The indentation located centrally on the upper lip, directly beneath the nasal septum, is known scientifically as the philtrum. This vertical groove is a distinctive characteristic of human facial anatomy, running from the base of the nose down to the vermilion border of the upper lip. The philtrum is often bordered by two subtle ridges that terminate at the tubercle, the slight projection in the center of the upper lip.
How the Philtrum Forms During Development
The philtrum is a direct result of complex facial construction that occurs very early in prenatal development. This formation begins during the first trimester of pregnancy, specifically between the fourth and seventh weeks of gestation. The philtrum represents the precise meeting point where three distinct masses of tissue, called facial prominences, come together and fuse.
The process involves the merging of the medial nasal prominences, which contribute the central component of the upper lip. Simultaneously, the maxillary prominences, which originate from the tissue that forms the cheeks and side portions of the upper jaw, move toward the midline.
The fusion of these prominences essentially “zips up” the central upper face. The philtrum is the visible external seam or indentation that remains after this fusion process is complete. The depth and definition of the philtrum can vary significantly among individuals, but the presence of this structure is universal in typical human development.
Function and Evolutionary Purpose
In humans, the philtrum has little functional significance and is largely considered a vestigial feature. It is a remnant of a structure that held a greater purpose in ancestral species. It may play a minor role in channeling moisture, such as sweat, from the area under the nose down to the mouth.
The homologous structure in many other mammals is a functional part of the olfactory system. For example, in canines and felines, the philtrum is a deep, moist groove extending down to the rhinarium, the wet surface around the nostrils. This groove helps draw moisture up to the rhinarium, enhancing the sense of smell by keeping the tissue damp.
As humans evolved, reliance on a highly sensitive, moist nose pad diminished, leading to the structure’s reduction to the relatively shallow groove seen today. The human philtrum remains a prominent marker of the face’s developmental history, even though its practical utility has been lost.
When Development Goes Awry
The philtrum serves as a highly visible indicator of whether the complex process of facial development proceeded correctly. The most well-known condition resulting from a failure in this process is Cleft Lip, which occurs when the facial prominences do not properly fuse. If the maxillary prominence fails to merge with the medial nasal prominence, a gap is left in the structure of the upper lip where the philtrum should have formed.
This developmental error can result in a unilateral or bilateral cleft, meaning the separation occurs on one or both sides of the midline. The severity of the cleft relates directly to the extent of the tissue’s failure to join during the early weeks of gestation. Proper fusion is necessary to create the seamless structure of the upper lip and palate.
Another important clinical indicator related to the philtrum involves its shape. A smooth or flattened philtrum is a recognized diagnostic feature for Fetal Alcohol Spectrum Disorders (FASD). The philtrum is present but lacks the distinct ridges and central groove typically observed. This smoothing suggests that alcohol consumption during pregnancy can disrupt the subtle morphological shaping of the philtrum, making its appearance an external sign of a congenital condition.