Fine structure of dendritic and axonal growth cones in embryonic chick spinal cord

Abstract

Our preliminary electron microscopic observations of embryonic chick spinal cord indicate that the majority of synapses in the cervical cord at the fourth and sixth days of incubation are found in the marginal layer rather than the mantle layer. These findings raise two basic questions about the type of connections in the marginal zone during the first week of development: are all these early synapses axondendritic, and are synaptic junctions present on axonal and dendritic growth cones? Since criteria for distinguishing between axonal and dendritic growth cones are scarce or nonexistent, serial section electron microscopy was employed to definitively identify the two types of terminals. Two types of dendritic growth cones are present in the marginal layer during the first week of development. The first type is distinguished by a dendritic stalk which expands into a club‐shaped or bulb‐like terminal. A characteristic feature of this type of dendritic ending is the presence of numerous vesicles which vary in diameter from about 1000 Å to 2500 Å. Its cytoplasm has a floccular appearance and is frequently electron lucent. In contrast, the second type of dendritic terminal is electron dense, has a consistently homogeneous appearance, and usually lacks the large vesicles of the first type. The shape of the second type is a finger‐like process (filopodium) roughly of the same diameter as the main dendritic trunk. Evidence is presented that some dendritic growth cones observed in th…