of their fibroblasts. although tooth eruption occurs at different times for different people, a general eruption timeline exists. typically, humans have 20 primary (baby) teeth and 32 permanent teeth. tooth eruption has three stages. the first, known as deciduous dentition stage, occurs when only primary teeth are visible. once the first permanent tooth erupts into the mouth, the teeth are in the mixed (or transitional) dentition. after the last primary tooth falls out of the mouth—a process known as exfoliation—the teeth are in the permanent dentition. primary dentition starts on the arrival of the mandibular central incisors, usually at eight months, and lasts until the first permanent molars appear in the mouth, usually at six years. the primary teeth typically erupt in the following order: (1) central incisor, (2) lateral incisor, (3) first molar, (4) canine, and (5) second molar. as a general rule, four teeth erupt for every six months of life, mandibular teeth erupt before maxillary teeth, and teeth erupt sooner in females than males. during primary dentition, the tooth buds of permanent teeth develop below the primary teeth, close to the palate or tongue. mixed dentition starts when the first permanent molar appears in the mouth, usually at six years, and lasts until the last primary tooth is lost, usually at eleven or twelve years. permanent teeth in the maxilla erupt in a different order from permanent teeth on the mandible. maxillary teeth erupt in the following order: (1) first molar (2) central incisor, (3) lateral incisor, (4) first premolar, (5) second premolar, (6) canine, (7) second molar, and (8) third molar. mandibular teeth erupt in the following order: (1) first molar (2) central incisor, (3) lateral incisor, (4) canine, (5) first premolar, (6) second premolar, (7) second molar, and (8) third molar. since there are no premolars in the primary dentition, the primary molars are replaced by permanent premolars. if any primary teeth are lost before permanent teeth are ready to replace them, some posterior teeth may drift forward and cause space to be lost in the mouth. this may cause crowding and/or misplacement once the permanent teeth erupt, which is usually referred to as malocclusion. orthodontics may be required in such circumstances for an individual to achieve a straight set of teeth. the permanent dentition begins when the last primary tooth is lost, usually at 11 to 12 years, and lasts for the rest of a person's life or until all of the teeth are lost (edentulism). during this stage, third molars (also called "wisdom teeth") are frequently extracted because of decay, pain or impactions. the main reasons for tooth loss are decay and periodontal disease. eruption times for primary and permanent teeth primary teeth teeth centralincisor lateralincisor canine firstpremolar secondpremolar firstmolar secondmolar thirdmolar maxillary teeth 10 mo 11 mo 19 mo - - -16 mo -29 mo - mandibular teeth 8 mo 13 mo 20 mo - - -16 mo -27 mo - permanent teeth teeth centralincisor lateralincisor canine firstpremolar secondpremolar firstmolar secondmolar thirdmolar maxillary teeth 7–8 yr 8–9 yr 11–12 yr 10–11 yr 10–12 yr 6–7 yr 12–13 yr 17–21 yr mandibular teeth 6–7 yr 7–8 yr 9–10 yr 10–12 yr 11–12 yr 6–7 yr 11–13 yr 17–21 yr immediately after the eruption enamel is covered by a specific film: nasmyth's membrane or 'enamel cuticle', structure of embryological origin is composed of keratin which gives rise to the enamel organ. nutrition and tooth development as in other aspects of human growth and development, nutrition has an effect on the developing tooth. essential nutrients for a healthy tooth include calcium, phosphorus, and vitamins a, c, and d. calcium and phosphorus are needed to properly form the hydroxyapatite crystals, and their levels in the blood are maintained by vitamin d. vitamin a is necessary for the formation of keratin, as vitamin c is for collagen. fluoride, although not a nutrient, is incorporated into the hydroxyapatite crystal of a developing tooth and bones. the dental theory is the low levels of fluoride incorporation and very mild fluorosis makes the tooth more resistant to demineralization and subsequent decay. deficiencies of nutrients can have a wide range of effects on tooth development. in situations where calcium, phosphorus, and vitamin d are deficient, the hard structures of a tooth may be less mineralized. a lack of vitamin a can cause a reduction in the amount of enamel formation. fluoride ingestion has been noted to delay eruption of teeth for as much as a year or more from the accepted eruption dates since the initial 1940s fluoridation trials. researchers theorize that the delay is a manifestation of fluoride's depressing impact on thyroid hormones. the delay in eruption has been suggested as the reason