Murine forebrain and midbrain crest cells generate different characteristic derivatives in vitro

Neural crest (NC) is a transient structure that gives rise to various types of tissues. Many NC cells are pluripotent in the sense that their progeny can generate more than one derivative. However, the potentiality to differentiate into certain derivatives, such as cartilage and bone, seems to be specified with respect to the neuraxial levels at which the NC generates. In order to compare the differentiation potentiality of different regions of head NC, the derivatives of forebrain and midbrain mouse NC have been investigated in vitro using explant cultures of neuroepithelial fragments. From morphology and expression of specific markers, the midbrain crest cultures obviously generated earlier and were greater in number of neuronal cells than were the forebrain ones. Moreover, collagen type II positive cells were detected in the midbrain but not in the forebrain crest cultures. Finally, pigment cells were only observed in the forebrain cultures. The results suggest that the forebrain and midbrain crest cells have a different potentiality to differentiate.

Mammalian craniofacial embryology in vitro

Our review demonstrates that the whole embryo culture system established by New and his colleagues, in combination with beneficial fluorescent dye cell-tracing techniques, has greatly contributed to many advancements in the field of mammalian craniofacial embryology, especially with regard to elucidating the developmental behavior of cephalic crest cells. In addition, based on recent results, further combining whole embryo culture with mandibular organ culture methods has allowed us to trace cranial crest cells for a much longer developmental period, i.e., presently up to the cap stage in odontogenesis.

Cranial anomaly of homozygous rSey rat is associated with a defect in the migration pathway of midbrain crest cells

Craniofacial development of vertebrates depends largely on neural crest contribution and each subdomain of the crest-derived ectomesenchyme follows its specific genetic control. The rat small eye (rSey) involves a mutation in the Pax-6 gene and the external feature of rSey homozygous embryos exhibits craniofacial defects in ocular and frontonasal regions. In order to identify the mechanism of craniofacial development, we examined the cranial morphology and migration of cephalic crest cells in rSey embryos. The chondrocranial defects of homozygous rSey embryos primarily consisted of spheno-orbital and ethmoidal anomalies. The former defects appeared to be brought about by the lack of the eye. In the ethmoid region, the nasal septum and the derivative of the medial nasal prominence were present, while the rest of the nasal capsule, as well as the nasal and lachrymal bones, were totally absent except for a pair of cartilaginous rods in place of the nasal capsule. This suggests that the primary cranial defect is restricted to the lateral nasal prominence derivatives. Dil labeling revealed the abnormal migration of crest cells specifically from the anterior midbrain to the lateral nasal prominence in homozygous rSey embryos. Pax-6 was not expressed in the crest cells but was strongly expressed in the frontonasal ectoderm. To determine whether or not this migratory defect actually resides in environmental cues, normal midbrain crest cells from wild-type embryos were labeled with Dil and were orthotopically injected into host rSey embryos. Migration of the donor crest cells into the lateral nasal prominence was abnormal in homozygous host embryos, while they migrated normally in wild-type or heterozygous embryos. Therefore, the cranial defects in rSey homozygous embryos are due to inappropriate substrate for crest cell migration towards the lateral nasal prominence, which consistently explains the cranial morphology of homozygous rSey embryos.

Contribution of early-emigrating midbrain crest cells to the dental mesenchyme of mandibular molar teeth in rat embryos

Teeth are formed by reciprocal interactions between the epithelium and mesenchyme in the first pharyngeal arch. Although the contribution of midbrain and hindbrain crest cells to the first pharyngeal arch has been previously examined in rodent embryos, no direct evidence exists that these cells are actually involved in the dental mesenchyme. In order to elucidate the contribution of the cranial neural crest cells in tooth formation, we first identified the emigration sites and stages providing the crest cells that migrate to the presumed tooth-forming region of the mandibular prominence. Focal labeling with DiI was performed at the midbrain and anterior hindbrain crests in rat embryos, and the labeled embryos were cultured for 30 or 60 hr. The resultant migration patterns indicated that posterior midbrain crest cells emigrating by the end of the 4-somite stage predominantly migrated to the region where tooth buds normally develop. Second, we established a new type of long-term culture system in which whole embryo culture is followed by a mandibular organ culture. Using this system, rat embryos were maintained from the early-somite stage and the molars in the explants were able to reach the bud stage within 8 days. Finally, to ascertain if posterior midbrain crest cells emigrating by the end of the 4-somite stage were involved in the dental mesenchyme, these cells were labeled with DiI and processed for the long-term culture. Labeled crest cells were clearly detectable in the dental mesenchyme. These findings indicate that the early-emigrating posterior midbrain crest cells contribute to mandibular molar tooth development in rat embryos.

