NO ONE KNOWS., bluedogeyes: In Which the Can is Opened When a...

bluedogeyes:

In Which the Can is Opened
When a sperm cell meets an egg cell, and they really love each other, they form a zygote. This cell begins to divide in a process known as cleavage, called this because at this stage, it does not grow in size. The zygote splits into enough cells to form a blastocyst, and things get interesting. To understand embryo development, and why most animals look the same early on, one has to understand that human DNA is very similar to the DNA of every other animal on the planet (this will also be covered in another comic). So how do we get so different?
Embryonic stem cells inside the blastocyst contain all the information needed to create the entire organism. Different cells don’t have different DNA, just different gene expressions. Think of the DNA chain like a blueprint, and as the blueprint is read, different switches are turned on or off. This is called cell signaling. Due to our genetic similarity to our animal relatives, this starts us off all looking remarkably similar, with tails and little nubs for limbs. But later instructions can override these initial similarities: Placental mammals begin life with a yolk sac, which disappears early on; our tail diminishes—leaving only the coccyx; whales and dolphins (letter D above) start life with hind leg nubs, which are re-absorbed into the embryo later, and like the tailbone, some vestigial traces remain. As each stem cell changes into another type of cell, so those cells can branch off again into more types, creating more specific features as the animal grows.
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(read all at Sci-ence!)

In Which the Can is Opened

When a sperm cell meets an egg cell, and they really love each other, they form a zygote. This cell begins to divide in a process known as cleavage, called this because at this stage, it does not grow in size. The zygote splits into enough cells to form a blastocyst, and things get interesting. To understand embryo development, and why most animals look the same early on, one has to understand that human DNA is very similar to the DNA of every other animal on the planet (this will also be covered in another comic). So how do we get so different?

Embryonic stem cells inside the blastocyst contain all the information needed to create the entire organism. Different cells don’t have different DNA, just different gene expressions. Think of the DNA chain like a blueprint, and as the blueprint is read, different switches are turned on or off. This is called cell signaling. Due to our genetic similarity to our animal relatives, this starts us off all looking remarkably similar, with tails and little nubs for limbs. But later instructions can override these initial similarities: Placental mammals begin life with a yolk sac, which disappears early on; our tail diminishes—leaving only the coccyx; whales and dolphins (letter D above) start life with hind leg nubs, which are re-absorbed into the embryo later, and like the tailbone, some vestigial traces remain. As each stem cell changes into another type of cell, so those cells can branch off again into more types, creating more specific features as the animal grows.

…

(read all at Sci-ence!)

NO ONE KNOWS.

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