2: Embryology and Histology

CHAPTER 2 Embryology and Histology

EMBRYOLOGY

Embryology is the study of prenatal development (gestation period), which begins with fertilization and continues until birth. Divided into three periods: preimplantation, embryonic, fetal.

• See CD-ROM for Chapter Terms and WebLinks.

• See Chapters 9, Pharmacology: drugs and placental barrier; 11, Clinical Treatment: pregnant patient.

A. Preimplantation period (FIRST week):

1. Union of ovum and sperm undergoes fertilization to form egg (zygote) where final stages of meiosis occur in ovum:

a. To allow formation of new individual, sperm and ovum when joined have CORRECT number of chromosomes (diploid number of 46).

b. Ovum and sperm have to reduce by one half the normal number of chromosomes (to haploid number of 23).

c. Thus zygote received half its chromosomes from female and half from male, with resultant genetic material a reflection of BOTH biological parents.

d. Photographic analysis of person’s chromosomes is done by orderly arrangement of pairs in a karyotype.

2. After fertilization, zygote undergoes mitosis (individual cell division); solid ball of cells is a morula; review of the stages and terms of mitosis

is recommended (see CD-ROM WebLinks).

3. Because of ongoing process of mitosis and secretion of fluid by the cells within morula, zygote becomes a blastocyst (blastula), which undergoes successive cell divisions by mitosis:

a. By end of first week, blastocyst stops traveling, undergoes implantation, and becomes embedded in prepared endometrium, which is the innermost layer of uterine lining.

b. After 1 week of cleavage, blastocyst consists of trophoblast layer (layer of peripheral cells) and embryoblast layer (small inner mass of embryonic cells).

B. Embryonic period (weeks 2 through 8): implanted blastocyst becomes embryo after first week.

1. Second week:

a. Increased number of cells creates embryonic cell layers (germ layers) and forms disc.

b. Bilaminar disc (also known as disk); two-layered, flattened, essentially circular plate of cells with superior epiblast layer (high columnar cells) and inferior hypoblast layer (small cuboidal cells).

c. Disc is suspended in uterus’s endometrium between two fluid-filled cavities: amniotic cavity faces epiblast layer, and yolk sac faces hypoblast layer; BOTH serve as initial nourishment for embryonic disc.

d. Placenta: prenatal organ that joins pregnant woman and developing embryo; permits selective exchange of soluble bloodborne substances (including drugs and nutrients) through the placental barrier.

a. Within the disc, primitive streak forms, which is a furrowed, rod-shaped thickening in the middle of the disc:

(1) Cells from epiblast layer migrate between the two layers, becoming mesenchyme, embryonic connective tissue.

(2) Mesenchyme layer has potential to proliferate and differentiate into IMPORTANT types of connective tissue-forming cells (e.g., fibroblasts, chondroblasts, osteoblasts).

b. Differentiation of new embryonic layer within disc from mesenchyme, called mesoderm:

(1) Trilaminar disc: three-layered disc with epiblast layer now ectoderm and hypoblast layer now endoderm.

(a) Ectoderm: forms epidermis of skin, nervous system, other structures.

(b) Mesoderm: forms muscle coats, connective tissues, vessels supplying tissues and organs, other tissues.

(c) Endoderm: forms epithelial linings of respiratory passages and digestive tract, including some glandular organ cells.

c. Disc now has a cephalic end (head) where oropharyngeal membrane (buccopharyngeal membrane, future mouth) forms and caudal end (tail) where cloacal membrane forms (future anus).

4. Embryo also undergoes folding, placing three types of tissues in proper positions; recognizably human at end of embryonic period (eighth week).

C. Fetal period (weeks 9 through birth): encompasses beginning of ninth week (third month) to ninth month; involves maturation of existing structures as embryo enlarges to become a fetus.