Differential expression of N-CAM, vimentin and MAP1B during initial pathfinding of olfactory receptor neurons in the mouse embryo

Olfactory receptor neurons extend their primary axons from the nasal epithelium to the olfactory bulb primordium via the frontonasal mesenchyme. In the present study, expression of neuronal markers (vimentin and MAP1B) and N-CAM was immunohistochemically investigated in the development of the olfactory system in mouse embryos. Expression of vimentin and MAP1B was first observed at early day 10 of gestation (D10) in the posterosuperior region of the medial nasal epithelium, while N-CAM was initially detected in the mesenchyme adjacent to the vimentin- and MAP1B-positive nasal epithelium. As development proceeded, the localization of neuronal marker-positive cells was mostly included in the N-CAM positive region. In addition, we adopted in situ labelling with vital dye (DiI) to directly determine the localization of the olfactory nerve and N-CAM on the same sections. We demonstrated that most extending axons were located in the N-CAM positive region. These results suggest that the expression of N-CAM plays a crucial role in the initial pathfinding of the olfactory nerve.

Localization of transforming growth factor-beta type I and type II receptors in mouse development

We have investigated the localization pattern of the transforming growth factor-beta (TGF-beta) receptors type I (T beta R-I) and type II (T beta R-II) during mouse organogenesis by immunohistochemical analysis. Staining of both receptors was found in many developing organs, e.g., bone, teeth, Meckel's cartilage, and neural tissues, where the expression of their ligands has been previously reported. During the investigated stages, expression of T beta R-I was more ubiquitous than that of T beta R-II. T beta R-II preferentially localized in the undifferentiated mesenchymal cells which subsequently differentiated into bone. There was no staining of T beta R-II in the central nervous system, while intense T beta R-I staining was found specifically in nervous tissues. Expression of T beta R-I and T beta R-II was mostly coincident with that of their ligands, suggesting that TGF-beta s act as multiple mediators during organogenesis. In addition, colocalization of both receptors in the epithelia of the tooth bud and submandibular gland, which were actively invaginating into the mesenchyme, leads us to speculate that both receptors may be necessary for dynamic epithelial morphogenesis.

Retinoic acid stage-dependently alters the migration pattern and identity of hindbrain neural crest cells

This study investigates the migration patterns of cranial neural crest cells in retinoic acid (RA)-treated rat embryos using DiI labeling. Wistar-Imamichi rat embryos were treated at the early (9.0 days post coitum, d.p.c.) and late (9.5 d.p.c.) neural plate stages with all-trans RA (2 × 10(−7) M) for 6 hours and further cultured in an RA-free medium. RA exposure stage dependently induced two typical craniofacial abnormalities; that is, at 9.0 d.p.c. it reduced the size and shape of the first branchial arch to those of the second arch, whereas, in contrast, at 9.5 d.p.c. it induced fusion of the first and second branchial arches. Early-stage treatment induced an ectopic migration of the anterior hindbrain (rhombomeres (r) 1 and 2) crest cells; they ectopically distributed in the second branchial arch and acousticofacial ganglion, as well as in their original destination, i.e., the first arch and trigeminal ganglion. In contrast, late-stage treatment did not disturb the segmental migration pattern of hindbrain crest cells even though it induced the fused branchial arch (FBA); labeled crest cells from the anterior hindbrain populated the anterior half of the FBA and those from the preotic hindbrain (r3 and r4) occupied its posterior half. In control embryos, cellular retinoic acid binding protein I (CRABP I) was strongly expressed in the second branchial arch, r4 and r6, while weakly in the first arch and r1-3. CRABP I was upregulated by the early-stage treatment in the first branchial arch and related rhombomeres, while its expression was not correspondingly changed by the late-stage treatment. Moreover, whole-mount neurofilament staining showed that, in early-RA-treated embryos, the typical structure of the trigeminal ganglion vanished, whereas the late-stage-treated embryos showed the feature of the trigeminal ganglion to be conserved, although it fused with the acousticofacial ganglion. Thus, from the standpoints of morphology, cell lineages and molecular markers, it seems likely that RA alters the regional identity of the hindbrain crest cells, which may correspond to the transformation of the hindbrain identity in RA-treated mouse embryos (Marshall et al., Nature 360, 737–741, 1992).

