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

Abstract

Small graft pieces (size: 0.15xO.2mm to 0.1 x0.4mm and 0.25x0.25 mm) were isolated from the prospective prosencephalic ectoderm of the definitive primitive streak (st. 4) and head process (st. 5) chick blastoderms, and cultured in vivo intracoelomically. In all 1437 graft pieces isolated from 448 donor blastoderms were transplanted into the coelom of 941 host embryos; 216 hosts died before reaching 12 days. The 725 surviving hosts, which carried 1241 implants, yielded 304 analysable grafts. The very small size of the isolates affected the recovery rate. 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. Eye structures (retina as well as tapetum) differentiated in an area measuring ca. 0 . 4~0 . 1 mm extending transversely about 0.1 mm anterior to the head process at st. 5.Structural elements of the lens were not observed in any graft.Attempts to understand the process of neural induction in the early chick embryo date back to the work of WADDINGTON (1 , 2), who first proposed the role of the head process mesoderm as neural inductor. 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. his median piece covered 3/4 of the width of the prospective neural ectoderm) and he did not study differentiation tendencies of various,components of the brain in more detail. In the present work, differentiation tendencies of much smaller pieces of prospective prosencephalic ectoderm have been tested; and an attempt has been made to analyse the pattern of neural differentiation in this region. Special attention has been paid to the localization of the eye potency field, and results are discussed in comparison with the earlier findings (5, 6, 7), which were based on the differentiation of test pieces comprising all three germ layers. 83

Patterning of the chick forebrain anlage by the prechordal plate

“…According to fate-mapping studies the eyefield in vertebrates lies near the anterior margin of the neural plate Le Douarin, 1985, 1987;Eagleson and Harris, 1990). In the chick the potential to differentiate eye structures is present at HH4, that is before the ingression of the prechordal plate (Khare and Choudury, 1985). Slightly later, at HH5, the eyefield becomes separated into two lateral parts (Rudnick, 1948).…”