ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Seventeen Outline INTRODUCTION All Cells in a Multicellular Organism Descend From a Single Cell The Developmental Program Unfolds With Precision fig 17.1 DEVELOPMENT IS A REGULATED PROCESS Multicellular Cell Specialization Controlled Via Gene Expression ` In fungi only reproductive cells are specialized Plant development is flexible and influenced by the environment Animal development is rigidly controlled with less influence by environment fig 17.2 Vertebrate Development Dynamic series of stages of cell movement and formation of organs fig 17.3 Cleavage Zygote is the initial vertebrate being One cell divides rapidly forming blastomeres fig 17.4 Embryo stays same size, cell number increases, cell size decreases Cells at animal pole form external body tissues Cells at vegetal pole from internal tissues Formation of the blastula Outer blastomeres connected by tight junctions Cell mass effectively separated from environment At sixteen-cell stage cells at interior pump Na+ to outside Forms osmotic gradient in intercellular spaces Water moves from cells to enlarging intercellular spaces Spaces combine to form a cavity in cell mass 17.3b Resulting hollow ball of cells is the blastula Gastrulation Gastrula forms when wall of blastula at vegetal pole pushes inward fig 17.3c Cell extensions called lamellipodia help in cell movement Process called gastrulation, embryo becomes bilaterally symmetrical Embryo develops three germ layers Endoderm forms tube of primitive gut, most internal organs Outer cells are ectoderm form skin and nervous system Mesoderm forms notochord, bones, blood vessels, connective tissue, muscles Neurulation Presence of notochord triggers thickening of an ectodermal zone fig 17.3d Cells elongate, form wedge shape and roll into a tube Neural tube formed through this process of neurulation Cell migration Variety of cells migrate to form distant tissues fig 17.3e Neural crest pinches off from neural tube forms sense organs Somites migrate from central blocks of muscle forming skeletal muscles Receptor proteins of migrating cells interact with destination tissues to cease movement Organogenesis and growth Basic vertebrate plan established when body is only a few millimeters long Tissues develop into organs size increases enormously fig 17.3f Insect Development Insects possess two distinctly different body forms Changes from a tubular eating machine to a form with wings and legs Change in body form called metamorphosis Exemplified by the fruit fly, Drosophila fig 17.5,6 Maternal genes Construction of egg begins development before fertilization Nurse cells move their mRNA into end of egg nearest them fig 17.6a After divisions daughter cells contain different maternal products Action of maternal, not zygotic, genes controls initial development Syncytial blastoderm Nuclear divisions without cytokinesis produce syncytial blastoderm fig 17.6b Produce 400 nuclei within a single cytoplasm Nuclei communicate freely, but experience different maternal products Hollow ball formed as nuclei spread apart and grow intervening membranes Development similar to that of vertebrates follows Tubular body form called a larva Larval instars As larva feeds it grows, sheds its outer chitinous skin Drosophila produce three larval instar stages in four days fig 17.6c Imaginal disks A dozen groups of cells are set aside in the abdomen of the larva fig 17.6d Have no role in the larva, form key parts of the adult body Metamorphosis Hard shell forms around larva, now called pupa fig 17.6e Cells break down, release nutrients used by imaginal disks Disks associate with each other to assemble adult fly Metamorphosis of larva to pupa to adult takes four days Adult emerges from split pupal shell Plant Development Plant body is fundamentally tubular like an animal body Consists of pipes that draw water from roots and send food outward Share key developmental elements with animals Developmental mechanisms different between plants and animals Animal cells move, plant cells encased in immoveable stiff cellulose walls Plants develop by building bodies outward from meristems Dividing meristems produce cells that differentiate into tissues Animals and plants have different reactions to their environment Animals move away from unfavorable circumstances Plants endure environment, change developmental strategies Assemble body from few simple modules like leaves, roots Each module has rigid structure and organization Utilization of modules is flexible Plant develops, adds modules influenced by environment Adjusts path of its development to local circumstances fig 17.7 Early cell divisions First division off-center, one daughter cell is small, cytoplasm dense fig 17.7a