Interaction of extraembryonic microglia and neonatal brain development

Microglia are resident immune cells in the central nervous system (CNS), which, in a healthy state, promote CNS homeostasis and respond to CNS injury. In contrast, microglia are also implicated in pathological conditions where they may contribute to neural damage. Primitive microglia arise from extraembryonic progenitors in the yolk sac (YS). The extraembryonic origins of primitive microglia are distinct from other tissue macrophages. The YS is the first site of hematopoiesis in development. Uniquely, microglial pregenital cells in the mouse derive from an early myeloid branch of the hematopoietic lineage in the YS. Microglia are critical in several key stages of physiological brain development, including embryonic vasculogenesis, immunosurveillance, and neurogenesis. Abnormal microglial function has been linked to neurodevelopmental and neurodegenerative diseases, although mechanistic roles in disease etiology remain incompletely understood. Knowledge of species-specific differences between human, murine and other animal models is also critical to understanding translational relevance to human health and disease as biomedical understanding of the importance of primitive microglia advances. This significance drives the importance of understanding, comparatively, the extraembryonic origins and developmental mechanisms whereby human primitive microglia differentiate and migrate to inform translational research. A better understanding of the molecular drivers may lead to biomarkers and/or preventative or therapeutic measures for neonatal brain development and neurodegenerative diseases. Herein, the role of microglia in neonatal brain development is discussed, current understandings of the developmental origins of microglia are described, the ontogeny and phylogeny of microglia, and implications of in vitro microglialike cell differentiation, with a specific interest on neurodegenerative diseases, are reviewed. Together, these emphasize the importance of leveraging the extraembryonic origins of microglia to not only better understand neurodevelopment and neurodegenerative diseases, but also to develop protective measures that are specific to human microglia.