10 – the biology of metamorphosis

Hypothalamic control of the pars distalis in fishes, amphibians, and reptiles.

Publisher Summary This chapter reviews recent studies on metamorphosis from the perspective of hormonal control. It also focuses on biochemical and cell-biological aspects of amphibian metamorphosis. The developmental stages of larvae of representative species are presented. Amphibian metamorphosis appears to be under multihormonal control and the secretion and function of these hormones are regulated in various ways. Thyroid hormone levels need to be low for premetamorphic tadpoles to grow without showing metamorphic changes. For the final phase of metamorphosis, known as climax, higher levels of thyroid hormones are required because no tail shortening occurs in thyroidectomized tadpoles with relatively low concentrations of thyroid hormone. Thyroid hormone mRNA concentrations decrease in most of the tissues that undergo substantial remodeling. However, in the developing ovary of the juvenile toad, high levels of thyroid hormone α-receptor mRNA are detected. The accumulation of thyroid hormone receptor transcripts in oocytes is sustained and continues to increase throughout further development. The presence of thyroid-stimulating hormone in the amphibian pituitary has been confirmed indirectly by evidence of a difference in thyroid function between normal and hypophysectomized tadpoles at TK stage III and by the stimulating effect of crude extracts prepared from the pituitaries of prometamorphic and climax Xenopus larvae on uptake of 131I by the thyroid.

Aspects of Amphibian Metamorphosis: Hormonal Control

The key component regulating vertebrate puberty and sexual maturation is the endocrine system primarily effectuated along the brain-pituitary-gonad (BPG) axis. By far most investigations on the teleost BPG axis have been performed on salmonids, carps, catfish and eels. Accordingly, earlier reviews on the BPG axis in teleosts have focused on these species, and mainly on females (e.g. 'Fish Physiology, vol. IXA. Reproduction (1983) pp. 97'; 'Proceedings of the Fourth International Symposium on the Reproductive Physiology of Fish. FishSymp91, Sheffield, UK, 1991, pp. 2'; 'Curr. Top. Dev. Biol. 30 (1995) pp. 103'; 'Rev. Fish Biol. Fish. 7 (1997) pp. 173'; 'Proceedings of the Sixth International Symposium on the Reproductive Physiology of Fish. John Grieg A/S, Bergen, Norway, 2000, pp. 211'). However, in recent years new data have emerged on the BPG axis in flatfish, especially at the level of the brain and pituitary. The evolutionarily advanced flatfishes are important model species both from an evolutionary point of view and also because many are candidates for aquaculture. The scope of this paper is to review the present status on the male teleost BPG axis, with an emphasis on flatfish. In doing so, we will first discuss the present understanding of the individual constituents of the axis in the best studied teleost models, and thereafter discuss available data on flatfish. Of the three constituents of the BPG axis, we will focus especially on the pituitary and gonadotropins. In addition to reviewing recent information on flatfish, we present some entirely new information on the phylogeny and molecular structure of teleost gonadotropins.

The brain-pituitary-gonad axis in male teleosts, with special emphasis on flatfish (Pleuronectiformes).

Publisher Summary
This chapter describes colloidal gold probes in immunocytochemistry. The interest in colloidal gold as an immunocytochemical marker has grown. Firstly, it has been shown that it is capable of strong emission of secondary electrons, making it a very useful marker for scanning electron microscopy. Secondly, accumulations of colloidal gold over antigen-containing sites can be seen in the light microscope. The typical red color forms without incubation with an enzyme substrate and its appearance can be monitored continuously. Recently, protein A–colloidal silver has also been used for light microscopical detection of pancreatic hormones and for double staining experiments. Thirdly, colloidal gold probes can be used to detect antigens in ultrathin plastic or frozen sections. It has also been shown that the smaller sized probes are able to penetrate into whole cultured cells, to label intracellular antigens in the pre-embedding mode. Fourthly, colloidal gold–antibody probes have been used in a novel way to label cell monolayers. Finally, monoclonal antibodies have been shown to provide a first choice material for labeling with colloidal gold.

