Gastrulation and Neurulation Movements In Xenopus laevis Gastrulation of Xenopus Involution Involuting Marginal Zone (IMZ) Migration Convergent Extension IMZ Neural Epiboly Animal Cap Early Mid Late Mid Mid Neurula Gastrulation and Neurulation Movements In Xenopus laevis Diagrams of midsagittal views of gastrulation,,showing dorsal at upper left, animal pole at upper right, vegetal pole at lower left, and prospective germ layers, mesoderm (red), endoderm (yellow, green for “bottle cells”), neural ectoderm (forebrain, light blue; hindbrain/spinal cord, dark blue), and epidermis (grey). Stages of development are as indicated; diameter of embryo is about 1.3 mm; time elapsed from 9 hr to 17 hr of development at 23 C. The prospective mesoderm (red) located in the involuting marginal zone (IMZ), involutes, or rolls over an internal blastoporal lip (curved arrow, top, Early), and migrates across the roof of the blastocoel (arrows), beginning on the dorsal side (Early), proceeding laterally and finally ventrally (curved arrow, Mid, Late Mid). Prospective posterior dorsal mesoderm (notochordal and somitic) also lengthens, or extends, in the anterior-posterior axis (oppositely pointing arrows), and narrows, or converges, around the blastopore (BP), thus simultaneously elongating the anterior-posterior axis of the embryo and squeezing the blastopore shut (Mid-Mid Neurula). As these movements occur, the mesoderm carries along with it the overlying endoderm (yellow, green) of the IMZ, forming the archenteron (gut) roof. Note that this thin layer of endoderm covering the mesoderm in the IMZ is called “suprablastoporal endoderm” and it forms the archenteron roof; the floor of the archenteron is formed from the large vegetal endodermal cells, called the “subblastorporal endoderm”. The green area designates the “bottle cells”, cells at constrict apically, causing a collection of pigment and a local bending in the epithelial sheet, an “invagination” that is the first sign of the blastopore. These cells are carried inside by virture of their attachment to the underlying mesoderm and later respread to form a large area in the anterior and lateral parts of the archenteron. As the prospective notochordal (axial) and somitic (paraxial) mesoderm converges and extends on the inside, the posterior neural tissues, consisting of the prospective hindbrain and spinal cord (dark blue) converge and extend in parallel with the dorsal mesodermal tissues (arrows, Mid-Mid Neurula). Beginning in the early n eurula, the convergent extension movements of the neural plate are accompanied by apical constriction of the superficial epithelial cells of the neural plate, a rolling of the epithelial sheet into a trough (Mid Neurula) and eventual apposition of the neural folds, which fuse, forming the neural tube.

Xenopus Fate Map and Gastrulation Movements Surface Midsagittal IMZ-Deep IMZ-Surface Late Blastula Early Gastrula Prospective fates and morphogenic movements of the late blastula and early gastrula are shown in views of the surface, a midsagittal section, the IMZ-Deep, and the IMZ-Superficial. Dorsal is to the left in all illustrations. The IMZ-Superficial (far right) is represented in the surface view (left) and the IMZ-Deep (near right) is the ring of deep prospective mesodermal cells lying beneath the IMZ-Superficial in the surface view (far left), and visible on edge in the midsagittal sectional view (near left). Movements are indicated by arrows. Special features illustrated include the animal pole (AP); archenteron (A); the blastocoel (BLC); the bottle cells (BC); the blastopore (pointers); the leading dorsal mesendoderm (Mesen); the head mesoderm (Hd); the heart mesoderm (Ht); the involuting marginal zone (IMZ), composed of multiple layers of deep, nonepithelial mesoderm (IMZ-Deep), and superficial, epithelial endoderm (IMZ-Surface), the lateral and ventral mesoderm (LV); the noninvoluting marginal zone (NIMZ); the vegetal pole (VP). Prospective tissues shown are epidermis (light blue), neural tissue (dark blue), dorsal NIMZ (blue green), notochord (red), somitic mesoderm (red orange), migrating mesendoderm at the leading edge of the mesodermal mantle (orange), suprablastoporal endoderm (yellow), a special region of suprablastoporal endoderm known as the bottle cells (green), and vegetal subblastoporal endoderm (yellow, divided into cells). About 30-40 prospective notochordal cells (black dots) and prospective somitic cells (white line), 1 to 3 per somite, are found in the superficial epithelial layer of the IMZ (IMZ-Superficial). Note that both the prospective notochordal and somitic cells move out of the epithelial surface layer and join the deep mesenchymal layer during mid and late neurulation (ingression). Note that the IMZ-Superficial is complex and consists of prospective endodermal cells of two types: those cells represented in green are destined to form bottle cells by apical constriction as the blastopore forms, are carried inside by virtue of their attachment to the underlying mesoderm, and latter respread to form a large area of the archenteron lining; those represented in yellow remain cuboidal and are involuted with the underlying mesoderm. In addition, those cells represented in black are prospective notochord destined to ingress and join the much larger deep component of the notochord during neurulation, and those cells represented by white are prospective somitic mesoderm destined to ingress to join the deep somitic mesoderm during neurulation.

