Intrapituitary interactions: another level of endocrine regulation

1. Factors, produced within the anterior pituitary, act locally to influence many acute and developmental changes in neighbouring cells. 2. Broad spectra of factors and activities characterize these paracrine interactions. 3. Cleavage products of pro-opiomelanocortin, other than those that act as systemic hormones, are produced by neonatal gonadotrophs and these peptides stimulate mitosis and differentiation of lactotrophs. 4. Activins and follistatins exert opposing effects on gonadotrophs. The stimulatory activity of activins is modulated extracellularly through protein-protein interactions with follistatin. Within gonadotrophs, the stimulatory activity of activins is self-limiting through receptor-mediated stimulatory and inhibitory signal transduction pathways. 5. During the oestrous cycle, the interactions between gonadotroph cells and epidermal growth factor (EGF), acting as a local signal, occur on multiple levels. Levels of expression of EGF receptors on cells vary as a function of the cycle, as do EGF-induced changes in gene expression and proliferation of cells.