Atlas of Human Fetal Jaw Development

Lawrence Freilich, David Hunt

• Publisher: Quintessence Publishing Co, Inc
• Year: 2019
• ISBN:
  9780867157161
• Topic: Medicine
• Subtopic: Dentistry

This atlas presents a study of human fetal jaw development with a primary focus on the hard tissues of the maxilla and mandible. High-definition photographs are featured throughout of the well-preserved upper and lower jaw elements from the human fetal skeletons contained in the collections of the Smithsonian National Museum of Natural History.

CHAPTER 1

Introduction

Project Goals: Documenting Human Fetal Jaw Development by Photographic and Computed Tomographic Illustration

The goal of this project is to present an ordered study of human fetal jaw development, primarily focused on the hard tissues of the maxilla and mandible, in an image-driven format not well presented in the current scientific literature. This is an investigation of the growth and development of the human maxilla and mandible from approximately 4 months in utero until term (circa 10 months) by photographs and descriptive observations of the changes occurring in the bony tissue.

Previous studies

Although there have been numerous studies that have presented the growth and development of the human gnathic region^1–10 (see Sperber et al¹¹ for a recent in-depth volume), the results presented are generally in histologic, radiologic, or drawing formats, and few illustrations are made of dissected and cleared bone. The most relevant comparisons would be in cephalometric radiologic illustrations.^5,8,12,13

In the recent past, a number of studies have employed varying techniques to examine human fetal craniofacial growth and development. Radlanski and colleagues have published several studies using three-dimensional (3D) computed tomography (CT) to obtain craniofacial reconstructions from serial sections of fetal specimens.^14–17 Other recent studies have documented craniofacial growth using fetal bones from frozen whole skulls¹⁸ or dried mandibles.¹⁹ A third popular technique has been the use of 3D ultrasonography to study fetal craniofacial structure.^20–22 Of these studies, McGahan et al²¹ included a new version known as multislice display to visualize both normal facial structure as well as specimens exhibiting both cleft lip and palate.

All of these studies and other comparable works provide valuable data on fetal craniofacial development. However, these publications have examined sequential human fetal jaw development in soft tissue specimens rather than by examining the dry bony tissue. This study focuses on the paired maxillae and mandibles from specimens of a single ancestry and presents the observations of change by means of high-definition photography. Well-preserved maxillary and mandibular jaw elements from human fetal skeletons contained in the collections of the Smithsonian Institution’s National Museum of Natural History (NMNH) Department of Anthropology were used in this study. The Smithsonian Collection constitutes one of the largest documented human fetal skeleton collections in any institution worldwide, containing well over 300 individual fetal skeletons obtained in the early 20th century from the Baltimore, Maryland, and Washington, DC, regions. There are roughly an equal number of specimens representing both Caucasian and African American ancestry and nearly equal numbers of males and females within each ancestral group.

Project phase I

In this study human fetal jaw development is examined and documented in three phases. First, the maxillae and mandibles of each of the examined individuals are photographed in close-up using a Nikon digital camera with a 60-mm macro-Nikkor lens (Nikon). Several aspects of each jaw element were recorded, including lateral, medial, and anterior views. At these early developmental stages, the right and left halves of the maxilla and mandible are not yet fused into single bones. Photographs of the individual halves and, in some instances, the articulated right and left maxillae and mandibles are presented. All photographic studies were conducted within the Anthropology Department of the NMNH with assistance from Dr Bruno Frohlich.

To control for possible effects of sex and ancestry differences, all the individuals examined were African American male fetal skeletons from the collection. The African American male group has the largest number represented in the collection and was selected solely on the fact that there would be a greater number of individuals to choose from for the best-preserved individuals in the collection. Within this group, the individuals were divided into three distinct growth stages between the ages of 3½ months in utero to term. Fetal age was established by measurement of femoral length in millimeters (from 10 to 20 mm at 3½ months to approximately 80 to 90 mm at term). Those specimens examined represent the approximate midpoint of femur length within each age group. Measurement of the maxilla and mandible were the maximum anteroposterior length, measured in millimeters. In the mandible the anteroposterior length was measured from the most posterior edge of the condylar process to the most anterior edge at the mental symphyseal protuberance. For the maxilla, the anteroposterior length was measured from the most posterior edge of the maxilla (usually either the posterior rim of the alveolus or the zygomatic process) to the most anterior edge of the maxilla (usually the anterior nasal spine or the palatine suture at the incisive midline).

Project phase II

The second phase of this study puts the developmental stages of the maxilla and mandible in perspective relating to the overall morphologic and physiologic changes in the gnathic region and the overall changes in the skull. The photographic illustration of eight mandibles in sequential order is displayed in chapter 4 to help evaluate the overall changes in structure. In addition, eight whole prepared skulls of known age are presented in chapter 5 to put in perspective the changes of the mouth region relative to the rest of the skull. This provides a comprehensive view of the gnathic changes accompanying overall cranial development. In addition, to associate the stages of cranial development with the overall growth and development of the entire fetal skeleton, chapter 6 provides photographic illustrations of three complete skeletons from individuals aged at 3½, 7, and 10 lunar months. These images are meant to exemplify the developmental changes that are taking place in the rest of the fetal skeletal system at these distinct ages.

Project phase III

The third phase of this study examines the internal structure of selected jaws using a special CT scanning method. In the recent past, several studies have used cone beam CT (CBCT) and standard CT scanning on fetal craniofacial structures^14,23–25 to produce 3D radiographic images or 3D surface renderings of fetal structures. These studies used full fetal heads for their reproduction, but for this study the sectional data and 3D surface renderings have been performed on dry bone specimens. To accomplish this phase of the study, the collaboration of personnel at the Naval Postgraduate Dental School of the National Naval Medical Center in Bethesda, Maryland, is gratefully acknowledged. The maxilla and mandible of each specimen were subjected to a CBCT scan and subsequent image production for analysis. These scans offer a variety of structural information about the osseous tissues, including an external 3D view of each bone as well as high-definition radiographic images at resolutions of up to 0.1 mm. This represents a significantly higher resolution than can be produced by conventional CT scanning. However, the output of the cone beam does have its limitations in the density values and metric interpretation (see studies such as Lee et al,²⁶ Weitz et al,²⁷ and Abboud et al²⁸). The scans were conducted using an Iluma CBCT system with Iluma 3D Vision software (Imtec). Additional studies were conducted on Kodak K9000 and K9500 CBCT scanners. Settings on the scanners varied from 90 kV on the Iluma to 63 or 65 kV and 3.0 or 4.0 mA on the K9000 and K9500.

Background on the Fetal Collections Housed at the NMNH

Prominent collectors and donors

When Aleš Hrdlička first arrived in 1903 at the United States National Museum—which was later identified as the NMNH—he corresponded and interacted with many of the anatomists and scholars at medical research and training institutions both nationally and internationally. However, he was most active in communicating with colleagues in Washington, DC, and the surrounding metropolitan areas. During the first 20 years of his tenure, Hrdlička received donations from, or made exchanges with, these medical scholars who closely associated themselves with him. With regard to the fetal collection, all but two of the...

Table of contents

• Cover
• Half Title Page
• Title Page
• Contents
• Preface
• 1. Introduction
• 2. Developmental Origins of the Maxilla and Mandible
• 3. Photographic Study of the Maxilla and Mandible: Three Ascending Age Specimens
• 4. Mandibles of Eight Ascending Age Groups
• 5. Whole Skulls of Eight Ascending Age Groups
• 6. Whole Skeletons of Three Ascending Age Groups
• 7. CBCT Scans: Maxilla, Mandible, and Whole Skull