Nuclear factor 1 X-type-associated regulation of myogenesis in developing mouse tongue

OBJECTIVES

The tongue contains skeletal myofibers that differ from those in the trunk, limbs, and other orofacial muscles. However, the molecular basis of myogenic differentiation in the tongue muscles remains unclear. In this study, we conducted comprehensive gene expression profiling of the developing murine tongue.

METHODS

Tongue primordia were dissected from mouse embryos at embryonic day (E)10.5-E18.5, while myogenic markers were detected via microarray analysis and quantitative polymerase chain reaction (PCR). In addition to common myogenic regulatory factors such as Myf5, MyoD, myogenin, and Mrf4, we focused on Nfix, which acts as a unique molecular switch triggering the shift from embryonic to fetal myoblast lineage during limb myogenesis. Nfix inhibition was performed using a specific antisense oligonucleotide in the organ culture of tongue primordia.

RESULTS

Microarray and ingenuity pathway analyses confirmed the significant upregulation of myogenic signaling molecules, including Nfix, associated with the differentiation of myoblasts from myogenic progenitor cells during E10.5-E11.5. Quantitative PCR confirmed that Nfix expression started at E10.5 and peaked at E14.5. Fetal myoblast-specific genes, such as Mck and Myh8, were upregulated after E14.5, whereas embryonic myoblast-specific genes, such as Myh3 and Myh7, were downregulated. When Nfix was inhibited in the organ culture of tongue primordia, subtle morphological differences were noted in the tongue. Such an observation was only noted in the cultures of E10.5-derived tongue primordia.

CONCLUSIONS

These results reveal the contribution of Nfix to tongue myogenesis. Nfix expression during early tongue development may play a vital role in tongue muscle development.

Tongue development in stillborns autopsied at different gestational ages

OBJECTIVES

This study aimed to analyze, through the morphometric method, the perimeter and length of the tongue, the collagen fibers, and the perimeter of blood vessels at different gestational ages and fetal weights.

MATERIAL AND METHODS

Tongues (n=55) of stillborns autopsied at 23-40 weeks of gestational age were macroscopically analyzed, and their length and perimeter were measured. Fifty-five tongue fragments were collected through a longitudinal section in the region that accompanies the median lingual sulcus and histologically processed. Slides were stained with picrosirius and immunolabeled with CD31 antibody. Quantification was performed on collagen fibers under polarized light, and on the perimeter of vessels with the CD31.

RESULTS

A positive and significant correlation of gestational age with tongue perimeter and length was found. There was a positive and significant correlation between collagen fibers and gestational age, as well as between gestational age and the perimeter of blood vessels. Between collagen fibers and fetal weight, a positive and significant increase was observed. Regarding the correlation between the perimeter of blood vessels and the fetal weight, an increase was observed.

CONCLUSION

As gestational age advances, there is an increase in tongue perimeter and length, in the percentage of collagen fibers, and in vascular perimeter, demonstrating that tongue formation is directly related to tongue growth and development.

Tongue Growth during Prenatal Development in Korean Fetuses and Embryos

Background:

Prenatal tongue development may affect oral-craniofacial structures, but this muscular organ has rarely been investigated.

Methods:

In order to document the physiology of prenatal tongue growth, we histologically examined the facial and cranial base structures of 56 embryos and 106 fetuses.

Results:

In Streeter’s stages 13–14 (fertilization age [FA], 28 to 32 days), the tongue protruded into the stomodeal cavity from the retrohyoid space to the cartilaginous mesenchyme of the primitive cranial base, and in Streeter’s stage 15 (FA, 33 to 36 days), the tongue rapidly swelled and compressed the cranial base to initiate spheno-occipital synchondrosis and continued to swell laterally to occupy most of the stomodeal cavity in Streeter’s stage 16–17 (FA, 37 to 43 days). In Streeter’s stage 18–20 (FA, 44 to 51 days), the tongue was vertically positioned and filled the posterior nasopharyngeal space. As the growth of the mandible and maxilla advanced, the tongue was pulled down and protruded anteriorly to form the linguomandibular complex. Angulation between the anterior cranial base (ACB) and the posterior cranial base (PCB) was formed by the emerging tongue at FA 4 weeks and became constant at approximately 124°–126° from FA 6 weeks until birth, which was consistent with angulations measured on adult cephalograms.

Conclusions:

The early clockwise growth of the ACB to the maxillary plane became harmonious with the counter-clockwise growth of the PCB to the tongue axis during the early prenatal period. These observations suggest that human embryonic tongue growth affects ACB and PCB angulation, stimulates maxillary growth, and induces mandibular movement to achieve the essential functions of oral and maxillofacial structures.

