Disruption of the pial basal lamina during early avian embryonic development inhibits histogenesis and axonal pathfinding in the optic tectum

Abstract

Bacterial collagenase was injected into the ventricular cavity of the optic tectum of chick and quail embryos. Histological examination up to 6 days after enzyme injection revealed that the collagenase disrupted the pial basal lamina, which was evident by the fragmented distribution of basal lamina proteins at the pial surface of the midbrain and the brainstem. Although the disrupted basal lamina was not reestablished at later stages of development, the pial basal lamina of the newly developing neuroepithelium in the caudal part of the tectum was continuous and intact. Western blot analysis showed that the collagenase digested collagens but spared noncollagenous proteins. The disruption of the pial basal lamina caused the neuroepithelial cells to retract their pial end feet and caused tectal axons to exit the brain tissue into the adjacent mesenchyme. The vertical migration of neuroblasts to the pial layers of the tectum was inhibited, leading to a disruption of the tectal histogenesis. In the developing optic pathways, retinal axons were misguided at the optic chiasma and terminated in the head mesenchyme instead of the tectum. None of the abnormalities in histogenesis and axonal pathways were observed when the basal lamina was disrupted at a later stage of embryonic development. The present experiments demonstrate that the pial basal lamina has an important function during brain morphogenesis in restricting axons to the brain, providing an anchoring of the neuroepithelial cells to the pial surface, and allowing the formation of a defined cytoarchitecture of the brain.

Basement Membrane–Dependent Survival of Retinal Ganglion Cells

“…In addition, the BMs prevent growing axons and migrating neurons from aberrantly exiting into the adjacent connective tissues. [12][13][14] The abundance of cortical and retinal dysplasias in mice with mutations in BM proteins 9,14 and their receptors [15][16][17] confirms the importance of BMs in containing axons, neurons, and glia cells within the confines of the CNS.…”

Biomechanical properties of native basement membranes

“…The fact that blood vessel breaks and CNS ectopias occur in mice with mutations of different BM proteins shows that these defects are linked to a general, mechanical property of BMs rather than the lack of a specific component. Furthermore, ectopias and blood vessel ruptures were also observed in chick embryos in which the ILM and the pial BM were enzymatically disrupted [40–42], confirming that this phenotype is not connected to the lack of a specific protein, but rather the weakness of the ILM as a structure. We propose that BMs have a critical role in stabilizing the cortical and retinal tissue borders as well as blood vessels in the CNS and the eye.…”

Assembly of Lamina-Specific Neuronal Connections by Slit Bound to Type IV Collagen

“…Radial glia are oriented perpendicular to the laminae and span the entire depth of the tectum. These cells extend processes with a `bottlebrush' morphology to the neuropil surface, where they are anchored to the basement membrane through their endfeet (Halfter and Schurer, 1998; Halfter et al, 2002). Radial glia have been implicated in supporting the layered architecture of the brain by virtue of their morphology and their intimate contacts with migrating cells (Rakic, 2003), but their exact signaling function remains unclear.…”