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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