D. Congenital malformations (birth defects):

1. Developmental problems evident at birth (or before because of prenatal diagnostic tests): occur during BOTH preimplantation and embryonic periods, thus in first trimester of pregnancy. Examples:

a. Ectopic pregnancy: implantation that occurs outside the uterus.

b. Down syndrome (trisomy 21): extra chromosome present after meiotic division.

c. Ectodermal dysplasia: abnormal development of one or more ectodermal structures.

d. Spina bifida: neural tube defect.

2. Malformations: caused by genetic factors such as chromosome abnormalities, environmental agents, teratogens.

a. Teratogens: infections (e.g., rubella virus), drugs (e.g., fetal alcohol syndrome [FAS] causing serious physical and mental disorders, systemic tetracycline antibiotic therapy causing intrinsic dental staining), radiation.

b. Women of reproductive age should AVOID teratogens at time of first missed menstrual period and thereafter, to protect developing child.

E. Amniocentesis: sampling of amniotic fluid.

1. MOST common invasive prenatal diagnostic procedure.

2. Fluid sampled during fourteenth to sixteenth weeks after last missed menstrual period.

3. Performed in older age groups: one or both parents have chromosomal abnormality or neural tube defect, previous child was affected, or parents are carriers of inborn errors of metabolism, e.g., X-linked disorders such as hemophilia.

DEVELOPMENT OF FACE AND ORAL CAVITY

Face and structures of the oral cavity begin development early in embryonic period. ALL three embryonic layers are involved in facial development (layers are discussed earlier). Depends on five facial processes (prominences) that form during fourth week and surround stomodeum (primitive mouth): (1) single frontonasal process; (2 and 3) paired maxillary processes; and (4 and 5) paired mandibular processes (Figure 2-1). In the future, the stomodeum will form the oral cavity.

A. Facial processes are centers of growth for the face:

1. If an adult’s face is divided into thirds, these portions roughly correspond to centers of facial growth:

a. Upper face: from frontonasal process.

b. Middle face (midface): from maxillary processes.

c. Lower face: from mandibular processes.

B. Overall growth of the face is in inferior and anterior direction in relation to cranial base:

1. Growth of upper face is MORE rapid, in keeping with association with developing brain; forehead ceases to grow much after age 12.

2. In contrast, middle and lower portions of the face grow MORE slowly over prolonged period of time:

a. Finally cease to grow late in puberty.

b. Eruption of permanent third molars at around 17 to 21 years marks end of major growth of lower two thirds of face.

C. Underlying facial bones also developing at this time depend on centers of bone formation by intramembranous ossification.

Figure 2-1 Embyro at third to fourth weeks of development.

(From Bath-Balogh M, Fehrenbach MJ: Illustrated dental embryology, histology, and anatomy, ed 2, St. Louis, 2006, Saunders/Elsevier.)

Mandibular Arch and Lower Face Formation

During fourth week, two bulges of tissue appear inferior to stomodeum; these paired mandibular processes are formed in part by neural crest cells that migrated to the facial region, covered externally by ectoderm and internally by endoderm (Figure 2-1).

A. Paired mandibular processes BOTH fuse at midline to form the mandibular arch (mandibular symphysis is indication):

1. Mandibular arch and related tissues (also known as first branchial arch) are first portions of the face to form after creation of stomodeum.

2. Directly forms lower face, including lower lip.

3. Will also form mandible, mandibular teeth, associated tissues, as well as tongue.

B. Meckel’s cartilage forms within each side of mandibular arch, IMPORTANT in alveolar bone development.

1. Makes contribution to mandible, and portion of cartilage participates in formation of middle ear bones.

2. Part of perichondrium surrounding Meckel’s cartilage becomes ligaments of the jaws and middle ear.

C. Mesoderm of mandibular arch forms muscles of mastication, as well as some palatal muscles and suprahyoid muscles. Because these muscles are derived from mandibular arch, they are innervated by nerve of the first arch, fifth (V) cranial nerve (trigeminal).

