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Animal Tissues & Development
Animal Cells
Tissues and Morphogenesis
•  Eukaryotic
•  No cell wall
No plastids
No central vacuole
•  Multicellular:
–  extensive
specialization &
differentiation
–  unique cell-cell
junctions
Animals
Cadherins & Cell junctions
•  Motile
Cadherins
•  “calcium-dependent adhesion”
transmembrane proteins
•  Highly differentiated
tissues
–  Tissue-specific
•  Δ Ca++ ➞ Δ adhesion strength
•  Intercellular junctions
–  Allow developmental
cell migration
–  tissue-specific cadherins
•  Δ cadherin type ➞ Δ binding
•  Extracellular protein
fibers
–  Allow developmental
tissue separation
–  collagen
•  Diploid life cycle
•  Blastula/gastrula
embryo
•  Collagen fibers
•  Elastin fibers
•  Fibronectin
• 
sea urchin
(echinoderm), a
model organism
–  Attachment / movement along ECM
Seconds
post-fertilization events
Extracellular Matrix (ECM)
1
Binding of sperm to egg
2
Acrosomal reaction: plasma membrane
depolarization (fast block to polyspermy)
3
4
6
8
10
Increased intracellular calcium level
20
Cortical reaction begins (slow block to polyspermy)
30
Sperm
head
40
50
1
Minutes
EGG CYTOPLASM
Formation of fertilization envelope complete
2
Increased intracellular pH
3
4
5
Increased protein synthesis
10
20
Fusion of egg and sperm nuclei complete
30
40
Onset of DNA synthesis
60
Figure 47.5
Heyer
90
First cell division
1
Animal Tissues & Development
Cleavage: DNA replication / mitosis / cytokinesis
with no growth phases
products of cytokinesis smaller & smaller blastomeres
Spiral vs. Radial Cleavage
•  Protostomes: “mouth first”
S
S
G1
M
–  most invertebrates
–  spiral cleavage
–  determinate
G2
M
Cell cycle during
cleavage stage
•  Deuterostomes: “mouth second”
Cell cycle after
cleavage stage
–  echinoderms
& vertebrates
–  radial cleavage
–  indeterminate
•  Little/no synthesis of new RNA or proteins
•  All cells dependent upon molecular machines from original ovum
Determinate cleavage in a protostome (round worm)
Time after fertilization (hours)
Determinate cleavage in a protostome (round worm)
Zygote
0
First cell division
Nervous
system,
outer skin,
musculature
Musculature,
gonads
Outer skin,
nervous system
Germ line
(future
gametes)
Musculature
20 µm
10
Cytoplasmic determinates – RNA-protein complexes
•  Define cell fate and body axis.
•  E.g., “P-granules” in round worm embryo
•  Dispersed in egg cell
•  After fertilization, aggregates at future posterior end
•  Upon each cleavage, partition to posterior-most cell
Hatching
Intestine
Intestine
Mouth
1 Newly fertilized egg
3 Two-cell embryo
2 Zygote prior to first division
4 Four-cell embryo
Anus
Vulva
Eggs
POSTERIOR
ANTERIOR
1.2 mm
Figure 47.19
Caenorhabditis elegans Indeterminate cleavage in a deuterostome (frog)
Blastulation — Sea Urchin
Experiment
Experimental egg
(side view)
Control egg
(dorsal view)
1a
1b
Gray
crescent
Gray
crescent
Figure 47.21
•  Cleavage partitions the cytoplasm of one large cell into
many smaller cells called blastomeres
•  Continued cleavage  hollow structure called a blastula
–  The hollow cavity is the blastocoel
Thread
2
Results
Figure 47.23
Heyer
(a)
Normal
Belly piece
Normal
Fertilized egg. Shown
here is the zygote shortly
before the first cleavage
division, surrounded by
the fertilization envelope.
The nucleus is visible in
the center.
(b) Four-cell
stage.
Remnants of the
mitotic spindle can be
seen between the two
cells that have just
completed the second
cleavage division.
(c) Morula. After
further
cleavage divisions, the
embryo is a multicellular
ball that is still surrounded
by the fertilization
envelope. The blastocoel
cavity has begun to form.
