Precursor Diversity and Complexity of Lineage Relationships in the Outer Subventricular Zone of the Primate

Abstract

Long-term ex vivo live imaging combined with unbiased sampling of cycling precursors shows that macaque outer subventricular zone (OSVZ) includes four distinct basal radial glial (bRG) cell morphotypes, bearing apical and/or basal processes in addition to nonpolar intermediate progenitors (IPs). Each of the five precursor types exhibits extensive self-renewal and proliferative capacities as well as the ability to directly generate neurons, albeit with different frequencies. Cell-cycle parameters exhibited an unusual stage-specific regulation with short cell-cycle duration and increased rates of proliferative divisions during supragranular layer production at late corticogenesis. State transition analysis of an extensive clonal database reveals bidirectional transitions between OSVZ precursor types as well as stage-specific differences in their progeny and topology of the lineage relationships. These results explore rodent-primate differences and show that primate cortical neurons are generated through complex lineages by a mosaic of precursors, thereby providing an innovative framework for understanding specific features of primate corticogenesis.

Unsupervised lineage‐based characterization of primate precursors reveals high proliferative and morphological diversity in the OSVZ

“…This larger diversity, largely resulting from subdividing existing categories, could correspond to precursors with as yet to be determined specialized developmental roles. In accordance with the previously reported nature of precursor type transitions (Betizeau et al, 2013), the HMT shows that each precursor type generates characteristic frequencies of daughter types. Importantly, the HMT transition diagrams confirm the presence of loops connecting members at different lineage depths, thereby speaking against a strict direction of transitions from less toward more fate‐constrained precursors.…”

“…The enlarged supragranular layers of the primates arise from a specialized precursor pool, the outer subventricular zone (OSVZ) (Smart et al, 2002; Lukaszewicz et al, 2005). The OSVZ is the major germinal zone of the developing primate cerebral cortex and harbors the bulk of cortical progenitors from midcorticogenesis onward (Smart et al, 2002; Lukaszewicz et al, 2005; Fietz et al, 2010; Hansen et al, 2010; Fietz and Huttner, 2011; Lui et al, 2011; Reillo and Borrell, 2012; Betizeau et al, 2013). Although significant progress has been made in identifying the OSVZ precursor types in human (Hansen et al, 2010; Fietz and Huttner, 2011) and non‐human (Betizeau et al, 2013) primates, understanding the cellular and molecular properties of OSVZ precursors that drive its expansion in the primate lineage remains a major challenge (Lui et al, 2011; Dehay et al, 2015).…”

“…bRGs were described as monopolar precursors with a basal process contacting the basal lamina, but lacking an apical process contacting the ventricle (Fietz et al, 2010; Hansen et al, 2010; Fietz and Huttner, 2011; Reillo and Borrell, 2012). A recent study in embryonic macaque, combining real‐time imaging of retrovirally infected green fluorescent protein (GFP)‐expressing progenitors from long‐term organotypic slice cultures with immunohistochemistry, showed that the bRG population is more diverse than previously reported in human studies (Betizeau et al, 2013). This study showed that, in addition to the previously described basal process bearing bRG precursor (the bRG‐ basal ‐P precursor), three additional categories of bRGs can be observed: 1) bipolar bRG precursors exhibiting an apically directed process in addition to the basal process (bRG– both ‐P precursors); 2) monopolar bRG precursors lacking a basal process and exhibiting an apically directed process (bRG– apical ‐P precursors); and 3) transient bRGs (tbRG precursors) where either a well‐developed apical or basal process (or both) retract or are formed during the lifetime of the precursor (Betizeau et al, 2013).…”

“…HMTs encode in their structure the underlying assumption that there exist discrete types of precursors that can be characterized both by their observable characteristic morphology and by the fate of their progeny. In this way the unsupervised machine learning algorithms for HMTs are able to identify unobservable precursor types when lineage‐based clustering is applied to a large database of monkey cortical precursors, collected for Betizeau et al (2013). One can speculate that molecular markers would identify these unobserved precursor types.…”

“…In the Betizeau et al (2013) database each cell is characterized by its observed features as well as its position in the lineage trees. Despite the simplicity of the HMT model, the learned probabilistic model accurately identifies and explains the roles of diverse morphological precursor types and their lineage relations in monkey cortex, reproducing the diversity revealed by empirical (manual) observation (Betizeau et al, 2013). In addition, HMT predicts the type of cells that previously could not be characterized because of their unknown fate.…”