2Early Concepts: Preformation vs Epigenesis The question of how a zygote becomes an animal has been asked for centuries.As recently as the 18th century, the prevailing theory was a notion called preformation – the idea that the egg or sperm contains an embryo.A preformed miniature infant, or “homunculus,” that simply becomes larger during development.
3Early Concepts: Preformation vs Epigenesis Kaspar Friederich Wolff (1759) demonstrated there was no preformed chick in the early egg.Undifferentiated granular material became arranged into layers.The layers thickened, thinned, and folded to produce the embryo.
4Early Concepts: Preformation vs Epigenesis Epigenesis is the concept that the fertilized egg contains building materials only, somehow assembled by an unknown directing force.Although current ideas of development are essentially epigenetic in concept, far more is known about what directs growth and differentiation.
5Key Events in Development Development describes the changes in an organism from its earliest beginnings through maturity.Search for commonalities.
6Key Events in Development Specialization of cell types occurs as a hierarchy of developmental decisions.Cell types arise from conditions created in preceding stages.Interactions become increasingly restrictive.With each new stage:Each stage limits developmental fate.Cells lose option to become something differentSaid to be determined.
7Key Events in Development The two basic processes responsible for this progressive subdivision:Cytoplasmic localizationInduction
8FertilizationFertilization is the initial event in development in sexual reproduction.Union of male and female gametesProvides for recombination of paternal and maternal genes.Restores the diploid number.Activates the egg to begin development.
9Fertilization Oocyte Maturation Egg grows in size by accumulating yolk.Contains much mRNA, ribosomes, tRNA and elements for protein synthesis.Morphogenetic determinants direct the activation and repression of specific genes later in post- fertilization development.Egg nucleus grows in size, bloated with RNA.Now called the germinal vesicle.
10FertilizationMost of these preparations in the egg occur during the prolonged prophase I.In mammalsOocyte now has a highly structured system.After fertilization it will support nutritional requirements of the embryo and direct its development through cleavage.After meiosis resumes, the egg is ready to fuse its nucleus with the sperm nucleus.
11FertilizationA century of research has been conducted on marine invertebrates.Especially sea urchins
12Contact Between Sperm & Egg Broadcast spawners often release a chemotactic factor that attracts sperm to eggs.Species specificSperm enter the jelly layer.Egg-recognition proteins on the acrosomal process bind to species-specific sperm receptors on the vitelline envelope.
13Fertilization in Sea Urchins Prevention of polyspermy – only one sperm can enter.Fast blockDepolarization of membraneSlow blockCortical reaction resulting in fertilization membrane
14Fertilization in Sea Urchins The cortical reaction follows the fusion of thousands of enzyme-rich cortical granules with the egg membrane.Cortical granules release contents between the membrane and vitelline envelope.Creates an osmotic gradientWater rushes into spaceElevates the envelopeLifts away all bound sperm except the one sperm that has successfully fused with the egg plasma membrane.
16Fertilization in Sea Urchins One cortical granule enzyme causes the vitelline envelope to harden.Now called the fertilization membrane.Block to polyspermy is now complete.Similar process occurs in mammals.
17Fertilization in Sea Urchins The increased Ca2+ concentration in the egg after the cortical reaction results in an increase in the rates of cellular respiration and protein synthesis.The egg is activated.
18Fusion of PronucleiAfter sperm and egg membranes fuse, the sperm loses its flagellum.Enlarged sperm nucleus is the male pronucleus and migrates inward to contact the female pronucleus.Fusion of male and female pronuclei forms a diploid zygote nucleus.
19Cleavage Cleavage – rapid cell divisions following fertilization. Very little growth occurs.Each cell called a blastomere.Morula – solid ball of cells. First 5-7 divisions.
20PolarityThe eggs and zygotes of many animals (not mammals) have a definite polarity.The polarity is defined by the distribution of yolk.The vegetal pole has the most yolk and the animal pole has the least.
21Body AxesThe development of body axes in frogs is influenced by the polarity of the egg.The polarity of the egg determines the anterior-posterior axis before fertilization.At fertilization, the pigmented cortex slides over the underlying cytoplasm toward the point of sperm entry. This rotation (red arrow) exposes a region of lighter-colored cytoplasm, the gray crescent, which is a marker of the dorsal side.The first cleavage division bisects the gray crescent. Once the anterior-posterior and dorsal-ventral axes are defined, so is the left-right axis.