Frontonasal Process and Upper Face Formation

During fourth week, frontonasal process is a bulge of tissue in upper facial area, at most cephalic end of embryo, which is also the cranial boundary of the stomodeum. In the future, frontonasal process will form upper face, which includes forehead, bridge of nose, primary palate, nasal septum, and all structures related to medial nasal processes (Figure 2-1).

A. Placodes become submerged, forming depression in center of each:

1. Form nasal (olfactory) pits, which develop into nasal cavities.

2. Deepening of nasal pits produces a nasal sac that grows internally toward developing brain; separated from the stomodeum by oronasal membrane.

B. On outer portion of nasal pits are two crescent-shaped swellings, lateral nasal processes that form the alae (sides of the nose).

1. Middle portion of the tissue growing around nasal placodes appears as medial nasal processes.

2. BOTH fuse together externally to form the middle portion of the nose from root to apex, center portion of upper lip, philtrum region.

C. BOTH paired medial nasal processes also fuse internally and grow inferiorly on inside of stomodeum:

1. Form intermaxillary (premaxillary) segment, MOST anterior portion of the tissues.

2. Involved in formation of maxillary incisor teeth and associated tissues, primary palate, nasal septum.

Branchial Apparatus Formation

Branchial apparatus consists of the branchial arches, branchial grooves and membranes, pharyngeal pouches.

A. Arches are six pairs of U-shaped stacked bilateral swellings of tissue appearing inferior to the stomodeum and include the mandibular arch (FIRST branchial arch), which will form the lower face; each arch will form into different structures.

1. Each arch contains cartilaginous core, aortic arch, definite cranial nerve; each cranial nerve will supply the structures that develop from the mesenchyme of the arch.

2. Second (hyoid) arch enlarges and grows so that by the sixth week it will overlap the third, fourth, and sixth arches and cover them.

3. Tracts of epithelial cells can remain after development of the second pharyngeal arch; on occasion, a fistula opens to both the external surface and the pharynx, connecting these structures and becoming a complete branchial fistula.

4. However, a fistula can open externally on the anterolateral surface of the neck or internally to the pharynx by way of a ruptured pharyngeal membrane and become an incomplete branchial fistula.

B. Between neighboring branchial arches, branchial (pharyngeal) grooves are noted on each side of the embryo; FIRST groove forms the external auditory meatus and mesenchyme of first and second arches, which are located on either side of this pharyngeal groove, and will also give rise to the external ear.

1. Four well-defined pairs of pharyngeal pouches develop as endodermal evaginations form the lateral walls lining the pharynx and form different structures.

2. Space between the second arch and the other three arches is called the cervical sinus and is lined by ectoderm; sinus can undergo enlargement at a later time and form cervical cysts.

Palatal Development

Formation of the palate starts in the embryo during fifth week, takes place during several weeks of prenatal development, and is completed in twelfth week in the fetus (Figure 2-2). Formed from two separate embryonic structures, primary palate and secondary palate. Palate is developed in three consecutive stages: formation of primary palate, formation of secondary palate, completion of final (definitive) palate.

A. Intermaxillary segment forms primary (primitive) palate, a triangular mass:

1. In the future, primary palate will form premaxillary portion of the maxilla, anterior one third of final (definitive) palate.

2. Consists of small portion of the hard palate, anterior to incisive foramen; contains maxillary incisor teeth.

B. During sixth week of prenatal development (within embryonic period), bilateral maxillary processes form two palatal shelves (lateral palatine processes), BOTH fusing together to form secondary palate (fusing from anterior to posterior):

1. Secondary palate will form MOST posterior two thirds of hard palate, contains maxillary canines and posteriors, posterior to incisive foramen as well as soft palate and its uvula (fusion line indicated by median palatine raphe).

2. If the palatal shelves do NOT fuse with the primary palate (or with each other), cleft palate can result, with varying degrees of disability (see next discussion).

Figure 2-2 Development of palate.

(From Bath-Balogh M, Fehrenbach MJ: Illustrated dental embryology, histology, and anatomy, ed 2, St. Louis, 2006, Saunders/Elsevier.)

