Hormones, produced by glands or cells, are messengers which act locally or at a distance to coordinate the function of cells and organs. Types of hormone include: peptides (e.g. hypothalamic releasing factors) and proteins (e.g. insulin, growth hormone)—these generally interact with membrane receptors located on the cell surface, causing activation of downstream signalling pathways leading to alteration in gene transcription or modulation of biochemical pathways to effect a physiological response; steroids (e.g. cortisol, progesterone, testosterone, oestradiol) and other lipophilic substances (e.g. vitamin D, retinoic acid, thyroid hormone)—these act by crossing the plasma membrane to interact with intracellular receptors, with hormone action via nuclear receptors altering cellular gene expression directly.
Chicken oviduct—the target tissue for growth hormone action: effect on cell proliferation and apoptosis and on the gene expression of some oviduct-specific proteins