22Amount of YolkDifferent types of animals have different amounts of yolk in their eggs.Isolecithal – very little yolk, even distribution.Mesolecithal – moderate amount of yolk concentrated at vegetal pole.Telolecithal – Lots of yolk at vegetal pole.Centrolecithal – lots of yolk, centrally located.
23Cleavage in FrogsCleavage planes usually follow a specific pattern that is relative to the animal and vegetal poles of the zygote.Animal pole blastomeres are smaller.Blastocoel in animal hemisphere.Little yolk, cleavage furrows complete.Holoblastic cleavage
24Cleavage in BirdsMeroblastic cleavage, incomplete division of the egg.Occurs in species with yolk-rich eggs, such as reptiles and birds.Blastoderm – cap of cells on top of yolk.
25Direct vs. Indirect Development When lots of nourishing yolk is present, embryos develop into a miniature adult.Direct developmentWhen little yolk is present, young develop into larval stages that can feed.Indirect developmentMammals have little yolk, but nourish the embryo via the placenta.
26BlastulaA fluid filled cavity, the blastocoel, forms within the embryo – a hollow ball of cells now called a blastula.
27GastrulationThe morphogenetic process called gastrulation rearranges the cells of a blastula into a three-layered (triploblastic) embryo, called a gastrula, that has a primitive gut.Diploblastic organisms have two germ layers.
28GastrulationThe three tissue layers produced by gastrulation are called embryonic germ layers.The ectoderm forms the outer layer of the gastrula.Outer surfaces, neural tissueThe endoderm lines the embryonic digestive tract.The mesoderm partly fills the space between the endoderm and ectoderm.Muscles, reproductive system
29Gastrulation – Sea Urchin Gastrulation in a sea urchin produces an embryo with a primitive gut (archenteron) and three germ layers.Blastopore – open end of gut, becomes anus in deuterostomes.
30Gastrulation - Frog Result – embryo with gut & 3 germ layers. More complicated:Yolk laden cells in vegetal hemisphere.Blastula wall more than one cell thick.
31Gastrulation - ChickGastrulation in the chick is affected by the large amounts of yolk in the egg.Primitive streak – a groove on the surface along the future anterior-posterior axis.Functionally equivalent to blastopore lip in frog.
32Gastrulation - Chick Epiblast and hypoblast Blastoderm consists of two layers:Epiblast and hypoblastLayers separated by a blastocoelEpiblast forms endoderm and mesoderm.Cells on surface of embryo form ectoderm.
33Gastrulation - Mouse In mammals the blastula is called a blastocyst. Inner cell mass will become the embryo while trophoblast becomes part of the placenta.Notice that the gastrula is similar to that of the chick.
34Suites of Developmental Characters Two major groups of triploblastic animals:ProtostomesDeuterostomesDifferentiated by:Spiral vs. radial cleavageRegulative vs. mosaic cleavageBlastopore becomes mouth vs. anusSchizocoelous vs. enterocoelous coelom formation.
35Deuterostome Development Deuterostomes include echinoderms (sea urchins, sea stars etc) and chordates.Radial cleavage
36Deuterostome Development Regulative development – the fate of a cell depends on its interactions with neighbors, not what piece of cytoplasm it has. A blastomere isolated early in cleavage is able to from a whole individual.
37Deuterostome Development Deuterostome means second mouth.The blastopore becomes the anus and the mouth develops as the second opening.
38Deuterostome Development The coelom is a body cavity completely surrounded by mesoderm.Mesoderm & coelom form simultaneously.In enterocoely, the coelom forms as outpocketing of the gut.
39Deuterostome Development Typical deuterostomes have coeloms that develop by enterocoely.Vertebrates use a modified version of schizocoely.
40Protostome Development Protostomes include flatworms, annelids and molluscs.Spiral cleavage
41Protostome Development Mosaic development – cell fate is determined by the components of the cytoplasm found in each blastomere.Morphogenetic determinants.An isolated blastomere can’t develop.
42Protostome Development Protostome means first mouth.Blastopore becomes the mouth.The second opening will become the anus.
43Protostome Development In protostomes, a mesodermal band of tissue forms before the coelom is formed.The mesoderm splits to form a coelom.SchizocoelyNot all protostomes have a true coelom.Pseudocoelomates have a body cavity between mesoderm and endoderm.Acoelomates have no body cavity at all other than the gut.
