Tag Info

Hot answers tagged

15

There is some evidence that fetal development under zero gravity conditions might be problematic. Wakayama S, Kawahara Y, Li C, Yamagata K, Yuge L, et al. (2009) Detrimental Effects of Microgravity on Mouse Preimplantation Development In Vitro. PLoS ONE 4(8): e6753. doi:10.1371/journal.pone.0006753 The paper is here. These authors studied aspects of ...


13

Great question, and one about which there has historically been a lot of speculation, and there is currently a lot of misinformation. I will first address the two answers given by other users, which are both incorrect but have been historically suggested by scientists. Then I will try to explain the current understanding (which is not simple or complete). My ...


9

Charnov and Ernest (2006) present data on offspring number per year and neonatal mass for 532 species of mammals. The two are related by the linear regression equation: ln(offspring/year) = 2.4 - ( 0.3 * ln(neonate mass) ) Giant panda neonates weigh 100-200 g and are weaned at 46 weeks. So, according to the regression, pandas should have, on average, 2.8 ...


9

Fetal testis produces testosterone from cholesterol. There is a peak of production around 15 weeks of gestation (the "masculinization programming window"). So the genotype of the fetus can affect testosterone levels directly via effects on the biosynthesis of the hormone, or indirectly by defective regulation of the pathway's activity. However, exposure to ...


8

While it might make more logical sense to have separate passageways for air and food/water, this did not happen in evolutionary history due to the peculiarities of lung development. Vertebrate lungs develop as an outpouching of the gut tube, which itself has a very long evolutionary history (probably homologous among all deuterostomes). In the image below, ...


6

This is a very interesting question. Many people have researched this topic, and many still are. But regardless, I had never heard of Alan Turing's contributions, so thank you! First of all, I cannot actually find who first coined the term morphogen. Though people had hypothesized that chemicals could play a critical role in development through much of the ...


6

The propensity for heterozygotic twins seems to be driven primarily by genetics, with additional factors playing a role (http://152.98.160.29/contents/p/staff/CV162Lewis_UQ_Copy.pdf', info site): hormones ("Mothers of fraternal twins tend to be taller, and have earlier and shorter menstrual cycles") ethnic background (which is really genetic) - African ...


5

In humans, the amnion (amniotic sac) persists from the primitive amniotic cavity1. One side of this is formed from the cytoblast (a prismatic epithelium) and the plasmodioblast. Together these two layers are the ectoplacenta or chorion. They are also referred to as Rauber's layer. These replace the lining epithelium of the uterus, whereupon internal ...


5

No. In fact the lens of the eye, which is nearly optically perfect in humans, does not change or grow after it is fully formed around week 26 of gestation. Interestingly this is why one of the cues for identifying young children is having small faces with large eyes. This also the case for puppies and cats and other animals, who are mostly cuter when they ...


4

Wikipedia actually covers this: Most bilateral animals, including all the vertebrates, are coelomates. Now, some coelomates have subsequently lost their coelom but primates (actually, I believe, all vertebrates) are not among them. In humans, the coelom forms, amongst others, the pleural cavity. So, yes: humans do have a coelom that partitions into ...


4

Plants have a simpler anatomical structure than mammals (is anatomical the right word, or would physiological be more appropriate?). Mammals on average don't have more genes than plants, so my understanding is that this additional complexity is the result of finer and more complex regulatory mechanisms. When you remove or duplicate an individual gene in an ...


4

There is variance in egg shape; sea-birds often use a more pear or conical shaped egg to prevent it from rolling off the cliff (where they nest) if it is disturbed. Some reptiles lay very spherical eggs (some turtles) whereas some reptiles lay very elongated ones (see black pine snake eggs). I imagine that we don't see square eggs because they would be ...


4

This is a very general question. The "developmental sequences" are just genes like any other. Like all genes they are semi-randomly distributed through the genome. While there are gene-rich and gene-poor areas in the genome, with some exceptions --notably the homeobox genes--, genes are not grouped by function. As to how they are accessed sequentially, that ...


4

There are molecular motors but the frequency is a function of energy input (ATP); similar to the angular velocity dependence on amount of current in electrical motors. The concept of molecular motor may not be suitable for a clock like device. There are clocks based on genetic circuits, which produce stable oscillations. Examples include the circadian ...


4

A few possible explanations are named in the Wikipedia article you link: Regarding spontaneous or natural monozygotic twinning, a recent theory posits that monozygotic twins are formed after a blastocyst essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half, leaving the same ...


