Animal Development

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Animal Development
A photo of a human embryo six to eight weeks
after conception. Brain formation (upper left)
and heart developing (red shape in the center)
Table of Content Overview Concept 47.1 Concept
47.2 Concept 47.3

• Sapphira Tsang
• April 14, 2012
• Biology/ Period 1

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Overview

• Question How does a zygote become an egg?
• Theories
• 18th centurypeople believed that the answer was
  preformation
• Preformation was the idea that the egg or sperm
  already had a mini human (called a homunculus)
  that grows and develops into a larger adult
  version
• Aristotle proposed his theory of epigenesis
• Epigenesis was the belief that an animals
  conformation is formed from a shapeless egg
• Answer
• Genome of the zygote and differences between
  early embryonic cells are factors that change the
  way an organism develops
• Cell differentiation differences between the
  functions, structures, and roles of cells
• Morphogenesis a process wherein an animal takes
  shape and the location of the cells are already
  determined

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Concept 47.1 Fertilization and three body
structuring stages

• Regulation of development occurs during
  fertilization
• Three stages occur that starts building animals
  body
• Cleavage cell divides from the zygote creates a
  hollow ball of cells called a blastula
• Gastrulation production of a three-layered
  embryo called the gastrula
• Organogenesis basic organs are forming which
  eventually grows into adult structures

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Fertilization

• Fertilization is when the sperm and egg, the
  gametes, unite
• Fertilization combines the haploid sets of
  chromosomes into a diploid cell (called the
  zygote)
• When the sperm comes in contact with the eggs
  surface, metabolic reactions are triggered within
  the egg which activates the development of the
  embryo
• Scientists study fertilization using sea urchins
• Two reactions occur during fertilization
• Acrosomal reaction
• Cortical reaction

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Acrosomal Reaction during sea urchin fertilization

1. Head of the sperm, acrosome, comes in contact
   with the egg, activating acrosomal reaction.
2. Acrosome releases hydrolytic enzymes?digest the
   jelly coat surrounding the egg.
3. Sperm to elongates a structure called the
   acrosomal process, made up of actin filaments,
   which will fully penetrate through the jelly
   coat. The acrosomal process contains molecules
   which bind to receptor proteins that are rooted
   into the vitelline layer.
4. The hole in the vitelline layer allows sperm
   membrane to fuse with egg membrane. The combined
   membranes are depolarized which provides a fast
   block to polyspermy. Polyspermy (fertilization of
   egg by more than one sperm) can lead to an
   abnormal number of chromosomes in the zygote.
5. Sperm enters and travels to cells nucleus.
6. Cortical reaction occurs.

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Cortical Reaction during sea urchin fertilization

• When sperm binds to eggs surface, signal
  transduction pathway activates? calcium is
  released into cytosol.
• High concentration of calcium initiates cortical
  reaction wherein fusion with the eggs plasma
  membrane of vesicles located in the eggs cortex
  (area beneath the cells membrane) occurs
• Cortical granules are formed during oogenesis
• Cortical granules release their contents into
  periveritelline space (area between the vitilline
  layer and the plasma membrane)
• Vitelline layer detaches from plasma membrane an
  osmotic gradient forces water into the
  perivitelline space, pushing it away from the
  membrane
• The vitelline layer becomes a fertilization
  envelope, preventing other sperms from entering
  by a slow block to polyspermy

A picture of calcium spreading out over the cell.
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Events of fertilization/ activation of a sea
urchin egg

• The rate of cell respiration and protein
  synthesis may increase as a result of the rise of
  calcium

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Fertilization in mammals

• Fertilization in mammals is internal but it is
  external for sea urchins
• Mammalian eggs are surrounded by follicle cells
• Follicle cells and the egg are released during
  ovulation
• Sperm travels through follicle cells in order to
  get to the zona pellucida, the eggs
  extracellular matrix?sperm binds to receptor
  molecules in the zona pellucida.
• Binding stimulates acrosomal reaction sperm
  release hydrolytic enzymes
• Zona pellucida is broken down by enzymes sperm
  membrane can fuse with the plasma membrane of egg
  by receptors.
• Sperm nucleus enters egg

