Development Lecture 3 Flashcards
What are the three mechanisms of cell commitment?
- localised determinants
- embryonic induction
- Morphogen gradients
What did Anthon van Leeuwenhoek discover?
sperm in 1677, he believed he could see the blood vessels and nervous tissue, supporting the idea of Preformation
Who first drew Homunculus and who thought that all future generations where in the Homunculi?
Nicolas Hartsoeker drew Homonculus
Nicolas Malebranche suggested the ‘Russian doll effect’
Who observed chick embryos under a microscope and incorrectly drew the Preformation conclusion from it?
Marcos Malpighi
Define Preformation
Organs and tissues are preformed in the egg/sperm.
Organs and tissues are correctly positioned in embryo
They simple enlarge during development
Define epigenesis
Organs and tissue are added gradually, the complexity increases through development
What five developments provided evidence for epigenesis?
- William Harvey (1578-1657) observed increasing complexity in deer embryos during gestation. He also studied chick and insect embryos and backed up Aristotle
- Cell theory - all living things are made from cells and new cells only arise though division of pre-exisiting cells.
Sperm and eggs are single cells and cannot contain tissues made up of thousands of cells - Hans Driesch - sea urchin embryos are capable of replacing parts that have been deleted. According to preformation, these parts should be permanently deleted.
- Gregor Mendel’s pea genetics - why offspring acquire characteristics
- Dyes and stains to study embryos
What ‘decisions’ are made that lead to the development of a neuron?
- which germ layer; ectoderm, endoderm, mesoderm?
ectoderm (outer) germ layer - Neuroepithelium or dermis?
neuroepithelium - forebrain, hindbrain, midbrain, spinal cord?
hindbrain - glia or neuron?
neuron
Define pluripotent
Cell can become any cell, including the germ layers but NOT extraembryonic tissue cell
Define totipotent, give an example
Cell can become any cell, including extra embryonic tissue cell
The fertilised egg
Define multipotent
Cell can become any call from a specific germ layer
Who came up with the idea of an epigenetic landscape?
Conrad Waddington
Define determination
The commitment to a specific cell type, mostly irreversible
Define specification
commitment to a particular fate that can be changed according to the environment
Define differentiation
The process whereby cells acquire their final structure and functional characteristics, often by producing tissue specific proteins
How can specified and determine cells be distinguished experimentally?
A determined neuron cell, when located to another environment, will form an ectopic neuronal cell.
A specified neuronal cell when transplanted will not form a neuron because its fate is still determined by the environment
What are the two main mechanisms responsible for specifying cell fate in embryos?
Cytoplasmic determinants
Embryonic induction
What is a local determinant?
A molecule that can enter nucleus to determine gene expression that will determine cell fate
What did Wilhelm Roux (1850-1924) suggest determines cell fate in embryos?
- There are molecules that tell a cell what to become
2. The egg was a mosaic of different, unequally distributed determinants for all the different cell types
How did Wilhelm Roux (1850-1824) test his theory of determinants?
Roux (1888) killed a single blastomere of a two-cell frog embryo with a hot needle. The surviving blastomere developed into a half-embryo
He concluded that the embryo has a mosaic of different determinants
In reality, what do determinants do in an embryo?
There are only few determinants in an embryo and they establish general polarities.
It is rare for determinants to determine cell type fate
Give examples of determinants that do establish polarities
In Drosophila, the germ plasm determines germ cells
In Ascidians, the myoplasm determines muscle cells
What evidence supports cytoplasmic inheritance?
Transplantation studies
1. Cytoplasm in moved to a new location in the embryo
- If the cytoplasm contains local determinants then ectopic cells will form in that place in the embryo
- aberration of the determinant, results in the absence of the tissue type which it determines
Who used the yellow cytoplasm of Ascidians to first observe local determinants?
- Edwin Conklin (1863-1952) - observed that ascidians have different coloured cytoplasm within the eggs
- the coloured cytoplasm always ended up in the same place
- any cells that acquired the yellow cytoplasm became muscle cells
- Conklin suggested that the yellow cytoplasm contained muscle determinant
What evidence had Laurent Chabry provided that supported Conklin’s observations in Ascidians?
Chary selectively killed blastomere from ascidian blastocysts and found that specific blastomeres were responsible for producing a particular set of tissues
What did Chary conclude from his experiments on ascidian blastomeres?
Each blastomere develops autonomously and does not require interactions with other cells
Who discovered macho-1?
Hiroki Nishida and Kaichiro Sawada (2001)
What is macho-1?
