Origin and morphogenesis of neurons in the frog cardiac ganglion

This paper reviews the cellular events underlying the formation of the cardiac ganglion in the frog. The first neurons become postmitotic at the end of embryogenesis and begin differentiating in a functioning heart. Neuronal precursors in the heart continue dividing and differenting at least through the beginning of metamorphosis. Since cell death is not a characteristic of early cardiac ganglion development, proliferation and differentiation alone regulate the addition of neurons to the heart. As new neurons are added, their position is unique and reflects their time of origin. The developmental addition of new neurons to the ganglion is neither time nor stage dependent, but matches the size of the growing heart, even into adult life. During this period, the shape and polarity of cardiac neurons is established through elimination of supernumerary axons that initially extend throughout the heart. The temporal and spatial accumulation of cardiac SIF cells occurs in parallel with cardiac neurons and may indicate a common mechanism underlying the origin of these neural crest derivatives. These studies contribute to our understanding of the cellular processes underlying the development of this important autonomic control mechanism and provide the background for current studies on the regulation of cardiac neurogenesis.