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Extra-axonal environment and fibre directionality in the early development of the chick embryo optic chiasm: a light
Insights
Early chick embryo optic chiasm development involves extracellular spaces and a temporary degenerative center. Primitive glial cells guide optic fiber reorientation at the midline.
Area of Science:
- Neuroscience
- Developmental Biology
- Ophthalmology
Background:
- The optic chiasm is a crucial brain structure where optic nerve fibers partially cross.
- Understanding its early development is key to comprehending visual system formation.
Purpose of the Study:
- To investigate the cellular and structural events during early optic chiasm development in chick embryos.
- To elucidate the role of extracellular spaces and primitive glial cells in optic fiber organization.
Main Methods:
- Light microscopy was used to observe cellular structures.
- Scanning electron microscopy provided high-resolution surface imaging of developing tissues.
Main Results:
- Extracellular spaces, defined by foot-shaped cell prolongations, are present before and during optic fiber arrival.
- A transient prechiasmic degenerative center causes disorganization, coinciding with optic fiber fascicle arrival.
- Primitive glial cells emerge and proliferate from stage 26, mediating optic fiber reorientation from lateromedial to rostrocaudal direction.
Conclusions:
- Early optic chiasm formation involves dynamic changes in extracellular matrix and cell populations.
- Primitive glial cells play a critical role in guiding optic nerve pathway formation.
- The observed degenerative center may be involved in tissue remodeling during chiasm development.
Abstract:
The events that occur during the early development of the optic chiasm of the chick embryo have been studied by light and scanning electron microscopy. In developmental stages previous to the arrival of the first optic fibres in the floor of the diencephalon, as well as during the arrival of the leading fibres, extracellular spaces can be seen in the diencephalon ventral wall. These spaces are defined by external cell prolongations which end in a foot-shaped formation. During stages 25 and 26 a prechiasmic degenerative centre appears in the area immediately rostral to the early chiasm, leading to a notable degree of disorganization in the diencephalon wall. This centre appears to be related to the reorganization of the system of external cell processes and extracellular spaces which become progressively more irregularly distributed, coinciding with the arrival of the first optic fibre fascicles to the midline of the floor of the diencephalon. The optic fibre fascicles change their latero-medial directionality in the medial-most regions of the ventral diencephalon, where their course becomes rostrocaudal. This reorientation of the optic fibres seems to be mediated by primitive glial cells which first appear in the ventrorostral region of the early chiasm (previously occupied by the system of external cell processes and extracellular spaces) in stage 26, increasing in number from this stage on. The morphology of the primitive glial cells is laminar in nature and the cells are seen to be densely packed together with no large extracellular spaces between them.