Summary
The neurohypophysis of Sphenodon forms a hollow sac with two lateral diverticula which project anteriorly from the pars nervosa. In the tuber and stalk the ependymal end feet form a complete cuff between the neurosecretory fibre terminals and the investing capillaries, but in the pars nervosa this cuff, although still extensive, is incomplete thus permitting the neurosecretory fibre terminals to make direct contact with the capillaries of the plexus intermedius. In addition to small diameter nerve fibres containing only neurotubules and neurofilaments, four types of granular and/or vesicular fibre can be distinguished. Two of these are probably peptidergic and one aminergic. The fourth fibre may represent a distinct cholinergic fibre or possibly agranular portions of one of the other three types. There are no pituicytes.
The pars intermedia surrounds the pars nervosa, separated from it by the capillaries of the plexus intermedius. Capillaries are also found on the outer surface of the pars intermedia in the investing connective tissue, but none of these fibres penetrate between the intermedia cells. Two types of cell are found. The Type I, glandular cells contain large numbers of secretory granules. These granules have a distinct fibrillar substructure, the fibrils being gathered together at the equator of the granule to form a dense band. The Type II cells are agranular and irregular in shape, their processes lying amongst the Type I cells and terminating at the pericapillary space of the plexus intermedius or the superficial plexus, where they effectively insulate much of the terminal surface of the Type I cells from the pericapillary space. These Type II cell vascular terminals contain large numbers of small electron lucent vesicles along their vascular margin, suggesting that the cells are engaged in the transport of material into or out of the capillaries. Within the pars intermedia, the Type II cells also bound colloid filled spaces which may represent the remains of the hypophysial cleft. No nerve fibres enter the pars intermedia.
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My thanks are due to the Secretary of State for Internal Affairs of the Government of New Zealand for permission to export a tuatara, and to Professor H. Heller, Sir Horace smirk and Professor J. Robb who all helped in obtaining the animal and transporting it to England. I am also extremely grateful to Professor D. R. Arthur for permission to examine Dendy's collection of tuatara material which is in the possession of the Zoology Department, King's College, London. The electron microscope facilities employed in this work were financed by a grant made by the Medical Research Council to Professor Sir Francis Knowles, Bt., F.R.S., who was also kind enough to read and criticise this manuscript.
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Weatherhead, B. Cytology of the neuro-intermediate lobe of the tuatara, Sphenodon punctatus gray. Z. Zellforsch. 119, 21–42 (1971). https://doi.org/10.1007/BF00330536
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DOI: https://doi.org/10.1007/BF00330536