(d) Blastula. A single
layer of
cells surrounds a large
blastocoel cavity.
Although not visible here,
the fertilization envelope
is still present; the embryo
will soon hatch from it and
begin swimming.
Figure 47.6
2
Animal Tissues & Development
Animal Morphogenesis
Morphogenesis
•  Creation of form - directed by genes
•  In plants, by differential growth
•  In animals, by both growth & cell migration
Cell
movement
Zygote
(fertilized egg)
Eight cells
– 
– 
– 
– 
Gut
Cell proliferation
Cell migration
Cell differentiation
Cell death (apoptosis)
Blastula
Gastrula
Adult animal
(cross section) (cross section) (sea star)
Cell division
Morphogenesis
Observable cell differentiation
Seed
leaves
Shoot
apical
meristem
Figure 21.4
Zygote
(fertilized egg)
Root
apical
meristem
Embryo
Plant
inside seed
Two cells
Blastulation & Gastrulation
Primary embryonic germ layers
•  Diploblastic: two germ layers
•  Early embryonic development in animals
–  Ectoderm: develops into epidermal & neural tissues
–  Endoderm: develops into feeding tissues
–  Blastocoel: becomes filled with acellular mesoglia
3 In most animals, cleavage results in the
formation of a multicellular stage called a
blastula. The blastula of many animals is a
hollow ball of cells.
1 The zygote of an animal
undergoes a succession of mitotic
cell divisions called cleavage.
Blastocoel
Cleavage
Cleavage
6 The endoderm
of the archenteron
develops into
the the animal’s
digestive tract.
Blastocoel
Zygote
Eight-cell stage
Blastula
Cross section
of blastula
Endoderm
Blastocoel
Examples:
Porifera & Cnidaria
Endoderm
5
The blind
pouch formed by
gastrulation, called
the archenteron,
opens to the outside
via the blastopore.
Ectoderm
Ectoderm
Gastrula
Blastopore
Figure 32.2
Gastrulation
4 Most animals also undergo gastrulation, a
rearrangement of the embryo in which one end of the
embryo folds inward, expands, and eventually fills the
blastocoel, producing layers of embryonic tissues: the
ectoderm (outer layer) and the endoderm (inner layer).
Blastopore
Primary embryonic germ layers
Gastrulation — Sea Urchin
•  Triploblastic: three germ layers
–  Ectoderm: develops into epidermal & neural tissues
–  Endoderm: develops into gut & accessory organs
–  Mesoderm — displaces blastocoel: develops into
muscle, endoskeleton, & connective tissues
Examples:
everything else
Archenteron
Mesoderm
Blastopore
Figure 32.10b
Heyer
Figure 47.8
3
Animal Tissues & Development
Triploblastic gastrulation forms
three germ layers
ECTODERM
• Epidermis of skin and its
derivatives (including sweat
glands, hair follicles)
• Epithelial lining of mouth
and rectum
• Sense receptors in
epidermis
• Cornea and lens of eye
• Nervous system
• Adrenal medulla
• Tooth enamel
• Epithelium or pineal and
pituitary glands
MESODERM
• Notochord
• Endoskeletal system
• Muscular system
• Muscular layer of
stomach, intestine, etc.
• Excretory system
• Circulatory and lymphatic
systems
• Reproductive system
(except germ cells)
• Dermis of skin
• Lining of body cavity
• Adrenal cortex
ENDODERM
• Epithelial lining of
digestive tract
• Epithelial lining of
respiratory system
• Lining of urethra, urinary
bladder, and reproductive
system
• Liver
• Pancreas
• Thymus
• Thyroid and parathyroid
glands
Figure 47.16
c.f., Figure 40.5
Epithelial Tissue
•  Continuous sheet
or layers of cells
with direct cellcell junctions
•  All three germ
layers start as
epithelia, so
epithelial tissues
may derive from
any germ layer.
Muscle Tissue
c.f., Figure 40.5
•  Specialized for
contraction.
•  Derived from
mesoderm.
