A quantitative analysis of the development of the pyramidal tract in the cervical spinal cord in the rat.

A quantitative electron microscopic analysis was undertaken of the development of the pyramidal tract, at the level of the third cervical spinal segment, in rats ranging in age from the day of birth to three months old. The axon number was calculated as the product of axon density, determined in a systematic random sample of electron micrographs, and tract area. During the first postnatal week the tract contains thin unmyelinated axons and growth cones. Growth cones are abundant in neonatal rats, but can still be observed occasionally at the end of the first postnatal week, indicating a continuous addition of pyramidal tract axons during the first postnatal week. Myelination starts around P10. By the end of the first postnatal month approximately 50% of the axons have already been myelinated. Myelination proceeds during further maturation, but in the three month old rat 28% of the axons are still unmyelinated. The total number of axons increases rapidly after birth up to 153,000 at the fourth postnatal day. Subsequently, the number of axons is reduced by nearly 50% to 79,000 in the adult rat. The axon loss is most prominent during the second postnatal week, when 32,000 axons are eliminated, but continues for several weeks at a slower rate.

On the development of the pyramidal tract in the rat. I. The morphology of the growth zone.

An electron microscopic study has been made of the tip of the growing pyramidal tract in the rat. This part of the developing bundle, designated as the growth-zone, has been examined at the levels of the medulla oblongata and the third spinal segment at embryonic day 20 and on the day of birth, respectively. The tip of the pyramidal tract contains, apart from axons, numerous larger profiles. An analysis of serial sections revealed that these represent either growth cones or preterminal periodic varicosities. In the growth cones of the corticospinal axons three zones can be distinguished: a proximal "tubular", an intermediate "vesicular-reticular" and a distal "fine-granular" zone. As distinct from the classical descriptions the corticospinal growth cones end in a single or, less frequently, in two more or less parallel filopodia. None of the growth cones analyzed in this study showed multiple filopodia radiating from the terminal expansion as observed at the end of growing axons in tissue cultures and in developing spinal fibre tracts of nonmammalian vertebrates. As regards the varicosities, most of these structures are characterized by a light cytoplasmic density. Others, however, contain a denser cytoplasm, closely resembling that of the vesiculo-reticular part of growth cones.