Xenopus Fate Map and Gastrulation Movements Surface Midsagittal IMZ-Deep IMZ-Surface Early Gastrula Late Gastrula Prospective fates and morphogenic movements of the early gastrula and late gastrula are shown in views of the surface, a midsagittal section, the IMZ-Deep, and the IMZ-Superficial. Dorsal is to the left in all illustrations. The IMZ-Superficial (far right) is represented in the surface view (left) and the IMZ-Deep (near right) is the ring of deep prospective mesodermal cells lying beneath the IMZ-Superficial in the surface view (far left), and visible on edge in the midsagittal sectional view (near left). Movements are indicated by arrows. Special features illustrated include the animal pole (AP); archenteron (A); the blastocoel (BLC); the bottle cells (BC); the blastopore (pointers); the leading dorsal mesendoderm (Mesen); the head mesoderm (Hd); the heart mesoderm (Ht); the involuting marginal zone (IMZ), composed of multiple layers of deep, nonepithelial mesoderm (IMZ-Deep), and superficial, epithelial endoderm (IMZ-Surface), the lateral and ventral mesoderm (LV); the noninvoluting marginal zone (NIMZ); the vegetal pole (VP). Prospective tissues shown are epidermis (light blue), neural tissue (dark blue), dorsal NIMZ (blue green), notochord (red), somitic mesoderm (red orange), migrating mesendoderm at the leading edge of the mesodermal mantle (orange), suprablastoporal endoderm (yellow), a special region of suprablastoporal endoderm known as the bottle cells (green), and vegetal subblastoporal endoderm (yellow, divided into cells). About 30-40 prospective notochordal cells (black dots) and prospective somitic cells (white line), 1 to 3 per somite, are found in the superficial epithelial layer of the IMZ (IMZ-Superficial). Note that both the prospective notochordal and somitic cells move out of the epithelial surface layer and join the deep mesenchymal layer during mid and late neurulation (ingression). Note that the IMZ-Superficial is complex and consists of prospective endodermal cells of two types: those cells represented in green are destined to form bottle cells by apical constriction as the blastopore forms, are carried inside by virtue of their attachment to the underlying mesoderm, and latter respread to form a large area of the archenteron lining; those represented in yellow remain cuboidal and are involuted with the underlying mesoderm. In addition, those cells represented in black are prospective notochord destined to ingress and join the much larger deep component of the notochord during neurulation, and those cells represented by white are prospective somitic mesoderm destined to ingress to join the deep somitic mesoderm during neurulation.

Xenopus Fate Map and Gastrulation Movements Surface Midsagittal IMZ-Deep IMZ-Surface Late Gastrula AP hd mesen BLC ht BC BC lv Neurula Prospective fates and morphogenic movements of the late gastrula and neurula are shown in views of the surface, a midsagittal section, the IMZ-Deep, and the IMZ-Superficial. Dorsal is to the left in all illustrations. The IMZ-Superficial (far right) is also represented in the surface view (left). The IMZ-Deep (near right) is the ring of deep prospective mesodermal cells lying beneath the IMZ-Superficial in the surface view (far left), and visible on edge in the midsagittal sectional view (near left). Movements are indicated by arrows. Special features illustrated include the animal pole (AP); archenteron (A); the blastocoel (BLC); the bottle cells (BC); the blastopore (pointers); the leading dorsal mesendoderm (Mesen); head mesoderm (Hd); heart mesoderm (Ht); the involuting marginal zone (IMZ), composed of multiple layers of deep, nonepithelial mesoderm (IMZ-Deep and superficial, epithelial endoderm (IMZ-Superficial), the lateral and ventral mesoderm (LV); the noninvoluting marginal zone (NIMZ); the vegetal pole (VP). Prospective tissues shown are epidermis (light blue), neural tissue (dark blue), dorsal NIMZ (blue green), notochord (red), somitic mesoderm (red orange), migrating mesendoderm at the leading edge of the mesodermal mantle (orange), suprablastoporal endoderm (yellow), a special region of suprablastoporal endoderm known as the bottle cells (green), and vegetal subblastoporal endoderm (yellow, divided into cells). About 30-40 prospective notochordal cells (black dots) and prospective somitic cells (white line), 1 to 3 per somite, are found in the superficial epithelial layer of the IMZ (IMZ-Superficial). Note that both the prospective notochordal and somitic cells move out of the epithelial surface layer and join the deep mesenchymal layer during mid and late neurulation (ingression). Note that the IMZ-Superficial is complex and consists of prospective endodermal cells of two types: those cells represented in green are destined to form bottle cells by apical constriction as the blastopore forms, are carried inside by virtue of their attachment to the underlying mesoderm, and latter respread to form a large area of the archenteron lining; those represented in yellow remain cuboidal and are involuted with the underlying mesoderm. In addition, those cells represented in black are prospective notochord destined to ingress and join the much larger deep component of the notochord during neurulation, and those cells represented by white are prospective somitic mesoderm destined to ingress to join the deep somitic mesoderm during neurulation. AR