Palatal fusion – Where do the midline cells go?

ABSTRACT The mouse genome contains two Sim genes, Sim1 and Sim2. They are presumed to be important for central nervous system (CNS) development because they are homologous to the Drosophila single-minded (sim) gene, mutations in which cause a complete loss of CNS midline cells. In the mammalian CNS, Sim2 and Sim1 are coexpressed in the paraventricular nucleus (PVN). While Sim1 is essential for the development of the PVN (J. L. Michaud, T. Rosenquist, N. R. May, and C.-M. Fan, Genes Dev. 12:3264-3275, 1998), we report here that Sim2 mutant has a normal PVN. Analyses of the Sim1 and Sim2 compound mutants did not reveal obvious genetic interaction between them in PVN histogenesis. However, Sim2 mutant mice die within 3 days of birth due to lung atelectasis and breathing failure. We attribute the diminished efficacy of lung inflation to the compromised structural components surrounding the pleural cavity, which include rib protrusions, abnormal intercostal muscle attachments, diaphragm hypoplasia, and pleural mesothelium tearing. Although each of these structures is minimally affected, we propose that their combined effects lead to the mechanical failure of lung inflation and death. Sim2 mutants also develop congenital scoliosis, reflected by the unequal sizes of the left and right vertebrae and ribs. The temporal and spatial expression patterns of Sim2 in these skeletal elements suggest that Sim2 regulates their growth and/or integrity.

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split ends encodes large nuclear proteins that regulate neuronal cell fate and axon extension in the Drosophila embryo

Development ◽

10.1242/dev.127.7.1517 ◽

2000 ◽

Vol 127 (7) ◽

pp. 1517-1529 ◽

Cited By ~ 7

Author(s):

B. Kuang ◽

S.C. Wu ◽

Y. Shin ◽

L. Luo ◽

P. Kolodziej

Keyword(s):

Cell Fate ◽

Egf Receptor ◽

Neuronal Cell ◽

Drosophila Embryo ◽

Neuronal Precursors ◽

Split Ends ◽

Recognition Motifs ◽

Midline Cells ◽

Wrong Number ◽

Nuclear Levels

split ends (spen) encodes nuclear 600 kDa proteins that contain RNA recognition motifs and a conserved C-terminal sequence. These features define a new protein family, Spen, which includes the vertebrate MINT transcriptional regulator. Zygotic spen mutants affect the growth and guidance of a subset of axons in the Drosophila embryo. Removing maternal and zygotic protein elicits cell-fate and more general axon-guidance defects that are not seen in zygotic mutants. The wrong number of chordotonal neurons and midline cells are generated, and we identify defects in precursor formation and EGF receptor-dependent inductive processes required for cell-fate specification. The number of neuronal precursors is variable in embryos that lack Spen. The levels of Suppressor of Hairless, a key transcriptional effector of Notch required for precursor formation, are reduced, as are the nuclear levels of Yan, a transcriptional repressor that regulates cell fate and proliferation downstream of the EGF receptor. We propose that Spen proteins regulate the expression of key effectors of signaling pathways required to specify neuronal cell fate and morphology.

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The CNS midline cells and Egfr signaling genes are required for establishment of the RP2 motoneuron lineage in the Drosophila central nervous system

Biochemical and Biophysical Research Communications ◽

10.1016/j.bbrc.2009.01.104 ◽

2009 ◽

Vol 380 (4) ◽

pp. 729-735 ◽

Cited By ~ 1

Author(s):

Jung Yun Huh ◽

Sang-Hak Jeon ◽

Sang Hee Kim

Keyword(s):

Central Nervous System ◽

Nervous System ◽

Egfr Signaling ◽

Cns Midline ◽

Midline Cells

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Closing the Gap: Mouse Models to Study Adhesion in Secondary Palatogenesis

Journal of Dental Research ◽

10.1177/0022034517726284 ◽

2017 ◽

Vol 96 (11) ◽

pp. 1210-1220 ◽

Cited By ~ 14

Author(s):

K.J. Lough ◽

K.M. Byrd ◽

D.C. Spitzer ◽

S.E Williams

Keyword(s):

Mouse Models ◽

Animal Studies ◽

Cleft Lip ◽

Ex Vivo ◽

Genetically Engineered ◽

Orofacial Clefts ◽

Orofacial Clefting ◽

Palatal Fusion ◽

Closing The Gap

Secondary palatogenesis occurs when the bilateral palatal shelves (PS), arising from maxillary prominences, fuse at the midline, forming the hard and soft palate. This embryonic phenomenon involves a complex array of morphogenetic events that require coordinated proliferation, apoptosis, migration, and adhesion in the PS epithelia and underlying mesenchyme. When the delicate process of craniofacial morphogenesis is disrupted, the result is orofacial clefting, including cleft lip and cleft palate (CL/P). Through human genetic and animal studies, there are now hundreds of known genetic alternations associated with orofacial clefts; so, it is not surprising that CL/P is among the most common of all birth defects. In recent years, in vitro cell-based assays, ex vivo palate cultures, and genetically engineered animal models have advanced our understanding of the developmental and cell biological pathways that contribute to palate closure. This is particularly true for the areas of PS patterning and growth as well as medial epithelial seam dissolution during palatal fusion. Here, we focus on epithelial cell-cell adhesion, a critical but understudied process in secondary palatogenesis, and provide a review of the available tools and mouse models to better understand this phenomenon.

