Development of Nerve Cells in Hydrozoan Planulae: I. Differentiation of Ganglionic Cells

¹ Department of Biological Sciences, University of Notre Dame, Notre Dame, Indiana 46556

The cytomorphosis of ganglionic cells in hydrozoan planulae of Halocordyle disticha is described at the fine-structural level. Ganglionic cells arise from undifferentiated interstitial cells (I-cells). I-cells originate at 8 hours postfertilization as a central core of cells in the endoderm. Such I-cells possess a centrally located nucleus with one to several tiny nucleoli, a few segments of rough endoplasmic reticulum, a few mitochondria, electron-dense granules, and free ribosomes. Interstitial cells destined to form ganglionic cells migrate as single cells through the mesoglea to the base of the ectoderm where they can divide and subsequently differentiate. Early stages of neural differentiation are characterized by the enlargement of the I-cell nucleoli, the loss of electrondense granules, and the appearance of a Golgi complex, numerous mitochondria, and microtubules. Next, neurites grow out from both sides of the ganglionic cell body and join neurites from adjacent ganglionic cells to form an extensive neural plexus just apical to the mesoglea in the planula. Neurites are rich in mitochondria and microtubules and extend in both a longitudinal and transverse direction with respect to the planular anterior-posterior axis. Some neurites extend down from the neural plexus and abut the mesoglea. The last phase of ganglionic cell development occurs when electron-dense droplets appear within the region of the Golgi and eventually in the neurites. Neurites contain both electron-dense droplets and dense cored vesicles located in clusters at specific intervals along their length. Such droplets and vesicles are often found in close association with the mesoglea in neurites which contact the mesoglea. Two morphological types of ganglionic cells are identifiable at the fine-structural level: a light ganglionic cell and a dark ganglionic cell. Light ganglionic cells possess an electronlucent cytoplasm and fewer mitochondria than dark ganglionic cells. Dark ganglionic cells have an electrondense cytoplasm and their cell bodies are more oblong than the rounder cell bodies of light ganglionic cells. Light ganglionic cells comprise the majority of the planular ganglionic population.

Neural differentiation begins in the planular ectoderm at 24 hours postfertilization and continues throughout larval development. The neural plexus is present at 24 hours and increases in size (i.e., number of neurites) and complexity as animals mature. Ganglionic cells are distributed along the entire length of the planula and steadily increase in number as planulae age. All stages of ganglionic cell differentiation are found throughout planular development (24-96 hours postfertilization). Ganglionic cells of the planula show combined morphological features of interneurons and neurosecretory cells, suggesting they are multifunctional neurons.