development definition and fertilisation process
aka ontogeny - growth and differentiation of tissues in multicellular organisms
formation of diploid zygote from haploid egg and sperm
sperm penetrates protective layer around egg
receptors on egg surface bind to molecules on sperm surface
changses in the egg surface prevent polyspermy
acrosomal reaction
triggered when sperm meets egg
acrosome at tip of sperm releases hydrolytic enzymes that digest jelly material around egg
the cytoskeleton bridge between sperm and egg allows movement of molecules
and fusion of membranes
sperm nucleus enters egg
Na+ ion channels open and Na+ enters and depolarises egg membrane
repels other positive sperm
cortical reaction
Sperm and egg fusion triggers vesicles in the egg to release their contents
This triggers a wave of Ca2+ across the egg
causes a fertilisation envelope to form - slow block to polyspermy
cleavage and gastrulation
in exam
a period of rapid cell division without growth, partitions embryo into smaller cells- blastomeres
blastula = hollow sphere of cells
blastocoel = fluid filled cavity in blastula
gastrulation rearranges cells of a blastula into layered embryo = gastrula
cell differentiation occurs
Gastrula in triploblastic organisms has ectoderm, endoderm and blastopore - see notes
Gastrulation:
Invagination of the blastoel by the archenteron
archenteron will become endoderm, outer layer will be ectoderm
blastopore forms
mouth, anus and primitve gut form
embryonic germ layers
germ layer = embryonic tissue
ectoderm = covers outer surface, forms epidermis, nervous system, hair, cornea
endoderm = innermost germ layer, forms digestive tract, organs and secretory organs
mesoderm = bilaterians only. Forms coelom, notochord, muscles, bone, kidney
gastrulation in chicks
embryo has upper and lower layer
epiblast = mostly ectoderm
hypoblast = mostly endoderm
epiblast cells move towards the middle of the blastoderm, then into embryo towards the yolk
the midline of embryo thickens - primitive streak
gastrulation in humans
blastocyst = human equivalent of blastula
trophoblast - outer layer of blastocyst that doesn’t contribute to development but triggers implantation.
Blastocyst has trophoblast, blastocoel and inner mass of undifferentiated cells
after implantation the trophoblast expands to make extraembryonic membranes and turn cells into epiblast and hypoblast
gastrulation happens like in chicks(?), and makes endoderm, mesoderm and endoderm.
extraembryonic membranes
chorion - outermost layer with 2 sublayers - trophoblast and mesoderm. forms the placenta
allantois - full of blood vessels, for nutrition, excretion and gas exchange
amnion - has amniotic fluid for protection
yolk sac - made of hypoblast cells, helps with initial circulation before placenta forms
developmental adaptions of amniotes/ land vertebrates
shelled egg in birds
uterus in mammals
amnion - helped with reproduction on dry land - developed later
morphogenesis - neurulation
regions of germ layers develop into rudimentary organs
adoption of particular developmental fates may cause cells to change shape and move to new locations
neurulation is the formation of brain and spinal cord
cells from the mesoderm form the notochord - rod extending along top of embryo
molecules secreted due to notochord signalling cause ectoderm to form the neural plate
neural plate curves inwards, forming the neural tube
example of induction - cells cause change in nearby cella
cell migration in morphogenesis
neural crest = cells that develop along the neural tube
neural cells migrate - forms nerves, teeth etc
mesoderm lateral to the notochord forms blocks called somites - form vertebrae, ribs and muscles near vertebrae.
domestication syndrome due to neural crest - selected dogs with specific pattern of migration of neural crest cells. Indirectly selected for other features
cell shape and death in morphogenesis
eg contraction of actin filaments in the cytoskeleton causes one end to become narrower. Happens in formation of neural tube
involves cell adhesion molecules
and extracellular matrix - meshwork of secreted glycoproteins and other molecules lying outside plasma membranes
apoptosis - programmed cell death - eg finger formation
cell differentiation as a result of gene expression
muscle cells/ myoblasts produce specific proteins which form muscles (morphogenesis)
MyoD is a transcription factor and master regulatory gene
causes gene cascades - regulates transcription of other transcription factors
GRNs - gene regulatory networks
development of fruit flies using morphogens and homeotic genes
polarities are established - higher conc of morphogen in head
Maternal gene - bicoid transcription factor.
morphogens = proteins that establish embryo’s axis using gradients of concentration.
