Dorsal lip blastopore3/14/2024 The following one (C), supposedly tortoise-like and ancestral to reptiles, is characterized by a fusion of the bilateral sites of mesoderm internalization, along a structure related to the blastoporal plate. The first one (B), proposed to be ancestral to amniotes, is characterized by a posterior restriction of the site of mesoderm internalization (black line). Two successive transition forms have been hypothesized in this model. The first line depicts the evolutionary scenario proposed by Arendt and Nübler-Jung (1999) to account for the transition between an amphibian-like (A) and an avian-like (D) gastrulation pattern. Scale bar: 500 µm.Ĭomparison of gastrulation and early neurulation stages between the turtle and the chick.Ĭomparison of the turtle gastrula (E,F) with hypothetical ancestral forms (C, D) accounting for the transition between amphibian- and avian-like gastrulation patterns. The planes of sections are indicated by dotted lines on the whole-mount view of each embryo. A1': higher magnification of A1 at the anterior lip level. A1, B1, C1: sections of hybridized embryos shown in A–C respectively. orbicularis embryos after in situ hybridization using the Otx5 probe (A, C: dorsal views B: ventral view). Otx5 expression during gastrulation in E. Black arrowhead in D2 points to the dorsal epiblast extension protruding between the lateral lips of the blastopore. A1, D1-2, F1, G1, H1-2: sections of hybridized embryos shown in A, D, F, G, H respectively. B1, C1: higher magnification of the region of the blastopore region of the embryo shown in B and C. orbicularis embryos after in situ hybridization using the Otx2 probe (A, C–I: dorsal views B: ventral view of the same embryo as in C). Otx2 expression during gastrulation in E. Black arrowheads in C, D, E, F, F3 point to the prospective intermediate mesoderm expressing Lim1 starting from stage 1+. Opened arrowheads in A1-2 and B1 point to the Lim1 positive mesenchymal cell population. Higher magnifications of A1 and A2 at the dorsal lip level are shown in A1' and A2'. A1-3, B1, C1-6, D1-4, F1-4: sections of the embryos shown in A, B, C, D, F respectively. orbicularis embryos after in situ hybridization using the Lim1 probe. Lim1 expression during gastrulation in E. The negative territories observed in the midline of both the dorsal lip and blastoporal canal floor are delimited by black arrows. The signal is ring-shaped at stages 0a–0b (A, B) but shows a clear bilateral symmetry at stage 1, with a discontinuity at medial levels. A1-4, B1-3, C1-3, D1-4: sections of embryos shown in A, B, C, D respectively. orbicularis embryos after in situ hybridization with the Brachyury probe (A–B, D–H: dorsal views C: ventral view). orbicularis.Ī–C, D–H: whole mount views of E. Scale bar : 500 µm.īrachyury expression during gastrulation in E. bl, blastocoel h, hypoblast bp, blastoporal plate e, epiblast. A–C, D–F : sagittal sections of respectively stage 0a and 0b embryos G–L : transverse sections of a late stage 0b embryo. Black arrowheads indicate the blastoporal cavity or canal. Section J is a tangential section of this protrusion. Red arrowheads in J points to a characteristic protrusion of the dorsal lip, which starts to form at stage 0b and appears as a narrow extension between the lateral lips at late stage 0b. The approximate location of the sections shown in G–L are also shown on a tranverse section (F) of a slightly younger embryo to help interpretation of the histologies. The level and plane of sections shown in A–L are indicated by dotted lines on the schematic views. Schematic dorsal views of turtle embryos at stages 0a to late 0b (0b+) are shown in the upper line, with the area opaca in blue and the area pellucida in white. Histological characteristics of stage 0 E. The mode of gastrulation in each group is shown on the right, as reviewed in. The phylogenetic relationships between the major amniote taxa are taken from. Gastrulation patterns of the major tetrapod groups.
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