The contribution of both forebrain and midbrain crest cells to the mesenchyme in the frontonasal mass of mouse embryos

Migration of cranial neural crest cells is a crucial event in the formation of facial organs such as the frontonasal mass and branchial arches. However, the source of the populating crest cells that occupy the frontonasal mass remains unclear in mammalian embryos. To elucidate this, we performed focal DiI injections at various sites in the prosencephalon (forebrain, including the future telencephalon and diencephalon), mesencephalon (midbrain), and the anterior part of the rhombencephalon (hindbrain) separated posteriorly by the preotic sulus (i.e., rhombomere A; future rhombomere 1 and 2) of cultured mouse embryos from the 3- to 10-somite stage. Results directly revealed that during these stages the lateral edge of the prosencephalon produced crest cells which migrated to the frontonasal mass. On the other hand, labeled cells at the anterior neural ridge in the prosencephalon contributed mainly to the head epithelium, including the nasal placode, Rathke's pouch, and oral epithelium. As for the crest cells of the mesencephalon and rhombomere A, their destinations were significantly dependent on the injection site and somite stage. At the 3- to 4-somite stage, the crest cells emigrating from both the mesencephalon and rhombomere A migrated to the first branchial arch. Moreover, the mesencephalic region, but never rhombomere A, produced another group of crest cells that migrated to the frontonasal mass. In the 5- to 10-somite stage, the destinations of late-emigrating crest cells were restricted depending on their premigratory positions, i.e., the region producing crest cells migrating toward the frontonasal mass was restricted to the anterior portion of the mesencephalon, and the crest cells from the posterior portion of the mesencephalon primarily migrated to the first branchial arch, while those from the rhombomere A predominantly migrated to the trigeminal ganglion. Migration toward the frontonasal mass from the mesencephalon ceased at the earliest in the 7-somite stage, followed by termination of mesencephalic and rhombencephalic crest cell migration toward the first branchial arch at the 8-somite stage, whereas the contribution from rhombomere A to the trigeminal ganglion continued even at the 10-somite stage. This behavior suggests that both the prosencephalic and mesencephalic crest contribute to the mesenchymal cells in the frontonasal mass and also that the migration patterns of crest cells released from the prosencephalon, mesencephalon, and rhombencephalon depend on their axial level and developmental stage at initial emigration.

Apical cell escape from the neuroepithelium and cell transformation during terminal lip fusion in the house shrew embryo

The house shrew embryo has many cells in the ventricular lumen and on the luminal surface of the fusing terminal lip of the cephalic neural tube. The origin and fate of these cells were studied by means of light and electron microscopy, and by DiI labeling in a whole-embryo culture system. The cells appeared at stage 11A and persisted until stage 12A. Most of the cells seemed to originate from the neuroepithelium, as shown by frequent observations of epithelial cell escape and DiI labeling analysis. The cells on the luminal surface sometimes showed apoptotic features, but were not subjected to phagocytosis. Some of the escaping cells seemed to migrate to the ventral part of the prosencephalic neuropore and insert themselves into it. Others separated from the luminal surface and floated into the lumen. It seems likely that the floating cells either become autolyzed, or else change into macrophage-like cells, the latter alternative being supported by the results of DiI labeling. The macrophage-like cells actively phagocytosed the other degenerating cells and apoptotic bodies. These observations suggest that the apical escape of cells may play an important role in the remodeling of the neural fold during the terminal lip fusion, and that early neuroepithelial cells may have the potential to become cells with vigorous phagocytic activity, like macrophages.

Uchida rat (rSey): a new mutant rat with craniofacial abnormalities resembling those of the mouse Sey mutant

A new mutant rat with small eyes (rSey) which was found in the course of breeding Sprague-Dawley rats is described. Genetic analysis demonstrates that rSey is inherited as an autosomal dominant mutation. Heterozygotes (rSey/+) have small eyes, while homozygotes (rSey/rSey) do not develop lens and nasal placodes, resulting in lack of eyes and the nose and perinatal death. rSey does not affect any other cranial regions including the maxilla, mandible, hyoid arch and otic vesicles. The genetics and phenotype of the mutant rat closely resemble the Sey mutation in the mouse, suggesting that rSey is the rat counterpart of the Sey mouse. Tissue recombination studies indicate that ectoderm from homozygotes (rSey/rSey) never differentiates into lens tissue even if it is cultured with normal optic vesicles from rSey/+ or +/+ embryos. In contrast, lens differentiation occurs when ectoderm from rSey/+ or +/+ as well as rSey/rSey embryos. These results suggest that the failure of head ectoderm from rSey/rSey embryos to differentiate into lens results from defects in the early differentiation signaling from the neural plate or underlying mesenchyme before the optic vesicle grows out to contact the head ectoderm.

