Chapter 47 Animal Development

Overview: A Body-Building Plan A human embryo at about 7 weeks after conception shows development of distinctive features © 2011 Pearson Education, Inc.

Figure 47.1 Figure 47.1 How did a single cell develop into this intricately detailed embryo? 1 mm

Development occurs at many points in the life cycle of an animal This includes metamorphosis and gamete production, as well as embryonic development © 2011 Pearson Education, Inc.

EMBRYONIC DEVELOPMENT Sperm Figure 47.2 EMBRYONIC DEVELOPMENT Sperm Zygote Adult frog Egg FERTILIZATION CLEAVAGE Metamorphosis Blastula GASTRULATION Figure 47.2 Developmental events in the life cycle of a frog. ORGANO- GENESIS Larval stages Gastrula Tail-bud embryo

Although animals display different body plans, they share many basic mechanisms of development and use a common set of regulatory genes Biologists use model organisms to study development, chosen for the ease with which they can be studied in the laboratory © 2011 Pearson Education, Inc.

Concept 47.1: Fertilization and cleavage initiate embryonic development Fertilization is the formation of a diploid zygote from a haploid egg and sperm © 2011 Pearson Education, Inc.

Fertilization Molecules and events at the egg surface play a crucial role in each step of fertilization Sperm penetrate the protective layer around the egg Receptors on the egg surface bind to molecules on the sperm surface Changes at the egg surface prevent polyspermy, the entry of multiple sperm nuclei into the egg © 2011 Pearson Education, Inc.

The Acrosomal Reaction The acrosomal reaction is triggered when the sperm meets the egg The acrosome at the tip of the sperm releases hydrolytic enzymes that digest material surrounding the egg © 2011 Pearson Education, Inc.

Basal body (centriole) Figure 47.3-1 Basal body (centriole) Sperm head Figure 47.3 The acrosomal and cortical reactions during sea urchin fertilization. Acrosome Jelly coat Vitelline layer Sperm-binding receptors Egg plasma membrane

Basal body (centriole) Figure 47.3-2 Basal body (centriole) Sperm head Figure 47.3 The acrosomal and cortical reactions during sea urchin fertilization. Acrosome Hydrolytic enzymes Jelly coat Vitelline layer Sperm-binding receptors Egg plasma membrane

Basal body (centriole) Figure 47.3-3 Sperm nucleus Acrosomal process Basal body (centriole) Actin filament Sperm head Figure 47.3 The acrosomal and cortical reactions during sea urchin fertilization. Acrosome Hydrolytic enzymes Jelly coat Vitelline layer Sperm-binding receptors Egg plasma membrane

Basal body (centriole) Figure 47.3-4 Sperm plasma membrane Sperm nucleus Acrosomal process Basal body (centriole) Actin filament Sperm head Fused plasma membranes Figure 47.3 The acrosomal and cortical reactions during sea urchin fertilization. Acrosome Hydrolytic enzymes Jelly coat Vitelline layer Sperm-binding receptors Egg plasma membrane

Fertilization envelope Figure 47.3-5 Sperm plasma membrane Sperm nucleus Fertilization envelope Acrosomal process Basal body (centriole) Actin filament Sperm head Cortical granule Fused plasma membranes Figure 47.3 The acrosomal and cortical reactions during sea urchin fertilization. Acrosome Hydrolytic enzymes Perivitelline space Jelly coat Vitelline layer Sperm-binding receptors EGG CYTOPLASM Egg plasma membrane

Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy © 2011 Pearson Education, Inc.

The Cortical Reaction Fusion of egg and sperm also initiates the cortical reaction Seconds after the sperm binds to the egg, vesicles just beneath the egg plasma membrane release their contents and form a fertilization envelope The fertilization envelope acts as the slow block to polyspermy © 2011 Pearson Education, Inc.

The reaction is triggered by a change in Ca2 concentration The cortical reaction requires a high concentration of Ca2 ions in the egg The reaction is triggered by a change in Ca2 concentration Ca2 spread across the egg correlates with the appearance of the fertilization envelope © 2011 Pearson Education, Inc.

10 sec after fertilization 25 sec 35 sec 1 min 500 m Figure 47.4 EXPERIMENT 10 sec after fertilization 25 sec 35 sec 1 min 500 m RESULTS 1 sec before fertilization 10 sec after fertilization 20 sec 30 sec 500 m Figure 47.4 Inquiry: Does the distribution of Ca2 in an egg correlate with formation of the fertilization envelope? CONCLUSION Fertilization envelope Point of sperm nucleus entry Spreading wave of Ca2

10 sec after fertilization 25 sec 35 sec 1 min 500 m Figure 47.4a EXPERIMENT 10 sec after fertilization 25 sec 35 sec 1 min 500 m RESULTS Figure 47.4 Inquiry: Does the distribution of Ca2 in an egg correlate with formation of the fertilization envelope? 1 sec before fertilization 10 sec after fertilization 20 sec 30 sec 500 m

Fertilization envelope Point of sperm nucleus entry Figure 47.4b CONCLUSION Spreading wave of Ca2 Fertilization envelope Point of sperm nucleus entry Figure 47.4 Inquiry: Does the distribution of Ca2 in an egg correlate with formation of the fertilization envelope?

