Vitamin AVitamin A is a fat-soluble vitamin and an essential nutrient for animals. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinal (also known as retinaldehyde), retinoic acid, and several provitamin (precursor) carotenoids, most notably beta-carotene. Vitamin A has multiple functions: it is essential for embryo development and growth, for maintenance of the immune system, and for vision, where it combines with the protein opsin to form rhodopsin the light-absorbing molecule necessary for both low-light (scotopic vision) and color vision.
Germ cellA germ cell is any cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads. There, they undergo meiosis, followed by cellular differentiation into mature gametes, either eggs or sperm. Unlike animals, plants do not have germ cells designated in early development. Instead, germ cells can arise from somatic cells in the adult, such as the floral meristem of flowering plants.
Halley's methodIn numerical analysis, Halley's method is a root-finding algorithm used for functions of one real variable with a continuous second derivative. It is named after its inventor Edmond Halley. The algorithm is second in the class of Householder's methods, after Newton's method. Like the latter, it iteratively produces a sequence of approximations to the root; their rate of convergence to the root is cubic. Multidimensional versions of this method exist.
RetinolRetinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family that is found in food and used as a dietary supplement. Retinol or other forms of vitamin A are needed for vision, cellular development, maintenance of skin and mucous membranes, immune function and reproductive development. Dietary sources include fish, dairy products, and meat. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xerophthalmia. It is taken by mouth or by injection into a muscle.
GametogenesisGametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic division of diploid gametocytes into various gametes, or by mitosis. For example, plants produce gametes through mitosis in gametophytes. The gametophytes grow from haploid spores after sporic meiosis.
