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???displayArticle.abstract??? Fibronectin, a major component of the extracellular matrix is critical for processes of cell traction and cell motility. Whole-mount confocal imaging of the three-dimensional architecture of the extracellular matrix is used to describe dynamic assembly and remodeling of fibronectin fibrils during gastrulation and neurulation in the early frog embryo. As previously reported, fibrils first appear under the prospective ectoderm. We describe here the first evidence for regulated assembly of fibrils along the somitic mesoderm/endoderm boundary as well as at the notochord/somitic mesoderm boundary and clearing of fibrils from the dorsal and ventral surfaces of the notochord that occurs over the course of a few hours. As gastrulation proceeds, fibrils are restored to the dorsal surface of the notochord, where the notochord contacts the prospective floor plate. As the neural folds form, fibrils are again remodeled as deepneural plate cells move medially. The process of neural tube closure leaves a region of the ectoderm overlying the neural crest transiently bare of fibrils. Fibrils are assembled surrounding the dorsal surface of the neural tube as the neural tube lumen is restored.
Fig. 1. Fibronectin fibrils surround the somitic mesoderm during gastrulation and
extension. A,B: Sagittal sections through the future anterior midline of the early- (A, stage
10 ) and mid-gastrula (B, stage 11 ) show fibronectin fibrils along the neural/mesoderm
interface into the cleft of Brachet (arrowheads) but not at the mesoderm/
endoderm interface (arrows). C: A projection of 251 transverse confocal sections
through the dorsal mesoderm at stage 13 (C) shows that fibronectin appears at the
somitic mesoderm/endoderm interface (arrow) in addition to the neural/somitic
mesoderm boundary (arrowhead). At the same time, fibrils are cleared from the ventral
(v) and are in the process of being cleared from the dorsal (d) surfaces of the
notochord (no). D: Progressive clearing of fibronectin from the notochord is better
seen in projections of en face confocal stacks of an early immature state (D), to a
more connected network (E), and finally to a more tightly connected array surrounding
the somitic mesoderm (F, so) when fibrils are cleared from both surfaces of the
notochord (no). G: Projections of subsets of slices from a single en face stack containing
only the neural/mesoderm boundary (G) or the mesoderm/endoderm boundary (H) show distinct fibrils, whereas a projection of confocal sections through the middle of the mesoderm (shown overexposed in I) show fibrils at the lateral walls of the notochord (no) and small fibronectin-rich puncta within the somitic mesoderm (so). J: Ontogeny of the fibrillar fibronectin matrix. A ut-awaytransverse view of fibrillar fibronectin matrix assembly from mid- to late-gastrula stages. bp, blastopore; ne, neural ectoderm; en, endoderm; a, anterior; p, posterior. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
Fig. 2. Fibrils elaborated under the mesendoderm and at the limit of involution. A: The blastocoele roof (bcr) of the animal cap ectoderm
with accompanying mesendodermal mantle (mese) is dissected from whole fixed embryos. B: A series of confocal sections were
collected and projected into a single image and show dense fibrils in the center of the roof (bcr) surrounded by a ring of lower density
matrix underneath the migrating mesendoderm (mese; arrows show direction of mesendoderm migration in B and D). C: High
magnification of the inset in B shows that the density difference is due to gaps (asterisks) in the matrix behind the leading edge cells of
the mesendoderm (indicated by the dashed line). D: A cross-section through the confocal stack (along the solid line in C) shows gaps
in the matrix (asterisks) immediately behind the migratory leading edge of the cell (arrowhead; arrow shows direction of migration).
E: Confocal from tissue samples cut from a lateral region of the a fixed embryo were collected in en face orientation. F: The resulting
confocal series are projected to a single plane and show the abrupt edge of fibronectin fibrils near the blastopore lip (arrowheads). Scale
of C and D are the same. ar, archenteron; bp, blastopore; bc, blastocoele; no, notochord.
Fig. 3. Fibril assembly and remodeling during neural fold formation and neural tube closure. A: Transverse confocal sections were
collected and assembled into montages of dorsal axial tissues from the flat neural plate stage (A) to tadpole stage (J). A,B: Fibronectinrich
fibrils localize to the ventral and dorsal surfaces of the somitic mesoderm of early neural fold stages (stage 14) but are absent from
the boundary between lateral plate and ventralmesoderm and endoderm (arrowheads in A). C: Fibrils are restored to the dorsal surface
of the notochord (arrowhead) where notoplate cells attach. Archenteron roof plate cells contact the notochord along the fibril-free
ventral surface (arrow in C). D: Somite morphogenesis continues and the neural plate converges toward the midline as a deep neural
groove forms. E: Fibrils are assembled under the ventral surface of the notochord (arrow) as the neural folds (asterisk) appose one
another. F: By the time the neural folds have fused, fibronectin-rich fibrils are formed along the inner surface of the lateral plate and ventralmesoderm (arrowheads). G,H: Fibrils, mostly absent from the dorsal surface of the neural tube (arrowheads in G), are assembled as the
neural tube lumen (asterisk in H) is reestablished. I: By late tail bud stage, fibrils sheath the neural tube, notochord, and somites, including
the intersomitic clefts (arrows) shown in this section. I,J: From late tail bud (I) to tadpole stages (J) fibrils are extensively assembled in the
fin (arrowheads) and contact both the dorsal surface of the neural tube and the dorsal-most surface of somites. ar, archenteron; d, dorsal;
ec, ectoderm; en, endoderm; ne, neural plate ectoderm; no, notochord; nt, neural tube; so, somitic mesoderm; v, ventral.
