Embryology is the study of prenatal development throughout the stages before birth. The first part of this chapter discusses development with emphasis on the formation of the teeth and structures of the oral cavity. Learning about the development of the oral structures provides the foundation for later understanding of developmental problems that may occur in these structures.

Histology is the study of the structure and function of tissues on a microscopic level. The second part of this chapter covers the histology of the teeth, their supporting structures, and the oral mucosa, which surrounds the teeth and lines the mouth. By understanding the histology of oral tissues, the dental assistant can better understand the disease processes that occur in the oral cavity.

Oral Embryology

Pregnancy begins with conception, which is also known as fertilization. Gestation, the period from fertilization to birth, has an average duration of 9 months from conception to birth, or 40 weeks from the last menstrual period (LMP). The due date is usually figured out with a convenient rule of thumb; count back three months from the day the LMP began, then add a year and a week. The date you come up with is just a guide; the baby may arrive anywhere from two weeks earlier to two weeks later. The sex of the baby is established at conception and will be apparent in a few weeks. This little 1-inch being weighs only about .04 ounce (1 gram) and could fit comfortably on a tablespoon.

A physician usually describes prenatal development in weeks on the basis of the date of the LMP. In embryology, developmental age is based on the date of conception, which is assumed to have occurred two weeks after the LMP. Developmental ages noted in this chapter are based on the date of conception.

Prenatal Development

Prenatal development consists of three distinct periods: (1) preimplantation, (2) embryonic, and (3) fetal (Fig. 8-1).

The preimplantation period takes place during the first week. At the beginning of the week, an ovum (egg) is penetrated by and united with a sperm during fertilization (Fig. 8-2). This penetration of the egg by a sperm cell has an immediate effect on the surface of the egg: The outer coating of the egg changes, so that no other sperm cell can enter. The union of the egg and the sperm subsequently forms a fertilized egg, or zygote. In the zygote, 23 chromosomes in the sperm unite with 23 chromosomes in the egg, providing a new life with a full complement of 46 chromosomes. These chromosomes will determine its inherited characteristics and will direct its growth and development. The process of joining the parents’ chromosomes is called meiosis. Meiosis ensures that the future embryo will have the correct number of chromosomes.

FIG. 8-2 Sperm fertilizes the ovum and unites with it to form the zygote after the process of meiosis and during the first week of prenatal development. Chromosomes from the ovum and sperm join to form a zygote—a new individual. (From Bath-Balogh MB, Fehrenbach MJ: Illustrated dental embryology, histology, and anatomy, ed 3, St Louis, 2011, Saunders.)

The embryonic period extends from the beginning of the second week to the end of the eighth week, and the new individual is known as an embryo. This period is the most critical time because during these weeks, development begins in all major structures of the body. The cells begin to proliferate (increase in number), differentiate (change into tissues and organs), and integrate (form systems). Many of these key developments occur before the mother even knows that she is pregnant. At about the end of the eighth week of pregnancy, a baby graduates from embryo to fetus. This name change signifies a change in the baby’s level of development. While an embryo, the baby looks very much like a tadpole, but as a fetus, it has a distinctly human appearance.

The fetal period continues from the ninth week and lasts until birth. During the fetal phase, body systems continue to develop and mature. The fetus has distinguishable ears, arms, hands, legs, and feet, as well as the fingerprints and footprints that will set it apart from other human beings. Because all organ systems are formed during the embryonic period, the fetus is less vulnerable than the embryo to malformations caused by radiation, viruses, and drugs (Table 8-1). The fetal stage is a period of growth and maturation (Fig. 8-3).

