"Considering what can go wrong, it's amazing that anybody gets born at all." -- Martha J. Burt

I. Developmental biology

1. We'll start with the development of a frog or salamander embryo -- useful for showing the basic patterns that appear, in modified form, in all vertebrate embryos, including humans. (Have a look at the Amphibian Embryology Tutorial for more on frog development.)
   1. A fertilized egg (a zygote) divides into two cells, then four, then eight, then sixteen. . . (these stages are called the two-cell stage, the four-cell stage. . . you get the idea. . .)
   2. . . . until the embryo is a solid ball of several hundred cells, more than can be counted conveniently. It's now called a morula.
   3. Cell movements cause the morula to hollow itself out, forming a hollow ball of cells called a blastula.
   4. At one end of the blastula, cells begin moving into the inside of the blastula. This looks something like denting a balloon with your fingertip. Now the embryo's a gastrula, and the process of forming the "dent" is gastrulation.
      1. The cells moving into the embryo form an expanding layer of cells, the endoderm, that will go on to form the gut (and, later, the lungs, liver, pancreas, etc., which all start out as branches from the gut).
      2. The cells left on the outside of the embryo form anotehr basic layer of cells, the ectoderm -- which goes on to form, not just the skin, but the nervous system as well.
      3. Additional cells move into the blastula, and arrange themselves around the gut, forming a third layer in between the ectoderm and the endoderm. This is the mesoderm that will give rise to muscle and bone eventually.
   5. Chemical signals sent by the mesoderm cause two folds of tissue on the outside of the gastrula to form, fold together, and pinch off. This forms the neural tube, which goes on to form the brain and spinal cord.
      1. An embryo at this stage is called a neurula, and the process is neurulation.
      2. If the neural tube doesn't fold ap and close up properly, birth defects can result such as spina bifida and anencephaly (absence of a brain).)
2. Development of "higher" vertebrates
   1. In a yolky egg -- say, of a chicken -- the egg cell can't possibly divide completely.
   2. The cell divides unequally, in such a way that a disc of cells (instead of a ball) forms on one side of the yolk.
   3. The disc of cells is analogous to a blastula, and a form of gastrulation happens (along a fold, rather than a pore), and then neurulation happens. . .
   4. The embryo eventually begins to "roll up," forming a 3-D body from what's basically been a 2-D disc of cells. . .
   5. BUT: Unlike the frog, most of the disc cells don't go on to form part of the embryo itself!
      1. Most of the cells end up forming a set of fluid-filled sacs around the actual embryo, the amniotic membranes.
      2. A vertebrate animal that develops in this way is called an amniote (i.e., a reptile, bird, or mammal).
3. Human development -- yet another variation on the theme. (See the Visible Human Embryo project for more details and pictures.)
   1. Human eggs don't have much yolk -- but, like vertebrates with big yolky eggs, humans develop from a disc of cells.
   2. Again, a human zygote divides into 2, 4, 8, 16, 32. . . cells and forms a solid morula.
   3. The morula hollows out into a blastula (well, technically, it's called a blastocyst, just to make things tricky) -- but again, most of the blastula cells don't become part of the embryo itself.
      1. Most of the cells form the amniotic membrane (a.k.a "bag of waters", a.k.a. "caul").
      2. Part of these membrane sacs will also form the structure by which the mother's body provides nourishment to the growing embryo, and removes its wastes: the placenta.
   4. By about three months after conception, a human embryo has formed virtually all its organs. In general, the next six months are a period of growth and maturation of organs and structures that are already present.

II. Holy cow, how does all that stuff happen? How do cells "know" how to develop?

1. This is a huge question, and one whose answer still isn't complete.
2. But the simplest answer is: Cells don't act in isolation!
   1. We can look at the process of development as resulting from cells communicating with each other, influencing each other, switching each other's genes on and off:
      1. Differentiation. As embryonic cells divide, they specialize into different types of cells, and genes that won't be needed end up being shut down. Normally this is irreversible (cloning animals depends on reversing the process, as we discussed). (Undifferentiated embryonic cells are stem cells, and they've been in the news quite a bit lately. . .)
      2. Cell movement. At various times, certain cells are able to move through a developing embryo to new regions.
      3. Apoptosis, also known as programmed cell death. At certain times during development, cells "commit suicide" in a patterned, orderly fashion. EXAMPLE: Your hand was once a flat flipper-like structure. The reason you have separate fingers is that, when you were an embryo, the cells in between your fingers died off. (If they hadn't, you'd have webbed fingers.)
      4. Induction. Cells receive signals from other cells that, in effect, tell them where they are in the embryo, and when to move, divide, etc. EXAMPLE: Formation of the lens of the eye.