Two embryological fates for cells of the neural tube are well established. Cells from the dorsal part of the developing neural tube emigrate and become neural crest cells, which in turn contribute to the development of the peripheral nervous system and a variety of non‐neural structures. Other neural tube cells form the neurons and glial cells of the central nervous system (CNS). This has led to the neural crest being treated as the sole neural tube‐derived emigrating cell population, with the remaining neural tube cells assumed to be restricted to forming the CNS. However, this restriction has not been tested fully. Our investigations of chick, quail and duck embryos utilizing a variety of different labelling techniques (DiI, LacZ, GFP and quail chimera) demonstrate the existence of a second neural tube‐derived emigrating cell population. These cells originate from the ventral part of the cranial neural tube, emigrate at the exit/entry site of the cranial nerves, migrate in association with the nerves and populate their target tissues. On the basis of its site of origin and route of migration we have named this cell population the ventrally emigrating neural tube (VENT) cells. VENT cells also differ from neural crest cells in that they emigrate considerably after the emigration of neural crest cells, and lack expression of the neural crest cell antigen HNK‐1. VENT cells are multipotent, differentiating into cell types belonging to all four basic tissues in the body: the nerve, muscle, connective and epithelium. Thus, the neural tube provides at least two cell populations – neural crest and VENT cells – that contribute to the development of the peripheral nervous system and various non‐neural structures. This review describes the origin of the idea of VENT cells, and discusses evidence for their existence and subsequent fates.

We investigated the contributions of a recently described population of neural tube cells, which participates in the development of a variety of tissues, to the development of the heart and great vessels. These cells, termed as the ventrally emigrating neural tube (VENT) cells, originate in the ventral part of the hindbrain neural tube, emigrate at the site of attachment of the cranial nerves, and populate their respective target tissues. VENT cells of the caudal hindbrain neural tube at the level of the vagus nerve, which were previously reported to migrate into the heart, were tagged with replication-deficient retroviruses containing the LacZ gene in chick embryos, after the emigration of neural crest from this region. In older embryos, VENT cells were detected in a variety of locations including the ventricles, atria, their septa, aorticopulmonary septum, and great vessels of the heart. Immunostaining with a specific marker revealed that VENT cells differentiated into smooth muscle cells of great vessels. Differentiation of VENT cells into cardiac muscle cells was reported previously. Extirpation of the VENT cells prior to their departure from the neural tube resulted in some common cardiovascular malformations: thin-walled ventricles and atria, ventricular and atrial septal defects, persistent truncus arteriosus, and stenosis of the great vessels. These results suggest that a novel population of neural tube cells also contributes to the normal development of the heart and great vessels. Thus, the heart and great vessels develop from three sources of cells: mesoderm, neural crest, and VENT cells.

Virtually all cell types in the inner ear develop from the cells of the otic vesicle. The otic vesicle is formed by the invagination of non-neural ectodermal cells known as the otic placode. We investigated whether a recently described cell population, originating from the ventral part of the hindbrain neural tube known as the ventrally emigrating neural tube (VENT) cells, also contributes cells to the otic vesicle. The ventral hindbrain neural tube cells were labeled with the fluorescent vital dye DiI or replication-deficient retroviruses containing the LacZ gene in chick embryos on embryonic day 2, after the emigration of neural crest from this region. One day later, the labeled cells were detected only in the hindbrain neural tube. Shortly thereafter, the labeled cells began to appear in the eighth (vestibulocochlear) cranial nerve and otic vesicle. From embryonic day 3.5-5, the labeled cells were detected in the major derivatives of the otic vesicle, i.e. the endolymphatic duct, semicircular canals, utricle, saccule, cochlea, and vestibulocochlear ganglion. That the emigrated cells originated from the ventral part of the hindbrain neural tube was confirmed by focal application of DiI impregnated filter paper and with quail chimeras. It is concluded that, in addition to the otic placode cells, the otic vesicle also contains the ventrally emigrating neural tube cells, and that both cell populations contribute to the structures and cell types in the inner ear. It is well known that inductive signals from the hindbrain are required for the morphogenesis of the inner ear. The migration of the hindbrain neural tube cells into the otic vesicle raises the possibility that the inductive effect of the hindbrain might be mediated, at least in part, by the ventrally emigrating neural tube cells and that, therefore, a mechanism exists that involves cells rather than diffusible molecules only.

