Expression of the neurotrophin brain-derived neurotrophic factor (BDNF) and of its high-affinity receptor TrkB

factor (BDNF) and of its high-affinity receptor TrkB have previously been described in the avian embryo. During early embryonic development, BDNF mRNA expression was identified by reverse transcriptase/polymerase chain reaction (RT-PCR) beginning at stage 1 (3–4 h of embryo incubation), while trkB expression was not detected until the eight-somite stage (29 h) (Baig and Khan, 1996). These observations compare favorably with those of Yao et al. (Yao et al., 1994), who described early (embryonic day 1 and 2) expression of neurotrophin in quail embryos. Further analysis of this neurotrophin in the chick embryo by northern blot analysis and in situ hybridization during embryonic development showed a peak of expression of BDNF at day 4.5 (Hallbook et al., 1993). BDNF has been reported to serve as a cell survival factor for chick embryonic motor neurons (Becker et al., 1998), and its expression has been associated with stato-acoustic ganglion development within the inner ear (Hallbook and Fritzsch, 1997). The results from these and other studies largely emphasize the role of BDNF as a target-oriented factor that promotes the development of neuronal tissues in the developing avian embryo, yet also support the emerging view that BDNF can modulate the development and differentiation of non-neuronal tissues. The expression and function of several neurotrophins and neurotrophin receptors, including BDNF and TrkB, within the rat ovary have previously been associated with the initial (pubertal) ovulation (Dissen et al., 1996). Moreover, expression of trkB mRNA has been demonstrated at the time of primordial follicle organization in the rat ovary, and it has been proposed that such expression is required for the organization of somatic and germ cells and/or for the initiation of follicle growth (Dissen et al., 1995; Ojeda et al., 1996). In contrast, the presence of BDNF and trkB has not previously been described in the adult avian ovary. Furthermore, neither the regulation nor the site of function of this neurotrophin 2087 The Journal of Experimental Biology 204, 2087–2095 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3367