–  Grouped into four major tissue types:
•  Epithelial
•  Connective
•  Muscle
•  Nervous
Connective Tissue
c.f., Figure 40.5
•  Cells are
suspended in an
extracellular
matrix.
–  often largely
composed of
collagen
fibers.
•  Derived from
mesoderm.
Nervous Tissue
c.f., Figure 40.5
•  Specialized to
conduct
electrochemical
nerve impulses.
•  Derived from
ectoderm.
Note: “Nerve” =
bundle of axons from
multiple neurons
Glia
Neuron:
Dendrites
Cell body
15
µm
Axons of
neurons
Axon
40
µm
•  Diploblastic
animals have
myo-epithelia
for contraction.
Triploblastic Animal Tissues
•  Typical mammalian body is composed of
~50,000,000,000,000 cells
•  Typical vertebrate body is composed of >100
specialized types of cells (tissue types)
(Fluorescent LM)
(Confocal LM)
Heyer
4
Animal Tissues & Development
Tissues  Organs  Organ Systems
External environment
CO2
Food
O2
Animal
body
Respiratory
system
Lung tissue (SEM)
Interstitial
fluid
Heart
Nutrients
250 µm
Mouth
Cells
Digestive
system
Excretory
system
50 µm
Lining of small
intestine (SEM)
100 µm
Circulatory
system
Blood vessels in
kidney (SEM)
Metabolic waste
products (nitrogenous waste)
Anus
Unabsorbed
matter (feces)
Body Symmetry
•  Developmental pattern formation results in
symmetry of growth and regional specialization
Bauplan:
Radial symmetry. The parts
of a radial animal, such as a
sea anemone (phylum
Cnidaria), radiate from the
center. Any imaginary slice
through the central axis
divides the animal into
mirror images.
Ger. “Life Plan” (pl: baupläne)
The arrangement, pattern, and
development of tissues, organs, and
systems unique to a particular type of
organism.
Bilateral symmetry. A
bilateral animal, such as
a lobster (phylum
Arthropoda), has a left
side and a right side.
Only one imaginary cut
divides the animal into
mirror-image halves.
Figure 32.7
Coelom
(c)
Acoelomate. flatworms
–  Formation of coelom (body
cavity) allows movement of
organs within the body,
esp. gut expansion & motility
Body covering
(from ectoderm)
•  Pseudocoelomate: cavity
between endoderm &
mesoderm
Pseudocoelomate. nematode worm
Pseudocoelom
Heyer
Body covering
(from ectoderm)
Muscle layer
(from
mesoderm)
Digestive tract
(from ectoderm)
•  Eucoelomate: cavity within
(a)
mesoderm
Coelomate. annelid worm
Figure 32.9
Eucoelomate Gastrulation
•  Coelom development in open vs. closed circulation
Digestive tract
(from endoderm)
(b)
•  Acoelomate: no body cavity
Variations in
Tissuefilled region
(from
mesoderm)
Coelom
Digestive tract
(from endoderm)
Body covering
(from ectoderm)
Tissue layer
lining coelom
and suspending
internal organs
(from mesoderm)
5
Animal Tissues & Development
(a) Cleavage
Protostome development
(examples: molluscs,
annelids)
Eight-cell stage
Deuterostome development
(examples: echinoderms,
chordates)
Eight-cell stage
•  Gastrovascular cavity (blind gut)
–  Blastopore remains only orifice to gut
•  Protostome (“mouth first”) development
Coelom
(b) Coelom
formation
•  Deuterostome (“mouth second”) development
Archenteron
–  The blastopore becomes the anus
–  Secondary invagination to form mouth
Coelom
Mesoderm
Anus
(c) Fate of the
blastopore
Protostome development
(examples: molluscs,
annelids)
Eight-cell stage
Anus
Anus develops
from blastopore.
Blastopore
Mesoderm
Folds of archenteron
form coelom.
Mouth
Anus
Digestive tube
Key
Mouth
Mouth develops
from blastopore.
Mouth
Mouth develops
from blastopore.
(a) Cleavage
Mouth
Blastopore
Solid masses of mesoderm
split and form coelom.
