Lip

The lips consist of an upper lip (labium superius) and a lower lip (labium inferius). The lips merge at the right and left corner of the mouth (angulus oris). They contain muscle tissue and the oral fissure (Rima oris) forms the entrance to the oral cavity. On the inside, they have an upper and lower labial frenulum (frenulum labii superioris et inferioris), a connection to the jaw.

Histology

The lips can already be divided macroscopically into two different tissue types. On the outside there is skin, which changes into mucous membrane on the inside. It can be divided into 3 sections, which merge smoothly into each other, so that a clear border is not recognizable.

The outer skin (pars cutanea) consists of a multi-layered horny squamous epithelium and forms a protective barrier to the outside world. It is characteristically composed of several layers of superimposed cells. In these cell layers and the underlying tissue, there are hair follicles, sebaceous glands (to keep the lips supple) and sweat glands, among other things.

The keratinization is caused by dying special cells called keratinocytes. Between the inside and outside is the transition zone in which the actual lip red is located. This area is also called pars intermedia.

This area also contains keratinized squamous epithelium, which is significantly thinner than the anterior zone. The connective tissue protrudes into the squamous epithelium, which is called lamina propria directly below the cell layers. Here, many arterial capillaries run through the thin upper layer.

This is how the strong red color of the lips is created. When the oxygen content in these capillary loops decreases, the typical phenomenon of blue lips (cyanosis) develops. The inner layer is lined with mucosal epithelium, an uncornified squamous epithelium.

In addition to numerous vessels and nerve fibers, there are many small salivary glands that produce viscous (mucous) saliva. This keeps the lips moist and, like the larger salivary glands, is involved in the breakdown of the food taken in. Beneath this layer lies the muscle of the lips (Musculus orbicularis oris) embedded in the connective tissue. This is responsible for the movement of the lips.