I talked with my mom and asked about my history and here it comes (long story very short):
My mother had placenta previa so she was hospitalized when she had been pregnant 26 weeks. When she had been waiting me for 30 weeks her placenta broke down and they had to get me out. So I was born quite early but they had given some cortisone to my mother to get my lungs developed enough and I was ok. Mom said that I recovered very fast.
And one story (my moms favorite one): I had some kind of doctors control and they noticed that my top of the head was still open so they could check my brains (since I was born so early, there was a huge risk for me to be abnormal) with ultra sound. And the doctor said "What a beautiful brain structure!" Mom always say that from that on she knew I'd be smart :D
But I was quite shocked, I always knew I was born early but THAT early...
Monday, October 1, 2007
Thursday, September 27, 2007
Gastrulation - 7th to 10th Day
Axis formation in mammals is poorly understood and as in eg. xenopus there is an animal-vegetal pole and dorsal-ventral axis (which I won't cover in here) there is no such things in mammalian embryo. Only "axis" that can be said to exist is embryonic-aemryonic axis according to the Inner Cell Mass. But this axis is only geometrical, it has nothing to do with the future germ layers.
Blastocyst implants into the uterine wall and the trophectoderm gives rise to extra-embryonic ectoderm. The gastrulation involves cell movements which yield a three-dimensional embryo, where the germ layers, ectoderm, mesoderm and endoderm are in correct positions. The gastrulation is initiated by the formation of primitive streak and the cells move inside as individual cells, giving rise to endoderm and mesoderm.
The embryo is now 10 days old.
This part of the embryo development wasn't covered in our course, only the gastrulation of sea urchins and xenopus was covered and as we are not interested on them, I will not go through them ;)
Blastocyst implants into the uterine wall and the trophectoderm gives rise to extra-embryonic ectoderm. The gastrulation involves cell movements which yield a three-dimensional embryo, where the germ layers, ectoderm, mesoderm and endoderm are in correct positions. The gastrulation is initiated by the formation of primitive streak and the cells move inside as individual cells, giving rise to endoderm and mesoderm.
The embryo is now 10 days old.
This part of the embryo development wasn't covered in our course, only the gastrulation of sea urchins and xenopus was covered and as we are not interested on them, I will not go through them ;)
Cleavage - First Week
The cleavage include rapid cell divisions and during these cell divisions the size of the embryo is no increasing. 
It is still the sized of an egg cell. The egg cell has nutrients and the embryo needs to rely on these nutrients during the first fast cell divisions. Normally cell division (mitosis) includes several steps and it lasts about 16 hours but during cleavage the cells skip most of the steps and and there is no transcription of the genes, thus the cell division lasts only 30 minutes (in mice). After the first few cell divisions the embryo is called morula.
At this stage the cells become compacted when the blastomeres maximize the cell contacts. The cells become polarized and compaction yields inner and outer cells. Some of the Inner Cell Mass (ICM) form gap junctions and these cells give rise to the embryonic tissues.
The outer cells become trophoblast and give rise to extra embryonic membranes. The trophoblast also secretes fluids into the morula and forms blastocoel in the late cleavage. The developing embryo becomes blastocyst (hollow ball of cells).
After the formation of blastocyst, the embryo is ready to go to gastrulation to form multilayered embryo.
Here's the time window for these events:
It is still the sized of an egg cell. The egg cell has nutrients and the embryo needs to rely on these nutrients during the first fast cell divisions. Normally cell division (mitosis) includes several steps and it lasts about 16 hours but during cleavage the cells skip most of the steps and and there is no transcription of the genes, thus the cell division lasts only 30 minutes (in mice). After the first few cell divisions the embryo is called morula.
At this stage the cells become compacted when the blastomeres maximize the cell contacts. The cells become polarized and compaction yields inner and outer cells. Some of the Inner Cell Mass (ICM) form gap junctions and these cells give rise to the embryonic tissues.
The outer cells become trophoblast and give rise to extra embryonic membranes. The trophoblast also secretes fluids into the morula and forms blastocoel in the late cleavage. The developing embryo becomes blastocyst (hollow ball of cells).
After the formation of blastocyst, the embryo is ready to go to gastrulation to form multilayered embryo.Here's the time window for these events:
- Day 1: First cell division
- Day 2: Second cell division (4-cell stage)
- Day 3: 6-12 cell stage
- Day 4: 16-32 cell stage and the embryo becomes morula
- Day 5: Embryo forms blastocyst
- Days 6-7: Embryo implants into the uterus wall
Fertilization - Day 1
Ok, lets look closer the fertilization.
When sperm enters the female reproductive tract, it is not yet mature. It goes over capacitation, which means that it matures when traveling towards the egg. But how does it find the egg? There might be some kind of chemoattractants the egg releases. When the sperm binds these attractants (one way of species specificity, the sperm has receptors only for the own species eggs proteins) it gets hyperactivated. Its internal pH concentration increases (due internal messenger pathways) which induces the usage of ATP which makes the flagella spin faster (activates dynein). When the sperm finds the egg, it first encounters the cumulus cells (somatic cells) that surround the egg and nourish it.
