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The development of the brain's vascular system is a predominantly prenatal process in mammalian species and is required for neurogenesis and further brain development. Our recent work on fetal pig has revealed that many neurodevelopmental processes start well before birth and proceed rapidly reaching near‐mature status already around birth. Here, we analyzed the development of neocortical vasculature from embryonic day (E) 45 onward (gestation in pig lasts 114 days) using qualitative and quantitative image analyses and protein blots. In all cortical layers, vessel volume from total brain volume at E100 resembled that of a postnatal day (P) 30 piglet. Endothelial cells expressed the tight junction protein claudin‐5 from E45 onward. GFAP+ and AQP4+ astrocytes, PDGFRβ+ pericytes, and α‐SMA+ smooth muscle cells are detectable near vessels at E60 suggesting an early assembly of blood–brain barrier components. The vascular system in the visual cortex is advanced before birth with an almost mature pattern at E100. Findings were confirmed by blots that showed a steady increase of expression of tight junction and angiogenesis‐related proteins (claudin‐5, occludin, VE‐cadherin, PECAM‐1/CD31) from E65 onward until P90. The expression profile was similar in visual and somatosensory cortex. Together, we report a rapid maturation of the vascular system in pig cortex. [ABSTRACT FROM AUTHOR]

Knowledge on cortical development is based mainly on rodents besides primates and carnivores, all being altricial. Here, we analyzed a precocial animal, the pig, looking at dorsoparietal cortex from E45 to P90. At E45, most ionized calcium‐binding adapter molecule 1‐positive (Iba1+) cells had a macrophage‐like morphology and resided in meninges and choroid plexus. Only a few cells were scattered in the ventricular and subventricular zone (VZ and SVZ). At E60/E70, all laminar compartments displayed microglia cells at a low‐to‐moderate density, being highest in VZ and SVZ followed by intermediate zone/white matter (IZ/WM). The cortical plate and marginal zone displayed only a few Iba1+ cells. Cells were intensely labeled, but still had poorly arborized somata and many resembled ameboid, macrophage‐like microglia. Concurrent with a massive increase in cortical volume, microglia cell density increased until E85, and further until E100/E110 (birth at E114) to densities that resemble those seen postnatally. A fraction of microglia colabeled with Ki67 suggesting proliferation in all laminar compartments. Cell‐to‐cell distance decreased substantially during this time, and the fraction of microglia to all nuclei and to neurons increases in the laminar compartments. Eventually, of all cortical DAPI+ nuclei 7–12% were Iba1+ microglia. From E70 onwards, more and more cells with ramified processes were present in MZ down to IZ/WM, showing, for instance, a close association with NeuN+, NPY+, and GAD65/67+ somata and axon initial segments. These results suggested that the development of microglia cell density and morphology proceeds rapidly from mid‐gestation onwards reaching near‐adult status already before birth. [ABSTRACT FROM AUTHOR]