Digestive tube
Gastrovascular
Cavity
Radial and indeterminate
Spiral and determinate
–  The blastopore becomes the mouth
–  Secondary invagination to form anus
Ectoderm
Mesoderm
Endoderm
Mouth
Mouth develops from blastopore.
Anus
Anus develops from blastopore.
Figure 32.10
Deuterostome development
(examples: echinoderms,
chordates)
Eight-cell stage
Radial and indeterminate
Spiral and determinate
Coelom
(b) Coelom
formation
Archenteron
Coelom
Mesoderm
Blastopore
Blastopore
Solid masses of mesoderm
split and form coelom.
(c) Fate of the
blastopore
Mesoderm
Folds of archenteron
form coelom.
Mouth
Anus
Digestive tube
Key
Ectoderm
Mesoderm
Endoderm
Mouth
Mouth develops from blastopore.
Anus
Anus develops from blastopore.
Figure 32.10
Gastrulation — Sea Urchin
Gastrulation — Sea Urchin
Figure 47.8
Heyer
6
Animal Tissues & Development
Protostome Larval Development
Protostomal development occurs in two distinct animal groups
•  Lophotrochozoa: have ciliated larval stages
– 
Usually with a distinct larval stage called a trochophore
Apical tuft
of cilia
•  Ecdysozoa: have no ciliated tissues
–  All stages have an external cuticle
–  Growth requires ecdysis (molting)
Mouth
Anus
trochophore larva
Figure 32.12
ecdysis
More Variations in
Deuterostome Gastrulation
Vertebrate
Development
From
Figure 47.2
EMBRYONIC DEVELOPMENT
Sperm
M.K. Richardson (1997) Anatomy & Embryology
Radial Cleavage & Blastulation — Frog
Zygote
• Large yolk
content
necessitates
asymmetrical
blastulation
N
Adult
frog
FER
TIL
IZA
TIO
Egg
GA
OR
Metamorphosis
GE
VA
EA
CL
GA
ST
RU
LA
TIO
SIS
ENE
G
NO
Larval
stages
Blastula
N
Gastrula
Tail-bud
embryo
Heyer
7
Animal Tissues & Development
47-22: Frog body polarity
Point of
sperm entry
— established during
oogenesis &
fertilization
Animal
hemisphere
Zygote Animal
hemisphere
Cleavage furrow
47-7: Frog embryo —
Animal pole
Cleavage planes &
asymmetrical
blastulation
0.25 mm
Vegetal
hemisphere
Gray
crescent
Ventral
4-cell
stage
forming
Animal
pole
Future
dorsal
side of
tadpole
Anterior
2-cell
stage
forming
Gray
crescent
Vegetal pole
Point of
sperm
entry
Right
Vegetal
hemisphere
0.25 mm
Blastocoel
8-cell
stage
First
cleavage
Dorsal
Left
Posterior
Body axes
47-10: frog
Blastula
(cross section)
Establishing the axes
CROSS SECTION
SURFACE VIEW
Animal pole
Blastocoel
1
gastrulation
Dorsal lip
of blastopore
Early
gastrula
Vegetal pole
Neural plate
Neural
fold
— unique to chordates
Neural folds
Blastopore
Blastocoel
shrinking
2
Neurulation
Dorsal lip
of blastopore
Archenteron
Neural crest
LM
1 mm
Neural
fold
Blastopore
3
Neural
plate
Outer layer
of ectoderm
Blastopore
Blastocoel
remnant
Neural crest
Ectoderm
Mesoderm
Endoderm
Notochord
Ectoderm
Mesoderm
Neural tube
Endoderm
Archenteron
Formation of the neural tube
Archenteron
Future ectoderm
Future mesoderm
Future endoderm
Late
gastrula
Blastopore
47-14: frog
Yolk plug
neurulation
Blastopore
Neural plate formation
47-18: frog fate map
Epidermis
Somites
Eye
Epidermis
Central
nervous
system
Mesoderm lateral to the
notochord forms blocks called
somites
Lateral to the somites, the
mesoderm splits to form the
coelom
Notochord
Mesoderm
Tail bud
Chordate mesoderm
segmentation
SEM
Neural tube
Notochord
Coelom
