Distribution of laminin in the murine pituitary.

The distribution and cellular localization of the glycoprotein laminin were investigated by light and electron microscopic immunocytochemistry in the adult murine pituitary gland. Immunoblots confirmed that laminin was the only protein in the pituitary gland of the adult male mouse to react with antilaminin serum. Laminin immunoreactivity was demonstrated at the light microscopic level simultaneously with that of beta-follicle stimulating hormone (beta-FSH) and beta-luteinizing hormone (beta-LH). In addition to its distribution is basal laminae, laminin immunoreactivity was coincidently expressed in gonadotrophs with the immunoreactivities of beta-FSH and beta-LH. Electron microscopic immunocytochemistry was employed on aldehyde-fixed sections embedded in L.R. White. Sites of binding of primary antisera to laminin were identified with affinity-purified secondary antisera directly coupled to 20 nm particles of colloidal gold. Three antisera recognizing laminin were compared and found to result in an identical pattern of immunoreactivity. Laminin was found extracellularly only in formed basal laminae in all three lobes of the pituitary and was not found in extracellular matrices of connective tissue. Laminin immunoreactivity was also found intracellularly in gonadotrophs but in none of the other endocrine or non-endocrine cells of the anterior lobe. Within gonadotrophs, only secretory granules were labeled. The majority, but not all, secretory granules were labeled in each of the gonadotrophs examined, and the proportion of granules labeled with laminin could not be increased by doubling the concentration of anti-laminin serum. Laminin immunoreactivity segregated with the subset of secretory granules containing beta-FSH. In contrast, laminin immunoreactivity was absent in the smaller subset of secretory granules that contain serotonin.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of hyaluronic acid and chondroitin sulfate proteoglycans in the presumptive aganglionic terminal bowel of ls/ls fetal mice: an ultrastructural analysis.

The terminal colon of the ls/ls mouse is aganglionic because an intrinsic defect prevents its colonization by cells migrating from the neural crest. Previous studies showed that laminin, type IV collagen, and glycosaminoglycans accumulate in the region of the presumptive aganglionic ls/ls bowel through which crest-derived cells would be expected to migrate. It was suggested that crest-derived cells might fail to enter the abnormal bowel because they receive inappropriate signals from a defective extracellular matrix. This hypothesis was evaluated by analyzing the ultrastructure of the extracellular matrix in mutant and control gut. Tissue was fixed in the presence of ruthenium red before or after selective enzymatic digestion. Heparan sulfate proteoglycan (diameter approximately equal to 15 nm) and chondroitin sulfate proteoglycan (diameter approximately equal to 20-50 nm) granules were found in both control and presumptive aganglionic gut. The heparan sulfate proteoglycan granules were primarily located within formed basal laminae, while chondroitin sulfate proteoglycan granules decorated plasma membranes and 5 nm hyaluronic acid microfibrils that formed a network in the extracellular matrix. At day E11.5, the mutant gut differed from the control in the following: 1) Hyaluronic acid microfibrils were longer and more numerous. 2) There were larger numbers of chondroitin sulfate proteoglycan granules associated with cell membranes and with hyaluronic acid microfibrils. By day E13 the spaces between mesenchymal cells of the outer wall of the control bowel contained a regular lattice of hyaluronic acid microfibrils studded with chondroitin sulfate proteoglycan granules. Instead of this lattice, tangles of excessively long hyaluronic acid microfibrils, coated more heavily than in the control with chondroitin sulfate proteoglycan granules, were found in the presumptive aganglionic gut. These results confirm that the extracellular matrix is abnormal in the presumptive aganglionic bowel of the ls/ls mouse; moreover, they also indicate that the defect involves not one, but several components of the extracellular matrix, as well as their distribution. The defective extracellular matrix is apparent at a time when crest-derived cells would be expected to be migrating in the terminal bowel and is located in their path. The observations thus support the idea that a localized abnormality of the extracellular matrix interferes with the colonization of the terminal bowel by crest-derived cells in the ls/ls mouse.

Human medullary thyroid carcinoma: characterization of the serotonergic and neuronal properties of a neurectodermally derived cell line.