10 sec after fertilization Figure 47.4c 10 sec after fertilization Figure 47.4 Inquiry: Does the distribution of Ca2 in an egg correlate with formation of the fertilization envelope?

Figure 47.4d Figure 47.4 Inquiry: Does the distribution of Ca2 in an egg correlate with formation of the fertilization envelope? 25 sec

Figure 47.4e Figure 47.4 Inquiry: Does the distribution of Ca2 in an egg correlate with formation of the fertilization envelope? 35 sec

Figure 47.4f Figure 47.4 Inquiry: Does the distribution of Ca2 in an egg correlate with formation of the fertilization envelope? 1 min

1 sec before fertilization Figure 47.4g 1 sec before fertilization Figure 47.4 Inquiry: Does the distribution of Ca2 in an egg correlate with formation of the fertilization envelope?

10 sec after fertilization Figure 47.4h 10 sec after fertilization Figure 47.4 Inquiry: Does the distribution of Ca2 in an egg correlate with formation of the fertilization envelope?

Figure 47.4i 20 sec Figure 47.4 Inquiry: Does the distribution of Ca2 in an egg correlate with formation of the fertilization envelope?

Figure 47.4j 30 sec Figure 47.4 Inquiry: Does the distribution of Ca2 in an egg correlate with formation of the fertilization envelope?

Egg Activation The rise in Ca2+ in the cytosol increases the rates of cellular respiration and protein synthesis by the egg cell With these rapid changes in metabolism, the egg is said to be activated The proteins and mRNAs needed for activation are already present in the egg The sperm nucleus merges with the egg nucleus and cell division begins © 2011 Pearson Education, Inc.

Fertilization in Mammals Fertilization in mammals and other terrestrial animals is internal Secretions in the mammalian female reproductive tract alter sperm motility and structure This is called capacitation and must occur before sperm are able to fertilize an egg © 2011 Pearson Education, Inc.

No fast block to polyspermy has been identified in mammals Sperm travel through an outer layer of cells to reach the zona pellucida, the extracellular matrix of the egg When the sperm binds a receptor in the zona pellucida, it triggers a slow block to polyspermy No fast block to polyspermy has been identified in mammals © 2011 Pearson Education, Inc.

Zona pellucida Follicle cell Sperm nucleus Cortical granules Figure 47.5 Zona pellucida Follicle cell Figure 47.5 Fertilization in mammals. Sperm nucleus Cortical granules Sperm basal body

The diploid nucleus forms after this first division of the zygote In mammals the first cell division occurs 1236 hours after sperm binding The diploid nucleus forms after this first division of the zygote © 2011 Pearson Education, Inc.

Cleavage Fertilization is followed by cleavage, a period of rapid cell division without growth Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres The blastula is a ball of cells with a fluid-filled cavity called a blastocoel © 2011 Pearson Education, Inc.

(a) Fertilized egg (b) Four-cell stage (c) Early blastula Figure 47.6 50 m (a) Fertilized egg (b) Four-cell stage (c) Early blastula (d) Later blastula Figure 47.6 Cleavage in an echinoderm embryo.

(a) Fertilized egg Figure 47.6a Figure 47.6 Cleavage in an echinoderm embryo. (a) Fertilized egg

(b) Four-cell stage Figure 47.6b Figure 47.6 Cleavage in an echinoderm embryo. (b) Four-cell stage

(c) Early blastula Figure 47.6c Figure 47.6 Cleavage in an echinoderm embryo. (c) Early blastula

(d) Later blastula Figure 47.6d Figure 47.6 Cleavage in an echinoderm embryo. (d) Later blastula

Cleavage Patterns In frogs and many other animals, the distribution of yolk (stored nutrients) is a key factor influencing the pattern of cleavage The vegetal pole has more yolk; the animal pole has less yolk The difference in yolk distribution results in animal and vegetal hemispheres that differ in appearance © 2011 Pearson Education, Inc.

The third cleavage is asymmetric, forming unequally sized blastomeres The first two cleavage furrows in the frog form four equally sized blastomeres The third cleavage is asymmetric, forming unequally sized blastomeres © 2011 Pearson Education, Inc.

Holoblastic cleavage, complete division of the egg, occurs in species whose eggs have little or moderate amounts of yolk, such as sea urchins and frogs Meroblastic cleavage, incomplete division of the egg, occurs in species with yolk-rich eggs, such as reptiles and birds © 2011 Pearson Education, Inc.

8-cell stage (viewed from the animal pole) Figure 47.7 Zygote 2-cell stage forming Gray crescent 0.25 mm 8-cell stage (viewed from the animal pole) 4-cell stage forming Animal pole 8-cell stage Figure 47.7 Cleavage in a frog embryo. 0.25 mm Blastula (at least 128 cells) Vegetal pole Blastocoel Blastula (cross section)

Figure 47.7a-1 Zygote Figure 47.7 Cleavage in a frog embryo.

