biologi - pertumbuhan & perkembangan
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• Introduction to animal development
Animal Development
How are new organisms produced?
– Development requires both cell growth and cell differentiation (different cells express different genes)
http://users.hartwick.edu/fauthp/BBBAnimDevelop.ppt
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Stages of development
Animal Development
How are new organisms produced?
– Neurulation • Ectoderm cells elongate into
neural plate as notochord develops from mesoderm below
• Cells in neural plate changes shape, eventually rolling into neural tube which then develops into brain and spinal cord
– Induction = one embryonic region influences development of adjacent region
• Stages of development
Animal Development
How are new organisms produced?
– Neurulation • During formation of neural
tube, mesoderm creates coelom and some organs
• In vertebrates, neural crest forms like ‘roof’ of neural tube
• Stages of development
Animal Development
How are new organisms produced?
– 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
• Cell communication during development
Animal Development
How are new organisms produced?
– 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)
• Cell communication during development
Animal Development
How are new organisms produced?
– 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?
• Cell communication during development
Animal Development
How are new organisms produced?
– 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)
» Some cells (e.g. egg cells) become determined early in development because of their location in embryo
» Other cells become committed later in development
» Early blastomere are totipotent (capable of expressing all genes)
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