Parafollicular (PF) cells have been found to be a good model system for the study of serotonergic cellular mechanisms relevant to neurons. PF cells are derived from the same region of the neural crest that gives rise to the neurons of the gut and are capable of extending neurofilament-bearing neuritic processes. PF cells also synthesize 5-hydroxytryptamine (5-HT) and costore 5-HT in the same vesicles as the specific 5-HT-binding protein, 45 kDa SBP. A hypothesis has been advanced that PF cells and enteric neurons share a common developmental precursor. The present investigation was undertaken in order to determine whether a human medullary thyroid carcinoma (MTC) cell line, which is derived from PF cells, sufficiently mimics PF cells that it can be substituted for them in investigations of serotonergic cellular biology. In contrast to PF cells, MTC cells can be propagated in vitro to provide adequate amounts of material for biochemical studies. MTC cells were found to contain neuropeptides, including calcitonin, calcitonin gene-related peptide, and somatostatin, which have also been reported to be present in PF cells and enteric neurons. MTC cells also were observed to store endogenous 5-HT, to be able to synthesize 3H-5-HT from 3H-L-tryptophan, and to take up 3H-5-HT from the ambient medium by a carrier-mediated mechanism very similar to that of serotonergic neurons. In addition, the longterm accumulation of 3H-5-HT in MTC cells was antagonized by reserpine, suggesting that the cells contain 5-HT storage vesicles that, like the synaptic vesicles of serotonergic neurons, are characterized by a reserpine-sensitive transporter of biogenic amines. MTC cells also contain type A, but not type B, monoamine oxidase. Finally, MTC cells were found to contain both 45 and 56 kDa SBP. MTC cells thus retain a great many of the properties of PF cells, and, like PF cells, they are serotonergic cells with characteristics similar to serotonergic neurons. Substantial differences were found in the content of immunoreactive 5-HT and neuropeptides in individual MTC cells. Moreover, the release of newly synthesized 5-HT to the medium exceeded the ability of the cells to store the amine. Studies of the ultrastructure of the MTC cells revealed a limited and highly variable number of secretory granules, probably accounting for their limited 5-HT storage capacity and for the heterogeneity of immunostaining with antisera to 5-HT or neuropeptides.(ABSTRACT TRUNCATED AT 400 WORDS)

Tissue effects on the expression of serotonin, tyrosine hydroxylase and GABA in cultures of neurogenic cells from the neuraxis and branchial arches.

The phenotypically diverse neurones of the enteric nervous system are developmentally derived from precursors that migrate to the bowel from the vagal and sacral regions of the neuraxis. In order to gain insight into the generation of enteric neuronal diversity, we examined the expression of serotonin (5-HT), tyrosine hydroxylase and GABA in vitro. In the mature avian intestine, intrinsic neurones contain 5-HT or GABA but not tyrosine hydroxylase. These markers were demonstrated immunocytochemically, singly or simultaneously. All three phenotypic markers developed in cultures of cranial, vagal or truncal neural crest when the cultures were grown in enriched medium, containing horse serum and chick embryo extract; however, 5-HT and GABA, but not tyrosine hydroxylase-immunoreactive cells, also developed in cultures that were grown in partially defined medium. Tyrosine hydroxylase immunoreactivity was seen when partially defined medium was supplemented with nerve growth factor (NGF). Cultures of branchial arches (III and IV) contained cells that displayed tyrosine hydroxylase immunoreactivity, but not that of 5-HT- or GABA-; however, 5-HT immunoreactivity was seen when branchial arches were cocultured with aneuronal hindgut (from 4-day chick embryos). Cultures of cells from chick gut dissociated at 7 days contained tyrosine hydroxylase as well as 5-HT and GABA immunoreactivities; however, no cultures of bowel dissociated at 8 days or later expressed tyrosine hydroxylase immunoreactivity. When neuraxial cells were cocultured with branchial arches or heart instead of gut, no 5-HT-immunoreactive cells were seen; nevertheless, the further addition of explants of gut to the heart/crest cocultures did permit the expression of 5-HT immunoreactivity. These results are consistent with the hypotheses that precursors with the potential to give rise to cells that express 5-HT, GABA and tyrosine hydroxylase are found at several levels of the neuraxis; however, the ability to express these phenotypes may be suppressed either while the crest cells are migrating (for example, 5-HT and GABA expression by crest cells passing through the branchial arches) or in their final destination (for example, tyrosine hydroxylase in the gut). This suppression may be transient and reversed by the microenvironment of the target organs.

Ultrastructural immunocytochemical studies of the localization and distribution of somatostatin, calcitonin, calcitonin gene-related peptide, and substance P in the bat thyroid follicle.

