Note: As we progress though this book, I shall close the comments on the previous week’s post once the current post goes up. By all means continue any ongoing conversations. This will streamline our conversations by restricting them to one thread. So, Last week’s comments are closed. We’re open for business on this week’s post. My thanks to co-author Dr. Tollefsen for checking in and aiding us in our discussions. He’s writing under ‘CT’.
There is no way for me to condense all of the biology that the authors present in Chapter 2, Fertilization. They discuss a great deal about which much has been written here. Click here for several lessons in mitosis, meiosis and fertilization. Apart from the recapitulation of the biological fundamentals, I wish to bring out of the chapter a few key points made by the authors.
First, the biology presented in the chapter is flawless! For any who would suggest that perhaps a couple of philosophers miss nuanced features of biology which might lead them into erroneous conclusions, let me state as a molecular biologist that this chapter could easily be mistaken to have been written by an embryologist.
Fertilization
Recalling that the sperm contains half the number of chromosomes and the egg the other half, these two haploid cells join to form a diploid cell, the zygote. A question arises asking when the zygote comes into being. Is it upon penetration of the egg by the sperm, or when the two gametic nuclei join to form a diploid nucleus? Some say the latter, but I tend to agree with the authors in claiming the former, since, as they rightly point out, that after penetration of the egg by the sperm, both gametes cease to be (both structurally and functionally) as they were before. They form a new entity, both structurally and functionally.
Both sperm and egg exist as such as parental tissue types. When joined, a new biological organism comes into existence, with its own unique genetic identity and intrinsically unfolding developmental trajectory.
Despite the slight differences of opinion, the authors note that, “…there is widespread agreement among embryologists both that a new human individual comes into existence when there is a single, unified, and self-integrated biological system, and that this happens no later than syngamy.” (Syngamy=The lining up of the 23 pairs of chromosomes.)
The authors then run us through the various stages of embryological development:
Zygote, cleavage, morula, blastula, gastrula, up to the formation of what is known as the primitive streak. Prior to this, the embryo is capable of twinning, an issue that will be dealt with substantially in chapter 6.
Twinning is an important issue in development, as some would posit that prior to this stage, an individual does not exist. However, the authors quote a number of embryology texts which mark fertilization, not gastrulation, as the beginning of a new human individual.
The events of development described by the authors can be viewed in both 4D sonograms and fiber-optic videography at The Endowment for Human Development.
The authors go on to make several points about the human embryo:
1. It is distinct from any maternal or paternal cell. It is growing and has its own distinct direction.
2. The embryo is human, with a genetic make-up characteristic of human beings.
3, The embryo is a complete or whole organism, though immature.
The bottom line: A human embryo is not something different in kind from a human being. A human embryo is a whole living member of the species Homo sapiens in the earliest stage of his/her natural development.
Are embryos produced by in vitro fertilization and cloning still human organisms?
The authors say ‘yes’.
For IVF embryos, they are the product of sperm and egg union in a Petri dish rather than a fallopian tube.
For cloned embryos, they are the result of an egg that has had its haploid nucleus replaced by a diploid nucleus from a diploid body cell. The resultant ‘clonote’ (as opposed to naturally occurring zygote) functions as any embryo. Because they are the same as any other embryo, they ought to enjoy the same moral worth as any embryo.
Those are the chapter highlights. Chris, if I missed anything, my apologies. It’s been a hectic day.
There’s the red meat of all pro-life argumenation.
Discuss.
Simon and Dan (and possibly others),
I know I started a big conversation on the first of these boards, but I may not be able to discuss your replies because we just had a little accident and have to take our daughter to the emergency room. Everything is okay, but I just hate it when people start a huge conversation and don’t respond to the responses, but I may be out for a bit.
Bobby,
I hope all is well. You’re living it today bro.
Prayers for you all.
Bobby, same here, hope all goes well.
No arguments here.
Again the twinning argument is only appealing to a technicality which can be easily corrected
All we have to say that the overwhelming majority of Homo Sapiens begin life at conception, but a small percentage begin life after the conception of the parent individual with a type of asexual cloning reproduction that leads to twinning.
Interesting from what I’ve read twinning is a process flaw not a genetic flaw, so I would again argue that since the very same process results in individuals 97% or so of the time, and is the biological goal of creating an individual, we in fact had a individual before the twinning but a freak process flaw creasted a pseudo death cloning event.
True we have no knowledge as to whether a twinning event will occur, but that ignorance substracts nothing from the individuality of the individual before the twinning even. No more say then whether any ignorance regarding when a particular bacteria will cease being an individual and divide and clone two offspring.
PS Gerard could you set up a bodily autonomy thread when Bobby is back on board? I’d rather take it up there than hijake this thread.
awww, hope she’s ok.
sending prayers your way.
Excellent discussion so far.
CT (and Gerry) – I have a question re: the penetration of egg vs fully conjoined nuclei. It seems your position is once penetration has occurred, the actual growing process is initiated so the completion of conjoining of the pronuclei is not as definitive as the actual arrival of the male pronuclei within the oocyte. Is that correct?
