The trabecular cartilages generally appear as a paired, rod-shaped cartilages at the ventral side of the forebrain and lateral side of the adenohypophysis in the vertebrate embryo. During development, their anterior ends fuse and form the trabecula communis. Their posterior ends fuse with the caudal-most parachordal cartilages.
Development
Most skeletons are of mesodermal origin in vertebrates. Especially axial skeletal elements, such as the vertebrae, are derived from the paraxial mesoderm, which is regulated by molecular signals from the notochord. Trabecular cartilages, however, originate from the neural crest, and since they are located anterior to the rostral tip of the notochord, they cannot receive signals from the notochord. Due to these specialisations, and their essential role in cranial development, many comparative morphologists and embryologists have argued their developmental or evolutionary origins. The general theory is that the trabecular cartilage is derived from the neural crestmesenchyme which fills anterior to the mandibular arch.
Other animals
Lamprey
As clearly seen in the lamprey, Cyclostome also has a pair of cartilaginous rods in the embryonic head which is similar to the trabecular cartilages in jawed vertebrates. However, in 1916, Alexej Nikolajevich Sewertzoff pointed out that the cranial base of the lamprey is exclusively originated from the paraxial mesoderm. Then in 1948, :de:Alf Johnels|Alf Johnels reported the detail of the skeletogenesis of the lamprey, and showed that the “trabecular cartilages” in lamprey appear just beside the notochord, in a similar position to the parachordal cartilages in jawed vertebrates. Recent experimental studies also showed that the cartilages are derived from the head mesoderm. The “trabecular cartilages” in the Cyclostome is no longer considered to be the homologue of the trabecular in the jawed vertebrates: the trabecular cartilages were firstly acquired in the Gnathostome lineage.
History
The trabecular cartilages were first described in the grass snake by :de:Martin Heinrich Rathke|Martin Heinrich Rathke at 1839. In 1874, Thomas Henry Huxley suggested that the trabecular cartilages are a modified part of the splanchnocranium: they arose as the serial homologues of the pharyngeal arches. The vertebrate jaw is generally thought to be the modification of the mandibular arch. Since the trabecular cartilages appear anterior to the mandibular arch, if the trabecular cartilages are serial homologues of the pharyngeal arches, ancestral vertebrates should possess more than one pharyngeal arch anterior to the mandibular arch. The existence of premandibular arch has been accepted by many comparative embryologists and morphologists. Moreover, :de: Erik Stensio|Erik Stensio reported premandibular arches and the corresponding branchiomeric nerves by the reconstruction of the Osteostracans However, the existence of the premandibular arch has been rejected, and the trabecular cartilages are no longer assumed to be one of the pharyngeal arches.
