Transcript 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
• 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?