The Processes of Development
► A multicellular organism develops through a series of embryonic stages and eventually into an adult. Development continues until death. Review Figure 19.1. See Web/CD Activity 19.1
► Growth results from a combination of cell division and cell expansion.
► Differentiation produces specialized cell types.
► Morphogenesis—the creation of the overall form of the multicellular organism—is the result of pattern formation.
► In many organisms, the fates of early embryonic cells have not yet been determined. These cells may develop into different tissues if transplanted to a different part of an embryo. Review Figure 19.2
► As the embryo develops, its cells gradually become determined—committed to developing into particular cell types. Following determination, cells eventually differentiate into their final, often specialized, forms.
The Role of Differential Gene Expression in Cell Differentiation
► The zygote is totipotent; it contains the entire genetic constitution of the organism and is capable of forming all adult tissues.
► Two lines of evidence show that differentiation does not involve permanent changes in the genome. First, nuclear transplantation and cell fusion experiments show that the nucleus of a differentiated cell retains the ability to act like a zygote nucleus and direct the production of an entire organism. Second, molecular investigations have shown directly that all cells contain all genes for the organism, but that only certain genes are expressed in a given tissue. Review Figures 19.3, 19.4, 19.5
► Embryonic stem cells are totipotent, and they can be cultured in the laboratory. With suitable environmental stimulation, these cells can be induced to form cells that differentiate into a particular type. Review Figure 19.6
The Role of Cytoplasmic Segregation and Induction in Cell Determination
► Unequal distribution of cytoplasmic determinants in the egg, zygote, or embryo can lead to cell determination. Experimentally altering this distribution can alter gene expression and produce abnormal or nonfunctional organisms. Review Figures 19.7, 19.8. See Web/CD Tutorial 19.1
► Some embryonic animal tissues direct the development of their neighbors by secreting inducers.
► Induction is often reciprocal: One tissue induces a neighbor to change, and the neighbor, in turn, induces the first tissue to change, as in eye formation in vertebrate embryos. Review Figure 19.9
► Induction in the nematode Caenorhabditis elegans can be very precise, with individual cells producing specific effects in just two or three neighboring cells. Review Figure 19.10
The Role of Pattern Formation in Organ Development
► Apoptosis is important in pattern formation. Some genes whose protein products regulate apoptosis have been identified.
► Plants have organ identity genes that interact to cause the formation of sepals, petals, stamens, and carpels. Mutations of these genes may cause meristem cells to form a different organ. Review Figure 19.12
► Plant organ identity genes code for transcription factors of the MADS box family.
► Both plants and animals use positional information as a basis for pattern formation. Gradients of morphogens provide this information.
The Role of Differential Gene Expression in Establishing Body Segmentation
► The fruit fly Drosophila melanogaster has provided much information about the development of body segmentation.
► The first genes to act in determining Drosophila segmentation are maternal effect genes, such as bicoid and nanos, which encode morphogens that form gradients in the egg. These morphogens act on segmentation genes to define the anterior-posterior organization of the embryo. Review Figure 19.14
► There are three kinds of segmentation genes. Gap genes organize broad areas along the anterior-posterior axis, pair rule genes divide the axis into pairs of segments, and segment polarity genes define the anterior-posterior axis of each segment.
Review Figure 19.15. See Web/CD Tutorial 19.2
► Segmentation develops as the result of a transcriptionally controlled cascade, with the product of each gene promoting or repressing the expression of the next.
► Activation of the segmentation genes leads to the activation of the appropriate homeotic genes in each segment. The homeotic genes define the functional characteristics of the segments.
► Mutations of homeotic genes often have bizarre effects, causing structures to form in inappropriate parts of the body.
► Homeotic genes contain a sequence called the homeobox, which encodes the homeodomain, an amino acid sequence that is part of many transcription factors.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.