The sperm releases hyaluronidase enzyme that degrades the hyaluronic acids and lets the sperm penetrate to the next layer of the zona pellucida. Zona pellucida has three different proteins, ZP1, ZP2 and ZP3. The ZP3 binds to mature sperm and causes acrosomal vesicle of the sperm to fuse and release its enzymes. These enzymes help the sperm penetrate the zona pellucida layer (yet another way of species specificity). ZP2 binds to sperm that has undergone acrosomal release and ZP1 is only a stucture protein.
After this the egg membrane elongates and engulfs the sperm with the help of microvilli. The sperm and egg membranes fuse and this triggers the fast block of polyspermy. Fusion initiates the release of Ca2+ ions which causes the egg cell membrane to depolarize. This prevents sperm from binding and initiates also the slow block of polyspermy; the exocytosis of cortical granules (also known as zona reaction). These cortical granules have different sets of enzymes that harden the zona pellucida so that sperm cannot penetrate it. The Ca2+ also activates the egg metabolism (Maturation Promoting Factor is degraded and the egg completes meiosis).
Now the egg has two pronuclei which fuse together and meiosis completes.
The formed zygote enters to the next phase, cleavage stage.
When sperm enters the female reproductive tract, it is not yet mature. It goes over capacitation, which means that it matures when traveling towards the egg. But how does it find the egg? There might be some kind of chemoattractants the egg releases. When the sperm binds these attractants (one way of species specificity, the sperm has receptors only for the own species eggs proteins) it gets hyperactivated. Its internal pH concentration increases (due internal messenger pathways) which induces the usage of ATP which makes the flagella spin faster (activates dynein). When the sperm finds the egg, it first encounters the cumulus cells (somatic cells) that surround the egg and nourish it.
The sperm releases hyaluronidase enzyme that degrades the hyaluronic acids and lets the sperm penetrate to the next layer of the zona pellucida. Zona pellucida has three different proteins, ZP1, ZP2 and ZP3. The ZP3 binds to mature sperm and causes acrosomal vesicle of the sperm to fuse and release its enzymes. These enzymes help the sperm penetrate the zona pellucida layer (yet another way of species specificity). ZP2 binds to sperm that has undergone acrosomal release and ZP1 is only a stucture protein.
After this the egg membrane elongates and engulfs the sperm with the help of microvilli. The sperm and egg membranes fuse and this triggers the fast block of polyspermy. Fusion initiates the release of Ca2+ ions which causes the egg cell membrane to depolarize. This prevents sperm from binding and initiates also the slow block of polyspermy; the exocytosis of cortical granules (also known as zona reaction). These cortical granules have different sets of enzymes that harden the zona pellucida so that sperm cannot penetrate it. The Ca2+ also activates the egg metabolism (Maturation Promoting Factor is degraded and the egg completes meiosis).Now the egg has two pronuclei which fuse together and meiosis completes.
The formed zygote enters to the next phase, cleavage stage.
Tuesday, September 25, 2007
From fertilization to gastrulation - Week 1
My future baby is only a dream for now.
But since I'm having sometimes even frustrating bad baby fever, and I'm having a course about Developmental Biology, this is good time to start this :) Though I will leave many things away (I'm not writing a book)...
So, lets start from the beginning. Here's how it all starts;
Sperm and egg meet after mom and dad love each other ;) The sperm must penetrate through many obstacles before it can fertilize the egg...
...and after fertilization we have a zygote.
After this the pronuclei of the egg and sperm fuse and the zygote starts to divide fast. During these fast cell divisions the embryo is in cleavage stage. The cell becomes a morula and at this time the future embryo has traveled from the fallobian tube to the uterus.
After the future embryo has reached the uterus, it transforms to blastula and implants (implantation) to the uterus. Also gastrulation begins and the future embryo becomes gastrula.
I believe that's enough for this time. Here's a schematic summary what has happened so far:
1. zygote
2. - 6. cleavage
6. morula
8. blastula
9.-11. gastrulation
But since I'm having sometimes even frustrating bad baby fever, and I'm having a course about Developmental Biology, this is good time to start this :) Though I will leave many things away (I'm not writing a book)...
So, lets start from the beginning. Here's how it all starts;
Sperm and egg meet after mom and dad love each other ;) The sperm must penetrate through many obstacles before it can fertilize the egg...
...and after fertilization we have a zygote.
After this the pronuclei of the egg and sperm fuse and the zygote starts to divide fast. During these fast cell divisions the embryo is in cleavage stage. The cell becomes a morula and at this time the future embryo has traveled from the fallobian tube to the uterus.
After the future embryo has reached the uterus, it transforms to blastula and implants (implantation) to the uterus. Also gastrulation begins and the future embryo becomes gastrula.
I believe that's enough for this time. Here's a schematic summary what has happened so far:
1. zygote
2. - 6. cleavage
6. morula
8. blastula
9.-11. gastrulation
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