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MACROSCOPE

From Research to Reality

To understand her son’s birth defect, a mother makes an emotional and scientific journey

Katherine E. Willmore

Shaping the Face

Facial development for a human embryo begins early in the fourth week of development, when five facial prominences appear around the area that will become the mouth. These swellings of embryonic tissues act as growth centers of the face. I like to think of them as blobs of Play-Doh—each a pliable mass that can be pushed and pulled into a variety of shapes. Above the future mouth are two maxillary prominences, which form the upper jaw and the cheek regions of the face. Sandwiched between them is the frontonasal prominence, which forms the forehead and nose. Below the opening of the future mouth lie the paired mandibular prominences that will fuse to form the lower jaw.

The first parts of the face to develop are the lower lip and jaw, which are essentially formed by the tenth week of development. The two mandibular prominences grow toward each other until they meet at what will be the center of the chin and lower lip, at which point they fuse. Our nostrils, which begin to develop at the end of the fourth week, start off as paired, oval-shaped thickenings of the frontonasal prominence, called nasal placodes. The outer margins of the placodes grow faster than the insides do. To visualize this, imagine an inflatable swimming pool. As you blow air into the pool, the sides rise as they take on most of the air, with very little air going to the bottom of the pool. Similarly, the fast growth of the outer margins of our nasal placodes forms the sides of our nostrils.

Meanwhile, the maxillary prominences are growing rapidly. They push against the nasal portion of the frontonasal prominence, causing the primitive nostrils to move closer to each other. At the end of the sixth week, the maxillary prominences start to fuse with the outer parts of the nostrils, joining the cheek regions with the sides of the nose. Then they ooze underneath the frontonasal prominence and finally meet at the midline underneath the primitive nose. Between the seventh and tenth weeks, the middle portions of the two nostrils fuse with each other and with the maxillary prominences below. These fusion events form separate nostrils with a single septum between, creating continuity between the nose, cheeks, upper jaw and lip.

Palate development takes place between the sixth and twelfth weeks, but the period from the end of the sixth week to the beginning of the ninth is most critical. The primary palate, located at the front of the mouth, begins to develop early in the sixth week. When the nostrils fuse together to form a single septum in the middle, a portion of the frontonasal prominence gets squished downwards. This portion, which now lies below the nose and between the maxillary prominences, is called the intermaxillary segment. The little divot between your nose and mouth, called the philtrum, is part of this structure, as is the primary palate, which includes the bone that houses your upper two front teeth.

Development of the secondary palate begins around the same time, but it is a more complicated process. Initially, the tissue that will form the secondary palate hangs vertically from each maxillary prominence, on either side of the tongue. These dangling bits of tissue are called the palatal shelves. As the lower jaw elongates with growth, the tongue is able to move forward and flatten so that it lies below the palatal shelves. Then, during the seventh and eighth weeks, one of the most incredible developmental events occurs: Over just a few hours, the palatal shelves actually elevate from a vertical to a horizontal position. Once the shelves are in place, they continue to grow until they meet in the middle of what will become the roof of the mouth, where they fuse with each other and with the primary palate in front.

All these acts of fusion require timed coordination of cell movement, cell death and cell growth. To understand the process, it may be easier to think of what it would look like for a part of the body that we can see. Imagine that your index and middle fingers are going to fuse along their sides. First, the skin that lies between them must break down. The cells that make up this skin die, leaving the tissue within each finger free to intermingle. The tissue of each finger then grows toward and into that of the other. Finally, skin grows over the site of fusion, creating one large finger. This scenario may read like science fiction, but the essential processes themselves are what create our very real-world faces, fusing the different elements of the palate and the rest of our facial structures.








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