Download Principles of Development Principles of Development Development

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Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Early Concepts: Preformation Versus Epigenesis n n
Preformation n Concept of a miniature n
CHAPTER
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Principles of Development adult being present in the sperm or egg, waiting to unfold n Some claimed they could n
see a miniature adult in the egg or sperm n A young animal merely n
unfolded the structures unfolded that are already there 8­2 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Early Concepts: Preformation Versus Epigenesis Early Concepts: Preformation Versus Epigenesis n Kaspar Friederich Wolff (1759) n
n Epigenesis n
n Concept that the fertilized egg contains n
n Demonstrate there was no preformed n
chick in the early egg n Undifferentiated granular material became n
arranged into layers n The layers thickened, thinned, and folded n
to produce the embryo 8­3 building materials only, somehow assembled by an unknown directing force n Although n
Although current ideas current ideas of development of development are essentially epigenetic in concept are essentially n Far more is known about what directs n
growth and differentiation 8­4 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Early Concepts: Preformation Versus Epigenesis Early Concepts: Preformation Versus Epigenesis
Early Concepts: Preformation Versus Epigenesis n Development n
n Series of n
Series of progressive progressive changes in an individual changes
from its beginning to from its beginning to maturity n Begins when a fertilized n
egg divides mitotically egg divides mitotically n Specialization occurs as a n
hierarchy of developmental “decisions” 8­5 n Cell types n
n Arise from conditions created in preceding stages n
n Interactions become increasingly n
restrictive n With each new stage n
n Each stage limits developmental fate n
n Cells n
Cells lose option to become something different lose option to become something different n Said to be n
Said to be determined determined n The n
The two basic processes two basic processes responsible for this progressive subsivision n Cytoplasmic localization n
n Induction n
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Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Fertilization Fertilization n Oocyte Maturation n
n Fertilization n
n Egg grows in size by accumulating yolk n
n Initial event n
Initial event in development in sexual in development in sexual n Contains much n
Contains much mRNA, ribosomes, mRNA, ribosomes, tRNA reproduction and elements for protein synthesis n Union of male and female gametes n
n Morphogenetic determinants n
Morphogenetic determinants direct the direct the activation and repression of specific genes later in postfertilization development n Egg nucleus n
Egg nucleus grows in size, bloated with grows in size, bloated with RNA n Provides for n
Provides for recombination recombination of paternal and maternal genes n Restores the diploid number n
n Activates n
Activates the egg to begin development n Now called the n
Now called the germinal vesicle germinal vesicle 8­7 8­8 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Fertilization Fertilization n Most of these preparations in the egg n Fertilization and Activation n
n A century of research has been conducted n
occur during the prolonged prophase occur during the
prolonged prophase I n Oocyte Oocyte now a highly structured system n Provided with a dowry which, after on marine invertebrates on marine invertebrates n Especially Especially sea urchins sea urchins n Contact and Recognition Between Egg n
and Sperm fertilization will support nutritional requirements of the embryo and direct its development through cleavage n After meiosis resumes n
After meiosis resumes, the egg is ready to , the egg is ready to fuse its nucleus with the sperm nucleus 8­9 n n
n Marine organisms n
n release enormous numbers of sperm in the ocean to fertilize eggs n Many eggs release a Many eggs release a chemotactic chemotactic molecule n Attract sperm of the same species 8­10 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Fertilization Fertilization
Fertilization Sea urchin sperm n Penetrate a n
Penetrate a jelly layer jelly layer surrounding egg surrounding egg n Next, contacts the n
Next, contacts the vitelline envelope vitelline envelope n Prevention of Polyspermy n
n Fertilization cone n
Fertilization cone forms where the sperm forms where the sperm contacts the vitelline membrane contacts the vitelline membrane n Sperm head drawn in and fuses with egg plasma membrane n Important changes in the egg surface block n
entrance to any additional sperm n Polyspermy, Polyspermy, the entry of more than one sperm n In the sea urchin, an n
In the sea urchin, an electrical potential electrical potential rapidly spreads across the membrane n “ n
“fast block” fast block” n Thin membrane above the egg plasma membrane n
n Egg n
Egg­ ­recognition proteins recognition proteins on the acrosomal process bind to species­ bind to species ­specific sperm receptors specific sperm receptors on the on the vitelline envelope n Ensures an egg recognizes only sperm of the same species n
