Contribution of maternal cytoplasmic factors and cellular interaction to determination of archenteron in a starfish embryo was analyzed by (1) examining temporal and positional pattern of expression of an endoderm-specific enzyme, alkaline phosphatase, (2) deleting the vegetal polar fragment from an immature oocyte and (3) changing the orientation of a blastomere within an early stage embryo. The archenteron (and the differentiated digestive tract) of Asterina pectinifera was divided into three areas based on the time of start of alkaline phosphatase expression. At 27 hours after 1-methyladenine treatment, the whole archenteron except the anterior end started to express alkaline phosphatase. The anterior negative area differentiated into mesodermal tissues such as mesenchyme cells and anterior coelomic pouches (anterior mesodermal area). The alkaline-phosphatase-positive area 1 gave rise to the esophagus and the anterior end of the stomach. Alkaline-phosphatase-positive area 2, which was gradually added to the posterior end of the archenteron after 30 hours, became alkaline-phosphatase- positive and formed the middle-to-posterior part of the stomach and the intestine. When the vegetal oocyte fragment, the volume of which was more than 8% of that of the whole oocyte, was removed from the immature oocyte, archenteron formation was strongly suppressed. However, when the volume deleted was less than 6%, most of the larvae started archenteron formation before the intact controls reached the mesenchyme-migration stage (30 hours). Although cells in the alkaline-phosphatase-positive area 2 are added to the posterior end of the archenteron after 30 hours in normal development (R. Kuraishi and K. Osanai (1992) Biol. Bull. Mar. Biol. Lab., Woods Hole 183, 258–268), few larvae started gastrulation after 30 hours. Estimation of the movement of the oocyte cortex during the early development suggested that the area that inherits the cortex of the 7% area coincides with the combined area of anterior mesodermal area and alkaline-phosphatase-positive area 1. When one of the blastomeres was rotated 180° around the axis of apicobasal polarity at the 2-cell stage to make its vegetal pole face the animal pole of the other blastomere, two archentera formed at the separated vegetal poles. Intracellular injection of tracers showed that cells derived from the animal blastomere, which gives rise to the ectoderm in normal development, stayed in the outer layer until 30 hours; a proportion of them then entered the archenteron gradually. The involuted animal cells expressed alkaline phosphatase and were incorporated into the middle-to-posterior part of the stomach and the intestine. These results suggest that anterior mesodermal area and alkaline-phosphatase-positive area 1 are determined by cytoplasmic factor(s) that had already been localized in their presumptive areas. In contrast, alkaline-phosphatase-positive area 2 becomes the endoderm by homoiogenetic induction from the neighboring area on the vegetal side, namely alkaline-phosphatase-positive area 1.

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