The enteric nervous system is believed to be derived solely from the neural crest cells. This is partly based on the belief that the neural crest cells are the sole neural tube-derived cells colonizing the gastrointestinal tract. However, recent studies have shown that after the emigration of neural crest cells an additional population of cells emigrate from the cranial neural tube. These cells originate in the ventral part of the hindbrain, emigrate through the site of attachment of the cranial nerves, and colonize a variety of developing structures including the gastrointestinal tract. This cell population has been named the ventrally emigrating neural tube (VENT) cells. We followed the fate of these cells in the gastrointestinal tract. Ventral hindbrain neural tube cells of chick embryos were tagged with replication-deficient retroviral vectors containing the LacZ gene, after the emigration of neural crest from this region. In control embryos, the viral concentrate was dropped on the dorsal part of the neural tube. Embryos were sacrificed from embryonic days 3-12 and processed for the detection of LacZ positive ventrally emigrating neural tube cells. These cells colonized only the foregut, specifically the duodenum and stomach. Immunostaining with the neural crest cell marker HNK-1 showed that they were HNK-1 negative, indicating that they were not derived from neural crest. Cells were detected in three locations: (1). the myenteric and submucosal plexus of the enteric nervous system; (2). circular smooth muscle cell layer; and (3). mucosal lining of the lumen. A variety of specific markers were used to identify their fate. Some ventrally emigrating neural tube cells differentiated into neurons and glial cells, indicating that the enteric nervous system in the foregut develops from an additional source of precursor cells. It was also found that some of these cells differentiated into interstitial cells of Cajal, which mediate impulses between the enteric nervous system and smooth muscle cells, whereas others differentiated into epithelium. Altogether, these results indicate that the ventrally emigrating neural tube cells are multipotential. More importantly, they reveal a novel source of precursor cells for the neurons and glial cells of the enteric nervous system. The developmental and functional significance of the heterogeneous origin of the cell types remains to be established.

This chapter reviews the development of the human brain from conception to adult age and its evolution across time and species. The size and the shapes of the various portions of the mature nervous system result from a variety of developmental processes such as cellular proliferation, cellular migration, and cell death, and also from the constraints imposed by the surrounding non-neural tissues. During brain development, neurons are dependent on external factors for guidance and survival. These factors include biochemical influences such as neurotransmitters and steroid hormones. The mechanisms involved are complex and require appropriate spatiotemporal relationships and feedback loops. Human brains appear to be products of evolutionary mechanisms such as selection and genetic variation. During the evolution of the nervous system, certain general trends seem to have occurred. Laterality in the cortex of humans represents a behavioral and morphologic reorganization within the brain.

A population of multipotential neuroepithelial cells originating in the ventral portion of the hindbrain neural tube has been shown recently to emigrate at the site of attachment of the trigeminal nerve. These ventrally emigrating neural tube cells populate the mesenchyme of the first pharyngeal (branchial) arch. Because the Meckel's and the quadrate cartilage develop from this mesenchyme, we sought to determine whether these ventrally emigrating neural tube cells contributed to their development. The ventral neural tube cells were tagged with a replication-deficient retroviral vector containing the LacZ gene. This method permanently labels the descendents of the neural tube cells; thus, they can be subsequently tracked during development. The viral concentrate was microinjected into the lumen of the rostral hindbrain of chick embryos, after the emigration of neural crest is finished, on embryonic day 2 (stage 14). In control embryos, the virus was placed on top of the neural tube. Embryos were killed on days 3, 4, and 7 and processed for the detection of LacZ-positive cells. By day 7, the Meckel's and the quadrate cartilage can be easily recognized. LacZ-positive cells were seen in both cartilages. They were located in perichondrium and in the cartilage. Immunostaining with the neural crest cell marker HNK-1 indicated that the LacZ-positive cells were HNK-1 negative. The HNK-1-positive neural crest–derived cells were located in the cartilage but not in the perichondrium. These results indicate that the chondrocytes in the Meckel's and the quadrate cartilage differentiate from two sources of cells; the ventrally emigrating neural tube cells and the neural crest. The developmental significance of differentiation of cartilage from the ventral neural tube cells and of the heterogeneous origin of chondrocytes in morphogenesis remains to be established. Dev Dyn 1999;216:37–44. © 1999 Wiley-Liss, Inc.