n In the marine environment n
n Many species may be spawning at the same time n
n Similar recognition proteins are found on sperm of n
vertebrate species 8­11 8­12 2
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Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Fertilization n The n
The cortical reaction cortical reaction follows follows n Fusion of thousands of Fusion of thousands of enzyme enzyme­ ­rich rich cortical granules with the egg membrane cortical granules with the egg membrane n Cortical granules release contents between the membrane and vitelline envelope n Creates an Creates an osmotic gradient n Water rushes into space n Elevates the envelope n Lifts away all bound sperm except the one sperm that has successfully one sperm that has successfully fused with the egg plasma membrane 8­13 8­14 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Fertilization Fertilization n One cortical granule enzyme n Causes the vitelline envelope to harden n Now called the fertilization membrane n Block to polyspermy is polyspermy is now complete n Similar process occurs in mammals occurs in mammals 8­15 n Fusion of Pronuclei and Egg Activation n
n After sperm and egg membranes fuse n
n Sperm loses its flagellum n Enlarged sperm nucleus n
Enlarged sperm nucleus is the is the male male pronucleus and migrates inward to contact pronucleus
the female pronucleus the female pronucleus n Fusion of male and female pronuclei forms a n
diploid zygote nucleus 8­16 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Fertilization Cleavage and Early Development n Fertilization n
n Sets in motion important changes in the egg n n
cytoplasm n Fertilized egg n
Fertilized egg called a called a zygote zygote n Inhibitors of metabolism Inhibitors of metabolism that kept egg in that kept egg in quiescent state are removed quiescent state are removed n Initiates Initiates burst of DNA and protein synthesis burst of DNA and protein synthesis n Using n
Using mRNA stored in the egg cytoplasm mRNA stored in the egg cytoplasm n Initiates reorganization of cytoplasm n Zygote has been divided into many hundreds n
n Repositions morphogenetic determinants n
Repositions morphogenetic determinants that that activate or repress genes as development proceeds or thousands of cells n Blastula n
Blastula is formed is formed
n Zygote now enters n
Zygote now enters cleavage cleavage 8­17 Cleavage n Embryo divides repeatedly n
n Large cytoplasmic mass converted into n
small maneuverable cells: blastomeres small maneuverable cells: blastomeres n No cell growth n
No cell growth occurs, only subdivision occurs, only subdivision until cells reach regular somatic cell size n At the end of cleavage n
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n n
Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Cleavage and Early Development Cleavage and Early Development An animal ­vegetal axis is visible on the An animal­ vegetal axis is visible on the embryo before cleavage begins n Due to presence of n
Due to presence of yolk yolk at only one end at only one end n n
n Vegetal pole n
n Establishes n
Establishes polarity polarity in the embryo in the embryo n Region containing mostly n
Region containing mostly cytoplasm cytoplasm n Animal pole n
n Cleavage is n
Cleavage is affected by affected by n Quantity and distribution of yolk present n
n Genes controlling the symmetry of cleavage n
8­19 How Amount and Distribution of Yolk Affects Cleavage n Isolecithal Eggs n
n Very little yolk Very little yolk distributed evenly in distributed evenly in cytoplasm n Cleavage is Cleavage is holoblastic holoblastic n Cleavage furrow extends completely through the egg n Echinoderms, tunicates, cephalochordates, molluscs, and mammals 8­20 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Cleavage and Early Development Cleavage and Early Development n Mesolecithal Eggs n
n Telolecithal Eggs n
n Moderate amount of yolk Moderate amount of yolk concentrated concentrated n Much yolk Much yolk concentrated at vegetal pole concentrated at vegetal pole in the vegetal pole n Cleavage is Cleavage is meroblastic meroblastic n Animal pole Animal pole is opposite vegetal pole is opposite vegetal pole n Contains cytoplasm and very little n Cleavage furrow does not cut through yolk n Eggs cleave Eggs cleave holoblastically holoblastically, but , but cleavage is slowed in yolk­ cleavage is slowed in yolk ­rich vegetal rich vegetal pole n Amphibians the heavy yolk n Cleavage is confined to a narrow disc on top of the yolk n Birds, reptiles, most fishes, and a few amphibians 8­21 8­22 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Cleavage and Early Development
Cleavage and Early Development Function of yolk is to nourish young Direct development n Embryo to miniature adult n
n Associated with n
Associated with telolecithal eggs telolecithal eggs n Indirect development n
n Embryo to larva to adult n
n Associated with n
Associated with isolecithal and isolecithal and mesolecithal eggs n n
n n
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Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. An Overview of Development Following Cleavage An Overview of Development Following Cleavage n n
