Vertebrate Development I: Amphibians and Fish PDF

C H A P T E R 11 Vertebrate development I: Amphibians and Fish  Vertebrate development 1 All vertebrates (animals with backbones) are chordates (phylum Chordata) but not all chordates are vertebrates. Chordates and echinoderms are deuterostomes Invertebrate chordates include the lan...

C H A P T E R 11 Vertebrate development I: Amphibians and Fish  Vertebrate development 1 All vertebrates (animals with backbones) are chordates (phylum Chordata) but not all chordates are vertebrates. Chordates and echinoderms are deuterostomes Invertebrate chordates include the lancets (i.e. Amphioxus) and the tunicates (sea squirts). All chordates process a notochord and dorsal hollow nerve chord at some time during development – Pharyngeal grooves and caudal appendage (post-anal tail) are also often included in chordate characteristics Figure 1.20 Transitional states over the course of animal evolution (Part 1) (A) Amphioxus, or the lancelet, has a rudimentary notochord and nerve cord structures and thus is related to the common ancestor of all vertebrates. Figure 8.1 A schematized tree of life focused primarily on the phylogenetic relationships of animals Figure 10.1 The echinoderms and tunicates represent deuterostome invertebrates  Tunicates are the closest evolutionary relatives of the vertebrates Tunicates lack vertebrae at all life stages Free-swimming larva (“tadpole”) has notochord and dorsal nerve chord When tadpole undergoes metamorphosis, nerve cord and notochord degenerate and it secretes a cellulose tunic  Vertebrate development 1 All vertebrates have similar body plan Segmented vertebral column surrounding spinal cord Brain at head end enclosed in bony or cartilaginous skull Exhibit bilateral symmetry with many paired structures Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Fig. 3.1 Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Timing is highly variable even within taxa Use developmental stages After gastrulation, all vertebrate embryos pass through a stage at which they all resemble each other Phylotypic stage At this stage head is distinct and neural tube as formed Just under it is notochord present in embryos of all chordates Earliest mesodermal structure Does not persist in adult vertebrates Induces overlying ectoderm  neural tube Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Fig. 3.2 Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Somites = blocks of mesodermal tissue flanking notochord muscles and skeleton Differences in development among vertebrate taxa due to Evolutionary Modes of reproduction Yolk Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Mammalian eggs are small with little yolk – Mammalian embryo nourished 1st by fluid in oviduct Then in uterus Then placenta Reptile, bird and mammals = amniotes – Have 4 extra-embryonic membranes  Amniote egg Reptiles, birds and mammals Evolved in amphib. Ancestor of reptiles 255 myo Allowed development entirely on land  Shell – prevents desiccation, protects against mech. shock, allows gas exchange Extraembryonic membranes – Yolk sac – stores nutrient – Amnion – fluid bathing embryo – aquatic environ in shell, shock absorb. – Allantois – safe storage of Nitrogenous waste and gas exchange – Chorion – interacts w/ outside environment Amniote egg  Amphibians and Fish Fish and Amphibians are anamniotic – Do not produce the amnion and other extraembryonic membranes that permit embryonic development on land They do employ many of the same processes and genes used by other vertebrates to generate body axes and organs Amphibians have been extremely important model organisms in Embryology Scientist Speak 11.1 Xenopus Dev (25 minutes) https://learninglink.oup.com/access/content/barresi-12e-student-resources/barresi-12 e-scientists-speak-11-1?previousFilter=tag_chapter-11  Frog life cycle Egg maturation and mating behavior depend on environ. and hormones – Female (environment pituitary  gonadotrophic hormone estrogen) Egg – polarity – Top = animal hemisphere Dark pigmented – Bottom = vegital hemisphere – contains most yolk – Animal- vegetal axis exists in egg Before fert – egg is enclosed in vitelline membrane – Embedded in gelatinous coat Frog egg is meiosis II – Has 1st polar body  Fig. 3.4 Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Fertilization (external) – Sperm-oocyte fusion – Egg compl. 