Developmental endocrinology of the dipnoan, Neoceratodus forsteri

American Zoologist, Dec 1997 by Joss, Jean M P, Rajasekar, P Sylvia, Raj-Prasad, R Ashni, Ruitenberg, Kirsty

The primary function of the pineal in the early post hatching stages of lungfish development is not fully understood. The larvae are heavily pigmented, with both epidermal and dermal melanophores. The latter do respond to melatonin and these young fish blanch in response to complete darkness in the same manner as has been described previously for larval amphibians (Bagnara, 1965). These studies are presently in preparation for publication. The behavioural studies which may indicate whether this blanching response is in fact the primary function of the pineal in hatchling lungfish remain to be carried out.

DEVELOPMENT OF THE PITUITARY

Griffiths (1938) described the development of the pituitary in N. forsteri at a purely structural level, including no histological description of cell types. The cell types in the adult pituitary have been defined immunocytochemically by several groups (Hansen and Hansen, 1994; Joss et al., 1990). We are now attempting to define the ontogeny of the differentiation of cell types in the pituitary of N. forsteri by immunocytochemical and immunogold techniques.

At hatching, the pituitary has separated from the epithelium of the roof of the mouth and comes to lie beneath the infundibular region of the hypothalamus. It comprises a hollow ball of cells which are as yet unidentified. At four weeks, four cell types can be distinguished ultrastructurally by the size of their secretory granules. Two of these have been identified by immunogold labelling as corticotropes and melanotropes. By 20 weeks, three additional cell types can be distinguished which have been identified as prolactin cells, somatotropes, and thyrotropes. Using antibodies raised against the beta chain of lungfish luteinising hormone (isolated by H. Kawauchi) and shown to be immunoreactive with putative gonadotropes in adult lungfish pituitaries, we have not yet identified gonadotropes in the pituitary of lungfish less than 40 weeks of age. The study of development of the pituitary is preliminary at the present moment but has been included here to indicate that the sequence of differentiation of the cell types appears to follow a similar pattern to that observed in amphibians (Oota and Saga, 1991; Ogawa et al., 1995; Miranda et al., 1996); i.e., corticotropes appearing first, followed by prolactin cells and thyrotropes. The latter will be discussed further in the next section on the development of the thyroid.

DEVELOPMENT OF THE THYROID

At hatching the thyroid is only apparent in those fish which hatch at a later stage of development. At stage 49, 2-3 follicles, comprising 9-12 cells each, make up the thyroid. Thereafter, follicle number increases slowly and quite variably between individuals.

Pituitary control of thyroid function Preliminary observations on thyroid follicle cell height (Fig. 3) suggest that an increase occurred at approximately 28 weeks of age, immediately preceding the second increase in growth rate of juvenile lungfish and following the appearance of TSH-immunoreactive cells in the pituitary. Therefore, experiments were carried out to explore the relationship between the pituitary and thyroid during this period of development. These experiments included fish from the first period of accelerated growth (16 weeks) for comparison.