Morphogenesis Of The Head

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At the initial embryonic stage, three tubes are aligned longitudinally: the neural tube, the notochord and the primitive digestive tube closed at both its caudal and cranial extremities. Two major developments will occur, drastically changing the morphology. First of all, the rapid overgrowth of the neural tube, particularly in its cranial part, will generate the five encephalic vesicles with the cephalic and pontine flexures after closing the neural tube like a 'zipper model' during primary neurulation (2-4 weeks) and secondary neurula-tion (4-6 weeks). Isolated on its lateral border is the neural crest, responsible for the spinal and cranial sensory ganglions as well as postganglionic fibers, glial cells and the adrenal medulla. The construction of the primitive skull base has two parts: first, the posterior part built around the notochord, which plays a role in the genesis of the vertebral bodies, ending in the clivus joining the occipital bone process and a part of the sphenoid at the level of the sella turcica; second, the anterior prechordal part built around the olfactory ethmoid bone and the orbital neurocranium with a typical human skull base angle (kyphotic skull base as opposed to the lordotic skull base of carnivores). The calvarium, the second part of the neurocranium, becomes osseous by a different osteogenesis than the cartilaginous matrix of the skull base, made by direct apposition of bone on the continuous dural membrane. Therefore the growth of the telencephalon is the 'motor' of the expansion of the skull vault which is possible because the cranial sutures are growth lines. The telencephalon initially has a smooth aspect and progressively makes folds called gyri to increase the surface of the cortex. When it finishes its growth, the sutures close, starting from the endocranial side. The anencephaly which is the malformative lack of telencephalon means the absence of a skull vault. The hydrocephalic deformation in young children with anomaly of the circulation of cerebrospinal fluid demonstrates the importance of intracranial pressure for the expansion of the skull. The suture drawing is linear on the endocranial side and sinuous on the exocranial side, due to the alternating tractions of the muscles during growth. However, after a certain age it is absolutely impossible to move the bones, and the explanation for the clinical success of cranial osteopathy has to be found in manipulation not of cranial bones but of the skin covering the skull.

The second major growth process takes place around the primitive intestinal digestive tube which will first be actively open, creating the primitive mouth at the cranial extremity, later dividing into nasal cavity and oral cavity, and the anal canal and its opening at the caudal extremity. The ecto-blast and mesoblast of the area will generate some folds around the digestive tube that we must call 'visceral arches' and not branchial arches. In fact, this denomination originated in Ernst Haeckel's theory of recapitulation, which said the human embryo follows different stages as a phylogenetic recapitulation, which is nonsense. The four visceral arches, contributing by their growth to the setting up of the embryonic face, correspond to the segmentation of the mesoblast with ectodermal and endodermal fissures.

The important point is to remember that every visceral arch will have a skin cover from ectoblast, skeleton and muscles from mesoblast building the

Fig. 2.1 Coronal section of the embryo's head showing the segmentation of the face isolating ectodermic and endodermic fissures and visceral arches.

Fig. 2.1 Coronal section of the embryo's head showing the segmentation of the face isolating ectodermic and endodermic fissures and visceral arches.

splanchnocranium, and a specific cranial nerve from brainstem having motor, sensory and vegetative fibers innervating the territory (Fig. 2.1). Inside, the separation between the respiratory and digestive visceral tubes organizes the mandatory aero-digestive crossroads within the pharynx, allowing first the larynx to inject sounds into the mouth to be 'masticated' as phonemes and second, food to be injected into the esophagus without entering the respiratory tract, explaining the crucial role of the soft palate. The appropriate nomenclature is as follows:

• First visceral arch: 'mandibular arch', later developing the superior maxillary bone, the malar bone, the jaw with the temporomandibular joint (TMJ) and all the masticatory muscles (temporal, masseter, lateral and medial pterygoid, tensor tympani and tensor veli, the anterior part of the digastric, mylohyoid). The cranial nerve here is the trigeminal nerve (CN V) with its motor root and sensory root originating from the trigeminal ganglion with its three branches (ophthalmic, maxillary, mandibular) serving the three areas of the face (Plates IA, IB). A para-sympathetic ganglion is appended to each tri-geminal branch. The orbits contain the eyeballs which are diencephalic nervous expansions and have a roof of neural origin and a floor of visceral origin.

