Pattern and localisation of expression of vascular endothelial growth factor and its receptor flt-1 in the ovine pituitary gland: expression is independent of hypothalamic control

The pituitary gland, a highly vascularised endocrine organ, contains permeable fenestrated endothelium that allows direct access of endocrine cells to the hemal milieu. Vascular endothelial growth factor (VEGF) has a mitogenic effect on endothelial cells and renders the endothelium more permeable. The following study investigated the expression of VEGF and its receptor flt-1 mRNA and protein in the pituitary gland of sheep. VEGF expression was localised, by in situ hybridisation and immunocytochemistry, mainly to the pars tuberalis/zona tuberalis (PT/ZT) region of the gland. No hybridisation signal was observed in the pars intermedia or pars nervosa. Reverse transcriptase-polymerase chain reaction (RT-PCR)/Southern blotting confirmed the predominant expression of VEGF in the PT/ZT compared with the pars distalis (PD). Western blot analysis with the VEGF antibody revealed major (48 kDa) and minor (24 kDa) bands representing the monomer and dimer forms of VEGF and also confirmed the differential expression of VEGF in the PT/ZT compared with the PD. Double immunocytochemistry with VEGF and prolactin or luteinising hormone- β (LH- β) antibodies demonstrated that the VEGF-secreting cells are not lactotrophs or gonadotrophs. However, co-localisation of VEGF with S-100 was observed in a proportion of cells suggesting that some VEGF secreting cells are follicular stellate. Immunocytochemistry with a flt-1 antibody confirmed the expression of this high affinity receptor for VEGF in endothelial cells across the pituitary gland. Immunocytochemistry with the VEGF antibody using pituitary glands from intact and hypothalamo-pituitary disconnected sheep demonstrated comparable expression patterns suggesting that the regulation of blood flow and vascular permeability in the pituitary gland is under local regulation and is independent of hypothalamic input.