Development of Nerve Cells in Hydrozoan Planulae: III. Some Interstitial Cells Traverse the Ganglionic Pathway in the Endoderm.

Hydrozoan planulae of Pennaria tiarella possess migratory stem cells--interstitial cells--that are capable of self renewal and can differentiate into either ganglionic nerve cells or nematocytes. The commitment and differentiation of a subpopulation of larval endodermal interstitial cells to the neural pathway were examined using light immunocytochemistry and transmission electron microscopy. Embryos of different ages, from 8 to 96 h, were tested for their ability to bind rabbit antiserum raised to the neuropeptide FMRFamide. A subpopulation of interstitial cells in the anterior endoderm of the planula begins to express a FMRFamide-like antigen between 48 and 72 h postfertilization. Concurrent with this endodermal interstitial cell expression, a subset of ectodermal ganglionic cells with FMRFamide-like immunoreactivity appears in the anterior end of the planula. Ultrastructural examination of the interstitial cell population in the anterior planular endoderm, at 48 h in development, indicates that, based upon morphology, there are at least three subsets of interstitial cells in this region: undifferentiated interstitial cells, interstitial cells traversing the nematocyte differentiation pathway, and interstitial cells traversing the neural differentiation pathway. The endodermal interstitial cells entering the neural pathway form a Golgi complex, electron-dense droplets, dense cored vesicles, and microtubules. Neurite formation does not occur in the endoderm; rather, neurites are only found in association with ectodermal ganglionic cells. Furthermore, planulae lack fully differentiated endodermal neurons. This study demonstrates that, during embryogenesis, some interstitial cells destined for neural differentiation are committed in the endoderm before their emigration to the ectoderm, begin to express cytochemical and morphological features of neural differentiation while in the endoderm, and migrate to the ectoderm as neuroblasts.