Small cell becomes embryo, divides rapidly forming ball of cells Other daughter cell forms suspensor linking embryo to nutrient tissue Cells near suspensor form roots, opposite end becomes shoot Tissue formation Plant embryo differentiates into three germ layers Outermost cells become epidermal cells Bulk of interior becomes ground tissue Cells at core of embryo become vascular tissue No cell migration involved as with animals Seed formation First set of leaves called cotyledons Development arrested, embryo packaged into a seed fig 17.7c Seed allows for dispersal and survival in harsh conditions Germination Embryo resumes development with germination Roots grow downward, shoot upward fig 17.7d Meristematic development Apical meristems generate cells to make all components of adult plant fig 17.7e Other meristems produce wood and secondary growth (circumference) Meristematic activity influenced by hormones Hormones allow plant to adjust to its environment Morphogenesis Form of plant body determined by to events Plane in which cells divide Changes in cell shape due to osmotic expansion fig 17.7f Plant growth-regulating hormones affect morphogenesis Influence orientation of microtubules on interior of membrane Microtubules guide deposition of cellulose in cell wall Orientation of cellulose fibers determines elongation of cell as it grows BASIC MECHANISMS OF DEVELOPMENT Multicellular Organisms Develop According to Molecular Mechanisms Mechanisms evolved early in the history of life Six mechanisms are of particular importance Cell Movement Cells move via cell adhesion molecules like cadherins Span plasma membrane, protrude into cytoplasm, extend from cell surface Cytoplasmic portion attached to cytoskeleton actin or intermediate filaments Extracellular portion has five 100 amino acid segments with Ca++ sites Ca++ binding sites attach cadherin to other cells fig 17.8 Cadherin links to another of same type, joining cytoskeletons of two cells Helps sort cells with different cadherins into separate groups Cadherins associated with desmosomes are strongest Migrating cells traverse intercellular matrix via integrins fig 17.9 Matrix: protein linked polysaccharides with embedded fibrous proteins Integrins attach to cytoskeleton actin filaments Protruding integrins attach to fibrous portion of matrix Binding can also initiate cellular changes Induction Mosaic development Shown by Drosophila, as well as other animals and plants Initial cells created by cleavage contain determinant developmental signals Individual cells set off on different developmental paths Regulatory development Occurs in mammals All blastomeres receive equal sets of determinants Body form determined by cell-cell interactions Demonstration of the importance of cell-cell interactions Separate cells of early blastula and allow to develop Ones from animal pole develop characteristics of ectoderm Ones from vegetal pole develop characteristics of endoderm Neither develop characteristics of mesoderm Mesoderm cells develop only from animal pole cells that grow next to vegetal pole cells Induction: switching cell from one path of development to another fig 17.10 Inducing cells secrete growth factor proteins, serve as intercellular signals Signals produce abrupt changes in patterns of gene transcription Mesoderm example involves series of four signals Organizers produce signal molecules that convey positional information Inform surrounding cells of their distance from organizer If close, concentration of signal molecule is greater Signal molecules called morphogens fig 17.11 Same morphogen can have different effect at different concentrations fig 17.12 In Xenopus low level causes cells to become epidermis Slightly higher levels make cells into muscles Higher level causes cells to become notochord Determination Totipotent: cells capable of expressing all genes of genome As in all cells of mammalian egg up to eight-cell stage If cells separated, can all develop into normal individual Can do reverse, combine cells of eight cell stage into one individual Called a chimera fig 17.13 Contains cells from different genetic lines After eight-cell stage mammalian cells become different Due to cell-cell interactions Future developmental fate of cells becomes irreversible Determination: commitment to a particular developmental path Differentiation: cell specialization produced at end of developmental path Cell can be determined but not yet differentiated Molecular mechanism of determination Gene regulatory proteins control patterns of gene expression, initiate developmental changes When genes are activated they further reinforce their own activation When switch is thrown cell is fully committed to developmental path Partial commitment to