Neurons in layer I of the developing occipital cortex of the rat

This paper describes the neurons in layer I of the rat occipital cortex, and traces postnatal changes in the numbers and morphology of the different cell types therein. Golgi-Cox and Nissl material from adults and from 0.5,2,4,6,8,10,11,12,13,14,15,16,18,20,24,28 and 35 day-old rats, was utilized. Horizontal, vertical and classical non-pyramidal cells without axons were recognized. Horizontal cells included foetal horizontal cells (Retzius-Cajal cells) and persisting horizontal cells. The former were fully differentiated and numerous at birth and were bipolar, with dendrite and axon extending from opposite poles, and fine ascending (vertical) branches. Some such cells possessed two axons. Almost all degenerated and disappeared over the first two to three postnatal weeks. An analogy with Rohon-Beard cells is drawn and it is suggested that these cells receive an early input which is subsequently withdrawn and/or concentrated on cells in deeper layers. There is no evidence of transformation of foetal to persisting horizontal cells, which are not numerous and most commonly give rise to two dendrites from opposite perikaryal poles. Vertical cells, with spinous dendrites and descending axons, differentiate and reach adult numbers by the end of the first postnatal week. Classical non-pyramidal cells include spinous and spine-free varieties, resemble those in deeper layers, and are mature by the end of the third postnatal week. In the upper stratum of layer I are small numbers of cells without axons.