Trabecular cartilages (trabeculae cranii, sometimes simply trabeculae, prechordal cartilages) are paired, rod-shaped cartilages, which develop in the

head A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals may no ...

of the

vertebrate Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

. They are the primordia of the anterior part of the

cranial base The base of skull, also known as the cranial base or the cranial floor, is the most inferior area of the skull. It is composed of the endocranium and the lower parts of the calvaria. Structure Structures found at the base of the skull are for e ...

, and are derived from the cranial neural crest cells.

Structure

The trabecular cartilages generally appear as a paired, rod-shaped cartilages at the ventral side of the

forebrain In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the rostral (forward-most) portion of the brain. The forebrain (prosencephalon), the midbrain (mesencephalon), and hindbrain (rhombencephalon) are the three prima ...

and lateral side of the

adenohypophysis A major organ of the endocrine system, the anterior pituitary (also called the adenohypophysis or pars anterior) is the glandular, anterior lobe that together with the posterior lobe (posterior pituitary, or the neurohypophysis) makes up the pi ...

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 A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...

are of

mesodermal The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical Embry ...

origin in vertebrates. Especially axial skeletal elements, such as the vertebrae, are derived from the

paraxial mesoderm Paraxial mesoderm, also known as presomitic or somitic mesoderm is the area of mesoderm in the neurulating embryo that flanks and forms simultaneously with the neural tube. The cells of this region give rise to somites, blocks of tissue runnin ...

(e.g.,

somites The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide in ...

), which is regulated by molecular signals from the

notochord In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord cons ...

. Trabecular cartilages, however, originate from the

neural crest Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, ...

, and since they are located anterior to the rostral tip of the notochord, they cannot receive signals from the notochord. Due to these specializations, and their essential role in cranial development, many comparative morphologists and

embryologists Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''-logia'') is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and ...

have argued their developmental or evolutionary origins. The general theory is that the trabecular cartilage is derived from the neural crest

mesenchyme Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every ...

which fills anterior to the mandibular arch (premandibular domain).

Other animals

Lamprey

As clearly seen in the

lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...

Cyclostome Cyclostome is a biological term (from the Greek for "round mouth") used in a few different senses: * for the taxon Cyclostomi, which comprises the extant jawless fishes: the hagfish (Myxini) and the lampreys (Petromyzontidae). This was thought for ...

also has a pair of cartilaginous rods in the embryonic head which is similar to the trabecular cartilages in jawed vertebrates. However, in 1916, Alexei Nikolajevich Sewertzoff pointed out that the cranial base of the lamprey is exclusively originated from the paraxial mesoderm. Then in 1948, 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 (true) trabecular cartilages were firstly acquired in the

Gnathostome Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...

lineage.

History

The trabecular cartilages were first described in the

grass snake The grass snake (''Natrix natrix''), sometimes called the ringed snake or water snake, is a Eurasian non- venomous colubrid snake. It is often found near water and feeds almost exclusively on amphibians. Subspecies Many subspecies are recogn ...

Martin Heinrich Rathke Martin Heinrich Rathke (25 August 1793, Danzig – 3 September 1860, Königsberg) was a German embryologist and anatomist. Along with Karl Ernst von Baer and Christian Heinrich Pander, he is recognized as one of the founders of modern embryolo ...

at 1839.Entwicklungsgeschichte der Natter. Konigsberg (1839) In 1874,

Thomas Henry Huxley Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist specialising in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution. The stor ...

suggested that the trabecular cartilages are a modified part of the

splanchnocranium The splanchnocranium (or visceral skeleton) is the portion of the cranium that is derived from pharyngeal arches. ''Splanchno'' indicates to the gut because the face forms around the mouth, which is an end of the gut. The splanchnocranium consists o ...

: they arose as the serial homologues of the

pharyngeal arches The pharyngeal arches, also known as visceral arches'','' are structures seen in the embryonic development of vertebrates that are recognisable precursors for many structures. In fish, the arches are known as the branchial arches, or gill arch ...

. The vertebrate

jaw The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serv ...

is generally thought to be the modification of the

mandibular arch The pharyngeal arches, also known as visceral arches'','' are structures seen in the embryonic development of vertebrates that are recognisable precursors for many structures. In fish, the arches are known as the branchial arches, or gill ar ...

(1st pharyngeal 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 (so-called "premandibular arches") anterior to the mandibular arch. The existence of premandibular arch(es) has been accepted by many comparative embryologists and morphologists (e.g.,

Edwin Stephen Goodrich Edwin Stephen Goodrich FRS (Weston-super-Mare, 21 June 1868 – Oxford, 6 January 1946), was an English zoologist, specialising in comparative anatomy, embryology, palaeontology, and evolution. He held the Linacre Chair of Zoology in the Univer ...

Gavin de Beer Sir Gavin Rylands de Beer (1 November 1899 – 21 June 1972) was a British evolutionary embryologist, known for his work on heterochrony as recorded in his 1930 book ''Embryos and Ancestors''. He was director of the Natural History Museum, Lon ...

). Moreover, Erik Stensio reported premandibular arches and the corresponding branchiomeric nerves by the reconstruction of the Osteostracans (e.g., ''

Cephalaspis ''Cephalaspis'' (from el, κεφαλή , 'head' and el, ἀσπίς , 'shield') is a possibly monotypic genus of extinct osteostracan agnathan vertebrate. It was a trout-sized detritivorous fish that lived in the early Devonian. Description ...

''; recently this arch was reinterpreted as the mandibular arch) However, the existence of the premandibular arch(es) has been rejected, and the trabecular cartilages are no longer assumed to be one of the pharyngeal arches.

Footnotes

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References

* Couly GF, Coltey PM, Le Douarin NM. 1993. The triple origin of skull in higher vertebrates: A study in quail-chick chimeras. Development 117:409?429. * de Beer GR. 1937. The Development of the Vertebrate Skull. Oxford University Press, London. Anatomy Animal morphology Evolution Animal developmental biology Embryology