The chicken trigeminal ganglion II. Fine structure of the neurons during development,

Abstract

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...

Altered neuronal lineages in the facial ganglia of Hoxa2 mutant mice

“…Placode-derived neuroblasts cease dividing, exhibit cytological properties of neurons and established peripheral projections before the end of the first week of development. These cells are situated in the more distal aspects of the ganglion (Gaik and Farbman, 1973;Noden 1980). In contrast, crest-derived neurons in the trigeminal ganglion do not stop dividing until the seventh day of embryonic development.…”

Making a head: Neural crest and ectodermal placodes in cranial sensory development

“…The trigeminal ganglion is the largest sensory ganglia, mediating much of what is sensed throughout the head and face, and is composed of three branches: the ophthalmic, the maxillary, and the mandibular [52,75,94]. The sensory nerve endings of this ganglion are either mechanoreceptors (involved in sensing touch and/or pressure), thermoreceptors (sensing changes in temperature), and nociceptors (sensing stimuli involved in pain) [61,63,123,140].…”