The Subventricular Zone in the Immature Piglet Brain: Anatomy and Exodus of Neuroblasts into White Matter after Traumatic Brain Injury

Abstract

Stimulation of postnatal neurogenesis in the subventricular zone (SVZ) and robust migration of neuroblasts to the lesion site in response to traumatic brain injury (TBI) is well established in rodent species; however, it is not yet known whether postnatal neurogenesis plays a role in repair after TBI in gyrencephalic species. Here we describe the anatomy of the SVZ in the piglet for the first time and initiate an investigation into the effect of TBI on the SVZ architecture and the number of neuroblasts in the white matter. Among all ages of immaturity examined the SVZ contained a dense mesh network of neurogenic precursor cells (doublecortin+) positioned directly adjacent to the ependymal cells (ventricular SVZ, Vsvz) and neuroblasts organized into chains that were distinct from the Vsvz (abventricular SVZ, Asvz). Though the architecture of the SVZ was similar among ages, the areas of Vsvz and Asvz neuroblast chains declined with age. At postnatal day (PND) 14 the white matter tracts have a tremendous number of individual neuroblasts. In our scaled cortical impact model, lesion size increased with age. Similarly, the response of the SVZ to injury was also age dependent. The younger age groups that sustained the proportionately smallest lesions had the largest SVZ areas, which further increased in response to injury. In piglets that were injured at 4 months of age and had the largest lesions, the SVZ did not increase in response to injury. Similar to humans, swine have abundant gyri and gyral white matter, providing a unique platform to study neuroblasts potentially migrating from the SVZ to the lesioned cortex along these white matter tracts. In piglets injured at PND 7, TBI did not increase the total number of neuroblasts in the white matter compared to uninjured piglets, but redistribution occurred with a greater number of neuroblasts in the white matter of the hemisphere ipsilateral to the injury compared to the contralateral hemisphere. At 7 days after injury, less than 1% of neuroblasts in the white matter were born in the 2 days following injury. These data show that the SVZ in the piglet shares many anatomical similarities with the SVZ in the human infant, and that TBI had only modest effects on the SVZ and the number of neuroblasts in the white matter. Piglets at an equivalent developmental stage to human infants were equipped with the largest SVZ and a tremendous number of neuroblasts in the white matter, which may be sufficient in lesion repair without the dramatic stimulation of neurogenic machinery. It has yet to be determined whether neurogenesis and migrating neuroblasts play a role in repair after TBI and/or whether an alteration of normal migration during active postnatal population of brain regions is beneficial in species with gyrencephalic brains.

Endogenous neurogenic cell response in the mature mammalian brain following traumatic injury

“…This age-related difference in the degree of injury-induced endogenous cell response has also been recently reported in species with gyrencephalic brains. In this study, Costine and colleagues have found that in a cortical impact injury piglet model, TBI significantly increased cell proliferation in the SVZ in piglets that were injured at post-natal day 7 but not in animals injured at 4 months old (Costine et al, 2015). Furthermore, we and others have found that injury-induced newly generated granular cells integrate into the existing hippocampal circuitry (Emery et al, 2005; Sun et al, 2007), and this endogenous neurogenesis is directly associated with the innate cognitive recovery observed following injury (Blaiss et al, 2011; Sun et al, 2015).…”

Neuroblast Distribution after Cortical Impact Is Influenced by White Matter Injury in the Immature Gyrencephalic Brain

“…Piglets received a total of 4 injections (Table 1); BrdU (50 mg/kg; IP) or vehicle (saline) was administered as previously described while piglets were lightly anesthetized with isoflurane once per day (Costine et al, 2015). The different time points in administration of BrdU was to compare the number of BrdU + neurons before relative to after injury, in order to identify whether injury itself stimulated the formation of new neurons from the SVZ within the first days after trauma.…”

“…In normal early postnatal development in gyrencephalic species, large populations of neuroblasts are found in a migratory morphology in the major white matter tracts (Sanai et al, 2011; Costine et al, 2015). The SVZ generates neuroblasts that migrate to distant brain regions via distinct pathways, migratory streams to the olfactory bulbs (where population continues past infancy), prefrontal cortex, and possibly the nucleus accumbens (De Marchis et al, 2004; Sanai et al, 2011).…”

Development of a Model of Hemispheric Hypodensity (“Big Black Brain”)

“…Male, Yorkshire piglets (Earle Parsons & Sons, Inc., Hadley, MA) were housed and fed as previously described after arrival at post-natal day (PND) 5, 19, or 28. 25 Milk replacer (PND 5) or grain pellets (PND 19 and 28) were removed the evening prior to surgery and piglets were given a liquid electrolyte solution (BlueLite; TechMix, Inc., Stewart, MN). A total of 48 piglets (Experimental Phase 1, n = 8; Experimental Phase 2, n = 25; Experimental Phase 3, n = 15) were used to develop this model in three experimental phases.…”

“…23,25 The brain was removed leaving the ventral dura intact and post-fixed at 4°C for up to 1 month. The eyes with attached optic nerves were removed by dissection through the floor of the anterior cranial fossa into the orbital space to expose the globe and contents of the orbit in a subset of injured piglets (n = 3).…”