Maternal macho-1 is localised to the yellow cytoplasm of the egg at the 16- and 32- cell stages
It is localised to cells that give rise to muscles
Macho-1 is a transcription factor that regulated muscle-specific genes (m-actin, m-myosin, myf, tbx6, snail)
What happens (and why) when macho-1 is injected into ascidian embryonic cells that aren’t yellow?
they become muscle cells because muscle-specifc genes are turned on by macho-1
What is mosaic development and give an example?
Ascidians
The egg is a mosaic of different local determinants that specific cell types
If you take blastomeres of the blastocyst and separate them from each other, each blastomere should develop into different part of the embryo
What scientists separated sea urchin blastomeres at the two cell stage, the four cell stage and the eight cell stage?
Hans Driesch
What happens when sea urchin blastomeres are separated and what is this called?
each blastomere forms a perfectly normal larva, just smaller than normal
REGULATIVE DEVELOPMENT
What type of development do sea urchins follow?
Regulative
True or false:
- Mammalian embryos are regulative
- If you separate ascidian blastomeres, each blastomere forms a normal, but smaller, larva
- true
2. false
How are identical twins formed
- Blastomeres are separated
2. the inner cell mass divides
What is a tetragametic chimera?
An individual that is made of two genetically distinct cell types
Define competence
the ability of a cell to respond to a signal
What is embryonic induction?
The process where cells communicate with each other to change their cell fates
What is necessary for embryonic induction?
- signalling cell
2. responding cell
Who separated newt blastomeres as they were undergoing the first cleavage division and found that two small but otherwise normal tadpoles grow?
Hans Spermann (1869-1941)
True or false:
Human blastomeres can be separated up to the 8 cell stage and form normal embryos
false - only up to the 4 cell stage
How can chimeras help geneticists?
Help to understand whether genes act autonomously (mutant cells continue to form the mutant phenotype)
or non autonomously (mutant phenotype is rescued by normal cell)
How did Hans _ demonstrate embryonic induction?
Spermann
He transplanted small regions of the amphibian embryo into new locations
e.g. the notochord of a new gastrula into the ventral side of a similarly stages host, which formed a second dorsal axis (i.e. a conjoined twin)
How did Spermann’s student _ demonstrate that the conjoined twin from the notochord transplantation was made of host cells or transplanted cells?
Hilde Mangold
- Transplanted prospective notochord cells from lightly pigmented gastrula into the ventral region of darkly-pigmented hosts
- She found that transplanted cells always formed the notochord of the conjoined twin but the nervous system of the conjoined twin was formed from host cells that would usually form the epidermis
THE GRAFTED CELLS HAD CHANGED THE FATE OF HOST CELLS
What is neural induction?
Grafted notochord instructs ectoderm to form nervous system
Why did Mangold & Spermann term the amphibian notochord ‘the organiser’?
The notochord changed the fate of host cells
How does neural induction work in amphibians?
- The prospective notochord involuted inside the embryo during gastrulation and moves beneath the dorsal ectoderm
- The prospective notochord induced the overlying ectoderm to form the neural plate
- The prospective notochord also acts on adjacent mesoderm cells, instructing them to form muscle
How do conjoined twins arise?
A transplant places a second notochord on the ventral side of the embryo
Who transplanted the primitive node to a new location in the epiblast?
What did he observe
Conrad Waddington
Induction by the notochord
The node is the source of notochord cells in the avian gastrulae
The transplanted node continues to form the notochord
IT ALSO recruited host cells to form a second embryo, including much of the nervous system
What is the structure of the distinct neuronal subtypes along the dorsal ventral access of the vertebrate spinal cord?
V3 interneurons MN V2 interneurons V1 interneurons V0 interneurons Floor plate Notochord
How can you distinguish between the different neuronal subtypes in the ventral half of the vertebrate spinal chord?
Each population expresses a different set of transcription factors
How was neuronal specification in the vertebrate spinal cord demonstrated?
A section of the notochord was removed from the chick embryo
The notochord was grafted adjacent to the lateral wall of the neural tube
Where the notochord was removed - neural tube failed to specify into floor plate, ventral interneurons or motoneurons
The floor plate was formed by the neural tube, adjacent to the grafted notochord
The floor plate induced ectopic ventral interneurons and motor neutrons when grafted into the ventral wall of the neural tube
What happens when a floor plate is grafted into the ventral side of the neural tube?
The floor plate induceds ectopic ventral interneurons and motor neutrons when grafted into the ventral wall of the neural tube
What happens when the notochord is grafted adjacent to the ventral side of the neural tube?
The notochord induced the floor plate and the floor plate indicted ventral interneurons and motor neurons
what signal is responsible for the inductive activities of the floor plate an notochord?
Sonic hedgehog (Shh)
How did _ use sea urchin embryos to support epigenesis?
Hans Driesch - sea urchin embryos are capable of replacing parts that have been deleted. According to preformation, these parts should be permanently deleted.
