Narrow Results

The respiratory system of mammalians is made of two successive branched structures with different physiological functions. The upper structure, or bronchial tree, is a fluid transportation system made of approximately 15 generations of bifurcations leading to the order of 2¹⁵ = 30,000 bronchioles with a diameter of order 0.5 mm in the human lung.¹ The branching pattern continues up to generation 23 but the structure and function of each of the subsequent structures, called the acini, is different. Each acinus is made of a branched system of ducts surrounded by alveolae and play the role of a diffusion cell where oxygen and carbon dioxide are exchanged with blood across the alveolar membrane.² We show in this paper that the bronchial tree presents simultaneously several optimal properties of totally different nature. It is first energy efficient;^3-6 second, it is space filling;⁷ and third it is "rapid" as discussed here. It is this multi-optimality that is qualified here as magic. The multi-optimality physical characteristic suggests that, in the course of evolution, an organ selected against one criterion could have been later used for a totally different reason. For example, once energetic efficiency for the transport of a viscous fluid like blood has been selected, the same genetic material could have been used for its optimized rapidity. This would have allowed the emergence of mammalian respiration made of inspiration–expiration cycles. For this phenomenon to exist, the rapid character is essential, as fresh air has to reach the gas exchange organs, the pulmonary acini, before the start of expiration.

Control of Reproduction in Mammals and Plants: Common Features Beyond Genes.

Improving the Nutritional Value of Crops by Genetic Modification: Problems and Opportunities Illustrated by Vitamin C.

Is Agricultural Biotechnology Worth the Risk?

Production of energy is a foundation of life. The metabolic rate of organisms (amount of energy produced per unit time) generally increases slower than organisms’ mass, which has important implications for life organization. This phenomenon, when considered across different taxa, is called interspecific allometric scaling. Its origin has puzzled scientists for many decades, and still is considered unknown. In this paper, we posit that natural selection, as determined by evolutionary pressures, leads to distribution of resources, and accordingly energy, within a food chain, which is optimal from the perspective of stability of the food chain, when each species has sufficient amount of resources for continuous reproduction, but not too much to jeopardize existence of other species. Metabolic allometric scaling (MAS) is then a quantitative representation of this optimal distribution. Taking locomotion and the primary mechanism for distribution of energy, we developed a biomechanical model to find energy expenditures, considering limb length, skeleton mass and speed. Using the interspecific allometric exponents for these three measures and substituting them into the locomotion-derived model for energy expenditure, we calculated allometric exponents for mammals, reptiles, fish, and birds, and compared these values with allometric exponents derived from experimental observations. The calculated allometric exponents were nearly identical to experimentally observed exponents for mammals, and very close for fish, reptiles and the basal metabolic rate (BMR) of birds. The main result of the study is that the MAS is a function of a mechanism of optimal energy distribution between the species of a food chain. This optimized sharing of common resources provides stability of a food chain for a given habitat and is guided by evolutionary pressures and natural selection.

To unravel the cellular/molecular mechanisms underlying the development of mammalian brain, we have improved the manipulation techniques of embryos/brain tissues whereby various gene expression vectors are easily transferred into the developing nervous system. These techniques allow us to perform time- and region-specific manipulations of candidate genes and are necessary for a better understanding of the roles of such genes in the developing cerebral cortex.

The vertebrate eye is derived from several types of embryonic tissues such as the neuroectoderm, surface ectoderm and mesenchyme. Previous studies have shown that most of the mesenchymal tissue in the eye is derived from neural crest in the avian embryos. In mammals, however, contribution of the neural crest cells to the eye structures has not been clarified. Here we have examined this issue by using reporter mouse lines in which neural crest cells express LacZ or EGFP specifically. The results show that neural crest-derived cells (NCDCs) gave rise to the formation of various ocular tissues as previously reported in avian eyes. Furthermore, the results indicate that midbrain-derived neural crest cells dominantly contribute to the eye structure, especially to the vitreous tissues.