Cadmium increases human fetal germ cell apoptosis

BACKGROUND

Cadmium (Cd) is a common environmental pollutant and a major constituent of tobacco smoke. Adverse effects of this heavy metal on reproductive function have been identified in adults; however, no studies have examined its effects on human reproductive organs during development.

OBJECTIVES

Using our previously developed organ culture system, we investigated the effects of cadmium chloride on human gonads at the beginning of fetal life, a critical stage in the development of reproductive function.

METHODS

Human fetal gonads were recovered during the first trimester (711 weeks postconception) and cultured with or without Cd. We used different concentrations of Cd and compared results with those obtained with mouse fetal gonads at similar stages.

RESULTS

Cd, at concentrations as low as 1 microM, significantly decreased the germ cell density in human fetal ovaries. This correlated with an increase in germ cell apoptosis, but there was no effect on proliferation. Similarly, in the human fetal testis, Cd (1 microM) reduced germ cell number without affecting testosterone secretion. In mouse fetal gonads, Cd increased only female germ cell apoptosis.

CONCLUSIONS

This is the first experimental demonstration that Cd, at low concentrations, alters the survival of male and female germ cells in humans. Considering data demonstrating extensive human exposure, we believe that current environmental levels of Cd could be deleterious to early gametogenesis.

Phthalates impair germ cell development in the human fetal testis in vitro without change in testosterone production

BACKGROUND

Several studies have described an increasing frequency of male reproductive disorders, which may have a common origin in fetal life and which are hypothesized to be caused by endocrine disruptors. Phthalate esters represent a class of environmental endocrine-active chemicals known to disrupt development of the male reproductive tract by decreasing testosterone production in the fetal rat.

OBJECTIVES

Using the organ culture system we developed previously, we investigated the effects on the development of human fetal testis of one phthalate--mono-2-ethylhexyl phthalate (MEHP)--an industrial chemical found in many products, which has been incriminated as a disruptor of male reproductive function.

METHODS

Human fetal testes were recovered during the first trimester (7-12 weeks) of gestation, a critical period for testicular differentiation, and cultured for 3 days with or without MEHP in basal conditions or stimulated with luteinizing hormone (LH).

RESULTS

Whatever the dose, MEHP treatment had no effect on basal or LH-stimulated testosterone produced by the human fetal testis in vitro, although testosterone production can be modulated in our culture system. MEHP (10(-4) M) did not affect proliferation or apoptosis of Sertoli cells, but it reduced the mRNA expression of anti-Müllerian hormone. MEHP (10(-4) M) reduced the number of germ cells by increasing their apoptosis, measured by the detection of caspase-3-positive germ cells, without modification of their proliferation.

CONCLUSIONS

This is the first experimental demonstration that phthalates alter the development of the germ cell lineage in humans. However, in contrast to results observed in the rat, phthalates did not affect steroidogenesis.

[Development and regulations of testicular functions in the human foetus]

Two major functions are assumed by the testis: the production of male gametes (that is, spermatozoa) and the production of steroid hormones. Both two functions are established during fetal life and are essential to the adult fertility and the masculinization of the internal tract and genitalia. For many years, our laboratory has been interested in the ontogeny of those two functions in rodents and, since 2003, in collaboration with gynecology and obstetrics service of professor R. Frydman in Antoine-Béclère hospital, we have studied them in human. The first aim of this work was to improve the global knowledge of the human fetal testis development by using both our experimental data and the literature. Then, we focused on the different defects that can occur during the fetal testis development. Indeed, male reproductive abnormalities have been steadily increasing since the last decades and are thought to be related to the concomitant increase of the concentration of contaminants and particularly of endocrine disruptors in the environment. Thus, we decided to study the effect of endocrine disruptors on human fetal testis and, more particularly, the effect of phthalates, by using an organ culture system developed for human. In contrast to the data obtained in rat, mono (ethylhexyl)-phthalate (MEHP), an active metabolite of the most widespread phthalate in the environment, does not disturb the steroidogenic function. On the other hand, it has a negative effect on the male germ cells number. This study is the first experimental demonstration of a negative effect of phthalates directly on human fetal testis.

