Figure 2: Light and electron microscopic patterns of dividing immature cells in the DG of hibernating animals. Mitotic phases are seldom found in normothermic animals (not shown here). (a and b) are light microscopic analysis of BrdU-immunoperoxidase-positive cells (arrowheads) in the DG of hibernating ground squirrel after injection of BrdU in interbout animal (on 50 μm thick coronal slices). Note the distribution of BrdU-labelled cells in the SGZ. (b) Higher magnification light micrograph demonstrating granule cells where low temperature has halted division showing clearly different mitotic phases: metaphase and telophase due to hypothermia causing entrance into the hibernation state. (c) Ultrastructural details from an electron micrograph (6000× magnification) of an early metaphase immature granule cell from rat hippocampus, (image adapted from a figure shown in Popov et al. [3]), with an interphase neuron located above the metaphase cell. Both immature interphase and metaphase granule neurons have synapses (interphase neuron-dendritic spine and metaphase one-shaft synapse) which are absent in mature granule cells of hippocampus of either ground squirrel or rat. BV: blood vessel; DG: dentate gyrus; SGZ: subgranular zone; mt: microtubules; and MT: mitochondria.