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24

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 ...


18

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 ...


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 ...


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 ...


8

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, ...


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

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 ...


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

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 ...


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

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

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

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

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

Embryonic cells "know" where they are relative to each other by chemical signals, same as in adults. These molecules are known as morphogens (specific examples include the sonic hedgehog and β-catenin). The amount of morphogen in a region of cells determines which gene gets turned on and thus what it develops into. And the amount varies by how far they are ...


4

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 ...


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


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

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 ...


3

Yes, you are correct. You can read the wikipedia article about an embryo: An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the >time of first cell division until birth, hatching, or germination. In humans, it is called an >embryo until about eight weeks after fertilization (i.e. ten weeks after the last menstrual ...


3

I am no expert on the topic, but according to an article published on the centenary of his study, it states that the microscopical structure and classification of these areas are in parallel to the evolutionary distinction between old and new cortical subdivisions. So as and when new subdivisions are deduced (based on its function, cytoarchitecture or the ...


3

Regarding option C: Although it is correct that testosterone is converted into DHT, it is the former, not the latter, which is responsible for differentiation of the mesonephric (a.k.a. Wolffian) ducts: Between 8 and 12 weeks, the initial secretion of testosterone stimulates mesonephric ducts to transform into a system of organs—the epididymis, vas ...


3

The dura mater is formed by the mesoderm. The mesoderm is the middle layer of the early trilaminar embryo. The three germ layers are the ectoderm, mesoderm and endoderm, formed by gastrulation. The middle germ layer forms connective tissues and muscle throughout the body, with the exception of the head region, where some of these structures have a neural ...



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