Cells at the Edge: The Dentin–Bone Interface in Zebrafish Teeth

Abstract

Bone-producing osteoblasts and dentin-producing odontoblasts are closely related cell types, a result from their shared evolutionary history in the ancient dermal skeleton. In mammals, the two cell types can be distinguished based on histological characters and the cells’ position in the pulp cavity or in the tripartite periodontal complex. Different from mammals, teleost fish feature a broad diversity in tooth attachment modes, ranging from fibrous attachment to firm ankylosis to the underlying bone. The connection between dentin and jaw bone is often mediated by a collar of mineralized tissue, a part of the dental unit that has been termed “bone of attachment”. Its nature (bone, dentin, or an intermediate tissue type) is still debated. Likewise, there is a debate about the nature of the cells secreting this tissue: osteoblasts, odontoblasts, or yet another (intermediate) type of scleroblast. Here, we use expression of the P/Q rich secretory calcium-binding phosphoprotein 5 (scpp5) to characterize the cells lining the so-called bone of attachment in the zebrafish dentition. scpp5 is expressed in late cytodifferentiation stage odontoblasts but not in the cells depositing the “bone of attachment”. nor in bona fide osteoblasts lining the supporting pharyngeal jaw bone. Together with the presence of the osteoblast marker Zns-5, and the absence of covering epithelium, this links the cells depositing the “bone of attachment” to osteoblasts rather than to odontoblasts. The presence of dentinal tubule-like cell extensions and the near absence of osteocytes, nevertheless distinguishes the “bone of attachment” from true bone. These results suggest that the “bone of attachment” in zebrafish has characters intermediate between bone and dentin, and, as a tissue, is better termed “dentinous bone”. In other teleosts, the tissue may adopt different properties. The data furthermore support the view that these two tissues are part of a continuum of mineralized tissues. Expression of scpp5 can be a valuable tool to investigate how differentiation pathways diverge between osteoblasts and odontoblasts in teleost models and help resolving the evolutionary history of tooth attachment structures in actinopterygians.

Plicidentine in the oral fangs of parrotfish (Scarinae, Labriformes)

“…Newly erupted, but not yet functional fangs do not contact the bony edges of the socket (Figure 6a). Later on, fangs fuse to the jaws via progressive mineralization of alveolar bone (“attachment bone” sensu Fink, 1981; “spongy bone” sensu Shellis, 1982; Rosa et al, 2021) between the external dentinal wall and the bony border of the socket (Figure 6b‐f). Once the fang is attached externally, a second phase of alveolar bone mineralization invades the pulp cavity (Figures 5c and 6c), which becomes entirely filled in with bone in the oldest functional teeth (Figure 6d).…”

Dental structure and tooth attachment modes in the common fangtooth Anoplogaster cornuta (Valenciennes, 1833) (Actinopterygii; Trachichthyiformes; Anoplogastridae)

“…The nature of the bone of attachment, i.e., its classification as either dentin, bone, or an intermediate type of mineralized tissue, has long been debated (for a review see [8]). Recent studies in zebrafish (Danio rerio) demonstrated that the cells that deposit the bone of attachment express genetic markers of osteoblasts but not of odontoblasts [8].…”

“…The highly variably feeding modes of teleost fishes and the associated manifold mechanical demands on their teeth are reflected by a huge diversity of tooth forms and different modes of attachment of the teeth to their supporting (dentigerous) bones [4,7,8]. These attachment modes range from a firm fixation (ankylosis) by a bridging mineralized tissue located between the dentinal tooth base and the dentigerous bone to specialized types of fibrous attachment that allow for a certain mobility of the teeth against their supporting jaw bones [4,7].…”

“…These attachment modes range from a firm fixation (ankylosis) by a bridging mineralized tissue located between the dentinal tooth base and the dentigerous bone to specialized types of fibrous attachment that allow for a certain mobility of the teeth against their supporting jaw bones [4,7]. Most authors consider ankylosis between tooth and dentigerous bone as the ancestral character state [4,7,8]. Accordingly, fibrous attachment modes are thought to represent a derived condition, brought about by a cessation of mineralization before complete ankylosis between tooth base and dentigerous bone is achieved.…”

“…Fink [7] distinguished four major types of tooth attachment in actinopterygian fishes, with Types 1 and 2 being the more frequent ones. In Type 1, the teeth are rigidly ankylosed to the underlying dentigerous bone by a mineralized tissue, referred to as attachment bone or bone of attachment [4,8]. In the Type 2 mode of attachment, a dividing zone at the base of the teeth remains completely unmineralized and the dentinal tooth base is connected to the underlying bone of attachment by a ringlike collagenous ligament, which allows some movement of the tooth against the bone.…”