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The Science of Pregnancy Timeline: Week 4– How does the embryo ‘know’ what it should be shaped like?

8 Aug

Adapted from Gray's Anatomy, this surface illustration of a rabbit embryo looks just like a human's around day 13, with the central wedge forming the primitive streak (pr).

“It is not birth, marriage or death, but gastrulation, which is truly the most important time in your life.” –Biologist Lewis Wolpert

Like teens backpacking for a year before college, the immature cells of the embryo go a-travelling before they commit to becoming specific cell types like eye cells and liver cells. As it turns out, where they end up plays a huge role in what they’ll become.

In the first weeks of pregnancy, as the embryo grows from the size of a poppy seed to that of a lemon seed, its duplicating cells are moving and folding in patterns.

Image shared via Flickr by user Mammaoca2008.

If you’ve ever made a paper crane you know that you start by making creases in the paper that establish a center point, making subsequent folds perfectly symmetrical. So it is with the bodies of most all of earth’s creatures, which have two symmetrical sides. The origami analogy applies well to the early development of the embryo: The body becomes organized as layers of cells migrate, fold, and turn inward.

Around 13 days after conception, the cells of the pear-shaped embryo begin to gather at the midline, establishing the axis of the embryo called the “primitive streak.”

This development marks the beginning of “gastrulation,” (scientists again with the unsexy wordsmithing), — the establishment of the basic, 3-layered body plan.

Two weeks after fertilization, gastrulation is in full swing. (Note: most pregnancy timelines call this “week four,” since they measure from the date of the mother’s last menstrual period.)

The embryo divides into three layers:
• The outer layer will become the skin, hair, brain, nerves and spinal cord.
• The middle layer will form muscles and bones, heart and lungs, kidneys, blood vessels, testicles or ovaries.
• The inner layer will become the stuff betwixt your piehole and your corn hole, like the tongue, tonsils, and digestive system.
(For those of you taking the quiz, these are the ectoderm, mesoderm, and endoderm.)

The migrations of cells in the early embryo are critical, because cells that “know” where they are begin to behave differently, and it’s this behavior that drives development. Two ways a cell can “know” where it is: Certain signals or proteins may be more or less ample according to their location along up-and-down or front-and-back axes, making it possible for cells to “read” their location in the scheme of the body the way we read latitude and longitude lines on a map. Or, a cell “knows” its location by “communicating” with its nearest neighbors, by sending or receiving chemical signals.  Larry the skin cell working in a subcutaneous cubicle knows he is third in line for the “corner office” on the surface of the elbow, and he is waiting to move up as his predecessors retire.  And Larry himself will change as he rises in the ranks.

Cells behave differently according to where they are located:  They change shape, they attract or repel one another, they influence cells around them. Some cells self-destruct, as they do to create the space between forming fingers. A cell appropriately placed in the mesoderm, becoming part of the team that will build the heart, will begin to activate the protein-making genes that will make it a muscle cell.

Three weeks after conception (in pregnancy week 5), the embryo begins to have curves and bumps, thanks to differentiation, and is working hard to create organs. It now has muscle cells and blood cells, neurons and glial cells. The baby will be born not just with 10 fingers and 10 toes, but with some 250 cell types. These cell types, the same 250 or so that we have as adults, have identical genes. But they behave differently, and acquire distinct traits because they are expressing different genes, and thus creating different proteins, according to their placement in the body and their relationships with nearest neighbors.