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Organogenesis involves using the basic body plan (organized embryo) to develop specific organs (limbs, heart, eyes) in specific regions that leads to the fully functional organism, capable of independent survival.1How are these processes involved?Pattern formation - directs cell identity and leads to ordered spatial pattern of cell activityPositional information -directs where organs will formInduction -direction of specific cell fateMorphogenesis -changing the form of cellsDifferentiation -acquire functional and structural identity distinct from their surrounding cells2Embryonic Germ LayersThe three layers produced by gastrulation are called embryonic germ layersThe ectoderm forms the outermost layer of the gastrulanervous system and epidermisThe endoderm forms the innermost layer of the gastrulainner linings of the digestive tract and other systemsThe mesoderm is the middle layermuscle, skeleton, circulatory, reproductive, excretory, connective tissues3OrganogenesisVarious regions of the three embryonic germ layersDevelop into the rudiments of organs during the process of organogenesis

EyeForebrainHeartBlood vesselsNeural tubeLate organogenesis. Rudiments of most major organs have already formed in this chick embryo, which is about 56 hours old and about 23 mm long (LM).4Organogenesis and the 3 Germ LayersMany different structuresAre derived from the three embryonic germ layers during organogenesis

ECTODERMMESODERMENDODERM Epidermis of skin and itsderivatives (including sweatglands, hair follicles) Epithelial lining of mouthand rectum Sense receptors inepidermis Cornea and lens of eye Nervous system Adrenal medulla Tooth enamel Epithelium or pineal andpituitary glands Notochord Skeletal system Muscular system Muscular layer of stomach, intestine, etc. Excretory system Circulatory and lymphaticsystems Reproductive system(except germ cells) Dermis of skin Lining of body cavity Adrenal cortex Epithelial lining ofdigestive tract Epithelial lining ofrespiratory system Lining of urethra, urinarybladder, and reproductivesystem Liver Pancreas Thymus Thyroid and parathyroidglands5MorphogenesisMorphogenesis in animals involves specific changes in cell shape, position, and adhesionMorphogenesis is a major aspect of development in both plants and animals, but only in animals does it involve the movement of cellsThe cytoskeleton drives changes in the shape of the cell and cell migration, or cell crawlingFibers of the extracellular matrix may function as tracks, directing migrating cells along particular routes6Cell DifferentiationTwo general principles underlie differentiation during embryonic developmentFirst, during early cleavage divisionsEmbryonic cells must somehow become different from one anotherSecond, once initial cell asymmetries are set upInteractions among the embryonic cells influence their fate, usually by causing changes in gene expressionThis process is termed induction7Sources of developmental information for the early embryo

Cytoplasmic determinants in the egg. The unfertilized egg cell has molecules in its cytoplasm, encoded by the mothers genes, that influence development. Many of these cytoplasmic determinants, like the two shown here, are unevenly distributed in the egg. After fertilization and mitotic division, the cell nuclei of the embryo are exposed to different sets of cytoplasmic determinants and, as a result, express different genes. (a)Induction by nearby cells. The cells at the bottom of the early embryo depicted here are releasing chemicals that signal nearby cells to change their gene expression.(b)8Restriction of PotencyIn many species that have cytoplasmic determinantsOnly the zygote is totipotent, capable of developing into all the cell types found in the adult9OrganogenesisOrganogenesis is the process by which parts of the 3 germ layers develop the rudiments of organs. Organogenesis involves localized morphogenic changes in tissue and cell shape versus the large scale mass movement of cells seen in gastrulation.

10OrganogenesisEvidence of organogenesis is seen in the appearance of folds splits and dense clustering of cells.Easy observations are made in chordates when the notochord, neural tube, etc. take form.

11OrganogenesisThe notochord is formed from dorsal mesoderm which condenses just above the archenteron.travismulthaupt.com

12OrganogenesisSignals sent from the notochord to the ectoderm cause the region of the ectoderm to form the neural plate.The neural plate will curve inward forming the neural tube which runs along the anterior-posterior axis of the embryo eventually forming the CNS.13

OrganogenesisIn vertebrates, the neural crest forms along the border where the neural tube pinches off from the ectoderm.Neural crest cells then migrate to various parts of the embryo giving rise to many structures such as peripheral nerves, teeth, skull bones, etc.15

OrganogenesisLateral to the notochord are condensations of mesodermal cells arranged in the block called somites.The somites give rise to cells that migrate to new locations forming vertebrae and muscles.