Gray crescent Zygote 2-cell stage forming Figure 47.7a-2 Figure 47.7 Cleavage in a frog embryo. 2-cell stage forming

2-cell stage forming 4-cell stage forming Figure 47.7a-3 Gray crescent Zygote Figure 47.7 Cleavage in a frog embryo. 2-cell stage forming 4-cell stage forming

2-cell stage forming 4-cell stage forming Figure 47.7a-4 Animal pole Gray crescent Vegetal pole Zygote Figure 47.7 Cleavage in a frog embryo. 2-cell stage forming 4-cell stage forming 8-cell stage

Blastula (cross section) Figure 47.7a-5 Animal pole Blastocoel Gray crescent Vegetal pole Zygote Figure 47.7 Cleavage in a frog embryo. 2-cell stage forming 4-cell stage forming 8-cell stage Blastula (cross section)

8-cell stage (viewed from the animal pole) Figure 47.7b 0.25 mm Animal pole Figure 47.7 Cleavage in a frog embryo. 8-cell stage (viewed from the animal pole)

Blastula (at least 128 cells) Figure 47.7c 0.25 mm Blastocoel Figure 47.7 Cleavage in a frog embryo. Blastula (at least 128 cells)

Regulation of Cleavage Animal embryos complete cleavage when the ratio of material in the nucleus relative to the cytoplasm is sufficiently large © 2011 Pearson Education, Inc.

Concept 47.2: Morphogenesis in animals involves specific changes in cell shape, position, and survival After cleavage, the rate of cell division slows and the normal cell cycle is restored Morphogenesis, the process by which cells occupy their appropriate locations, involves Gastrulation, the movement of cells from the blastula surface to the interior of the embryo Organogenesis, the formation of organs © 2011 Pearson Education, Inc.

Gastrulation Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula © 2011 Pearson Education, Inc.

Each germ layer contributes to specific structures in the adult animal The three layers produced by gastrulation are called embryonic germ layers The ectoderm forms the outer layer The endoderm lines the digestive tract The mesoderm partly fills the space between the endoderm and ectoderm Each germ layer contributes to specific structures in the adult animal © 2011 Pearson Education, Inc.

Video: Sea Urchin Embryonic Development © 2011 Pearson Education, Inc.

ECTODERM (outer layer of embryo) Figure 47.8 ECTODERM (outer layer of embryo) • Epidermis of skin and its derivatives (including sweat glands, hair follicles) • Nervous and sensory systems • Pituitary gland, adrenal medulla • Jaws and teeth • Germ cells MESODERM (middle layer of embryo) • Skeletal and muscular systems • Circulatory and lymphatic systems • Excretory and reproductive systems (except germ cells) • Dermis of skin • Adrenal cortex Figure 47.8 Major derivatives of the three embryonic germ layers in vertebrates. ENDODERM (inner layer of embryo) • Epithelial lining of digestive tract and associated organs (liver, pancreas) • Epithelial lining of respiratory, excretory, and reproductive tracts and ducts • Thymus, thyroid, and parathyroid glands

Gastrulation in Sea Urchins Gastrulation begins at the vegetal pole of the blastula Mesenchyme cells migrate into the blastocoel The vegetal plate forms from the remaining cells of the vegetal pole and buckles inward through invagination © 2011 Pearson Education, Inc.

The newly formed cavity is called the archenteron This opens through the blastopore, which will become the anus © 2011 Pearson Education, Inc.

Mesenchyme (mesoderm forms future skeleton) Digestive tube (endoderm) Figure 47.9 Animal pole Blastocoel Mesenchyme cells Vegetal plate Vegetal pole Blastocoel Filopodia Mesenchyme cells Archenteron Blastopore Figure 47.9 Gastrulation in a sea urchin embryo. 50 m Blastocoel Ectoderm Archenteron Key Blastopore Mouth Future ectoderm Mesenchyme (mesoderm forms future skeleton) Digestive tube (endoderm) Future mesoderm Anus (from blastopore) Future endoderm

Animal pole Blastocoel Mesenchyme cells Vegetal pole Vegetal plate Key Figure 47.9a-1 Animal pole Blastocoel Mesenchyme cells Vegetal pole Vegetal plate Figure 47.9 Gastrulation in a sea urchin embryo. Key Future ectoderm Future mesoderm Future endoderm

Animal pole Blastocoel Mesenchyme cells Vegetal pole Vegetal plate Key Figure 47.9a-2 Animal pole Blastocoel Mesenchyme cells Vegetal pole Vegetal plate Figure 47.9 Gastrulation in a sea urchin embryo. Key Future ectoderm Future mesoderm Future endoderm

Animal pole Blastocoel Mesenchyme cells Filopodia Vegetal pole Figure 47.9a-3 Animal pole Blastocoel Mesenchyme cells Filopodia Vegetal pole Vegetal plate Archenteron Figure 47.9 Gastrulation in a sea urchin embryo. Key Future ectoderm Future mesoderm Future endoderm

