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CHAPTER 47: ANIMAL
DEVELOPMENTAP Biology 2013
ZYGOTE TO ADULT• Preformation - 18th century theory that
the egg or sperm contained an embryo
• The embryo was thought to be a preformed miniature infant (homunculus) that becomes larger during development
• We now know:
• An organism’s development is determined by the genome of the zygote and by differences that arise between early embryonic cells
• Cell differentiation - specialization of cells in their structure and function
• Morphogenesis - process by which an animal takes shape
Fig. 47.2EMBRYONIC DEVELOPMENT
Sperm
Adult frog
Egg
Metamorphosis
Larval stages
Zygote
Blastula
Gastrula
Tail-bud embryo
FERT
ILIZ
ATIO
N
CLEAVAGE
GASTRULATION
ORG
ANO-
GENESIS
DEVELOPMENTAL EVENTS• Fertilization - main function is to bring the haploid nuclei of sperm and
egg together to form a diploid zygote
• Contact of the sperm on the egg’s surface initiates metabolic reactions within the egg that trigger embryonic development
• Acrosomal reaction - when sperm meets egg, hydrolytic enzymes that digest material surrounding the egg are released
• Gamete contact blocks polyspermy
Fig. 47.3
Basal body (centriole)
Sperm plasma membrane
Sperm nucleus
Sperm head
Acrosome Jelly coat
Sperm-binding receptors
Fertilization envelope
Cortical granule Fused
plasma membranes
Hydrolytic enzymes Vitelline layer
Egg plasma membrane
Perivitelline space
EGG CYTOPLASM
Actin filament
Acrosomal process
1
2
3
DEVELOPMENTAL EVENTS• Fertilization:
• Fusion of egg and sperm also initiates the cortical reaction which causes a rise in Ca2+ that stimulates cortical granules to release their contents outside the egg
• These changes cause the formation of a fertilization envelope that also acts as a block to polyspermy Fig. 47.4
10 sec after fertilization
25 sec 35 sec 1 min 500 µm
500 µm 30 sec 20 sec 10 sec after
fertilization 1 sec before fertilization
Point of sperm nucleus entry
Spreading wave of Ca2+
Fertilization envelope
EXPERIMENT
RESULTS
CONCLUSION
DEVELOPMENTAL EVENTS
• Activation of the Egg
• Because of the rise in Ca2+ in the egg’s cytosol, the rate of cellular respiration and protein synthesis increases substantially
• In mammals, the cortical reaction modifies the zona pellucida as a slow block to polyspermy
Fig. 47.5
Zona pellucida
Follicle cell
Sperm basal body
Sperm nucleus
Cortical granules
DEVELOPMENTAL EVENTS• Cleavage - period of rapid cell division without growth
• Many animals (not mammals) have defined polarity (distribution of yolk with vegetal pole having the most and the animal pole having the least)
Figs. 47.6
(a) Fertilized egg (b) Four-cell stage (c) Early blastula (d) Later blastula
50 µm
4
5
6
DEVELOPMENTAL EVENTS• Cleavage planes follow
a specific pattern relative to the animal and vegetal poles
• Meroblastic cleavage - incomplete division of the egg (yolk-rich eggs like reptiles and birds)
• Holoblastic cleavage - complete division of the egg (little or moderate amounts of yolk like sea urchins and frogs)
Fig. 47.7
Zygote
2-cell stage forming
4-cell stage forming
8-cell stage
Vegetal pole
Blastula (cross section)
Gray crescent
Animal pole
Blastocoel
0.25 mm
0.25 mm
8-cell stage (viewed from the animal pole)
Blastula (at least 128 cells)
DEVELOPMENTAL EVENTS
• Morphogenesis - cells occupy their appropriate locations
• Gastrulation - rearranges the cells of the blastula into a three-layered embryo called a gastrula that has a primitive gut
• Three embryonic germ layers:
• Ectoderm - outer layer of gastrula
• Endoderm - lines the embryonic digestive tract
• Mesoderm - partially fills the space between the ectoderm and endoderm
Figs. 47.8-47.9
Key
Animal pole Blastocoel
Mesenchyme cells
Vegetal plate
Vegetal pole
Archenteron
Filopodia
Archenteron
Blastocoel
Blastopore Mouth
Mesenchyme (mesoderm forms future skeleton)
Anus (from blastopore)
Digestive tube (endoderm)
Ectoderm
Future ectoderm
Future mesoderm
Future endoderm
ECTODERM (outer layer of embryo)
MESODERM (middle layer of embryo)
ENDODERM (inner layer of embryo)
• Epidermis of skin and its derivatives (including sweat glands, hair follicles)
• Epithelial lining of digestive tract and associated organs (liver, pancreas) • Epithelial lining of respiratory, excretory, and reproductive tracts and ducts
• Germ cells • Jaws and teeth • Pituitary gland, adrenal medulla • Nervous and sensory systems
• Skeletal and muscular systems • Circulatory and lymphatic systems • Excretory and reproductive systems (except germ cells) • Dermis of skin • Adrenal cortex
• Thymus, thyroid, and parathyroid glands
GASTRULATION FROG VS. CHICK
Key
Future ectoderm Future mesoderm Future endoderm
SURFACE VIEW CROSS SECTION Animal pole
Vegetal pole Early gastrula
Blastocoel
Dorsal lip of blasto- pore
Blastopore Dorsal lip of blastopore
Blastocoel shrinking
Archenteron
Archenteron
Blastocoel remnant
Ectoderm Mesoderm Endoderm
Blastopore Yolk plug Blastopore
Late gastrula
3
2
1 Fig. 47.10
Future ectoderm
Migrating cells (mesoderm)
Blastocoel
Epiblast
YOLK
Endoderm Hypoblast
Primitive streak
Fertilized egg Primitive streak
Embryo
Yolk
Fig. 47.11
7
8
9
GASTRULATION IN HUMANS• Human eggs have very
little yolk
• Blastocyst - human equivalent of blastula
• Inner cell mass - cluster of cells at one end of the blastocyst
• Trophoblast - outer epithelial layer that does not contribute to embryo but instead initiates implantation
• Gastrulation
Blastocyst reaches uterus. 1
2
3
4
Blastocyst implants (7 days after fertilization).
