Early Unguided Human Brain Organoid Neurovascular Niche Modeling into the Permissive Chick Embryo Chorioallantoic Membrane

Engrafting organoids into vascularized tissues in model animals, such as the immunodeficient mouse or chick embryo chorioallantoic membrane (CAM), has proven efficient for neovascularization modeling. The CAM is a richly vascularized extraembryonic membrane, which shows limited immunoreactivity, thus becoming an excellent hosting model for human origin cell transplants. This paper describes the strategy to engraft human brain organoids differentiated at multiple maturation stages into the CAM. The cellular composition of brain organoids changes with time, reflecting the milestones of human brain development. We grafted brain organoids at relevant maturation stages: neuroepithelial expansion (18 DIV), early neurogenesis (60 DIV), and early gliogenesis (180 DIV) into the CAM of embryonic day (E)7 chicken embryos. Engrafted brain organoids were harvested 5 days later and their histological features were analyzed. No histological signs of neovascularization in the grafted organoids or abnormal blood vessels adjacent to the graftings were detected. Moreover, remarkable changes were observed in the cellular composition of the grafted organoids, namely, an increase in the number of glial fibrillary acidic protein-positive-reactive astrocytes. However, the cytoarchitectural changes were dependent on the organoid maturation stage. Altogether, these results suggest that brain organoids can grow in the CAM, and they show differences in the cytoarchitecture depending on their maturation stage at grafting.

Molecular complexity of visual mapping: a challenge for regenerating therapy

Investigating the cellular and molecular mechanisms involved in the development of topographically ordered connections in the central nervous system constitutes an important issue in neurobiology because these connections are the base of the central nervous system normal function. The dominant model to study the development of topographic maps is the projection from the retinal ganglion cells to the optic tectum/colliculus. The expression pattern of Eph/ephrin system in opposing gradients both in the retina and the tectum, labels the local addresses on the target and gives specific sensitivities to growth cones according to their topographic origin in the retina. The rigid precision of normal retinotopic mapping has prompted the chemoaffinity hypothesis, positing axonal targeting to be based on fixed biochemical affinities between fibers and targets. However, several lines of evidence have shown that the mapping can adjust to experimentally modified targets with flexibility, demonstrating the robustness of the guidance process. Here we discuss the complex ways the Ephs and ephrins interact allowing to understand how the retinotectal mapping is a precise but also a flexible process.