Cellular circadian clocks in the pineal.

Ependym und Circumventriculäre Organe

The pineal has been identified as a major circadian pacemaker within the circadian system of a number of lower vertebrates although other pacemaking sites have been implicated as well. The rhythmic synthesis and secretion of the pineal hormone, melatonin, is suggested as the mechanism by which the pineal controls circadian oscillators located elsewhere. Both light and temperature cycles can entrain the pineal melatonin rhythm. The pineal, therefore, acts as a photo and thermoendocrine transducer which functions to synchronize internal cycle with cycles in the environment. A model is presented which portrays the pineal as a major component of a ‘multioscillator’ circadian system and which suggests how these multiple circadian clocks are coupled to each other and to cycles of light and temperature in the external world.

The pineal and melatonin: Regulators of circadian function in lower vertebrates

1 Introduction.- 1.1 Phylogeny.- 1.1.1 The Chelonian Bauplan.- 1.1.2 Fossil History and Evolution.- 1.1.3 Systematics and Biogeography.- 1.1.4 Body Size.- 1.2 Chelonian Reproductive Traits Compared with Other Reptiles.- 1.3 Anatomy of the Sexual Organs.- 1.3.1 Males.- 1.3.2 Females.- 1.4 Secondary Sexual Characteristics.- 2 Methods to Study Reproduction.- 2.1 Radiography.- 2.2 Ultra-sound Scanning.- 2.3 Laparoscopy and Biopsies.- 2.4 Blood Sampling.- 2.5 Considerations Regarding Captive Maintenance.- 3 Gonadal Cycles and Gamete Production.- 3.1 Sexual Maturity.- 3.2 Ovarian Cycle and Egg Production.- 3.2.1 Oogenesis and Folliculogenesis.- 3.2.2 Vitellogenesis.- 3.2.3 Oocyte Maturation and Ovulation.- 3.2.4 Luteal Phase.- 3.2.5 Oviducal Period.- 3.2.6 Regression and Follicular Atresia.- 3.3 Testicular Cycle and Sperm Production.- 3.3.1 Germinal Quiescence.- 3.3.2 Gonial Proliferation.- 3.3.3 Spermatocytogenesis.- 3.3.4 Spermiogenesis.- 3.3.5 Spermiation.- 3.3.6 Maturation and Storage of Sperm in the Epididymis.- 3.4 Accessory Secretions and Insemination.- 3.5 Sperm Storage in the Female Genital Tract.- 4 Reproductive Behaviour.- 4.1 Migration.- 4.2 Sexual Behaviour.- 4.2.1 Timing of Mating.- 4.2.2 Finding a Partner.- 4.2.3 Courtship and Copulation.- 4.2.4 Sexual Selection and Sexual Size Dimorphism.- 4.3 Nesting.- 4.3.1 Nesting Area and Selection of Nest Site.- 4.3.2 Conditions for Nesting.- 4.3.3 Preparation of Nest Site.- 4.3.4 Digging the Nest Hole.- 4.3.5 Oviposition.- 4.3.6 Covering the Nest.- 4.3.7 Nest Guarding.- 4.4 Emergence from the Nest.- 5 Reproductive Cycles and Environment.- 5.1 Temperate Zone.- 5.2 Tropical and Subtropical Zone.- 5.3 Multi-Year Reproductive Cycles.- 5.4 Unpredictable Habitats.- 5.5 Conclusions.- 6 Control of Reproduction.- 6.1 Exogenous Factors.- 6.1.1 Temperature and Photoperiod.- 6.1.2 Water Levels, Rain and Moisture.- 6.1.3 Food Availability.- 6.1.4 Social Factors and Population Density.- 6.2 Endogenous Timing Mechanisms.- 6.3 Effects of Stress on Reproduction.- 6.4 Conclusions.- 7 Hormones and Reproduction.- 7.1 Gonadal Steroids.- 7.1.1 Males.- 7.1.2 Females.- 7.2 Gonadotropins.- 7.3 Gonadotropin-Releasing Hormone.- 7.4 Pineal Hormones.- 7.5 Other Hormones.- 7.6 Hormones and Behaviour.- 7.7 Conclusions.- 8 Eggs and Embryonic Development.- 8.1 Egg Characteristics.- 8.1.1 Eggshell.- 8.1.2 Egg Contents.- 8.2 Development and Hatching.- 8.2.1 Development Rates and Incubation Times.- 8.2.2 Descriptive Embryology.- 8.2.3 Pipping and Hatching.- 8.3 Sex Determination.- 8.4 Effect of Incubation Environment on Neonates.- 9 Reproductive Patterns and Life History.- 9.1 Resource Allocation.- 9.1.1 Growth and Maturity.- 9.1.2 Egg Size, Clutch Size and Clutch Frequency.- 9.1.3 Reproductive Effort and Cost of Reproduction.- 9.1.4 Mortality, Longevity and Ageing.- 9.1.5 Somatic Repair.- 9.2 Population Structure and Dynamics.- 10 Threats, Reproduction and Conservation.- 10.1 Environmental Degradation.- 10.1.1 Case Study: Pseudemydura umbrina.- 10.2 Exploitation.- 10.2.1 Case Study: Erymnochelys madagascariensis.- 10.3 Conservation Strategies.- 10.3.1 Conservation of Wild Populations.- 10.3.2 Sustainable Use.- 10.3.3 Conservation in Captivity.- 10.3.4 Case Study: Pseudemydura umbrina.- 10.3.5 Case Study: Erymnochelys madagascariensis.- 10.4 Commercial Captive Breeding Operations.- 10.5 Risk Minimisation in Assessing Reproduction in Live Chelonians.- 10.5.1 Risks and Limitations of Currently Used Methods.- 10.5.2 Routine Screening of Reproductive Condition.- 11 Conclusions and Outlook.- References.- Systematic Index.

The reproductive biology of the Chelonia

The presence of melatonin is demonstrated in the pineal gland, the retina and the Harderian gland in some mammalian and non-mammalian vertebrates, using a specific fluorescence labelled antibody technique. Four different potent antibodies against melatonin have been used and compared. In the pineal gland of hamsters, mice, rats and snakes, specific fluorescence, mostly restricted to the cytoplasm of the cells, is detected in pinealocytes. Fluorescence is also detected in the pineal organ of fishes, tortoises and lizards, but it has not been possible, from cryostat sections of fresh tissue, to assert which kind of cell is reacting (photoreceptor cells or interstitial ependymal cells). In the retina, fluorescence is almost exclusively restricted to the outer nuclear layer. In the Harderian gland of mammals and reptiles, fluorescence is localized in the secretory cells of the alveoli and mostly restricted to the cytoplasm surrounding the nucleus. These results are discussed in relation to the concept of melatonin synthesis at extrapineal sites independent of pineal production.

Immunohistochemical evidence for the presence of melatonin in the pineal gland, the retina and the Harderian gland.

Circadian and circannual fluctuations of pineal indoleamines (serotonin and melatonin) in Testudo hermanni Gmelin (Reptilia, Chelonia). I. Under natural conditions of photoperiod and temperature.

J. Arendt

Papers

Circadian and circannual fluctuations of pineal indoleamines (serotonin and melatonin) in Testudo hermanni Gmelin (Reptilia, Chelonia). I. Under natural conditions of photoperiod and temperature.