Animal Development Introduction to animal development Development requires

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 similarsized 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?