Team Members: Thomas Venables &
Parag Joshi
Team Members: Thomas Venables &
Parag Joshi
References & reference Images:
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Comments :
Fertilization, perhaps the most important step in developmental
biology, is composed of several complex processes. The reactions
involved in the combination of the sperm and the egg are the starting
point for the entire developmental process. The complexities of
these interactions will be analyzed through the animation of several
diagrams from Gilbert's Sixth Edition of Developmental
Biology.
The first diagram, found on page 192, figure 7.8 B, illustrates and example of a mammalian sperm cell crossing the cumulus layer, the zona pellucida, and fusing with the egg cell membrane. The animation begins with a sperm cell, composed of a nucleus, acrosome, and flagellum, approaching the cumulus layer (extra-cellular coat) of the egg. The sperm cell travels through this layer and upon reaching the zona pellucida, releases the acrosomal contents. The zona pellucida is a specialized extra-cellular matrix that surrounds the egg, containing recognition proteins for the incoming sperm cell. The reaction between the sperm and the zona pellucida causes the release of proteolytic enzymes that carve through the zona pellucida, further guiding the sperm cell to the egg membrane. The final scene in the animation depicts the fusion of the sperm and egg membranes for the delivery of nuclear contents. One shortcoming recognized in the animation of this model is the lack of an appropriate time scale.
The next diagram addressed in the series of animations is found on page 195, figure 7.12. This diagram provides a hypothetical explanation of the initiation of sperm capacitation, a "set of physiological changes that allow the sperm to be competent to fertilize the egg" (Gilbert 194). The animated sequence begins with the efflux potassium (cause unknown). The resulting decrease in membrane potential combined with an efflux of albumin-cholesterol activates ion channels that pump calcium and bicarbonate ions into the cell. Once in the cell, these ions stimulate the protein Adenylate Cyclase to convert AMP to cyclic AMP. The cAMP then activates protein kinase A (PKA), which is already present within the cell. Subsequently, PKA inhibits phosphotyrosine phosphatase while activating protein tyrosine kinases. The protein tyrosine kinases then trigger a cascade of events involved in capacitation. Including the lack of a time scale in the animation, there is also the need for some explanation regarding the necessity of the inactivation of phosphotyrosine kinase.
The next image provided is a diagram of the structure of the mammalian zona pellucida as seen on page 198, Figure 7.17. It is apparent that the zona pellucida is composed of fibrillar structures composed of repeating ZP2 and ZP3 glycoproteins linked together by the occasional ZP1. The diagram, however, does not provide any of the quantitative dimensions regarding the depth and size of these proteins.
Figure 7-24 on page 204 was the basis of the third animation. The animation is a representation of cortical granule exocytosis involved in the slow block to polyspermy. This reaction is necessary in order to prevent multiple fertilization on one egg. The animation starts after fertilization has occurred elsewhere on the egg. It depicts sperm approaching the plasma membrane of the egg while cortical granules containing hyalin and enzymes are releasing their contents into what was the membrane of the cell. This creates a thick barrier between the cytoplasmic contents of the egg and the approaching sperm. The sperm is then released from the newly formed "fertilization envelope" thus preventing polyspermy. The animation does not show the role of the actin microfilaments in this process. This animation would be improved greatly with a time scale because it would demonstrate how quickly this process occurs in order to prevent polyspermy.
The flow chart on page 209, Figure 7-27, is the next item presented in the series of animations and diagrams. This diagram indicates the pathways of the slow and fast blocks to polyspermy as well as the initiation of gamete metabolism. It is a nice, succinct presentation of these processes.
The final topic covered in the animations of fertilization involves the activation of gamete metabolism. The first animation provided, demonstrates the conversion of the membrane bound protein Posphotidylinositol 4,5-bisphosphate (PIP2) into diacylglycerol (DAG) and Inositol 1,4,5-trisphosphate (IP3) by Phospholipase C (PLC). The IP3 formed in this conversion then proceeds to its receptor on the endoplasmic reticulum, which controls the release of calcium into the cytoplasm of the cell. The calcium release combined with the DAG produced from PIP, activates a sodium/hydrogen pump (Na into cell, H out of cell) located in the lipid bilayer. The actions of the pump lead to a basic environment in the cytoplasm, which causes the series of events that are known as egg activation. The calcium release also stimulates the cortical granule exocytosis discussed previously. In the animation, it should be noted that it was not known if PLC traveled along the bilayer to PIP2 or vice versa. The exact process of the initiation of this event is unknown, however, the next four animations cover the four hypotheses presented in Gilbert's text (Figure 7-29, pg. 209). These animations deal with the nature of the interaction between the sperm cell and the protein tyrosine kinase, which is thought to activate PLC. In the first example, the receptor for the sperm is thought to have tyrosine kinase activity. The second example presents the tyrosine kinase as an associated cytoplasmic protein with the receptor. These two examples are prior to sperm fusion. The final two animations are hypotheses of the mechanism occurring after sperm fusion. In the first of these, the active tyrosine kinase originated from the contents of the sperm cell and becomes activated through soluble factors from the egg. The final animation presents the idea that soluble factors from the sperm activate PLC, which in turn activates the calcium release.
In conclusion, the animations and diagrams presented although not without their problems, are effective in their representations of the information provided. The one significant problem we encountered is the time that is involved in these examples and if we were to do this in the future, this is something we would do more research on.