for the apparent difference in decay among the youngest children. fluoride ingestion during tooth development can lead to a permanent condition known as fluorosis with varying levels of severity, the result of fluoride's interference with the normal osteoblast development. undiagnosed and untreated celiac disease often causes dental enamel defects and can be the only manifestation of the disease, in absence of gastrointestinal symptoms or malabsorption signs. bisphenol a (bpa) is a hormone-disrupting chemical that has been implicated in having negative effects on human health, including, but not limited to, fetal development. as shown in animal studies which mimic human enamel, the mother's consumption of products with bpa during pregnancy can lead to the child's tooth development being obstructed. those children are shown to be prone to incisor and first molar hypomineralization, a weakened state of the enamel. additionally, it is most important for mother's to avoid bpa during pregnancy, but also avoid bpa-use in the child's products up to five months of age. developmental disturbances the absence of third molars is very common, occurring in 20–23% of the population, followed in prevalence by the second premolar and lateral incisor. anodontia is a complete lack of tooth development. it is rare, most often occurring in a condition called hypohidrotic ectodermal dysplasia. hypodontia is a lack of some tooth development (not including third molars). it is one of the most common developmental abnormalities, affecting 3.5–8.0% of the population. hypodontia is often associated with the absence of a dental lamina, which is vulnerable to environmental forces like infection and chemotherapy medications. it is also associated with many syndromes, such as down syndrome and crouzon syndrome. hyperdontia is the development of extraneous teeth. it occurs in 1–3% of caucasians and is more frequent in asians. about 86% of these cases involve a single extra tooth in the mouth, most commonly found in the maxilla, where the incisors are located. hyperdontia is believed to be associated with an excess of dental lamina. dilaceration is an abnormal bend found on a tooth, and is nearly always associated with trauma that moves the developing tooth bud. as a tooth is forming, a force can move the tooth from its original position, leaving the rest of the tooth to form at an abnormal angle. cysts or tumors adjacent to a tooth bud are forces known to cause dilaceration, as are primary (baby) teeth pushed upward by trauma into the gingiva where it moves the tooth bud of the permanent tooth. enamel hypoplasia or hypomineralization is a defect of the teeth caused by a disturbance in the formation of the organic enamel matrix, clinically visible as enamel defects. it may be caused by nutritional factors, some diseases (such as undiagnosed and untreated celiac disease, chicken pox, congenital syphilis), hypocalcemia, fluoride ingestion, birth injury, preterm birth, infection or trauma from a deciduous tooth. in some circumstances enamel hypoplasia can be so severe that last sections of enamel is missing, exposing the underlying dentin. some systemic conditions may cause delayed tooth development, such as nutritional factors, endocrine disorders (hypothyroidism, hypopituitarism, hypoparathyroidism, pseudohypoparathyroidism), undiagnosed and untreated celiac disease, anemia, prematurity, low birth weight, renal failure, heavy metal intoxication or tobacco smoke, among others. regional odontodysplasia is rare, but is most likely to occur in the maxilla and anterior teeth. the cause is unknown; a number of causes have been postulated, including a disturbance in the neural crest cells, infection, radiation therapy, and a decrease in vascular supply (the most widely held hypothesis). teeth affected by regional odontodysplasia nevamelogenesis imperfecta is an autosomal dominant disease characterized by a defect in dental enamel formation. teeth are often free of enamel, small, misshapen, and tinted brown. the cause of these deformities is due to a mutation in enamel in expression. dental patients with this disease should be especially cautious and visit their dentist frequently. natal and neonatal teeth are an anomaly that involves teeth erupting in a newborn infant's mouth earlier than usual. the incidence ranges from 1:2,000 to 1:3,500 births. natal teeth are more frequent, approximately three times more common than neonatal teeth. some authors reported a higher prevalence in females than males. the most common location is the mandibular region of the central incisors. natal teeth and neonatal teeth are associated with genetics, developmental abnormalities and certain recognized syndromes. additional names for this condition include precocious dentition, baby teeth, and milk teeth.