鸡胚胚盘外胚层对诱导的反应能力的实验分析

EXPERIMENTAL ANALYSIS ON THE REACTIVE CAPACITY OF THE EPIBLAST OF THE CHICK BLASTODERM

将4期原结或原条小块(平均为0.3—0.4平方毫米)分别作卵内移植至5、6、7、8期胚盘的原结前区(预定头区),原结区及原条区(预定躯干区)等不同平面的左侧或右侧外胚层下,在手术后6—30小时内的不同间隔时间取材观察,分析外胚层反应能力的区域性差异及其与胚龄的关系。所得结果简述如下:1.鸡胚5、6、7、8期胚盘外胚层对原结或原条移植块的诱导呈现不同的反应状态。5—6期胚盘均有神经反应能力,形成分化程度不同的神经组织。7期胚盘除个别标本外,几乎完全丧失神经反应能力,8期胚盘已完全丧失神经反应能力,只出现外胚层板状增厚反应。表明随宿主胚龄的增高,外胚层的反应能力相应下降以至于全部丧失。2.神经反应能力终止于第6期末期或第6—7期之间。6期胚盘的前后区均仍有不同程度的神经反应能力,7期胚盘除原结前区(预定头区)外胚层似仍有微弱的反应外,已基本丧失神经反应能力。到8期早期,外胚层板状增厚反应能力亦告消失。3.胚盘外胚层对诱导的神经反应能力,随胚龄的增高而逐渐出现了区域性差异,5—6期胚盘预定头区外胚层的神经反应能力较高,向后有沿头—尾轴逐渐成梯度递减的趋向。胚盘左右侧外胚层的反应能力无显著差别。4.本文还就胚壁内胚层对原结诱导的可能反应予以讨论。

It is well known that the competence of the prospective ectoderm of early amphi-bian gastrulae for neuralization changes with time and region of the embryo.In chickembryo,Woodside(1937)demonstrated also that the neural competence of the epiblastdecreases with increasing age.However,no experimental data bearing on the regionaldifference in competence have been offered.This report represents an attempt to givean analysis of the reactive capacity of the ectoblast of the chick blastoderm with specialreference to time and space.The grafts of Hensen’s node at the primitive streak stage,ranging in size from0.3--0.4 mm square were transplanted to the host blastoderms of different ages,varyingfrom the early head process stage up to 4 somites stage(i.e.corresponding to the deve-lopmental stages of 5,6,7 and 8 of Hamburger and Hamilton).The grafts were placed underncath the ectoderm at various levels(i.c.antero-latcral to the node or lateral to thenode and streak)of the host.They were then removed at various intervals from 6--30hours after operation,preserved in Bouin’s fixatives and stained with dilute Delafieldhematoxylin.It was observed that neural formation,e.g.neural epithelium,neural plate or neuralfold could be induced in the whole epiblast of host blastoderms of stages 5 and 6.Thereactivity changed quickly during the neural fold formation stage(stage 7),the epiblast atthis stage produced only placodal thickenings,except in a few cases the neural formationwas induced in the anterior epiblast of the blastoderm.The placodal thickening couldalso be called forth in the anterior epiblast of host blastoderm of stage 8(2--4 somitesstage),but the posterior epiblast was incapable of any response to inductors.Ifl otherwords,the competence for neuralization vanished shortly after the late head fold stage(stage 6--7),and the capacity for placodal thickening response vanished in the 4-somitesstage(stage 8).These results clearly demonstrated that the reactive capacity of the epi-blast to inductors decreased with increasing age of the host at the time of transplanta-tion.The results presented here together with those obtained in our previous findingsshowed that there was no regional difference in neural competence in the epiblast of theprimitive streak stage,but such difference did develop gradually with increasing age ofthe blastoderms.Our data indicated that the frequencies of neural response to inductorsof the epiblast were higher at regions anterior and lateral to the Hensen’s node andlower at the posterior region in the blastoderm of stages 5 and 6.The frequenciestended to decrease along a gradient axis in a cephalocaudal direction,and appeared tobe no difference in capacity in the epiblast between the left and right side of the blasto-derm.Finally,we also observed in some instances that when a node graft was transplantedto the posterior border of the pellucida area of the blastoderm,a gut-like epitheliumcould be induced from host germ wall endoderm which had been in contact with thechordal mesoderm of the graft.The possible role of endoderm differentiation uponchordal mesoderm and the significance of their relationship are discussed.