In this article, let's discuss today some important questions related to the first week of embryonic development and these are the basic questions also. So, the first question you can see here when does a secondary oocyte complete its second meiotic division to become a mature ovum? So, these are the options. Very important is the calcium wave in oocyte. So here you can see here that this calcium wave in oocyte when it begins that is when this pump so here penetrates this corona radiata cell layer and the zona pellucida and when it makes contact with the foetal end membrane. So therefore, we've written so here it is written that the calcium wave in oocyte begins or happens after penetration of corona radiata cell layer and zona pellucida and the result of this calcium wave in oocyte is that so here basically it causes the resuming of second meiotic division of oocyte and you already know that so here due to the second meiotic division of oocyte only that is secondary oocyte converted into a mature ovum. So therefore, the correct answer is the at fertilisation. Why? Because that this penetration of so here a penetration of zona pellucida is also a step of so here penetration of zona pellucida is also a step of fertilisation. So now you can see here the correct answer is option C and now for understanding I've written in this way so here see that is when the sperm penetrates the zona pellucida then calcium wave of oocyte occurs and this calcium wave of oocyte resumes the second meiosis sorry meiosis to the secondary oocyte and since penetration of zona pellucida is a step of fertilisation so therefore the correct answer is option C that is at fertilisation. Now third question so here next question you can see how soon after fertilisation occurs within the uterine tube does the blastocyst begin implantation. If you see basically implantation period is between 6 to 12 days after fertilisation so therefore and you already know basically the first week of embryonic development is from the day 1 to day 6 so therefore almost at the last day of the first week of embryonic development that is implantation begins so therefore that implantation period is between 6th and 7th day sorry 7th sorry 6th and 12 days after fertilisation. That's a question now here if you see the question so here basically now how soon after fertilisation occurs within the so therefore that since it is not written about the day 6 but now if you see that within minutes no by 12 by 12 hours no now basically if you see that all these are the processes or the days a period so first week of embryonic development but this occurs at the almost at the last or the beginning of the second week so therefore the so here almost for the best answer is the option E day 7 so therefore I also have so see friends so here since implantation occurs almost at the last day of first week of embryonic development and the first week of first week is from day 1 to day 6 so therefore the correct answer is almost that is by day 7 option E. Now the next question is very simple where does a blastocyst normally implant so before this you should already know that for that is implantation that is the endometrium the layer of endometrium has to undergo some cyclic changes and that layer of so here endometrium which can undergo such cyclic changes so here in that layer only so here implantation normally occurs and since the so here layer is the function layer of the endometrium only and all the remaining layers they don't undergo such changes so therefore the correct answer is option B so therefore I have written that in this way so here that since the functional layer of endometrium is the upper layer of uterus and it undergoes cyclic changes during menstruation whereas the other layers like the basal layer also in fact doesn't undergo any changes so therefore the correct answer is option B. This is a very interesting question which of the following events is involved in the cleavage of the gygo during the week 1 of embryonic development. If you see here that this is the fertilised egg now here so as you know that through the cleavage process that this fertilised egg undergoes an unequal number of divisions and also you can see here that and as a result so here basically that from this a single cell first of all two cells formed and three cells and finally at the last so here basically multiple cells are formed and this so here basically cells which are formed after the cleavage process and all these cells are the blastomeres. Now here if you see that the size of the blastomeres so here is smaller than the normal fertilised egg or a normal gygo so therefore you can mention that through the process of cleavage so here blastomeres are formed and these blastomeres they have smaller size so therefore so a cleavage process creates blastomeres having size smaller than the fertilised egg or the gygo. So therefore, we have written that since through the cleavage process the gygo fertilised egg undergoes unequal divisions to produce numerous cells called blastomeres and since the size of blastomeres smaller than the gygo so therefore I can say that cleavage involved in the event decrease so here because the size is less in size of blastomeres and blastomeres what basically all the cells which are produced by the unequal division of the gygo so therefore the correct answer is option E. Now here also if you want to see so here this is the correct answer so here basically that a decrease in size of blastomeres if you want to see basically option D not an increase in size of blastomeres no a series of meiotic divisions forming blastomeres it is also not because it is not a meiotic divisions so that's all yes products on a highly differentiated blastomeres no so here if you see that so here from the cell stage 1 to cell stage 8 all the cells they have similar structures but for the formation of morula that is cell cycle differentiated but not for a cleavage process. Now the next question if you see that so here basically which of the following structures must degenerate for blastosis implantation to occur so therefore that first of all that you should know that the complete implantation occurs when this blastosis the whole portion of blastosis is within the endometrial stroma. Now it is a it is one of the processes of implantation there are many processes of so here