Melanocytes in the Leptomeningeal Region of Fetal Sheep Cerebral Cortex: Response to Exposure to Bacteria

Over the past several decades, there has been growing evidence in support of the in utero colonization hypothesis, stating that womb is not sterile and that low levels of bacteria may be found in fetal tissues without harming the fetus. Exposure of the fetus to bacteria would likely stimulate an immune response. Melanin, produced in the brain by several cell types, may play a role in the inflammatory response as it does in the epidermis. Melanocytes are phagocytic cells that synthesize melanin and are thought to function in response to inflammation. We hypothesize that we can find melanocytes in fetal brain tissues, and that their numbers are increased in fetuses exposed to bacteria. We assessed tissues from 31 fetal sheep that were catheterized and exposed to experimental manipulation (n=23) or were non‐catheterized (naïve; n=8). Identified histologically using Fontana‐Masson stain and quantified as number of cells in a 40x field (10 fields/animal), we found melanocytes in the leptomeningeal region of all fetuses. There was a higher presence of (leptomeningeal) melanocytes in catheterized animals compared to non‐catheterized animals (25±4 vs 3±1 cells/field) per 32,000 um 2 . We conclude the leptomeningeal melanocytes in the fetal cerebral cortex are part of the fetal innate immune response to exposure of the fetus to pathogen‐associated molecular patterns. Support or Funding Information NIH Grant HD033053