Blastulation n Cleavage creates a cluster of cells called n
the blastula the blastula n Formation of a blastula stage, with one n
layer of germ cells n Occurs in n
Occurs in all multicellular animals all multicellular animals n Blastula stage typically consists of a n
Blastula stage typically consists of a few few n In most animals n
hundred to several thousand cells n Development continues to form n
Development continues to form 1 or 2 more 1 or 2 more n In n
In mammals mammals, cell cluster called a , cell cluster called a germ layers in a gastrula stage germ layers in a gastrula stage blastocyst n In most animals n
n Cells are arranged around a fluid n
Cells are arranged around a fluid­ ­filled cavity filled cavity called the blastoceol 8­25 8­26 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. An Overview of Development Following Cleavage An Overview of Development Following Cleavage n n
Gastrulation and Formation of Two Germ Layers n Gastrulation n
n Results in the formation of a Results in the formation of a second germ second germ layer n Involves an invagination an invagination of one side of of one side of blastula n The only opening into embryonic gut is the blastopore the blastopore n Blind or incomplete gut n
n Some animals retain the Some animals retain the blind gut blind gut n Most develop a Most develop a complete gut complete gut with a with a second opening, the anus n Forms a new internal cavity n
n Archenteron or gastrocoel n
n Opening into the cavity: n
Blastopore n Gastrula has an outer layer of Gastrula has an outer layer of ectoderm ectoderm and an inner layer of endoderm and an inner layer of endoderm 8­27 8­28 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. An Overview of Development Following Cleavage An Overview of Development Following Cleavage
An Overview of Development Following Cleavage n n
Formation of a Complete Gut n Inward movement of the n
Inward movement of the archenteron archenteron continues until the end of the archenteron reaches the ectodermal reaches the ectodermal wall of the gastrula n An n
An endodermal endodermal tube, the gut, is tube, the gut, is surrounded surrounded by the blastocoel by the blastocoel n The n
The endodermal endodermal tube has tube has two openings two openings n n
n Diploblastic n
n Most animals add a n
Most animals add a 3 3 rd germ layer n Triploblastic n
n Mesoderm n
n 3 rd germ layer n
n Blastopore n
n Forms n
Forms between between the endoderm and the the endoderm and the n Second opening forms by n
Second opening forms by merging of the merging of the ectoderm archenteron tube with the ectoderm 8­29 Formation of Mesoderm n Animals with n
Animals with two germ layers two germ layers 8­30 5
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Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. An Overview of Development Following Cleavage An Overview of Development Following Cleavage n Mesoderm forms in n
Mesoderm forms in two ways two ways n n
n Proliferation of cells from near the lip of the blastopore into the space between the blastopore into the space between the archenteron and the outer body wall n Pushing of the central region of the archenteron wall into the space between the archenteron and the outer body wall n Regardless of method of formation n
n Initial mesoderm cells Initial mesoderm cells arise from arise from endoderm Formation of the Coelom n Coelom n
n Body cavity surrounded by mesoderm n
n Coelomic cavity n
Coelomic cavity appears within the appears within the mesoderm by n Schizocoely n
Schizocoely or or enterocoely enterocoely n The method by which the coelom forms is n
an inherited character an n Important in grouping organisms based on n
developmental characters n Upon completion of coelom formation n
n Body has n
Body has 3 tissue layers 3 tissue layers and and 2 cavities 2 cavities 8­31 8­32 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Suites of Developmental Characters Two major groups of triploblastic animals Two major groups of triploblastic animals n Protostomes n
Protostomes and and deuterostomes deuterostomes n The groups are identified by a suite of four n
developmental characters n Cleavage Patterns n
n Fate of Blastopore n
n Coelom Formation n
n Mesoderm Formation n
n n
8­33 8­34 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Suites of Developmental Characters Deuterostome Development n Cleavage Patterns n
n Radial cleavage n
n Embryonic cells are arranged in n
Embryonic cells are arranged in radial symmetry radial symmetry around the animal around the animal­ ­vegetal axis vegetal axis n After third cleavage n
n Upper tier of cells sits directly on top of the n
tier of cells below n Most deuterostomes have n
Most deuterostomes have regulative development regulative development n Fate of a cell depends on n
Fate of a cell depends on interactions with interactions with neighboring cells n Early blastomeres n
n Capable of producing an entire embryo n
Capable of producing an entire embryo if separated from the other cells 8­35 8­36