1 mm
Neural
crest
Somite
Endoderm
Blastula
Neural tube stage
(transverse section)
Archenteron
(digestive cavity)
Fate map of a frog embryo
Coelom formation by mesoderm separation
Heyer
47-14c: frog segmentation
Somites
8
Animal Tissues & Development
Meroblastic cleavage
Holoblastic cleavage
•  Only non-yolk cytoplasm of the egg cleaved
•  Complete division of the egg into blastomeres
–  Occurs in species whose eggs have little or moderate amounts of yolk
–  Occurs in species whose large eggs have abundant yolk
Fish
•  Most of yolk partitioned into vegetal pole blastomeres
Development
–  Establishes anterior/posterior axis
50 µm
Sea urchin: • small egg; little yolk
• Symmetrical holoblastic cleavage
Frog: • larger egg; moderate yolk
• Asymmetrical holoblastic cleavage
250 µm
Source: http://www.ucalgary.ca/UofC/eduweb/virtualembryo/why_fish.html
Another way — asymmetric
blastulation in many vertebrates
Fish
•  Large, yolk-rich eggs
•  Extreme meroblastic
cleavage forms the
blastoderm.
•  Separation of the
epiblast from the
hypoblast forms the
blastocoel.
Development
Fertilized egg
Figure 47.10
Disk of
cytoplasm
1 Zygote. Most of the cell’s volume is yolk,
with a small disk of cytoplasm located at the
animal pole.
2
Four-cell stage.
3 Blastoderm. The many cleavage
divisions produce the blastoderm, a mass of
cells that rests on top of the yolk mass.
BLASTODERM
YOLK MASS
Epiblast
Blastocoel
Hypoblast
Cutaway view of the
blastoderm. The cells of the
blastoderm are arranged in two
layers, the epiblast and
hypoblast, that enclose a fluidfilled cavity, the blastocoel.
Source: http://www.ucalgary.ca/UofC/eduweb/virtualembryo/why_fish.html
Gastrulation — Chick
Gastrulation — Chick
Eye
Epiblast
Forebrain
Neural tube
Notochord
Somite
Heart
Coelom
Future
ectoderm
Primitive
streak
Archenteron
Endoderm
Mesoderm
Lateral fold
Blood
vessels
Ectoderm
Yolk stalk
Migrating
cells
(mesoderm)
Endoderm
Hypoblast
YOLK
Figure 47.11
Heyer
Form extraembryonic
membranes
Somites
Yolk sac
YOLK
(a) Early organogenesis. The archenteron forms when
lateral folds pinch the embryo away from the yolk.
Neural tube
(b) Late organogenesis. 56 hours old
chick embryo, about 2–3 mm long (LM).
Figure 47.15
9
Animal Tissues & Development
Amniotes: extra-embryonic membranes
Amnion
Allantois
Endometrium
(uterine lining)
47-12: mammalian
blastulation
Embryo
Inner cell mass
Trophoblast
Blastocoel
Amniotic
cavity
with
amniotic
fluid
Albumen
Maternal
blood
vessel
Shell
Yolk
(nutrients)
Chorion
Yolk sac
47-13: chick extra-embryonic membranes
Expanding
region of
trophoblast
47-12: mammalian
gastrulation
Blastocyst
reaches uterus.
Amniotic
cavity
Amnion
Expanding
region of
trophoblast
Epiblast
Hypoblast
Trophoblast
Blastocyst
implants.
Mammalian Gastrulation
Epiblast
Hypoblast
Chorion (from
trophoblast
Yolk sac (from
hypoblast)
Extraembryonic
membranes start
to form and
gastrulation
begins.
Extraembryonic mesoderm cells
(from epiblast)
Allantois
Amnion
Chorion
Ectoderm
Mesoderm
Endoderm
Yolk sac
Extraembryonic
mesoderm
Gastrulation has produced a
three-layered embryo with four
extraembryonic membranes.
Mammalian embryo &
extraembryonic membranes
Heyer
Amniote embryo &
extraembryonic membranes
10