Cavernous sinus and abducens nerve in human fetuses near term.

A long tortuous course of the abducens nerve (ABN) crossing a highly curved siphon of the internal carotid artery is of interest to neurosurgeons for cavernous sinus surgery. Although a "straight" intracavernous carotid artery in fetuses can change into an adult-like siphon in infants, there is no information on when or how the unique course of ABN is established. Histological observations of 18 near-term fetuses (12 specimens of frontal sections and 6 specimens of sagittal sections) demonstrated the following: (I) the ABN consistently took a straight course crossing the lateral side of an almost straight intracavernous carotid artery; (II) the straight course was maintained when sympathetic nerves joined; (III) few parasellar veins of the developing cavernous sinus separated the ABN from the ophthalmic nerve; and (IV) immediately before the developing tendinous annulus for a common origin of extraocular recti, the ABN bent laterally to avoid a passage of the thick oculomotor nerve. Since the present observations strongly suggested morphologies at birth and in infants, major angulations of the ABN as well as the well-known course independent of the other nerves in the cavernous sinus seemed to be established during childhood. In the human body, the ABN might be a limited example showing a drastic postnatal change in course. Consequently, it might be important to know the unique course of ABN before performing endovascular interventions and skull base surgery for petroclival and cavernous sinus lesions without causing inadvertent neurovascular injuries to neonates or infants.

Sensory pathways in the human embryonic spinal accessory nerve with special reference to the associated lower cranial nerve ganglia.

BACKGROUND AND PURPOSE: Muscles supplied by the spinal accessory nerve are particularly prone to the development of trigger points characteristic of myofascial pain. This study aimed to confirm sensory pathways in the spinal accessory nerve and to describe sensory ganglion cell distributions along the lower cranial nerve roots. METHODS: Using sagittal sections of ten human embryos at 6-7 weeks and horizontal sections of three 15- to 16-week-old embryos, we analyzed ganglion cell distributions along the lower cranial nerve roots, including the spinal accessory (XI) nerve. RESULTS: In all ten 6- to 7-week-old embryos, the XI nerve root contained abundant ganglion cells, which were evenly distributed along the XI nerve root at levels between the jugular foramen and the dorsal root of the second cervical nerve. However, the hypoglossal (XII) nerve roots did not contain ganglion cells and did not communicate with nearby roots in the dural space. Thus, the so-called Froriep's occipital ganglion is unlikely to be associated with the XII nerve but rather with the XI nerve roots. According to observations of three larger fetuses (15-16 weeks), most of Froriep's ganglion cells seemed to have degenerated during early fetal life. CONCLUSION: Nociceptive sensory pathways in the adult human XI nerve may be much more limited in number than would be expected based on previous animal studies. However, it is possible that sensory ganglion cells in the embryonic XI nerve root send axons toward the developing spinal accessory nerve fibers outside of the jugular foramen.

Intrinsic properties guide proximal abducens and oculomotor nerve outgrowth in avian embryos.

Proper movement of the vertebrate eye requires the formation of precisely patterned axonal connections linking cranial somatic motoneurons, located at defined positions in the ventral midbrain and hindbrain, with extraocular muscles. The aim of this research was to assess the relative contributions of intrinsic, population-specific properties and extrinsic, outgrowth site-specific cues during the early stages of abducens and oculomotor nerve development in avian embryos. This was accomplished by surgically transposing midbrain and caudal hindbrain segments, which had been pre-labeled by electroporation with an EGFP construct. Graft-derived EGFP+ oculomotor axons entering a hindbrain microenvironment often mimicked an abducens initial pathway and coursed cranially. Similarly, some EGFP+ abducens axons entering a midbrain microenvironment mimicked an oculomotor initial pathway and coursed ventrally. Many but not all of these axons subsequently projected to extraocular muscles that they would not normally innervate. Strikingly, EGFP+ axons also took initial paths atypical for their new location. Upon exiting from a hindbrain position, most EGFP+ oculomotor axons actually coursed ventrally and joined host branchiomotor nerves, whose neurons share molecular features with oculomotor neurons. Similarly, upon exiting from a midbrain position, some EGFP+ abducens axons turned caudally, elongated parallel to the brainstem, and contacted the lateral rectus muscle, their originally correct target. These data reveal an interplay between intrinsic properties that are unique to oculomotor and abducens populations and shared ability to recognize and respond to extrinsic directional cues. The former play a prominent role in initial pathway choices, whereas the latter appear more instructive during subsequent directional choices.

