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The fine structure of the spinal cord in human embryos and early fetuses
Insights
Human spinal cord development shows significant maturation during the embryonic period, with the first synapses forming. This early synaptic development correlates with the onset of human reflex movements.
Area of Science:
- Neuroscience
- Developmental Biology
- Embryology
Background:
- The human spinal cord undergoes complex development from embryonic to fetal stages.
- Understanding early maturation is crucial for comprehending neurological development.
Purpose of the Study:
- To investigate the ultrastructural maturation of the developing human spinal cord.
- To identify the timing and location of the first synaptic connections.
Main Methods:
- Electron microscopy was used to examine human embryos (18-28 mm) and fetuses (35-45 mm).
- Analysis focused on cellular differentiation, organelle development, and synapse formation.
Main Results:
- Distinct maturation, including organelle development and glial differentiation, occurs in the late embryonic spinal cord.
- The first axodendritic synapses appear in the cervical spinal cord of a 28 mm embryo.
- Synapses, including axosomatic types, are found in the alar plate during the fetal period.
- Maturation and synapse formation proceed in a craniocaudal direction.
Conclusions:
- Synaptogenesis begins during the embryonic period, preceding the fetal period.
- The observed synaptic development aligns with the emergence of human reflex movements.
- Early spinal cord maturation is a dynamic process involving cellular differentiation and complex network formation.
Abstract:
The developing human spinal cord was studied by electron microscopy in 7 embryos (18--28 mm (C.-R.) and 5 fetuses (35--45 mm). It was found that, during the last week of the embryonic period, the spinal cord undergoes distinct maturation, as evidenced by cytoplasmic organelles, glial differentiation, and the formation of synaptic junctions. The ventricular (ependymal) cells are ciliated; densely grouped cells with scanty processes occupy the early intermediate (mantle) zone; and glioblasts are present in the marginal zone. Degenerating cells are evident in the spinal cord. The first axondendritic synapses appear in the marginal layer of the basal plate in the cervical part of the spinal cord in a human embryo of 28 mm, i.e., during the embryonic period proper. In the fetal period, synapses are found also in the alar plate, and axosomatic synapses are present. The formation of synapses and the general maturation of the spinal cord seem to proceed craniocaudally. The appearance of synapses is in keeping with the finding of reflex movements in the human during the late embryonic and early fetal periods.