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

Animal Development Stages of development Fertilization Fertilization = union of gametes (sperm & egg) 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 Stages of development Fertilization 3 steps involved in fertilization 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 Stages of development Fertilization 3 steps involved in fertilization 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 Stages of development Cleavage Cleavage begins within an hour of fertilization; series of mitotic divisions 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 Stages of development Cleavage Pattern of cleavage divisions depends on amount of yolk in zygote If little yolk (lancelets), holoblastic cleavage 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 Stages of development Cleavage Pattern of cleavage divisions 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 Stages of development Cleavage Pattern of cleavage divisions 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 Stages of development Blastula Stage characterized by hollow ball of cells 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 embryonic stem cells How are new organisms produced?

Animal Development Stages of development Gastrulation Gastrulation = initial movement of cells in embryo 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 Stages of development Gastrulation Cell movement during gastrulation: 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 Stages of development Gastrulation in primitive chordates (e.g. lancelets) 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 Stages of development Gastrulation in most aquatic vertebrates 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 Stages of development Gastrulation in most aquatic vertebrates Major steps: Opening of blastopore becomes filled with inner yolk-filled cells creating yolk plug 3 tissue layers established; prepares embryo for tissue differentiation and development of organs How are new organisms produced?

Animal Development Stages of development Gastrulation in terrestrial vertebrates 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 primitive streak to produce mesoderm How are new organisms produced?

Animal Development Stages of development Neurulation Ectoderm cells elongate into neural plate as notochord develops from mesoderm below 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 Stages of development Neurulation During formation of neural tube, mesoderm creates coelom and some organs In vertebrates, neural crest forms like ‘roof’ of neural tube How are new organisms produced?

Animal Development Stages of development Cell migration, organogenesis and growth 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 Stages of development (review) How are new organisms produced?

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

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

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

Animal Development Cell communication during development All cells of embryo genetically identical How does embryo develop specialized cells? 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?