1 What are the three periods of prenatal development?

2 Which period of prenatal development is the most critical, and why?

TABLE 8-1

Developmental Disturbances

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Developmental Period		Examples of Disturbances
Preimplantation period	Child with Down syndrome	If any disturbances occur in meiosis during fertilization, major congenital malformations result. Down syndrome is caused by an extra chromosome. A child with this syndrome has a flat, broad face with wide-set eyes, a flat-bridged nose, oblique eyelid fissures, and other defects. An affected child can have various levels of mental retardation. Children with Down syndrome have increased levels of periodontal (gum) disease and abnormally shaped teeth. Implantation may occur outside the uterus; this is called ectopic (ek-TOP-ik) pregnancy. Most such pregnancies occur within the fallopian tube. Tubal pregnancies usually rupture, causing loss of the embryo and threatening the life of the pregnant woman.
Embryonic period	Child with fetal alcohol syndrome	Developmental disturbances during this period may cause major congenital malformations. Caused by the rubella (roo-BELL-ah) virus, German measles in the mother can result in cardiac defects and deafness in the child. Exposure to high levels of radiation may result in cell death and retardation of mental development and physical growth. Fetal alcohol syndrome can occur when a pregnant woman ingests alcohol.
Fetal period	Endogenous developmental stain: tetracycline	In amniocentesis (am-nee-o-sen-TEE-sis), a common prenatal diagnostic procedure, amniotic fluid is sampled during the 14th to 16th weeks to determine whether a birth defect is present in the fetus. When a pregnant woman takes the systemic antibiotic tetracycline (tet-rah-SI-kleen) during the fetal period, permanent staining of the child’s primary teeth may result.

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Figures from Zitelli BJ, Davis HW: Atlas of pediatric physical diagnosis, ed 5, St Louis, 2008, Mosby; and Daniel SJ, Harfst SA, Wilder R: Mosby’s dental hygiene: concepts, cases, and competencies, ed 2, St Louis, 2008, Mosby. (Courtesy Dr. George Taybos, Jackson, MS.)

FIG. 8-3 A fetus at various weeks of development.

Embryonic Development of the Face and Oral Cavity

The face and its related tissues begin to form during the fourth week of prenatal development within the embryonic period. During this time, the rapidly growing brain of the embryo bulges over the oropharyngeal membrane, beating heart, and stomodeum (Fig. 8-4).

FIG. 8-4 Scanning electron micrograph of the head and neck of an embryo at four weeks shows development of the brain, face, and heart. Note the stomodeum (ST), or “primitive mouth,” and the developing eye.

Primary Embryonic Layers

During the third week of development, the cells of the embryo form the three primary embryonic layers: ectoderm, mesoderm, and endoderm. The cells within each layer multiply and differentiate into specialized cells needed to form the organs and tissues of the body.

3 What are the three primary embryonic layers?

Structures Formed by Specialized Cells of Primary Embryonic Layers

Ectoderm (outer layer)

Skin, brain, spinal cord

Hair, nails

Enamel of teeth

Lining of oral cavity

Mesoderm (middle layer)

Lining of abdominal cavity

Dentin, pulp, and cementum of teeth

Endoderm (inner layer)

Lining of digestive system

Lining of lungs

Parts of urogenital system

Early Development of the Mouth

During the fourth week, the stomodeum (primitive mouth) and the primitive pharynx merge, and the stomodeum extends into part of the mouth. By the beginning of the fifth week, the embryo is approximately 5 mm. The heart is prominent and bulging (Fig. 8-5).

FIG. 8-5 A human embryo during the fifth week of development. (From Bath-Balogh MB, Fehrenbach MJ: Illustrated dental embryology, histology, and anatomy, ed 3, St Louis, 2011, Saunders.)

The site of the face is indicated from above by the region just in front of the bulging forebrain (future forehead) and from below by the first pair of branchial arches (future jaws).

Branchial Arches

By the end of the fourth week, six pairs of branchial arches have formed. The term pharyngeal arches is often used instead of branchial arches in discussions of these tissues. The first two of these arches give rise to the structures of the head and neck (see Chapter 9).

The first branchial arch, also known as the mandibular arch, forms the bones, muscles, and nerves of the face. The first arch also forms the lower lip, the muscles of mastication, and the anterior portion of the alveolar process of the mandible.

The second branchial arch, also known as the hyoid arch, forms the styloid process, stapes of the ear, stylohyoid ligament, and part of the hyoid bone. The second arch also forms the side and front of the neck, as well as some muscles of facial expression.

The third branchial arch forms the body of the hyoid and the posterior tongue. The fourth, fifth, and sixth branchial arches form the structures of the lower throat, including the thyroid cartilage, and the muscles and nerves of the pharynx and larynx.

4 Which branchial arch forms the bones, muscles, nerves of the face, and lowe/>

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Jan 8, 2015 | Posted by mrzezo in Dental Nursing and Assisting | Comments Off

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8. Oral Embryology and Histology

Oral Embryology and Histology

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