An electron-microscopical study of the developing transitional region in feline S1 dorsal rootlets
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The peripheral nervous system (PNS)-central nervous system (CNS) transitional region in cat dorsal roots undergoes significant remodeling before adopting adult organization by the 5th-6th postnatal week, with myelination beginning around day 60.
Area of Science:
- Neuroscience
- Developmental Biology
- Cell Biology
Background:
- The peripheral nervous system (PNS) and central nervous system (CNS) are distinct but connected, with a critical transitional region in dorsal roots.
- Understanding the development and organization of this transitional zone is crucial for comprehending nerve development and potential regeneration.
Purpose of the Study:
- To investigate the electron-microscopic organization of the PNS-CNS transitional region in cat S1 dorsal roots during fetal and early postnatal development.
- To identify the developmental timeline for the maturation of this transitional region and compare it to adult ultrastructure.
Main Methods:
- Electron microscopy was used to examine the ultrastructure of the PNS-CNS transitional region in cat fetuses and kittens.
- Developmental stages studied ranged from 25-47 days gestation to the 5th-6th postnatal week.
Main Results:
- The adult organization pattern was established by the 5th-6th postnatal week.
- Before this, the transitional region showed extensive remodeling, with unmyelinated axons and Schwann cell clusters.
- Myelination of axons in the transitional region commenced around the 60th day of gestation, characterized by high cellularity and the emergence of CNS and glial components.
Conclusions:
- The PNS-CNS transitional region in dorsal roots undergoes a dynamic period of reorganization during development.
- The formation of the CNS compartment and the development of short internodes and glial fringes highlight a complex maturation process.
- Further quantitative studies are needed to fully elucidate the reorganization of the proximal root.
Abstract:
The organization of the PNS-CNS transitional region in S1 dorsal roots was studied electron-microscopically in cat foetuses and in kittens. The adult organization pattern was recognized first during the 5th-6th postnatal week. Before this date the transitional region underwent a period of conspicuous remodelling. In 25- to 47-days-old foetuses the transitional region was characterized by large clusters of Schwann cells clinging to bundles of unmyelinated axons. This part of the root then remained unmyelinated after the more distal PNS part and the more proximal CNS part had acquired myelin. Axons of the transitional region started to myelinate first around the 60th day after conception. At this stage the transitional region was characterized by its cellularity: Schwann cells, glioblasts and fibroblasts were abundant. The CNS compartment started to grow out into the root during the 1st postnatal week. Concomitant with the distal expansion of the CNS compartment - calculated to be about 5 micrometer/day during the 1st month - there appeared in the adjacent PNS compartment numerous extraordinarily short internodes carrying myelin sheaths. A glial fringe began to develop and encapsulate PNS-borderline paranodes. The observations are discussed with respect to the adult ultrastructure. It is suggested that there is a conspicuous reorganization of the proximal part of the root. The need for supplementary quantitative data is emphasized.