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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 ...


25

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 ...


12

You may want to look into "Evolutionary - Developmental Biology", which deals with how an embryo develops into a grown organism. Thus, by combinatorial specifying the identity of particular body regions, Hox genes determine where limbs and other body segments will grow in a developing embryo or larva. A paragon of a toolbox gene is Pax6/eyeless, ...


10

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 ...


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 ...


9

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, ...


9

All animals 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 develops on ...


8

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 ...


7

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 ...


7

The first differentiation in human embryogenesis is from early blastomeres into trophoblast, which forms the outer layer of the blastocyst, and inner cell mass (ICM). It may be unsurprising then that cells on the inside of the 8-16 cell stage morula differentiate into ICM whereas those on the outside differentiate into trophoblast. However it is currently ...


7

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 ...


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 ...


6

I wanted to add some helpful references. The 6th edition of the Gilbert Developmental Biology textbook is available on NCBI bookshelf. It's a bit old (2000), but much of the information is still relevant. You can search this textbook for specific terms but not browse. There is also a collaborative science/fashion project between the Storey sisters, called ...


6

Identical twins Twinning occurring at the two cell stage or afterwards, up to and including the 16 cell stage, which translates to days 1 to 3 after fertilization, results in diamniotic, dichorionic twins. Twinning at the 32 cell stage (day 4), up to and including day 6 results in diamniotic, monochorionic twins. The majority of identical twins split at ...


5

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 ...


5

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 ...


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

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 ...


5

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 ...


5

We can be sure it's a tail because it is an extension of the vertebrae, and we define "tail" as an extension of the vertebrae, among other things. tail tāl/ noun: tail; plural noun: tails the hindmost part of an animal, especially when prolonged beyond the rest of the body, such as the flexible extension of the backbone in a vertebrate, ...


4

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 ...


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 https://web.archive.org/web/20151024041339/http://www.vetmed.vt.edu/education/curriculum/vm8054/Labs/Lab5/Lab5.htm http://www.pitt.edu/~sshostak/biosci1450/hislec03.html


4

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 ...


4

The animal is very suggestive for Dog whelk-like mollusc removed from its shell. I try to mark the organs, do not rely on these marks as an absolut. Testis Renal vein Gill Mantle Prostate Osphradium 10 and others - digestive and reproductive systems, cannot mark precisely. Digestive gland source for comparing etc


4

Yes it is real, in the sense that this protocol has been through a peer reviewed journal. Note that the embryo is developing outside the shell, not outside the egg. It is the fertilized egg that develops into the embryo. The shell only provides protection and allows exchange of gases. You can replace the shell with any other material that does the same job. ...


4

First, there is some confusion on your part about heart cells and pain perception. Heart cells generate an action potential intrinsically; they do not need the central nervous system to beat (your second article explains this; read the part about the importance of calcium.) So yes, long before a fetus can feel pain, the heart is beating, because there must ...


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