An mnr2b/hlxb9lb enhancer trap line that labels spinal and abducens motor neurons in zebrafish.

BACKGROUND: The developing nervous system consists of a variety of cell types. Animal models that allow the visualization of specific classes of neurons are crucial for the study of neuronal networks. RESULTS: We performed an enhancer trap screening in zebrafish and generated a collection of transgenic lines that expressed GFP in a spatially and temporally restricted manner. Among the fish generated, we identified an insertion of the enhancer trap construct in the vicinity of the mnr2b/hlxb9lb gene encoding the mnx class of homeodomain transcription factor. The insertion gave rise to GFP expression predominantly in spinal motor neurons and abducens motor neurons. During embryogenesis, GFP expression was also detected in endodermal and mesodermal tissues, where mnr2b is known to be expressed. CONCLUSION: These results show that the enhancer trap construct recapitulated the expression pattern of the mnr2b gene and this transgenic line should be useful for the visualization of the spinal and abducens motor neurons in the developing nervous system.

Cranial nerve fasciculation and Schwann cell migration are impaired after loss of Npn-1.

Interaction of the axon guidance receptor Neuropilin-1 (Npn-1) with its repulsive ligand Semaphorin 3A (Sema3A) is crucial for guidance decisions, fasciculation, timing of growth and axon-axon interactions of sensory and motor projections in the embryonic limb. At cranial levels, Npn-1 is expressed in motor neurons and sensory ganglia and loss of Sema3A-Npn-1 signaling leads to defasciculation of the superficial projections to the head and neck. The molecular mechanisms that govern the initial fasciculation and growth of the purely motor projections of the hypoglossal and abducens nerves in general, and the role of Npn-1 during these events in particular are, however, not well understood. We show here that selective removal of Npn-1 from somatic motor neurons impairs initial fasciculation and assembly of hypoglossal rootlets and leads to reduced numbers of abducens and hypoglossal fibers. Ablation of Npn-1 specifically from cranial neural crest and placodally derived sensory tissues recapitulates the distal defasciculation of mixed sensory-motor nerves of trigeminal, facial, glossopharyngeal and vagal projections, which was observed in Npn-1(-/-) and Npn-1(Sema-) mutants. Surprisingly, the assembly and fasciculation of the purely motor hypoglossal nerve are also impaired and the number of Schwann cells migrating along the defasciculated axonal projections is reduced. These findings are corroborated by partial genetic elimination of cranial neural crest and embryonic placodes, where loss of Schwann cell precursors leads to aberrant growth patterns of the hypoglossal nerve. Interestingly, rostral turning of hypoglossal axons is not perturbed in any of the investigated genotypes. Thus, initial hypoglossal nerve assembly and fasciculation, but not later guidance decisions depend on Npn-1 expression and axon-Schwann cell interactions.

Electrophysiological properties of morphologically-identified medial vestibular nucleus neurons projecting to the abducens nucleus in the chick embryo.

