Facial clefts in the embryo and adult
Early in the development of the embryo, we’re basically just a tube, with a notochord (the precursor to our nervous system) and three layers of tissue. The branchial arches, neural crest, and somites join together as they develop to form the head and neck.
To become a creature with a face, this tube must close in a very specific way, and in mammals, the way that it comes together is very similar between the species. In fact, most mammals can end up developing very similar facial deformities.
In the human, there are 15 “clefts” (separations) that join together very early on in development. The primary regions of the face are completely brought together before the embryo is even the size of a penny (17 mm - about 2/3 of an inch). Most clefts are brought together before the fetus is 1 cm long - less than the diameter of a AAA battery.
Since most of the facial clefts are some of the earliest possible non-lethal deformities, the failure of one or more of them to fuse does not stop the development, unlike many other early congenital problems. For most serious deformities at this stage, the genes simply stop giving cogent signals to one another, and the female’s body self-aborts, as it is not evolutionarily advantageous to create a non-viable fetus.
While all facial clefts have the potential to fail to fuse, the ones that most frequently result in viable fetuses are the clefts of the palate and the maxillary processes. These can occur bilaterally (on both sides), unilaterally (only on one side), and can involve part of the palate, only the palate, only the lip, or the complete palate and lip structure. The most common defect is an isolated unilateral cleft lip, formerly known as a "harelip" (though that term is often seen as derogatory these days).
Due to the fact that the face is the way most people identify one another, its importance in human culture can make any facial deformity a problem for the person who has it. While cleft lips have been fixed for as long as people have had stitches (at least 5000 years), fixing the bone clefts has been much more difficult and daunting. In 1827, John Peter Mettauer completed the first successful bone-flap transplant to fix a cleft palate, and reconstructive surgery has only improved since then.
While most facial cleft anomalies are now routinely fixed at a very young age in industrialized nations, there are many parts of the world where hospital access is limited, and cleft deformities are not easily repaired. The loss of nutrition that suckling difficulty in children with cleft palates have can have life-long consequences. And that’s assuming the infant survives.
Charitable organizations such as Operation Smile attempt to fill that gap - literally.