Development of Multicellular Organisms Flashcards
Genome of all cells in an organism are ____ but expression of genes is ____
identical
different
what controls development
differential gene expression
four major phases in development
- cell division and proliferation: producing many cells from one
- cell specialization/differentiation: creating cells with different characteristics
- cell interaction: influences behaviors of other cells
- cell movement/migration and rearrangement: forms layers which give rise to structures such as tissues and organs
*all happens simultaneously in different parts of the developing embryo
development
- cells have a memory
- cells retain record of signals their ancestors received during embryonic development
- genes expressed by cell depend on environment, both present and past
functional interchange of homologous proteins:
homologous proteins are functionally interchangeable
- basic machinery for development similar for all organisms
- e.g. mouse lacking cerebellum gene –> cerebellum gene from fly inserted into mouse –> mouse developed cerebellum
conserved mechanisms for development
- after fertilization the zygote divides rapidly
- results in formation of many small cells that are dependent on food stored in egg by the mother
- genome is inactive initially
- later genome becomes activated and cells divide and cohere to form a blastula
- blastula then undergoes massive rearrangements to form gastrula
blastula
ball of cells surrounding a hollow cavity
gastrula
blastula that undergoes massive rearrangements that has 3 major layers
blastula consists of a sheet of ______ facing the external medium; which gives rise to the ____
sheet of epithelial cells
ectoderm
ectoderm
precursor of nervous system and epidermis
“attract-o-derm: everything that many people find attractive (hair, skin, eyes, brain)”
part of epithelial sheet becomes tucked into the interior which gives rise to the
endoderm
endoderm
precursor of gut, lung, and liver
“endternal organs: most of the internal organs”
group of cells that move into the space between the ectoderm and endoderm (break off from the endoderm) gives rise to the
mesoderm
mesoderm
precursor of muscle and connective tissues
“means of getting around: movement, cardiovascular system, skeleton and muscles “
gastrulation
transformation of the blastula, a hollow sphere of cells into a layered structure with a gut
50 percent genes in a fruit fly nematode worm, and human have recognizable ____ in the other species
homologs
higher organisms have several homologs of the same gene
gene duplication
gene regulatory proteins
most important for development
instructions for producing a multicellular animal is contained in the
non-coding regulatory DNA sequences associated with each gene
_____ sequences in DNA similar in most organisms but _____ sequences make one organism different from another and provide uniqueness
coding
non-coding
cells make developmental decisions long before they should any outward signs of _____
differentiation
e.g. appearance of the red cell is the final act
determined
cells that are fated to develop into a specialized cell type despite changes in environment
e.g. fated to be a red cell –> if put in another environment –> still becomes a red cell
completely undetermined (not determined)
cells that can change rapidly due to alterations in environment
committed
cells that have some attributes of a particular cell type but can change with environment
memory of positional values
- before acquiring a particular fate, cells express genes that are markers of their location e.g. they are ‘regionally determined’
- position specific character of cell called positional value
- cells retain ‘memory’ of positional value
cellular differentiation:
asymmetric division (e.g. development of germ cells) and inductive signaling
asymmetric division
-significant sets of molecules distributed unequally between daughter cells
inductive signaling
- definition: induction of a different developmental program in select cells in a homogeneous group leading to altered character
- cells born the same can become different due to change in environment after birth (different molecules induced)
- these molecules then directly or indirectly alter the pattern of gene expression between the 2 cells
- most important environmental cues are signals from neighboring cells
- few cells closest to the source take on induced character - signal is limited in time and space
types of signals in inductive signaling:
- short range: cell-cell contacts
- long range: substances that can diffuse through the extracellular medium
morphogen
- definition: long rand inductive signal that imposes a pattern on a field of cells
- exert graded effects by forming gradients of different concentrations
- each concentration can direct the target cells into a different developmental pathway
- need an “on” and “off” system
- antagonists or extracellular inhibitors bind to the signal or its receptor and block interaction
what forms gradients:
- localized production of an inducer that diffuses away from its source
- localized production of an inhibitor that diffuses away from its source and blocks the action of a uniformly distributed inducer
p granules
form germ cells
lateral inhibition and positive feedback
steps:
- system starts off homogeneous and symmetrical
- environment imposes weak asymmetry
- positive feedback amplifies effect
- broken asymmetry is ‘all or none’ phenomenon
- irreversible: once achieved, external signal becomes irrelevant
e. g. delta-notch signaling (uses lateral inhibition)
factors underlying diversity in patterns
- combinatorial control: response of a cell to a given signal may differ based on the presence of other signals (combinations create variety)
- cell memory: effect of a given signal depends on previous experiences of the cell (which may have altered its chromatin, regulatory proteins, transcription and RNA)
- sequential induction: different signals formed/secreted in a spatial and temporal manner
receptor tyrosine kinase (RTK) signaling pathway:
- ligand family
- receptor family
ligand family –> receptor family
EFG –> EGF receptors
FGF (Branchless) –> FGF receptors (Breathless)
Ephrins –> Eph receptors
TGFBeta superfamily signaling pathway:
- ligand family
- receptor family
ligand family –> receptor family
TGFBeta –> TGFBeta receptors
BMP (Dpp) –> BMP receptors
Nodal
Wnt signal pathway
ligand family –> receptor family
Wnt (Wingless) –> Frizzled