Animal pole Blastocoel Mesenchyme cells Filopodia Vegetal pole Figure 47.9a-4 Animal pole Blastocoel Mesenchyme cells Filopodia Vegetal pole Vegetal plate Archenteron Blastocoel Archenteron Figure 47.9 Gastrulation in a sea urchin embryo. Blastopore Key Future ectoderm Future mesoderm Future endoderm

Mesenchyme (mesoderm forms future skeleton) Anus (from blastopore) Figure 47.9a-5 Animal pole Blastocoel Mesenchyme cells Filopodia Vegetal pole Vegetal plate Archenteron Blastocoel Digestive tube (endoderm) Archenteron Ectoderm Figure 47.9 Gastrulation in a sea urchin embryo. Blastopore Key Mouth Mesenchyme (mesoderm forms future skeleton) Future ectoderm Future mesoderm Anus (from blastopore) Future endoderm

Blastocoel Filopodia Mesenchyme cells Archenteron Blastopore 50 m Figure 47.9b Blastocoel Filopodia Mesenchyme cells Archenteron Figure 47.9 Gastrulation in a sea urchin embryo. Blastopore 50 m

Gastrulation in Frogs Frog gastrulation begins when a group of cells on the dorsal side of the blastula begins to invaginate This forms a crease along the region where the gray crescent formed The part above the crease is called the dorsal lip of the blastopore © 2011 Pearson Education, Inc.

These cells become the endoderm and mesoderm Cells continue to move from the embryo surface into the embryo by involution These cells become the endoderm and mesoderm Cells on the embryo surface will form the ectoderm © 2011 Pearson Education, Inc.

Dorsal lip of blasto- pore Figure 47.10 SURFACE VIEW CROSS SECTION Animal pole 1 Blastocoel Dorsal lip of blasto- pore Dorsal lip of blastopore Blastopore Early gastrula Vegetal pole 2 Blastocoel shrinking Archenteron Figure 47.10 Gastrulation in a frog embryo. Ectoderm 3 Blastocoel remnant Mesoderm Endoderm Key Future ectoderm Blastopore Future mesoderm Late gastrula Yolk plug Archenteron Blastopore Future endoderm

Dorsal lip of blasto- pore Future ectoderm Figure 47.10a SURFACE VIEW CROSS SECTION Animal pole 1 Blastocoel Key Dorsal lip of blasto- pore Future ectoderm Future mesoderm Dorsal lip of blastopore Blastopore Early gastrula Future endoderm Figure 47.10 Gastrulation in a frog embryo. Vegetal pole

2 Blastocoel shrinking Archenteron Key Future ectoderm Future mesoderm Figure 47.10b 2 Blastocoel shrinking Archenteron Key Future ectoderm Future mesoderm Future endoderm Figure 47.10 Gastrulation in a frog embryo.

Ectoderm 3 Blastocoel remnant Mesoderm Key Endoderm Future ectoderm Figure 47.10c Ectoderm 3 Blastocoel remnant Mesoderm Key Endoderm Future ectoderm Future mesoderm Blastopore Future endoderm Late gastrula Yolk plug Archenteron Figure 47.10 Gastrulation in a frog embryo. Blastopore

Gastrulation in Chicks Prior to gastrulation, the embryo is composed of an upper and lower layer, the epiblast and hypoblast, respectively During gastrulation, epiblast cells move toward the midline of the blastoderm and then into the embryo toward the yolk © 2011 Pearson Education, Inc.

The midline thickens and is called the primitive streak The hypoblast cells contribute to the sac that surrounds the yolk and a connection between the yolk and the embryo, but do not contribute to the embryo itself © 2011 Pearson Education, Inc.

Migrating cells (mesoderm) Endoderm Hypoblast Figure 47.11 Fertilized egg Primitive streak Embryo Yolk Primitive streak Epiblast Future ectoderm Figure 47.11 Gastrulation in a chick embryo. Blastocoel Migrating cells (mesoderm) Endoderm Hypoblast YOLK

Gastrulation in Humans Human eggs have very little yolk A blastocyst is the human equivalent of the blastula The inner cell mass is a cluster of cells at one end of the blastocyst The trophoblast is the outer epithelial layer of the blastocyst and does not contribute to the embryo, but instead initiates implantation © 2011 Pearson Education, Inc.

These enclose specialized structures outside of the embryo Following implantation, the trophoblast continues to expand and a set of extraembryonic membranes is formed These enclose specialized structures outside of the embryo Gastrulation involves the inward movement from the epiblast, through a primitive streak, similar to the chick embryo © 2011 Pearson Education, Inc.

