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The fate of each of the blastomeres in the 16-cell stage Xenopus embryo which had been carefully selected for stereotypic cleavages was determined by intracellularly marking a single blastomere with horseradish peroxidase and identifying the labeled progeny in the tailbudembryo by histochemistry. Each blastomere populated all three primary germ layers. The progeny of each blastomere were distributed characteristically both in phenotype and in location. For example, most organs were populated by the descendants of particular sets of blastomeres. Furthermore, within an organ the progeny of a single blastomere were restricted to defined spatial addresses. This study describes the fates of identified 16-cell stage blastomeres and demonstrates that they are distinct and predictable if embryos are preselected for stereotypic cleavages.
Fig. 1. The names of the blastomeres of the 16-cell stage Xenopus embryo, as assigned by Hirose and Jacobson (1979). Although Xenopus embryos need not cleave in this pattern, the embryos used in this study all did.
IG. 3. The distribution of HRP-labeled cells in rostra1 trunk sections of tailbud embryos. (A) Dl.1: Labeled cells were located bilaterally in the central region of the somite and in the ventral spinal cord. Notochord cells were labeled faintly. One ventralhindgut cell was labeled in this section. X52. (B) 01.2: In a caudal head section, labeled cells were in the trigeminal ganglion (cg), ventralretina, and lens. Ventralpharynx and branchial arch mesoderm contained several labeled cells. Arrow points to a single cell in the dorsal rhombencephalon. X56. (C) 02.1: In a section near the spinomedullary junction labeled cells were in the headsomite, foregut, and heart. X54. (D) 02.2: In a section near the spinomedullary junction, large numbers of labeled neural crest cells migrated along the rhombencephalon and otocyst. The entire headsomite was labeled. Caudal branchial mesoderm, dorsal foregut, and heart contained labeled cells. X66. (E) V1.1: In the midtrunk region numerous labeled epidermal cells were scattered over the ventral and lateral regions. Neural crest cells, dorsal and ventral myotubes, and ventrolateral hindgut cells were labeled. X59. (F) V1.2: In a section near the spinomedullary junction, numerous neural crest cells, cranial ganglion cells, brachial mesoderm cells, heart cells, and epidermal cells were labeled. X60. (G) V2.1: In a section near the spinomedullary junction, labeled cells only were in the dorsal region of the lateral plate mesoderm and in the nephric tubules. X55. (H) V2.2: In a section near the spinomedullary junction, labeled cells were in the neural crest, lateral plate mesoderm, and nephrotome. A few labeled cells were in the headsomite at this level. X51. Abbreviations as in Fig. 2.
IG. 4. The distribution of HRP-labeled cells in caudal trunk or tail sections of tailbud embryos. (A) D1.1: Labeled cells in the tail region were in the central somite, ventral spinal cord and notochord. The dark coloration in the epidermis and hindgut is due to melanin granules, not to the HRP reaction product. X57. (B) Dl.2: Labeled neurons were located throughout the ipsilateral spinal cord and central somite. Labeled neural crest cells, dorsal to the spinal cord and betwen it and the somites were located bilaterally. A few labeled cells were in the dorsal finepidermis. X55. (C) 02.1: Labeled cells were only in the neural crest and ventralhindgut at this level. X53. (D) 02.2: Labeled cells were in the intermediate and ventral spinal cord, mostly the central somite, the neural crest, and ventralhindgut. X57. (E) Vl.1: In the tail, labeled cells in the epidermis and ventralsomite occurred bilaterally in some embryos. The only labeled cells in the spinal cord were Rohon-Beard neurons (arrow). X79. (F) Irl.~: Many cells in the lateral plate mesoderm, epidermis, and dorsal somite were labeled. A few cells were labeled in the hindgut, neural crest, and dorsal spinal cord. Arrow points to a labeled Rohon-Beard neuron. X59. (G) VZ.1:Many labeled cells were in the dorsal and ventralsomite, neural crest, lateral plate mesoderm, and hindgut. X52. (H) V2.2:Labeled myotubes were mostly in dorsal and ventralsomite at this level, but more caudally they were mostly in the central somite. Labeled neural crest were between the epidermis and somite. Arrow points to a labeled dorsal commissural neuron in the spinal cord. Large regions of the hindgut, including archenteron roof, were labeled. X52. Abbreviations as in Fig. 2.
