Principles of Development

1
Principles of Development

• Chapter 8

2
Early 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.

3
Early 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.

4
Early 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.

5
Key Events in Development

• Development describes the changes in an organism
  from its earliest beginnings through maturity.
• Search for commonalities.

6
Key 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 different
• Said to be determined.

7
Key Events in Development

• The two basic processes responsible for this
  progressive subdivision
• Cytoplasmic localization
• Induction

8
Fertilization

• Fertilization is the initial event in development
  in sexual reproduction.
• Union of male and female gametes
• Provides for recombination of paternal and
  maternal genes.
• Restores the diploid number.
• Activates the egg to begin development.

9
Fertilization

• 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.

10
Fertilization

• Most of these preparations in the egg occur
  during the prolonged prophase I.
• In mammals
• Oocyte 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.

11
Fertilization and Activation

• A century of research has been conducted on
  marine invertebrates.
• Especially sea urchins

12
Contact Between Sperm Egg

• Broadcast spawners often release a chemotactic
  factor that attracts sperm to eggs.
• Species specific
• Sperm enter the jelly layer.
• Egg-recognition proteins on the acrosomal process
  bind to species-specific sperm receptors on the
  vitelline envelope.

13
Fertilization in Sea Urchins

• Prevention of polyspermy only one sperm can
  enter.
• Fast block
• Depolarization of membrane
• Slow block
• Cortical reaction resulting in fertilization
  membrane

14
Fertilization 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 gradient
• Water rushes into space
• Elevates the envelope
• Lifts away all bound sperm except the one sperm
  that has successfully fused with the egg plasma
  membrane.

15
Fertilization in Sea Urchins
16
Fertilization 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.

17
Fertilization 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.

18
Fusion of Pronuclei

• After 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.

19
Cleavage

• Cleavage rapid cell divisions following
  fertilization.
• Very little growth occurs.
• Each cell called a blastomere.
• Morula solid ball of cells. First 5-7 divisions.

20
Polarity

• The 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.

21
Body Axes

• The 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.
22
Amount of Yolk

• Different 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.

23
Cleavage in Frogs

• Cleavage 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

24
Cleavage in Birds

• Meroblastic 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.

25
Direct vs. Indirect Development

• When lots of nourishing yolk is present, embryos
  develop into a miniature adult.
• Direct development
• When little yolk is present, young develop into
  larval stages that can feed.
• Indirect development
• Mammals have little yolk, but nourish the embryo
  via the placenta.

26
Blastula

• A fluid filled cavity, the blastocoel, forms
  within the embryo a hollow ball of cells now
  called a blastula.

27
Gastrulation

• The 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.

28
Gastrulation

• The three tissue layers produced by gastrulation
  are called embryonic germ layers.
• The ectoderm forms the outer layer of the
  gastrula.
• Outer surfaces, neural tissue
• The endoderm lines the embryonic digestive tract.
• The mesoderm partly fills the space between the
  endoderm and ectoderm.
• Muscles, reproductive system

29
Gastrulation 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.

30
Gastrulation - Frog

• Result embryo with gut 3 germ layers.
• More complicated
• Yolk laden cells in vegetal hemisphere.
• Blastula wall more than one cell thick.

31
Gastrulation - Chick

• Gastrulation 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.

32
Gastrulation - Chick

• Blastoderm consists of two layers
• Epiblast and hypoblast
• Layers separated by a blastocoel
• Epiblast forms endoderm and mesoderm.
• Cells on surface of embryo form ectoderm.

33
Gastrulation - 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.

34
Suites of Developmental Characters

• Two major groups of triploblastic animals
• Protostomes
• Deuterostomes
• Differentiated by
• Spiral vs. radial cleavage
• Regulative vs. mosaic cleavage
• Blastopore becomes mouth vs. anus
• Schizocoelous vs. enterocoelous coelom formation.

35
Deuterostome Development

• Deuterostomes include echinoderms (sea urchins,
  sea stars etc) and chordates.
• Radial cleavage

36
Deuterostome 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.

37
Deuterostome Development

• Deuterostome means second mouth.
• The blastopore becomes the anus and the mouth
  develops as the second opening.

38
Deuterostome 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.

39
Deuterostome Development

• Typical deuterostomes have coeloms that develop
  by enterocoely.
• Vertebrates use a modified version of
  schizocoely.

40
Protostome Development

• Protostomes include flatworms, annelids and
  molluscs.
• Spiral cleavage

41
Protostome Development

• Mosaic development cell fate is determined by
  the components of the cytoplasm found in each
  blastomere.
• Morphogenetic determinants.
• An isolated blastomere cant develop.

42
Protostome Development

• Protostome means first mouth.
• Blastopore becomes the mouth.
• The second opening will become the anus.

43
Protostome Development

• In protostomes, a mesodermal band of tissue forms
  before the coelom is formed.
• The mesoderm splits to form a coelom.
• Schizocoely
• Not 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.

44
Two Clades of Protostomes

• Lophotrochozoan protostomes include annelid
  worms, molluscs, some small phyla.
• Lophophore horseshoe shaped feeding structure.
• Trochophore larva
• Feature all four protostome characteristics.

