2. A recurring theme in human reproduction derived from outcome results from lit
ID: 267253 • Letter: 2
Question
2. A recurring theme in human reproduction derived from outcome results from literally millions of IVF treatments is that the developmental fate of the embryo, i.?., whether it will progress to a normal birth, is largely determined (i) in the oocyte before fertilization, (ii) at fertilization, or (iii) by the 8-cell stage. Considering the discussions in class and from the assigned readings, what specific molecular, cellular, and genetic processes or events occurring at the level of the plasma membrane, cytoplasm or DNA need to occur o)a papogfese thro stlages in order to promolt normal and succesfil embryogenesis (10) (b) what specific dysfunction(s) at each of these three stages have been shown to cause embryogenesis to fail and why? (10)Explanation / Answer
Ans; Events occurring in the oocyte before fertilisation
Plasma membrane: expands and attains 20 µm in diameter and microvilli regions are formed which are the site of entry for spermatozoa.
Cytoplasm: Oocytes acquire the ability to fuse with sperm when they reach 20 µm in diameter
DNA: becomes condensed and cell division gets arrested at prophase II of meiosis
Events occurring in during fertilisation
Plasma membrane: Spermatozoa penetrates zona pellucida and encounters plasma membrane of oocyte. There are several proteins involved in plasma membrane fusion. Cluster differentiation (CD) molecules (tetraspanin superfamily) are sperm and oocyte membrane surface clusters of differentiation molecules involved in the fusion of plasma membranes of spermatozoa and oocyte and are produced in male and female genital tracks. Especially CD9 and CD81 (human CD151) interact with each and recognises microvilli region on the oocytes plasma membrane and makes this part of plasma membrane to fuse. CD9 and CD81 trtraspanin proteins separately can fuse plasma membranes but less efficiently than their combinatorial interacting manner. Fertilisation failure in IVF will be higher if these two proteins not interacting. Another CD46 expressed in acrosome-reacted spermatozoa interacts with CD11b/CD18 (?M?2), which is an integrin expressed on oocytes. One of the sperm surface epididymal protein, which is a cysteine-rich secretory protein, known as cysteine-rich secretory protein 1 (CRISP1) involved in acrosomal reaction. Another protein IZUMO1, which is an approximately 56 KDa testis-specific member of the immunoglobulin superfamily (IgSF) involved in acrosomal reaction
Plasma membrane interaction procedure involves two distinct steps: The sperm binding where adhesion molecules are the main factors bringing membranes in a close apposition and the second step of membrane fusion. The second step is distinguishable from the first and involves the cytoplasmatic continuity between spermatozoon and oocyte. Briefly, on the sperm surface there is a fusion protein that contains a folded hydrophobic fusion domain. Before fusion occurs, the adhesion procedure should take place in order that the two plasma membranes come into close apposition. Adhesion is mediated by two surface proteins, which function as a receptor-Ligand pair in each plasma membrane. One of the pair proteins, the one that is closer to the site where the fusion opening will be made, makes a bend in the lipid bilayers so the two membranes come in contact. At that time the fusion protein undergoes a structural change exposing the fusion domain and inserting it in the opposite membrane bilayer. Next, the outer leaflets of the two bilayers intermingle (hemi-fusion) and create an opening known as the fusion pore. The incorporation of one membrane into the other results in the connection between the cytoplasms of the two gametes. These process if fails may fail fertilisation
Cytoplasm: The cytoplasm swells and forms colliculus (~7 microns in length and 2 microns wide) resembling a so-called fertilization cone. If fertilisation cone is not seen in IVF then fertilisation is failed
DNA: becomes condensed and homologoes chromosomes starts pairing in metaphase II of meiosis
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