Figure 2.
Developmental changes in macro- and microstructural measurements of myelin in the prefrontal cortex of male rats. A, Embedded coronal brain tissue slices of a pre-adolescent rat (PD15, left) and an adolescent rat (PD43, right; scale bar = 1 mm). The myelinated portion of the CCFM is outlined with a black dotted line in the pre-adolescent tissue slice and outlined with a red dotted line in the adolescent tissue slice to illustrate how myelinated cross-sectional area was analyzed. Small squares on the embedded slices indicate the location of the images shown in C, D. B, There was a significant increase in the myelinated cross-sectional area of CCFM in adolescent rats compared to pre-adolescent rats (t(3.95) = 5.65, *p = 0.005). C, D, Representative images of myelin development over adolescence in Cg1 and CCFM regions (scale bar = 20 μm). Neurons (black arrows), astrocytes (white arrows), and oligodendrocytes (orange arrows) were identified and distinguished using an algorithm created by García-Cabezas et al. (2016) that is based on cytological features. Examples of myelinated axons that crossed through the plane of section in the 1.5-μm semi-thin sections are labeled with small orange arrowheads. The number of myelinated axons and area-based g-ratio were determined from cross-sectioned axons such as these. Myelinated axons that were parallel to the plane of the semi-thin sections were not included in the analysis (not labeled in this figure). There was a significant developmental increase in the number of myelinated axons in Layer V of the Cg1 (C, t(3.00) = 14.17, *p = 0.0008) and CCFM (D, t(3.29) = 8.88, *p = 0.002). Data expressed as mean ± SEM. PrL, prelimbic cortex; IL, infralimbic cortex; aca, anterior commissure.