Figure 47.12 1 Blastocyst reaches uterus. Endometrial epithelium (uterine lining) Inner cell mass Uterus Trophoblast Blastocoel 2 Blastocyst implants (7 days after fertilization). Expanding region of trophoblast Maternal blood vessel Epiblast Hypoblast Trophoblast Expanding region of trophoblast 3 Extraembryonic membranes start to form (10–11 days), and gastrulation begins (13 days). Amniotic cavity Epiblast Hypoblast Yolk sac (from hypoblast) Extraembryonic mesoderm cells (from epiblast) Figure 47.12 Four stages in the early embryonic development of a human. Chorion (from trophoblast) 4 Gastrulation has produced a three-layered embryo with four extraembryonic membranes. Amnion Chorion Ectoderm Mesoderm Endoderm Yolk sac Extraembryonic mesoderm Allantois

Endometrial epithelium (uterine lining) Figure 47.12a Endometrial epithelium (uterine lining) Inner cell mass Uterus Trophoblast Blastocoel Figure 47.12 Four stages in the early embryonic development of a human. 1 Blastocyst reaches uterus.

Expanding region of trophoblast Figure 47.12b Expanding region of trophoblast Maternal blood vessel Epiblast Hypoblast Trophoblast Figure 47.12 Four stages in the early embryonic development of a human. 2 Blastocyst implants (7 days after fertilization).

Expanding region of trophoblast Figure 47.12c Expanding region of trophoblast Amniotic cavity Epiblast Hypoblast Yolk sac (from hypoblast) Extraembryonic mesoderm cells (from epiblast) Chorion (from trophoblast) Figure 47.12 Four stages in the early embryonic development of a human. 3 Extraembryonic membranes start to form (10–11 days), and gastrulation begins (13 days).

Extraembryonic mesoderm Figure 47.12d Amnion Chorion Ectoderm Mesoderm Endoderm Yolk sac Extraembryonic mesoderm Allantois Figure 47.12 Four stages in the early embryonic development of a human. 4 Gastrulation has produced a three-layered embryo with four extraembryonic membranes.

Developmental Adaptations of Amniotes The colonization of land by vertebrates was made possible only after the evolution of The shelled egg of birds and other reptiles as well as monotremes (egg-laying mammals) The uterus of marsupial and eutherian mammals © 2011 Pearson Education, Inc.

In both adaptations, embryos are surrounded by fluid in a sac called the amnion This protects the embryo from desiccation and allows reproduction on dry land Mammals and reptiles including birds are called amniotes for this reason © 2011 Pearson Education, Inc.

The four extraembryonic membranes that form around the embryo The chorion functions in gas exchange The amnion encloses the amniotic fluid The yolk sac encloses the yolk The allantois disposes of waste products and contributes to gas exchange © 2011 Pearson Education, Inc.

Organogenesis During organogenesis, various regions of the germ layers develop into rudimentary organs Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm © 2011 Pearson Education, Inc.

Figure 47.13 Neural folds Eye Somites Tail bud Neural fold Neural plate SEM 1 mm 1 mm Neural tube Neural crest cells Neural fold Neural plate Notochord Neural crest cells Coelom Notochord Ectoderm Somite Figure 47.13 Neurulation in a frog embryo. Mesoderm Outer layer of ectoderm Archenteron (digestive cavity) Endoderm Neural crest cells (c) Somites Archenteron (a) Neural plate formation Neural tube (b) Neural tube formation

(a) Neural plate formation Figure 47.13a Neural folds 1 mm Neural fold Neural plate Figure 47.13 Neurulation in a frog embryo. Notochord Ectoderm Mesoderm Endoderm Archenteron (a) Neural plate formation

The neural plate soon curves inward, forming the neural tube The neural tube will become the central nervous system (brain and spinal cord) © 2011 Pearson Education, Inc.

Video: Frog Embryo Development © 2011 Pearson Education, Inc.

(b) Neural tube formation Figure 47.13b-1 Neural fold Neural plate Figure 47.13 Neurulation in a frog embryo. (b) Neural tube formation

(b) Neural tube formation Figure 47.13b-2 Neural fold Neural plate Neural crest cells Figure 47.13 Neurulation in a frog embryo. (b) Neural tube formation

Outer layer of ectoderm Figure 47.13b-3 Neural fold Neural plate Neural crest cells Figure 47.13 Neurulation in a frog embryo. Neural crest cells Outer layer of ectoderm Neural tube (b) Neural tube formation

Mesoderm lateral to the notochord forms blocks called somites Neural crest cells develop along the neural tube of vertebrates and form various parts of the embryo (nerves, parts of teeth, skull bones, and so on) Mesoderm lateral to the notochord forms blocks called somites Lateral to the somites, the mesoderm splits to form the coelom (body cavity) © 2011 Pearson Education, Inc.

Archenteron (digestive cavity) Figure 47.13c Eye Somites Tail bud SEM 1 mm Neural tube Neural crest cells Notochord Coelom Figure 47.13 Neurulation in a frog embryo. Somite Archenteron (digestive cavity) (c) Somites

Neural folds 1 mm Figure 47.13d Figure 47.13 Neurulation in a frog embryo. 1 mm

Eye Somites Tail bud SEM 1 mm Figure 47.13e Figure 47.13 Neurulation in a frog embryo. SEM 1 mm

Organogenesis in the chick is quite similar to that in the frog © 2011 Pearson Education, Inc.

