# Heterogeneity of astrocyte density, morphology and connexins in the mouse hippocampus

## TL;DR

This study maps the distribution and features of astrocytes and their connexins in the mouse hippocampus, revealing spatial patterns linked to brain function.

## Contribution

The paper provides the first comprehensive anatomical mapping of astrocyte density, morphology, and connexin distribution in specific hippocampal subregions.

## Key findings

- The highest astrocyte density and connexin expression were found in the stratum lacunosum moleculare and subgranular zone.
- Astrocytic processes showed the largest radial extension in layers with high synaptic density.
- Cx30 and Cx43 levels correlated with astrocyte density and microvessel presence, suggesting roles in metabolic and barrier functions.

## Abstract

The hippocampal formation is crucial for episodic learning and memory. In addition to neurons, astrocytes have also received increasing attention in recent years as essential components of brain networks by regulating the blood-brain barrier, eliminating waste products via the glymphatic system, supporting neuronal activity by providing energy supply and metabolic substrates, and regulating extracellular neurotransmitter levels. Astrocytes are heterogeneous and highly dynamic cells that respond to neuronal activity and dysfunction via morphological and functional changes. Astrocytic connexins (Cx) 30 and 43 form the molecular basis for gap junctions and hemichannels and are, thus, central to coupling, intercellular communication and network integration of astrocytes in the brain. However, little is known about the spatial heterogeneity of astrocyte density, morphology and Cx expression in the subregions and layers of the hippocampus. Therefore, in this study, we used immunohistochemistry to analyze the density and detailed morphological features of astrocytes and the spatial distribution of Cx30 and Cx43 in the layers of CA1, CA3 and dentate gyrus (DG). Astrocyte density correlated positively with the intensity of Cx30- and Cx43-immunoreaction (Ir). The stratum lacunosum moleculare (SLM) of CA1 and CA3 and the subgranular zone (SGZ) of DG showed the highest density of GFAP-positive (+) astrocytes and the strongest Cx30- and Cx43-Ir. The GFAP+ astrocytic processes had the largest radial extent in the pyramidal layer of CA1 and CA3 and in the granular layer of the DG. Our study provides a comprehensive anatomical and comparative mapping of astrocytic density, morphology and Cx distribution in the mouse hippocampus and provides an important basis for further studies on the dynamics of neuron-glial interaction under different physiological and pathological conditions.

Schematic illustration showing the spatial diversity of astrocytes where the highest density of GFAP+ astrocytes, the smallest radial extension of GFAP+ processes, and the highest density of Cx30 and Cx43 are found in the subregions where the most microvessels exist and, thus, are correlated to blood-brain barrier (BBB) function and higher metabolic activity of astrocytes in these layers. The largest radial extension of GFAP+ processes is found in subregions that have the highest synaptic density (Created by Biorender.com).Diagram illustrating relationships between synaptic density, astrocyte radial extent, astrocyte density with Cx30/Cx43, and microvessel density. Astrocytes interact with synapses and microvessels, transporting nutrients and molecules through fine processes.

Schematic illustration showing the spatial diversity of astrocytes where the highest density of GFAP+ astrocytes, the smallest radial extension of GFAP+ processes, and the highest density of Cx30 and Cx43 are found in the subregions where the most microvessels exist and, thus, are correlated to blood-brain barrier (BBB) function and higher metabolic activity of astrocytes in these layers. The largest radial extension of GFAP+ processes is found in subregions that have the highest synaptic density (Created by Biorender.com).