biologi - pertumbuhan & perkembangan

• 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


– 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


Animal 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


Animal 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


Animal 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


Animal 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


Animal 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


Animal 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


Animal 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


Animal 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


Animal Development



Animal 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


Animal 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


Animal 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


Animal 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


Animal 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


Animal Development


– 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


Animal Development


– Neurulation • During formation of neural

tube, mesoderm creates coelom and some organs

• In vertebrates, neural crest forms like ‘roof’ of neural tube


Animal 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

• Stages of development (review)

Animal Development


• Cell communication during development

Animal 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)


Animal 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?


Animal 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)

» 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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