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Stephanie Bobbitt
Chapter 47) Animal Development p. 998-1017
- The Stages of Early Embryonic Development
o From egg to organism, an animal’s form develops gradually: the concept of epigenesis
 preformation: old idea that the embryo already had everything it needed, and development just
meant it got bigger; also included the idea that the embryo contains all of its descendents
 epigenesis: the form of an animal comes gradually from a formless egg
 development includes cell division and differentiation and morphogenesis (process where an animal
takes shape); overall development process is epigenesis
o Fertilization activates the egg and brings together the nuclei of sperm and egg
 purpose of fertilization is to combine haploid sets of chromosomes into one diploid cell
 when sperm makes contact with egg’s surface, metabolic reactions are initiated in the egg to start
embryonic development
 The Acrosomal Reaction
 acrosomal reaction (in sea urchins): sperm cell meets jelly coat around an egg  vesicle at
sperm tip called acrosome discharges contents by exocytosis; reaction releases hydrolytic
enzymes that enable the acrosomal process structure to penetrate the jelly coat
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lock-and-key recognition ensures eggs are fertilized by sperm of same species
membranes of the sperm and egg fuse together  ion channels open in egg cell’s plasma
membrane  allows sodium
ions to flow into cell
 fast block to polyspermy: aka
depolarization that was
described above; prevents
more than one sperm from
fusing with the egg’s
membrane
The Cortical Reaction
 cortical reaction: series of
changes in cortex of egg
cytoplasm
 fusion of egg and sperm
trigger egg’s ER to release
calcium into cytosol; released calcium causes a change in cortical granules  cortical granules
fuse with plasma membrane and release contents into space between membrane and vitelline
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layer  causes perivitelline space to swell  vitelline layer becomes the fertilization envelope
(resists entry of more sperm)
 slow block to polyspermy: this fertilization envelope and changes in egg surface
 Activation of the Egg
 the rise in calcium concentration in the cytosol quickens the egg’s metabolism
 it is possible to artificially activate an egg by injecting calcium
 egg does not have a nucleus, but it makes new kinds of proteins when activated – inactive
mRNA is stored in cytoplasm before activation
 sperm nucleus
swells and merges
with egg nucleus,
then DNA synthesis
begins
 Fertilization in
Mammals
 capacitation:
enhancing sperm
function in female
reproductive tract
 sperm cell goes
through layer of
follicle cells around
egg to get to zona
pellucida: egg’s
extracellular matrix
made of three
glycoproteins that
form cross-linked
filaments (one
glycoprotein, ZP3, is a sperm
receptor)
 when sperm binds to receptor
molecule, acrosome of sperm is
induced to release contents –
enzymes from acrosome allow
sperm to penetrate zona pellucida
to get to the egg’s membrane
 microvillia (fingerlike extensions
of egg) take sperm into egg
 chromosomes from both gametes
share a spindle apparatus during
first mitotic division of zygote –
chromosomes from both parents
do not come together until after
this first division
Cleavage partitions the zygote into many
smaller cells
 building animal’s body occurs in three
stages: (1) cleavage to make
multicellular embryo (2) gastrulation
to make a three-layered embryo (3)
organogenesis to make organs
 cleavage: rapid cell division after
fertilization; cells go through cell cycle,
skipping G1 and G2 phases; divides
the zygote cell into smaller cells called blastomeres
yolk: stored nutrients; influences pattern of cleavage
vegetal pole has a bigger concentration of yolk than the opposite pole called animal pole
(see figure on previous page) plasma membrane and cortex rotate to where sperm entered 
exposes a region of the cytoplasm called gray crescent, which marks the dorsal side of embryo
 yolk slows down cell division, so cleavage occurs faster in animal hemisphere
 morula: solid ball of cells produced by continued cleavage
 blastocoel: fluid-filled cavity forms in morula – making hollow ball called blastula
 meroblastic cleavage: incomplete division of a yolk-rich egg (ex: in birds)
 holoblastic cleavage: complete division of eggs with small or moderate amount of yolk (ex: in frogs)
Gastrulation rearranges the blastula to form a three-layered embryo with a primitive gut
 gastrulation: major rearrangement of cells of blastula; involves changes in cell motility, cell shape,
and cellular adhesion to other cells and molecules
 gastrula: three-layered embryo made by gastrulation
 ectoderm: outer layer of gastrula; later develops into nervous system and epidermis
 endoderm: lines embryonic digestive tract; later develops into digestive tract and related organs
 mesoderm: partly fills space between ectoderm and endoderm; later develops into organs like
kidney, heart, muscles, etc.
 sea urchin gastrulation begins at vegetal
pole where some cells detach from blastula
wall and go to blastocoel as cells called
mesenchyme cells; other cells flatten to
make a plate that buckles inward (process
is called invagination)  buckled plate’s
cells are rearranged that transforms the
invagination into a deep pouch called the
archenteron (open end of this is called the
blastopore and will later become the anus)
 an opening forms at the other end of
archenteron which will be the mouth end of
the digestive tube
 right figure shows sea urchin gastrulation
 figure on next page shows frog gastrulation
 frog gastrulation starts with a small crease
on one side of blastula that is caused by
invagination that becomes the dorsal side
of blastopore (called dorsal lip; forms
where gray crescent was in zygote) 
involution occurs (cells on embryo surface
roll into embryo interior and move away
from blastopore  cells or organized into
the mesoderm and endoderm layers  yolk
plug made of big cells move inward and
cells on outside develop into the ectoderm)
In organogenesis, the organs of the animal
body form from the three embryonic germ
layers
 organogenesis: regions of the layers
develop into organs
 figure on next page shows early
organogenesis in frogs
 notochord: made from dorsal mesoderm that condenses, eventually become the vertebral discs
 neural tube starts off as a plate of dorsal ectoderm which will fold inward to become the neural tube,
which becomes the central nervous system (brain and spinal cord)
 somites: blocks that some mesoderm separates into; cells from these give rise to vertebrae and make
muscles associated with axial skeleton
 neural crest: band of cells that migrate to parts of embryo and form pigment cells of skin, skull,
teeth, adrenal glands, etc.
