We have examined the biomechanical development of the notochord of Xenopus early tail-bud embryos by: (1) quantifying morphological and mechanical changes in the embryo during stages 20–28, and (2) conducting manipulative experiments to elucidate mechanical roles of various components of the notochord. The notochord, which is composed of a stack of flat cells surrounded by a connective tissue sheath, elongates dramatically and begins straightening between stages 21 and 25. At this time the fiber density in the notochord sheath goes up, the osmotic activity of the notochord cells increases, vacuoles within these cells swell, the internal pressure of the notochord increases 2- to 3-fold, and the flexural stiffness of the notochord rises by an order of magnitude. We suggest that the tendency of the notochord cells to osmotically swell is resisted by the sheath, thereby permitting the internal pressure to rise. This pressure increase results in the greater stiffness that permits the notochord to elongate and straighten without being buckled by the surrounding tissues.

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