Elsevier

Neuroscience Research

Volume 58, Issue 2, June 2007, Pages 207-214
Neuroscience Research

Sex difference in cellular proliferation within the telencephalic ventricle zone of Bengalese finch

https://doi.org/10.1016/j.neures.2007.02.001Get rights and content

Abstract

Cellular proliferation within the ventricular zone (VZ) may contribute to sex differences through the net addition of neurons in song control nuclei. To address this issue, we administered [3H]thymidine to Bengalese finches of both sexes, and estradiol benzoate (EB) to females 15 days post hatching. The birds were killed 2 h later to examine thymidine labeled cells within the VZ at three brain levels, HVC, anterior commissure and Area X. Our results indicated that: (1) cell proliferation in the VZ was significantly higher in the three studied brain levels in males and EB implant females relative to intact or empty implant females, respectively; (2) proliferation in the dorsal half of the VZ, in proximity to HVC, was notably higher than that in the ventral half of the VZ; (3) proliferation in the ventral VZ (VVZ), which is relatively close to Area X was higher relative to other subregions of VZ (dorsal and intermediate). Our study suggests that sex differences in cell proliferation in the VZ may contribute to the net growth of HVC and Area X in males, and estradiol may play an important role in sexual difference in cellular proliferation within the VZ.

Introduction

Sexual differentiation in song behavior is reflected by high sexual dimorphism of song control nuclei. Male song control nuclei are larger in volume (Nottebohm and Arnold, 1976), cell size, and cell number relative to those of females (Gurney, 1981, Nordeen et al., 1987). Sexual differentiation of song nuclei is produced in two ways. (1) HVC and Area X, whose neurons are largely born during the first 20 days post-hatching (Alvarez-Buylla and Nottebohm, 1988), exhibit sex differences as a result of large increases in cellular numbers in males, but not in females (Nordeen and Nordeen, 1988; Burek et al., 1995, Burek et al., 1997; Kim et al., 2004, Wade and Arnold, 2004). (2) Other song control nuclei, such as the robust nucleus of the arcopallium (RA) and lateral magnocellular nucleus of anterior nidopallium (LMAN), exhibit sexual dimorphism after neurons have finished migration and initial differentiation, resulting in neuronal growth in males and neuronal shrinkage or death in females (Alvarez-Buylla and Nottebohm, 1988, Konish and Akutagawa, 1990, Alvarez-Buylla et al., 1994; Burek et al., 1994, Burek et al., 1995). Female birds can be masculinized by administration of estradiol early in development or during the critical period of development (Konishi and Akutagawa, 1985, Konish and Akutagawa, 1988, Nordeen and Nordeen, 1989).

Cell survival, migration and specification are known to contribute to sex differences in the net addition of song control nuclei neurons (Goldman and Nottebohm, 1983, Nordeen and Nordeen, 1988, Kirn et al., 1994). However, cellular proliferation within the ventricular zone (VZ), which is another potential mechanism underlying sexual differentiation in song control nuclei such as HVC and Area X, has not been studied in detail (DeWulf and Bottjer, 2002, DeWulf and Bottjer, 2005). To address this issue we administered [3H]thymidine to juvenile bengalese finches (Lonchura striata) of both sexes 15 days post-hatching, within the critical period of sexual differentiation of the song control system. The birds were killed 2 h later to examine proliferation activity uncombined with cell death or migration away from the VZ. In addition, to further disclose the potential role that estradiol may play in cell proliferation within the VZ, we compared cellular proliferation between females implanted with estradiol benzoate (EB) at age of 5 days post-hatching and those receiving empty implants.

Section snippets

Thymidine labeling and tissue preparation

Fifty-five Bengalese finches were taken from our breeding colony to measure levels of cellular proliferation in VZ. Of these, twelve birds received a 3 mm subcutaneous Silastic implant filled with EB (75 μg, Sigma), while twelve birds received an empty Silastic implant. Each bird then received a single intramuscular injection of [3H]thymidine (2.5 μCi/g dose; specific activity, 6.7 Ci/mmol; New England Nuclear) at 15 days of age. All individuals were killed 2 h later. Two hours survival is short

Cellular proliferation at the brain level of HVC

Clusters of thymidine labeled cells were observed within the VZ (Fig. 2). Although scattered clusters of thymidine labeling were distributed along the entire extent of the VZ, and not localized to a specific region (Fig. 1c), a greater number of labeled cells appeared at the dorsal VZ. Because labeled cells did not overlap (Fig. 2), we could count labeled cells individually.

There was a significant trend toward decreased number of thymidine labeled cells along the seven dorsal–ventral bins in

Discussion

Since new HVC and Area X neurons require 1–3 weeks to arrive in song-control nuclei after they are produced within the VZ (Burd and Nottebohm, 1985, Alvarez-Buylla and Nottebohm, 1988, Burek et al., 1994, Kirn et al., 1999), such long post-generation intervals may cause varied numbers of neurons in targeted regions, due to subsequent cell division, differentiation, cell death and migration (Alvarez-Buylla and Nottebohm, 1988, Konish and Akutagawa, 1990; Burek et al., 1994, Burek et al., 1995).

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      However, sex differences in proliferation have been reported in juvenile zebra finches in anatomically discrete brain regions, confined to the ventral and rostral part of the VZ at the level of Area X (DeWulf and Bottjer, 2002, 2005). Studies of 15-day-old Bengalese finches also revealed localized sex differences in cell proliferation that the authors related to the development of sex differences in the morphology of the song nuclei HVC and Area X (Zeng et al., 2007). Because adult females are often used to investigate cellular effects of testosterone on adult plasticity in songbirds, it is useful to confirm that cells in both sexes respond similarly to the steroid.

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    Grant sponsor: National Natural Science Foundation of China to SJ Zeng (No.: 30470226), MX Zuo (No.: 30670685) and XW Zhang (No.: 30460042), and Beijing Natural Science Foundation of China to MX Zuo (No. 5052016).

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