Abstract
Myoblast-lineage cells involved in the development of tongue muscles are known to derive from the occipital somites. The present study aimed at tracing the long-range migration pathway that the myoblast-lineage cells take prior to development of the tongue primordium. For this purpose, we used immunohistochemistry combining both anti-desmin and anti-MyoD antibodies as myoblast-lineage specific markers, as well as Ki 67 as a cell-proliferation marker. ICR mouse embryos at E 9.8 through E 12.3 were subjected to analyzation. A cell migration pathway that initiates in the occipital somites and terminates in the lateral lingual swellings of tongue primordium was monitored on desminimmunostained serial histologic sections prepared from embryos obtained at discrete embryonic days. The results showed that a troop of the myoblast-lineage cells migrating away from the occipital somites was characterized as desmin-positive/MyoD-negative cells with development of lamellipodialike pseudopodia, which we called herein muscle progenitor cells. These muscle progenitor cells were found to take the following route after their departure from the occipital somites: the mesenchyme of body column→the ventral areas of the fourth and the third branchial archs→the medial areas of the second branchial archs. On E 10.3, a leading group of the muscle progenitor cells first reached the medial portion of mandibular processes, where they remained MyoD-negative but lost the pseudopodia to develop mutual cellular contact. On E 11.4, the lateral lingual swellings of the tongue primordium first became discernible in the histologic views. At that developmental stage, MyoD-positive myoblasts started to differentiate within the muscle progenitor population in the mandibular processes. During the period of E 11.4 through E 12.3, the volume of the tissue of the tongue primordium increased rapidly accompanied by the locational shift of myogenic cell population comprising both muscle progenitor cells and myoblasts. These cells exhibited proliferation activities as evidenced by the Ki 67-positive immuno-reaction. On E 12.3 and there after, myoblasts within the tongue primordium started to exit the cell cycle and then to display elongated morphologies prior to myotube formation. On the basis of the present observations, we are now investigating the molecular networks operating to regulate of cell-motility and phenotypic modulation of tongue myoblast-lineage cells.