44Two Clades of Protostomes Lophotrochozoan protostomes include annelid worms, molluscs, & some small phyla.Lophophore – horseshoe shaped feeding structure.Trochophore larvaFeature all four protostome characteristics.
45Two Clades of Protostomes The ecdysozoan protostomes include arthropods, roundworms, and other taxa that molt their exoskeletons.Ecdysis – shedding of the cuticle.Many do not show spiral cleavage.
46Building a Body PlanAn organism’s development is determined by the genome of the zygote and also by differences that arise between early embryonic cells.Different genes will be expressed in different cells.
47Building a Body PlanUneven distribution of substances in the egg called cytoplasmic determinants results in some of these differences.Position of cells in the early embryo result in differences as well.Induction
48Restriction of Cellular Potency In many species that have cytoplasmic determinants only the zygote is totipotent, capable of developing into all the cell types found in the adult.
49Restriction of Cellular Potency Unevenly distributed cytoplasmic determinants in the egg cell:Are important in establishing the body axes.Set up differences in blastomeres resulting from cleavage.
50Restriction of Cellular Potency As embryonic development proceeds, the potency of cells becomes progressively more limited in all species.
51Cell Fate Determination and Pattern Formation by Inductive Signals Once embryonic cell division creates cells that differ from each other,The cells begin to influence each other’s fates by induction.
52InductionInduction is the capacity of some cells to cause other cells to develop in a certain way.Dorsal lip of the blastopore induces neural development.Primary organizer
53Spemann-Mangold Experiment Transplanting a piece of dorsal blastopore lip from a salamander gastrula to a ventral or lateral position in another gastrula developed into a notochord & somites and it induced the host ectoderm to form a neural tube.
54Building a Body PlanCell differentiation – the specialization of cells in their structure and function.Morphogenesis – the process by which an animal takes shape and differentiated cells end up in their appropriate locations.
55Building a Body Plan The sequence includes Cell movementChanges in adhesionCell proliferationThere is no “hard-wired” master control panel directing development.Sequence of local patterns in which one step in development is a subunit of another.Each step in the developmental hierarchy is a necessary preliminary for the next.
56Hox GenesHox genes control the subdivision of embryos into regions of different developmental fates along the anteroposterior axis.Homologous in diverse organisms.These are master genes that control expression of subordinate genes.
57Formation of the Vertebrate Limb Inductive signals play a major role in pattern formation – the development of an animal’s spatial organization.
58Formation of the Vertebrate Limb The molecular cues that control pattern formation, called positional information:Tell a cell where it is with respect to the animal’s body axes.Determine how the cell and its descendents respond to future molecular signals.
59Formation of the Vertebrate Limb The wings and legs of chicks, like all vertebrate limbs begin as bumps of tissue called limb buds.The embryonic cells within a limb bud respond to positional information indicating location along three axes.
60Formation of the Vertebrate Limb One limb-bud organizer region is the apical ectodermal ridge (AER).A thickened area of ectoderm at the tip of the bud.The second major limb-bud organizer region is the zone of polarizing activity (ZPA).A block of mesodermal tissue located underneath the ectoderm where the posterior side of the bud is attached to the body.
61MorphogenesisMorphogenesis is a major aspect of development in both plants and animals but only in animals does it involve the movement of cells.
62The Cytoskeleton, Cell Motility, and Convergent Extension Changes in the shape of a cell usually involve reorganization of the cytoskeleton.
63Changes in Cell ShapeThe formation of the neural tube is affected by microtubules and microfilaments.
64Cell MigrationThe cytoskeleton also drives cell migration, or cell crawling.The active movement of cells from one place to another.In gastrulation, tissue invagination is caused by changes in both cell shape and cell migration.
65Evo-DevoEvolutionary developmental biology - evolution is a process in which organisms become different as a result of changes in the genetic control of development.Genes that control development are similar in diverse groups of animals.Hox genes
66Evo-DevoInstead of evolution proceeding by the gradual accumulation of numerous small mutations, could it proceed by relatively few mutations in a few developmental genes?The induction of legs or eyes by a mutation in one gene suggests that these and other organs can develop as modules.
67The Common Vertebrate Heritage Vertebrates share a common ancestry and a common pattern of early development.Vertebrate hallmarks all present briefly.Dorsal neural tubeNotochordPharyngeal gill pouchesPostanal tail
68AmniotesThe embryos of birds, reptiles, and mammals develop within a fluid-filled sac that is contained within a shell or the uterus.Organisms with these adaptations form a monophyletic group called amniotes.Allows for embryo to develop away from water.
