Download Animal Development

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 Ch 47
• 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
– Cortical Reaction:
Cortical granules in egg
fuse w/ PM. The granules
clip off sperm-binding
receptors. Slow block to
Polyspermy
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
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
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
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
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 blastodisc)
» Cleavage pattern called
meroblastic; embryo
forms like a cap on yolk
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
Animal Development
• Stages of development
– Blastula
• Stage characterized by
hollow ball of cells
– At about 16-cell stage of cleavage
– 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
• 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
Animal Development
Germ Layers
Animal Development
• Stages of development
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
– Opening created by cell
migration is archenteron
– Opening of archenteron is
blastopore
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
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
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)
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
» Early blastomere are totipotent (capable of expressing all genes)