Developmental changes in the vascular network of the rat visual areas 17, 18 and 18a

The present study examines quantitatively the areal and the laminar fluctuations of the vascular network in the visual areas 17, 18 and 18a of the rat cerebral cortex, from postnatal day (P) 1 to P60. For this purpose, the detailed vascular networks of the visual areas, marked after transcardial perfusion of India ink, are analyzed with the use of an image analysis system in order to measure the total vascular density (VD) and the relative density of capillaries (CD), of medium (MD)- and large (LD)-sized vessels in combination with changes in the mean diameter of all three types of vessels. Comparative quantitative microscopy showed that both VD and CD do not exhibit significant interareal differences in the adult rat brain. However, while VD reaches adult values much earlier in area 18a (P21) than in areas 17 and 18 (P60), CD obtains adult values at P31 in areas 17 and 18a, but later (P60) in area 18. Maturation process of laminar VD, CD, MD and LD was not found to follow a simple (i.e. inside-out or mediolateral) sequence, and, in each cortical area, laminar fluctuations of vessels density revealed a complicated developmental pattern, which might be attributed to their changing structural and functional status. Developmental changes in the diameter of capillaries, examined in conjunction with concomitant changes of vascular and capillary density in each area, suggest the existence of angiogenesis in all three visual areas during the third postnatal week of age.