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Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Suites of Developmental Characters Suites of Developmental Characters n Fate of Blastopore n
n Coelom Formation n
n Enterocoely n
n Deuterostome embryos n
n Mesoderm and coelom form at the same time n
n Develop a complete gut n
n In enterocoely n
n Blastopore n
Blastopore becomes the becomes the anus anus n Gastrulation begins with one side of the blastula n
bending inward forming the archenteron bending inward forming the archenteron n Second opening n
Second opening becomes the becomes the mouth mouth n As archenteron elongates n
n Sides push outward and expand into a n
Sides push outward and expand into a pouch pouch­ ­like like coelomic compartment n Pouch n
Pouch­ ­like compartment pinches off like compartment pinches off n Forms a mesoderm bound space surrounding the n
gut 8­37 8­38 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Suites of Developmental Characters Suites of Developmental Characters n Bilateral cleavage n
n Anterior n
Anterior­ ­posterior axis posterior axis n Examples of Deuterostome Development n
n Established prior to fertilization by n
Established prior to fertilization by unequal unequal distribution of cytoplasmic components n First cleavage furrow n
Variations in Deuterostome Cleavage n Passes through the n
Passes through the animal animal­ ­vegetal axis vegetal axis n Radial Cleavage n
n Divides the n
Divides the asymmetrically distributed asymmetrically distributed n Characteristic of the Characteristic of the Deuterostomia Deuterostomia cytoplasm between two blastomeres cytoplasm n Echinoderms n Determines the n
Determines the future right and left side future right and left side n Hemichordates n (Half n
(Half­ ­embryo on one side is mirror image of the embryo on one side is mirror image of the half embryo on the other n Chordates 8­39 n Characteristics of n
Characteristics of Ascidians (Tunicates) Ascidians (Tunicates) 8­40 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Suites of Developmental Characters Suites of Developmental Characters
Suites of Developmental Characters n Rotational Cleavage n Early divisions may be asynchronous may be asynchronous n Characteristic of Characteristic of mammals mammals n Embryo may contain odd numbers of cells n Cleavage is Cleavage is slower in mammals slower in mammals than in any than in any other animal group n 1 st cleavage division n Passes through Passes through animal animal­ ­vegetal axis vegetal axis n 2 nd cleavage division n One blastomere n Divides Divides through the through the animal animal­ ­vegetal axis vegetal axis n One blastomere n Divides Divides perpendicular to the animal perpendicular to the animal­ ­ vegetal axis 8­41 n After the After the 3 3 rd cleavage division n Cells form Cells form tightly packed cluster n Outer layer of cells forms Outer layer of cells forms trophoblast n Not part of embryo proper n Embryonic contribution to the placenta n Cells that give rise to the embryo n Inner cell mass 8­42 7
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Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Suites of Developmental Characters Suites of Developmental Characters n Discoidal Cleavage n
n Ectoderm n
n Telolecithal eggs (reptiles, birds, most fish) n
n Epithelium and nervous system n Large mass of yolk Large mass of yolk in each egg in each egg n Endoderm n
n Cleavage Cleavage confined to a small disc of n Epithelial lining of the digestive tube cytoplasm lying on top of yolk cytoplasm
n Early cleavage furrows n
n Carve the disc into a single layer of cells called the blastoderm called the blastoderm n Further cleavage divides blastoderm into 5 to 6 layers 8­43 n Mesoderm n
n Muscular system, reproductive system, peritoneum, calcareous plates sea endoskeleton. 8­44 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Suites of Developmental Characters n Coelom Coelom in in vertebrates vertebrates forms by forms by n Schizocoely n Coelom Coelom in in nonvertebrate chordates nonvertebrate chordates forms by n Enterocoely 8­45 8­46 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Suites of Developmental Characters Suites of Developmental Characters
Suites of Developmental Characters Protostome Development n n
Cleavage Patterns Spiral Cleavage (in most) n Blastomeres cleave n
Blastomeres cleave obliquely to the animal obliquely to the animal­ ­ vegetal axis vegetal axis n Produces quartets of cells that n
Produces quartets of cells that lie in the furrows lie in the furrows between cells of underlying layer n Mosaic development n
n n
n n
n n
n n
n n
Blastopore becomes the Blastopore
becomes the mouth mouth Anus forms from a second opening Anus Coelom Formation Band of mesoderm forms around the gut before a coelom forms n If a coelom exists, it is formed by n
If a coelom exists, it is formed by schizocoely schizocoely n Endodermal cells move by ingression into the n
space between the archenteron walls (endoderm) and outer body wall (endoderm) and outer body wall (ectoderm) n Cells divide and produce mesodermal n
precursors which give rise to mesoderm precursors which give rise to mesoderm n n
n Characteristic of most n
n Cell fate n