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Cleavage

• After fertilization, cell division occurs
• Process of cleavage takes place cells undergo S
  (DNA synthesis) and M (mitosis) phase
• Skip over G1 and G2 phases (no protein synthesis
  is occurring)
• Embryo doesnt enlarge cytoplasm divides into
  smaller cells called blastomeres
• Each has its own nucleus
• First 5-7 divisions cluster of cells is known as
  a morula
• Blastocoel (fluid filled cavity) begins to form
  within morula
• it is fully formed in the blastula (a hollow ball
  of cells)
• In animals, the distribution of yolk (stored
  nutrients) affects pattern of cleavage
• Vegetal pole one pole of the egg where yolk is
  most abundant
• Animal pole opposite end of the egg where yolk
  concentration is significantly less
• Gray crescent a light gray area of the cytoplasm
  that helps the cell mark the dorsal side
• Many zygotes and eggs of animals have a definite
  polarity. However, mammals do not.
• Polarity distribution of yolk to the vegetal
  pole, having most yolk, and animal pole, having
  less yolk

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Cleavage in echinoderm embryo
The egg is fertilized and this photo shows the
zygote prior to cleavage division.
This is a picture of post-second cleavage
division. The cell has divided and is at the four
cell stage.
The morula has formed and the blastocoel is
beginning to form.
A fully formed blastula is present and the embryo
will later hatch from the fertilization envelope.
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Polarity determines body axes in an amphibian
A picture of the body axes of a fully developed
tadpole embryo.

• Polarity of the egg helps with the determination
  of the anterior and posterior ends of an animal.
• 2. Gray crescent on the cytoplasm marks the
  future dorsal site of the organism.
• 3. Cleavage division begins left-right axis
  is defined after the dorsal-ventral and
  anterior-posterior ends are defined.

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Cleavage in a chick embryo
Cleavage in a frog embryo

• Zygote- cell is mostly made out of yolk. A small
  disk is located on the area of the animal pole.
  Egg white provides extra nutrients.
• Four cell stage- early cell divisions are
  meroblastic (or incomplete). Holoblastic cleavage
  occurs when eggs containing very little yolk
  divides completely. The cleavage furrow forms
  through the cytoplasm and not through the yolk.
• Blastoderm- cleavage divisions occur a
  blastoderm is produced which is a group of cells
  covering the top of the yolk.
• Blastoderm cells make up two layers the epiblast
  and hypoblast, which enfold the blastocoel.

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Gastrulation

• Cells of blastula rearranges
• Gastrulation produces embryonic tissues/
  embryonic germ layers.
• Gastrula is the three layered embryo
• Process is determined by
• Changes in cell motility
• Changes in cell shape
• Changes in cellular adhesion to other cells
• Cells near the blastula surface will move into
  the interior regions and start forming three cell
  layers

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Gastrulation in sea urchin embryo

• Gastrulation starts at vegetal pole?Cells from
  blastula wall travels into blastocoel they are
  referred to as mesenchyme cells in the
  blastocoel the cells still in blastula wall make
  up the vegetal plate.
• Vegetal plate folds into itself, a process called
  invagination? starts to form an archenteron, a
  primitive gut. The open end of archenteron
  becomes the anus.
• Endoderm cells make up archenteron. The
  mesenchyme cells send thin extensions called
  filopodia towards the ectoderm cells of the
  blastocoel wall.
• Filopodia contracts and pulls archenteron across
  the blastocoel space.
• Archenteron combines with the blastocoel wall,
  forming digestive tube.

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Gastrulation in frog embryo

1. Gastrulation starts on dorsal side of blastula.
   Invagination starts at gray crescent region, and
   becomes the dorsal lip. Involution process that
   occurs when cells roll over the lip of the
   blastospore and goes into the embryo?forms the
   endoderm and mesoderm. In the animal pole,
   ectoderm transforms and covers entire surface.
2. Blastopore lip grows and invagination is still
   taking place. Once lips meet on either side,
   blastopore transforms into a circle which shrinks
   when the ectoderm starts covering
   surface?archenteron forms, endoderm and mesoderm
   continues to expand by involution and blastocoel
   shrinks.
3. Archenteron is replaced by blastocoel all three
   germ layers are formed the blastospore
   encompasses a group of yolk-filled cells.

