Gradual transition of pyramidal cell types in the dorsal hippocampal area CA2b of the C57BL/6 mouse.

Hippocampal area CA2 is important for social recognition memory. It has classically been defined as the region between areas CA3 and CA1 where pyramidal cells have larger cell bodies than CA1 neurons, but lack mossy fiber input and thorny excrescences (TEs) typical for CA3 neurons. Based on molecular signatures, the borders of area CA2 have been redefined, with area CA2b now covering parts of former area CA3a. Functional data suggest that CA2b is a mixture of CA3 and CA2 cells, yet information about the spatial distribution of these cells within CA2b remains vague. In the present study, we filled pyramidal cells in dorsal CA3-CA2-CA1, post-hoc identified CA2 cells using Purkinje Cell Protein 4 (PCP4), and analyzed their proximal apical spines using confocal and electron microscopy. We found that dorsal CA2b resembles an intermediate zone between CA3 and CA2a, where CA3 neurons with large TEs are gradually replaced by CA2 neurons. These CA2 neurons are heterogeneous in their scarce display of spines and larger spiny protrusions, rarely also carry TEs, and form synapses with mossy fibers. Thus, morphology needs to be combined with molecular markers to identify CA2 neurons in area CA2b with certainty.