Gene expression of vascular endothelial growth factor-A in the pituitary during formation of the vascular system in the hypothalamic-pituitary axis of the rat

Techniques involving fluorescein-5-isothiocyanate-conjugated gelatin injection, immunohistochemistry, and in situ reverse transcription/polymerase chain reaction (RT-PCR) revealed a close relationship between vascular endothelial growth factor (VEGF)-A-expressing cells and microvessels in the hypothalamic-pituitary axis of the rat. In situ RT-PCR clearly indicated the presence of VEGF-A mRNA-expressing cells in the pars tuberalis and in the pars distalis both at embryonic day 15.5 (E15.5) and in later developmental stages. The primary capillaries extended along the developing pars tuberalis, whereas the portal vessels penetrated into the pars distalis at E15.5 and subsequently expanded into the lobe to connect with the secondary capillary plexus, emerging in the pars distalis. At the same time, several VEGF-A-positive cells appeared in the pars distalis. These VEGF-A-positive cells were found to correspond to a portion of adrenocorticotropin (ACTH) cells by dual-staining for in situ RT-PCR and immunohistochemistry, suggesting that some ACTH cells have the potential to produce VEGF-A. Thus, the present study suggests that VEGF-A is involved in the development of the primary capillaries and in the vascularization of the pars distalis, but not in the portal vessels since the formation of portal vessels begins at E13.5, before the appearance of VEGF-A in the rostral region of the pars distalis.