Die Epiphyse und die circumventrikul�ren Strukturen des Epithalamus im Gehirn des Elefanten (Loxodonta africana)

Functional or Vestigial? The Genomics of the Pineal Gland in Xenarthra

Vestigial organs are historical echoes of past phenotypes. Determining whether a specific organ constitutes a functional or vestigial structure can be a challenging task, given that distinct levels of atrophy may arise between and within lineages. The mammalian pineal gland, an endocrine organ involved in melatonin biorhythmicity, represents a classic example, often yielding contradicting anatomical observations. In Xenarthra (sloths, anteaters, and armadillos), a peculiar mammalian order, the presence of a distinct pineal organ was clearly observed in some species (i.e., Linnaeus's two-toed sloth), but undetected in other closely related species (i.e., brown-throated sloth). In the nine-banded armadillo, contradicting evidence supports either functional or vestigial scenarios. Thus, to untangle the physiological status of the pineal gland in Xenarthra, we used a genomic approach to investigate the evolution of the gene hub responsible for melatonin synthesis and signaling. We show that both synthesis and signaling compartments are eroded and were probably lost independently among Xenarthra orders. Additionally, by expanding our analysis to 157 mammal genomes, we offer a comprehensive view showing that species with very distinctive habitats and lifestyles have convergently evolved a similar phenotype: Cetacea, Pholidota, Dermoptera, Sirenia, and Xenarthra. Our findings suggest that the recurrent inactivation of melatonin genes correlates with pineal atrophy and endorses the use of genomic analyses to ascertain the physiological status of suspected vestigial structures.

Evidence of melatonin secretion in cetaceans: plasma concentration and extrapineal HIOMT-like presence in the bottlenose dolphin Tursiops truncatus

The pineal gland is generally believed to be absent in cetaceans, although few and subsequently unconfirmed reports described the organ in some species. The recent description of a complete and photographed pineal body in a bottlenose dolphin (Tursiops truncatus) prompted us to examine a series of 29 brains of the same species, but no gland was found. We then decided to investigate if the main product of the gland, melatonin, was nevertheless produced and present in the plasma of this species. We collected plasma and serum samples from a series of captive bottlenose dolphins for a period of 7 months spanning from winter to summer and we determined the indoleamine concentration by radio-immunoassay (RIA). The results demonstrated for the first time a quantitative assessment of melatonin production in the blood of a cetacean. Melatonin levels were comparable to those of terrestrial mammals (5.15-27.74 pg/ml daylight concentration), with indications of both seasonal and daily variation although the presence of a circadian rhythm remains uncertain. Immunohistochemical analyses using as a marker hydroxyindole-O-methyl-transferase (HIOMT, the key enzyme involved in the biosynthesis of the hormone), suggested extrapineal melatonin production by the retina, the Harderian gland and the gut. The enzyme was unequivocally localized in all the three tissues, and, specifically, ganglion cells in the retina showed a very strong HIOMT-immunoreactivity. Our results suggest that further research might reveal unexplored aspects of melatonin production in cetaceans and deserves special attention and further efforts.

Elephant brain

We report morphological data on brains of four African, Loxodonta africana, and three Asian elephants, Elephas maximus, and compare findings to literature. Brains exhibit a gyral pattern more complex and with more numerous gyri than in primates, humans included, and in carnivores, but less complex than in cetaceans. Cerebral frontal, parietal, temporal, limbic, and insular lobes are well developed, whereas the occipital lobe is relatively small. The insula is not as opercularized as in man. The temporal lobe is disproportionately large and expands laterally. Humans and elephants have three parallel temporal gyri: superior, middle, and inferior. Hippocampal sizes in elephants and humans are comparable, but proportionally smaller in elephant. A possible carotid rete was observed at the base of the brain. Brain size appears to be related to body size, ecology, sociality, and longevity. Elephant adult brain averages 4783 g, the largest among living and extinct terrestrial mammals; elephant neonate brain averages 50% of its adult brain weight (25% in humans). Cerebellar weight averages 18.6% of brain (1.8 times larger than in humans). During evolution, encephalization quotient has increased by 10-fold (0.2 for extinct Moeritherium, approximately 2.0 for extant elephants). We present 20 figures of the elephant brain, 16 of which contain new material. Similarities between human and elephant brains could be due to convergent evolution; both display mosaic characters and are highly derived mammals. Humans and elephants use and make tools and show a range of complex learning skills and behaviors. In elephants, the large amount of cerebral cortex, especially in the temporal lobe, and the well-developed olfactory system, structures associated with complex learning and behavioral functions in humans, may provide the substrate for such complex skills and behavior.

An overview of the central nervous system of the elephant through a critical appraisal of the literature published in the XIX and XX centuries

The two species of elephants (Indian: Elephas maximus and African: Loxodonta africana) possess the largest brain among land mammals. Due to its size, the elephant brain is discussed in virtually every paper dealing with the evolution of the central nervous system of mammals and comparative brain size. Studies on the social habits of elephants also deal with the skills and the "intelligence" and brain size of these species. Yet most of the descriptions and conclusions reported in comparative studies rely on second-hand data derived from investigations performed several decades before, often dating as far back as the XIX century. Furthermore, many of the original papers actually describing gross and detailed features of the brain of elephants are either no longer available, are written in languages other than English, or are difficult to trace. The present study gives a short description of the anatomy of the central nervous system of elephants, with special attention to its distinctive features, reports all available literature on the subject, and briefly discusses its origins and rationale.

The ultrastructure of mammalian pinealocytes: a systematic investigation

Pinealocytes are not only the principal cellular components of the pineal gland, but they are also the principal synthetic machinery of this enigmatical gland with highly diverse and often questionable empyreal roles assigned to it. Ultrastructural descriptions of pinealocytes belonging to some 70 species of mammals (a mere 2% or less of the over 4,200 mammalian species) have been summarized from the available literature with new observations on 12 species of chiropterans. Space limitation precluded any treatment of the supporting glia, neural elements, and the perivascular spaces. A detailed table lists nearly all mammalian species whose pineal ultrastructure has been investigated. Blanks in this table point to the necessity of studies on those particular groups. A tabular listing of unusual structures reported within the pinealocyte cytoplasm points out the impending experimental work on these species. Such studies using the latest techniques might provide clearer insights into the functional role of the pineal gland as an important and integral component of the neuroendocrine axis. Whereas sufficient structural information now exists on cytoplasmic organelles such as synaptic ribbons and spherules, annulate lamellae, subsurface cisterns, and the several types of synaptic arrangements seen in relation to the pinealocyte soma and its processes, the functional role of these structures in pineal synthetic processes remains to be elucidated.