A population of ventral neural tube cells has recently been shown to migrate out of the hind brain neural tube via the vagus nerve and contribute to the developing gastrointestinal tract. Since liver is also innervated by the vagus nerve, we sought to determine if these cells also migrate into the liver. Ventral neural tube cells in the caudal hindbrain of chick embryos were tagged with a replication-deficient retroviral vector containing the LacZ gene on embryonic day 2. Embryos were processed for detection of labeled cells on embryonic day 5 and 11. Labeled cells were seen in the liver on both days and identified as hepatocytes. Previously, it was believed that all hepatocytes develop from the gut endoderm. Results of the present study show an additional source for the formation of liver cells.

A population of ventrally emigrating neural tube cells has been shown to migrate along the vagus nerve and contribute to the development of the gastrointestinal tract. Since the vagus also goes to the heart, we sought to determine if these cells migrated into the heart. Neural tube cells were tagged with replication-deficient retroviral vectors containing the LacZ gene, to permanently label their progeny. The virus was microinjected into the lumen of the caudal hindbrain of chick embryos on day 2. Embryos were later processed for the detection of LacZ positive cells. Labeled cells were initially confined to the neural tube. Later, they migrated in association with the vagus nerve into the heart, where they were located in the myocardium. Labeled cells were identified as cardiac muscle cells of non-neural crest origin, with specific markers. It is concluded that some cardiac muscle cells differentiate from the neural tube cells.

It is generally believed that after the emigration of neural crest, the neuroepithelial cells of the neural tube are committed to differentiate only as neurons and supporting cells of the central nervous system. Neural crest cells arise from the dorsal portion of the developing neural tube and contribute to the formation of the peripheral nervous system and a variety of non-neural structures. In contrast to this view we have recently shown, by focal application of the vital dye Dil in duck embryos, that an additional population of cells emigrates from the neural tube. By using an entirely different technique we confirm and extend these observations in the chick embryo. Replication-deficient retroviral vector LZ12 containing the gene LacZ was utilized to label the neural tube cells. The viral concentrate was microinjected into the lumen of the rostral hind-brain neural tube, considerably after the completion of emigration of neural crest cells. The labeled cells were monitored in whole mounts and histological sections. Initially, the labeled cells were restricted to the neuroepithelium of the hindbrain neural tube. Subsequently, they were seen in the neural tube and in the ganglion of the fifth cranial nerve (trigeminal ganglion). Later, they migrated beyond the trigeminal ganglion, i.e., into the mesenchyme of the first pharyngeal arch. Immunostaining with the neural crest cell marker, HNK-1, indicated that the emigrated neuroepithelial cells were HNK-1 negative. It is concluded that in the chick embryo some neuroepithelial cells emigrate at the site of attachment of the trigeminal nerve, migrate into the ganglion and then into the mesenchyme of the first arch. This cell population differs antigenically from the neural crest cells.

Craniofacial skeletal muscle cells are believed to develop from mesoderm. A population of ventral neural tube cells has recently been shown to migrate out of the hindbrain and populate the craniofacial mesenchyme in chick embryos. Since skeletal muscle cells develop from this mesenchyme, we sought to determine if the emigrated neural tube cells contributed to their development. Ventral neural tube cells in the hindbrain of chick embryos were labeled on embryonic day 2 with replication-deficient retroviral vectors containing the gene LacZ, which provides a permanent marker for the progeny. On day 7 embryos were processed for the detection of labeled cells. Labeled cells were seen in craniofacial skeletal muscles. By using muscle-specific markers, the labeled cells were confirmed to be skeletal muscle cells. Thus, some muscle cells are derived from the ventral neural tube cells of the hindbrain.

BACKGROUND & AIMS:Smooth muscle cells in the walls of the gastrointestinal tract are thought to derive solely from mesoderm surrounding the primitive gut. A population of neuroepithelial cells has recently been shown to migrate from the ventral part of the neural tube in the region joined by the vagus nerve. We sought to determine if these cells contributed to the development of the stomach and intestine.METHODS:Cells of the ventral hindbrain of chick embryos were tagged by replication-deficient retroviral vectors containing the lacZ gene, providing a permanent label that is transmitted without dilution as the cells divide. Embryos were processed for detection of labeled cells. Specific markers were used to determine differentiation of progeny in the gastrointestinal tract.RESULTS:Cells labeled in the ventral neural tube migrate in association with the vagus nerve. Labeled cells are found in the intestine and stomach after time for further migration and differentiation. Using a specific marker, they were clearly identified as smooth muscle cells.CONCLUSIONS:Some of the smooth muscle cells of the gastrointestinal tract are derived from precursor cells that originate in the ventral part of the hindbrain neural tube. Their developmental importance and functional significance remain to be determined.