Electron microscope immunocytochemistry was used to determine the intracellular localization and distribution among follicular elements of four peptides: calcitonin, somatostatin, calcitonin gene-related peptide (CGRP), and substance P in the thyroid glands of bats captured in the prehibernation phase of their annual life cycle. Previous studies have shown that this period of the hibernation-activity cycle is characterized by the accumulation and storage of secretory granules in parafollicular cells. Sites of binding of primary antisera to each of the four peptides were identified by means of affinity-purified secondary antisera directly coupled to colloidal gold particles. Calcitonin and somatostatin immunoreactivities were found in all parafollicular cells examined and in every secretory granule within these cells. CGRP was also found in all parafollicular cells examined (n = 75) but only in about half of their secretory granules. In contrast to these peptides, substance P immunoreactivity was not found in any parafollicular cells, but was localized exclusively in nerve endings within the basement membrane of the follicle.

Accumulation of components of basal laminae: association with the failure of neural crest cells to colonize the presumptive aganglionic bowel of ls/ls mutant mice.

Aganglionosis occurs in the terminal colon of the ls/ls mouse because an intrinsic defect of the presumptive aganglionic tissue prevents the entry and colonization of this portion of the bowel by migrating neural crest cells. The current study was undertaken to determine if abnormalities of the extracellular matrix could be identified in this segment that might account for migratory failure. Since basal laminae of the muscularis mucosa are overproduced in the aganglionic segment of adult ls/ls mice, we examined components of basal laminae in fetal gut from Day E 11 to Day E 16 of gestation. This period spans the time of enteric ganglion formation. Laminin and collagen type IV were studied by immunocytochemistry and proteoglycans by staining glycosaminoglycans with Alcian blue. Abnormalities of each of these components occur during development of the presumptive aganglionic bowel in the ls/ls mouse and could be detected as early as Day E 11. These defects consist mainly of an overabundance of these materials, both in defined basal laminae and throughout the extracellular space of the mesenchyme. Electron microscopic observations in the presumptive aganglionic ls/ls colon revealed a thickening of basal laminae and exceptionally wide intercellular spaces between smooth muscle myoblasts that contained an irregular fibrillar material, consisting of 4.5- to 6.0-nm filaments associated with 14- to 20-nm granules. Fibrillar and flocculant material was continuous with formed basal laminae, and was concentrated in the same areas found to have an overabundance of laminin immunoreactivity. These observations indicate that there is an accumulation of extracellular matrix material, including components of basal laminae, that (i) precedes the formation of enteric ganglia, (ii) is in the path through which enteric neural precursors from the crest would have to migrate, and (iii) is limited to the aganglionic and hypoganglionic ls/ls bowel. These data are consistent with the hypothesis that components of basal laminae contribute to the inability of crest cells to colonize the terminal bowel of ls/ls mice.

Two types of secretory granules in gonadotrophs: discrimination by the simultaneous EM immunocytochemical localization of serotonin and beta-follicle stimulating hormone.

The hypothesis that the secretory granules of mammalian gonadotrophs are heterogeneous was tested. Previous studies had shown that all of the granules contain beta-luteinizing hormone (beta-LH) immunoreactivity, and some contain beta-follicle stimulating hormone (beta-FSH) or 5-HT immunoreactivity. Moreover, differential release of beta-LH and beta-FSH has also been demonstrated. In the current study the pituitary glands of mice were investigated immunocytochemically at the ultrastructural level with antisera directed against human growth hormone (GH), beta-LH, beta-FSH, and 5-HT. The immunoreactivities of beta-LH, beta-FSH, and 5-HT were restricted to gonadotrophs. No 5-HT immunoreactivity was seen in somatotrophs, identified by the immunoreactivity of GH in the secretory granules of these cells. Antisera to beta-LH labeled all gonadotroph granules; however, anti-beta-FSH and anti-5-HT sera labeled only subsets of the granules. The proportion of granules labeled could not be increased by doubling the concentration of anti-beta-FSH serum. The incidence of double labeling of granules by antisera to beta-FSH and 5-HT was significantly less than that predicted from the incidence of granule labeling by either reagent alone. It is concluded that beta-FSH and 5-HT immunoreactivities do not co-exist in the same secretory granules of gonadotrophs; therefore, these granules are heterogeneous and there must be at least two types of granules. It is possible that the two types of granules may be responsive to different second messengers, thereby explaining the differential release of LH and FSH.

Occurrence of cells containing paracrystalloid material in the intestinal lamina propria of the hibernating bat Myotis lucifugus.

