What do we mean by the directions “cranial” and “caudal” on a vertebra?

In illustrating vertebrae, it is important to consistently depict their orientation, so we can objectively assess and compare the slope of the neural arch, neural canal, or articular surfaces. However, differing vertebral shapes across taxa and across regions of the spinal column make it difficult to maintain consistency, or even define what we mean by the directions “cranial” and “caudal”. Consequently, characters such as “Neural arch slopes cranially 30° relative to the vertical” are disputable rather than objective measurements. Cranial and caudal are defined as directed along the horizontal axis, but several different notions of “horizontal” are possible: 1. Long axis of centrum is horizontal. This is appealing for elongate vertebrae such as sauropod cervicals, but is not always well defined, and is difficult to determine for craniocaudally short vertebrae such as most caudals. 2. Articular surfaces of centrum are vertical. Difficult to determine when dealing with facets that are concave or (worse) convex; and ambiguous for “keystoned” vertebrae in which the facets are not parallel. 3. Neural canal is horizontal. Anatomically informative, but difficult to determine in vertebrae that have not been fully prepared or CT-scanned, and impossible to see in lateral view. Ambiguous for vertebrae where the dorsal and ventral margins of the canal are not straight or not parallel. 4. Similarity in articulation (“horizontal” is defined as a line joining the same point on two similarly oriented copies of the same vertebra when optimally articulated). This is less intuitive than definitions 1–3, but takes the entire vertebra into account. We advocate explicitly stating a definition and using it consistently. In most cases, definition 3 (“Neural canal is horizontal”) best reflects anatomical and developmental realities, and it is therefore preferred. Low-tech techniques can be used to determine neural canal orientation with adequate precision for most purposes.

What do we mean by the directions “cranial” and “caudal” on a vertebra?

In illustrating vertebrae, it is important to consistently depict their orientation, so we can objectively assess and compare the slope of the neural arch, neural canal, or articular surfaces. However, differing vertebral shapes across taxa and across regions of the spinal column make it difficult to maintain consistency, or even define what we mean by the directions “cranial” and “caudal”. Consequently, characters such as “Neural arch slopes cranially 30° relative to the vertical” are disputable rather than objective measurements. Cranial and caudal are defined as directed along the horizontal axis, but several different notions of “horizontal” are possible: 1. Long axis of centrum is horizontal. This is appealing for elongate vertebrae such as sauropod cervicals, but is not always well defined, and is difficult to determine for craniocaudally short vertebrae such as most caudals. 2. Articular surfaces of centrum are vertical. Difficult to determine when dealing with facets that are concave or (worse) convex; and ambiguous for “keystoned” vertebrae in which the facets are not parallel. 3. Neural canal is horizontal. Anatomically informative, but difficult to determine in vertebrae that have not been fully prepared or CT-scanned, and impossible to see in lateral view. Ambiguous for vertebrae where the dorsal and ventral margins of the canal are not straight or not parallel. 4. Similarity in articulation (“horizontal” is defined as a line joining the same point on two similarly oriented copies of the same vertebra when optimally articulated). This is less intuitive than definitions 1–3, but takes the entire vertebra into account. We advocate explicitly stating a definition and using it consistently. In most cases, definition 3 (“Neural canal is horizontal”) best reflects anatomical and developmental realities, and it is therefore preferred. Low-tech techniques can be used to determine neural canal orientation with adequate precision for most purposes.

What do we mean by the directions “cranial” and “caudal” on a vertebra?

In illustrating vertebrae, it is important to consistently depict their orientation, so we can objectively assess and compare the slope of the neural arch, neural canal, or articular surfaces. However, differing vertebral shapes across taxa and across regions of the spinal column make it difficult to maintain consistency, or even define what we mean by the directions “cranial” and “caudal”. Consequently, characters such as “Neural arch slopes cranially 30° relative to the vertical” are disputable rather than objective measurements. Cranial and caudal are defined as directed along the horizontal, but several different notions of “horizontal” are possible: 1. Long axis of centrum is horizontal. This is appealing for elongate vertebrae such as sauropod cervicals, but is difficult to determine for craniocaudally short vertebrae such as most caudals. 2. Articular facets of centrum are vertical. Difficult to determine when dealing with facets that are concave or (worse) convex; and ambiguous for “keystoned” vertebrae in which the facets are not parallel. 3. Neural canal is horizontal. Anatomically informative, but difficult to determine in vertebrae that have not been fully prepared or CT-scanned, and impossible to see in lateral view. Ambiguous for vertebrae where the dorsal and ventral margins of the canal are not parallel. 4. When two instances of the vertebra are optimally articulated, the same points are at the same height on both. This is less intuitive than definitions 1–3, but more precise and can be determined for any vertebra. We advocate explicitly stating a definition and using it consistently.

