Ontogenetic development of TRH-like immunoreactive nerve terminals in the median eminence of the rat.

Ontogenetic development of TRH-like immunoreactive nerve terminals in the median eminence of the rat was studied immunocytochemically. By light microscopy, TRH-like immunoreactivities were first detected on the 1st day after birth in the external layer of the median eminence. By electron microscopy, TRH-like immunoreactive nerve fibers and terminals were visible on the 0.5th day after birth. The nerve terminals were first found in direct contact with the perivascular basal lamina of the portal vessel on the 2nd day. TRH-like immunoreactivities were only localized on dense granular vesicles about 105 nm in diameter in the axoplasm throughout the developmental stages. The immunoreactive nerve fibers with TRH-like immunoreactive granular vesicles gradually increased in number with development. The physiological significance of TRH as a hormone is discussed in relation to the presence of TRH-like immunoreactive nerve terminals in the median eminence of the developing rat.