High radiosensitivity of germ cells in human male fetus

CONTEXT

Germ cells formed during human fetal life are essential for fertility of the adult, and several studies have described an increasing frequency of male reproductive disorders, which may have a common origin in fetal life and which are hypothesized to be caused by endocrine disruptors. However, factors inducing a genotoxic stress may also be implicated.

OBJECTIVES

We investigated the effect of gamma-irradiation on the functions of human fetal testis during the first trimester of gestation by using an organ culture system. Then we focused on the role of the p53 pathway in the observed effects.

RESULTS

Germ cells were highly sensitive to irradiation even at doses as low as 0.1 and 0.2 Gy. Indeed, for these doses, one third of germ cells died by apoptosis. Other germ cells were blocked in their cycle, but no repair seemed to occur, and longer culture with the highest dose used showed that they were destined to die. Sertoli cells were less affected, although their proliferation and the level of anti-Müllerian hormone were reduced. Irradiation had no effect on testosterone secretion or on the expression of steroidogenic enzymes by Leydig cells. After irradiation, p53 phosphorylated on serine 15 was detected from 1-24 h in all cell types. This activation of p53 was accompanied by an increase in mRNA levels of proapoptotic factors Bax and Puma, whereas that of antiapoptotic Bcl-2 remained unchanged. P21, which is responsible for cell cycle arrest, was also up-regulated 6, 30, and 72 h after irradiation. Finally, when we added pifithrin-alpha, a specific inhibitor of p53 functions, a significant decrease in irradiation-induced apoptosis in both germ and Sertoli cells was observed, indicating the involvement of the p53 pathway in irradiation-induced apoptosis.

CONCLUSIONS

This study demonstrated here for the first time the great sensitivity of human fetal germ cells to genotoxic stress caused by ionizing radiation.

Caspase-2 involvement during ionizing radiation-induced oocyte death in the mouse ovary

In mammals, the pool of primordial follicles at birth is determinant for female fertility. Exposure to IR during oogonia proliferation and the diplotene stages of ovarian development induced the virtual disappearance of primordial follicles in the postnatal ovary, while half the follicular reserve remained present after irradiation during the zygotene/pachytene stages. This sensitivity difference was correlated with the level of caspase-2 expression evaluated by immunohistochemistry. At the diplotene stage, Western blot and caspase activity analysis revealed that caspase-2 was activated 2 h after irradiation and a significant increase in the number of oocytes expressing cleaved caspase-9 and -3 occurred 6 h after treatment. Inhibition of caspase-2 activity prevented the cleavage of caspase-9 and partially prevented the loss of oocytes in response to irradiation. Taken together, our results show that caspase-2-dependent activation of the mitochondrial apoptotic pathway is one of the mechanisms involved in the genotoxic stress-induced depletion of the primordial follicle pool.

A new method for toxicity assays on human and mouse fetal testis

Exposure to environmental pollutants (EP) is associated with a wide range of toxic effects, in particular in testis development. Uranium is a potential pollutant of nuclear industry and over the last few years, its environmental concentrations have increased. In animals, the current procedures for evaluating the potential developmental toxicity of uranium are based on in vivo studies. These methods do not allow to know the direct effects on testicular cells and are obviously excluded for human experiments. Consequently, we have developed an in vitro culture system of the whole testis. In the present study we characterized and validated this organ culture system in both mouse fetal testes and human fetal testes recovered during the first trimester (6-12 weeks) of gestation. We compared the histological aspect, the number of germ cells and the testosterone production, before and after culture. Testicular architecture and intercellular communications were preserved, and organ culture appears as a powerful method for studying the early development of testicular gametogenesis and steroidogenesis in both species. Thus by using this method we will be able to investigate the effects of uranium on mouse and human developing testis. The mouse model will allow us to determine the dose range of interest without restriction of material.

Use of organ culture to study the human fetal testis development: effect of retinoic acid

CONTEXT

In human, the chronology of the testicular development has been extensively studied, but the factors implicated in the onset and the regulation of gametogenesis and steroidogenesis remain hardly known.