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Induction of primitive streak and Hensen's node by the posterior marginal zone in the early chick embryo

Development ◽

10.1242/dev.125.17.3521 ◽

1998 ◽

Vol 125 (17) ◽

pp. 3521-3534 ◽

Cited By ~ 10

Author(s):

R.F. Bachvarova ◽

I. Skromne ◽

C.D. Stern

Keyword(s):

Chick Embryo ◽

Marginal Zone ◽

Lower Layer ◽

Primitive Streak ◽

Normal Embryo ◽

Anterior Pole ◽

Posterior Edge ◽

Amphibian Embryo ◽

Midline Cells ◽

Marginal Zones

In the preprimitive streak chick embryo, the search for a region capable of inducing the organizer, equivalent to the Nieuwkoop Center of the amphibian embryo, has focused on Koller's sickle, the hypoblast and the posterior marginal zone. However, no clear evidence for induction of an organizer without contribution from the inducing tissue has been provided for any of these structures. We have used DiI/DiO labeling to establish the fate of midline cells in and around Koller's sickle in the normal embryo. In the epiblast, the boundary between cells that contribute to the streak and those that do not lies at the posterior edge of Koller's sickle, except at stage X when it lies slightly more posteriorly in the epiblast. Hypoblast and endoblast (a second lower layer formed under the streak) have distinct origins in the lower layer, and goosecoid expression distinguishes between them. We then used anterior halves of chick prestreak embryos as recipients for grafts of quail posterior marginal zone; quail cells can be identified subsequently with a quail-specific antibody. Anterior halves alone usually formed a streak, most often from the posterior edge. Quail posterior marginal zones without Koller's sickle were grafted to the anterior side of anterior halves. These grafts were able to increase significantly the frequency of streaks arising from the anterior pole of stage X-XI anterior halves without contributing to the streak or node. Stage XII anterior halves no longer responded. A goosecoid-expressing hypoblast did not form under the induced streak, indicating that it is not required for streak formation. We conclude that the marginal zone posterior to Koller's sickle can induce a streak and node, without contributing cells to the induced streak.

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The fate of medial edge epithelial cells during palatal fusion in vitro: an analysis by DiI labelling and confocal microscopy

Development ◽

10.1242/dev.114.2.379 ◽

1992 ◽

Vol 114 (2) ◽

pp. 379-388 ◽

Cited By ~ 5

Author(s):

M.J. Carette ◽

M.W. Ferguson

Keyword(s):

Laser Scanning ◽

Time Course ◽

Laser Scanning Microscopy ◽

Confocal Laser ◽

Palatal Fusion ◽

Morphological Criteria ◽

Medial Edge ◽

Mesenchymal Transformation

Fusion of bilateral shelves, to form the definitive mammalian secondary palate, is critically dependent on removal of the medial edge cells that constitute the midline epithelial seam. Conflicting views suggest that programmed apoptotic death or epithelial-mesenchymal transformation of these cells is predominantly involved. Due in part to the potentially ambiguous interpretation of static images and the notable absence of fate mapping studies, the process by which this is achieved has, however, remained mechanistically equivocal. Using an in vitro mouse model, we have selectively labelled palatal epithelia with DiI and examined the fate of medial edge epithelial (MEE) cells during palatal fusion by localisation using a combination of conventional histology and confocal laser scanning microscopy (CLSM). In dynamic studies using CLSM, we have made repetitive observations of the same palatal cultures in time-course investigations. Our results concurred with the established morphological criteria of seam degeneration; however, they provided no evidence of MEE cell death or transformation. Instead we report that MEE cells migrate nasally and orally out of the seam and are recruited into, and constitute, epithelial triangles on both the oral and nasal aspects of the palate. Subsequently these cells become incorporated into the oral and nasal epithelia on the surface of the palate. We hypothesize an alternative method of seam degeneration in vivo which largely conserves the MEE population by recruiting it into the nasal and oral epithelia.

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TGF-β Signaling and the Epithelial-Mesenchymal Transition during Palatal Fusion

International Journal of Molecular Sciences ◽

10.3390/ijms19113638 ◽

2018 ◽

Vol 19 (11) ◽

pp. 3638 ◽

Cited By ~ 8

Author(s):

Akira Nakajima ◽

Charles F. Shuler ◽

Alexander Gulka ◽

Jun-ichi Hanai

Keyword(s):

Cell Fate ◽

Transforming Growth Factor ◽

Epithelial Mesenchymal Transition ◽

Genetically Engineered ◽

Fusion Process ◽

Mesenchymal Transition ◽

Morphological Process ◽

Palatal Fusion ◽

Genetically Engineered Mice

Signaling by transforming growth factor (TGF)-β plays an important role in development, including in palatogenesis. The dynamic morphological process of palatal fusion occurs to achieve separation of the nasal and oral cavities. Critically and specifically important in palatal fusion are the medial edge epithelial (MEE) cells, which are initially present at the palatal midline seam and over the course of the palate fusion process are lost from the seam, due to cell migration, epithelial-mesenchymal transition (EMT), and/or programed cell death. In order to define the role of TGF-β signaling during this process, several approaches have been utilized, including a small interfering RNA (siRNA) strategy targeting TGF-β receptors in an organ culture context, the use of genetically engineered mice, such as Wnt1-cre/R26R double transgenic mice, and a cell fate tracing through utilization of cell lineage markers. These approaches have permitted investigators to distinguish some specific traits of well-defined cell populations throughout the palatogenic events. In this paper, we summarize the current understanding on the role of TGF-β signaling, and specifically its association with MEE cell fate during palatal fusion. TGF-β is highly regulated both temporally and spatially, with TGF-β3 and Smad2 being the preferentially expressed signaling molecules in the critical cells of the fusion processes. Interestingly, the accessory receptor, TGF-β type 3 receptor, is also critical for palatal fusion, with evidence for its significance provided by Cre-lox systems and siRNA approaches. This suggests the high demand of ligand for this fine-tuned signaling process. We discuss the new insights in the fate of MEE cells in the midline epithelial seam (MES) during the palate fusion process, with a particular focus on the role of TGF-β signaling.

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