, and later the dental roots, may serve as a model to approach general questions of bone formation. Twenty-five heads of human embryos and fetuses (Radlanski-Collection, Berlin) ranging from 19mm to 270mm (crown-rump-length) CRL were prepared as histological serial sections. For each stage, virtual 3D-reconstructions were made in order to study the morphogenesis of the mandibular molar primordia with their surrounding bone. Special focus was given to recording the bone-remodeling pattern, as diagnosed from the histological sections. In early stages (19-31mm CRL) developing bone was characterized by appositional only. At 41, in the canine region, mm CRL bony extensions were found forming on the bottom of the trough. Besides general apposition, regions with resting surfaces were also found. At a fetal size of 53mm CRL, septa have developed and led to a compartment for canine development. Furthermore, one shared compartment for the incisor primordia and another shared compartment for the molars also developed. Moreover, the inner surfaces of the dental crypts showed resorption of bone. From this stage on, a general pattern became established such that the compartmentalizing ridges and septa between all of the dental primordia and the brims of the crypts were noted, and were due to appositional growth of bone, while the crypts enlarged on their inner surfaces by resorption. By 160mm CRL, the dental primordia were larger, and all of the bony septa had become reduced in size. The primordia for the permanent teeth became visible at 225mm CRL and shared the crypts of their corresponding deciduous primordia.