OBJECTIVES

To identify these factors, we developed an organ culture system for human fetal testes recovered during the first trimester (6-12 wk) of gestation. We first aimed at investigating the characteristics of this system by comparing the in vivo and in vitro gametogenesis and steroidogenesis. Second, we used organ culture to investigate the effect on the human testicular functions of retinoic acid (RA), previously described as a regulator of gonadal development in rodents.

RESULTS

Organ culture proved to be an efficient tool for studying the early development of the testicular functions. Indeed, this system was able to maintain satisfactory development of the germ cells and Leydig cells in the absence of any added factor. For older fetuses, the number of germ cells decreased in culture and the LH was necessary to maintain the steroidogenic activity. The addition of 10(-6) m RA decreased the total number of germ cells in the fetal testis at all studied stages. This resulted from an increase in apoptosis, which slightly exceeded the increase of proliferation. However, RA had a stimulatory effect on the steroidogenic function for the youngest fetuses over a short period of time by increasing the expression of P450 cholesterol side-chain cleavage, 17 alpha-hydroxylase/C17-20 lyase, and steroidogenic acute regulatory protein.

CONCLUSIONS

Thus, RA appears as a potential regulator of both gametogenesis and steroidogenesis in human fetal testis. Our organ culture is an interesting tool for studying the effects of various factors on the development of human fetal testis, in particular the effect of hormone-disrupting chemicals.

Organotypic culture, a powerful model for studying rat and mouse fetal testis development

The key role of the fetal testis in the masculinization of genital organs has been known for a long time. More recently, the observed increases in male reproductive disorders has been postulated to be the result of changes in fetal and neonatal testis development in response to increasing environmental pollution. However, few tools are available for studying fetal testis development and the effects of physiological or toxic substances. We have developed an organ culture system in which rat fetal testis is grown on a filter floating on a synthetic medium containing no serum, hormones or biological factors. In this study, we have compared the long-term morpho-functional development of the various testicular cell types in this system with that observed in vivo and have extended this system to the mouse. Rat Leydig, Sertoli and germ cells and macrophages develop normally over a period of 1-2 weeks in this system. Fewer cells are produced than in vivo but the level of differentiated function is similar. Germ cells, which are difficult to culture in vitro, resume mitosis after a quiescent period, at the same time as in vivo. Similar results have been obtained with mouse fetuses, except that Leydig cells dedifferentiate in vitro if the testis is explanted after 13.5 days post conception. Testicular architecture and intercellular communication are sufficiently preserved for the development of the main fetal and neonatal testicular cell types in vitro with no added factors. Our floating-filter organotypic culture system in synthetic medium therefore allows the morpho-functional development of somatic and germ cells in fetal testis explants taken at all developmental stages in rat and at early stages in mouse. This method is potentially useful for studies of the effects of various factors, and of xenobiotics, in particular.

Estrogen receptor beta-mediated inhibition of male germ cell line development in mice by endogenous estrogens during perinatal life

Epidemiological, clinical, and experimental studies have suggested that excessive exposure to estrogens during fetal/neonatal life can lead to reproductive disorders and sperm abnormalities in adulthood. However, it is unknown whether endogenous concentrations of estrogens affect the establishment of the male fetal germ cell lineage. We addressed this question by studying the testicular development of mice in which the estrogen receptor (ER) beta or the ERalpha gene was inactivated. The homozygous inactivation of ERbeta (ERbeta-/-) increased the number of gonocytes by 50% in 2- and 6-d-old neonates. The numbers of Sertoli and Leydig cells and the level of testicular testosterone production were unaffected, suggesting that estrogens act directly on the gonocytes. The increase in the number of gonocytes did not occur during fetal life but instead occurred just after birth, when gonocytes resumed mitosis and apoptosis. It seems to result from a decrease in the apoptosis rate evaluated by the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling method and cleaved caspase-3 immunohistochemical detection. Last, mice heterozygous for the ERbeta gene inactivation behaved similarly to their ERbeta-/- littermates in terms of the number of gonocytes, apoptosis, and mitosis, suggesting that these cells are highly sensitive to the binding of estrogens to ERbeta. ERalpha inactivation had no effect on the number of neonatal gonocytes and Sertoli cells. In conclusion, this study provides the first demonstration that endogenous estrogens can physiologically inhibit germ cell growth in the male. This finding may have important implications concerning the potential action of environmental estrogens.

