Organogenesis PDF

# ORGANOGENESIS ## ORGANOGENESIS - The mechanisms of organogenesis in invertebrates are similar, but the body plan is very different. - For example, the neural tube develops along the ventral side of the embryo in invertebrates, rather than dorsally as occurs in vertebrates. ## MORPHOGENESIS - Morph...

# ORGANOGENESIS ## ORGANOGENESIS - The mechanisms of organogenesis in invertebrates are similar, but the body plan is very different. - For example, the neural tube develops along the ventral side of the embryo in invertebrates, rather than dorsally as occurs in vertebrates. ## MORPHOGENESIS - Morphogenesis in animals but not plants involves movement of cells ### Mechanisms of Morphogenesis - Reorganization of the cytoskeleton is a major force in changing cell shape during development - For example, in neurulation, microtubules oriented from dorsal to ventral in a sheet of ectodermal cells help lengthen the cells along that axis ## Organogenesis in the chick is quite similar to that in the frog. Below is an image with the following labels: - Neural tube - Notochord - Archenteron - Lateral fold - Somite - Coelom - Endoderm - Mesoderm - Ectoderm - Yolk stalk - Yolk sac - YOLK The image shows all of these labels pointing to their respective parts of an early organogenesis in the chick. ## Organogenesis in a chick embryo An image that shows several parts of a chick embryo after late organogenesis. - Eye - Forebrain - Heart - Blood vessels - Somites - Neural tube ## Change in cell shape during morphogenesis Shows an image with the following labels: - Ectoderm - Neural plate - Microtubules - Actin filaments - Neural tube The image shows several stages in the development from an ectoderm to a Neural tube. ## Convergent extension of a sheet of cells - The cytoskeleton promotes elongation of the archenteron in the sea urchin embryo - This is convergent extension, the rearrangement of cells of a tissue that cause it to become narrower (converge) and longer (extend) - Convergent extension occurs in other developmental processes - The cytoskeleton also directs cell migration ## Programmed Cell Death - Programmed cell death is also called **apoptosis** - At various times during development, individual cells, sets of cells, or whole tissues stop developing and are engulfed by neighboring cells. - For example, many more neurons are produced in developing embryos than will be needed. - Extra neurons are removed by apoptosis. ## Cytoplasmic determinants and inductive signals contribute to cell fate specification - **Determination** is the term used to describe the process by which a cell or group of cells becomes committed to a particular fate. - **Differentiation** refers to resulting specialization in structure and function. ## Cells in a multicellular organism share the same genome - Differences in cell types are the result of the expression of different sets of genes ## Fate Mapping - **Fate maps** are diagrams showing organs and other structures that arise from each region of an embryo - Classic studies using frogs indicated that cell lineage in germ layers is traceable to blastula cells ## Fate Map of a frog embryo - The image contains an embryo with a blastula section labelled with the following: - Epidermis - Central nervous system - Notochord - Mesoderm - Endoderm - The image also contains a neural tube stage section labelled with the following: - Epidermis - Neural tube ## Cell lineage analysis in a tunicate An image of a tunicate that shows the lineage from a 64-cell embryo to larvae. - 64-cell embryos - Blastomeres injected with dye - Larvae ## Later studies of Caenorhabditis elegans used the ablation (destruction) of single cells to determine the structures that normally arise from each cell. - The researchers were able to determine the lineage of each of the 959 somatic cells in the worm. ## Germ Cells - Germ cells are the specialized cells that give rise to sperm of eggs. - Complexes of RNA and protein are involved in the specification of germ cell fate. - In C. elegans, such complexes are called P granules, persist throughout development, and can be detected in germ cells of the adult worm. ## Determination of germ cell fate in C. elegans - P granules are distributed throughout the newly fertilized egg and move to the posterior end before the first cleavage division. - With each subsequent cleavage, the P granules are partitioned into the posterior-most cells. - P granules act as cytoplasmic determinants, fixing germ cell fate at the earliest stage of development. ## Partitioning of P granules during C. elegans development - This image shows a series of four images of a developing embryo with the following labels: - 20 μm - Newly fertilized egg - Zygote prior to first division - Two-cell embryo - Four-cell embryo ## Axis Formation - A body plan with bilateral symmetry is found across a range of animals. - This body plan exhibits asymmetry across the dorsal-ventral and anterior-posterior axes. - The right-left axis is largely symmetrical. ## Axis Formation - The anterior-posterior axis of the frog embryo is determined during oogenesis. - The animal-vegetal asymmetry indicates where the anterior-posterior axis forms. - The dorsal-ventral axis is not determined until fertilization. - Upon fusion of the egg and sperm, the egg surface rotates with respect to the inner cytoplasm. - This cortical rotation brings molecules from one area of the inner cytoplasm of the animal hemisphere to interact with molecules in the vegetal cortex. - This leads to expression of dorsal- and ventral-specific gene expression. ## The body axes and