The ultrastructural changes in the neurohypophysis after destruction of the paraventricular nuclei in normal and castrated rats

Summary In normal and castrated rats an electrolytic lesion of 1–1.5 mm in diameter was placed in the hypothalamus using bilateral electrodes. Such a lesion destroyed both paraventricular nuclei and surrounding tissue. The electron microscope study of the neurohypophysis revealed that about 1/5 of the neurosecretory axons and terminals degenerated. In the castrated rats, the clear axons described in a previous paper (Zambrano andDe Robebtis, 1968), were those that underwent degeneration. This finding supports our previous assumption that such axons belong to the paraventricular system. The sequence of the ultrastructural changes occurring in the degenerating axons and terminals is described. Special features were the early lysis of the neurotubules, the breakage of the membrane of the elementary granules and of the axolemma. Disrupted axonic material was observed in the interstitial tissue. The degeneration of the clear axons from the paraventricular system is discussed in relation to their possible oxytocinergic nature.An early cellular reaction was found in the perivascular microglial type of cell. These increased in number and underwent ultrastructural changes indicative of active phagocytosis and pinocytosis. Both the protoplasmatic and the fibrous pituicytes did not participate in the removal of the degenerating axons. This was apparently done by extracellular digestion and engulfment by microglial cells.Supported by grants from the Consejo Nacional de Investigaciones Científicas y Técnicas and by the Air Force Office of Scientific Research (AF-AFOSR 963-67).The Authors are grateful to Dr.S. Taleisnik, Instituto de Investigaciones Médicas Mer cedes y Martín Ferreira, Córdoba, Argentina for encouraging this investigation and for the technical help so generously offered.