Figure 6

The developmental process of cranial dermal denticles in Ancistrus sp. (A) Cranial dermal denticle morphogenesis progresses through the evolutionarily conserved odontode developmental stages. First, the cranial epithelium thickens to form the epithelial placode. The placode invaginates and forms a bud-shaped epithelial layer (EM early morphogenesis). Subsequently, condensed mesenchymal cells invade the lumen of the denticle germ, leading to the formation of denticle papilla (LM late morphogenesis). The denticle germ forms circular epithelial tissue enclosing the denticle papilla (ED early differentiation). The denticle papilla then differentiates into dentin inside the denticle germ, forming a bell-shaped structure (LD late differentiation). At this stage, underlying dermal bony plates differentiate underneath the dental germ. LM, ED, and LD may correspond to the bud, cap, and bell stage of tooth development, respectively. Dentin finally erupts from the epithelium, being anchored to dermal bony plates (eruption stage). The development of primary and secondary cranial dermal denticles is regulated by evolutionarily conserved oGRN. Among oGRN, pitx2 exhibits the earliest expression in the placode and induces the development of cranial dermal denticles. Pitx2 and shha are expressed in the epithelial layer surrounding the dentin cone. Dlx2a, Wnt10a, b-catenin, and bmp4 are broadly expressed in both the epithelium and underlying mesenchyme. Pax9 and fgf3 transcripts were detected in the epithelium and mesenchyme laterally to the dentin cone. (B) The formation of cranial dermal denticles in the course of Ansitrus sp. development. From the embryonic to the early larva stage (14 dpf), primary cranial dermal denticles form in the restricted positions of the cranial epidermis. After primary cranial dermal denticles shed and disappear, secondary cranial dermal denticles begin to form. The number of secondary cranial dermal denticles increases along with the expansion of underlying dermal bony plates, constructing the head armor.