I want to be clear when discussing this idea of a “fertilized egg”. It’s a horribly overused term, but for clarification: does the oocyte cease to exist or is it transformed over a brief period of time and how long would that be?
In that context of transformation – is it accurate to even use the term “fertilized egg”?
Thanks.
FYI – fiber-optic videography is also known as embryoscopy. I was absolutely stunned when I first saw a brief video of an 8 week old made at Yale (presented by Steve Wagner, who at that time was at Stand to Reason).
Hi, this is my first post to your blog. I like what I have read and am very interested in this particular conversation. I wrote my Master’s thesis on Frozen Embryo Adoption and did look at this book while doing my research.
You say in your synopsis:
“Because they are the same as any other embryo, they ought to enjoy the same moral worth as any embryo.”
You are paraphrasing the authors, correct? It is the use of the word ‘ought’ that made me wonder. It makes the statement sound unclear. Those embryos are alive, albeit by extraordinarily unusual and immoral means, yet they are human life.
Can you clarify that for me, please?
Hi Faith,
Thanks for posting. Here is the quote from page 52, paragraph 1:
“Because cloned embryos are the same as other embryos, they ought to be treated as having the same moral status, whatever that might be, as other human embryos.”
We know from the remainder of the book that the authors are advancing the argument for full moral standing for human embryos.
About your thesis, what was the position that you staked out? And do you know Elizabeth Rex, President of the Children First Foundation, and her leadership in embryo adoption?
Chris Arsenault
Interesting question.
In light of my other post regarding self-assembly what desciptiive term would best illustrate the process by which the zygote is created through the fusion on the pronuclie?
Does saying they simply fuse convey the complexity esp if we consider what we would call it if we asked a two machines to combine to make a single entity.
That then contructs/self-assembles through modular production and configuration until it gets additional outside resources.
BTW I’ve been thinking self-assembly could be stretched to mean self building, so this could mean either with fixed resources or additional resouces.
Chris Arsenault:
“I have a question re: the penetration of egg vs fully conjoined nuclei. It seems your position is once penetration has occurred, the actual growing process is initiated so the completion of conjoining of the pronuclei is not as definitive as the actual arrival of the male pronuclei within the oocyte. Is that correct?”
Yes and No. I would argue the following:
Once the sperm penetrates the Zona Pellucida it enters the perivitelline space. At that point, the Izumo protein on sperm and CD9 on egg allow binding and fusion to occur. This triggers cortical granules to release enzymes that change the property of the Zona Pellucida, making it impervious to penetration by other sperm. This cortical reaction is very rapid, almost instantaneous.
I tend to view zygotic formation as being initiated by the cortical reaction. The egg is a transformed egg from that moment on. It is no longer virgin and has begun to see its potential actuated. The biochemical, and histological processes having begun, the presence of the growing sperm pronucleus means that human development is underway.
From my perspective development exists on a continuum, beginning with penetration and the cortical reaction. All the rest is drawing hard and fast lines in an ever-changing histological landscape. Some would argue the fusion of the pronuclei heralds a true cell.
I would argue that a cell in metaphase of mitosis has seen its nuclear envelope disintegrate, and will not reappear until cleavage. Is a metaphase, anaphase and telophase cell not to be considered a cell because a defined nucleus temporarily ceases to exist? Of course not.
Neither, then, should the newly penetrated egg not be considered a zygote proper simply because this cell emjoys a unique histologic character.
Thanks Gerry.
So if I understand you correctly, the cortical reaction has fundamentally altered the receptive property of the egg sufficiently that it should no longer be consider it an egg?
So any other point in the process is sort of splitting hairs…(Cells? heh)
Maybe to put it in another context – the dance of each life begins when the groom enters the room and the band strikes up the music.
Bobby,
I just saw your post up above. I hope your daughter is OK.
Best wishes,
Dan
Hi Gerard,
“Once the sperm penetrates the Zona Pellucida it enters the perivitelline space. At that point, the Izumo protein on sperm and CD9 on egg allow binding and fusion to occur. This triggers cortical granules to release enzymes that change the property of the Zona Pellucida, making it impervious to penetration by other sperm. This cortical reaction is very rapid, almost instantaneous.”
How long does this process take, from the time that the sperm first contacts the egg until the cortical reaction is complete?
How long does the cortical reaction itself take? If this is truly the event that marks the beginning of a new organism, then only during this time period would there be any ambiguity regarding the existence or non-existence of a new organism.
Dan, my understanding is that the cortical reaction is almost instantaneous and needs to be in order to prevent polyspermy.
Gerard: what is the time scale involved for the cortical reaction (order of magnitude)? Seconds? Milliseconds? Hard to imagine that it could be shorter than that.
I’d also appreciate an answer to my first question, ie. how much time typically elapses from the time that the sperm first contacts the egg until the cortical reaction is complete? There must be observations of this, eg. from in vitro fertilization.