Neurons in the medial vestibular nucleus (MVN) show a wide range of axonal projection pathways, intrinsic firing properties, and responses to head movements. To determine whether MVN neurons participating in the vestibulocular reflexes (VOR) have distinctive electrophysiological properties related to their output pathways, a new preparation was devised using transverse brain slices containing the chicken MVN and abducens nucleus. Biocytin Alexa Fluor was injected extracellularly into the abducens nucleus so that MVN neurons whose axons projected to the ipsilateral (MVN/ABi) and contralateral (MVN/ABc) abducens nuclei were labeled selectively. Whole-cell, patch-clamp recordings were performed to study the active and passive membrane properties, sodium conductances, and spontaneous synaptic events in morphologically-identified MVN/AB neurons and compare them to MVN neurons whose axons could not be traced (MVN/n). Located primarily in the rostral half of the ventrolateral part of the MVN, MVN/AB neurons mainly have stellate cell bodies with diameters of 20-25 µm. Compared to MVN/n neurons, MVN/ABi and MVN/ABc neurons had lower input resistances. Compared to all other MVN neuron groups studied, MVN/ABc neurons showed unique firing properties, including type A-like waveform, silence at resting membrane potential, and failure to fire repetitively on depolarization. It is interesting that the frequency of spontaneous excitatory and inhibitory synaptic events was similar for all the MVN neurons studied. However, the ratio for miniature to spontaneous inhibitory events was significantly lower for MVN/ABi neurons compared to MVN/n neurons, suggesting that MVN/ABi neurons retained a larger number and/or more active inhibitory presynaptic neurons within the brain slices. Also, MVN/ABi neurons had miniature excitatory postsynaptic currents (mEPSCs) with slower decay time and half width compared to MVN/n neurons. Altogether, these findings underscore the diversity of electrophysiological properties of MVN neuron classes distinguished by axonal projection pathways. This represents the first study of MVN/AB neurons in brain slice preparations and supports the concept that the in vitro brain slice preparation provides an advantageous model to investigate the cellular and molecular events in vestibular signal processing.

Development of oculomotor axon projections in the chick embryo.

The pattern of innervation of the extraocular muscles is highly conserved across higher vertebrate species and mediates sophisticated visuomotor processes. Defects in oculomotor development often lead to strabismus, a misalignment of the eyes that can cause partial blindness. Although it has been intensively studied from a clinical perspective, relatively little is known about how the system develops embryonically. We have therefore mapped the development of the oculomotor nerve (OMN) in chick embryos by using confocal microscopy. We show that OMN development follows a series of stereotyped steps that are tightly regulated in space and time. The OMN initially grows past three of its targets to innervate its distal target, the ventral oblique muscle, only later forming branches to the more proximal muscles. We have also investigated spatiotemporal aspects of the unusual contralateral migration of a subpopulation of oculomotor neurons by using molecular markers and have found the semaphorin axon guidance molecules and their receptors, the neuropilins, to be expressed in discrete subnuclei during this migration. Finally, we have created an embryological model of Duane retraction syndrome (DRS), a form of strabismus in which the OMN is believed to innervate aberrantly the lateral rectus, the normal target of the abducens nerve. By ablating rhombomeres 5 and 6 and hence the abducens, we have mimicked a proposed oculomotor deficit occurring in DRS. We find that the absence of the abducens nerve is not sufficient to produce this inappropriate innervation, so other factors are required to explain DRS.

Differential expression of LIM homeobox genes among motor neuron subpopulations in the developing chick brain stem.

During development of the chick brain stem, cranial motor neuron subpopulations differentiate at distinct axial levels and extend their axons along specific pathways into the periphery. Differences in phenotype and axonal trajectory of these neuronal populations might be governed by the expression of different repertoires of transcription factors. In 2- to 7-day chick embryos, we find that genes of the LIM homeobox family are expressed differentially among cranial motor nuclei. Whereas Islet-1 is expressed by motor neurons of all cranial nerves, Islet-2 is expressed only in nuclei that contain somatic motor neurons and transiently in a specialized population of contralateral vestibuloacoustic efferent neurons. Lim-3 is expressed in the hypoglossal and accessory abducens nuclei only, and Lim-1 and Lim-2 are not expressed by cranial motor neurons. Our findings are consistent with a role of these transcription factors in determining neuronal phenotype and axonal pathfinding.

Developmental relations between sixth nerve motor neurons and their targets in the chick embryo.

