Follicle-stimulating isohormones: regulation and biological significance

Follicle-stimulating hormone (FSH) is a key hormone in the regulation of fo llicular development. Although the existence of FSH heterogeneity is well e stablished, the physiological significance of this pleomorphism remains unk nown. Observed changes in circulating FSH heterogeneity during critical rep roductive events such as puberty and reproductive cyclicity suggest that di fferent combinations of FSH isoforms reach the target sites during differen t physiological states to influence a variety of biological end points such as cellular growth, development, steroidogenesis and protein synthesis. Co nsidering that these FSH isoforms have different physicochemical properties and potential to bind not only their cognate receptors but also structural ly related, non-FSH receptors with various affinities, the regulatory impli cations of FSH heterogeneity in modulating the various FSH-induced function s are enormous. However, assigning functional significance to FSH heterogen eity has been hampered because of (1) difficulties associated with procurem ent of highly purified, naturally occurring, circulating FSH isoforms; (2) absence of reference standards that contain the entire repertoire of FSH is oforms present in biological fluids; and (3) specificity issues inherent to the detection systems used. if particular FSH isoforms do possess selectiv e biological functions, specific combinations of FSH isoforms could be gene rated to regulate fertility in farm animals and humans.