Lip pits were first reported in 1845 by Demarquay who thought they were indentations made by the upper incisors. Today it is known that they occur during embryogenesis due to IRF6 mutations. On day 32 of embryonic development there are four growth centers of the lower lip divided by two lateral grooves and one median. By day 38 of normal development, the grooves have disappeared. However, when growth of the mandibular process is impeded, a lip pit occurs. Lip pits begin to develop on day 36. Cleft lip begins to develop on day 40, and cleft palate emerges on day 50 of development. There are three types of lip pits, which are classified according to their location: midline upper, comissural, and lower lip. The most common phenotype is two symmetrical lower lip pits flanking both sides of the midline in the bilateral paramedial sinuses. Lower lip pits may also be bilaterally, unilaterally, or medially asymmetrical. The occurrence of a single lip pit is considered incomplete expression, and it typically occurs on the left side of the lower lip. There are also three different shapes for lip pits, the most common being circular or oval; less common forms include slit-like or transverse. The lip pits extend into the orbicularis oris muscle, ending in blind sacs surrounded by mucous glands. In some cases mucous is excreted when the muscles contract.
Van der Woude syndrome
Helsmoortel-Van der Aa syndrome
Van Den Bosch syndrome
Koolen–De Vries syndrome
Van Wyk and Grumbach syndrome
Wildervanck syndrome
Van Gogh syndrome
Nezelof syndrome