Cells of the neural tube are thought to be committed to form only the central nervous system, whereas the peripheral nervous system is believed to be derived from neural crest cells and from placodes, which are specialized regions of the surface ectoderm. Neural crest cells arise early from the dorsal part of the neural tube. The possibility that after emigration of the neural crest cells, another population of cells arising from the ventral part of the neural tube also emigrates via a different route was examined. Here we report that, after labeling cells of the ventral neural tube in the rostral hindbrain of E3 duck embryos with DiI, they were later found in the trigeminal ganglion of the fifth cranial nerve. A trail of labeled cells could be traced from the ventral part of the neural tube to the peripheral ganglion. Further, expression of the homeobox gene Islet-1 in cells of the neural tube and the ganglion also indicated that some ventral neural tube cells may normally emigrate to the trigeminal ganglion. It is concluded that not all neural tube cells are committed to form the central nervous system; the ventral part of the neural tube also provides cells for the formation of the trigeminal ganglion. These results raise the possibility that the ventral neural tube may serve as an additional source of cells for the formation of various other components of the peripheral nervous system.

The formation of motor neurons in the spinal cord is dependent on inductive signals from the floor plate and notochord. Motor neurons in the brain stem, on the other hand, develop in the absence of both structures. This suggests that either the germinal epithelium is specified intrinsically to form specific cranial motor nuclei or that the inductive signals for the formation of cranial motor neurons arise from some other structure. These possibilities were investigated experimentally by using the formation of trochlear motor neurons in the midbrain of duck embryos as a model system. The trochlear motor neurons, which form the nucleus of the fourth cranial nerve, developed normally after early damage to the prospective germinal epithelium, suggesting that it is unlikely to be specified intrinsically to form these cranial motor neurons. Instead, their development was found to be dependent on the cells within, or associated with, the ventromedial region of the brain stem, as the extirpation of this region results in the absence of motor neuron formation. These results show that structures other than the floor plate and notochord provide inductive signals for the cellular differentiation and patterning of the developing central nervous system. They raise the possibility that the inductive signals for motor neuron differentiation in the spinal cord and the brain stem may not be necessarily identical.

The inductive signals for the differentiation of motor neurons in the spinal cord have been experimentally shown to arise from cells in the midventral region of the neural tube, often referred to as the floor plate, and from the notochord. Although the prevailing view is that a similar mechanism accounts for the differentiation of motor neurons in the brain stem, supporting experimental evidence is lacking. Here, using the formation of the trochlear nucleus in the midbrain of duck embryos as a model system, we report that the floor plate and the notochord are not necessary for the development of these motor neurons in the brain stem. Early damage to the floor plate or extirpation of the floor plate and notochord does not prevent the development of these cranial motor neurons. Thus, either the inductive signals for the formation of these cranial motor neurons arise from some other structure or the germinal epithelium of the cranial neural tube is intrinsically programmed to form specific cranial motor nuclei.

A loss of about half of the trochlear motor neurons occurs during the course of normal development in duck and quail embryos. The role of the size of the target muscle in controlling the number of surviving motor neurons was examined by making motor neurons innervate targets either larger or smaller in size than their normal target. In one experiment the smaller trochlear motor neuron pool of the quail embryo was forced to innervate the larger superior oblique muscle of the duck embryo. This was accomplished by grafting the midbrain of a quail embryo in the place of the midbrain of a duck embryo. Results indicated that no additional quail trochlear motor neurons were rescued in spite of a considerable increase in target size. In another experiment the larger trochlear motor neuron pool of the duck embryo was made to innervate the smaller superior oblique muscle of the quail embryo. This resulted in loss of some additional neurons; however, the number of surviving motor neurons was not proportionate to the reduction in target size. These experiments failed to provide support for the hypothesis that the size of the target muscle controls the number of surviving motor neurons. Although contact with target is necessary for survival of neurons, factors other than the number or size of target cells are involved in the control of motor neuron numbers during development.