of implantation occurring before this process. Now one of the process is the hatching of blastosis basically disappearing of blast sorry hatching of blastosis where basically zona pellucida disappears or a disappearing of zona pellucida degeneration of zona pellucida. Second is a decidual reaction and the next is the penetration of blastosis through uterine epithelium and then finally due to this penetration of blastosis through uterine epithelium the whole portion of blastosis so here comes or enters with inside the endometrial stroma so therefore that is the disappearing of zona pellucida is very essential for a complete implantation to occur. So therefore that I have written that answer in this manner that since the implantation completes only when the whole portion of blastosis enters in the endometrial stroma and before the complete enter blastosis in the endometrial stroma the zona pellucida removes. So therefore the appropriate answer is option B. So therefore the option B is so here will be zona pellucida. Now this question is very interesting which of the following is the origin of mitochondrial DNA of all human adult cells? Now I will tell you why the answer is this and how. So friends you should know that before this sperm head fertilises with a nucleus of this ovum or a nucleus of this ovum or the basically the ovum yes so here till now it is at the secondary site so here not a so here future nucleus of ovum future because so here as you know that as there is no penetration so therefore it is still the secondary site okay but nevertheless so here that is before the head of sperm so here that is futures with the future nucleus of ovum that it has to penetrate these three fertilisation barriers. What are the three fertilisation barriers? The first one is the corona radiata cell, second is the zona pellucida and third is the perivital and membrane and all this penetration occurs through the acrosomal reaction. Now as you can see here that is after the penetration only the sperm head enters inside this secondary site that is nucleus and all the components tail mitochondria are destroyed. So therefore and you already know that is a sperm comes from the father and here and in the mitochondria only of sperm that is the maternal sorry paternal mitochondria present paternal DNA present and it was destroyed so therefore mitochondrial component or a DNA component is mostly from the mother. So therefore mitochondrial DNA is from maternal in origin. So therefore that I have written there during the penetration of fertilisation barriers through the acrosomal reaction only the sperm head survives arrest all the components of the sperm like mitochondria till destroyed and sperm from father and egg from mother so therefore mitochondrial DNA of a zygote obtained from mother's DNA mitochondria so therefore the correct answer is so therefore the correct answer is so here basically the origin of the mitochondrial DNA of all human adult is only maternal type not paternal because the mitochondria of the sperm destroyed during the fertilisation. So here also I have seen that answer is maternal only. So now so friends that you see here that this is a zygote so here cleavage process is going on now here that this is the four cell stage. So as I mentioned that this D is a four cell stage and here you can see here that at the four cells stage all the cells they are similar to each other now if we culture each cell and then if we make that is a baby or that is a foetus from this cell each of the cell then all the all the babies will have same genetic components why because the cells from which they are originated they are of same same or they are of similar to each other so therefore that I have since since at four cell stage all the blastomere cells produced are similar to one another so therefore if each blastomere cell made to produce baby then all the four babies will have same genetic components so therefore the answer is option C you can see here that so here birth of four genetically identical babies. Now the last question is about in oogenesis which of the following events occurs immediately following the completion of second meiosis so first of all that you should remove the suppose let the answer is suppose this suppose any suppose C then you will have to eliminate the remaining options so let's eliminate the option B first so as you know that this calcium wave of oocyte occurs after the penetration of corona radiata and zona zona pellucida now and then this calcium wave of oocyte so here causes the completion of second meiosis so that means that this step occurs before the completion of second meiosis so therefore option B is eliminated now so therefore as you know that this completion of cleavage, appearance of blastocyst, degeneration of zona pellucida all these steps occur after fertilisation now if you see that this formation of female pronucleus occurs before fertilisation but now we have to ask basically occurs immediately following the completion of second meiosis so therefore that as you already know that after the completion of second meiosis so here female pronucleus is formed and then when this female pronucleus so here fertilises with the sperm basically male pronucleus then only fertilisation occurs and after the fertilisation that these steps occurs so therefore option E, D and E are also eliminated so therefore the correct answer is option C. So therefore, we have written that since the meiosis to completed after the penetration of corona radiata now and completion of cleavage, blastocyst formation and degeneration of zona pellucida occur after fertilisation and the female pronucleus formation occurs before fertilisation so therefore the correct answer is option C because this occurs immediately after the completion of second meiosis. Understanding the fundamentals of fertilization, cleavage, blastocyst formation, implantation and early embryonic development is essential for students and professionals aspiring to build a career in reproductive medicine and embryology. Medline Academics offers comprehensive infertility fellowship in India designed to bridge the gap between theoretical knowledge and clinical practice. Through expert-led teaching, simulation training and hands-on experience, learners gain strong foundation. 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