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Gastrulation in chick embryo

• Some cells of the epiblast travel towards the
  inside of the embryo producing a primitive streak
  (a bunch of moving into the middle of the
  blastoderm).
• Some cells form the endoderm some form the
  mesoderm.
• The cells that stay on the embryos surface
  becomes the ectoderm.

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Organogenesis

• the three germ layers develop into basic organs

Organogenesis in frog embryo

• Somites-
• neural tube formed
• lateral mesoderm separates, making the coelom
• mesoderm forms the somites
• somites form axial skeleton muscles

• Neural plate forms-
• notochord grows from dorsal mesoderm
• notochord signals for dorsal ectoderm to form
  neural plate

Neural tube forms- neural plate folds into itself
and detaches itself, resulting in neural
tube?neural tube becomes the central nervous
system.
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Organogenesis in chick

• Organogenesis in a chick is similar to
  organogenesis in a frog

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Structures formed from the three embryonic germ
layers in vertebrates

• Ectoderm

• Mesoderm

Endoderm

• Notochord
• Skeletal system
• Muscular system
• Muscle that make up the stomach, intestines, etc
• Excretory system
• Circulatory and lymphatic system
• Reproductive system (except for germ cells)
• Dermis of skin
• Body cavity lining
• Adrenal cortex

• Sweat glands
• Hair follicles
• Epidermis of skin
• Lining of mouth and rectum
• Sensory receptors
• Eye cornea and lens
• Nervous system
• Adrenal medulla (part of the adrenal gland which
  secrete hormones)
• Tooth enamel
• Epithelium of pineal and pituitary glands

• Digestive tract lining
• Respiratory system lining
• Urethra, urinary bladder, and reproductive system
  lining
• Liver
• Pancreas
• Thymus
• Thyroid and parathyroid glands

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Developmental adaptations of amniotes
Amnion
Allantois

• Terms to know
• Amnion serves as protection and cushion for
  embryo prevents dehydration
• Allantois basically a garbage can?stores
  embryos waste functions with chorion as a
  lung
• Chorion exchange gases with allantois provide
  embryo with oxygen and carbon dioxide
• Yolk Sac stores nutrients and feeds the embryo
• Amniotes animals that develop as embryos in
  fluid-filled sacs in eggs or a uterus

Chorion
Yolk sac

• All vertebrates develop in aqueous environments
• Evolution of animal movement onto land requires
• Shelled eggs
• Uterus of marsupial and placental mammales

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Mammalian Development

• Mammalian egg cell and zygote
• Do not contain polarity in their cytoplasm
  contents
• Have yolk-lacking, holoblastic zygote cleavages
• Have small eggs
• Gastrulation and organogensis are similar to
  those processes in birds and reptiles
• Embryo development in early stages
• Cleavage formed
• Embryo traveled down oviduct to the uterus
• Inner cell mass a group of cells at one end of
  the cavity
• Inner cell mass develops into embryo proper and
  add to all extra embryonic membranes
• Implantation occurs
• Trophoblast (outer epithelium of blastocyst)
  initiates implantation when it secretes enzymes
  that break down endometrium molecules (uterus
  lining)
• Blastocyst can then enter the endometrium
• Trophoblast thickens and it extends fingerlike
  projections into maternal tissues rich in blood
  vessels
• Invasion by trophoblast results in erosion of
  capillaries in endometrium? the blood spills out
  and covers trophoblast tissue
• Gastrulation starts
• Implantation completed
• Cells from epiblast move inward through the
  primitive streak, forming the mesoderm and
  endoderm
• Germ layers are formed

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Concept 47.2 Morphogenesis in animals involves
specific changes in cell shape, position, and
adhesion

• Only in animals, morphogenesis involves movement
  of cells
• Movement can determine cell shape or allow cells
  to travel within embryo
• When the cytoskeleton changes, so does the cell
  shape