Extraembryonic membranes start to form (10–11 days), and gastrulation begins (13 days).
Gastrulation has produced a three-layered embryo with four extraembryonic membranes.
Uterus
Maternal blood vessel
Endometrial epithelium (uterine lining) Inner cell mass
Trophoblast
Blastocoel
Expanding region of trophoblast
Epiblast Hypoblast Trophoblast
Expanding region of trophoblast Amniotic cavity Epiblast Hypoblast Yolk sac (from hypoblast)
Extraembryonic mesoderm cells (from epiblast) Chorion (from trophoblast)
Amnion Chorion Ectoderm Mesoderm Endoderm Yolk sac Extraembryonic mesoderm
Allantois
Fig. 47.12
ORGANOGENESIS• Regions of the three germ layers develop into the rudiments of organs during
organogenesis
• Vertebrates form a notochord from the mesoderm and a neural plate from the ectoderm
• Neural plate curves inward forming the neural tube Fig. 47.13
Mesoderm also gives rise to the
somites (later form vertebrae and
muscle) and the coelom
Neural folds
1 mm
Neural fold
Neural plate
Notochord Ectoderm Mesoderm
Endoderm
Archenteron
(a) Neural plate formation
(b) Neural tube formation
(c) Somites
Neural fold
Neural plate
Neural crest cells
Outer layer of ectoderm
Neural crest cells
Neural tube
Eye Somites Tail bud
SEM
Neural tube
Notochord
Coelom
Neural crest cells
Somite
Archenteron (digestive cavity)
1 mm
MORPHOGENESIS• Involves changes in shape,
position, and adhesion
• Changes in shape involve reorganization of the cytoskeleton
• Formation of the neural tube involves microtubules and microfilaments
• Also impacts cell migration (movement of cells from one place to another) ex. convergent extension
• Tissue invagination is caused by changes in both cells shape and migration during gastrulation
Ectoderm
Neural plate
Microtubules
Actin filaments
Neural tube
Extension Convergence
Figs. 47.15 & 47.16
10
11
12
MORPHOGENESIS
• Apoptosis - programmed cell death
• At various times during development, individual cells, sets of cells, or whole tissues stop developing and are engulfed by neighboring cells
DEVELOPMENTAL FATE• Determination - cell or group
of cells becomes committed to a particular fate
• Differentiation
• Embryonic cells must become different from on another
• Interactions with other embryonic cells influence the fate of cells by causing changes in gene expression
• Fate maps - territorial diagrams of embryonic development Fig. 47.17
Epidermis Epidermis Central nervous system Notochord
Mesoderm
Endoderm
Blastula Neural tube stage (transverse section)
(a) Fate map of a frog embryo
64-cell embryos
Blastomeres injected with dye
Larvae
(b) Cell lineage analysis in a tunicate
P GRANULES IN C. ELEGANS
Newly fertilized egg
Zygote prior to first division
Two-cell embryo
Four-cell embryo
20 µm
2
1
3
4 Fig. 47.20
13
14
15
AXES OF EMBRYOS
• Nonamniotic vertebrates - body axes are determined during oogenesis or fertilization
• Amniotes - environmental differences play a role in establishing differences between cells and body axes
• Uneven cytoplasmic determinants are important in establishing body axes
Figs. 47.21 & 47.22
Dorsal Right
Anterior Posterior
Ventral Left
(a) The three axes of the fully developed embryo
(b) Establishing the axes
Animal hemisphere
Vegetal hemisphere
Animal pole
Vegetal pole
Point of sperm nucleus entry
Gray crescent
Pigmented cortex
Future dorsal side
First cleavage
Figure 47.21
Control egg (dorsal view)
2
1a 1b
Gray crescent
Control group
Experimental group
Experimental egg (side view)
Gray crescent
Thread
Normal Normal Belly piece
EXPERIMENT
RESULTS
THE “ORGANIZER”• Initiates a chain reaction of
inductions that result in the formation of the notochord, neural tube, and other organs
• Plays a major role in pattern formation (spatial organization)
• Positional information tells a cell where it is with respect to the animal’s body axes
• Wings and legs of chicks begin as limb buds
• Limb buds respond to positional information
Fig. 47.24 & 47.25
Limb buds 50 µm
Anterior Limb bud
AER
ZPA Posterior
Apical ectodermal ridge (AER)
(a) Organizer regions (b) Wing of chick embryo
Digits
Anterior
Proximal
Dorsal Posterior
Ventral
Distal
2
3 4
Donor limb bud
Host limb bud
ZPA
Anterior
Posterior
New ZPA
4
4
3
3
2 2
EXPERIMENT
RESULTS
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