19.1 Development is a regulated process.
Overview of Development
• Organisms in all three multicellular kingdoms utilize gene expression (i.e., different cells express different genes at different times) to achieve cell specialization. (p. 382)
Vertebrate Development
• At different sites in the vertebrate embryo, particular cells proceed to form the body's tissues and organs, and then the body grows to a size and shape that will allow survival after birth. (p. 383)
• Vertebrate zygotes divide rapidly to form a blastula. Subsequent stages in vertebrate development include gastrulation, neurulation, cell migration, organogenesis, and growth. (pp. 383-385)
Insect Development
• Many insects go through metamorphosis from a larva to a fully functioning, reproductive adult. (p. 386)
• The development of Drosophila melanogaster proceeds from an egg to a syncytial blastoderm, to larval instars, which have imaginal discs, and finally through metamorphosis to an adult. (pp. 386-387)
Plant Development
• Plant cells are encased in cellulose walls. Instead of using cell migration, plants develop by building their bodies outward, creating new parts from meristems. (p. 388)
• Plants cope with environmental change by adjusting the type, number, size, and location of its structures to accommodate local circumstances. (p. 388)
• Three basic tissue types of plants are epidermal cells, ground tissue, and vascular tissue. (p. 388)
• Seeds germinate in response to environmental changes due to water, temperature, and other factors. (p. 388)
Nematode Development
• Nematodes develop a known number of somatic cells from a single fertilized egg in a complex series of cell divisions. (p. 390)
19.2 Multicellular organisms employ the same basic mechanisms of development.
Cell Movement and Induction
• Cell movement is largely a matter of changing patterns of cell adhesion. As a migrating cell travels, it continuously extends projections that probe the environment, and thus the cell feels its way toward a target. (p. 391)
• In some cases, particular groups of cells called organizers produce diffusible signal molecules that convey positional information to other cells. (p. 392)
• Induction occurs when a cell switches from one developmental path to another as a result of interactions with an adjacent cell. (p. 392)
Determination
• Totipotent cells are potentially capable of expressing all the genes of their genome. (p. 393)
• Determination is the commitment of a particular cell to a specialized developmental path. (p. 393)
• Recent experiments have shown that determination is reversible. (p. 394)
Pattern Formation
• All animals appear to use positional information to determine the basic pattern of body compartments and the overall body architecture. (p. 395)
Homeotic Genes
• Homeotic genes encode transcription factors that activate blocks of genes specifying particular body parts. (p. 399)
Programmed Cell Death
• Necrosis is cell death due to injury-caused swelling and bursting, while apoptosis refers to programmed cell death. (p. 400)
• The mechanism of apoptosis appears to have been highly conserved during the course of animal evolution. (p. 400)
19.3 Aging can be considered a developmental process.
Theories of Aging
• Several theories of aging that are being intensively investigated include the accumulated mutation hypothesis, the telomere depletion hypothesis, the wear-and-tear hypothesis, the gene clock hypothesis, and trade-offs between body size and life span, and between fecundity and life span. (p. 402)