The developmental relations between abducens (VI) nerves and their targets, the lateral rectus, quadratus, and pyramidalis muscles, have been examined in the chick embryo from early neural tube stages through 10 days of incubation. Sites of myoblast origins were determined by microinjection of replication-incompetent retroviruses containing the LacZ reporter into paraxial mesoderm corresponding to somitomeres 3-5. Motor neurons and axons were identified by Bodian staining, immunocytochemistry, and application of DiI and DiO to dissected peripheral nerves. Anlage of the dorsal oblique originate in somitomere 3, close to the ventrolateral margin of the mid-to-caudal mesencephalon. Precursors of the lateral rectus arise deep within somitomere 4, beside the future metencephalon (rhombomere "A"). Quadratus and pyramidalis precursors are located between and partially segregated from these other two anlage. VIth nerve axons exit rhombomeres 5 and 6 via multiple median roots, fasciculate, and by stage 17 have elongated rostrally beneath the hindbrain. Immediately caudal to a mesenchymal pre-muscle condensation located deep to rhombomere 2, the VIth nerve separates into two branches. One branch enters the rostral portion of the condensation, from which quadratus and pyramidalis muscles will segregate. This branch projects exclusively from rhombomere 5 and is the accessory abducens nerve. The other branch enters the caudal, presumptive lateral rectus, region of the condensation. This is the abducens nerve, and it projects from cells located in both rhombomeres 5 and 6. These findings indicate that specific matching of motor nerves with their presumptive targets begins prior to the differentiation and segregation of myogenic populations, and that spatial organization of developing eye muscles is initiated well before they interact with connective tissue precursors derived from the neural crest.

Development of the brain stem in the rat. II. Thymidine-radiographic study of the time of origin of neurons of the upper medulla, excluding the vestibular and auditory nuclei.

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational days 12 and 13 (E12 + 13) until the day before birth (E21 + 22). In radiographs from adult progeny of these rats the proportion of neurons generated on specific days was determined in the major nuclei of the upper medulla, with the exception of the vestibular and auditory nuclei. The neurons of the motor nuclei are generated over a brief period. Neurons of the retrofacial nucleus are produced first, with more than 60% of the cells arising on day E11 or earlier. Peak generation time of abducens neurons is day E12 and of the neurons of the facial nucleus is day E13. In contrast, the neurons of the superior salivatory nucleus are produced late, predominantly on day E15 and some on day E16. The generation of the (sensory relay) neurons of the nucleus oralis of the trigeminal complex takes place over an extended period between days E12 and E15; the last generated cells include the largest neurons of this nucleus. Neurons of the raphe magnus are produced between days E11 and E14, the neurons of the rostral medullary reticular formation between days E12 and E15. The latest generated neurons of the upper medulla (excluding the cochlear nuclei) belong to a structure identified as the granular layer of the raphe. Combining these results with those of the preceding paper (Altman and Bayer, '80a) and with additional data, it is postulated that the laterally and ventrally situated motor nucleus of the trigeminal, the facial nucleus, and the nucleus ambiguous form a single longitudinal zone of branchial motor neurons with a rostral-to-caudal cytogenetic gradient. In contrast, the medially and dorsally situated (juxtaventricular) hypoglossal nucleus and abducens nucleus (together with the other nuclei of the ocular muscles) form a longitudinal somatic motor zone with a caudal-to-rostral gradient. The dorsal nucleus of the vagus and the superior salivatory nucleus may constitute a preganglionic motor zone, also with a caudal-to-rostral cytogenetic gradient.

[Embryonic development of the brain of Scyliorhinus canicula (L.). III. The optical system and adjacent nuclei in the tegmentum mesencephali]. / Untersuchungen zur Embryonalentwicklung des Gehirns von Scyliorhinus canicula (L.) III. Das optische System und angrenzende Nuclei im mesencephalen Tegmentum.

The ontogenesis of the optic system in the brain has been studied with reference to the general biology of the embryo and to the cytoarchitecture of the optic tectum. 1. Sharks vary in terms of retinal structure (Franz 1931). Scyliorhinus shows densely packed and large cellular layers alternating with thin fiber layers. 2. The optic tracts, which are not excessively large, cross in the optic chiasm by approximately 95%. The optic fibers, forming a medial and a lateral tract, enter the tectum in zones 9 and 11. 3. The eye muscle nuclei do not differ from the general vertebrate pattern. The oculomotor nucleus represents the largest portion; the abducens nucleus is small. 4. The nucleus interstitialis of the fasciculus longitudinalis medialis is remarkably developed and consists of large neurons. Together with the interpreduncula nucleus it receives fibers of fasciculus retroflexus.