Some aspects of the development of the notochord in mouse embryos

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Abstract
Prenotochordal cells are derived by proliferation from the pluripotential ectoderm in the Hensen’s node area, and they migrate in the cephalic direction to be incorporated into the roof of the archenteron. Later, after separation from the archenteron, the notochord probably includes some endodermal cells. Topographically it occupies an intermediate position between the ectoderm and endoderm. In this way it assumes a mesodermal-like character. After separation fiom the archenteron, the differentiation of the organ and its cells follows a cranio-caudal gradient. At the time of separation the basal lamina, originally covering the dorsal side of both the prenotochordal and endodermal cells, remains continuous between the endoderm and the notochord. Its presence from the earliest stages of morphogenesis of the notochord presumably indicates that it plays an important formative role. The cells of Hensen’s node contain numerous microtubules, centrioles and cilia. In later stages fibrillogenesis in the prenotochordal and notochordal cells and excortication (ecdysis) occur. The intracellular fibrillar material in the notochordal cells may first form the basal lamina and later the perichordal sheath. In 12 to 13-day-old embryos, during excortication of the fibrillar material, the basal lamina undergoes disruption and eventually disappears. From the 10th day onwards, the outer plasma membrane shows an active endocytosis, by the formation of micropinocytic vesicles. Vacuolization occurs in the later stages of morpho-genesis and it is suggested that this is due to dilation of endoplasmic reticulum and mito-chondria. There is no evidence of a syncytial stage in the development of the notochord in mouse embryos.