These layers form extraembryonic membranes. Figure 47.14 Neural tube Eye Notochord Forebrain Somite Archenteron Coelom Heart Lateral fold Endoderm Mesoderm Blood vessels Ectoderm Somites Yolk stalk Yolk sac These layers form extraembryonic membranes. Figure 47.14 Organogenesis in a chick embryo. Neural tube YOLK (a) Early organogenesis (b) Late organogenesis

These layers form extraembryonic membranes. Figure 47.14a Neural tube Notochord Somite Archenteron Coelom Lateral fold Endoderm Mesoderm Ectoderm Yolk stalk Yolk sac Figure 47.14 Organogenesis in a chick embryo. These layers form extraembryonic membranes. YOLK (a) Early organogenesis

(b) Late organogenesis Figure 47.14b Eye Forebrain Heart Blood vessels Somites Figure 47.14 Organogenesis in a chick embryo. Neural tube (b) Late organogenesis

The mechanisms of organogenesis in invertebrates are similar, but the body plan is very different For example, the neural tube develops along the ventral side of the embryo in invertebrates, rather than dorsally as occurs in vertebrates © 2011 Pearson Education, Inc.

Mechanisms of Morphogenesis Morphogenesis in animals but not plants involves movement of cells © 2011 Pearson Education, Inc.

The Cytoskeleton in Morphogenesis Reorganization of the cytoskeleton is a major force in changing cell shape during development For example, in neurulation, microtubules oriented from dorsal to ventral in a sheet of ectodermal cells help lengthen the cells along that axis © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

Figure 47.15-1 Ectoderm Figure 47.15 Change in cell shape during morphogenesis.

Ectoderm Neural plate Microtubules Figure 47.15-2 Figure 47.15 Change in cell shape during morphogenesis.

Ectoderm Neural plate Microtubules Actin filaments Figure 47.15-3 Figure 47.15 Change in cell shape during morphogenesis.

Ectoderm Neural plate Microtubules Actin filaments Figure 47.15-4 Figure 47.15 Change in cell shape during morphogenesis.

Ectoderm Neural plate Microtubules Actin filaments Neural tube Figure 47.15-5 Ectoderm Neural plate Microtubules Actin filaments Figure 47.15 Change in cell shape during morphogenesis. Neural tube

Convergent extension occurs in other developmental processes The cytoskeleton promotes elongation of the archenteron in the sea urchin embryo This is convergent extension, the rearrangement of cells of a tissue that cause it to become narrower (converge) and longer (extend) Convergent extension occurs in other developmental processes The cytoskeleton also directs cell migration © 2011 Pearson Education, Inc.

Convergence Extension Figure 47.16 Figure 47.16 Convergent extension of a sheet of cells.

Programmed Cell Death Programmed cell death is also called apoptosis At various times during development, individual cells, sets of cells, or whole tissues stop developing and are engulfed by neighboring cells For example, many more neurons are produced in developing embryos than will be needed Extra neurons are removed by apoptosis © 2011 Pearson Education, Inc.

Concept 47.3: Cytoplasmic determinants and inductive signals contribute to cell fate specification Determination is the term used to describe the process by which a cell or group of cells becomes committed to a particular fate Differentiation refers to the resulting specialization in structure and function © 2011 Pearson Education, Inc.

Cells in a multicellular organism share the same genome Differences in cell types are the result of the expression of different sets of genes © 2011 Pearson Education, Inc.

Fate Mapping Fate maps are diagrams showing organs and other structures that arise from each region of an embryo Classic studies using frogs indicated that cell lineage in germ layers is traceable to blastula cells © 2011 Pearson Education, Inc.

Blastomeres injected with dye Figure 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 Figure 47.17 Fate mapping for two chordates. Blastomeres injected with dye Larvae (b) Cell lineage analysis in a tunicate

Central nervous system Figure 47.17a Epidermis Epidermis Central nervous system Notochord Mesoderm Endoderm Blastula Neural tube stage (transverse section) Figure 47.17 Fate mapping for two chordates. (a) Fate map of a frog embryo

Blastomeres injected with dye Figure 47.17b 64-cell embryos Blastomeres injected with dye Larvae Figure 47.17 Fate mapping for two chordates. (b) Cell lineage analysis in a tunicate

Figure 47.17c Figure 47.17 Fate mapping for two chordates.

Figure 47.17d Figure 47.17 Fate mapping for two chordates.

Later studies of C. elegans used the ablation (destruction) of single cells to determine the structures that normally arise from each cell The researchers were able to determine the lineage of each of the 959 somatic cells in the worm © 2011 Pearson Education, Inc.

Time after fertilization (hours) Figure 47.18 Zygote First cell division Nervous system, outer skin, muscula- ture Muscula- ture, gonads Outer skin, nervous system Germ line (future gametes) Time after fertilization (hours) Musculature 10 Hatching Intestine Figure 47.18 Cell lineage in Caenorhabditis elegans. Intestine Anus Mouth Eggs Vulva ANTERIOR POSTERIOR 1.2 mm

Figure 47.18a Figure 47.18 Cell lineage in Caenorhabditis elegans.

