Abstract
The claustrum is an enigmatic brain structure thought to be important for conscious sensations. Recent studies have focused on gene expression patterns, connectivity, and function of the claustrum, but relatively little is known about its development. Interestingly, claustrum-enriched genes, including the previously identified marker Nurr1, are not only expressed in the classical claustrum complex, but also embedded within lateral neocortical regions in rodents. Recent studies suggest that Nurr1 positive neurons in the lateral cortex share a highly conserved genetic expression pattern with claustrum neurons. Thus, we focus on the developmental progression and birth dating pattern of the claustrum and Nurr1 positive neurons in the lateral cortex. We comprehensively investigate the expression of Nurr1 at various stages of development in the rat and find that Nurr1 expression first appears as an elongated line along the anterior-posterior axis on embryonic day 13.5 (E13.5) and then gradually differentiates into multiple sub-regions during prenatal development. Previous birth dating studies of the claustrum have led to conflicting results, therefore, we combine 5-ethynyl-2′-deoxyuridine (EdU) labeling with in situ hybridization for Nurr1 to study birth dating patterns. We find that most dorsal endopiriform (DEn) neurons are born on E13.5 to E14.5. Ventral claustrum (vCL) and dorsal claustrum (dCL) are mainly born on E14.5 to E15.5. Nurr1 positive cortical deep layer neurons (dLn) and superficial layer neurons (sLn) are mainly born on E14.5 to E15.5 and E15.5 to E17.5, respectively. Finally, we identify ventral to dorsal and posterior to anterior neurogenetic gradients within vCL and DEn. Thus, our findings suggest that claustrum and Nurr1 positive neurons in the lateral cortex are born sequentially over several days of embryonic development and contribute toward charting the complex developmental pattern of the claustrum in rodents.
Highlights
The claustrum is an irregularly shaped sheet of neurons located in the basolateral forebrain of the mammalian brain
We study the birth dating of the claustrum, endopiriform nucleus, and related cortical regions that are labeled by Nurr1 gene expression by injecting ethynyl-2 -deoxyuridine (EdU) at different time points in timed-pregnant rats
We find that injections of 10 mg/kg or 25 mg/kg EdU show little effects on litter size, while higher doses correlate with reduced litter size (Figure 1B)
Summary
The claustrum is an irregularly shaped sheet of neurons located in the basolateral forebrain of the mammalian brain. It is thought to be involved in four main functions: regulation of sleep, consciousness, attention and salience, and memory (Mathur, 2014; Jackson et al, 2020; Smith et al, 2020; Wong et al, 2021). This notion is supported by the dense connectivity of the claustrum with other cortical regions (Smith and Alloway, 2010; Patzke et al, 2014; Torgerson et al, 2015; Kitanishi and Matsuo, 2017; Wang et al, 2017; Zingg et al, 2018). Despite the wealth of new gene expression data, the development of claustrum cell types remains incompletely understood
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