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October, 2006
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Sexual Maturation: Achievement of full sexual capacity in animals and in humans.

Life Histories

JoVE 10941

Constrained by limited energy and resources, organisms must compromise between offspring quantity and parental investment. This trade-off is represented by two primary reproductive strategies; K-strategists produce few offspring but provide substantial parental support, whereas r-strategists produce much progeny that receives little care. These strategies are related to an organism’s survival likelihood across its lifespan, which is represented by a survivorship curve. Three general types of survivorship curves are exhibited by organisms that: tend to live long lives (Type I, K-strategists); are equally likely to die at all ages (Type II); or have high early mortality rates, but long lifespans if they survive into adulthood (Type III, r-strategists). An organism’s life history includes all the events occurring across its lifespan, including birth, development, sexual maturation, reproduction, and death. Trade-offs involving the patterns and timing of life history events (notably survival and reproduction) across different ages and developmental stages represent different life history strategies. R-strategists and K-strategists make distinct reproductive compromises between the number of offspring and level of parental care, or offspring quantity versus quality. R-strategists (r

 Core: Biology


JoVE 10906

In human women, oogenesis produces one mature egg cell or ovum for every precursor cell that enters meiosis. This process differs in two unique ways from the equivalent procedure of spermatogenesis in males. First, meiotic divisions during oogenesis are asymmetric, meaning that a large oocyte (containing most of the cytoplasm) and minor polar body are produced as a result of meiosis I, and again following meiosis II. Since only oocytes will go on to form embryos if fertilized, this unequal distribution of cell contents ensures that there are enough cytoplasm and nutrients to nourish the early stages of development. Second, during oogenesis, meiosis “arrests” at two distinct points: once during embryonic growth and a second time during puberty. In mammals, oocytes are suspended in prophase I until sexual maturation, at which point meiosis I continues under hormonal influence until an egg precursor cell is released into a fallopian tube. At ovulation, the precursor exits the ovary and, only if fertilization occurs, is stimulated to complete meiosis II and form a complete egg. Defects during oogenesis can result in severe consequences. In particular, problems with chromosome segregation during either meiosis I or meiosis II may lead to an embryo being aneuploid, meaning that it contains an abnormal number of chromosomes. Increased age elevates a woman

 Core: Biology

Reproductive Techniques for Ovarian Monitoring and Control in Amphibians

1Reproductive Sciences, San Diego Zoo Institute for Conservation Research, 2Recovery Ecology, San Diego Zoo Institute for Conservation Research, 3Menagerie du Jardin des Plantes, Museum National d'Histoire Naturelle, 4Cincinnati Zoo & Botanical Garden, Center for Conservation and Research of Endangered Wildlife

JoVE 58675


Transforming, Genome Editing and Phenotyping the Nitrogen-fixing Tropical Cannabaceae Tree Parasponia andersonii

1Laboratory of Molecular Biology, Department of Plant Sciences, Wageningen University & Research, 2Center of Technology for Agricultural Production, Agency for the Assessment and Application of Technology (BPPT), 3Department of Ecological Science, Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, 4Beijing Advanced Innovation Center for Tree Breeding by Molecular Design, Beijing University of Agriculture

JoVE 59971

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