Neural tube formation in vertebrates
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Cell Crawling

• Cell crawling the movement of cells to other
  places
• Convergent extension type of morphogenetic
  movement wherein tissue layer cells arrange as a
  thin, long sheet
• Cell crawling is involved in convergent extension

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Extracellular Matrix (ECM) and Cell Adhesion
Molecules

• Roles of ECM fibers
• Guide cells in morphogenetic movements
• Function as tracks to direct migrating cells
• Migrating cells moving along specific paths have
  receptor proteins that receive direction signals
• Signals can direct the orientation of the
  cytoskeleton so that it can move the cell forward
• Cell adhesion molecules (CAMs) located on cell
  surface and binds to other CAMs on neighboring
  cells
• Help regulate movements and tissue building due
  to differences in amount of CAMs and chemical
  identity
• Cadherins require calcium for work
• Gene for cadherins is differentiated by their
  location at certain times during embryo
  development

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• Cell migration using fibronection

• Frog blastula formation through cadherin

• Fibronecton provides anchorage for cells

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Concept 47.3 The developmental fate of cells
depends on their history and inductive signals

• Two principles of differentiation during
  embryonic development
• In early cleavage divisions, embryonic cells must
  become different from each other.
• After asymmetries are determined, interactions
  between embryonic cells determine fate by causing
  changes in gene expression

Fate Mapping

• Fate map territorial diagrams of embryonic
  development
• Scientists studied fate maps while manipulating
  embryo parts to see whether a cells fate can be
  changed by moving it
• Two conclusions were made
• Founder cells give rise to specific tissues in
  older embryos
• As development proceeds, cells development
  potential becomes restricted
• Developmental potential range of structures it
  can form

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Fate mapping
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Establishing Cellular Asymmetries

• In nonamniotic vertebrates, body axes
  determination are made early during oogenesis or
  fertilization
• Example locations of melanin and yolk in the
  unfertilized egg of a frog determines the vegetal
  hemispheres
• In amniotes, body axes are not determined until
  later
• Environmental factors determine the axes
• Gravity establishes anterior-posterior axis of
  chicks in the eggs
• pH differences between blastoderm cells determine
  the dorsal-ventral axis

Restrictions of cellular potency

• Totipotent zygote is capable of developing into
  all adult cell types
• Only the zygote is totipotent
• Mammalian embryo cells remain totipotent until
  16-cell stage this is when they arrange into
  precursors of trophoblast and inner cell mass of
  blastocyst
• Location determines cell fate
• At 8-cell stage, each blastomeres can develop an
  embryo if isolated

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Cell Fate determination and pattern formation by
inductive signals

• Embryonic cell division creates cells that
  differ? cells influence each others fates by
  induction
• Inductive signals affect pattern formation
• Pattern formation development of spatial
  organization in an animal
• Positional information signals the cell its
  position in the animals body axes and determines
  how the cell will respond to molecular signals
• Signal molecules
• Affect gene expression in receiving cells
• Result in differentiation
• Can develop certain structures

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Spemann and Mangolds Experiment
Spemann and Mangolds experiment concluded that
the dorsal lip of the blastopore acts as an
organizer of the embryo
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Vertebrate Limb Development
A chicks wing and legs start off as limb buds.

• Limp bud provides a model of pattern formation
• Made up of a core mesodermal tissue surrounded by
  ectoderm layer
• Two organizer regions in limp bud of vertebrate
  limbs
• Apical ectodermal ridge (AER) thick region of
  ectoderm at tip of the bud produces secreted
  protein signals that promote limb-bud outgrowth
• Zone of polarizing activity (ZPA) a block of
  mesodermal tissue underneath ectoderm needed for
  proper pattern formation and produces posterior
  structures

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Tissue Tranplantation experiment
Tissue transplantation experiment supports the
idea that ZPA produces an inductive signal
message with information for posterior positions
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Works Cited

• www.campbellbiology.com
• Campbell Biology textbook
• Pictures from www.campbellbiology.com