PART 2 - GENES' GUIDE TO EMBRYO DEVELOPMENT

Let's use common sense and some bits of genetic reference materials to construct a developing embryo. The embryo that we will describe will not be a real embryo but it will go through several stages of development just as a real embryo must. Several specific genes will be cited as being expressed during this embryo's development. These genes will be used only as examples of real genes, in life they may come from many different animals. So the animal that will be the final form of our embryo will be a mixture of several animals, a genetic chimera.

Let's use some common sense and some bits of genetic reference materials to construct a developing embryo. The embryo that we will describe will not be a real embryo but it will go through several stages of development just as a real embryo must. Several specific genes will be cited as being expressed during this embryo's development. These genes will be used only as examples of real genes, in life they may come from many different animals. So the animal that will be the final form of our embryo will be a mixture of several animals, a genetic chimera.

Several specific stages or steps in the development of an embryo will be mentioned. These include the development of hatching, a neural plate (the neural plate is the beginning of the nervous system), head and tail orientation, blood, a kidney, bone and also a nervous system. There are hundreds more genes of importance to the developing embryo. However, by limiting our examples we can understand the sequence of their actions, clearly.

Since we are going to describe an hypothetical embryo, we may draw our genetic examples from any species we choose. The genes presented below are actual genes studied in many laboratories. The animals of their origin are noted. Some of these genes are found in many very different specie. The genes selected for this article represent only a very small number of possible genes that determine the development of any embryo.

By way of a quick review, all most multi-cellular animals, certainly any animal with a spine, goes through several distinct stages of embryonic development. An egg becomes fertilized, the cell begins to divide, forming a small ball of identical (undifferentiated) cells, these cells form a hollow ball (a blastula), some of these cells begin to become specialized cells (differentiation), and body parts develop until the embryo becomes able to exist somewhat independent of it's yolk sac or mother. Each of these stages has at least one specific gene associated with it. These genes guide the cells through the production of proteins through each stage of embryo development. As we describe some of these genes and the roles they play in the developing life, please note that the sequence of gene expression is as important as the expression of the genes. In other words, if growing an embryo were like building a house, you would not start by building a roof; you would begin with a solid foundation, then the walls, and then the roof. The same is true for a growing embryo. Each gene contributes its bit of protein at the precisely appropriate moment to grow a healthy animal.

HATCHING ENZYME: Since all embryos begin as fertilized eggs, we will need an enzyme to help dissolve the membrane of the egg. Some eggs (indeed the ones with which we are very familiar, chicken eggs) have hardened shells that must also be broken as the grown embryo emerges. But for this 'hatching,' we are talking about hatching from within the membrane that houses the original single celled egg as the single cell divides to form a small ball of cells. Hatching enzyme, envelysin, is produced by the gene similar to the MMP gene found in mammals. The study of this enzyme in the laboratory, however, uses sea urchins as a source. This enzyme dissolves the egg membrane allowing the growing cells to push into their new environment and expand.

NEURAL PLATE DEVELOPMENT: As the blastula grows some cells form a distinct area destined to become the nervous system of the animal. These specialized cells form something called the neural plate. Some proteins, such as noggin, produced by the noggin gene, signal these cells to differentiate. Without this gene all the cells in the blastula would remain essentially identical. The noggin gene has been studied in the Xenopus frog and the chicken.

HEAD AND TAIL ORIENTATION: Once a small flat section of cells has formed the neural plate, something tells all the cells which end is going to be a head and which is going to be a tail. The expression of the Xenopus LIM motif-containing protein kinase (Xlimk1) gene seems to direct this choice. In the Xenopus frog the deactivation of this gene causes embryos to develop without any clear head and tail orientation.

BLOOD: Blood seems to be a basic component of most animal life. If we consider any body fluid that functions to exchange nutrients and metabolites within a body as some form of blood; we could say that all animals (even single celled animals) have some form of blood. The HEM gene directs the production of part of a hemoglobin molecule an essential ingredient of blood within humans and other animals. If for some reason the production of hemoglobin began well before the development of blood vessels and fluid plasma, this globular protein would just accumulate within the developing embryo and not provide for its usual oxygen transport.

KIDNEY: Having begun to make some blood our embryo (and the young adult) will need a kidney to filter that blood and remove unwanted ions and substances for excretion. The Wilm's tumor suppresser gene (WT1) is instrumental in the development of the mammalian kidney. In other animals, the Xenopus frog for example, a similar gene xWT1, is expressed during the late tadpole phase of development and guides the formation of cells that become the nephric (kidney) system.

BONE: After many of the soft tissues have been formed, bone begins to form. The BSP gene (BSP stands for Bone Sialoprotein) initiates bone matrix formation in the chicken. Bones enable an animal to use leverage as it moves within its environment. Bones are not required for movement but they do help. Consider how well the boneless octopus gets around. You might also consider how birds would fly without some specially fitted bones.

NERVOUS SYSTEM: The coordination of all these body components is directed by a nervous system. This biologic computer is built from cells, much like all the other organs within a body. The development of the nervous system of the Drosophila (fruit fly) and is directed by the Drosophila neurospecific receptor kinase (Dnrk) gene.

All of these genes direct and develop the specific body parts that we have described. Many more are needed to construct even a simple living creature. It is also important to note that the sequence of gene expression plays a vital role in the development of any embryo. Some genes may function at specific periods to initiate the growth of a certain organ. The sequence of other genes may not be as critical as that of others. The absence of one may kill the growing embryo or allow it to develop into some new previously unknown animal.