Two inbred mouse strains, A/J and C57BL/6J, and a set of 27 AXB/BXA RI strains (derived from reciprocal intercrossing C57BL/6J and A/J followed by inbreeding progeny for ≥ 20 generations) were obtained from The Jackson Laboratory (Bar Harbor, ME, USA). Male and female mice were kept under a 12-h light/dark cycle and were given ad libitum access to food and water. Animals studied were between 60 and 150 days old (n = 118), but the majority of them (98) were 80 ± 20 days old. All experimental procedures were conducted under an Institutional Animal Care and Use Committee (IACUC)-approved protocol from the University of Tennessee as well as the Canadian Council on Animal Care (CCAC)-approved protocol VX 809 from the University of
British Columbia. The thymidine analog BrdU, which is actively incorporated into the S phase of dividing cells, was used to label and quantify constitutively proliferating cells in the RMS of C57BL/6J, A/J and click here AXB/BXA RI strains. All mice received a single intraperitoneal injection of BrdU (Sigma-Aldrich, St Louis, MO, USA) at a dosage of 50 mg BrdU/kg body weight using a stock solution of 5 mg BrdU/mL in 0.9% NaCl containing 0.007 N NaOH.
One hour later, animals were anesthetized with an overdose of Avertin (Sigma-Aldrich; 0.2 mL/10 g body weight), and perfused transcardially with 0.1 m phosphate buffer (PB; pH ∼7.2) followed by a solution of 95% alcohol/acetic acid (3 : 1). Brains were removed from the skull and postfixed in the same acid alcohol solution at 4°C overnight before being bisected and processed for paraffin embedding. Brains were dehydrated through a graded alcohol series and xylenes, and then infiltrated with paraffin (Paraplast Plus). Each brain hemisphere
was embedded separately, serially sectioned in the sagittal plane at 8 μm and then mounted on Superfrost/Plus slides. BrdU was also used of to determine the cell cycle length of rapidly dividing cells in the RMS by adopting the cumulative BrdU labeling protocol developed by Nowakowski et al. (1989). BrdU was administered to a new batch of 2–3-month-old male C57BL/6J and A/J mice (5 mg/mL BrdU in 0.9% NaCl and 0.007 N NaOH; 50 mg/kg body weight) every 2 h for a total period of 10 h to ensure that every dividing cell entering the S-phase has the chance to be labeled. Animals were anesthetized with Avertin and perfused transcardially at 0.5, 2.5, 4.5, 6.5, 8.5 and 10.5 h after the first BrdU injection. Sixty animals were used for the cell cycle analysis (five A/Js and five C57BL/6Js at each time point). Brain tissues were prepared as described above. Sections were deparaffinized in xylenes, rehydrated in a graded series of alcohol, treated with 1 m HCl for 30 min at 37°C to denature DNA, rinsed with 0.1 m PBS, treated with 1% H2O2 in PBS to block endogenous peroxidase, and washed for 5 min in 0.1 m PBST. Sections were then treated with incubation buffer (30% BSA 1 : 100, NGS 1 : 20, NaN3 1 : 100, in 0.