Figure 3 The neurocephalic rotation during neurulation centred on the ...
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Figure 3 The neurocephalic rotation during neurulation centred on the chordal apex (above) and the chondrocranial flexure acquired during the 8 th week (Carnegie stage 23) (below pictures from Levi 1900 in Dambricourt Malassé 1988, 1993, 2006a). H1: horizontal axis or the cephalic segment of the notochord, H2 vertical axis crossing the chordal apex, H3: pre-chordal horizontal axis. Cm: Meckel’s cartilaginous, Ms: mesencephalon, Met: metencephalon, My: myelencephalon, N: notochorde, P: prosencephalon, R: rhombencephalon, Tel: telencephalon.

The mammalian embryo distinguishes itself by an anterior-posterior polarity, noticeable along the notochord and by a dorsal-ventral polarity. In quadrupedal mammals, antero-posterior polarity is preserved until the adult stage. In humans, however, it disappears well before birth, reduced by the distance between the prosthion and the opistocranion, and it merges with the dorsal-ventral polarity, entirely redistributed along the vertical axis. This reduction of the antero-posterior polarity in humans is related to the verticalization of the cephalic termination of the metamerized axial skeleton (Dambricourt Malassé 1988, 2006, 2009a, b). Until the sixth week of the embryonic development, the cephalic skeleton is formed only by the future base, the cartilaginous blastema with the body of the sphenoid and the basi-occipital in the same alignment (Figure 3). We showed that the breaking of this plane (the planum basale) takes place during the closure of the neural groove at the apex of the notochord and is caused by the interaction of cellular tensions occurring between the cells of the neural plate, just above the discontinuity between the notochord and the pre-chordal plate (Dambricourt Malassé 1987, 1988, 1993, 2006, 2009). The neural groove does not close in this area where specific constraints develop, taking place between the segmented axial skeleton (metamerized) and the pre-chordal head skeleton, i.e. between the body of the sphenoid and the basi-occipital. This neural kinetics, located in front of the notochord and centred on its apex, follows an anteroposterior rotation which straightens the head of the embryo. The vector expands beyond a vertical line passing through the apex of the notochord. The neural groove continues with a reverse rotation forward and downward. The underlying planum basale formed by the cartilaginous blastema initiates two kinetics “mirroring each other”. The posterior sphenoid initiates a rotation in the direction of the neural kinetics at the spheno-occipital synchondrosis causing the downward and forward bending of the basioccipital and of the future cerebellar fossa. The anterior sphenoid (pre-sphenoid) initiates a reversed kinetics.

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Original Synthetic Article: The relationships between occlusion and posture in the hominid lineage, implications for the transition between mesolithic and neolithic …

Abstract
Background and Aim: Connecting the occlusion to the posture during growth consists in studying the correlations between the contact of both dental arches and the constraints acting within the skull base, at the level of the sphenoidal synchondroses. They articulate the ethmoid, sphenoid and basi-occipital anlages which are in the same alignment during the first weeks of the embryonic period. The basi-occipital belongs to the metamerised axial skeleton (petrous bones, exo-occipital, vertebral column and sacrum), the sphenoid and ethmoid belong to the prechordal territories. The simiiforms are the only mammals where embryonic kinetics breaks the alignment by a flexure sitting at the chordal-prechordal limit because of increasing tensions acting during neurulation. The degree of verticality of both facial and axial skeletons (i.e. the position of the cerebellar fossa) of the two temporo-mandibular joints and the mandibular dental arch depend on these late embryonic kinetics. The future occlusion between the maxillar and the mandibular dental arches requires coherent dynamics between the three sphenoidal synchondroses. Materials and Methods: These correlations were analyzed by using a metric methodology applied to x-rays or CT scan images of extant and fossil skulls. Results and conclusions: This approach presents major interests for the study of occluso-postural balances in fossil hominids as well as for the analysis of the evolutionary mode, tempo and phylogenetic association, in particular when dealing with the emergence of the contemporary occlusion at about 7,000 years BP. A better knowledge of this transition can be useful to understand the increasing ageneses and occluso-postural dysharmonies in modern populations.