6 – Colloidal Gold Probes in Immunocytochemistry

Immunohistochemical detection, by light and electron microscopy, of pituitary hormones in cold-blooded vertebrates. I. Fish and amphibians.

Immunohistochemical detection, by light and electron microscopy, of pituitary hormones in cold-blooded vertebrates. II. Reptiles.

Rabbit antiserum to synthetic LHRH was used with the immunofluorescence technique to identify the LHRH-secreting neurons and their axonal pathways in the brain of Xenopus laevis. Three groups of immunoreactive neurons were identified: the first, in the telencephalon, is a paired group of cells scattered near the two telencephalic ventricles; the second group lies near the preoptic recess; the third group occurs in the ventral wall of the infundibulum. Two principal neuronal pathways were observed: Fibres originating from the dorsally located telencephalic neurons converge on the cephalic median plane where they form a single bundle behind the telencephalic furrow. This bundle descends towards the anterior border of the preoptic recess where it divides into two nerve bundles which pass on either side of the preoptic recess, run above the optic chiasma then cross the infundibular floor and finally terminate in the median eminence. The second pathway is more direct. The more ventrally located telencephalic LHRH cells give rise to this second pathway. Their axons converge with the other LHRH fibres near the lateral border of the preoptic recess. Most of the LHRH nerve fibres terminate in the median eminence although some terminate near the paired pars tuberalis. No reaction was observed after the use of antiserum absorbed with synthetic antigen.

LHRH-like system in the brain of Xenopus laevis daud

Les antiserums prepares a l'egard de trois hormones de synthese [β(1–24) corticotropine, α et β MSH] nous ont servi pour localiser par immunofluorescence dans l'adenohypophyse des Amphibiens les sites d'elaboration de l'ACTH, de l'α et de la β MSH.

Identification par immunofluorescence des cellules corticotropes et mélanotropes de l'hypophyse des Amphibiens

In the brain of Rana temporaria, two distinct systems reactive with α- and β-endorphin antisera, respectively, and with a met-enkephalin antiserum, have been detected immunohistochemically. Neurons reacting with α- and β-endorphin antisera are located (1) in the preoptic nucleus, and (2) in the pars ventralis of the tuber cinereum. Immunoreactive nerve fibres of both groups of perikarya end in the infundibular floor near the capillaries and the preoptico-hypophysial tract. Control reactions have shown that the immunoreactivity is suppressed by the corresponding antigens, but also by β-LPH. In view of these results the immunoreactive systems examined correspond to an α/β-endorphin system or a lipotropinergic system. Neurons reacting with the met-enkephalin antiserum are located in the paraventricular organ. Intense immunofluorescence was observed in the infundibular floor. Controls show that the labelling by met-enkephalin antiserum is exclusively suppressed by met-enkephalin. In the pituitary gland, on the other hand, α- and β-endorphin antisera reveal: 1) the MSH/ACTH-like cells of the pars intermedia and 2) the ACTH-like cells of the pars distalis.

Immunohistochemical localisation of substances reactive to antisera against alpha- and beta-endorphin and met-enkephalin in the brain of Rana temporaria L.

Jeannine Doerr-Schott

Papers

Immunohistochemical detection, by light and electron microscopy, of pituitary hormones in cold-blooded vertebrates. I. Fish and amphibians.

Immunohistochemical detection, by light and electron microscopy, of pituitary hormones in cold-blooded vertebrates. II. Reptiles.

LHRH-like system in the brain of Xenopus laevis daud

Identification par immunofluorescence des cellules corticotropes et mélanotropes de l'hypophyse des Amphibiens

Immunohistochemical localisation of substances reactive to antisera against alpha- and beta-endorphin and met-enkephalin in the brain of Rana temporaria L.