Paulette Kan

Induction of oligodendrocyte progenitors in the trunk neural tube by ventralizing signals: effects of notochord and floor plate grafts, and of sonic hedgehog.

Recent evidence indicates that oligodendrocytes originate initially from the ventral neural tube. We have documented in chick embryos the effect of early ventralization of the dorsal neural tube on oligodendrocyte differentiation. Notochord or floor plate grafted at stage 10 in dorsal position induced the development of oligodendrocyte precursors in the dorsal spinal cord. In vitro, oligodendrocytes differentiated from medial but not intermediate neural plate explants, suggesting that the ventral restriction of oligodendrogenesis is established early. Furthermore, quail fibroblasts overexpressing the ventralizing signal Sonic Hedgehog induced oligodendrocyte differentiation in both the intermediate neural plate and the E4 dorsal spinal cord. These results strongly suggest that the emergence of the oligodendrocyte lineage is related to the establishment of the dorso-ventral polarity of the neural tube.

Differentiation in microgravity of neural and muscle cells of a vertebrate (amphibian)

The CELIMENE space experiment (CELulles en Impesanteur: Muscle Et Neurone Embryonnaires) was devoted to the study of the influence of gravity on the differentiation, the organisation and the maintenance of the highly specialised nervous system and muscular system. CELIMENE was carried out during the first flight of the IBIS hardware (Instrument for BIology in Space) with the fully automatic space mission PHOTON 10 in February 1995. Using the amphibian Pleurodeles waltl as a vertebrate model, in vitro experiments involved immunocytochemical detection of glial-, neuronal- and muscle-specific markers, and neurotransmitters in cells developed under conditions of microgravity compared with 1g controls, on-board and on the ground. We observed that the altered gravity did not disturb cell morphogenesis or differentiation.

Effets de divers facteurs de croissance (FGF, IGF-1) sur les ébauches des membres de l'embryon d'orvet (Anguis fragilis L.)

Abstract The present experiments were carried to investigate the effects of some growth factors (FGFs, IGF-1) on the development of limb buds in the slow-worm ( Anguis fragilis L.). This serpentiform reptile is devoid of legs in adulthood; but anlagen of limbs appear during embryonic life; their existence is only temporary: their growth ceases, they regress and disappear before hatching. Treatment of embryos was performed either by injection of the drugs around the limb buds or by application of small fragments of cellulosic paper soaked in the growth factors. The embryos were treated (27 by injection, 24 by application of cellulosic paper) at the stage of the allantoic bud 0.2 mm to 0.5 mm long and at an older stage (allantoic bud 1.8 mm to 4 mm long) (21 embryos treated). The administered growth factors were FGF-2, FGF-4 and IGF-1. Dosages were around 1 000 to 3 900 ng. Anterior limb buds display only very weak sensitivity to the effect of the applied growth factors: only a small proportion of the treated embryos presented a weak hypertrophy of these buds; however, after application of a fragment of cellulosic paper soaked in FGF-2, two thickening of the somatopleure in a embryo and two salient buds in another developed in the territory of the limb, propably representing anlagen of supernumerary limbs. In 25% of the embryos treated at the stage of the allantoic bud 1.8 to 4 mm long, the anlagen of the posterior limbs were greatly stimulated under the action of FGFs and IGF-1: the volume of the treated limbs was several times greater than the one of control limbs; histological study showed in the hypertrophied buds, numerous mitoses in the mesoblast and an apical ridge which did not degenerate. These results are in agreement with previous experiments and they show that it is possible to check experimentally the evolutive regression of the limbs of Anguis embryos.

Somites et prolongements somitiques impliqués dans la formation des membres chez les reptiles. L'amélie chez les serpents

Summary A new study was carried out on an increased number of young embryos of Reptile taxa either with well developed limbs or limbless to complete the previous observations devoted to the relationships between the somatic extensions and the formation of limb buds. Results are summarized in table I. In the embryos of Reptiles, the somites involved in limb formation send ventral processes in the somatopleural territory of the future limb and the limb develops only in front of these somatic extensions. In lizards with well developed limbs, there are always eight somites (S6–S13, S1 being the first postotic somite) which send ventral processes in the somatopleure and are thus involved in anterior limb formation. In serpentiform lizards such as Anguis fragilis, Ophisaurus apodus, Scelotes brevipes, only four or five somites (S6–S9 or S6–S10) form ventral extensions; the limb bud develops only in front of these somites, and its size is thus reduced from this early stage. Macroscopic and histological studies of young embryos belonging to four species of snakes (Natrix natrix, Natrix tessellata, Natrix maura and Vipera aspis) shows that none of the thoracic somites of the series S6–S13 form ventral extensions. Several observations suggest that in Reptiles with well developed limbs, the somitic extensions may initiate limb development by liberating a growth factor of the FGF family. In snake embryos, which are devoid of somitic extensions, no growth factor would be liberated and thus no anterior limb would develop. In some species, some pelvian somites form short extensions, which induce a feeble pelvian thickening or a rudimentary posterior limb bud (e.g. Boidae). It is an embryonic mechanism which is responsible for the limbless condition in Snakes.