Thanks.
Dan,
Sorry about not getting how much detail you were looking for.I found a good paper with excellent references for you that may answer many of your questions in great detail. The process of fertilization and development is well-established in other animal models, such as the sea urchin, for obvious ethical reasons.
The process in humans is pretty much the same. The eating away at the outer layer is an enzyme-mediated response that happens in a matter of minutes. Once penetration occurs, there is a two-stage cortical response as detailed in the introduction to the paper (part of which is copied and pasted below).
If you have any other questions, let me know. Here’s the link to the paper in PDF. It’s an EXCELLENT molecular paper:
http://www.brown.edu/Research/Wessel_Lab/Research/papers/Wong%20and%20Wessel,%202004.pdf
INTRODUCTION
If multiple sperm fuse with an egg at fertilization, the resultant zygote usually dies. This lethality is usually caused by the multiple paternal centrioles inherited by the embryo, which compete for the extra chromosomes and disrupt the establishment of the cleavage furrow. Thus, eggs have evolved barriers that are erected immediately after first insemination to block such polyspermy. Two mechanisms exist within the animal kingdom to achieve this block to polyspermy: a fast and a slow block. The fast block, first identified in echinoderms, is a rapid change in membrane potential that impairs additional sperm binding (Jaffe 1976; Schatten and Hulser 1983; Jaffe and Schlichter 1985; Glahn and Nuccitelli 2003). This event occurs within milliseconds of sperm–egg fusion and endures for about 60 sec. The slow block is a physical modification of the egg’s extracellular matrix that results in a new physical barrier between the fertilized egg and the unsuccessful sperm (Shapiro et al. 1989). This barrier persists through early development and protects the embryo against additional sperm penetration, invasive pathogens, and physical damage.
Two spatially distinct protein populations, both made during oogenesis, are needed to assemble the physical block to polyspermy. One population is an extracellular glycoprotein coat known as the zona pellucida in mammals and marsupials, the vitelline membrane in amphibians and avians, the chorion in teleosts, and the vitelline layer in echinoderms. This layer protects the oocyte, functions in sperm recognition and sperm activation, and scaffolds assembly of the slow block to polyspermy. The second population of proteins necessary for the physical block is derived from secretory vesicles found near the egg cortex, usually referred to as egg cortical granules.
The physical block to polyspermy in sea urchins involves the creation and elevation of a fertilization envelope, a shell
that prevents extra sperm from reaching the new zygote. As with other egg-derived barriers, the sea urchin fertilization
envelope is assembled from secreted cortical granule components laid onto a preexisting matrix, the carbohydrate-rich vitelline layer or glycocalyx. Little is known about the constituents of the vitelline layer, although one- and two- dimensional gel electrophoresis demonstrate that it consists of at least 25 different proteins (Gache et al. 1983; Niman et al. 1984).
The major constituents of the sea urchin fertilization envelope originate from the cortical granules. In ascending
order by mass from Strongylocentrotus purpuratus, they are ovoperoxidase (Somers et al. 1989; LaFleur et al. 1998), the cortical granule component of rendezvin (rendezvincg; unpublished data), SFE9 (Wessel 1995), SFE1 (Wessel et al.,
Look at the rest of the intro. Good Stuff.
Dear Gerry et. at.,
Again, apologies for only coming late to this conversation — though Gerry is far more competent than I in this area, and provides very helpful info and a great reference/link — thanks!
On the same subject of the transformation from oocyte to embryo, here’s a paragraph from a recent paper of mine:
To biologists witnessing this in context, it is clear that we have to this point [that is, up to penetration of the zona] the reproductive systems of both male and female at work in bringing about the conditions necessary for reproduction, and, further, that the gametic cells play their own functional role in this process. But upon sperm penetration of the zona, there is a radical change in the operation of what is often, somewhat misleadingly, called the fertilized egg. The zona pellucida immediately undergoes the first of two transformations that will render it impenetrable by further sperm; polyspermy would result at a minimum in grave harm to the developing organism and possibly to its demise. In other words, the zona ceases to act as the permeable boundary of a gametic cell engaged in the process of reproduction and begins to act as the impermeable boundary of an individual whose biological welfare is now distinct from that of the mother (or father).
Gerry’s analysis is a bit more biologically fine grained than this, but I think the overall point is the same. The nature and function of gametes is to find complementary gametes to unite with; the nature and function of a complete organism is, at its earliest stages, to protect itself against external threats, and to initiates the developmental processes that will further its growth as a member of its biological kind.
A quick point on assembling vs. developing here — from this point on for the next several days the embryo does not get any bigger — its development is quite self contained until it hatches from the zona. Again, it seems less like an assembly out of disparate parts than a development from within.
best,
CT
Gerard and CT: thanks for your detailed answers. I understand this much more clearly now.
Also, Gerard, thanks for the link to that excellent paper. I’m still working my way through it!
CT, it may seem that way but I can tell you that the people working with artificial systems are already thinking in these terms.
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