Studies of the distribution and functional roles of transitory amoeboid microglial cells in developing rat brain using exogenous horseradish peroxidase as a marker
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Abstract:
When HRP was injected intraperitoneally (i. p.), labelled amoeboid microglial cells (AMC) were consistently localized in the subcortical white matter and circumventricular zones in early postnatal (1 and 7 days old) but absent in late postnatal (14-day-old) rats. The ingested HRP disappeared from the labelled cells 5 days after IP injection. Subcutaneous injection of HRP had also resulted in the labelling of amoeboid microglial cells in the corpus callosum of early postnatal rats. When the injected HRP was followed ultrastructurally over a time course sequence in intravenously (i. v.) injected rats, it was first detected in the invaginations on the luminal side of endothelium and in the endothelial cytoplasm 30 min after injection. HRP was present both in the endothelium and amoeboid microglial cells 3 hours later. With time, the tracer was progressively accumulated in the cytoplasm of AMC and it was sequestered in the vacuoles and lysosomes. It is concluded from this study that when injected i. p., i. v. or subcutaneously in the early postnatal rats, HRP is circulated to the blood vessels in the subcortical white matter and circumventricular zones where it gains access into the nervous tissues by transendothelial transport. The extravasated tracer is then phagocytosed by the residential AMC and subsequently degraded. The absence of labelling of AMC is attributed to the maturation of BBB which impedes the entry of exogenous tracer by the 14th postnatal day.
Insights
Horseradish peroxidase (HRP) injected into early postnatal rats labels amoeboid microglial cells (AMC) in specific brain regions. This labeling is absent in older rats due to blood-brain barrier maturation.
Area of Science:
- Neuroscience
- Developmental Biology
- Immunology
Background:
- Microglia play crucial roles in brain development and immune surveillance.
- The blood-brain barrier (BBB) regulates the passage of substances into the central nervous system.
- Early postnatal development involves significant changes in microglial function and BBB permeability.
Purpose of the Study:
- To investigate the uptake and fate of systemically administered horseradish peroxidase (HRP) by amoeboid microglial cells (AMC) in the developing rat brain.
- To determine the developmental stage at which BBB maturation affects HRP uptake by microglia.
- To elucidate the mechanism of HRP entry into the brain and its subsequent handling by AMC.
Main Methods:
- Intraperitoneal, subcutaneous, and intravenous injections of HRP in rats at different postnatal ages (1, 7, and 14 days).
- Ultrastructural analysis using electron microscopy to track HRP distribution and cellular localization.
- Time-course studies to observe the dynamic process of HRP uptake and degradation by AMC.
Main Results:
- HRP was localized in amoeboid microglial cells (AMC) in the subcortical white matter and circumventricular zones of early postnatal rats (1 and 7 days old).
- HRP uptake by AMC was absent in 14-day-old rats, indicating a developmental change.
- Ultrastructural analysis revealed HRP entry via transendothelial transport and subsequent phagocytosis and sequestration within vacuoles and lysosomes of AMC.
Conclusions:
- Systemically injected HRP gains access to the developing brain via transendothelial transport across blood vessels in specific regions of early postnatal rats.
- Extravasated HRP is phagocytosed and degraded by resident amoeboid microglial cells (AMC).
- Maturation of the blood-brain barrier (BBB) by postnatal day 14 prevents exogenous tracer entry, explaining the absence of AMC labeling in older rats.