Retinoid-Sensitive Steps in Steroidogenesis in Fetal and Neonatal Rat Testes: In Vitro and In Vivo Studies

Retinoic acid (RA) was recently shown to modify testosterone secretion of the fetal testis in vitro. We characterized this effect by culturing rat testes explanted at various ages, from Fetal Day 14.5 to Postnatal Day 3. In basal medium, RA inhibited, in a dose-dependent manner, both basal and acute LH-stimulated testosterone secretion by testes explanted on Fetal Days 14.5, 15.5, and 16.5. It had no effect on testes from older animals. The negative effect of RA did not result from a diminution in the number of Leydig cells but from a decrease in P450c17 mRNA levels and in LH-stimulated cAMP production. However, the RA-induced decrease in P450C17 mRNA levels was also observed with neonatal testes, suggesting that this enzymatic step is no longer rate limiting at this developmental stage. To study the physiological relevance of RA effects, we used fetuses and neonates issued from mothers fed a vitamin A-deficient (VAD) diet, resulting in a threefold decrease of plasma retinol concentration. On Fetal Day 18.5 and on Posnatal Day 3, testosterone secretion by the testis ex vivo was significantly increased in VAD animals. This shows that the endogenous retinol inhibits differentiation and/or function of fetal Leydig cells before Fetal Day 18.5 and is required for the normal regression of fetal Leydig cell function that occurs after Fetal Day 18.5. In conclusion, our results show that retinoids play a negative role on the steroidogenic activity during the differentiation of rat fetal Leydig cells.

Development of the foetal and neonatal testis

The foetal testis originates from a proliferation of the mesonephric and the coelomic epithelia which are colonized by the primordial germ cells. In the foetal testis, the development and functions of the three main cell type precursors (Leydig, Sertoli and germ cells) do not depend upon gonadotropins. Numerous intra- and extra-testicular factors are candidates for the control of its development and functions. To study the potential involvement of these factors, we developed an organotypic culture system. In absence of any growth factors or hormone, this system allows a development of the three main cell types which mimics that observed in vivo. The effects of different regulators (gonadotropin-releasing hormone, follicle-stimulating hormone, transforming growth factor-beta, insulin-like growth factor-I, anti-Mullerian hormone, retinoic acid, oestrogens) were tested in this system. Whether or not some of the effects observed in vitro have a physiological relevance was evaluated using appropriate transgenic mice. It is concluded that the foetal testis cannot be considered as an adult mini-testis since it has a specific physiology which largely differs from that of the immature or adult testis.

Regulation and perturbation of testicular functions by vitamin A

In addition to playing a fundamental role in very diverse processes such as vision and the growth and differentiation of numerous types of cell, vitamin A (retinol) and its principal biologically active derivative, retinoic acid, are clearly involved in the regulation of testicular functions in rodents. An excess of vitamin A leads to testicular lesions and spermatogenetic disorders, and a deficiency induces early cessation of spermatogenesis and adversely affects testosterone secretion. Furthermore, mice mutant for retinoic acid alpha receptors and retinoid X beta receptors are sterile. Retinoids appear to exert an action on the three main testicular types of cell (Sertoli, germinal and Leydig cells), as they act on the signalling pathways and Sertoli cell metabolism, and modify numerous factors secreted in Sertoli cells. Retinoids also appear to be necessary for the proliferation and differentiation of A spermatogonia, and for spermiogenesis. In addition, vitamin A deficiency leads to atrophy of the accessory sex organs after decreased testosterone production. Recent studies have shown that retinoids already affect these three types of cell in fetuses. Curiously, the effects of retinoids on fetal and adult testis seem opposed.