69AmniotesIn these three types of organisms, the three germ layers also give rise to the four extraembryonic membranes that surround the developing embryo.
70AmniotesAmnion – fluid filled membranous sac that encloses the embryo. Protects embryo from shock.Yolk sac – stores yolk and pre-dates the amniotes by millions of years.
71Amniotes Allantois - storage of metabolic wastes during development. Chorion - lies beneath the eggshell and encloses the embryo and other extraembryonic membrane.As embryo grows, the need for oxygen increases.Allantois and chorion fuse to form a respiratory surface, the chorioallantoic membrane.Evolution of the shelled amniotic egg made internal fertilization a requirement.
72The Mammalian Placenta and Early Mammalian Development Most mammalian embryos do not develop within an egg shell.Develop within the mother’s body.Most retained in the mother’s body.MonotremesPrimitive mammals that lay eggs.Large yolky eggs resembling bird eggs.Duck-billed platypus and spiny anteater.
73The Mammalian Placenta and Early Mammalian Development MarsupialsEmbryos born at an early stage of development and continue development in abdominal pouch of mother.Placental MammalsRepresent 94% of the class Mammalia.Evolution of the placenta required:Reconstruction of extraembryonic membranes.Modification of oviduct - expanded region formed a uterus.
74Mammalian Development The eggs of placental mammals:Are small and store few nutrients.Exhibit holoblastic cleavage.Show no obvious polarity.
75Mammalian Development Gastrulation and organogenesis resemble the processes in birds and other reptiles.
76Mammalian Development Early embryonic development in a human proceeds through four stages:Blastocyst reaches uterus.Blastocyst implants.Extraembryonic membranes start to form and gastrulation begins.Gastrulation has produced a 3-layered embryo.
77Mammalian Development The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and have similar functions.
78Mammalian Development AmnionSurrounds embryoSecretes fluid in which embryo floatsYolk sacContains no yolkSource of stem cells that give rise to blood and lymphoid cellsStem cells migrate to into the developing embryoAllantoisNot needed to store wastesContributes to the formation of the umbilical cordChorionForms most of the placenta
79OrganogenesisVarious regions of the three embryonic germ layers develop into the rudiments of organs during the process of organogenesis.
80OrganogenesisMany different structures are derived from the three embryonic germ layers during organogenesis.
81Derivatives of Ectoderm: Nervous System and Nerve Growth Just above the notochord (mesoderm), the ectoderm thickens to form a neural plate.Edges of the neural plate fold up to create an elongated, hollow neural tube.Anterior end of neural tube enlarges to form the brain and cranial nerves.Posterior end forms the spinal cord and spinal motor nerves.
82Derivatives of Ectoderm: Nervous System and Nerve Growth Neural crest cells pinch off from the neural tube.Give rise toPortions of cranial nervesPigment cellsCartilageBoneGanglia of the autonomic systemMedulla of the adrenal glandParts of other endocrine glandsNeural crest cells are unique to vertebrates.Important in evolution of the vertebrate head and jaws.
83Derivatives of Endoderm: Digestive Tube and Survival of Gill Arches During gastrulation, the archenteron forms as the primitive gut.This endodermal cavity eventually produces:Digestive tractLining of pharynx and lungsMost of the liver and pancreasThyroid, parathyroid glands and thymus
84Derivatives of Endoderm: Digestive Tube and Survival of Gill Arches Pharyngeal pouches are derivatives of the digestive tract.Arise in early embryonic development of all vertebrates.During development, endodermally-lined pharyngeal pouches interact with overlying ectoderm to form gill arches.In fish, gill arches develop into gills.In terrestrial vertebrates:No respiratory function1st arch and endoderm-lined pouch form upper and lower jaws, and inner ear.2nd, 3rd, and 4th gill pouches form tonsils, parathyroid gland and thymus.
85Derivatives of Mesoderm: Support, Movement and the Beating Heart Most muscles arise from mesoderm along each side of the neural tube.The mesoderm divides into a linear series of somites (38 in humans).
86Derivatives of Mesoderm: Support, Movement and the Beating Heart The splitting, fusion and migration of somites produce the:Axial skeletonDermis of dorsal skinMuscles of the back, body wall, and limbsHeartLateral to the somites the mesoderm splits to form the coelom.