Patient with Cleft Palate and/or Cleft Lip

Cleft palate and/or cleft lip are craniofacial deformities that occur during fourth to twelfth weeks of prenatal development. Etiology is unknown; may involve genetics and exposure to environmental factors such as drugs or toxins (tobacco use, retinoic acid analogues, alcohol, anticonvulsants) during early prenatal development. MOST common class of congenital malformations in United States.

A. Cleft lip: failure of fusion of maxillary processes with medial nasal process:

1. Located at side of upper lip; unilateral or bilateral; may range from LEAST severe case, notch in vermilion border of upper lip (incomplete cleft), to MORE severe cases (complete cleft) that extend into floor of the nostril and through alveolar process of maxilla.

2. Cleft lip, with or without cleft palate, occurs in about 1 in 1000 live births.

3. Cleft lip is MORE common and MORE severe in males and is MORE frequently unilateral, occurring on left side.

B. Cleft palate: failure of fusion of palatal shelves with primary palate (or with each other):

1. Cleft uvula (bifid uvula) is the LEAST severe form.

2. Occurs, with or without cleft lip, in 1 in 2500 live births.

3. Isolated forms are LESS common than cleft lip and are MORE common in females.

C. Surgical intervention:

1. Plastic surgery: lip closure (adhesion) is performed before third month of life; palatal closure is performed at 1 to 5 years of age.

2. Bone grafting, orthodontics, and speech and hearing evaluation and treatment occur at 6 to 15 years of age.

3. Final plastic surgical procedures and replacement of teeth occur after age of 15, when physical growth is complete.

4. Medical anomalies such as heart, ear, skeletal, and genitourinary tract deformities may also occur.

D. Oral signs: increased risk of oral infection (including periodontal disease, dental caries) from malpositioning of the teeth, wearing of a full or partial denture to replace teeth or use of an obturator (dental appliance covering clefted area), mouth breathing, associated oral deformities; all complications also make oral hygiene procedures MORE difficult.

E. Risk factors for oral health: upper respiratory and middle ear infections and inadequate nourishment (before completion of surgical procedures), since cleft complicates nursing and/or feeding.

F. Barriers to care:

1. Economic issues; multiple oral and facial surgeries and care by professionals from different disciplines are required to correct defect and associated conditions.

2. Difficult communication because of inadequate speech production, hearing loss related to defect, self-consciousness.

G. Professional and homecare:

1. Frequent oral prophylaxis (every 3 or 4 months) to reduce risk of infection.

2. When premaxilla is unfixed or immediately after surgical procedures, fulcruming during instrumentation at the site SHOULD be avoided or limited.

3. Fluoride and calcium products to reduce incidence of caries.

H. Patient or caregiver education:

1. Supervision and/or performance of oral hygiene procedures, depending on age and abilities.

2. Care of dental appliance or prosthesis similar to care of denture; removal after meals to cleanse thoroughly and reduce halitosis (see Chapter 15, Dental Biomaterials).

3. Risk for infection and need for excellent daily homecare, frequent professional oral prophylaxis and examination; care to reduce periodontal disease or caries.

I. Commissural lip pits (dimple like) can occur at one or both commissures and are hereditary.

CLINICAL STUDY

Age	21 YRS	SCENARIO
Sex	Male Female	Upon extraoral examination during an initial appointment, the dental hygienist notes that there is a notch in the upper lip on the left side of the patient. When asked about it, the patient says it has been there since he was born. Nothing else is noted.
Height	5’8”
Weight	150 LBS
BP	115/75
Chief Complaint	None
Medical History	Hay fever
Current Medications	OTC allergy medications prn
Social History	College student in engineering

1. Why is there a notch noted the patient’s upper lip?

2. How did this occur in the patient?

3. How will this affect the patient’s treatment?

1. Notch in the vermilion border of upper lip may be an incomplete cleft of the lip.

2. Each maxillary process fuses with each medial nasal process to contribute to the sides of upper lip, and two medial nasal processes contribute to middle of upper lip. Failure of fusion of maxillary processes with medial nasal process can result in cleft lip. Clefts of the lip are located at one or both sides of upper lip; they are unilateral or bilateral and may range from notch in vermilion border of upper lip (incomplete cleft) to more severe cases (complete cleft) that extend into floor of nostril and through alveolar process of maxilla; sometimes associated with cleft palate. Cleft lip, with or without cleft palate, occurs in about 1 in 1000 live births.

