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CSB430H1 Study Guide - Final Guide: Olfactory Receptor Neuron, Olfactory Bulb, Neurogenesis

Cell and Systems Biology
Course Code
Vincent Tropepe
Study Guide

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CSB430 Lecture 1Brain Evolution (vertebrate brain evolution)
Changes in the brain overtime make a brain
Evolutionarily biology affects how we think about neurogenesis and brain development
Organization of vertebrate brain
Evolved >500 million years ago
Are divisions within each division of the brain; regions have similar function in living vertebrates
Neurogenesis in olfactory bulb, developing neocorte
Anterior/rostral to posterior/caudal
o Forebrain (prosencephalon)
Cerebrum (telencephalon)
Olfactory bulb
Optic chiasm
Thalamus, hypothalamus, epithalamus (diencephalon)
o Midbrain (mesencephalon)
Tectum (superior/inferior colliculi)
Tegmentum (SN, VTA, PAG, red nucleus, reticular formation)
o Hindbrain (rhombencephalon)
Medulla oblongata
Brains of vertebrates
Edinger (neuroanatomist, 1900) “linear phylogenetic scale”
o Looked at vertebrate brains of different species
o A (fish) B (frog) C (reptile) D (bird) E (cat) F (human)
o Part of rhombencephalon is very similar across all animals
Absolute size of structures different
Structure & organization of rhombencephalon (orange) common
o Forebrain (consciousness, complex behaviour) very different across species
Fish/frogs, forebrain parts small or non-existent
Only as you climb ladder of evolution, get sequential addition of parts of brain
More forebrain structures increased complexity of vertebrates
Ariens-Kappers, Herrick
o Developed nomenclature to describe structures relative to their evolutionary age
e.g. neocortex (only in human, some in cat), paleocortex
Theory of sequential addition of brain parts = scala naturae
Are some common hindbrain structures found in all vertebrates
As animals become more complex, evolutionarily advanced, have more structures added sequentially
(pallium = dorsal forebrain (incl. cerebral cortex) | sub-pallium = ventral forebrain)
o Forebrain of many vertebrate species have little or no pallium/neocortex
most of forebrain in lower vertebrates is mostly sub-pallium structures or striatum
e.g. small fish (bony fish, e.g. Salmon) would have the simplest/most primitive brains
o all vertebrates have good sense of smell, necessary for survival
prediction: telencephalon of fish has only olfactory projections
o Classic view: human brain has a lot of pallium, sub-pallium ventrally
Songbird brain has a lot of sub-pallium, little pallium
o Modern view: human and songbird brain have pallial regions (disproved Edinger)
Songbird brain has a lot of thalamic projections to the pallium
Early experiments only used anatomy, not even histology
Newer experiments use gene expression studies, histochemical staining
(AChEenzyme breaks down ACh at synapses), expression is marker for sub-pallium (basal ganglia)
thalamus projects to cortex; no cortex = no thalamic projections to cortex
Olfactory projections in fish telencephalon/forebrain
Fish have a complex dorsal pallium and sub-pallium
Fish have thalamic projections to the dorsal pallium
Forebrain in fish receives info from many other sense modalities and other connections
(Edinger predicted only olfactory projections would project to the forebrain)
OE (in nose), olfactory neurons project through cribriform plate bone (tiny holes)
OB neurons project to forebrain/ telencephalon
Experiment: anterograde/retrograde tracing dyes to see OB projections
Edinger predicted projections all over forebrain, saw only discrete projections to forebrain
(ventral ventral forebrain, dorsal-posterior forebrain)
Other parts of forebrain for other functions, not just olfaction
Modern theory, not sequential addition
All parts in species highly conserved,
but relative sizes/structures/anatomy different