Regulation and perturbation of testicular functions by vitamin A

In addition to playing a fundamental role in very diverse processes such as vision and the growth and differentiation of numerous types of cell, vitamin A (retinol) and its principal biologically active derivative, retinoic acid, are clearly involved in the regulation of testicular functions in rodents. An excess of vitamin A leads to testicular lesions and spermatogenetic disorders, and a deficiency induces early cessation of spermatogenesis and adversely affects testosterone secretion. Furthermore, mice mutant for retinoic acid alpha receptors and retinoid X beta receptors are sterile. Retinoids appear to exert an action on the three main testicular types of cell (Sertoli, germinal and Leydig cells), as they act on the signalling pathways and Sertoli cell metabolism, and modify numerous factors secreted in Sertoli cells. Retinoids also appear to be necessary for the proliferation and differentiation of A spermatogonia, and for spermiogenesis. In addition, vitamin A deficiency leads to atrophy of the accessory sex organs after decreased testosterone production. Recent studies have shown that retinoids already affect these three types of cell in fetuses. Curiously, the effects of retinoids on fetal and adult testis seem opposed.

Luteinizing hormone-dependent activity and luteinizing hormone-independent differentiation of rat fetal Leydig cells

Addition of 5x10(-2) U/ml recombinant luteinizing hormone (LH) to testes from fetuses at 16.5 day post conception (dpc) cultured for 5 days increased the number of Leydig cells by 34% and the acute LH-stimulated testosterone production by 600%. To determine whether these positive effects of LH in vitro are physiologically relevant in vivo, fetuses were decapitated on days 16.5 pc (before the onset of LH expression in the hypophysis) or 18.5 pc (before the surge of LH in the fetal plasma) and removed at 21.5 dpc. The number of fetal Leydig cells per testis and the acute LH-stimulated testosterone production by the testes ex vivo were unaltered by decapitation. Since, in all groups, the number of Leydig cells doubled between 16.5 and 18.5 dpc and between 18.5 and 21.5 dpc, these results suggest that neither the appearance of new fully differentiated fetal Leydig cells nor the maintenance of differentiated functions in existing fetal Leydig cells depend on LH during late fetal life, although this hormone is present in the plasma. Decapitation reduced the testosterone concentrations in the plasma (-56%) and in the testis in vivo (-67%) and the basal testosterone secretion of the testis ex vivo (-70%). This suggests that LH is required to maintain the physiological activity of the Leydig cell during late fetal life. However, the decrease of the in vivo testosterone production after decapitation was not sufficient to impair the growth of the Wolffian ducts and the lengthening of the anogenital distance. In conclusion, during late fetal life in the rat, Leydig cells are LH-independent for their functional differentiation and LH-dependent for their activity.

Transforming growth factor beta3 in the fetal and neonatal rat testis: immunolocalization and effect on fetal Leydig cell function

The localization of transforming growth factor beta3 (TGFbeta3) in the fetal and neonatal testis (from fetal day 13.5 to postnatal day 6) was investigated by immunohistochemical staining with a specific polyclonal antibody raised against a synthetic peptide corresponding to residues 50-75 of TGFbeta3. This antibody recognized 0.5 ng TGFbeta3 in western blot analysis, but did not detect 25 ng TGFbeta1 or TGFbeta2. The immunolocalization of TGFbeta3 in the fetal and neonatal testis changed throughout development. Immunostaining was present in the gonocytes by fetal day 13.5, persisted until postnatal day 3, and was heterogeneous in spermatogonia on postnatal day 6. The Sertoli cells contained no immunoreactivity at any age. The fetal-type Leydig cells were first immunostained for TGFbeta3 on day 16.5 and staining became very intense from day 18.5 onward. Staining disappeared when the antibody was presaturated with the synthetic peptide, but persisted when the antibody was presaturated with a tenfold excess of the corresponding peptide from TGFbeta2. Furthermore, we researched whether TGFbeta3 could act as a local regulator of fetal Leydig cell function. In a dispersed fetal testicular cell system, TGFbeta3 inhibited the LH-stimulated testosterone production by Leydig cells from 20.5-day-old fetuses. The inhibitory effect of TGFbeta3 was equal to that observed with TGFbeta1 or TGFbeta2. When compared with our previous studies showing the immunolocalization of TGFbeta1 and TGFbeta2, the present study shows that TGFbeta3 may have a specific role in the developing rat testis, but may also overlap the action of TGFbeta1 and TGFbeta2.