Hedgehog signal pathway
ligand family –> receptor family
Hedgehog –> patched, smoothened
notch signaling pathway
ligand family –> receptor family
delta –> notch
brain has ____ neurons
10^11
each neuron makes connections with 1000 other neurons
phases of neural development
Phase 1 genesis of neurons: different cell types develop independently at widely separate locations in embryo according to local program and are unconnected
Phase 2 outgrowth of axons and dendrites/synapse formation: axons and dendrites grow out along specific routes setting up a provisional but orderly network of connections between various parts of the nervous system
Phase 3 refinement of synaptic connections: continues into adult life; connections are adjusted and refined through interactions with distant regions via electric signals
neurons are produced in association with
glial cells (providing supporting framework and nutrition)
CNS consists of _______ and is derived from _____
brain, spinal cord, and retina
neural tube
PNS consists of ______ and is derived from _____
nerves, sensory neurons
neural crest
formation of neural tube
- neural tube derived from a single layered epithelium
- starts with a neural groove on the dorsal side of embryo
- gradually deepens as neural folds become elevated
- ultimately the folds meet and coalesce in the middle line and convert the groove into a closed tube (neural tube)
- center of the neural tube is the neural canal
programming associated with neural tube
- delta notch signaling controls differentiation into neurons (lateral inhibition and positive feedback)
- signal proteins secreted from ventral and dorsal side of neural tube act as opposing morphogens, causing neurons at different dorso-ventral positions to express different gene regulatory proteins
dorsal cells of neural tube secrete
Bone morphogenic protein (BMP) (TGFbeta family): pattern formation, differentiation, secretion of extracellular matrix
ventral cells of neural tube secrete
sonic hedgehog: depend on proteoglycans for function; activate latent genes; transcription repression to activation
where do neural crest cells originate and what do they do during or shortly after closure of the neural tube
originate at the dorsal end of the neural tube
they migrate extensively
define neurulation
closing of neural tube
are neural crest cells differentiated or undifferentiated
undifferentiated because when they migrate, they assume characteristics of their environment, e.g. adrenal glands
development of neural crest cells
- cells that will become neural crest cells (dorsal end) replicate
- migrate
- aggregate
- differentiate
movement of neurons occurs be virtue of
glial cells
growth cone
tip of axon/dendrite that has an irregular spiky enlargement
growth cone crawls through surrounding tissue, trailing the axon or dendrite behind
one of the growth cone starts migrating fast and develops axon-specific proteins- this will form the axon
growth cone behavior is dictated by ______ machinery
cytoskeletal
monomeric GTPases that control the assembly/disassembly of actin filaments which control movement of growth cone
Rho and Rac
GDP form: inactive
GTP form: active
axon is rich in what protein
microtubules
lamellipodia and filopodia is rich in what protein
actin
extracellular matrix environment in regards to migration of growth cones
sensed by receptors present on membrane
chemotactic factors in regards to migration of growth cones
released by neighboring cells
can be attractive or repulsive
two classes of extracellular matrix
immunoglobulin superfamily
cadherin family
contact guidance
growth cones often follow a path taken by other cells (e.g. nerve fibers are usually found in bundles)
laminin
favor axonal outgrowth
chondroitin sulfate proteoglycans
inhibit axonal growth
mechanisms of growth cone guidance
- extracellular matrix adhesion: growing axon glides over extracellular matrix
- cell surface adhesion: growing axon comes into contact with group of cells and grows over the surface of those cells
- guidance by pioneer neuron: neuron follows same route as previous neuron
- chemoattraction: group of cells secrete molecules that attract growing axon
- contact inhibition: growing axon comes into contract with group of cells and is repelled
- chemorepulsion: group of cells secrete molecules that repel growing axon
dorsal neurons of spinal cord
receive and relay sensory information from sensory neurons located in the periphery of the body
ventral clusters of neurons of spinal cord
develop as motor neurons and send out long axons to connect with specific subset of muscles
intermediate location of neurons of spinal cord
develop into interneurons that connect specific set of nerve cells to each other
guidance of commissural neurons are controlled by
chemotactic factors
nectrin
secreted by cells of the floor plate –> guides movement of commissural cells down towards floor plate
once growth cones reaches floor plate, neighboring cells secrete _____ as a repellant to push growth of axon towards brain
slit and semaphorin
binding of nectrin to its receptor causes opening of
calcium channels
target cells (of growing neurons) release
neurotrophic factors that further draw growing neurons to target cells to synapse
most neurons are made in excess and up to ____ die after they reach target cell
50%
growing neurons that do not receive enough neurotrophic factors
die by programmed cell death and do not synapse
first neurotrophic factor to be identifed was
nerve growth factor (NGF)
NGF works through
tyrosine kinase (TrkA) receptor
NGF promotes survival of
specific sensory and sympathetic neurons
short term effect of NGF
effect on growth cone and neurite extension. effect is local, direct, rapid, and independent of communication with cell body
long term effect of NGF
effect on cell survival. mediated by its receptor, uptake into cells via endocytosis and stimulation of downstream signaling pathways
synaptic remodeling is dependent on 2 rules that create spatial order
- axons from cells in different regions of retina compete for tectal neurons
- axons from neighboring sties which are excited at same time cooperate/collaborate to retain and strengthen synapse with tectal neurons
activity dependent synaptic remodeling depends on
electrical activity and synaptic signaling
what receptor plays an important role in memory formation
glutamate receptor (NMDA receptor) via entry of calcium triggers lasting change in synaptic strength