Germ cells are the specialized cells that give rise to sperm or eggs Complexes of RNA and protein are involved in the specification of germ cell fate In C. elegans, such complexes are called P granules, persist throughout development, and can be detected in germ cells of the adult worm © 2011 Pearson Education, Inc.

Figure 47.19 100 m Figure 47.19 Determination of germ cell fate in C. elegans.

P granules are distributed throughout the newly fertilized egg and move to the posterior end before the first cleavage division With each subsequent cleavage, the P granules are partitioned into the posterior-most cells P granules act as cytoplasmic determinants, fixing germ cell fate at the earliest stage of development © 2011 Pearson Education, Inc.

Zygote prior to first division Figure 47.20 20 m 1 Newly fertilized egg 2 Zygote prior to first division Figure 47.20 Partitioning of P granules during C. elegans development. 3 Two-cell embryo 4 Four-cell embryo

20 m 1 Newly fertilized egg Figure 47.20a Figure 47.20 Partitioning of P granules during C. elegans development. 1 Newly fertilized egg

Zygote prior to first division Figure 47.20b 20 m Figure 47.20 Partitioning of P granules during C. elegans development. 2 Zygote prior to first division

20 m 3 Two-cell embryo Figure 47.20c Figure 47.20 Partitioning of P granules during C. elegans development. 3 Two-cell embryo

20 m 4 Four-cell embryo Figure 47.20d Figure 47.20 Partitioning of P granules during C. elegans development. 4 Four-cell embryo

Axis Formation A body plan with bilateral symmetry is found across a range of animals This body plan exhibits asymmetry across the dorsal-ventral and anterior-posterior axes The right-left axis is largely symmetrical © 2011 Pearson Education, Inc.

The dorsal-ventral axis is not determined until fertilization The anterior-posterior axis of the frog embryo is determined during oogenesis The animal-vegetal asymmetry indicates where the anterior-posterior axis forms The dorsal-ventral axis is not determined until fertilization © 2011 Pearson Education, Inc.

Upon fusion of the egg and sperm, the egg surface rotates with respect to the inner cytoplasm This cortical rotation brings molecules from one area of the inner cytoplasm of the animal hemisphere to interact with molecules in the vegetal cortex This leads to expression of dorsal- and ventral-specific gene expression © 2011 Pearson Education, Inc.

(a) The three axes of the fully developed embryo Figure 47.21 Dorsal Right Anterior Posterior Left Ventral (a) The three axes of the fully developed embryo Animal pole First cleavage Animal hemisphere Pigmented cortex Point of sperm nucleus entry Figure 47.21 The body axes and their establishment in an amphibian. Future dorsal side Vegetal hemisphere Gray crescent Vegetal pole (b) Establishing the axes

In chicks, gravity is involved in establishing the anterior-posterior axis Later, pH differences between the two sides of the blastoderm establish the dorsal-ventral axis In mammals, experiments suggest that orientation of the egg and sperm nuclei before fusion may help establish embryonic axes © 2011 Pearson Education, Inc.

Restricting Developmental Potential Hans Spemann performed experiments to determine a cell’s developmental potential (range of structures to which it can give rise) Embryonic fates are affected by distribution of determinants and the pattern of cleavage The first two blastomeres of the frog embryo are totipotent (can develop into all the possible cell types) © 2011 Pearson Education, Inc.

Experimental egg (side view) Figure 47.22-1 EXPERIMENT Control egg (dorsal view) Experimental egg (side view) 1a Control group 1b Experimental group Gray crescent Gray crescent Thread Figure 47.22 Inquiry: How does distribution of the gray crescent affect the developmental potential of the first two daughter cells?

Experimental egg (side view) Figure 47.22-2 EXPERIMENT Control egg (dorsal view) Experimental egg (side view) 1a Control group 1b Experimental group Gray crescent Gray crescent Thread Figure 47.22 Inquiry: How does distribution of the gray crescent affect the developmental potential of the first two daughter cells? 2 RESULTS Normal Belly piece Normal

In mammals, embryonic cells remain totipotent until the 8-cell stage, much longer than other organisms Progressive restriction of developmental potential is a general feature of development in all animals In general tissue-specific fates of cells are fixed by the late gastrula stage © 2011 Pearson Education, Inc.

Cell Fate Determination and Pattern Formation by Inductive Signals As embryonic cells acquire distinct fates, they influence each other’s fates by induction © 2011 Pearson Education, Inc.

The “Organizer” of Spemann and Mangold Spemann and Mangold transplanted tissues between early gastrulas and found that the transplanted dorsal lip triggered a second gastrulation in the host The dorsal lip functions as an organizer of the embryo body plan, inducing changes in surrounding tissues to form notochord, neural tube, and so on © 2011 Pearson Education, Inc.