3. Since it is only notch in the vermilion border of the lip, this will not affect treatment. However, the dental hygienist should ask if the patient has any surgical repair history or if any complications have resulted from the notch. Consideration should be given to the way the question is asked, since the patient may be sensitive about this facial difference. The dental hygienist may also ask if she can answer any questions about the disorder and note the presence of the notch in the chart. Cleft lip is more common and more severe in males, is more frequently unilateral, and occurs mostly on left side. If severe, it can complicate nursing and/or feeding, speech development, appearance, and oronasal infection levels. Treatment includes plastic surgery with dental intervention and speech and hearing therapy.

TOOTH DEVELOPMENT

Child’s primary (deciduous) dentition develops during prenatal period and consists of 20 teeth, which erupt and are later shed (lost). As primary teeth are shed and jaws grow and mature, permanent dentition, consisting of as many as 32 teeth, gradually erupts and replaces primary dentition (Figure 2-3). Primary dentition begins on average with eruption of the mandibular central incisor at 6 to 10 months and is completed with eruption of the maxillary second molar at 25 to 33 months. Permanent dentition begins on average with the eruption of the mandibular first molar or central incisor at 6 to 7 years and is completed with the eruption of the third molars at 17 to 21 years. Discussion of tooth development will center first on the primary dentition and then include the permanent dentition.

• See Chapters 4, Head, Neck, and Dental Anatomy: dentitions; 6, General and Oral Pathology: developmental dental disorders.

A. Initiation stage:

1. FIRST stage for primary dentition begins between the sixth and seventh weeks (during embryonic period).

2. Stomodeum (primitive mouth) is lined by ectoderm, with outer portion of ectoderm forming oral epithelium.

3. Deep to this is ectomesenchyme, influenced by neural crest cells; basement membrane separates the two tissues, future site of dentinoenamel junction (DEJ).

4. During seventh week, oral epithelium grows deeper into the ectomesenchyme and is induced to produce the dental lamina for the formation of the succedaneous teeth (all teeth EXCEPT permanent molars).

B. Bud stage:

1. Second stage occurs at eighth week (during embryonic period).

2. Growth of dental lamina into buds penetrating into ectomesenchyme; each will develop into a tooth germ.

C. Cap stage:

1. Third stage occurs between ninth and tenth weeks (during fetal period).

2. Differentiation of the tooth bud’s dental lamina into the enamel organ, which will produce enamel, an ectodermal product, in the future.

a. Enamel organ covers dental papilla from the ectomesenchyme.

b. Dental papilla will produce future dentin and pulp tissue, which have mesenchymal origin.

c. Remaining ectomesenchyme surrounding enamel organ (outside of the cap) forms into dental sac (follicle), which will produce periodontium, cementum, periodontal ligament (PDL), alveolar bone; all of mesenchymal origin.

3. Together, enamel organ, dental papilla, and dental sac form the tooth germ (tooth primordium), giving the tooth bud its overall shape.

4. Extension of dental lamina into ectomesenchyme lingual to developing primary tooth germs forms the successional dental lamina for the succedaneous permanent teeth; for nonsuccedaneous permanent molars, which have NO primary predecessors, these teeth will form from a posterior extension of the dental lamina.

D. Bell stage:

1. Fourth stage occurs between eleventh and twelfth weeks (during fetal period).

2. Stage marked by extensive proliferation, with differentiation of the enamel organ into four layers:

a. Outer and inner enamel epithelium surrounds inner cells; inner cells become preameloblasts.

b. Stellate reticulum and stratum intermedium are located between the two outer and inner layers; provide support for ameloblasts.

