The study of prenatal development throughout the stages before birth. EMBRYOLOGY The study of prenatal development throughout the stages before birth.

ORAL EMBRYOLOGY Pregnancy starts with conception, or fertilization. Gestation is the period from fertilization to birth. 9 Months or 40 Weeks Pre-natal developmental age is described in weeks

3 Prenatal Development Periods Pre-implantation Period Occurs during first week Ovum (egg) is penetrated and united with a sperm during fertilization. Zygote, fertilized egg, is formed. Meiosis-process that ensures future (fetus) will have correct number of chromosomes.

Prenatal Development Periods… Embryonic Period Beginning of 2nd week to 8th week. Individual is now an embryo. Most critical time because of the development of major structures: Cells proliferate (increase in number) Differentiate (change into tissues and organs) Integrate (form systems)

Prenatal Development Periods… Fetal Period 9th week until birth. Body systems continue to develop and mature Distinguishable ears, arms, hands, legs, feet, fingerprints and footprints. Less vulnerable to malformations.

PRIMARY EMBRYONIC LAYERS ECTODERM (outer layer) MESODERM (middle layer) ENDODERM (inner layer) Skin, brain, spinal cord, hair, nails, enamel of teeth, lining of oral cavity Bones, muscles, circulatory system, kidneys, ducts, reproductive system, lining of abdominal cavity, dentin, pulp and cementum of teeth Lining of digestive system, lining of lungs, parts of urogenital system

WEEK FOUR! Stomodeum (primitive mouth) and the primitive pharynx merge. Stomodeum extends to part of the mouth.

Branchial Arches/Pharyngeal Arches By the end of 4th week of prenatal development: First Branchial Arch or Mandibular Arch- forms bones, muscles, nerves of the face, lower lip, muscles of mastication, and anterior portion of alveolar process of mandibular. Second Branchial Arch or Hyoid Arch- forms styloid process, stapes of the ear stylohyoid ligament, part of hyoid bone, side and front of neck, and some muscles of facial expression. Third Branchial Arch- forms body of hyoid and posterior tongue. (Hyoid-supports tongue and aids in articulation) Fourth, Fifth, and Sixth Brachial Arch- forms lower throat, thyroid cartilage, and muscles and nerves of pharynx and larynx.

3 Stages of Palate Formation 5th week of prenatal development Formation of Primary Palate Formation of Secondary Palate Fusion of the Palate Starts week 9 Completed week 12 Union of the Palates forms the Hard and Soft Palate.

Facial Development Occurs between 5th and 8th week. 5-6 weeks the forehead is the dominant feature. 8 weeks growth of the nose and upper and lower jaw. By the third month the fetus appears to look more human.

The development of the face and palate

TOOTH DEVELOPMENT Embryo is 5-6 weeks old: first signs of tooth development in anterior mandibular region. Anterior maxillary tooth development follows. Tooth development process continues towards the posterior in both jaws. 17th week: all primary teeth are developed and development of permanent teeth has begun.

DEVELOPMENTAL DISTURBANCES GENETIC FACTORS: Tooth/Jaw size relation Disturbances in meiosis during fertilization

DEVELOPMENTAL DISTURBANCES ENVIRONMENTAL FACTORS: Teratogens include: infections, drugs (prescription and OTC), exposure to radiation, alcohol. Toxins from periodontal disease linked to low birth weight. Malnutrition.

Facial Development After Birth Disposition-adding new bone Osteoblasts-new bone formation (healing fracture) Remodeling-changes of existing bone due to force on tooth within socket. Resorption-removal of bone Osteoclasts-resorb bone (root resorption) Modeling-displacement (changes as bones increase in size and shape)

Life Cycle of a Tooth Odontogenesis Process of tooth formation Divided into three primary periods: 1. Growth I. Bud Stage II. Cap Stage III. Bell Stage 2. Calcification 3. Eruption

Bud Stage: aka-Initiation Starts with formation of dental lamina Dental lamina-thickened band of epithelium that flows the curve of each developing dental arch. Dental lamina produces ten enlargements in each arch, tooth buds for primary teeth the grows in posterior direction and continues to form buds for permanent molars. Succedaneous teeth-permanent teeth developed on the lingual side of primary teeth begin forming at 24 weeks.

Cap Stage: aka-Proliferation Cells of the tooth grow and increase in number. Solid looking bud now looks like a hollow cap Enamel organ will form enamel of tooth Dental papilla will form pulp and dentin of tooth Dental sac will form cementum and periodontal ligament

Bell Stage: aka-Histodifferentiation Epithelial cells become ameloblasts (form enamel) Peripheral cells of the dental papilla become odontoblasts (dentin forming cells) Inner cells of dental sac differentiate into cementoblasts (cementum forming cells) Dental organ changes and resembles the shape of a bell

Cont… Morphodifferentiation-basic shape and relative size of tooth is established. Dentinoenamel juntion (DEJ) and cementodentinal junction are blue print for developing tooth. Process starts at top of tooth and works downward Roots begin developing after formation reaches cementoenamal junction (CEJ) Inner cells of dental sac differentiate to cementoblasts and form cementum to cover root.

Calcification The structural outline that was formed during the growth stage is hardened by the deposits of calcium and other mineral salts. Ameloblasts build enamel layer by layer. If a tooth has multiple cusps a cap is formed over each cusp and they eventually fuse to form solid enamel. A pit and fissure is formed. Fissure is a fault along the developmental groove on the occlusal surface. A Pit results when 2 developmetal grooves cross one another. Decay usually begins in the pits and fissures due to the inability to keep them clean. o

PRIMARY TEETH ERUPTION: The movement of a tooth into its functional position in the oral cavity. Eruption takes place in chronological order, as in permanent teeth. Eruption causes tenderness and swelling of local tissues. Exfoliation: Normal process by which primary teeth are lost and succedaneous (permanent) teeth develop. Osteoclasts cause resorption starting at apex and continuing towards the crown. Osteoblasts replace resorbed bone, odontoclasts and cementoblasts replace resorbed portions of tooth