Mosaic development of the olfactory cortex with Pax6-dependent and -independent components

doi:10.1016/S0165-3806(02)00304-8

Developmental Brain Research

Volume 136, Issue 1, 30 May 2002, Pages 17-26

https://doi.org/10.1016/S0165-3806(02)00304-8 Get rights and content

Abstract

The olfactory cortex is the target area of olfactory bulb axons and is suggested to be derived from neuroepithelial progenitors of various ventricular domains during development. In the present study, we examined the development of the olfactory cortex, using the newly developed monoclonal antibody (mAb) 9-4c, which recognizes reticulon 1-A and -B. The mAb labeled neuroepithelial progenitors at the pallio-subpallial boundary (PSB) and their putative descendants in the deep layers of the olfactory cortex. In the Pax6 mutant embryo, labeling at the PSB was specifically lacking, and the number of immunopositive cells in the olfactory cortex was markedly reduced. In contrast, the guidepost neurons of olfactory bulb axons, lot cells, developed relatively normally in the superficial layer of the olfactory cortex in the mutant embryo. These guidepost neurons have been recently shown to originate in the pallium and eventually guide the initial projection of olfactory bulb axons. The olfactory bulb projection in the Pax6 mutant embryo also suggested the dualistic nature of the olfactory cortex development; the initial projection of olfactory bulb axons developed relatively normally, whereas the final projection of their collateral branches was severely defective.

Introduction

One of the early events in the development of the central nervous system is regionalization of the neural tube into organized domains [23], [37], [52]. The telencephalic primordium is divided into two major domains: the pallium and subpallium [10]. Recent studies [32], [53] using various molecular markers have recommended division of the pallium into medial, dorsal, lateral and ventral pallium (MP, DP, LP and VP, respectively), as well as the subpallium into the lateral and medial ganglionic eminences (LGE and MGE) (Fig. 1A). In each domain, various genes are expressed in a specific manner and assign different developmental potentials to ventricular progenitors.

Pax6 is a transcription factor primarily expressed in the pallium [2], [42] (see also review in Ref. [31]). When Pax6 is disrupted in animals known as Small eye (Sey) mice and rats [14], [27], [30], the telencephalic domains are disorganized especially at the pallio-subpallial boundary (PSB). For example, expressions of some molecular markers are specifically lost at the PSB, and subpallial markers are expressed dorsally beyond the PSB in the homozygous mutant [41], [44], [48], [53]. Furthermore, radial glial fascicles at the PSB are morphologically disorganized in the mutant embryo [11], [43]. These observations suggest that the VP domain is severely defective and presumably respecified into a more ventral character in the Sey/Sey mutant.

The LP is thought to give rise to the olfactory cortex at maturity [32]. The olfactory cortex is the target region of olfactory bulb axons, consisting of morphologically distinct areas such as the anterior olfactory nucleus, the piriform cortex and the tubercle (Fig. 1A,B) [3]. Axons of the olfactory bulb form the prominent lateral olfactory tract (LOT) on the surface of the piriform cortex and innervate all olfactory cortical areas with collateral branches sprouted from the LOT [15], [39], [45]. In addition to the PSB domains, other domains are also thought to contribute to the generation of the olfactory cortex. Guidepost neurons of olfactory bulb axons, known as lot cells, are one example of such neurons. These cells originate in the DP domain and migrate tangentially and ventrally down to the future site of the LOT on the piriform cortex [47], where they guide the following olfactory bulb axons into the LOT pathway [38]. There are also cells of LGE origin that occupy the olfactory cortex [7]. These results demonstrate the mosaic nature of the olfactory cortex, although it remains to be elucidated how individual cells of different origins collaborate to construct a functional olfactory cortex.

In the present study, we investigated the development of the olfactory cortex in wild-type and Sey/Sey mouse embryos, using a newly developed monoclonal antibody (mAb), 9-4c. This novel regional marker labels the ventricular progenitors at the PSB that project radial fibers to the LOT position and their putative descendants in the deep layers of the olfactory cortex. A search for the antigen recognized by mAb 9-4c identified reticulon 1 (Rtn 1), an endoplasmic reticulum (ER) protein [36]. In the Sey/Sey mutant embryo, expression of this antigen was completely absent at the PSB, and the number of immunopositive cells in the olfactory cortex was markedly reduced. Nevertheless, the lot cells of DP origin developed relatively normally in the superficial layer of the olfactory cortex in the mutant embryo. Subsequent projection of olfactory bulb axons in the Sey/Sey embryo also supported the mosaic nature of the olfactory cortex development; the axons initially projected via the normal pathway and formed the LOT bundle, but later failed to develop collateral projections over the wide olfactory cortical areas.

Section snippets

Mice

Sey/+-CBA mice [14] were provided by Drs Veronica van Heyningen and Pen Rashbass of MRC Human Genetics Unit and maintained by crossing with +/+-CBA mice in our laboratory. The homozygous embryos were obtained by intercrosses between the heterozygotes and distinguished from Sey/+ and +/+ littermate embryos by the eyeless phenotype. The midday on which a vaginal plug was found was designated as E0.5. The experimental protocol was approved by the Ethics Review Committees for Animal Experimentation

Specific staining of radial glial fibers at the PSB with mAb 9-4c

In the E12.5 telencephalon, strong expression of Pax6 clearly marked the PSB extending ventrally from the pallio-subpallial boundary (Fig. 1C). The mAb 9-4c strongly stained radial fibers projecting from the ventricular zone between the PSB and morphological sulcus (Fig. 1D). Although radial fibers in the LGE and MGE were also occasionally stained, the staining intensity was weaker than that of the PSB. The cell bodies of 9-4c-positive radial fibers were located on the ventricular edge and

Discussion

Our results reveal the mosaic nature of the olfactory cortex with at least three populations (Fig. 7); one of the three is Pax6-dependent and the other two are Pax6-independent. The Pax6-dependent population is derived from the VP domain that is marked with 9-4c mAb in the wild-type telencephalon. The cells generated from the VP domain seem to form the deep layers of the olfactory cortex in the wild-type embryo (red cells in Fig. 7) since these cells expressed the common molecular marker. In

Acknowledgments

We thank Dr Veronica van Heyningen and Dr Pen Rashbass of MRC Human Genetics Unit for the generous gift of the Sey/+ mutant mice, Dr Jun-ichi Miyazaki of Osaka University for the gift of pCAGGS and Dr Kunimasa Ohta for helpful advice. We also thank Dr John Rubenstein for mouse Dlx1 cDNA, and Dr Naoki Takahashi for mouse Dbx1 cDNA. This work was supported by grants from the Ministry of Education, Sports, Science and Technology, PRESTO and CREST of the Japan Science and Technology Corporation.

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Research reportMosaic development of the olfactory cortex with Pax6-dependent and -independent components

Abstract

Introduction

Section snippets

Mice

Specific staining of radial glial fibers at the PSB with mAb 9-4c

Discussion

Acknowledgments

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Mosaic development of the olfactory cortex with Pax6-dependent and -independent components