Animal Development Chapter 47 1 mm Figure 47.1 Figure 47.1 How did a single cell develop into this intricately detailed embryo? Chapter 47 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

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

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

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 reaction is triggered by a change in Ca2 concentration The fertilization envelope acts as the slow block to polyspermy © 2011 Pearson Education, Inc.

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 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 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

Cleavage Fertilization is followed by cleavage, a period of rapid cell division without growth In mammals the first cell division occurs 1236 hours after sperm binding 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.

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 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)

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 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.

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 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.

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 chick embryo © 2011 Pearson Education, Inc.

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 In both adaptations, embryos are surrounded by fluid in a sac called the amnion This protects the embryo from desiccation and allows reproduction on 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 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.

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

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, etc) 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

(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 involves movement of cells 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.

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

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 Differences in cell types are the result of the expression of different sets of genes Differentiation refers to the resulting specialization in structure and function © 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

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 © 2011 Pearson Education, Inc.

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 The anterior-posterior axis of the frog embryo is determined during oogenesis The animal-vegetal asymmetry indicates where the anterior-posterior axis forms 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 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) 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 © 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 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.