development associated with positional labels Reflect cell's location in embryo Influence how pattern of body develops Example: chick embryo cell transplantation Leg cell (To become thigh) transplanted to wing tip Cell becomes leg tip (toe) rather than wing tip Cell committed to be leg, but not necessarily a particular part of leg Pattern Formation Use of positional labels in pattern formation in Drosophila Egg has initial asymmetry due to maternal mRNA deposited by nurse cells Maternal mRNA from bicoid gene marks embryo's anterior end mRNA translated into bicoid protein upon fertilization Diffuses through syncytial blastoderm, forming morphogen gradient Without bicoid protein no had or thorax develops, embryo is two-tailed Injection of protein causes embryo to be normal Effect of bicoid protein occurs by activating gap genes fig 17.14 gap genes map out subdivisions of embryo Hunchback and nanos genes establish thoracic and abdominal segments Pair-rule genes alter every other body segment into zones Segment polarity genes subdivide these zones Cascade of gene activity results in segmentation of fly's body plan Activation of genes depends on morphogen diffusion in syncytial blastoderm fig 17.15 Expression of Homoeotic Genes Homeotic genes determine the form each segment will take Code for proteins that function as transcription factors Activates a particular module of the genetic program producing body parts Mutations in Drosophila homeotic genes Bithorax: fly grows extra set of wings fig 17.16 Antennapedia: legs grow out of head instead of antennae Bithorax complex: affect body parts of thorax and abdomen Discovered by Lewis in 1950 Order of genes is order of body parts, as if genes are activated in order fig 17.17 Antennapedia complex Discovered by Kaufman in 1980 Governs anterior end, also serially activated Homeotic genes typically contain homeobox sequence of amino acids Codes for homeodomain: an amino acid DNA-binding peptide domain fig 17.18 Function as transcription factors, ensuring genes are transcribed at right time Distinguishes portion of genome devoted to pattern formation Homeotic genes also found in mice and humans Similar genes function in flowering plants Genes in mammals aligned in same order as segments they control fig 17.19 Ordered nature of homeotic gene clusters is highly conserved in evolution fig 17.20 Programmed Cell Death Many cells in are ultimately destined to die Examples: webbing between digits, vertebrate neurons Presence of cells and death required for proper development Necrosis Cell death due to injury Cell swells and bursts, contents released into extracellular spaces Apoptosis Planned cell death Cell shrinks, surrounding cells absorb remains fig 17.20 Animals all experience developmentally regulated suicide Example: nematode worm Same 131 cells die during development Controlled by three genes: ced-3, ced-4, ced-9 Example: human cells bax gene encode cell death program bcl-2 represses cell death program bcl-2 may prevent damage by destroying free radicals Antioxidant: molecule that destroys free radicals MODEL DEVELOPMENTAL SYSTEMS The Nematode Caenorhabditis elegans Tiny animal composed of 959 somatic cells Entire genome mapped, complete DNA sequencing in progress Organism is transparent, Migration of cells easy to follow Complete linage map determined for each cell and its divisions Each worm has exact same number of cells with identical program The Fruit Fly Drosophila melanogaster Key organisms to understand cellular mechanisms of development Examine how genes expressed early in development determine adult plan Imaginal disks float in larva, grow into adult body parts in pupa Characteristic segmentation of adult established early in development Chemical gradients create polarity that directs development Series of segmentation genes react to chemical gradient Two clusters of homeotic genes Anterior end = antennapedia complex; posterior end = bithorax complex Organization of genes corresponds to order of segments The Mouse Mus musculus Possess battery of homeotic HOX genes Closely related to homeotic genes of Drosophila Same genes seem to operate in same order Creation of chimeric mice Contain cells from two genetic lines Chimeric mice essentially have four parents The Flowering Plant Arabidopsis thaliana Small relative of the mustard plant Easy to grow and cross, has short generation time Able to self-fertilize Can produce thousands of offspring in two months Genome same size as C. elegans and Drosophila Library of genes clones available to researchers Numerous gene mutations altering pattern formation are known Mechanisms in early development similar to animals Development of organs parallels that of animals Possess similar sets of homeotic genes