Developmental endocrinology of the dipnoan, neoceratodus forsteri

The development of the pineal, pituitary and thyroid glands of the extant lungfish, Neoceratodus forsteri, are being studied both morphologically and functionally. This paper presents data from hatching to 40-52 weeks for a standardised series of lungfish, bred at Macquarie University. At hatching, the pineal comprises a single organ attached to the roof of the diencephalon, with well-developed photoreceptor, supporting and ganglion cells. The photoreceptors gradually degenerate, giving way to secretory cells which contain electron dense granules. These latter are immunoreactive to melatonin antibodies and digestable with protease. The pituitary at hatching comprises a hollow ball of cells lying beneath the infundibular region of the hypothalamus. Ultrastructurally, four cell types can be distinguished by cytoplasmic granule size after the first four weeks of development posthatching. By 20 weeks, a further three cell types are recognisable. Immunogold labelling has identified corticotropes and melanotropes at four weeks and, at 20 weeks, prolactin cells, thyrotropes and somatotropes also can be identified. The thyroid is only just apparent at hatching, containing 2-3 follicles. The numbers of follicles increases gradually, and variably between animals, with age. Iodine uptake in methimazole-treated animals did not exceed that of controls at any of the three stages tested, indicating a lack of feedback control between thyroid hormones and pituitary thyrotropes at, or before, 40 weeks of age. Thyroid hormone receptors in the liver at 40 weeks are predominantly immunoreactive to human TRa antibodies. These findings taken together suggest that, up to 40 weeks post hatching, lungfish development is equivalent to amphibian premetamorphic development. This would be consistent with lungfish neoteny, but cannot be taken as evidence for neoteny until confirmed at later stages of development.