Embryonic expression of alkaline phosphatase activity in the oligochaete annelid Tubifex

Temporal and spatial expression of embryonic alkaline phosphatase (ALP) activity in the oligochaete Tubifex hattai was examined by means of histochemical staining. During the course of development up to 7 days (at 22 degrees C), ALP activity emerged in three distinct structures in the following order: dorsal surface linear structures running along the midline, nephridium-like structures located in the mesodermal territory of segments 7 and 8, and ventral vessels located in the space between ventral ganglia and the endoderm. Stage-by-stage observation showed that the dorsal linear structures originate as a bilateral pair of small cell clusters that appear at the anterior end of the early gastrula embryo. Likewise, each nephridium-like structure was traced back to a single ALP-positive cell that emerges at the ventrolateral region in each of segments 7 and 8 at the late gastrula stage. Ventral vessels became ALP-positive, first in segments 5-13 and then in more posterior segments. No ALP activity was detected in the endoderm during embryogenesis.