Representation of Geometric Borders in the Developing Rat

• Border cells are present in MEC from the outset of navigation in rat pups
• Key properties of border cells are present from the beginning
• Grid cells mature slower than border cells

SummaryLocal space is represented by a number of functionally specific cell types, including place cells in the hippocampus and grid cells, head direction cells, and border cells in the medial entorhinal cortex (MEC). These cells form a functional map of external space already at the time when rat pups leave the nest for the first time in their life, at the age of 2.5 weeks. However, while place cells have adult-like firing fields from the outset, grid cells have irregular and variable fields until the fourth week, raising doubts about their contribution to place cell firing at young age. Recording in MEC of juvenile rats, we show that, unlike grid cells, border cells express adult-like firing fields from the first days of exposure to an open environment, at postnatal days 16–18. Thus, spatial signals from border cells may be sufficient to maintain spatially localized firing in juvenile hippocampal place cells.