Dorsal lip of blastopore Primary embryo Figure 47.23 EXPERIMENT RESULTS Dorsal lip of blastopore Primary embryo Secondary (induced) embryo Pigmented gastrula (donor embryo) Primary structures: Nonpigmented gastrula (recipient embryo) Neural tube Notochord Figure 47.23 Inquiry: Can the dorsal lip of the blastopore induce cells in another part of the amphibian embryo to change their developmental fate? Secondary structures: Notochord (pigmented cells) Neural tube (mostly nonpigmented cells)

Formation of the Vertebrate Limb Inductive signals play a major role in pattern formation, development of spatial organization The molecular cues that control pattern formation are called positional information This information tells a cell where it is with respect to the body axes It determines how the cell and its descendents respond to future molecular signals © 2011 Pearson Education, Inc.

The wings and legs of chicks, like all vertebrate limbs, begin as bumps of tissue called limb buds © 2011 Pearson Education, Inc.

Apical ectodermal ridge (AER) 3 4 Anterior Figure 47.24 Anterior Limb bud AER ZPA Limb buds Posterior 2 50 m Digits Apical ectodermal ridge (AER) 3 4 Anterior Figure 47.24 Vertebrate limb development. Ventral Proximal Distal Dorsal Posterior (a) Organizer regions (b) Wing of chick embryo

Apical ectodermal ridge (AER) Figure 47.24a Anterior Limb bud AER ZPA Limb buds Posterior 50 m Apical ectodermal ridge (AER) Figure 47.24 Vertebrate limb development. (a) Organizer regions

The embryonic cells in a limb bud respond to positional information indicating location along three axes Proximal-distal axis Anterior-posterior axis Dorsal-ventral axis © 2011 Pearson Education, Inc.

(b) Wing of chick embryo Figure 47.24b 2 Digits 3 4 Anterior Ventral Figure 47.24 Vertebrate limb development. Proximal Distal Dorsal Posterior (b) Wing of chick embryo

Apical ectodermal ridge (AER) Figure 47.24c 50 m Apical ectodermal ridge (AER) Figure 47.24 Vertebrate limb development.

One limb bud–regulating region is the apical ectodermal ridge (AER) The AER is thickened ectoderm at the bud’s tip The second region is the zone of polarizing activity (ZPA) The ZPA is mesodermal tissue under the ectoderm where the posterior side of the bud is attached to the body © 2011 Pearson Education, Inc.

Tissue transplantation experiments support the hypothesis that the ZPA produces an inductive signal that conveys positional information indicating “posterior” © 2011 Pearson Education, Inc.

EXPERIMENT Anterior New ZPA Donor limb bud Host limb bud ZPA Posterior Figure 47.25 EXPERIMENT Anterior New ZPA Donor limb bud Host limb bud ZPA Posterior RESULTS 4 Figure 47.25 Inquiry: What role does the zone of polarizing activity (ZPA) play in limb pattern formation in vertebrates? 3 2 2 3 4

EXPERIMENT Anterior New ZPA Donor limb bud Host limb bud ZPA Posterior Figure 47.25a EXPERIMENT Anterior New ZPA Donor limb bud Host limb bud ZPA Figure 47.25 Inquiry: What role does the zone of polarizing activity (ZPA) play in limb pattern formation in vertebrates? Posterior

Figure 47.25b RESULTS 4 3 2 2 Figure 47.25 Inquiry: What role does the zone of polarizing activity (ZPA) play in limb pattern formation in vertebrates? 3 4

Sonic hedgehog is an inductive signal for limb development Hox genes also play roles during limb pattern formation © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

Cilia and Cell Fate Ciliary function is essential for proper specification of cell fate in the human embryo Motile cilia play roles in left-right specification Monocilia (nonmotile cilia) play roles in normal kidney development © 2011 Pearson Education, Inc.

Normal location of internal organs Location in situs inversus Figure 47.26 Lungs Heart Liver Spleen Stomach Figure 47.26 Situs inversus, a reversal of normal left-right asymmetry in the chest and abdomen. Large intestine Normal location of internal organs Location in situs inversus

Cortical granule release (cortical reaction) Figure 47.UN01 Sperm-egg fusion and depolarization of egg membrane (fast block to polyspermy) Cortical granule release (cortical reaction) Figure 47.UN01 Summary figure, Concept 47.1 Formation of fertilization envelope (slow block to polyspermy)

2-cell stage forming Animal pole 8-cell stage Vegetal pole Blastocoel Figure 47.UN02 2-cell stage forming Animal pole 8-cell stage Vegetal pole Figure 47.UN02 Summary figure, Concept 47.1 Blastocoel Blastula

Figure 47.UN03 Figure 47.UN03 Summary figure, Concept 47.2

Neural tube Neural tube Notochord Notochord Coelom Coelom Figure 47.UN04 Neural tube Neural tube Notochord Notochord Coelom Coelom Figure 47.UN04 Summary figure, Concept 47.2

Species: Stage: Figure 47.UN05 Figure 47.UN05 Test Your Understanding, question 8 Species: Stage:

Figure 47.UN06 Figure 47.UN06 Appendix A: answer to Test Your Understanding, question 8