ANIMAL DEVELOPMENT
CHAPTER 47
Figure 47.0 Human embryo
SEA URCHIN FERTILIZATION
• Acrosomal Reaction – sperm
release hydrolytic enzymes
that allow acrosomal process
to penetrate jelly coat of egg
• Membranes of sperm and egg
fuse together
• Causes membrane depolarization
(change in membrane potential)
which is the fast block to
polyspermy
• Cortical reaction – fusion triggers
signal transduction that causes ER
to release Ca2+ that in turn causes
cortical granules to release their
contents into space between
membrane and vitelline layer
• Causes osmosis and swelling of
space which is the slow block
to polyspermy
• High Ca2+ also causes
metabolic activity to begin in
egg
Figure 47.2 The acrosomal and cortical reactions during
sea urchin fertilization
Figure 47.3 A wave of Ca2+ release during the cortical
reaction
MAMMAL FERTILIZATION
• Differences between sea urchin
and mammals:
– Oocyte cloaked in follicle cells
– Zone pellucida is like the vitelline
layer
– Haploid nuclei of sperm and egg
do not immediately fuse, but only
after first cell division
Figure 47.5 Fertilization in mammals
Figure 47.4 Timeline for the fertilization of sea urchin eggs
VOCABULARY
• Cleavage – cell division after
fertilization; embryo doesn’t grow
• Yolk – where nutrients are stored;
most plentiful in birds, reptiles
and fish
• Gastrulation – forms three layers
(ectoderm, endoderm, and
mesoderm)
• Organogenesis – generation of
rudimentary organs
Figure 47.6 Cleavage in an echinoderm (sea urchin)
embryo
Figure 47.6x Sea urchin development, from single cell to
larva
CLEAVAGE
• Meroblastic cleavage –
incomplete cell division due to
yolk
• Holoblastic – complete cell
division (little, moderate, or
no yolk)
• Most animals (except
mammals) have definite
polarity due to
heterogeneously distributed
materials (such as yolk)
• Animal pole – no yolk end of
embryo
• Vegetal pole – yolk end of
embryo
• In frogs, plasma membrane
and cortex rotate to form
grey crescent
• In birds, cleavage is
meroblastic
• In humans and sea urchins,
holoblastic cleavage
Figure 47.7 The establishment of the body axes and the
first cleavage plane in an amphibian
Figure 47.8x Cleavage in a frog embryo
Figure 47.8d Cross section of a frog blastula
GASTRULATION
• Sea Urchin
– Single-celled blastula wall
– Mesenchyme cells migrate
to form mesoderm
– Invagination forms
archenteron
– Blastopore becomes anus
Figure 47.9 Sea urchin gastrulation
• Frog
– More yolk and wall of blastula
more than one cell thick
– Invagination forms blastopore
and dorsal lip
– Involution – cells on outer
surface rolling over lip to inside
of embryo
– Blastopore sealed off by yolk
plug
Figure 47.10 Gastrulation in a frog embryo
• Bird
– Primitive streak, a groove,
forms rather than a blastopore
(round)
– Only epiblast cells contribute
to embryo, while hypoblast
cells direct formation of
primitive streak and aid in
other development
Figure 47.12 Cleavage, gastrulation, and early
organogenesis in a chick embryo
ORGANOGENESIS
• First organs formed are
notochord and neural tube
• Notochord – skeletal rode
common to all chordates
• Neural tube becomes the brain
and spinal cord
• Somites become vertebrate
• Neural crest (only
vertebrates) – cells that
migrate to form a variety of
things
Figure 47.11 Organogenesis in a frog embryo
TISSUE LAYERS
• Only part of each layer
contributes to actual embryo
• Extraembryonic membranes
support growing embryo:
– Yolk sac, Amnion, Chorion,
and Allantois
Figure 47.14 The development of extraembryonic
membranes in a chick
Early development of a human embryo and its extraembryonic membranes
MAMMALIAN DEVELOPMENT
• Amnion – encloses embryo in
fluid filled sac (“water breaks”)
• Chorion – cushions embryo
(outside of amnion)
• Allantois – extension of hind gut;
incorporated into umbilical cord
(stores uric acid waste in birds)
• Yolk sac - encloses fluid filled
cavity or yolk (in birds etc.)
MORPHOGENESIS
• Involves specific changes in cell shape,
position, motility, and adhesion
• Developmental fate depends on cytoplasmic
determinants
• Fate mapping can reveal cell genealogies
• Cell fate and pattern formation determined
by inductive signals
• Remember the ZPA and Hox gene
Figure 47.16 Change in cellular shape during
morphogenesis
Figure 47.20 Fate maps for two chordates
Figure 47.21 Experimental demonstration of the
importance of cytoplasmic determinants in amphibians
Figure 47.22 The “organizer” of Spemann and Mangold
Figure 47.23 Organizer regions in vertebrate limb
development
Figure 47.24 The experimental manipulation of positional
information
This undated computed
axial tomography (CT
scan) provided on
Thursday, Nov. 8, 2007,
by the Sparsh Hospital
shows 2-year-old
Lakshmi before she
underwent surgery in
Bangalore, India . The
Indian girl born with
four arms and four
legs.
kidneys and other body
parts of the
undeveloped fetus.
Lakshmi, who has been revered by some in her village as a reincarnation of
the four-armed Hindu goddess she was named for, was born joined at the
pelvis to a “parasitic twin” that stopped developing in her mother’s womb.
The surviving fetus absorbed the limbs,
Mutated Hox gene (HoxD13)