45
Two 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.

46
Building a Body Plan

• An organisms 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.

47
Building a Body Plan

• Uneven 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

48
Restriction 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.

49
Restriction 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.

50
Restriction of Cellular Potency

• As embryonic development proceeds, the potency of
  cells becomes progressively more limited in all
  species.

51
Cell 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 others fates
  by induction.

52
Induction

• Induction 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

53
Spemann-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.

54
Building a Body Plan

• Cell 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.

55
Building a Body Plan

• The sequence includes
• Cell movement
• Changes in adhesion
• Cell proliferation
• There 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.

56
Hox Genes

• Hox 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.

57
Formation of the Vertebrate Limb

• Inductive signals play a major role in pattern
  formation the development of an animals
  spatial organization.

58
Formation of the Vertebrate Limb

• The molecular cues that control pattern
  formation, called positional information
• Tell a cell where it is with respect to the
  animals body axes.
• Determine how the cell and its descendents
  respond to future molecular signals.

59
Formation 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.

60
Formation 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.

61
Morphogenesis

• Morphogenesis is a major aspect of development in
  both plants and animals but only in animals does
  it involve the movement of cells.

62
The Cytoskeleton, Cell Motility, and Convergent
Extension

• Changes in the shape of a cell usually involve
  reorganization of the cytoskeleton.

63
Changes in Cell Shape

• The formation of the neural tube is affected by
  microtubules and microfilaments.

64
Cell Migration

• The 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.

65
Evo-Devo

• Evolutionary 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

66
Evo-Devo

• Instead 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.

67
The Common Vertebrate Heritage

• Vertebrates share a common ancestry and a common
  pattern of early development.
• Vertebrate hallmarks all present briefly.
• Dorsal neural tube
• Notochord
• Pharyngeal gill pouches
• Postanal tail

68
Amniotes

• The 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.

69
Amniotes

• In these three types of organisms, the three germ
  layers also give rise to the four extraembryonic
  membranes that surround the developing embryo.

70
Amniotes

• Amnion 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.

71
Amniotes

• 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.

72
The Mammalian Placenta and Early Mammalian
Development

• Most mammalian embryos do not develop within an
  egg shell.
• Develop within the mothers body.
• Most retained in the mothers body.
• Monotremes
• Primitive mammals that lay eggs.
• Large yolky eggs resembling bird eggs.
• Duck-billed platypus and spiny anteater.

73
The Mammalian Placenta and Early Mammalian
Development

• Marsupials
• Embryos born at an early stage of development and
  continue development in abdominal pouch of
  mother.
• Placental Mammals
• Represent 94 of the class Mammalia.
• Evolution of the placenta required
• Reconstruction of extraembryonic membranes.
• Modification of oviduct - expanded region formed
  a uterus.

74
Mammalian Development

• The eggs of placental mammals
• Are small and store few nutrients.
• Exhibit holoblastic cleavage.
• Show no obvious polarity.

75
Mammalian Development

• Gastrulation and organogenesis resemble the
  processes in birds and other reptiles.

76
Mammalian 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.

77
Mammalian Development

• The extraembryonic membranes in mammals are
  homologous to those of birds and other reptiles
  and have similar functions.

78
Mammalian Development

• Amnion
• Surrounds embryo
• Secretes fluid in which embryo floats
• Yolk sac
• Contains no yolk
• Source of stem cells that give rise to blood and
  lymphoid cells
• Stem cells migrate to into the developing embryo
• Allantois
• Not needed to store wastes
• Contributes to the formation of the umbilical
  cord
• Chorion
• Forms most of the placenta

79
Organogenesis

• Various regions of the three embryonic germ
  layers develop into the rudiments of organs
  during the process of organogenesis.

80
Organogenesis

• Many different structures are derived from the
  three embryonic germ layers during organogenesis.

81
Derivatives 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.

82
Derivatives of Ectoderm Nervous System and Nerve
Growth

• Neural crest cells pinch off from the neural
  tube.
• Give rise to
• Portions of cranial nerves
• Pigment cells
• Cartilage
• Bone
• Ganglia of the autonomic system
• Medulla of the adrenal gland
• Parts of other endocrine glands
• Neural crest cells are unique to vertebrates.
• Important in evolution of the vertebrate head and
  jaws.

83
Derivatives of Endoderm Digestive Tube and
Survival of Gill Arches

• During gastrulation, the archenteron forms as the
  primitive gut.
• This endodermal cavity eventually produces
• Digestive tract
• Lining of pharynx and lungs
• Most of the liver and pancreas
• Thyroid, parathyroid glands and thymus

84
Derivatives 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 function
• 1st 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.

85
Derivatives 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).

86
Derivatives of Mesoderm Support, Movement and
the Beating Heart

• The splitting, fusion and migration of somites
  produce the
• Axial skeleton
• Dermis of dorsal skin
• Muscles of the back, body wall, and limbs
• Heart
• Lateral to the somites the mesoderm splits to
  form the coelom.