An anatomical analysis of the chicken trigeminal ganglion was made using light microscopy on specimens prepared by usual chemical fixation or freeze-drying methods and by electron microscopy. Two types of neurons were consistently seen, dark and light cells. Dark cells contained a dense cytoplasm with Nissl substance distributed evenly throughout, whereas light cells had a less dense cytoplasm containing clumps of Nissl substance. The Nissl bodies in light cells contained only a few small cisternae of granular endoplasmic reticulum as compared with many stacked cisternae in Nissl bodies of dark cells. The ratio of dark to light cells was approximately 62:38 in all regions of the ganglion. Dark cells were consistently smaller than light cells. In the seven-day old chick, the mean diameters of the dark and light neurons were

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The localization of neuronal cell bodies in the trigeminal ganglion for the pulpal nerves of incisors and first and second molars in the rat was determined by utilization of retrograde axonal transport of horseradish peroxidase. Transmedian labeling was found with all teeth, but labeling was greater in the ipsilateral ganglion.

A morphological study of the developing chick trigeminal ganglion was done. For descriptive convenience, four stages of development have been defined: Phase I (2 to 4 days of incubation) -Both the neural crest and epidermal placode contribute to the forming trigeminal ganglion, which is comprised of clusters of undifferentiated cells and primitive neuroblasts. Toward the end of this phase, satellite cells and unmyelinated nerve fibers are seen.Phase I1 (5 to 10 days of incubation) -In the lateral portion of the ganglion, large differentiating neuroblasts and young bipolar and pseudounipolar neurons were present. The latter were characterized by a large, eccentrically located nucleus at one pole, a long axonal process at the opposite pole, and a clear centrosphere region between the two. In the medial region of the ganglion, most of the neurons were smaller and more closely packed together. Phase I11 (11 to 17 days of incubation)-The ganglion was constituted of bipolar and pseudounipolar cells with an apparently random distribution of small and large neurons. Nissl bodies appeared in developing neurons. Phase IV (18 days of incubation to 3 weeks post-hatching) -Light and dark neurons first made their appearance. Dark neurons were characterized by (1) clear perinuclear cytoplasm containing a Golgi apparatus, vesicles and mitochondria and (2) a peripheral cytoplasm of densely packed stacks of Nissl substance. Light neurons contained individual clumps of Nissl substance distributed in a pale cytoplasmic matrix.In a series of elegant experiments, Hamburger ('61) demonstrated that the trigeminal ganglion of the chick has a dual origin from cranial neural crest and from placodal head ectoderm. He showed that in the 6-to 11-day chick embryo the ganglion is composed of two cell populations, a mediodorsal group of small cells with poor affinity for silver and a ventrolateral group of large cells with a strong argyrophilia. In order to demonstrate the separate origin of the two cell types, Hamburger performed ablation experiments in two groups of younger embryos, aged 40-48 hours. In one group, he extirpated the rostral medulla, thus effectively removing most of the cranial neural crest. In the other group, he removed a strip of head ectoderm anterior to the otocyst, thus effectively removing the presumptive placodal contribution to the ganglia. Examination of the embryos six to seven days after operation revealed that the ganglia forming in the first group (rostral medulla removed) contained almost exclusively J. MORPH., 141: 57-76, large neurons, with a strong affinity for silver, and cytologically identical to the large neurons in normal ganglia from 6-to 11-day embryos. Conversely, in the experimental group from which strips of placodal ectoderm had been removed, Hamburger found the majority of the cells were small neurons with poor affinity for silver and only a few large cells were present. He suggested the few large cells present had originated from placodal ectoderm, which had regenerated after surgery. Rec...

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