The ultrastructure of the eel pituitary at the elver stage with special reference to its neurosecretory innervation.

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The present study deals with the development of the pituitary of the eel at the following stages: immediately after the elvers have left their marine environment and after 8 and 16 weeks in freshwater. The following results have been obtained: 1. The pars nervosa of the neuro-intermediate lobe of elvers is highly differentiated. As in adult eels, 3 types of neurosecretory fibre can be distinguished (A1 fibres: size of granules c. 1700 A; A2 fibres: c. 1200 A; B fibres: c. 750 A) and synapses between A2 fibres and pituicytes are present. 2. In the pars intermedia, 3 types of cell are present (type I, II and III). Type I and type III cells show striking changes which appear to be consistent with the view that type I cells are involved in production of MSH and that type III cells are affected by osmotic changes. 3. Already in elvers of stage I, the nerve tracts innervating the pars distalis contain an abundance of type B neurosecretory fibres and a small amount of type A fibres. In elvers of stage II, type A neurosecretion has increased and can be demonstrated also by light microscopy. 4. In the proximal pars distalis of elvers of stage I only STH cells and a few undifferentiated cells are present. In the following stages the latter ones appear to have been transformed into gonadotrophic cells. 5. In the rostral pars distalis of all stages investigated ACTH, TSH, and ‘prolactin’ cells are present. In elvers of stage I, at the level of ultrastructure, TSH cells can be demonstrated in abundance but not all of them have developed their typical staining pattern. In elvers of stage II the differentiation of the TSH cells appears to be finished. ‘Prolactin’ cells are relatively undifferentiated in elvers of stage I but are well developed and show signs of great activity in elvers of stage III. These observations are discussed in view of the possible role of a prolactin-like hormone for the adaptation to freshwater. 6. The development of intervascular channels which link neurosecretory tracts and endocrine cells is described and the possible role of neurosecretion for the function of pars distalis cells is discussed.