E. Apposition stage:

1. Enamel, dentin, cementum are secreted in successive layers.

a. Initially secreted as a matrix, extracellular substance that is partially calcified yet serves as a framework for later calcification:

(1) Preameloblasts repolarize and differentiate into ameloblasts and then induce dental papilla cells to repolarize and become odontoblasts.

(2) Thus odontoblasts produce predentin before ameloblasts produce enamel from Tomes’ process.

b. With enamel matrix in contact with predentin, mineralization of disintegrating basement membrane now occurs, forming DEJ.

2. Structure responsible for root development is the cervical loop, most cervical portion of enamel organ:

a. Bilayer rim consisting of ONLY inner and outer enamel epithelium.

b. Grows deeper into the dental sac to become Hertwig’s epithelial root sheath (HERS):

(1) Function of HERS is to shape the root(s).

(2) Also induces dentin formation in root area so that it is continuous with coronal dentin, as well as cementum on roots overlying the newly formed dentin.

c. After the disintegration of HERS, its cells may become epithelial rests of Malassez, which are located in the mature PDL and may become cystic in the future (periapical cyst formation).

3. Ectomesenchyme from dental sac (follicle) begins to form PDL, adjacent to newly formed cementum.

4. After enamel apposition ceases in crown area, ameloblasts place an acellular dental cuticle on new enamel surface and the enamel organ is compressed, forming reduced enamel epithelium (REE).

5. The REE fuses with oral epithelium lining the oral cavity to allow eruption process for both dentitions.

a. Primary teeth act as guides for the developing permanent teeth; premature loss of primary teeth can have a serious effect on eruption of permanent teeth and their position in the dental arch.

b. To allow removal of the primary for replacement by the permanent tooth, odontoclast resorption of primary teeth (MAINLY root, possibly crown) occurs.

c. Succedaneous permanent tooth erupts into oral cavity in position lingual to roots of shed primary tooth, just as it will develop, EXCEPT permanent maxillary incisors, which move into a MORE facial position for eruption.

d. After a permanent tooth erupts in the oral cavity, its root begins to form in the cervical area after crown formation. Thus prevention of traumatic injury to the permanent teeth before they are fully anchored into the jaws is very important (see Chapter 11, Clinical Treatment).

e. Nasmyth’s membrane: residue of the eruption process that may form on newly erupted teeth of BOTH dentitions; leaves teeth with extrinsic green stain that is MAINLY removed by first brushing.

F. Maturation: reached when the dental tissues mineralize into their mature hard tissues.

Figure 2-3 Tooth development.

(From Fehrenbach MJ, ed: Dental anatomy coloring book, St. Louis, 2008, Saunders/Elsevier.)

GENERAL HISTOLOGY

Histology is the study of the structure, composition, and function of tissues that form organs and then into systems. Tissues consist of a group of similar cells that combine in a characteristic pattern and perform a specific function. In addition to cells, intercellular substance and tissue fluid tissues are found in tissues.

• See CD-ROM for Chapter Terms and WebLinks.

Cell Structure

A cell is the smallest unit of organization in the body. Cell structure includes cell components, such as organelles, plasma, and cytoskeletal elements. All are IMPORTANT in cell function (Figure 2-4).

• See Chapter 3, Anatomy, Biochemistry, and Physiology: cell physiology.

A. Plasma membrane: surrounds each cell; selectively allows movement of water and other substances into and out of cells and is involved in cell-to-cell recognition:

1. Fluid mosaic model is the current working model of a membrane.

2. Contains proteins that consist of channels, pumps, and transport systems; these regulate composition of the intracellular fluid.

3. Components: phospholipids, which are polar and have charged heads; lipids, which are hydrophobic; cholesterol, which affects membrane fluidity; proteins, such as channels, enzymes, receptors, carbohydrates.

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