The document discusses early concepts of development including preformation versus epigenesis. It was once thought that the embryo was preformed in the egg, but experiments demonstrated that undifferentiated material in the egg becomes arranged through epigenesis. Key developmental stages are described including fertilization, cleavage, blastula formation, and gastrulation which establishes the three germ layers. Differences in protostome and deuterostome development are outlined, focusing on differences in cleavage, fate determination, and body axis formation. The role of induction in patterning the embryo is also summarized.

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.