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Amniote embryos develop in a fluidfilled sac within a shell or uterus
 all vertebrate embryos need an
aqueous environment in order to
develop
 amniotes: vertebrates with amnions
(sac of fluid)
 Avian Development
 see figure on next page
 blastodisc: cap of cells made by
early cleavage divisions
 blastomeres sort into the
epiblast and hypoblast layers
 gastrulation involves cells of the
epiblast to move to the middle
of the blastodisc, detach, and
move toward yolk – this
movement makes groove the primitive streak (marks where anterior-posterior axis will be)
 all cells that make the embryo come from the epiblast; hypoblast directs formation of primitive
streak and its cells later make a sac around yolk and a stalk that connects the yolk to the embryo
 borders of embryonic disc fold down to pinch embryo into a three-layered tube joined in the
middle to the yolk
 tissues outside the embryo develop into four extraembryonic membranes for further
development (four are yolk sac, amnion, chorion, and allantois – see figure on next page)
 yolk sac digests yolk, amnion protects embryo from drying out, chorion cushions embryo against
mechanical shocks, and allantois is a disposal sac for uric acid
Mammalian Development
 cleavage in mammals is slower
 compaction: process that tightly sticks
cells together; involves making new
proteins on cell surface like cadherins
 see diagram on next page of development
 (1) a cluster of cells called the inner cell
mass protrudes into blastocyst (stage
where embryo has over 100 cells
surrounding a central cavity); trophoblast
surrounds the cavity; trophoblast +
mesoderm = fetal part of placenta
 (2) trophoblast secretes enzymes that let
the blastocyst go through the
endometrium and expands to extend
fingerlike projections into maternal tissue; blastocyst makes disc with epiblast (upper layer of
cells) and hypoblast (lower)
 (3) trophoblast gives rise to chorion; epiblast starts making amnion
 (4) gastrulation – cells move from epiblast through primitive streak to make mesoderm and
endoderm; 4 extraembryonic membranes formed – chorion, amnion, yolk sac (for mammals site of early formation of blood cells), and allantois (incorporated into umbilical cord)
The Cellular and Molecular Basis of Morphogenesis and Differentiation in Animals
o Morphogenesis in animals involves specific changes in cell shape, position, and adhesion
 morphogenesis involves cell movement only in animals
 changes in cell shape usually involve reorganizing cytoskeleton
 cells crawl through embryo with cytoskeletal fibers to extend and retract cellular protrusions that
are usually flat sheets (lamellipodia) or spikes (filopodia)
 convergent extension: morphogenic movement where a tissue layer’s cells rearrange so the cell
sheet narrows (converges) and elongates (extends)
 ECM may be involved in convergent extension since it guides cells in morphogenetic movements
 EC glycoproteins help cells move by providing anchorage and hold cells together when they reach
their destination
 certain substances inhibit migration of cells in certain directions
 cell adhesion molecules (CAMs): glycoproteins that bind to CAMs on other cells for stable tissue
structure and cell migration
 cadherins: cell-to-cell adhesion molecule that need calcium ions to function correctly
o The developmental fate of cells depends on cytoplasmic determinants and cell-cell induction: a review
 in many animal species, heterogeneous distribution of cytoplasmic determinants in unfertilized egg
leads to regional differences in early embryo
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interactions between embryonic cells induce changes in gene expression  ultimately cause cells to
differentiate into specialized cells
Fate mapping can reveal cell
genealogies in chordate embryos
 fate maps: territorial diagrams
of embryonic development –
shows which parts of embryo
will be derived from the parts
of the zygote or blastula
 early cells make specific
tissues of the older embryo
 a cell’s developmental
potential (range of structures
it can give rise to) is restricted
as development continues
The eggs of most vertebrates have
cytoplasmic determinants that
help establish the body axes and
differences among cells of the
early embryo
 Polarity and the Basic Body
Plan
 establishing a basic body
plan (determining where
anterior-posterior axis,
dorsal-ventral axis, and left
and right sides are) is
needed before the
organism can develop
 in mammals, polarity isn’t
obvious until after
cleavage; in other species,
basic instructions are
known earlier
 Restriction of Cellular Potency
 first cleavage could make
two identical blastomeres
(equal developmental
potential)
 ex: first two blastomeres
are similar and when separated, both can develop into a normal tadpole (aka blastomeres are
totipotent)
 in mammals, embryos stay totipotent until arranged into trophoblast and blastocyst’s inner cell
mass; other species, only zygote is totipotent
 tissue-specific fates of cells in late gastrulae are fixed
Inductive signals drive differentiation and pattern formation in vertebrates
 induction: one group of cells influences development of another
 The “Organizer” of Spemann and Mangold
 Spemann and Mangold discovered that blastopore’s dorsal lips in early gastrula plays a role in
embryonic development (initiation inductions that form neural tube and other organs)
 see diagram on next page for their experiment
 blastopore was called the primary organizer of the embryo
 growth factor BMP-4 is active only on ventral side of gastrula because organizer cells inactive
BMP-4 on dorsal side or embryo by making proteins that bind to it
 inductions typically are a sequence of inductive steps
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(1) transplant piece of dorsal lip of a nonpigmented newt gastrula to ventral side of early
gastrula of pigmented newt
(2) recipient embryo
made a second
notochord and neural
tube where the
transplantation
occurred; cells in the
recipient were still
made from itself, so
the nonpigmented
embryo had organized
the recipient