Expression of type I and II receptors for transforming growth factor beta in the adult rat testis

After having established the specificity of the antibodies for the rat testis by western blot analysis, the potential target cells for transforming growth factors (TGFbetas) were identified by immunohistochemical detection of both type I (TbetaRI) and type II (TbetaRII) transducing receptors for TGFbetas in the adult rat testis in situ. Leydig cells showed a strong TbetaRII immunoreactivity whereas the TbetaRI staining was weak. Only TbetaRII was detectable in Sertoli cells. In germ cells, staining for TbetaRI was stronger than for TbetaRII and the expression of both receptors depended on the seminiferous cycle stage. TbetaRI first appeared in pachytene spermatocytes and was absent in elongated spermatids from stage XIV onwards. Labelling for TbetaRII was observed as early as the spermatogonia stage; it increased in pachytene spermatocytes at the onset of TbetaRI and disappeared in elongating spermatids from stage XI onwards. These results show that TGFbetas can affect somatic cells functions and suggest that these factors are involved in the control of meiosis and early spermiogenesis, exerting a direct effect on germ cells.

Immunolocalization of aromatic L-amino acid decarboxylase, tyrosine hydroxylase, dopamine, and serotonin in the forebrain of Ambystoma mexicanum

To improve basic knowledge about the neurochemical organization of the urodele brain, and to study discrepancies in the localization of monoaminergic markers, we immunohistochemically charted the distribution of four such markers (tyrosine hydroxylase, aromatic L-amino acid decarboxylase, dopamine, and serotonin) in the axolotl (Ambystoma mexicanum) forebrain. Catecholaminergic and serotoninergic systems were found in similar locations to those seen in other Urodela. As seen in other vertebrates, the localization of the different monoaminergic markers reveals some inconsistencies. Cells that are exclusively tyrosine hydroxylase-immunoreactive are observed in the olfactory bulb, anterior olfactory nucleus/nucleus accumbens region, the epichiasmatic portion of the preoptic nucleus, and in the pars intercalaris thalami, whereas cells that are only labelled by aromatic L-amino acid decarboxylase are seen in the anterior olfactory nucleus/nucleus accumbens region, the bed nuclei of the anterior commissure, the posterior portion of the preoptic nucleus, the ventral hypothalamus, and the pars intercalaris thalami. The presence of cells solely serotonin (5-HT)-immunoreactive is suggested for the nucleus infundibularis dorsalis. Conversely, there were no areas that appeared to be exclusively immunoreactive for dopamine. Double-labelling for aromatic L-amino acid decarboxylase/tyrosine hydroxylase and aromatic L-amino acid decarboxylase/serotonin, together with cell counting, confirmed the existence of neurons that express only one monoaminergic marker in amphibian, supporting the hypothesis that these cells are universally present in the central nervous system of vertebrates.

Transforming growth factor beta1 and beta2 reduce the number of gonocytes by increasing apoptosis

Transforming growth factors beta1 and beta2 (TGFbetas) have recently been detected by immunohistochemistry in the fetal and neonatal rat testis, and the aim of the present study was to determine whether these factors can act as local regulators to control the number of gonocytes. Testes were kept in organ culture, and TGFbeta1 was found to have dose-dependent inhibitory effect on the number of gonocytes in testes explanted on fetal day 13.5. Either TGFbeta1 or beta2 at 10 ng/ml reduced the number of gonocytes by half after 2 days culture. TGFbetas did not decrease the BrdU labeling index of gonocytes or Sertoli cells, whereas these factors significantly increased the DNA fragmentation in gonocytes (TUNEL method). The other testicular cell types showed no positive TUNEL reaction. TGFbeta1 did not reduce the number of gonocytes in testes explanted on fetal day 17.5 (i.e. during the quiescent phase), but it did so in testes explanted on postnatal day 3 (i.e. stage of resumption of mitosis). To determine the potential cell type targets for TGFbetas, type I and type II TGFbeta receptors were immunolocalized in developing testis from fetal day 13.5 to postnatal day 3. Both receptors were present in the gonocytes throughout the whole period studied, and in the Leydig cells from fetal day 16.5 onward, but they were not detected in the Sertoli cells. Taken together, these results suggest that TGFbetas directly increase apoptosis in gonocytes without changing their mitotic activity during the developmental phases of proliferation.

