Jennifer B. Esty
Before any discussion can begin about the development of the brain, a general understanding of embryology is essential. Because the brain and, more generally, the nervous system are some of the earliest components to emerge in an embryo, the two stories are inevitably linked. This is how I plan to begin teaching this curriculum unit, so I present here a very abbreviated version of the stages of human reproduction.
Fertilization and the Zygote
When sperm penetrates an oocyte (the human egg), fertilization occurs and the fertilized egg, now called a zygote, is formed, a single cell with a full compliment of chromosomes. The zygote divides through the process of mitosis, producing two blastomeres. About every 20 hours the cells of the zygote divide again. When the ball of cells has reached about 16 cells, it is called a morula. The cells of the morula continue to divide and continue to make their way through the fallopian tube into the uterus. After about 4 days and at about 100 cells, the morula develops a cavity in the center of the ball and a disk of cells at one side of the ball. The cells that form the disk are called the embryoblast, the future embryo; the cells that form the walls of the rest of the cavity are the trophoblast, the future placenta and fetal membranes. The entire structure is now called a blastocyst (or blastula).
Blastocyst
The blastocyst enters the uterus at about day 6. Within a day or two the trophoblast begins burrowing into the lining of the uterus, destroying cells and stimulating capillary production. The capillaries will become the beginning of the maternal part of the placenta.
By about 13 days, the placenta and umbilical cord have begun to form and the embryoblast has three layers: the ectoderm, the medoderm, and the endoderm. At about 16 days the ectoderm begins to form the neural crest, which is the first stage in the formation of the nervous system. This is also about when a woman will notice that she has not had her period.
Embryo
Shortly after the neural plate forms, cell migrations alter the shape of the embryo, forming bumps and grooves along the length of the neural crest. These cell migrations are a sign of differentiation. Differentiation is the process that leads to the development of the specialized tissues that make up the human body. It is during this period that the fetus is most sensitive to agents causing birth defects; although this sensitivity will continue to some extent throughout the pregnancy. By about three weeks, the circulatory system and the heart begin to form. The cells which form the beginnings of the heart begin to pulse in a rhythm which will become a heartbeat. By about 5 weeks limb buds appear, the beginnings of the digestive and urinary systems also appear. By about 7 weeks, the ossification of the skeletal system begins, the kidneys begin to produce urine, the heart is mostly formed, the arms and legs, hands and feet are forming. Shortly thereafter, the sex of the embryo can be seen by the presence or absence of testes; however, these gonads are very small and not readily visible on ultrasound at this stage.
Fetus
At about 10 weeks, the embryo is considered a fetus. By this time, all of the body systems have begun to develop. The brain still has a smooth surface, but the fissures will begin to form soon. The heart is pumping blood through a primitive circulatory system. The kidneys are processing urine. The digestive organs are largely formed and are practicing their functions. The lungs have formed and begin to inhale and exhale, moving small amounts of amniotic fluid in and out of the lungs. The skeleton has begun to ossify and many muscles are capable of twitching. The skin has formed and some hair follicles are forming, too.
Over the next few months, the body systems continue to develop, grow and become more functional. The development at this point is like a sketch of a masterpiece. All the parts are in place and are proportional; now, all that is needed are the intricate details and coloration that will bring the piece to life. More to the point for this unit, the parts of the brain are sketched in, but the detailed connections that will make it a functional unit have yet to be connected. The detailed connections will come "online" as they get built over the next twenty years and will be continually modified over the lifetime of the owner. Most of the growth occurs before the age of 3, but much modification to a lesser extent continues into adolescence and early adulthood. The details of this development, the subject of this unit, follow.
Because of the unique nature of my students, this section will probably have a greater weight in my teaching than it may in a regular classroom. However, as most anatomy and physiology curricula cover reproduction as well as the brain, this section may be useful as a part of another unit when taught in a more usual classroom setting. It is for that purpose that I include how I plan to teach this section. There are three major concepts I want my students to understand when the complete this part of the unit. First, the students should be able to break the process up into the commonly recognized stages of development from fertilization through birth. Second, my students should have some idea of what is happening during each of the stages. Third, my students should be able to correctly identify the order of the stages that occur. The lesson I suggest for this stage of the unit is a small project. The teacher should break up the process of development into recognizable stages; the ones I have used above would work through the first trimester. The names of the stages should go on squares with enough room for a written description of the stage to be included as well. When I tried this with my class, as I was writing this unit, I printed out the squares with the stage names on a piece of paper and had the students take notes using them during a lecture, but, in future, I might have the students describe the stages based on their own observations from readings in the text book and from the visible embryo website. At any rate, the students should describe the growth that occurs during the stages in their own words in the space left on the square. As the students are exploring what is occurring during the stage, they should also be thinking about what embryo looks like because the next stage of the project is to illustrate the squares. The illustrations could be actually pictures of what a developing embryo looks like, or they could be a visual metaphor for what is happening. The final stage of the project is to have the students create a poster using their squares and illustrations. The poster should tell the story of the process of the formation of the embryo.
