Neural Crest and Placodes

During embryological development, the neural tube forms by a process of invagination of the ectoderm along the dorsal midline of the body and the head. At the edges of the invaginating neural tube tissue, a population of neural crest cells arises from the dorsomedial wall of the neural folds and migrates laterally and ventrally to contribute to many different structures of the adult, including the dorsal root ganglion cells (bipolar neurons) for all the spinal cord sensory nerves. In the head region, neural crest cells likewise migrate away from the region of the neural tube and give rise to multiple structures. A second tissue present only in the head region is a set of neurogenic placodes, which are thickened patches of ectodermal cells. The bipolar sensory neurons for all the cranial nerves except II and the epiphyseal are derived from the neurogenic placodes and/or neural crest tissues.

In the region of the nose, an olfactory placode forms that gives rise to the bipolar neurons of the olfactory and terminal nerves, and additional sets of placodes form along the sides of the head. Neural crest and/or placodally derived cells give rise to the bipolar neurons for the sensory ganglia of the trigeminal (V), facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX), and vagus (X) nerves. A trigeminal placode contributes neurons to the trigeminal ganglion. The sensory neurons within the vestibular and spiral ganglia for the vestibulocochlear nerve arise from a dorsolateral, otic placode; this is a much simpler situation than in fishes, in which a series of dorsolateral placodes gives rise to the vestibulocochlear nerve and to the set of mechanoreceptive and electroreceptive lateral line nerves. The receptor hair cells for the vestibulocochlear system likewise are derived from this dorsolateral placode.

The gustatory system is more variable across vertebrates in terms of both its peripheral and its central components. Mammals have a relatively limited gustatory system. A set of ventrolateral, or epibranchial, placodes contributes sensory neurons to ganglia of cranial nerves VII, IX, and X, but, unlike the situation in the vestibulocochlear system, the taste receptor cells of the taste buds are all derived locally from the pharyngeal endoderm in the oral cavity. This system thus is classified here as visceral. Fishes, in comparison, have much more elaborate taste systems than mammals, and in some groups, such as catfishes, the taste receptor cells are distributed over the entire body surface. The gustatory afferent fibers project into elaborate, laminated lobes in the brain stem and preserve a topographic map of the inputs.

Breaking Bulimia

Breaking Bulimia

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