Limulus polyphemus, the American horseshoe crab, has a single type of circulating blood cell, the granular amebocyte, which is the horseshoe crab's primary cellular defense against microbial infection. On exposure to gram-negative bacteria or their endotoxins, the amebocytes degranulate, releasing the clotting protein coagulogen and a number of proteases. The protease cascade converts the soluble coagulogen to insoluble coagulin, which forms fibrous clots that seal off the site of infection. The first description of this clotting reaction in the 1950s initiated development of Limulus amebocyte lysate and spurred an intensive study of the amebocytes. However, the site or sites and timing of amebocyte production have yet to be determined. We report here that during embryonic development in Limulus polyphemus, amebocyte production begins at stage 18. The first amebocytes detected are found in developing hemocoel cavities, and the cells may derive from previously undifferentiated yolk nuclei.

. Classical studies of horseshoe crab development have provided relatively little information about the earliest stages, and the contribution of yolk cells and yolk nuclei—a deficiency due in large part to the difficulty of preparing the eggs and embryos for sectioning. Using newly developed histological resins, we show that the yolk nuclei undergo a series of changes during embryogenesis, before cellularizing and forming the midgut epithelium during the first larval stage. The digestive diverticulum forms in a 2-step process. A mesodermally derived lamina divides the yolk mass into distinct lobes, defining the boundaries of the digestive caeca. The yolk nuclei then cellularize to form the midgut epithelium.

We have labeled microtubules in living Dictyostelium amoebae by incorporation of a GFP-alpha-tubulin fusion protein. The GFP-alpha-tubulin incorporates into microtubules and, as reported by others [Neujahr et al., 1998], the labeled microtubules are highly motile. Electron microscopy (EM) analysis of the distribution and organization of microtubules in the amoebae shows that some cytoplasmic microtubules form close associations. These associations could allow motor proteins attached to one microtubule to walk along an adjacent microtubule and thus generate some of the observed motility. Protein blot analysis indicates that the GFP-alpha-tubulin incorporates into microtubules at a lower efficiency than does the endogenous alpha-tubulin. EM and immunofluorescence (IF) analyses suggest that the GFP-alpha-tubulin interferes with microtubule nucleation. We have also observed an increased sensitivity of the GFP-alpha-tubulin expressing cells to blue light, as compared to wild-type cells. These results suggest that although GFP-alpha-tubulin can be used as a marker for microtubules in living cells, the use of this marker is not recommended for certain types of studies such as assembly dynamics.