2nd meiotic div – Sperm and egg pronuclei fuse Produces 2nd polar body – Axes of embryo are set up at fertilization More later – Fusion also activates cleavage – Cortical shift  gray crescent opposite site of sperm entry unpigmented area of cortical cytoplasm Landmark of early embryo (dorsal surface) Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press Frog life cycle  Fig. 3.3 Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Amphibian Cleavage 1st div – vert an to veg – bisects gray crescent rate slows in veg hemi 2nd vert an to veg perp to 1st 3rd horizontal perp to 1st 2 – offset toward animal pole  4 small blastomeres in an, 4 lg in veg cont synchronously until 16-64 cell stage = morula – loose synch because cell cycle longer in veg hemi Scanning electron micrographs of frog egg cleavage http://www.youtube.com/ watch?v=fUpc93T12bk  Fig. 3.5 Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Amphibian Cleavage gets ahead by a couple cycles in an hemi blastocoel forms in animal hemi above yolk mass– now blastula fluid filled can be divided into 3 regions – around an pole above blastocoel – future ectodermal cells – around veg pole – incld cells of yolk mass – future endodermal cells – Marginal zone - ring of subequatorial cells incld gray crescent – future mesoderm at ~ 10000-15000 cell stage – gastrulation begins Specification of the germ layers All tissues of the body form from the 3 primary germ layers Fate maps produced through research show which tissues develop from various regions of the early embryo Xenopus is best known but other vertebrates are similar Process begins in early embryo, but the germ layers become established during gastrulation Xenopus fate map 3 regions of frog blastula – Yolky veg region in lower 1/3 of blastula  most of the endoderm Yolk is present in all cells and is gradually consumed as it nourishes embryo – Animal hemi –> ectoderm – Marginal zone  future mesoderm Most dorsal meso  notochord Then somites, lateral plate, blood islands Fate maps of the blastula of the frog Xenopus laevis  Gastrulation moves endoderm and mesoderm inward Ectoderm surrounds outside Ventral ectoderm epidermis Dorsal  nervous system Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Amphibian Gastrulation cells move over surface and into blastopore – other cells follow – Process = involution cells of the blastula are destined to become certain tissues determined by experiments w/ markers – vital dyes – used to construct fate maps prospective (presumptive) endoderm – around most of ventral margins of blastopore and ventral part of embryo migrates to interior and lines archenteron most cells passing over dorsal lip of blastopore = chordamesoderm mesoderm that will form notochord and cephalic mesoderm Other mesoderm = lateral plate mesoderm –  internal organs (kidneys heart) most of dorsal cells = primitive ectoderm Ectoderm spreads downward to cover whole embryo – Process= epiboly  Fig. 3.6 Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press 7.9 Epiboly of the ectoderm (Part 2)  Fate maps in dift vertebrates are similar Differences due to amount of yolk  dift cleavage patterns  shapes of embryos Principles of Development 4e Fig. 4.23 Wolpert/Tickle Copyright © 2011 by Oxford University Press  http://www.youtube.com/watch?v=qisrNX3 QjUg http://www.youtube.com/watch?v=YlUsUH 9G1Mo Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Frog life cycle contd. Organogenesis begins w/ neurulation – emb= neurula – Notochord (dorsal mesodermal rod formed from first cells to pass over the dorsal lip of the blastopre) induces overlying ectoderm  neural plate and neural foldsneural tube (becomes cns) = “primary embryonic induction” – Neural crest cells break off of neural tube Migrate away to form other structures later – Nt induces neighboring tissue – Meso next to notochordsomites Precursers of back muscles, vertebrae, and dermis (inner layer of skin) Continued development of Xenopus laevis (Part 2)  Fig. 4.1 Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Elongates into Starts looking like a tadpole Brain divided into regions Eyes and ear development has begun Three branchial arches on each side Mouth breaks through and anus from blastopore Tail bud forms in posterior Various Organs form, Nerves, gills form Tadpole hatches and begins to feed Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Fig. 3.7 Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Fig. 3.8 Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press Metamorphosis  Early development of fish Zebra fish is a good model Develops rapidly – Whole lc-12wks Embryo is transparent Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Fig. 3.9 Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Fish Cleavage Most fish eggs are telolecithal and have discoidal meroblastic cleavage – Embryo from blastodisc only Many waves of Ca++ control cell division 1st cell divisions are synchronous distinct patterns of meridionial and equatorial cleavage of animal cells Forms mound of cells (blastoderm) at animal pole that sits on a single yolk cell At first all cells share connection with yolk that allows diffusion of molecules 1st divisions all vertical First horizontal division (6th) 64cell Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Fig. 3.10 Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  At ~ 10th division  mid-blastula transition Zygotic gene expression begins Cell division slows Cell movement begins  3 distinct cell popns yolk syncytial layer (YSL) From fusion of cells of vegetal edge of blastoderm to underlying yolk cells Later forms internal and external YSL Enveloping layer Most superficial cells from blastoderm Forms epithelial sheet 1 cell thick Shed during later dev Deep cells  embryo proper Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Fish Gastrulation First cell movements = epiboly of blastoderm cells over yolk – Inner blastoderm cells move outward and intercalate with superficial – Causes them to move vegetally over surface of yolk cell and envelop it – Blastoderm margin migrates vegetally due to epiboly of YSL Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Germ layer formation When blastoderm cells have covered about ½ of yolk cell, thickening occurs at margin of epibolizing blastoderm = germ ring – Germ ring composed of Epiblast –superficial layer Hypoplast – inner layer – Epi and hypo cells intercalate on future dorsal surface and form thickening = embryonic shield Emb shield functions like dorsal lip of blastopore in amphibs Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  – Convergence and extension of hypoblast cells  chordomesoderm = precursor of notochord – Adjacent cells –> mesodermal somites – Convergence and extension in epiblast brings presumptive neural cells to dorsal midline neural keel – Cells remaining in epiblastectoderm Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Fig. 3.11 http://www.luriechildrensresearch.org/topczewski/fish_facility /stages/ https://www.youtube.com/watch?v=ahJjLzyioWM Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Axis Formation in Vertebrates Anterior-posterior (craniocaudal), dorso- ventral, and mediolateral (left-right) axes must be determined in all vertebrate embryos must arise from spherical egg Some is established through polarity of egg – How much varies among vertebrate groups Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Xenopus (amphibians) Early development in Xenopus (and zebra fish) are exclusively under maternal factors in egg Embryonic genes do not begin being transcribed until mid-blastula transition Egg cont. large amounts of – proteins (ex histone), – maternal mRNAs Localized along an-veg axis during oogenisis Most dev impt in veg hemisphere Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  XENOPUS DEVELOPMENT Zygotic genes expression begins at mid-blastula transition After fert rate of protein synth increases 1.5X Due to expression of Maternal mRNA Little new mRNA synth until after 12th div (4096 cells) – Zygotic gene expression begins now = mid-blastula transition Timing not due to cell division directly or cell-cell interactions Appears to depend on ratio of DNA to cytoplasm Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  polarity in amphibian embryo Animal-vegital axis established in unfert ovum – already polarized into animal and vegetal half = primary polarity of egg animal hemisphere – nuc near an pole – denser conc of pigment granules, ribosomes and glycogen granules Vegital hemisphere – size and conc of yolk platelets greater toward vegetal pole – Antero-posterior axis of emb is comparable to an-veg axis of egg but not exactly the same veg – tail , an- head roughly – Precise location of a-p axis requires establishment of Dorso-vent axis Happens just after fert  Dorso-vent axis in Xenopus Unfert egg is spherical and radially symmetrical around A-V axis Changed by fertilization Sperm entry can occur anywhere in animal hemi Sets off chain of events Dorsal surface form opposite point of sperm entry Principles of Development 4e Wolpert/Tickle Copyright © 2011 by Oxford University Press  Reorganizations of cyt after fertilization and before cleavage – Cortical rotation convergence of cyt just (