Birth Defects

Knowledge of normal development will aid in the understanding of the potential reasons and timing of abnormal occurrences. During the third and fourth weeks of embryonic development the face and mouth form. Three important germ cell layers, ectoderm, mesoderm and endoderm are all essential in developing parts of the face and mouth. Along with the branchial arches the mouth, lips, parts of the nose and jaws will form between weeks three and six. At the same time development of the palate is taking place that encloses the future tongue which appears at four weeks.

Clefts of the lips, jaw or palate occur during this early time frame. While heredity plays a major role, nutritional deficiencies, infections, disease, and trauma in utero may contribute. The tongue may show a red rhomboid shape or may be bifid due to fusive irregularities. Thyroid tissue may be present at the base of the tongue.

Teeth begin (around week 6) to develop from a band of oral epithelium on the upper and lower jaws. From this tissue tooth buds form and eventually a tooth germ develops with ameloblasts (enamel forming cells) and odontoblasts (forms dentin and pulp). With the dentin mineralizing and enclosing the pulp, the ameloblasts will begin to form enamel.

Alterations in the enamel content during development can affect the clinical appearance of the teeth (shape, color, hardness) and the susceptibility to caries development.

At birth, the infant has all the primary teeth and many of the permanent teeth at different stages of development.

Dental enamel consists of 96% inorganic material, 4% organic enamel matrix and water. Its crystalline mineral salts make it the hardest calcified tissue in the body, yet at the same time it is a semi-permeable membrane. It’s thickness varies over the tooth (2-2.5mm on cusps of molars, to knife edge at the necks of teeth).

Changes in development of enamel (amelogenesis) can cause hypoplasia (pitting, furrowing or total absence of enamel) and hypocalcification (opaque or chalky areas on normal enamel surfaces). Nutritional deficiencies, endocrinopathies, febrile diseases and certain chemicals (excessive fluoride <1.5ppm antibiotics) may cause this.

Systemic influences causing enamel hypoplasia frequently occur during the 1st year. Therefore, the permanent teeth most frequently affected are incisors, canines, and first molars. The upper lateral incisor, since it develops later, is often not affected.

The dentin, a living tissue, constitutes the bulk of the tooth. It closely resembles bone. It is yellow in color, is highly elastic and is harder than bone. It contains 30% organic material and 70% inorganic. The primary cells of dentin are odontoblasts. By exposing 1mm of dentin 30,000 odontoblast cells are damaged. Dentin is formed throughout the life of the tooth.

The pulp furnishes nourishment to dentin, contains nerves, blood vessels and cells that form dentin.

The third molar is the most common congenitally missing tooth followed by the lateral incisor and second premolar.