Questions & Answers


Developmental Biology

>I am having a really hard time trying to understand the

>relationship between the mitochondrial cloud and the germ plasm. I

>would really appreciate if you could help clarify this for me.

The mitochondrial cloud contains within it components that resemble the germ plasm, distributed between the mitochondria. Circumstantial evidence implies that the germ plasm resides within the mitochondrial cloud before becoming localized to the vegetal cortex late in oogenesis.

>I'm relatively new at developmental biology and am trying to stage mouse
>embryos. We count days after plugging, but sometimes this is clearly
>not good enough based on size of the embryos we get. Is there a nice
>life-size picture of well-staged mouse embryos that I can use to compare
>our embyos to in order to properly stage them? Thanks for any help you
>can lend.

For early (preimplantation) stage embryos, the Cold Spring Harbor Manual on "Manipulating the Mouse Embryo" by Brigid Hogan et al. is very good. This manual also is useful for some aspects of peri- and postimplantation stage embryos. However, an excellent guide for implantation stage embryos from day 6.0 to 8.0 is: Downs, K. M. and Davies, T. (1993) Staging of gastrulating mouse embryos by morphological landmarks in the dissecting microscope. Development 118: 1255-1266. Finally, for other (and later stages), I suggest that you follow the criteria in The Biology of the Laboratory Mouse (1966) by the Staff of The Jackson Laboratory, 2nd Edition, McGraw Hill, New York. Chapter 12 by George Snell and Leroy Stevens deals with embryos from day 4 to day 9 based on sections at 6-hour intervals. Other chapters deal with later stage embryos greater than 10 somites. Gilbert A. Schultz, Ph.D, Professor and Head, Department of Medical Biochemistry, The University of Calgary.

>In your learning objectives following the oogenesis module, you ask what
>the difference is in timing between spermiogenesis and oogenesis.

The important thing here is to recognize that differentiation of the gametes in the two sexes occurs at different times relative to meiosis: during meiosis in the female and after meiosis in the male. LWB

>I am interested in your opinion of activin and its role during
>early development (Xenopus in particular). What are the "latest"
>thoughts about how this molecule fits into our current ideas of mesoderm

I asked this question of Jim Smith after a presentation he gave recently. He said that at this time, we should think of Vg-1 and activin as representing the same activity. We simply don't know the identity of the native inducer. LWB

>So, then you would consider activin a dorsal mesoderm inducer, but not THE
>inducer ( or the native inducer)??? I am asking because recently all the
>literature revolves around FGF and the BMPS, and everyone seems to have
>forgotten activin, yet the more specific experiments with truncated
>activin receptors (while not interfering with the BMP pathway like the
>original truncated receptor) still have effects on mesoderm formation. Is
>Dr. Smith suggesting all those effects are due to Vg1 interaction with activin?

According to Smith, activin (or its native counterpart) at intermediate levels activates X-bra production. At high or low levels, there is no induction. Thus, if there is an activin source, X-bra is produced at a moderate distance away from the source. X-bra then activates downstream targets, including Xwnt-11. He thinks that FGF is involved in the pathway from X-bra to Xwnt-11. Xwnt-11 is necessary for gastrulation. For further information on the relationship between FGF and X-bra, see Isaacs, HV. 1997. New perspectives on the role of the fibroblast growth factor family in amphibian development. Cell Mol Life Sci Apr;53(4):350-361. LWB

>I'm a physicist and recently, reflecting about symmetry in nature, asked
>myself where the (in most cases bilateral) symmetry in organisms derives
>from, i.e. more specific (and not in the evolutionary perspective), how
>it is encoded in the DNA. I suppose that symmetric organs or limbs are
>NOT due to a twofold representation in the genome, i.e. simply spoken,
>one for the left and one for the right leg. Still, I'd like to know in
>which way then symmetry arises, but having a quick look at some text
>books on genetics I didn't find anything at all on the subject.

See the paper by Isaac et al. and the Perspective by Robertson in the 28 February, 1997 issue of Science. LWB

>I understand that a complete linage map has been made for all the cells

>in C. elegans. Can you tell me where I can get it?

The complete lineage map for C. elegans can be found in Sulston, J.E., E. Schierenberg, J.G. White and J.N. Thomson. 1983. The embryonic cell lineage of the nematode Caenorhabditis elegans. Develop. Biol. 100: 64-119. LWB

>And, can you tell me where to find a recent compilation of gene

>regulation data for C. elegans?


>In today's lecture, I think I heard you mention that a secondary

>oocyte does not undergo meiosis II until it is fertilized. After

>fertilization, it will then undergo the second division to give rise to

>the third polar body and an ovum. Did I hear wrong? If that was true,

>wouldn't the zygote have 3n number of chromosomes at the moment of


A secondary oocyte is in meiosis II. Meiosis in many species (including man) arrests at metaphase of meiosis II in the unfertilized egg (ovum). Sperm entry triggers the completion of meiosis II. At that point, the egg nucleus becomes haploid and can fuse with the haploid sperm nucleus to restore the diploid condition. LWB

>In maintaining the protamine transcript in its repressed state, do the RNA

>binding proteins bind to the whole sequence in the 3'UTR or does it bind

>to a specific sequence? If it binds to a specific sequence is this

>sequence universally similar or different depending on the type of protein

>that binds and the species?

It binds to a specific sequence that is highly conserved among the two protamine transcripts and the transition protein transcript. The same protein binds to all these sequences. LWB

>Since the RNA binding proteins bind at the 3'UTR which is at the end of

>the transcript, could it be possible that translation of the coding

>sequences takes place but it does not go all the way to completion and

>partial proteins exist till the transcript is later derepressed?

The limiting step in protein synthesis is initiation. Binding of the protein to the 3' UTR prevents initiation at the 5' end. This indicates that the two ends of the RNA interact in some way. LWB

>Why do Sertoli cells release SGF and react to it by producing SGP2, when

>they could save the effort and instead release only SGP2 directly in

>adults? Is there any other known regulatory pathway that makes the

>release of SGF necessary?

Good question. I assume that there is a regulatory pathway involved. LWB

>What is the fate/role of polar bodies in production of a functioning

>ovum? Can fertilization occur if the first polar body was destroyed at

>the end of Meiosis I? What causes the uneven meiotic division of the

>primary oocyte into the secondary oocyte and a polar body and does this

>same mechanism account for the same uneven division during MII where the

>secondary oocyte becomes the ovum and another polar body? Meiotic

>division of the first polar body appears to happen evenly - what

>mechanism accounts for this and how can the two mechanisms coexist in

>such close proximity? Thanks for your help.

The polar bodies are a means to obtain haploidy without loss of cytoplasm. They have no additional demonstrated role. The uneven division is a consequence of displacement of the germinal vesicle to the animal pole (in both meiotic divisions). No such situation occurs in the first polar body, so it divides evenly. LWB

>When I was reviewing oocyte maturation I couldn't figure this one out. I

>know that when MPF is active it causes the phosphorylation of proteins

>like lamnins and histone H1 which allows the GVBD and meiosis untill

>metaphase II. I also realize that cmos protein stabilizes cyclin

>which keeps MPF active and arrests the cell at metaphase. Now, if MPF

>remains active through all of this, how does the oocyte get past metaphase

>I in order to arrest at metaphase II before fertilization?

Cyclin levels drop to allow continuation of meiosis. LWB

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