Proliferation of nasal epithelial and mesenchymal cells during primary palate formation

Proliferation of nasal epithelial and mesenchymal cells in mouse embryos was analyzed during primary palate formation using immunohistochemical demonstration of the thymidine analogue, 5-bromodeoxyuridine. Pulse labeling was employed to determine cell proliferation rates, with cell density of the nasal mesenchyme also being measured. To represent the entire nasal groove and prominences, four levels along the superior-inferior direction of three regions were utilized, i.e., the lateral and medial nasal prominences (LNP and MNP) and the bottom of the nasal groove. During the formation period, the labeling indices of the LNP and MNP epithelium decreased with respect to the development stage, whereas those of the bottom epithelium only slightly did. The epithelial cells in the prospective fusion area particularly showed decreased DNA synthesis in comparison with those in the nonfusing areas. In addition, the corresponding activity in the presumptive fusion area of the LNP epithelium was less than that in the MNP epithelium. The time at which a definitive decrease in the labeling index of the presumptive fusion area is believed to occur between tail somite (TS) stages TS5-7. A similar yet smaller decreasing tendency was observed in the labeling indices of the nasal mesenchyme. The cell density of the mesenchyme, however, slightly increased in all examined regions. Our results suggest that epithelial cell proliferation converts to a differentiation-type pattern, especially in the presumptive fusion area.(ABSTRACT TRUNCATED AT 250 WORDS)

A mutation in the Pax-6 gene in rat small eye is associated with impaired migration of midbrain crest cells

The rat small eye strain (rSey) lacks eyes and nose in the homozygote, and is similar to the mouse Sey strain with mutations in the Pax-6 gene. We isolated Pax-6 cDNA clones from an rSey homozygote library, and found an internal deletion of about 600 basepairs in the serine/threonine-rich domain. At the genomic level, a single base (G) insertion in an exon generates an abnormal 5' donor splice site, thereby producing the truncated mRNA. Anterior midbrain crest cells in the homozygous rSey embryos reached the eye rudiments but did not migrate any further to the nasal rudiments, suggesting that the Pax-6 gene is involved in conducting migration of neural crest cells from the anterior midbrain.

Distribution of F-actin during mouse facial morphogenesis and its perturbation with cytochalasin D using whole embryo culture

Histological and experimental studies were performed in mouse embryos to elucidate possible roles of actin filaments in the nasal epithelium during facial morphogenesis. C57BL/6 mouse embryos (8.5-11.5 days of gestation) were fixed and frozen sections were stained with rhodamine-phalloidin. Before formation of the nasal placode, there was no specific localization of F-actin. After the nasal placode was formed, intense staining of F-actin was observed at the apical side of the placode. Conversely, it was located at the basal side of the epithelium of developing nasal prominences. By using the whole embryo culture system, perturbation experiments were conducted with cytochalasin D (CD), which inhibits the polymerization of actin filaments. When day-10 embryos were exposed to CD at several concentrations for 24 hr, fusion of nasal prominences was inhibited in a dose-dependent manner. Treatment with a high dose of CD for 2 hr also prevented the same development irreversibly. In contrast, when day-9 embryos were exposed to CD at several concentrations for 24 hr, invagination of the nasal placode was not perturbed at all. The results suggest that apical F-actin plays an essential role in maintaining the close apposition state of the nasal prominences and in the following fusion. During the invagination stage, F-actin might be important in maintaining the epithelial structure, but is not crucial to the initiation of placode invagination.

Expression of retinoic acid receptor genes in neural crest-derived cells during mouse facial development

Retinoic acid (RA) is known as a teratogen that induces abnormalities in facial structures which are made up mainly of neural crest-derived mesenchyme. We investigated expression patterns of RA receptor (RAR) genes (subtypes alpha, beta, gamma) during mouse facial development. The expression of the RAR beta gene is specific for the mesenchyme around developing eyes and nose, whereas the RAR gamma gene is expressed in the mesenchyme differentiating to facial cartilages and bones. In contrast, the RAR alpha gene is expressed weakly and uniformly over the facial region. These results suggest that crucial roles of endogenous RA in facial development depend on differential functions of the RAR subtypes.