A light and electron microscope study of the small intestine of the little brown bat, Myotis lucifugus, was carried out at several stages in the animal's annual life cycle. An unusual morphological observation was the presence of cells in the lamina propria of the small intestine which were packed with a conspicuous basophilic granular material that appeared crystalline. Moreover, such cells were present only during the hibernation period and were therefore called "hibernation crystalloid" (HC) cells. By light microscopy, the crystal-like material was not sudanophilic, did not stain for nucleic acids, and did not contain acid phosphatase; it did show reactivity when stained by the periodic acid-Schiff procedure. By electron microscopy, the crystal-like material was found to be present in smooth, membrane-enclosed vacuoles along with an amorphous, dense granular substance. The crystalline material occasionally formed rigid-appearing rods that reached lengths of 10 microns. The crystal-containing cells were contacted by axonal varicosities. It is suggested that these innervated HC cells represent a unique cell type with a gastrointestinal function, yet to be determined, that may be related to hibernation.

Origin and morphology of nerve fibers in the aganglionic colon of the lethal spotted (ls/ls) mutant mouse.

The lethal spotted mutant mouse (ls/ls) develops congenital megacolon because of the absence of ganglia in the terminal colon. This aganglionosis results from a failure of neural crest cells to colonize this area during fetal life. We have postulated that the microenvironment of the aganglionic segment of bowel is abnormal. Our hypothesis suggests that this abnormal enteric microenvironment fosters the sprouting of neuritic processes. We further propose that neural and glial precursors cease to migrate once they have extended their definitive processes. As a result, the area distal to the site where neurite extension is favored does not become colonized by neural or glial precursors. A prediction of this hypothesis is that the aganglionic tissue should be innervated by axons from neurons located both in the more proximal ganglionated bowel and in ganglia located outside the gut. Neurons and their processes in control and ls/ls terminal gut were located by the histochemical demonstration of acetylcholinesterase (AChE) activity and their structure was classified as intrinsic (enteric) or extrinsic in type by electron microscopy. In ls/ls mice the submucosal plexus was much more severely affected than the myenteric plexus. No submucosal ganglia were found within 30 mm of the anus. In contrast, myenteric ganglia extended to within 4 mm of the anus on the mesenteric side of the gut and to within 15 mm on the antimesenteric side. Rostral to the areas that were absolutely aganglionic, both plexuses were hypoganglionic, especially the submucosal plexus, which was hypoganglionic throughout the entire colon. Both the aganglionic and caudal hypoganglionic zones of the ls/ls bowel were penetrated by large nerve trunks that had the ultrastructural characteristics of extra-enteric peripheral nerve. Unusual ganglia, outside the enteric musculature in the adventitia of the colon, were connected to these trunks. The location of the cell bodies of origin of the nerve fibers in the terminal colon of control mice and in the aganglionic segment of the bowel in ls/ls mice was determined by following the retrograde transport of tracers injected as close as possible to the anus. An extrinsic innervation originating from the inferior mesenteric ganglion and dorsal root ganglia (L6-S1) was found in both types of animal. In control but not ls/ls mice retrograde labeling was also observed in the sacral parasympathetic nucleus of the spinal cord. In addition, neuritic processes were traced to neurons in myenteric ganglia. In control mice, these labeled neurons were present in ganglia within the injection site as well as in bowel rostral and caudal to it.(ABSTRACT TRUNCATED AT 400 WORDS)

Inability of neural crest cells to colonize the presumptive aganglionic bowel of ls/ls mutant mice: requirement for a permissive microenvironment.

The enteric system is formed by cells that migrate to the bowel from the neural crest. In order to gain insight into intraenteric factors that influence this migration, the colonization of the bowel of the ls/ls mouse was investigated. The terminal 2 mm of ls/ls intestine fails to become colonized by crest cells and thus remains aganglionic. The entire bowel of control mice and ls/ls mice was explanted before the appearance in situ of recognizable neurons and grown in organotypic tissue culture. Neurons, detected by the histochemical demonstration of acetylcholinesterase activity, developed throughout the length of the control gut, but, even in vitro, were excluded from the terminal segment of the ls/ls intestine. Co-culture experiments were done, in which primary and secondary sources of crest cells were combined with recipient segments of bowel, to test the ability of the recipient tissue to become colonized by neural precursors. The primary source was murine crest cells migrating away from an explant of the neuraxis. Secondary sources included avian and murine foregut (control and ls/ls) containing migratory crest cells as well as the quail ganglion of Remak. Recipient segments of bowel included control avian and murine hindgut, explanted before the tissue had become colonized by crest cells in situ, as well as the presumptive aganglionic bowel of ls/ls mice. Both primary and secondary sources of crest cells proved to be able to contribute neurons to the control segments of recipient hindgut. Species differences were no barrier to the colonization of the bowel in vitro. Moreover, the ls/ls foregut was as good a source of neural precursors for a normal recipient bowel, as was control avian or murine foregut. In contrast, none of the sources of crest cells that were utilized contributed neurons to the presumptive aganglionic gut of ls/ls mice. Both cells and processes of enteric neurons developing in vitro (detected by demonstrating neurofilament immunoreactivity) tended to be excluded from the presumptive aganglionic tissue. On the other hand, neurites, but not cell bodies, of dorsal root ganglia co-cultured with presumptive aganglionic ls/ls bowel did enter the abnormal zone. These data are consistent with the hypothesis that nonneuronal elements of the wall of the presumptive aganglionic region of the ls/ls gut are abnormal and prevent the colonization of this segment of the gut with viable neural precursors from the neural crest.(ABSTRACT TRUNCATED AT 400 WORDS)