4

I think CT is an abbreviation for connective tissue. Some examples of its use in that fashion: http://en.wikipedia.org/wiki/Connective_tissue http://www.vetmed.vt.edu/education/curriculum/vm8054/Labs/Lab5/Lab5.htm http://www.pitt.edu/~sshostak/biosci1450/hislec03.html


4

First of all, I should quote the sentence from the MOST reliable ophthalmology sourcing in the world - American Academy of Ophthalmology: Section 11 - "Lens and Cataract" "The equatorial diameter of the unfixed human lens measures 2 mm at 12 weeks and 6 mm at 35 weeks. Both the growth and the maturation of lenticular fibers continue throughout life." The ...


4

All animals that develop in this way, whether they are oviparous (developing in an egg) or viviparous (developing inside their mother, or live-birth). From Wild Birds Unlimited: All mammals have navels or belly buttons where the umbilical cord distributes nutrients between a mother and her fetus. After birth, the umbilical cord is cut and a scar ...


3

It's old and I can't get access to this issue of the Annals of the New York Academy of Science, but it looks like it has some relevant information. Sifting through the abstracts it seems the vasculature of the fetal liver is completed at around 8 weeks although is still different to the adult vasculature because of the umbilical vein. The growth of the organ ...


3

Know your databases! This is just the query fit for OMIM. http://omim.org/entry/206500 Anencephaly is considered an extreme form of neural tube defect (182940), which has been associated with variation in several genes. The entity described here is believed to show autosomal recessive inheritance. The genes are those associated with neural tube ...


3

No, it is not possible. The reason is the genetic imprinting, which takes place on the parental DNA in the egg and the sperm. Here small modifications are added to the DNA which marks them as silenced. So some genes are expressed from the mother (while the fathers copy is not expressed) and vice versa. There are even genetic diseases which are caused by ...


2

Nutrition and environment don't have a huge affect, if any, that I could find. Age can, as women above 35 have a greater propensity for bearing twins. http://en.wikipedia.org/wiki/Twin#Fraternal_.28sororal.2Fdizygotic.29_twins However, because fraternal twins can be the result of a gene on the X-Chromsome, the answer is yes in this specfic case. ...


2

I would think this is very much still "used." 60 years later, we finally have the first experimental support for it: In this blog article about this journal piece the authors studied the ridges that form on the roof of mouse mouths. They manipulated the signaling molecules that induce their formation and observed changes in line with Turing's theory. Of ...


2

It is a mnemonic of two words: discoidal + blastula = discoblastula. So it suggests that cleavage is partial discoidal in birds. Embryo forms disc. Cells called blastodisc are on top of the yolk. It seems that the formed disc is blastodisc.


2

In humans, the coelom forms by the splitting of the lateral plate mesoderm, which occurs during weeks 4-5 (Sweeney, 1998). Sweeney, LJ. 1998. Basic Concepts in Embryology. McGraw-Hill.


2

In animals, polyploidy is not tolerated and very few polyploid species are known to exist. Those that do exist are usually asexual, parthenogenetic, or hermaphroditic. Most of the problems resulting from polyploidy occur during synapsis of homologues during prophase I. As plants do not have a chromosomal mechanism for sex determination, ...


2

Human embryogenesis is much more complicated than the more general and simplified picture given in your 1st figure. For details, please study http://en.wikipedia.org/wiki/Human_embryogenesis To point out the most distinctive features which might have led to your confusion: Before gastrulation, the blastocyst cavity is formed. Only the inner cell mass ...


2

From what I've read and I know from my Biology classes, during the formation of the embryo a whole bunch of things may go wrong. In these cases, during the neurula stage, in which the brain is supposed to begin its development, something goes wrong while the neural tube is forming. The causes? Some people argue that a lack of folic acid may lead to ...


2

Basically a summary and details-filled version of the above. Based on ln(number of offspring/year) = 2.4 - ( 0.3 * ln(neonate mass) ) (c.f. Charnov EL and SKM Ernest. 2006. The Offspring‐Size/Clutch‐Size Trade‐Off in Mammals. American Naturalist 167:578-582.) litter/year = 1 / (pregnancy duration in years + weaning time in years) it can be deduced ...


2

In addition to @mgkrebbs response, I'd add that many genes are controlled by epigenetic mechanisms which may mask one of the parental copies by methylation. This can be a specific effect where, the active gene is specifically from either the father or the mother. Mosaicism, where one copy of gene is randomly inactivated in any given cell, is also possible. ...



Only top voted, non community-wiki answers of a minimum length are eligible