• Second visceral arch: 'hyoid arch'. This refers to the upper half of the hyoid bone with the muscles coming from the mandible (geniohyoid, mylo-hyoid) and the styloid process (stylohyoid, digastric). The cranial nerve which serves this area is the facial nerve (CN VII) with its motor root innervating all the muscles of facial expression, the stylohyoid, the posterior part of the digastric muscle and the stapedian muscle. The facial nerve during its trajectory on the face is a pure motor nerve because the facial muscles responsible for expression and language have no proprio-ceptive receptors and the goniometric control needed for movement coordination is given by CN V innervating the skin in which the muscles are included. The sensitive root of CN VII has a ganglion known as the geniculate ganglion from its location on the flexure of the facial nerve within the ear. It serves the anterior two-thirds of the tongue and a part of the auricle and external auditory meatus. The intermediate nerve (CN VII bis) or Wrisberg nerve should not be further mentioned as such, because it is really the sensory root of CN VII and not a correction of a mistake in counting the cranial nerves. The vegetative centers related to salivary glands (submaxillary and sublingual) are in the part of the brainstem related to the facial nucleus, but the distribution of the postganglionic fibers is made by CN V.

• Third visceral arch: 'hyothyroid arch'. This concerns the lower half of the hyoid bone and the upper part of the thyroid cartilage with the stylo-pharyngeal muscle, the upper and middle pharynx constrictor muscles and the levator veli. The cranial nerve in this area is the glossopharyngeal nerve (CN IX) with motor and sensory root for the posterior third of the lingual mucosa, the upper part of the pharynx and middle ear mucosa and vegetative salivary center for the parotid gland. However, the extrinsic laryngeal muscles (thyrohyoid, sternothyroid, sternocleidohyoid, omohyoid) are innervated by branches of the cervical plexus (C2, C3) which use the hypoglossal nerve (CN XII) as a tutor to make their distribution by forming a loop.

• Fourth visceral arch: 'thyrocricoid arch'. This concerns the lower part of the thyroid cartilage and cricoid cartilage with all the intrinsic laryngeal muscles (thyroarytenoid as vocal cord, posterior and lateral cricoarytenoid, interarytenoid). The cranial nerve here is the vagus nerve (CN X) which has three unequal territories. First, it has a sensory cranial distribution corresponding to its two sensory ganglions (upper jugular and lower plexiform) for mucosa of the pharynx, larynx by the superior laryngeal nerve, tuba auditiva and part of the external auditory meatus. Second, it supplies all the intrinsic laryngeal muscles by the recurrent inferior laryngeal nerve corresponding to the motor root of CN X visible on the lower part of the lateral aspect of the myelencephalon. This nerve has a recurrent trajectory to reach the lower part of the larynx, which represents the anatomical limit of the visceral portion of the head and neck. Third, CN X is the most important parasym-pathetic nerve, conveying vegetative fibers to all viscera of the trunk as far as the pelvis.

Particular mention has to be made of the so-called spinal accessory nerve or accessory nerve (CN XI). It derives from the cervical spinal cord (C1-C4) through fibers going out laterally and traveling up to the foramen magnum and then to the jugular foramen to reach two cranial muscles: the sternocleido-mastoid (SCM) and the trapezius. It contains mainly motor fibers and needs for its proprioceptive part to carry out anastomosis with sensory fibers coming from the cervical plexus which it is possible to isolate by dissection before entering the SCM and the trapezius in the subclavicular space. It is important to understand that the same applies to all the purely motor cranial nerves: the oculomotor (CN III), trochlear (CN IV), abducens (CN VI) and hypo-glossal (CN XII). These all innervate muscles of somitic origin: the six extraocular and the 17 tongue muscles. The mandatory sensory anastomosis is provided by the sensory nerve of the territory, the trigeminal nerve with its three branches.

As a consequence of this description, a new functional classification of cranial nerves can be adopted, with four groups:

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