Embryonic Development Flashcards
What is embryonic development and how do we study it?
We study it using transgenic mice. It’s when the undifferentiated ball of cells turns into a fetus, meaning that all the major organ systems are established.
Outline embryonic development in mice
Happens very quickly. Implantation at embryonic day 4.5 and gastrulation (embryo forming multiple layers) 2 days later. Basic body plan established at E7-8.5 and organogenesis begins. At gastrulations midpoint (E11.5) all major organ systems are identifiable and growing, and the embryo is a fetus
How does embryonic development in humans differ from mice in terms of timescale?
It takes much longer in humans, implantation at days 6-9, gastrulation at 14, and the fetal stage at 8 weeks after fertilisation. Fetal growth represents 2/3 of gestation
Describe the development of the post implantation epiblast
Pre implantation have 10-20 cells (mice) and 50-100 (humans). After implantation, these cells proliferate significantly, initially expanding as undifferentiated pluripotent cells with identical properties - however their morphology and molecular profiles differ substantially from the pre-implantation epiblast. They form a true epithelium which in the mouse expands to form a cup shape but in the human it forms a disk. Apart from this they appear to be the same. Also, the gene expression and epigenetic characteristics of the post-implantation epiblast are distinct from pre in both species - the embryo is advancing developmentally
What are the two axis that the main body plan is split into?
Anterior-Posterior axis and Proximal-Distal axis. They both result from extraembryonic tissues signalling to epiblast cells - the ones closest to the signalling centre get the highest concentration of signals
Describe the proximal-distal axis
It is the first to emerge, and the proximal side of the embryo is the side closest to the extraembryonic endoderm
Describe the development of the anterior-posterior axis
Signals from the visceral endoderm initiate the asymmetry. Distal tip of the embryo is the distal visceral endoderm. The cells here migrate upwards into what will become the head. They form the anterior visceral endoderm. In real embryos, this is a bulge on one side of the embryo (identifier that the axis is forming). The AVE becomes an important signalling centre, helping to pattern the epiblast (with the extra embryonic epiblast). Critical in determine where the primitive streak forms in gastrulation (posterior side of embryo)
Explain the start of gastrulation
After the axes are established, a key group of cells receive the appropriate signals to form the primitive streak, which forms in the proximal/posterior side of the embryo. This is the start of gastrulation, a crucial part of embryogenesis. Each germ layer forms particular tissues - ectoderm for skin and neural tissue, mesoderm for blood and muscle, endoderm for gut.
Describe the process of gastrulation
Cells begin to move towards to midline, where they lose cell-cell contact, leaving the epithelium - epithelial to mesenchymal transition. When the cells emerge on the other side, they begin to migrate inbetween the epiblast and visceral endoderm layer. The primitive streak begins at the proximal-posterior side and expands distally. During the migration of cells, differentiation occurs. The position at which the cells cross the streak determines their fate. The first cells to exit become heart and blood, more distally they form trunk muscles and then endoderm. The wings of the mesoderm wrap around the epiblast and move anteriorally. The definitive endoderm cells intermingle with the visceral endoderm cells underneath, displacing them. This means that the definitive endoderm develops as the bottom layer which is eventually internalised (gut). The node also forms which is an important signalling centre for more specialised differentiation. The cells not passing through the streak, remaining on the outside, become ectoderm (skin). The mesoderm cells are in the middle of the 3 layered structure and the endoderm is on the inside. Definitive endoderm and mesoderm cells migrate forward, extending the structure anteriorly and a special type of mesoderm called the notochord signals to the overlying ectoderm to stimulate neural tissue (neuroectoderm) formation
How does Waddington’s landscape represent the germ layers?
A pluripotent cell guided at the top is separated into 3 valleys representing the 3 germ layers. These then differentiate further into other cell types.
What is the defining factor of an fetus?
When the major organ systems are established. Organogenesis happens toward the end of gastrulation and is an asynchronous process. In mice the fetus is at mid gestation (day 11-12) and in humans at the end of the third trimester
