Organization of the cerebral ganglion of the colonial ascidian Polyandrocarpa misakiensis.

The cerebral ganglion of a budding styelid ascidian, Polyandrocarpa misakiensis, whose phylogenetic location is midway between vertebrates and invertebrates, was studied by light and electron microscopy to obtain some insight into the evolution of the central nervous system. The lateral and ventral sides of the ganglion are surrounded by blood sinuses. The ganglion is covered with a thin fibrous sheath through which many nerve fibers run. The ganglion is composed of a cellular cortex and a fibrous medulla. The cortex consists of three to six layers of large and small neurons. Some neurons are also scattered within the medulla. Many neurons are monopolar, and some are bi- or multipolar. The cytoplasm of the large neurons is dense with extensive rough endoplasmic reticulum, free ribosomes, mitochondria, one or more Golgi complexes, large dense bodies, and many clear or dense vesicular structures. Some neurons send their processes directly into the lumen of the sinuses. The medulla is composed of loosely arranged nerve fibers without cellular wrappings. The medullary fibers contain vesicles and granules of various sizes, and microtubules. At the anterior and posterior ends of the ganglion, the medullary fibers are assembled into thick peripheral nerve fiber bundles. The peripheral nerve fibers are enveloped and subdivided by fibrous structures. Synapses are found in the medulla, in the cortex, and between the peripheral nerve fibers. The presence of neurons and axodendritic or axoaxonic synapses in the peripheral nerve fibers is consistent with a diffuse organization of the central nervous system of the ascidians. The morphology of the central nervous system synapses is comparable to that of other invertebrates, but the locations of the synapses are similar to those of vertebrates.