Dr. Morris Maduro, UC Riverside Biology 168 – 10F – Lecture 16, page 1Lecture #16: Xenopus– Mesoderm induction, gastrulation Textbook: 2nd Ed.: pp. 32-35; 67-76; 89-105; 125-133. 3rd Ed.:109-117; 125-143.Some figures in these notes are redrawn from Wolpert et al. (2nd edition). Formation of the blastocoel and mesoderm induction At the blastula stage, a fluid-filled cavity in the animal half of the embryo is formed (the blastocoel). The origin of this cavity has been traced to the first cell division: The cleavage furrow widens in the animal hemisphere and over the next cell divisions fills with water, becoming the blastocoel. Water enters the blastocoel because NaCl is actively pumped into the space by the cells surrounding it. Ions are not free to diffuse because the cells are connected by tight junctions – hence, osmotic pressure causes water to enter the blastocoel. What purpose does the blastocoel serve? Nieuwkoop and others performed experiments to address the importance of the blastocoel by examining the fates adopted by cells as if the blastocoel were not present: If the cells at the very animal part of the embryo (called the animal cap) are removed and allowed to develop in isolation (an explant experiment), ectoderm fates result. Similarly, ventral and dorsal marginal(equatorial) cells give rise to ventral and dorsal mesoderm, respectively. Ventral mesoderm = blood, mesenchyme; Dorsal mesoderm = notochord, muscle. Similarly, vegetal cellsgive rise to undifferentiated vegetal tissue. If the presumptive ectoderm cells are recombined with vegetal cells, they give rise to mesoderm instead. If ventral vegetal cells are used, ventral mesoderm results; if dorsal vegetal cells are used, dorsal mesoderm results: Properties: There must be a certain amount of animal cap cells (about 200) present for mesoderm induction to occur (the community effect), which suggests that a factor must accumulate to a certain threshold for induction of mesoderm genes to occur. The animal cap is able to respond to the induction (i.e., competent) up to 11 hours after fertilization, or up to the time gastrulation starts. Contact need only occur for a short time (~2h) to induce some mesoderm, with full induction at 5h. The signal for induction must be a secreted molecule -- if filters with pores too small for cells to develop junctions are used between the animal cap and vegetal cells, induction still occurs. The conclusion is that vegetal cells can inducethe specification of mesoderm in adjacent cells (meso-endoderm induction), and furthermore, that this induction is different depending on whichvegetal cells (prospective ventral or dorsal) are in contact with the ectoderm. Hence, the blastocoel isolates a subset of
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