Colocalization of luteinizing hormone and serotonin in secretory granules of mammalian gonadotrophs.

Previous studies have demonstrated the uptake of exogenous and storage of endogenous 5-hydroxytryptamine (5-HT) in parenchymal cells of the anterior pituitary. The present experiments were undertaken to test the hypothesis that the endogenous 5-HT of the anterior lobe of the pituitary is costored with beta-luteinizing hormone (beta-LH) within the same secretory granules of gonadotrophs. Electron microscope immunocytochemistry was used to detect 5-HT and beta-LH immunoreactivities in the anterior pituitary glands of mice and bats. Primary antisera generated in different species of animals to these two antigens were localized with appropriate species-specific secondary antisera coupled to colloidal gold particles of different sizes. This enabled 5-HT and beta-LH immunoreactivities to be demonstrated simultaneously on ultrathin sections of fixed anterior lobe tissue mounted on electron microscope (EM) grids. In both bats and mice 5-HT immunoreactivity, identified by immunostaining of beta-LH, was found in gonadotrophs, and in no other cell type. Within gonadotrophs about 25% of the secretory granules were labeled by antisera to both 5-HT and beta-LH, although 100% of granules reacted with the antiserum to B-LH. No secretory granules were found that were immunostained only by the antiserum to 5-HT. It is concluded that endogenous 5-HT may be a normal constituent of mammalian gonadotrophs and that it is colocalized with beta-LH in at least a subset of the secretory granules of these cells. It cannot yet be concluded that gonadotrophs synthesize 5-HT as well as taking it up from the ambient medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonergic elements of the mammalian pituitary.

Serotonin [5-hydroxytryptamine (5-HT)] is a constituent of the mammalian pituitary gland. We have used light and electron microscopic immunocytochemistry to locate immunoreactivity in the pituitary glands of mice and bats. In addition, we have compared the distribution of endogenous 5-HT immunoreactivity with that of exogenous [3H]5-HT taken up by elements of the pituitaries and detected by radioautography. 5-HT immunoreactivity was found in neurites in the posterior and intermediate lobes of the glands. These neurites also took up [3H]5-HT. The neural elements reactive with antisera to 5-HT or which labeled with [3H]5-HT differed in their distribution from neurites that labeled with [3H]dopamine or [3H]norepinephrine; moreover, lesioning of catecholaminergic neurites with 6-hydroxydopamine had no effect on the serotonergic structures. 5-HT immunoreactivity was also found in endocrine cells of the anterior lobes of the pituitaries of both species. No 5-HT was detected in neural elements of the anterior lobe, and in fact, no staining with nerve-specific antibodies to neurofilament proteins was found in the anterior pituitary. The ultrastructure of the granules of the 5-HT-immunoreactive endocrine cells suggested that they might be a subset of gonadotrophs, the cell type previously shown to take up exogenous [3H] 5-HT. 5-HT immunoreactivity was found within the granules of these cells. In bats, 5-HT immunoreactivity was present in the anterior pituitaries of active animals when 5-HT levels were high, but was greatly diminished in glands of hibernating animals when the 5-HT content was low. It is concluded that some cells that may be gonadotrophs contain endogenous 5-HT and also have an uptake mechanism for the amine. The function of 5-HT in the anterior pituitary remains to be determined, but 5-HT has previously been shown to inhibit the secretagogue action of LHRH on gonadotrophs.

Human serotonectin: a blood glycoprotein that binds serotonin and is associated with platelets and white blood cells.