Then uses zygotic genes.
Establishes major regions like head, thorax, tail
establishes segments and boundaries of segments
Establishes organ identities using homeotic genes - specify position along axis
uses nested spacial expression - different genes expressed in different body parts
french flag model - each cell has chance of becoming a colour, depends on concentration of transcription factor, which depends on position in body
Hox code - combinations of different transcription factors specify different organs
homeobox equivalents in plants
MADS-box
SNARE genes - involved in membrane fusion
independently evolved in plants and animals
differences in plant and animal development
animals - cell division
cell movement, embryonic shape and death determines shape
grow to fixed size and shape
plants - cell expansion
cell growth, cell division plane and post-embryonic shape determines shape
grows to variable size and shape
Evo devo
synteny is in bilaterians only
- when gene loci are on the same part of chromosome in different species/individuals
hox gene clusters are indicators of whole genome duplications
noradrenaline and acetylcholine
noradrenaline - sympathetic nervous system
causes positive chronotopy/increased heart rate
activates G protein - coupled receptor
ATP to cAMP via AC
Ca+ influx, allowing myosin to bind to actin
More calcium = more muscle contraction
Acetylcholine - parasympathetic nervous system
negative chronotopy/decreased heart rate
activates g protein
ATP to cAMP reduced because AC is inactivated
reduced calcium influx
Fick’s law of diffusion
R = DA delta p /d
R= rate of diffusion
A = area over which diffusion takes place
delta p = pressure difference between 2 sides
D = diffusion constant
d= distance over which diffusion takes place
Evolutionary changes to optimise R:
Increase SA - increases A
Decrease d
Increase concentration difference - increase delta p
specialised organs for external respiration
external - gas exchange between animal and envrionment
internal - transport of gases in blood
Gills - evaginated from body
Can have internal gills covered by body cavity
present in polychaete annelids, molluscs and decapod crustaceans
Passive ventilation- tentacles can function as gills and collect food particles
Active ventilation - gastropods and cephalopods- gill leaflets hang in mantle cavity. Ventilated by cilia current. In cephalopods there is movement and ventilation via muscle contractions of mantle
Lungs - invaginated
in gastropods the mantle cavity evolved into lungs
Tracheal systems - in most terrestrial arthropods
invertebrate circulatory systems
closed - some annelids, and cephalopod molluscs
gills
systemic heart - oxygen rich
systemic tissues
branchial heart - oxygen poor
gills
Blood remains in vessels
Open - all arthopods and most molluscs
low resistance so blood flows easily
cardio-arterial valves to control bloodflow
Hemolymph is pumped from tubular heart to body cavities
Then returns via blood vessels to be recirculated
osmotic pressure
osmolarity
tonicity definitions
measure of a solution’s tendency to take in water by osmosis
number of osmotically active moles of solute per litre of solution
measure of a solution’s ability to change the volume of a cell by osmosis
hypertonic
hyptotonic
isotonic
higher osmotic pressure (of outside solution)- outside solution more concentrated. water moves out.
lower osmotic pressure - outside solution less concentrated. water moves in
equitable osmotic pressure
osmoregulators and osmoconformers
maintain constant blood osmolarity despite different concentrations in their environment
Most freshwater invertebrates are hyperosmotic regulators - blood is more concentrated than freshwater
organisms that are in osmotic equilibrium with their environment
Most marine invertebrates are isoosmotic - have same osmotic pressure as seawater
Energetically less costly
But cells experience changing environment