their establishment in an amphibian Shows an image of an amphibian embryo with the following labels: - Anterior - Dorsal - Right - Left - Ventral - Posterior - Animal pole - Animal hemisphere - Point of sperm nucleus entry - Pigmented cortex - Future dorsal side - Vegetal hemisphere - Vegetal pole - Gray crescent - First cleavage ## Axis Formation - In chicks, gravity is involved in establishing the anterior-posterior axis. - Later, pH differences between the two sides of the blastoderm establish the dorsal-ventral axis. - In mammals, experiments suggest that orientation of the egg and sperm nuclei before fusion may help establish embryonic axes. ## Restricting Developmental Potential - Hans Spemann performed experiments to determine a cell's developmental potential (range of structures to which it can give rise). - Embryonic fates are affected by distribution of determinants and the pattern of cleavage. - The first two blastomeres of the frog embryo are totipotent (can develop into all the possible cell types). ## Inquiry: How does distribution of the gray crescent affect the developmental potential of the first two daughter cells? - An image of a frog embryo at different stages is shown. - It focuses on the effects of distributing the "gray crescent" to different sides of an embryo. - The image contains the following labels: - Experiment - Control egg (dorsal view) - Gray crescent - Control group - Experimental group - Experimental egg (side view) - Gray crescent - Thread - Results - Normal - Bellypiece - Normal ## Restricting Developmental Potential - In mammals, embryonic cells remain totipotent until the 8-cell stage, much longer than other organisms. - Progressive restriction of developmental potential is a general feature of development in all animals. - In general tissue-specific fates of cells are fixed by the late gastrula stage. ## Cell Fate Determination and Pattern Formation by Inductive Signals - As embryonic cells acquire distinct fates, they influence each other's fates by induction. ## The "Organizer" of Spemann and Mangold - Spemann and Mangold transplanted tissues between early gastrulas and found that the transplanted dorsal lip triggered a second gastrulation in the host. - The dorsal lip functions as an organizer of the embryo body plan, inducing changes in surrounding tissues to form notochord, neural tube, and so on. ## Inquiry: Can the dorsal lip of the blastopore induce cells in another part of the amphibian embryo to change their developmental fate? ## The "Organizer" of Spemann and Mangold - An image shows the process of transplanting the dorsal lip of the blastopore into a new embryo. - The labels on the image are: - Experiment - Dorsal lip of blastopore - Pigmented gastrula (donor embryo) - Nonpigmented gastrula (recipient embryo) - Results - Primary embryo - Secondary (induced) embryo - Primary structures: - Neural tube - Notochord - Secondary structures: - Notochord (pigmented cells) - Neural tube (mostly nonpigmented cells) ## Formation of the Vertebrate Limb - Inductive signals play a major role in pattern formation, development of spatial organization. - The molecular cues that control pattern formation are called positional information. - This information tells a cell where it is with respect to the body axes. - It determines how the cell and its descendents respond to future molecular signals. - The wings and legs of chicks, like all vertebrate limbs, begin as bumps of issue called limb buds. ## Formation of the Vertebrate Limb - An image that shows a chick embryo with limb buds. - Labels: - Anterior - Limb bud - AER - ZPA - Limb buds - 50 μm - Posterior - A micrograph of the apical ectodermal ridge (AER) is shown for further details. ## Limb Development - The embryonic cells in a limb bud respond to positional information indicating location along three axes: - Proximal-distal axis - Anterior-posterior axis - Dorsal-ventral axis ## Limb Development - One limb bud-regulating region is the apical ectodermal ridge (AER). - The AER is thickened ectoderm at the bud's tip. - The second region is the zone of polarizing activity (ZPA). - The ZPA is mesodermal tissue under the ectoderm where the posterior side of the bud is attached to the body. - Tissue transplantation experiments support the hypothesis that the ZPA produces an inductive signal that conveys positional information indicating "posterior". ## Wing of chick embryo An infographic of a chick embryo showing the wing's development, with the following parts labelled: - Digits - Anterior - Ventral - Distal - Proximal - Dorsal - Posterior ## Inquiry: What role does the zone of polarizing activity (ZPA) play in limb pattern formation in vertebrates? - An image shows experiments conducted to observe the roles of the ZPA in limb pattern formation. - Image labels: - Experiment - Donor limb bud - ZPA - Anterior - Posterior - New ZPA - Host limb bud - Results - 4 - 3 - 2 - 2 - 4 - 3 ## Limb Development - Sonic hedgehog is an inductive signal for limb development. - Hox genes also play roles during limb pattern formation. ## Cilia and Cell Fate - Ciliary function is essential for proper specification of cell fate in the human embryo. - Motile cilia play roles in left-right specification. - Monocilia (nonmotile cilia) play roles in normal kidney development. ## Situs inversus, a reversal of normal left-right asymmetry in the chest and abdomen. - An image shows two human bodies comparing a typical configuration of the chest and abdomen with a situs inversus one. - Shows the location of the following organs: - Lungs - Heart - Liver - Spleen - Stomach - Large intestine