A glycoprotein that circulates in blood, binds to the surface of platelets, and also binds serotonin with high avidity and specificity, has previously been found in rats. This glycoprotein has been called serotonectin. We now report the purification and characterization of a similar circulating glycoprotein in human blood that we propose be called human serotonectin, to distinguish it from the rat protein. Human serotonectin binds serotonin with high affinity (Kd1 = 36 nM; Kd2 = 1.1 microM). Monospecific antisera were raised in rabbits to purified human serotonectin. These antibodies were used to locate human serotonectin immunocytochemically, for quantitative estimation of the glycoprotein, and for rapid preparation of material purified by affinity chromatography. Evidence was obtained that indicated that human serotonectin circulates in plasma and also binds to the surfaces of white blood cells and platelets but not to red blood cells. In bone marrow it is found on megakaryocytes and on developing white cells of the eosinophil line. The protein can be completely removed by washing with isotonic sucrose or salt solutions from the surfaces of white cells but similar treatment only partially (63% sucrose wash/73% salt wash) removes human serotonectin from platelets. Antibodies to human serotonectin antagonize the uptake of serotonin by platelets but do not inhibit platelet aggregation. These data show that humans, like rats, have a circulating serotonin-binding glycoprotein that is also present as a peripheral membrane protein on platelets. The human also differs from the rat serotonectin in binding to white cells. The material may function in platelet serotonin uptake in both humans and rats; however, its function, if any, with respect to white cells is obscure.

Neurofilament expression in vagal neural crest-derived precursors of enteric neurons.

In order to gain insight into the potential role of the enteric microenvironment in the neuronal determination of the neural crest-derived precursor cells of enteric neurons, an attempt was made to ascertain when and where along the migratory route of these cells that they first express neuronal properties. The immunocytochemical detection of the 160-kDa component of the triplet of the chick neurofilament peptides served as a neuronal marker. In addition, neurogenic potential was assessed by growing explants of tissue suspected of containing presumptive neuroblasts in culture or as grafts on the chorioallantoic membrane of chick embryonic hosts. Neurofilament immunoreactivity was first detected in the foregut by Day 4 of development and spread to the hindgut by Day 7. Within the hindgut, development was more advanced within the colorectum than within the more proximal terminal ileum and caecal appendages. This probably reflects the distal-proximal migration of sacral neural crest cells in the postumbilical bowel. The ability of enteric explants to show neuronal development in vitro correlated with whether or not cells containing neurofilament immunoreactivity had reached that segment of gut at the age of explantation. These data suggest that enteric neuronal precursors have already begun to differentiate as neurons by the time they colonize the gut. Prior to the appearance of fibrillar neurofilament immunoreactivity in the foregut, cells that express this marker were found transiently within the mesenchyme of branchial arches 3, 4, and 5. These cells had disappeared from this region by developmental Day 6. The neurogenic potential of branchial arches 3 and 4 was demonstrated by the correlation that was found between the ability of explants of these arches to show neuronal development in vitro and the presence within them of cells that display neurofilament immunoreactivity. No similar neurogenic potential was found in the more rostral branchial arches which lacked the masses of neurofilament-immunoreactive cells. The location of the caudal branchial arches below the migrating vagal neural crest, the transience of the neurofilament immunoreactivity in them, and the coincident transience of their neurogenic potential in vitro, suggested that the masses of neurofilament immunoreactive cells in the caudal branchial arches might be vagal neural crest-derived neuronal precursor cells en route to the pharynx and the rest of the gut.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of the enteric nervous system.

The mature enteric nervous system (ENS) is characterized by a degree of neuronal phenotypic diversity and independence of central nervous system control unequaled by any other region of the peripheral nervous system. Studies that have utilized the immunocytochemical demonstration of neurofilament protein and explanation of primordial gut with subsequent growth in culture have indicated that the neural crest precursors of enteric neurons are already committed to the neuronal lineage when they colonize the bowel; however, neuronal phenotypic expression occurs within the gut itself. It is likely that precursors able to give rise to each type of neuron found in the mature ENS are present among the earliest neural crest émigrés to reach the bowel. In mice a proximodistal wave of neuronal phenotypic expression occurs that does not appear to reflect the descent of neuronal precursors. This observation, the known plasticity of developing neural crest-derived neurons, and the demonstration of a persistent population of proliferating neuroblasts in the gut raise the possibility that enteric neuronal phenotypic expression is influenced by the enteric microenvironment.