The Prechordal Cranium
The neurocranium arises from the cephalic crest cells and the head mesoderm. The prechordal cranium is located rostral (toward the oral or nasal region) to the notochord and originates from the cranial neural crest. This differs from the chordal cranium, which is derived from head mesoderm. Trabeculae, or small rod-like tissue elements, have been thought to fuse at the midline around Rathke’s pouch to form the Trabecula Communis (TC). The TC is a precursor for the nasal septum and the interorbital septum in nonmammalian amniotes. However, the origins of TC development remain controversial, specifically regarding the possibility of dual origins of this structure. This study examines the origins of the prechordal central stem by using molecular markers to determine the fate of those neural crest cells involved in its formation.
Figure 1 illustrates the prechordal chondrocranium development in the chicken embryo at stage 29 (A and A’). Embryos were stained with alcian blue to observe catilaginous elements. The first detected procartilaginous condensation occurs at this stage, which indicates the formation of the two trabecular cartilages. These two structures meet at the midline with the lighter-stained Trabecula Commons.
Origins of the Prechordal Central Stem in the Chicken Embryo
Molecular markers can be used to observe the initial process of the formation of the prechordal central stem. These molecular markers were specific to early chondrocytes, the cells that are found in cartilage. By analyzing the fate of the neural crest cells, which are multipotent cells that eventually specialize to become specific cells, such as melanocytes, peripheral neurons, and glia, it is possible to better clarify the process of the stem formation.
Neural crest cells were found to migrate along two different pathways from the neural crest: the preoptic pathway, rostral to the optic vesicle, and the postoptic pathway, caudal (toward the tail) to the optic vesicle. The fate of the neural crest cells of each pathway might have different cartilaginous components and were tested for this by neural crest cell transplantations between chicken and quail embryos and focal labeling of neural crest cell pathways.
Labeled neural crest cells transplanted from quail to chicken embryos were found in the maxillary and mandibular processes, but not in the medial region of the prechordal cranium. These cells contribute only to trabecular formation.
Neural crest cells transplanted from quail to chicken embryos were found in the medial region of the oral roof and were not found in the posterolateral roof or maxillary process. This indicates that these cells form the single median intertrabecula.
Figure 2 shows the preoptic pathway (light blue) and the postoptic pathway (dark blue) taken by neural crest cells from the neural crest in the chicken embryo (Fig 2 A and B). It also illustrates how the cells are distributed in the final structure of the prechordal central stem of the zebrafish embryo and the chicken embryo according to the pathway used to migrate (Fig. 2C).
The prechordal central stem was previously thought to be one continuous structure. Results suggest that two types of cartilaginous components contribute to form the prechordal central stem in chicken embryos. These two components are the paired trabeculae and a single median intertrabecula. These components are derived from trigeminal neural crest cells that migrate along two different pathways, which are the postoptic and preoptic streams, respectively. However, some mixing of cells might have occurred and cannot be disproven in the prechordal cranium formation. Whether the mammalian prechordal cartilage has dual origins similar to the chicken embryo is yet untested, but cannot be concluded from these results. Future studies in mammalian embryos might find homologous dual origins.
Tarazona O A, Fabrezi M, Ramirez-Pinilla M P. 2008. Cranial morphology of Bachia bicolor (Squamata: Gymnophthalmidae) and its postnatal development. Zoological Journal of Linnean Society, 152: 775-792.
Wada N, Nohno T, Kuratani S. 2011. Dual origins of the prechordal cranium in the chicken embryo.Developmental Biology, 356(2):529-540.
Yakota Y, Saito D, Tadokoro R, Takahashi Y. 2011. Genomically integrated transgenes are stably and conditionally expressed in neural crest cell-specific lineages. Developmental Biology, 353(2):382-395.