Incisor

Overview
Incisors (from Latin incidere, "to cut") are the first kind of tooth in heterodont mammals. They are located in the premaxilla.

Function
In many herbivorous or omnivorous mammals, such as the human and the horse, they are adapted for shearing sharply. In cats, the incisors are small and do not do much; biting off meat is done with the canines and the carnassials. In elephants, the upper incisors are modified into curved tusks, just as is the case with Narwhals, where normally one of them develops into a straight and twisted tusk. The incisors of rodents grow throughout life and are worn by gnawing.

In humans
Humans normally have eight (8) incisors, two of each type. The types of incisors are:
 * maxillary central incisor
 * maxillary lateral incisor
 * mandibular central incisor
 * mandibular lateral incisor

In animals
Among other animals, some other primates, cats and horses have twelve. The rodents have four. Rabbits and hares (lagomorphs) were once considered rodents, but are distinguished by having eight--1 small pair, called "peg teeth" is directly behind the most anterior pair.

The Rodent incisor
The rodent incisor is one of the evolutionary adaptations that make rodents such a successful group. There are two incisors in the upper jaw and two in the lower jaw. The incisors are separated from the molars by a diastema region, an area without any teeth. The tissue of the incisor is regenerated from the apical end and constantly wears down at the distal tip. The ever-growing incisor can be subdivided into two areas, the crown analogue and the root analogue.

The crown analogue is the labial half of the incisor. It is characterized by an enlarged cervical loop at the apical end. The cervical loop is the epithelial stem cell niche. The epithelial progeny of the crown analogue's cervical loop differentiates into ameloblasts that produce enamel.

The root analogue is the lingual half of the incisor. It's cervical loop is much smaller and the epithelium does not differentiate into ameloblasts, but instead forms a root sheath and fragments into epithelial cell rests of Malassez typical of root epithelium. The root analogue is covered in dentin and cementum like a normal root.