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Organogenesis is the formation of the organs.

The layers are germ layers; they have specific fates in the developing embryo.Organogenesis18Imaginal disc and wing development19During the event of metamorphoses, organisms need to develop new tissues which were not present during larval phase. Unlike metamorphoses observed in Amphibians where remodeling of existing tissues is observed, insect metamorphosis often involves the destruction of larval tissues by apoptosis and their subsequent replacement by an entirely different population of cells.

20There are two different population of cells exists in larvae of an insect : the cells which are used during larval phase of insects ,we can call them larval cells and Group of imaginal cells which are set aside during larval phase and wait for the signal to differentiate into adult structures. Imaginal discs dont contribute significantly to the larval life but later after metamorphosis contributes to part or complete adult appendage.

22The cells that give rise to the new epidermal tissues of the adult are termed as histoblasts and if histoblasts are organized into morphologically distinct clusters, these structures are called as imaginal discs.23It is the ability of a cell to contribute to the formation of adult structure makes it a histoblast and separates from rest of the neighboring cells in larval epidermis.The timing of appearance of the imaginal discs varies among different types of insects . Generally imaginal discs are observed during larval phase but in some taxa early imaginal discs can be traced in embryos itself. A typical imaginal disc is a bag of cells that has invaginated from the larval epidermis and is destined to form part or all of an adult appendage ( wings, legs, haltere, antenna etc), compound eye and genitalia.24Histoblasts or imaginal cells contribute to the entire structure of adult insect. The precursor cells of the abdomen and the internal organs of the adult, such as the gut, salivary glands and brain, arise from nests or rings of cells intimately associated with larval structures. Eg: the salivary gland imaginal rings are embedded in the larval salivary glands. Each segment of the adult abdomen is formed from four pairs of small histoblast nests.

25Like many other aspects, majority of information regarding Imaginal discs has come from the study in Drosophila .There are ten pairs of imaginal discs, which construct many of the adult organs, and an unpaired genital disc , making total 21 imaginal discs (if you count eye and antennal discs as separate, which is fused imaginal disc or else the count will be 19 ) in Drosophila larvae.26In D. melanogaster the imaginal disc primordia are formed during embryonic development, rather than during the last instar as they are in some other Holometabola. Each imaginal disc primordium contains 10 to 40 cells, which divide during the three instars to form as many as 60,000 cells by late third instar.27During the pupal stage, many larval structures are broken down, and adult structures, including the discs, undergo rapid development. Each disc everts and elongates, with the central portion of the disc becoming the distal part of whichever appendage, wing, leg, antenna, etc., it is forming. During the larval stage, the cells in the growing disc appear undifferentiated, but their developmental fate in the adult is already determined.

28The study of imaginal discs in the fruit fly Drosophila melanogaster led to the discovery of homeotic mutations such as antennapedia, where the developmental fate of a disc could sometimes change.It is of great interest that the kinds of developmental switches that occur are very specific, leg to antenna for instance. Study of this phenomenon led to the discovery of the homeobox genes, and started a revolution in the understanding of development in multi-celled animals that is still underway.

29Insulin and ecdysone are the key extrinsic regulators of growth for the wing imaginal disks of insects. In vitro tissue culture studies have shown that these two growth regulators act synergistically: either factor alone stimulates only limited growth, but together they stimulate disks to grow at a rate identical to that observed in situ.30It is generally thought that insulin signaling links growth to nutrition, and that starvation stops growth because it inhibits insulin secretion. At the end of larval life feeding stops but the disks continue to grow, so at that time disk growth has become uncoupled from nutrition. 31In insects, as in other organisms, nutrition is necessary for normal growth. In vitro studies have shown that nutrition does not act directly on cells but typically exerts its effect via hormonal signals such as insulin-like peptides and ecdysteroids . When insect larvae enter metamorphosis feeding stops and somatic growth ceases, but the imaginal disks continue growing at their normal rate. Evidently growth of the imaginal disks becomes uncoupled from nutrition at some time during the last larval instar. 32In Drosophila, ablation of the insulin producing cells in the brain leads to reduced larval growth and small adult flies . Mutations in the insulin receptor or the insulin receptor substrate likewise result in a reduction of body size .Over-expression of the insulin-like peptides during larval development results in large but normally proportioned adult flies.3334