When an embryo develops, what controls when certain organs start to develop?

Question

At fertilisation, the egg and sperm form just one cell. It divides, so what initiates it to divide? Or any cell to divide? The cell also starts to form 2 layers, one for the placenta, one to become the embryo itself. What causes the cells to do that? thanks

Answer 1

ISTRC the first division happens right after fusion of the nuclei.

The plane of division of cells can be pre-set by a gradient of substances in the cell and/or the orientation of the cytoskeleton scaffold. During gastrulation, cells not only divide but actually migrate pulling the layers toward the inside. They orienteer following, again, a concentration gradient. The movement is done by depolimerizing and repolymerizing the cytoskeleton, just like an amoeba or a macrophage would.

Answer 2

The hormones released by the ovary of the fetus.

Answer 3

This is a difficult question to answer without going into some very technical concepts. Firstly, after the embryo develops, it forms a ball of ever growing cells called a blastocyst. This ball of cells will naturally (by chance) have some cells that end up on the periphery (outside) of the group, and some that end up on the inside. The location of the cells affects what they develop into. The outer most cells, because of the hormone signals given off by the uterine environment, become placenta and associated cells. The inner-most cells of the blastula become vary body parts of the embryo. As the embryo develops (after implanting in the uterine wall), there are always some cells are relatively more central and some that are more to the periphery. It turns out that what the location of the cells predetermines what they become. Those stem cells that are along the central line of the embro will develop into the spinal cord and backbone, and those more on the outside develop into skin etc...So,....to make a long story short, the location of the cells determines what they become, and the structures that the cells have physical contact with also affect how the same effect.

Answer 4

The majority of development is controled via reactions to different hormones and other cells.  For example, cells on the outside of a zygote will develop into the placenta due to hormones in the uterus/fallopian tubes.  Cells on the inside will develop into embryotic cells.  

As the spinal column develops first, I'd hazard a guess that the structures that develop after this react based on their position in relation to the developing spine.

Zygote cells naturally divide at a regular pace but do not cause the zygote to grow in size - the egg cell is generally the largest cell in the body and is divided to make the smaller cells.