The anterior pituitary contains cells that produce and secrete growth hormone (GH) into the circulating blood that plays a critical role in muscle accretion, lipolysis and lean growth in the pig. GH cells (somatotrophs) characterized as specific topographical localizations in the anterior pituitary gland store GH in the 350-500 nm in diameter secretory vesicles that may dock and transiently fuse at the plasma membrane fusion pores [POROSOME] to secrete GH. The earlier AFM study demonstrated 40% increase in size of the fusion pore after stimulation of GH secretion. The objectives of our immunocytochemical study using light microscopy (LM) and transmission electron microscopy (TEM) are (1) to identify the spatial distribution patterns of GH cells during rapid growth in pigs and (2) to demonstrate the fate of secretory vesicles after secretion. Immunoreactive somatotrophs were consistently counted under LM and total number of filled, empty, and partly empty vesicles before and after stimulation of GH secretion was determined in TEM. All quantitative counting data were subjected to analysis of variance (ANOVA) and Student's t test. Somatotrophs were densely distributed in lateral wings of anterior lobe from proximal distal depth (43.8±1.2 per 30,495 [mu]m2, Mean±SEM), whereas rarely distributed in shoulder regions (21.8±1.4 per 30,495 [mu]m2) in day 1, day 42, and day 100. However a significant increase in median region at distal was observed in all groups (55.2%>, P < 0.05). TEM results showed that control pituitary cells contained more than twice as many filled vesicles that did the stimulated cells whereas stimulated cells contained nearly twice as many empty vesicles and 2.5 x more partly empty vesicles than did control cells. Immunogold pGH antibody labeling showed that empty vesicles were devoid of electron dense particles. There was no significant difference in total number of vesicles between the controls and stimulated. Our current investigations reveal (1) a regional/spatial specificity of cellular differentiation and transformation to facilitate GH secretion in young growing pigs and (2) GH containing vesicles in pituitary transiently fuse at the POROSOME and secrete GH without loss of secretory vesicles.