AnFgf8mouse mutant phenocopies human 22q11 deletion syndrome

Deletion of chromosome 22q11, the most common microdeletion detected in humans, is associated with a life-threatening array of birth defects. Although 90% of affected individuals share the same three megabase deletion, their phenotype is highly variable and includes craniofacial and cardiovascular anomalies, hypoplasia or aplasia of the thymus with associated deficiency of T cells, hypocalcemia with hypoplasia or aplasia of the parathyroids, and a variety of central nervous system abnormalities. Because ablation of neural crest in chicks produces many features of the deletion 22q11 syndrome, it has been proposed that haploinsufficiency in this region impacts neural crest function during cardiac and pharyngeal arch development. Few factors required for migration, survival, proliferation and subsequent differentiation of pharyngeal arch neural crest and mesoderm-derived mesenchyme into their respective cardiovascular, musculoskeletal, and glandular derivatives have been identified. However, the importance of epithelial-mesenchymal interactions and pharyngeal endoderm function is becoming increasingly clear.Fibroblast growth factor 8 is a signaling molecule expressed in the ectoderm and endoderm of the developing pharyngeal arches and known to play an important role in survival and patterning of first arch tissues. We demonstrate a dosage-sensitive requirement for FGF8 during development of pharyngeal arch, pharyngeal pouch and neural crest-derived tissues. We show that FGF8 deficient embryos have lethal malformations of the cardiac outflow tract, great vessels and heart due, at least in part, to failure to form the fourth pharyngeal arch arteries, altered expression of Fgf10 in the pharyngeal mesenchyme, and abnormal apoptosis in pharyngeal and cardiac neural crest.The Fgf8 mutants described herein display the complete array of cardiovascular, glandular and craniofacial phenotypes seen in human deletion 22q11 syndromes. This represents the first single gene disruption outside the typically deleted region of human chromosome 22 to fully recapitulate the deletion 22q11 phenotype. FGF8 may operate directly in molecular pathways affected by deletions in 22q11 or function in parallel pathways required for normal development of pharyngeal arch and neural crest-derived tissues. In either case, Fgf8 may function as a modifier of the 22q11 deletion and contribute to the phenotypic variability of this syndrome.

Synergy betweenHoxa1andHoxb1: the relationship between arch patterning and the generation of cranial neural crest

Hoxa1 and Hoxb1 have overlapping synergistic roles in patterning the hindbrain and cranial neural crest cells. The combination of an ectoderm-specific regulatory mutation in the Hoxb1 locus and the Hoxa1 mutant genetic background results in an ectoderm-specific double mutation, leaving the other germ layers impaired only in Hoxa1 function. This has allowed us to examine neural crest and arch patterning defects that originate exclusively from the neuroepithelium as a result of the simultaneous loss of Hoxa1 and Hoxb1 in this tissue. Using molecular and lineage analysis in this double mutant background we demonstrate that presumptive rhombomere 4, the major site of origin of the second pharyngeal arch neural crest, is reduced in size and has lost the ability to generate neural crest cells. Grafting experiments using wild-type cells in cultured normal or double mutant mouse embryos demonstrate that this is a cell-autonomous defect, suggesting that the formation or generation of cranial neural crest has been uncoupled from segmental identity in these mutants. Furthermore, we show that loss of the second arch neural crest population does not have any adverse consequences on early patterning of the second arch. Signalling molecules are expressed correctly and pharyngeal pouch and epibranchial placode formation are unaffected. There are no signs of excessive cell death or loss of proliferation in the epithelium of the second arch, suggesting that the neural crest cells are not the source of any indispensable mitogenic or survival signals. These results illustrate that Hox genes are not only necessary for proper axial specification of the neural crest but that they also play a vital role in the generation of this population itself. Furthermore, they demonstrate that early patterning of the separate components of the pharyngeal arches can proceed independently of neural crest cell migration.

Homeotic transformation of branchial arch identity after Hoxa2 overexpression

Overexpression of Hoxa2 in the chick first branchial arch leads to a transformation of first arch cartilages, such as Meckel's and the quadrate, into second arch elements, such as the tongue skeleton. These duplicated elements are fused to the original in a similar manner to that seen in the Hoxa2 knockout, where the reverse transformation of second to first arch morphology is observed. This confirms the role of Hoxa2 as a selector gene specifying second arch fate. When first arch neural crest alone is targeted, first arch elements are lost, but the Hoxa2-expressing crest is unable to develop into second arch elements. This is not due to Hoxa2 preventing differentiation of cartilages. Upregulation of a second arch marker in the first arch, and homeotic transformation of cartilage elements is only produced after global Hoxa2 overexpression in the crest and the surrounding tissue. Thus, although the neural crest appears to contain some patterning information, it needs to read cues from the environment to form a coordinated pattern. Hoxa2 appears to exert its effect during differentiation of the cartilage elements in the branchial arches, rather than during crest migration, implying that pattern is determined quite late in development.

sucker encodes a zebrafish Endothelin-1 required for ventral pharyngeal arch development

Mutation of sucker (suc) disrupts development of the lower jaw and other ventral cartilages in pharyngeal segments of the zebrafish head. Our sequencing, cosegregation and rescue results indicate that suc encodes an Endothelin-1 (Et-1). Like mouse and chick Et-1, suc/et-1 is expressed in a central core of arch paraxial mesoderm and in arch epithelia, both surface ectoderm and pharyngeal endoderm, but not in skeletogenic neural crest. Long before chondrogenesis, suc/et-1 mutant embryos have severe defects in ventral arch neural crest expression of dHAND, dlx2, msxE, gsc, dlx3 and EphA3 in the anterior arches. Dorsal expression patterns are unaffected. Later in development, suc/et-1 mutant embryos display defects in mesodermal and endodermal tissues of the pharynx. Ventral premyogenic condensations fail to express myoD, which correlates with a ventral muscle defect. Further, expression of shh in endoderm of the first pharyngeal pouch fails to extend as far laterally as in wild types. We use mosaic analyses to show that suc/et-1 functions nonautonomously in neural crest cells, and is thus required in the environment of postmigratory neural crest cells to specify ventral arch fates. Our mosaic analyses further show that suc/et-1 nonautonomously functions in mesendoderm for ventral arch muscle formation. Collectively our results support a model for dorsoventral patterning of the gnathostome pharyngeal arches in which Et-1 in the environment of the postmigratory cranial neural crest specifies the lower jaw and other ventral arch fates.