In vivo cell tracking of mouse embryonic myoblasts and fast fibers during development

Fast and slow TnI are co-expressed in E11.5 embryos, and fast TnI is present from the very beginning of myogenesis. A novel green fluorescent protein (GFP) reporter mouse lines (FastTnI/GFP lines) that carry the primary and secondary enhancer elements of the mouse fast troponin I (fast TnI), in which reporter expression correlates precisely with distribution of the endogenous fTnI protein was generated. Using the FastTnI/GFP mouse model, we characterized the early myogenic events in mice, analyzing the migration of GFP+ myoblasts, and the formation of primary and secondary myotubes in transgenic embryos. Interestingly, we found that the two contractile fast and slow isoforms of TnI are expressed during the migration of myoblasts from the somites to the limbs and body wall, suggesting that both participate in these events. Since no sarcomeres are present in myoblasts, we speculate that the function of fast TnI in early myogenesis is, like Myosin and Tropomyosin, to participate in cell movement during the initial myogenic stages. genesis

Myosin heavy chain composition of tongue muscle in microphthalmic (mi/mi) mice before and after weaning

To elucidate the effects of teeth on muscle fibers in the tongue during the developmental process, we examined the expression of muscle contractile proteins and the genes for those proteins in normal mice and microphthalmic (mi/mi) mice with impaired tooth eruption. The mice were observed during the growth period, including weaning, which is when feeding movements undergo major changes. Expression of the myosin heavy chain (MyHC)-2a protein, whose contraction speed is relatively slow, disappeared after weaning in normal mice, while it remained in high concentrations even after weaning in mi/mi mice. The presence of MyHC-2a after weaning in mice with no tooth eruption was attributed to a compensation for lack of proper masticatory function and sucking-like movements, as MyHC-2a is necessary for these movements.

Normal and aberrant craniofacial myogenesis by grafted trunk somitic and segmental plate mesoderm

Our research assesses the ability of three trunk mesodermal populations– medial and lateral halves of newly formed somites, and presomitic(segmental plate) mesenchyme – to participate in the differentiation and morphogenesis of craniofacial muscles. Grafts from quail donor embryos were placed in mesodermal pockets adjacent to the midbrain-hindbrain boundary,prior to the onset of neural crest migration, in chick host embryos. This encompasses the site where the lateral rectus and the proximal first branchial arch muscle primordia arise. The distribution and differentiation of graft-derived cells were assayed using QCPN and QH1 antibodies to identify all quail cells and quail endothelial cells, respectively. Chimeric embryos were assayed for expression of myf5, myod, paraxis and lbx1, and the synthesis of myosin heavy chain (MyHC), between 1 and 6 days later (stages 14-30). Heterotopic and control (orthotopic) transplants consistently produced invasive angioblasts, and contributed to the lateral rectus and proximal first branchial arch muscles; many also contributed to the dorsal oblique muscle. The spatiotemporal patterns of transcription factor and MyHC expression by these trunk cells mimicked those of normal head muscles. Heterotopic grafts also gave rise to many ectopic muscles. These were observed in somite-like condensations at the implant site, in dense mesenchymal aggregates adjacent to the midbrain-hindbrain boundary, and in numerous small condensations scattered deep to the dorsal margin of the eye. Cells in ectopic condensations expressed trunk transcription factors and differentiated rapidly, mimicking the trunk myogenic timetable. A novel discovery was the formation by grafted trunk mesoderm of many mononucleated myocytes and irregularly oriented myotubes deep to the eye. These results establish that the head environment is able to support the progressive differentiation of several distinct trunk myogenic progenitor populations, over-riding whatever biases were present at the time of grafting. The spatial and temporal control of head muscle differentiation and morphogenesis are very site specific, and head mesoderm outside of these sites is normally refractory to, or inhibited by, the signals that initiate ectopic myogenesis by grafted trunk mesoderm cells.

Molecular regulation of tongue and craniofacial muscle differentiation

The molecular regulation of muscle development is tightly controlled at three distinct stages of the process: determination, differentiation, and maturation. Developmentally, specific populations of myoblasts exhibit distinct molecular phenotypes that begin to limit the ultimate characteristics of the muscle fibers. The expression of the myogenic regulatory factor family of the transcription process plays a key role in muscle development and, ultimately, in the subset of contractile genes expressed in a specific muscle. Craniofacial muscles have distinct functional requirements and associated molecular phenotypes that distinguish them from other skeletal muscles. The general principles of muscle molecular differentiation with specific reference to craniofacial muscles, such as the tongue, are discussed in this review.