Secretory and morphological heterogeneity of porcine somatotropes during postnatal development

Previous studies from our laboratory have demonstrated that porcine somatotropes can be separated into two subpopulations of low (LD) and high density (HD) by centrifugation in a Percoll gradient. The two subsets are present throughout porcine postnatal growth, although their relative proportions vary with age. In prepubertal animals, HD cells exhibit higher secretory granule content and release more GH than LD cells under basal culture conditions. In the present study, we analysed the ultrastructure of separated LD and HD cells from neonate and mature female pigs, and quantified cell size as well as the relative abundance of several subcellular organelles on immunoidentified somatotropes. Subsequently, GH release under basal conditions was assessed for cultures of unseparated cells and also for LD and HD somatotropes obtained at different stages of postnatal development. Results from the morphometric study demonstrated that LD somatotropes were significantly smaller in size, contained less secretory granules and displayed a more developed endoplasmic reticulum than their HD counterparts, regardless of the age of the pituitary donors. In terms of secretory ability, a significant age-associated decrease in GH release was observed in monolayer cultures of unseparated cells from prepubertal and mature pigs compared to neonates. A similar decline in GH-releasing ability was detected for cultures of HD cells. For LD cells, GH secretion only decreased significantly in mature animals. In spite of the divergent pattern followed by both subpopulations during growth, HD somatotropes released significantly more GH than LD somatotropes at the three ages studied. Taken together, our findings demonstrate that the population of porcine somatotropes is mainly composed of two subtypes, LD and HD, which differ in density, morphology and basal secretory activity. These differences are essentially maintained during porcine postnatal development. The progressive reduction in the secretory capacity of HD and LD somatotropes, coupled to the decrease in the relative abundance reported for the HD subpopulation, provides the cytological basis for a better understanding of the decline in GH release associated with age in pigs.