Protoplasmic Astrocytes Are Poorly Understood Cells in Adult Human Brain Tissue

Background/Objectives: The traditional classification of astrocytes was based on morphological differences between astrocytes and their location in brain tissue. Astrocytes stained by impregnation techniques were divided into protoplasmic and fibrous astrocytes. We still use this classification, often supplemented by GFAP immunostaining. However, protoplasmic astrocytes have been found in the human cerebral cortex as GFAP-negative cells. Methods: In this study, astrocytes were identified using Cajal’s gold sublimation method and GFAP immunostaining. Biopsy samples of normal brain tissue (n = 25) were obtained from adult patients diagnosed with traumatic brain injury, stroke, gliomas and brain metastases. Results: In all samples, GFAP-positive fibrous astrocytes were found in the subpial region (layer I-I) and in the white matter. GFAP-positive protoplasmic astrocytes were absent or occurred only rarely in the cortical gray matter (layer III–VI) in samples from patients diagnosed with a tumor. Similar staining was also observed using the Cajal method. However, in samples from patients with traumatic brain injury accompanied by high intracranial pressure, GFAP-positive areas with numerous astrocytic processes and cells with a morphology similar to protoplasmic astrocytes were found. Conclusions: We can conclude that protoplasmic astrocytes are GFAP-negative cells that respond to brain injury by GFAP expression. We consider this finding to be a sign of protoplasmic astrocyte differentiation. On the other hand, fibrous astrocytes are GFAP-positive and respond to brain injury with increased GFAP expression. These results raise questions regarding the classification of astrocytes and, in particular, the histological visualization of the neuro-glial-vascular unit.