MCDB 423 Lecture 2: Lecture 2

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Molecular, Cellular and Developmental Biology
MCDB 423
John Kuwada

Lecture 2 Slide 1 This is a desropholia embryo The green are the nuclei and at this point in time, the embryo consists of a monolayer of cells All the cells look identical to each other but they are not At this stage, the embryo is already divided into different regions The ventral region of this egg consists of ectodermal cells that are going to give rise to the entire nervous system -This is called the neurogenic region The way neurons are generated is that some of the cells in the neurogenic region end up delaminating and going to the interior and eventually give rise to the entire nervous system Slide 2 We’ll look at the grasshopper Their embryo is 10x the size so the cells are easy to manipulate The embryo consist of a brain and many ganglia which are mini-brains that consist in many segments of the body -They control those segments Each of these are given rise to by a segment of the neurogenic region where ½ of it that contains a 150 ectodermal cells -This is a hemi-segment of the neurogenic region -They give rise to 30 unique neuroblast A neuroblast is a neural stem cell -So when it divides, one of its progeny will always be a neuroblast -They are also unique because each of these neuroblast gives rise to a specific subset of the neurons in the thoriac section as an example in this case -They are arranged in rows and columns which allows us just by looking at them to say it’s neuroblast 1-1, 1-2, 1-3, etc. Each neuroblast gives rise to a unique collection of neuron Lets examine neuroblast 1-1 -It gives rise to two progenies where one of them is a neuroblast 1-1 again -The other is a smaller cell called gaglion mother cell (GMC 1) -It will divide again giving rise to neuroblast 1-1 as well as GMC 2 and so on Eventually a GMC which is a neuro precursor cell (a progenitor cell) will divide and give rise to two neurons The first GMC gives rise to two neurons: ACC neuron and PCC neuron -ACC is a motor neuron -PCC is a neuron that is completely within the nervous system called interneurons Every single neuroblast 1-1 has this exact pattern and gives rise to neurons in the same pattern Slide 3 The monolayer in the neurogenic region, some of the cells will delaminate -This is a drawing of the process There are about 150 ectodermal cells that give rise to 30 neuroblasts -Approximately 5 end up in the formation of a single neuroblast Here you see a group of 4-5 neural ectodermal cells -One will start to delaminate meaning that it’s cell body starts to swell up and moves into the interior of the egg -Then it differentiates into a neuroblast cell This is a scanning electron micrograph -The surrounding neural ectodermal cells become supporting cells slide 4 The neuroblast are derived in three waves -First wave is the black neuroblast -Second wave is the hatch You can find neuroblast that divide even after all the surroundings neuroblasts around it have already developed So if you look early one, you will see that 4 or 5 are neural ectoderm cells -This allowed people to demonstrate that cell-cell interaction are the basis of the development of neuroblast slide 5 Each of the neuroblast are unique because each gives rise to a unique set of progenies It turns out that 7-3 are the only neuroblast that give rise to serotonin neurons -Serotonin are NTs The first 3 out of the 4 progeny from 7-3 are serotnogine neurons using an antibody that recognizes serotonin The image is showing one segment (the thoracic segment) -So you can see the presence of 7-3 using the presence of this type of progeny slide 6 Exp 1: -When one of the 4-5 neural ectodermal cells began to enlarge, they blated these cells (destroyed them) and would wait -Subsequently, a neuroblast 7-3 developed -He knew this because if he waited longer, they could screen for serotonin Exp 2: -At an earlier stage when none of the neural ectodermal cells started to enlarge, he blated all of them in that region -They discovered that no neuroblast developed and there were no serotonin neuron Answers: 1) True 2) False 3) True Slide 8 Possible experiment to test question 3 from the previous slide This is after the 2 wave but before the 3 wave -1 out of the 5 of the ectodermal cells will become 6-1 and 1 of the other 5 will become 7-3 To test if the cells are constricted to become 6-1 or 7-3, what can you do? Slide 9 This is a possible experiment You could ablate all the ectodermal cells in 6-1 and take 1 of the ectodermal cells from 7-3 and transplant them to 6-1 Shown above is two possible results you can get Answer: A Slide 10 One of the neuroblasts start to enlarge -It can inhibit its neighbors from enlarging and becoming a neuroblast -So it will be the only one that becomes a neuroblast So what is the inhibitory signal? Slide 11 Now we’re moving to dresophila and looking at the peripheral nervous system -This is the wings -This is because they have an irregular array of sensory neurons -It turns out that mutant screens were done in which the wings have many more sensory neurons than normal Two such mutations were in notch and delta A notch -/- will have more sensory neurons than a wild type as will the delta -/- -Since there are more neurons due to a mutation, these genes are called neurogenic genes -A loss of delta and notch gives rise to more neurons -So they may be part of the inhibitory signals slide 12 Notch and delta genes were cloned and sequenced They are both transmembrane proteins Notch has a large extracellular domain and a heft intracellular domain Delta has very small intracellular domain and a extracellular domain Given they both give you the same phenotype and they are cell surface proteins, it suggest
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