The appearance and extension of neural differentiation tendencies in the neurectoderm of the early chick embryo

Abstract

1. Recently the direct analysis of neural induction in the early chick embryo has become possible by the perfection of a technique for mechanical germ layer separation (HARA, 1961). This technique made it possible to interrupt the inductive process at various stages of development by separating the ectoderm from the inducing mesoderm anterior toHENSEN'S node. By subsequently culturing pieces of the isolated ectodermin vivo a study was made of the temporal sequence of appearance and the spatial extension of neural differentiation tendencies in the prospective neural ectoderm. 2. An area of the blastoderm anterior to the node was carefully freed of endo- and mesoderm with tungsten needles, and the remaining ectoderm was then cut into pieces of definite sizes with a glass needle. These pieces were culturedin vivo by means of transplantation into the coelome of 2 1/2 days old host embryos in order to assess their self-differentiation. 3. Donor blastoderms of four carefully defined stages (I-IV) were used: the nearly definitive and definitive streak stages, and the early and medium headprocess stages. The ectodermal area concerned was divided into a median zone and two lateral zones, and each of these zones was subdivided into two, three, or four anteroposterior areas, according to the stage used. The areas were designated with the letters 'A'-'D' for the median ones, and 'LA'-'LD' for the lateral ones. The 'A', 'LA', 'B', and 'LB' areas together always included the prospective prosencephalic region of the neurectoderm, whereas the remaining areas included the prospective mesencephalic region and part of the prospective rhombencephalic region (cf. Figs. 14, 16, 18, 20). 4. A total of 903 grafts prepared from 114 donor blastoderms were transplanted intracoelomically; 202 grafts were lost due to the death of the host embryos; out of the remaining 701 grafts 304 were recovered and studied histologically after 12 days of culturing. 5. There was a marked difference in the rates of recovery between grafts from the lateral and anteriormost median areas containing peripheral parts of the prospective neural anlage, and grafts from the more posterior median areas. The possible reasons for this difference were discussed. 6. The regional neural structures differentiating in the grafts were identified with the help of criteria established byHARA (1961) and extended in the course of the present study. 7. The results may be summarized as follows (cf. Figs. 14, 16, 18, 20): a. In all stages the relative numbers of grafts forming neural structures were lower in the lateral and anteriormost median graft areas than in the more posterior median areas. They generally increased from stage to stage in all graft areas, ranging from 0% in the anterolateral areas of stage I and II, to 100% in the posteromedian areas from stage III onwards. Among the lateral areas in any one stage it was always the area located at the level of the prechordal mesoderm (the 'LB' area) which showed the highest relative number of grafts forming neural structures. b...

Establishment of the Neural Differentiation Pattern in the Prospective Prosencephalic Ectoderm of the Chick Embryo (Gallus domesticus)

“…13). However, structural elements of the eye were encountered mainly in Dmp and Fml grafts at St. 4 and Cmm, Cmp, Dmp, Dml and Fmp grafts at stages 5-and 5. Structural elements of telencephalic cortex were found in the anterior median C/D/F grafts only.…”

“…Operation were performed according to the prescribed techniques (3,4). Staging of donor blastoderms was done according to HAMBURGER and HAMILTON (8), with the following finer sub-division.…”

“…It is known that the first inductive stimulus for prosencephalic differentiation emanates from the primary hypoblast before the laying down of the prechordal mesoderm (17,18,19,20); and it is also known that the embryonic endoderm invaginated through the node spreads centrifugally (21,22). Thus, the centrifugal differentiation pattern of neural structures in the prospective prosencephalic ectoderm may have some relation with the centrifugal extension of the embryonic endoderm at st. 4 and st. 5. The inducing stimulus of the prechordal mesoderm brings about the appearance of various structural elements of the prosencephalon in the overlying ectoderm at st. 4.…”

“…The histological analysis of the recovered grafts reveals that the frequency of neuralization was higher in the grafts isolated from the central region of the prospective prosencephalic ectoderm than in those from its peripheral region. A centrifugal extension of Merentiation tendencies of prosencephalic structures was observed from st. 4 to st. 5. At st. 5 frequency of telencephalic structures was high in the grafts of the anterior region, whereas that of diencephalic structures was high in those of the more posterior region.…”

“…HARA (3), based on his sandwich experiments, demonstrated that prechordal mesoderm induces prosencephalic differentiation tendencies in the competent ectoderm; anterior part of the head process induces mes-/rhombencephalic differentiation tendencies ; while its posterior part induces rhombencephalic and spinal cord differentiation tendencies. RAO (4) analysed the pattern of appearance and extension of regional differentiation tendencies in the prospective neural ectoderm. However, his isolates were large in size (e.g.…”