This presentation if from my professor. This is very useful

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
 Specializationof 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.
 Containsmuch 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
 Mostof these preparations in the egg
occur during the prolonged prophase I.
 In mammals
 Oocyte now has a highly structured
system.
 Afterfertilization it will support nutritional
requirements of the embryo and direct its
development through cleavage.
 Aftermeiosis 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
 Aftersperm 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
 Inmammals 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.
 Inenterocoely, the coelom forms as
outpocketing of the gut.

39. Deuterostome Development
 Typicaldeuterostomes 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 can’t
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 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.

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 other’s
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
 Inductivesignals play a major role in
pattern formation – the development of
an animal’s spatial organization.

58. Formation of the Vertebrate
Limb
 The molecular cues that control pattern
formation, called positional
information:
 Tella cell where it is with respect to the
animal’s 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.
 Ingastrulation, 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
 Insteadof 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 mother’s body.
 Most retained in the mother’s 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
 Embryosborn 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
 Variousregions 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.