[Expression of vimentin and GFAP and development of the retina in the trout]

The glial cell development was studied during the edification of the retina and the optic tract, in a teleost, the rainbow trout. The intermediate filament proteins, vimentin and glial fibrillary acidic protein (GFAP) were visualized by an indirect immunohistochemical method. Results show that both vimentin and GFAP are early expressed in the developing retina and, particularly in the Müller cells, a coexpression of vimentin and GFAP is observed from embryonic to adult stages. The ganglion cell layer and the optic fiber layer both exhibit GFAP-positive structures. The deep staining for GFAP is also seen in the optic nerve and induces us to credit astrocyte-like cells with a leading role in the pattern formation of this tract.

Organization of the serotoninergic system in the brain of two amphibian species, Ambystoma mexicanum (Urodela) and Typhlonectes compressicauda (Gymnophiona)

An immunocytochemical investigation was made of the distribution of serotonin (5-HT) in the brain of larval and adult Ambystoma mexicanum and adult Typhlonectes compressicauda. Immunoreactive perikarya can be identified in the caudal diencephalon (paraventricular organ and infundibular nucleus), in the ventral mesencephalon (interpeduncular nucleus) and in the raphe of the rhombencephalon. Immunopositive fibers and terminal arborizations are widely distributed, extending from the whole telencephalon to the spinal lemniscus area. However, the retinorecipient structures of the thalamus and mesencephalon are either very weakly innervated (Ambystoma) or completely immunonegative (Typhlonectes). The habenular system also exhibits very few 5-HT-positive structures. The major serotoninergic neuron clusters, in both Urodela and Gymnophiona, tend to gather, from the paraventricular organ to the raphe, on both sides of the sagittal plane, showing no tendency to "lateralization". A new interpretation of the limited development of the serotoninergic system in amphibians is given.

[Retinotopic projections of the trout (Salmo gairdneri)]

The retinotopic organization of the thalamic and tectal and visual centres was studied by Fink and Heimer method and radioautography. Quadratic lesions were achieved in the right retina of 21 adult trouts and 23 six month old specimens. Fourteen animals (adults) were kept alive for 20 to 25 days after operation, in running water at 13 degrees C and prepared for the Fink technique. In the remaining 7 adults and in the 23 six month specimens, we injected, 48 hours after the lesion, respectively, 2,5 microCi (in 20 microliters) and 0,5 microCi (in 1 microliter) of 14 C proline into the operated eye. The brains were prepared for radioautography. Results show that the thalamic pathways is not correlated with a peculiar quadrant. Nevertheless the temporo dorsal hemiretina sends the more numerous projections at this level. A topographic parallelism exists between retino-geniculate and retino-tectal projections. At the tectal level our data are somewhat at variance with those of electrophysiological mapping, since quadratic projections show a partial overlapping. In the optic tract, the fibres from every quadrant follow always the same pathway.

Membrane differentiation in the pregastrula of the teleost, Brachydanio rerio Hamilton-Buchanan (Teleostei: Cyprinidae). A scanning electron microscope study

The pregastrula stages of the teleost, Brachydanio rerio, have been studied in intact or fractured eggs by scanning electron microscopy. The internal surface of the enveloping blastomeres was smooth, while the external surface was covered with cristae; these cristae disappeared during epiboly but reappeared when the cells were in mitosis. At the end of the morula stage, the smooth yolk cytoplasmic layer formed a ring of periblastic cristae; up to the late blastula stage that ring was attached to "star-like" complexes. Light and transmission electron microscopy images of these complexes are given and we propose hypotheses regarding their function. At first closely apposed, the deep cells seemed to be maximally dispersed during the mid-blastula stage. At the blastula stage, the yolk syncitium formed both enveloping and deep cells. This suggests a primordial role of the yolk syncitium layer in pregastrula differentiation. A new hypothesis concerning the functions of the membrane protrusions is proposed: they might play a role in the postmiotic reassociation of the blastomeres. We suggest that, during early development, the three modes of cell association, i.e. monolayer, mass aggregation, syncitium, might be correlated with the composition of the three media (perivitelline liquid, blastocoelic liquid, yolk) present at that time.