The transient corticospinal projection from the occipital cortex during the postnatal development of the rat

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Abstract
The transient occipital cortical component of the pyramidal tract which we previously had identified during the postnatal development of the rat (Stanfield et al., '82) has been studied with anterograde as well as retrograde techniques. A continuous band of retrogradely labeled layer V neurons which spans the entire cortex including the occipital cortex is seen following injections of the fluorescent marker Fast Blue into the pyramidal decussation during the first postnatal week. No labeled cells are found in the occipital cortex following similar injections made on postnatal day 20 (P20), although such injections label many neurons in the more rostral cortical fields. However, if the Fast Blue injection is made on P2 and the animal is allowed to survive until P25 a large number of Fast Blue‐labeled layer V neurons is found in the occipital cortex, even though an acute, second injection of the retrograde tracer Nuclear Yellow made into the pyramidal decussation shortly before the animal is killed results in no occipital cortical labeling. When Fast Blue injections confined to the mid‐ or upper‐cervical spinal cord are made on P4 and the animals are killed on P9, again many retrogradely labeled neurons are found in the occipital cortex. Further, when injections of 3H‐proline or wheat germ agglutinin conjugated to horseradish peroxidase (WGA‐HRP) confined to the occipital cortex are made during the first 2 postnatal weeks, anterogradely transported label is seen within the pyramidal tract. At all stages examined the amount of such label and its caudal extent are less than that seen following similar injections into the parietal or frontal cortex. The greatest extent of the labeled occipital cortical fibers is reached at about the end of the first postnatal week and the number of these fibers seems to peak at about this same time. At this stage many of these labeled axons extend for a considerable distance down the spinal cord with some reaching as far caudal as lower lumbar levels, and at this stage some of these labeled occipital corticospinal fibers enter into the spinal gray. Over the next week the number of occipital cortical fibers in the pyramidal tract rapidly decreases and by P17 occipital cortical injections of 3H‐proline or WGA‐HRP result in virtually no transported label caudal to the pons. We conclude that during the early postnatal period many layer V neurons in the occipital cortex extend corticospinal axons well down the spinal cord and some of these axons enter the spinal gray, but that all of these occipital corticospinal fibers are subsequently eliminated even though many of their neurons of origin remain intact. Thus, the elimination of long axonal collaterals appears to be an important feature of the development of cortical projection systems and plays a major role in the determination of the tangential distribution of cortical projection neurons.