Cell fate determined distribution of morphogenetic determinants, in the egg cytoplasm n If separated, individual blastomeres n
If separated, individual blastomeres not capable of directing development of normal embryo 8­47 Fate of Blastopore 8­48 8
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Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Suites of Developmental Characters Mechanisms of Development n Mesodermal precursors in many organisms n
with spiral cleavage with n Believed to originate from a n
Believed to originate from a single large single large n Embryonic Induction n
n Induction n
blastomere, the 4d cell n Present in a 29 n
Present in a 29­ ­ to 64 to 64­ ­ cell embryo cell embryo n Capacity of some cells to evoke a specific n
developmental response in other cells n Some protostomes are n
Some protostomes are acoelomate acoelomate n Hans Spemann (1924) n
n Flatworms n
n Worked with salamander gastrula n
n In others, mesoderm lines only one side of n
n Took piece of the dorsal blastopore lip n
the blastocoe n Leaves a fluid n
Leaves a fluid­ ­filled cavity, the filled cavity, the pseudocoelom, pseudocoelom, around gut n Lined on the outer edge by n
Lined on the outer edge by mesoderm mesoderm n Lined on the inner edge by the n
Lined on the inner edge by the endodermal endodermal gut gut lining 8­49 8­50 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Mechanisms of Development Mechanisms of Development n Dorsal lip n
Dorsal lip was the was the primary organizer primary organizer n Transplanted it into a n
Transplanted it into a ventral or lateral ventral or lateral n Only tissue to induce growth n
position of another gastrula n Called this event n
Called this event primary induction primary induction n Developed a n
Developed a notochord and somites notochord and somites n Considered it to be the n
Considered it to be the first inductive event in first inductive event in n Induced n
Induced host ectoderm host ectoderm to form a to form a neural tube neural tube development n Whole system of organs developed at n
Whole system of organs developed at site of site of n Differentiated cells n
Differentiated cells act as inductors for act as inductors for adjacent undifferentiated cells graft n Timing is critical n
n Nearly complete n
Nearly complete secondary embryo secondary embryo formed formed n Primary induction sets in motion n
Primary induction sets in motion secondary secondary n Composed of n
Composed of grafted tissue grafted tissue and and induced induced induction host tissue 8­51 8­52 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Mechanisms of Development
Mechanisms of Development n The sequence includes n
n Cell movement n
n Changes in adhesion n
n Cell proliferation n
n There is n
There is no “hard no “hard­ ­wired” master control wired” master control panel directing development panel n Sequence of local patterns n
Sequence of local patterns in which one step in in which one step in development is a subunit of another n Each step n
Each step in the developmental hierarchy is in the developmental hierarchy is necessary preliminary for the next 8­53 8­54 9
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Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Gene Expression During Development Gene Expression During Development After fertilization, n Proteins are translated from stored mRNA n
transcribed from maternal genome transcribed from maternal genome n In many animals, maternal mRNA n
n Directs protein synthesis through n
cleavage and to mid­ cleavage and to mid ­blastula stage blastula stage n Protein synthesis then switches from n
maternal to zygotic control maternal to zygotic control as the nucleus as the nucleus transcribes its own mRNA n Homeotic Genes and Hox Genes n
n n
8­55 n Gene expression is regulated to ensure n
orderly development n Mutations of homeotic genes in fruit flies n
revealed they controlled overall body plan of legs, wings, etc n The n
The homeobox homeobox is a 180 is a 180­ ­nucleotide DNA nucleotide DNA sequence that occurs in most animals n These are n
These are master genes that control expression of subordinate genes 8­56 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Gene Expression During Development Gene Expression During Development n Homeotic genes n
Homeotic genes are remarkably similar are remarkably similar n Hox n
Hox and and homeobox genes homeobox genes play a role in play a role in across diverse species n Proteins coded by homeobox genes n
contain a highly conserved 60 contain a highly conserved 60­ ­amino acid amino acid sequence: the sequence : the homeodomain homeodomain n The homeodomain proteins all bind to n
specific promoter sequences of DNA shaping individual organs and limbs n Switch subordinate genes on or off n
n Mice and humans have four clusters of n
homeobox ­containing genes on separate homeobox­ containing genes on separate chromosomes 8­57 8­58 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Development of Systems and Organs Germ layers should not be confused with germ cells n Germ layers do not alone determine n
differentiation but rather the position of embryonic cells n n
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