Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 CHAPTER 52 ANIMAL DEVELOPMENT

2 Embryonic development  Fertilized egg becomes an organism with physiological systems and body parts Cellular differentiation  Different cells in the developing organism acquire specialized forms and functions due to cell-specific genes

3 Most animals have similar embryonic development processes Most modern animals develop from embryos with 3 cell layers  Vertebrates, arthropods, echinoderms, and mollusks 5 stages  Fertilization, cleavage, gastrulation, neurulation, organogenesis May also include metamorphosis of larval into adult form

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5 Fertilization Sperm can contact the egg inside or outside the female body Initial contact followed by acrosomal reactions where enzymes dissolve region of jelly-like layer allowing sperm to contact the plasma membrane of the egg Sperm head fuses with egg membrane and penetrates Fast block to polyspermy – fusion depolarizes eggs so that no more sperm may enter

6 Slow block to polyspermy – cortical reaction creates calcium wave across egg resulting in cortical granules inactivating sperm-binding proteins and hardening of vitelline layer/ zona pellucida Burst of calcium also activates pathways initiating first cell cycle Nucleus of sperm and egg fuse creating diploid zygote

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8 Cleavage Repeated cell cycles without cell growth “Biphasic” alternating only between mitotic (M) phase and DNA synthesis (S) phase (no G 1 or G 2 ) Blastomeres – half-size daughter cell produced at each division Blastula – hollow ball of cells Blastocoel – space inside ball

9 Vegetal pole – where yolk is more concentrated in the egg Animal pole – where there is less yolk and more cytoplasm in the egg Poles determine future axes of embryo

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11 Meroblastic cleavage – only the animal pole undergoes cell division  Birds and some fish with large amounts of yolk  Creates flattened disk called blastoderm

12 Holoblastic cleavage – complete cell division creating 2 equal sized blastomeres in the first division  Amphibians and mammals  Mammals undergo compaction creating a morula and forming a blastocyst (mammalian blastula)

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14 In mammals, fertilization and cleavage occur in the oviduct Blastocyst consists of  Outer trophectoderm which gives rise to placenta  Inner cell mass become embryo Embeds in uterine wall in implantation

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16 Little or no transcription occurs during early cleavage Maternal factors – cytoplasmic components (mRNAs and proteins) stockpile in egg during oogenesis Toward the end of cleavage, embryo shifts from relying on maternal factors to products from its own genome

17 Gastrulation Hollow ball forms gastrula with 3 distinct cell layers  Endoderm – forms epithelial lining of gut, liver, pancreas, lungs, and bladder  Mesoderm – forms heart, limbs, muscles, kidneys, blood, and connective tissues  Ectoderm – forms epidermis and nervous system And primordial germ cells

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19 Invagination  Gastrulation begins when band of tissue pinches in creating opening, blastopore  Apical constriction forces cells toward the center of the embryo  Initiating site of invagination becomes dorsal lip of blastopore

20 Involution  Cells of animal pole spread out and are forced back toward animal pole  Mediated convergent extension resulting in an embryo with 3 distinct layers

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22 In chordates and echinoderms blastopore becomes the anus Archenteron displaces blastocoel to become organism’s digestive tract Cells in 3 layers express different genes encoding distinct cadherin molecules Cells from one layer will only adhere to each other and not cells in the other 2 layers

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24 Primordial germ cells (PGCs)  Specialized group of cells migrates to future site of gonads  Often arise independently of 3 cell layers  2 functions Protect and propagate genetic content of species Undergo meiosis to produce gametes  Stem cells can undergo mitosis to replicate themselves

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26 Notochord – mesodermal structure providing structural rigidity along anteroposterior axis on dorsal side of embryo  Presence defines phylum Chordata  Produces signaling proteins to establish tissue patterns  Hox genes  By the time the notochord forms, the dorsal ectoderm overlying the notochord thickens and next stage begins

27 Neurulation Formation of neural tube from ectoderm located dorsal to notochord All neurons and their supporting cells in the CNS originate from neural precursor cells derived from neural tube Ectoderm overlaying notochord thickens forming neural plate Neural plate forms neural tube through apical constrictions Fusion – dorsal most cells on either side of neural tube fuse to close neural tube

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29 Neural crest unique to vertebrates Cells overlying dorsal side of neural tube Migrate to other regions of embryo to form all neurons and supporting cells of peripheral nervous system Also form melanocyctes, cells forming facial cartilage and parts of adrenal gland Sometimes called “fourth germ layer”

30 Somites  Mesoderm blocks become segmented at the anterior end first, giving rise to blocklike structures  Segmentation allows individual body segments to have more specialized functions  Number of somite pairs constant in a given species but can vary widely among species  Stages of development often standardized according to number of somites formed

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32 Organogenesis Organs have 2 or more tissue types Each germ layer gives rise to different organs Many organs form during or just after neurulation Organs can become functional at different times during development (lungs do not function until after birth) Development controlled by Hox genes Proteins or growth factors induce cells in local vicinity to differentiate along a specific pathway

33 Control of cell differentiation Fate of each cell determined by  Autonomous specification – differential acquisition of various cytoplasmic factors during cell division  Conditional specification – acquisition of properties through cell-to-cell signaling mechanisms Work together so that distinct cells differentiate into each of the numerous cell types with unique functions

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35 Morphogens – substances eliciting cellular responses based on concentration  In conditional specification, cell or group of cells makes a signaling protein, morphogen, that induces a response in other cells  Animal cap assay used to identify proteins that induce cells in animal pole to differentiate into mesoderm

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37 Morphogenetic field  Group of embryonic cells that ultimately produce a specific body structure  Removing limb field produces an embryo lacking that limb and transplanting the field to other areas produces the limb in that area  Cells within fields are uniquely specified to become particular embryonic structures before any physical evidence of that structure can be observed

38 Spemann’s organizer  Hans Spemann discovered this field in the early gastrula  “Organizer” secretes morphogens responsible for inducing formation of embryonic axis  First Nobel Prize in developmental biology

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Richard Harlan and Coworkers Identified Genes Expressed Specifically in the Organizer Hypothesized that morphogens in the organizer would promote formation of anterior structures (head) Used expression cloning  Purified mRNA from organizer, constructed cDNA library and transcribed each back into mRNA Injected mRNAs into UV treated eggs that would not have formed dorsal mesoderm Looked for evidence of “rescue” of UV treated embryos Noggin protein later revealed to act by inhibiting 2 other morphogens known to induce ventral mesoderm Antagonistically acting proteins specify structures in a concentration dependent fashion

Protein Molecules Called Frzb Can Inhibit Developmental Pathways Activated by Other Morphogens Fine balance between positively and negatively acting proteins must be established for proper development to occur Frzb is a protein about 50% similar to a larger protein call Frizzled receptor Frizzled receptor part of Wnt pathway controlling many aspects of embryonic development Binding of Wnt to Frizzled receptor indices mesoderm development Frzb prevents this by binding to free Wnt Ratio of Frzb and Frizzled receptor fine tune Wnt morphogen activity

44 Public Health Spina bifida – failure of neural tube to close  1 in 1,000 births in US  Up to 75% may be prevented if women take folic acid Thalidomide affected cells in limb field and produced babies with malformed limbs  Prescribed for morning sickness in pregnant women before side effect known

45 Fetal alcohol syndrome (FAS) is the leading cause of mental retardation  Morphological features caused by effects of ethanol on cell division  Cognitive deficits result from death of developing CNS neurons Down syndrome – trisomy 21 results in multiple physical and neurological disorders