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Modern theory of brain development
Brain structure and function conserved between species, have different size/structure/anatomy
e.g. Lamprey has a small cerebrum, but is still there (old Edinger theory would think there was none)
e.g. mouse brain is lissencephalic, human brain is gyrencephalic
Closely related species more similar brains
Similarities more obvious at lower levels (gene sequence, gene expression, basic brain regions)
Embryonic brains have clearer patterns of similarity than adult brains
Are differences at all levels (gene, structure, function)
Allometric scaling of brain and body across species
Is a positive relationship between body weight and brain weight (bodies bigger, most organs get bigger)
True in most vertebrate species
Compartmentalizing of brain regions as they get larger
Small brain, has certain number of neurons, neurons can inter-connect
one neuron can receive multiple synapses
but is a spatial limit & metabolic/energetic limit
Adding more cells to grow brain,
but keep same connections/neuron
would exponentially deplete resources
If you add neurons and make brain bigger,
have to reduce connections between neurons in some regions, keep other regions connected
brain begins to compartmentalize in structure and function
lamination occurs (laminar structure, e.g. 6 layers of cerebral cortex)
laminar structure
Neurons have parallel and perpendicular axons/dendrites
More spatial efficiency
More temporal precision for info processing
better functionality, can form spatial/topographic maps
Primary visual area, primary auditory area, primary somatosensory areas
Present in many mammalian species in different relative proportions of cortex
e.g. human brain primary visual area small area in the back
less connected primary sensory processing area = compartmentalized
areas in between = association areas of the cortex
primary visual area contributes less to visual processing, now have V2, V3, etc., visual association areas integrate other sensory modalities to process
auditory, somatosensory, emotional, memory information integrated in association areas
Brain gets bigger
proportion of neurons that are interconnected tends to decrease
limit on number of connections one neuron can make (spatial & energy cost)
less dense connections cause brain to become modular/compartmentalized
cannot compartmentalize entirely because regions cannot function independently of other regions
MRI-DTI of human fetal brain
Would not see different colours if every part of brain was equally connected to every other part of brain
Sometimes some regions are connected almost exclusively
Concerted vs. Mosaic brain evolution
Left: ancestral organization of the brain
Right: derived species over course of evolution
Concerted brain evolution
Change size of one part of brain, all other structures change size in the same direction
(all interconnected, so if one structure gets bigger, all have to get bigger to preserve the connectivity proportion)
o Brain regions evolve in concert because are interconnected (lamprey example for olfactory bulb)
o Allometric scaling (brains get bigger, parts of brain get bigger in concerted way)
o Gene pleiotropy (gene polymorphisms selected, what it does in one part may do same in other part)
Mosaic brain evolution
Some parts of brain can evolve independently from other parts
Are selective pressures to specialize in something, need some brain structures more
(e.g. in murky water, have to evolve other methods to move in environment evolve part of brain)
o Brain evolution less constrained than thought
o Individual structures can be affected by evolution independently
o Brain structures for specialized behaviours in fish
o High density linkage mapping in mice
o Differential gene expression during development

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Brain region’s proportional size tends to change with absolute brain size
Support for concerted brain evolution
Regions have characteristic slopes against total brain size
Experiment: lamprey species
o Different life stages of lamprey
o Measured volume of brain structures vs. volume of whole brain
o Regression lines’ steepness different
Steep = brain getting bigger, that region getting bigger
Not steep = brain getting bigger, region not really changing
e.g. olfactory bulb (OB) 45-degree regression line, changes in concert
Gene pleiotropy
Support for concerted brain evolution
Pax6 null mice have no eyes
Pax6 null in fruit flies have no eyes
Pax6 null heterozygous in humans, have eye structures but cannot see
Pax6 important for all linked parts of eye, even though many genes encode eye genes
Brain structure adaptations in fish
Support for mosaic brain evolution
Cichlid fishes in Lake Tanganyika
o Undergo speciation in different parts of the same lake
o Some Cichlid live near surface, others in the bottom of the lake
o Individual lobes differ in size between brains, but very subtle
Rates of evolution for different brain structures were different
Total brain size accounted for only 86% of brain structure volume
Differences in pairwise allometry (pairs of structures grew at different rates)
Selective pressure on visual system, speciated 1 million years ago
Blind cave fish vs. surface-dwelling fish
o Blind cave fish almost transparent, almost no eye structures, same size
o Surface-dwelling fish have pigment, have eyes, same size & brain volume
Brains differ
o Blind cave fish
Smaller primary visual area (blue)
Larger lateral line (balance/equilibrium, sense vibration in water)
o Surface-dwelling fish
Larger primary visual area (blue)
Smaller lateral line
High density linkage mapping in mice
Support for mosaic brain evolution
10,000 mice, from recombinant inbred mice strains
o Took pure inbred X different pure inbred
o Cross many times, creation many recombinant mice with a lot of variability in brain/brain structure size
o Generate series of lines of mice with different combinations of DNA from both strains, all mapped, & measure brains
o See if sequences co-vary with size of brain or size of brain structure
o Are regions of the genome that correlate highly with size of particular brain structures
Ch6 sequence co-varies with striatum size, Ch15 sequence co-varies with LGN size
o Could not find any example of same sequence of genome co-varying with multiple structures
Individual regions of genome able to acquire polymorphisms and change region size independently of others
If concerted, would see same sequence co-varying with sizes of multiple structures’ sizes (did NOT see this)
Differential gene expression during brain development
Gene pleiotropy occurs when a gene product interacts with multiple other proteins or catalyzes multiple reactions - Wiki
gene expression can be changed in many ways (enhancerscan be close or far awayand micro RNAs)
Regions of genome, including enhancers, can evolve independently
Can have enhancers that express Pax6 in retina, different enhancer that expresses Pax6 in eye
change when and where Pax6 is expressed, and how much
Can also regulate Pax6 at level of RNA (micro RNAs)
Micro RNAs encoded in different genes
can change Pax6 expression
Sequence of Pax6 not only way to change Pax6 expression
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