Term
| Describe the phases of fertilization |
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Definition
| There are 2 phases. The first is when the sperm get through the corona radiata. The corona actually helps the sperm by producing hormones that the sperm recognize and go to. Once they get through the corona they enter phase 2. This is where they have to get through the innermost protective covering of the ovum called the zona pellucida. This one is much harder to get through. Both the zona and the sperm have receptors on them that help the sperm bind to the zona. Once this happens there is a sac of enzymes in the head of the sperm called the acrosome that is released. These enzymes basically eat their way through the ovum until they get through the zona into the cytoplasm of the ovum. |
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Term
| What is the cortical reaction |
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Definition
| This is part of phase 2 of fertilization. It occurs after the first sperm has broken through the zona pellucida. The first part is that there is a fast shocking reaction which makes all the sperm that are surrounding the ovum back up. The slow reaction is due to a bunch of enzymes that are released from the zona that destroy all of the receptors on the outside of the zona so that the sperm cannot find it/bind to it anymore. This prevents polyspermy. |
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Term
| Describe the conversion of the zygote to the blastocyst |
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Definition
| Once the sperm is inside the cytoplasm it releases its tail and finds the nucleus of the ovum. The two pronuclei(haploid) fuse to form the complete diploid nucleus. After this happens mitosis can begin to occur. All the way through 8 cells they are distinct cells, but at 16 the blastomere goes through compaction. This makes a morula of cells that are blurred together. After the morula stage the cells on the outside of the morula begin to differentiate from those on the inside. This is what is going to make a the blastocyst. Once the cells are differentiated they are called pluripotent. They cannot go back once they have differentiated. The cells on the outside are called the trophoblast and the cells on the inside are called the inner cell mass(ICM). There is a cavity called the blastocyst cavity. The morula is now a blastocyst. The blastocyst is what is going to implant into the uterus, but before it can do that it has to get out of the zona pellucida which is still surrounding it. It does this by a process called hatching. The blastocyst produces proteases that eat through the zona and then it squeezes out of the zona freeing itself to implant into the uterus. |
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Term
| Describe the conversion of the ICM to the hypoblast, epiblast and bilaminar germ disc. |
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Definition
| During wk 1 there is 1 outer layer(trophoblast), 1 inner layer(ICM) and 1 cavity (blastocyst cavity). At wk 2 there will be 2 outer layers, 2 inner layers, and 2 cavities. The ICM differentiates into the hypoblast and the epiblast. The hypoblast is the layer that will be facing the blastocyst cavity. The epiblast will be facing the inner part of the trophoblast that has now differentiated into what is called the cytotrophoblast. Where the epiblast and the hypoblast touch is called the bilaminar germ disc. |
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Term
| Describe the proliferation and differentiation of the trophoblast |
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Definition
| The trophoblast differentiates into two layers. The outermost layer is called the syncitiotrophoblast. This is the part that will invade the uterus, search out capillaries, and fill up with blood for oxygen supply. The innermost part is called the cytotrophoblast. This is the part that will add a protective covering around the amniotic cavity(embryo). |
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Term
| Describe the process of gastrulation (formation of the trilaminar disc) |
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Definition
| Gastrulation is when the body begins to form. This starts somewhere around wk 3. At this point there are 3 outer layers, 3 inner layers, and 3 cavities. The outer layers starting from the outermost are syncitiotrophoblast, cytotrophoblast, and the XE mesoderm. the 3 inner layers are the epiblast, mesoderm, and hypoblast. These three layers are the trilaminar embryonic disc. The 3 cavities are the amniotic cavity, the yolk sac, and the chorionic cavity. Gastrulation is the process of how this trilaminar embryonic disc undergoes morphogenesis and turns into a baby! |
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Term
| Describe the primitive pit, primitive node, and primitive streak and their involvement in the formation of the primary germ layers. |
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Definition
| The primitive streak forms first. This is a layer of cells on the dorsal side of the epiblast(facing the amniotic cavity). This is what marks the beginning of gastrulation. From the primitive streak come the primitive node and the primitive pit. The streak is elongating at the same time. The primitive pit is at the anterior end of the streak and the pit is a groove that forms along the streak. The way that this helps create the 3 primary germ layers is through several steps. The cells on the epiblast(now called the ectoderm) migrate to the primitive streak and basically fall into the primitive node. From there they move in between the epiblast and the hypoblast and form a layer of tissue called the mesoderm. This continues to happen over and over again. Eventually the hypoblast is replaced by these migrating cells from the epiblast. This creates a layer under the mesoderm called the endoderm. Now we have the three layers, all of which are derived from epiblast. From top to bottom the layers are ectoderm(epiblast), mesoderm, and endoderm. The endoderm faces the yolk sac, and the ectoderm faces the amniotic cavity. |
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Term
| What is the difference between congenital and genetic defects. |
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Definition
| Congenital simply means present at birth. There are numerous ways for defects to occur that have nothing to do with genetics. Teratogens are the most obvious one. This includes things like alcohol, smoke, drugs(both prescribed and recreational), etc. Accidents could be another cause. Genetic defects are due to issues in the genes. Things like down syndrome(trisomy 21) are included here. |
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Term
| This begins to develop in wk 3 has a critical stage through wk 17 and continues to develop all the way to birth |
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Definition
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Term
| This begins to develop in wk 3 has a critical period through wk 6 1/2 and continues to develop until wk 9 |
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Definition
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Term
| This begins to develop in wk 4 has a critical period through wk 9 1/2 and continues to develop until wk 17. |
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Definition
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Term
| This begins to develop in wk 4 has a critical period through wk 7 and continues to develop up to wk 9. |
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Definition
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Term
| This begins to develop in wk 4 has a critical period through wk 8 1/2 and continues to develop until birth |
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Definition
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Term
| This begins to develop in wk 4, has a critical period through wk 7 and continues to develop until wk 9 |
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Definition
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Term
| This begins to develop in wk 6 has a critical period up to wk 9 and continues to develop until birth |
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Definition
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Term
| This begins to develop in wk 6 has a critical period to wk 9 and continues to develop until wk 15 |
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Definition
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Term
| This begins to develop in wk 7 has a critical period through wk 15 and continues to develop until birth. |
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Definition
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Term
| Describe what makes up the embryonic part of the placenta |
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Definition
| The chorion consists of the chorionic stalk which connects the placenta to the embryo, the XE mesoderm, cytotrophoblast and syncitiotrophoblast. |
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