jak2a expression in the developing zebrafish. The expression of jak2a in embryos at various developmental stages was examined by whole mount in situ hybridization. (a) 10 hpf embryo, dorsal view with anterior to the top. jak2a riboprobe gives a widespread, low-level signal. Elevated expression is apparent in the dorsal axis, but this corresponds to the thickest region of the embryo and does not reflect an increase in the density of transcript. (b) 14 hpf embryo, lateral view with anterior to the top and left. Arrowheads indicate the jak2a-riboprobe labeled line of cells in the medial lateral plate mesoderm that gives rise to primitive blood lineages. (c) 14 hpf dorsal view of dissection of dorsal and posterior axial structures. Anterior (A) is to the left and posterior (P) is to the right. jak2a transcript is evident in a narrow ribbon of cells at the medial edge of the lateral plate mesoderm (arrow) that extend to a distinct anterior limit (arrowhead). (d) 20 hpf embryos, lateral view. jak2a riboprobe signal is detected at high level in the medially converging cells of the ICM (arrowheads) and in the eye. A lower level of signal is seen in the remainder of the anterior CNS, but not in the trunk or posterior body. (e) 24 hpf embryo, lateral view showing the labeling of the mature ICM (bracket) and in the eye and anterior CNS at a lower level by jak2a probe. (f) 24 hpf embryos, lateral view showing a comparison of jak2awith scl staining. Both jak2a and scl probes label the anterior ICM (see [e] for reference); however, sclis also detected in a dorso-anterior pair of bilateral stripes (open arrowhead) and in the posterior ICM (solid arrowhead), whereasjak2a is not. Expression of scl is also seen in cells of the CNS. (g) 36 hpf embryo, lateral view showing detection ofjak2a message in circulating primitive erythrocytes (ce), cells can be detected in the axial vessels of the trunk and tail, on the yolk sac, and in the heart (open arrowhead). Elevated signal is also seen in the eyes and in the midbrain (solid arrowhead). (h) 2.5 dpf embryo, lateral view showing detection of jak2a transcript in the finbud (open arrowhead), the midbrain (solid arrowhead), and the eye. Note that, in contrast to (g), there is no jak2a signal from circulating blood. (i) 3.5 dpf larva, lateral view showingjak2a message restricted to the eye and elements of the pharyngeal arches (arrowhead). (j) 8 dpf larva, lateral view.jak2a transcript can be detected in the pronephros (arrow) and in circulating cells lodged in the vasculature of the tail (bracket). A, anterior; aICM, anterior intermediate cell mass; ce, circulating erythroblasts; cv, caudal vascular plexus; e, eye; fb, fin bud; ICM, intermediate cell mass; lpm, lateral plate mesoderm; P, posterior; p, pronephros; pICM, posterior intermediate cell mass.