Animal Development

Animal Development • Development requires both cell growth and cell differentiation (different cells express different genes) • Introduction to animal development How are new organisms produced?

Animal Development • Fertilization • Fertilization = union of gametes (sperm & egg) • Stages of development • 3 steps involved in fertilization • Penetration: head of sperm (acrosome) release enzymes to digest glycoprotein layer (zona pellucida) surrounding egg How are new organisms produced?

Animal Development • Fertilization • 3 steps involved in fertilization • Stages of development • Activation: sperm physically contacts egg plasma membrane • final meiotic division produces two egg nuclei, one retained as haploid egg nucleus within egg (mammals) • Cytoplasm rotates toward site of sperm entry; creates gray crescent on opposite side, establishing dorsal-ventral orientation • Produces increase in protein synthesis to prepare for cell divisions How are new organisms produced?

Animal Development • Fertilization • 3 steps involved in fertilization • Stages of development • Nuclei fusion: fusion of sperm nucleus with egg nucleus to form diploid zygote • Two hemispheres of zygote • Animal pole: pole where cells divide faster and are smaller; develops into ectoderm • Vegetal pole: pole where cells contain yolk, divide slower and are larger; develops into endoderm How are new organisms produced?

Animal Development • Cleavage • Cleavage begins within an hour of fertilization; series of mitotic divisions • Stages of development • Zygote divides into 2, 4, 8, … smaller and smaller blastomeres without increasing size of embryo • Cleavage involves about 12 divisions resulting in solid ball of blastomeres How are new organisms produced?

Animal Development • Cleavage • Pattern of cleavage divisions depends on amount of yolk in zygote • Stages of development • If little yolk (lancelets), holoblasticcleavage produces similar-sized blastomeres • If lots of yolk in vegetal pole (frogs), holoblastic cleavage produces different-sized blastomeres How are new organisms produced?

Animal Development • Cleavage • Pattern of cleavage divisions • Stages of development • In birds and reptiles... • Egg all yolk except for small area at one pole, cleavage only occurs in this area (called bastodisc) • Cleavage pattern called meroblastic; embryo forms like a cap on yolk How are new organisms produced?

Animal Development • Cleavage • Pattern of cleavage divisions • Stages of development • In mammals... • Holoblastic cleavage except concentrated at one pole (inner cell mass analogous to blastodisc) • Cells surrounding inner cell mass (trophoblast) become part of the placenta How are new organisms produced?

Animal Development • Blastula • Stage characterized by hollow ball of cells • Stages of development • At about 16-cell stage of cleavage, interior cells create osmotic gradient causing water to fill extracellular spaces • fluid-filled part of blastula is blastocoel • For short period of time, cells of mammalian blastula can develop into most of the cells types in the body; these are embryonicstemcells How are new organisms produced?

Animal Development • Gastrulation • Gastrulation = initial movement of cells in embryo • Stages of development • Invagination: dent inward • Involution: roll inward • Creates anterior-posterior orientation of embryo (bilateral symmetry) and archenteron (gut of embryo) • Cell movement during gastrulation creates three embryonic tissue layers: • Endoderm: digestive, respiratory, and most other organs • Ectoderm: skin, nervous system • Mesoderm: notochord, bones, blood vessels, muscles How are new organisms produced?

Animal Development • Stages of development How are new organisms produced?

Animal Development • Gastrulation • Cell movement during gastrulation: • Stages of development • Migrating cells move by changing shape • migrating cells have surface molecules that adhere to adjacent cells; cells move as a unit How are new organisms produced?

Animal Development • Gastrulation in primitive chordates (e.g. lancelets) • Stages of development • Surface of blastula invaginates into blastocoel • 2-layered (endoderm, ectoderm), cup-shaped embryo is gastrula • Opening created by cell migration is archenteron • Opening of archenteron is blastopore How are new organisms produced?

Animal Development • Gastrulation in most aquatic vertebrates • Stages of development • Gastrulation more complex because of size differences between animal and vegetal cells • Major steps: • Layer of surface cells invaginates at blastopore (dorsal lip of blastopore) • Cells from animal pole involute over dorsal lip, causing inner cells to fill blastocoel and create archenteron • Inner cells involuted over dorsal lip move between layers to form mesoderm How are new organisms produced?

Animal Development • Gastrulation in most aquatic vertebrates • Stages of development • Major steps: • Opening of blastopore becomes filled with inner yolk-filled cells creating yolkplug • 3 tissue layers established; prepares embryo for tissue differentiation and development of organs How are new organisms produced?

Animal Development • Gastrulation in terrestrial vertebrates • Stages of development • Embryo develops in blastodisc or inner cell mass • Lower and upper cells layers differentiate into endoderm and ectoderm without cell movement • Cells of ectoderm invaginate and involute at primitivestreak to produce mesoderm How are new organisms produced?

Animal Development • Neurulation • Ectoderm cells elongate into neuralplate as notochord develops from mesoderm below • Stages of development • Induction = one embryonic region influences development of adjacent region • Cells in neural plate changes shape, eventually rolling into neural tube which then develops into brain and spinal cord How are new organisms produced?

Animal Development • Neurulation • During formation of neural tube, mesoderm creates coelom and some organs • Stages of development • In vertebrates, neural crest forms like ‘roof’ of neural tube How are new organisms produced?

Animal Development • Cell migration, organogenesis and growth • Stages of development • Neural crest cells migrate to different areas of embryo • Anterior: produce forebrain, sense organs • Posterior: produce gill arches, muscle somites • Ventral: produce Schwann cells, adrenal medulla How are new organisms produced?

Animal Development • Stages of development (review) How are new organisms produced?

Animal Development • Induction • Certain act as organizers; induce changes in adjacent cells • Cell communication during development • Organizers produce protein that binds to target cells; cause changes in gene expression (based on concentration) How are new organisms produced?

Animal Development • Induction • Primary induction involves embryonic tissue types: endoderm, mesoderm and ectoderm (e.g. mesoderm of notochord inducing ectoderm to produce neural tube) • Cell communication during development • Secondary induction involves already differentiated tissue What would happen if cells of optic stalk moved elsewhere? How are new organisms produced?

Animal Development • All cells of embryo genetically identical • How does embryo develop specialized cells? • Cell communication during development • Different genes expressed in different cells • As cells specialize they lose ability to express more and more genes (development involves progressive restriction of gene expression) • Early blastomere are totipotent (capable of expressing all genes) • Some cells (e.g. egg cells) become determined early in development because of their location in embryo • Other cells become committed later in development How are new organisms produced?

Animal Development