Craniofacial Growth and Development

A better understanding of growth and development can create an easy path in gaining superior knowledge of diseases and abnormal developmental processes. In some cases, early detection of abnormalities can prevent complex treatments. This chapter provides a brief summary of craniofacial growth and tooth development.

What is Growth?

Over the years, there have been several definitions and justifications of growth. Some refer to growth as an increase in the size or number or changes in the amount of living substance occurring in a process of development.

Human growth refers to numerous sequential, developmental stages involving tissue and cell differentiations for formation of various organs and systems. Development is an evolutionary process from initiation to maturation. However, growth and development are not always about an increase in size or multiplication of cells. In some cases, certain cells and tissues must differentiate, change or decrease in size or number for other cells to form completely without abnormalities; for example, if remnant cells (remaining cells) persist during development and differentiation, they may result in cysts and complicate the growth process.

What Factors Affect Growth?

Several factors can influence growth. These factors can be categorised into two major groups of genetics and environmental. Hereditary factors or genetics play a significant role in regulating growth patterns. Thus, people are distinct because of their unique genetic make‐up and individualised growth and development. Environmental dynamics can modify the outcomes of normal growth patterns depending on the time and type of environmental influence. Human growth consists of two phases of prenatal (before birth) and postnatal (after birth).

Prenatal Developmental Phases

The normal human prenatal development consists of three phases, beginning with fertilisation and proceeding to formation of the three germ layers (ectoderm, mesoderm and endoderm). The first three weeks are considered to be the first prenatal phase. From the fourth prenatal week, the second phase initiates and extends to the eighth week. Any disturbances during the second prenatal phase may lead to various abnormalities and defects, depending on the type and timing of the disruption. This period is vital, as the three germ cells differentiate into several tissues and gives rise to the organs and systems. The prenatal development ends with the fetal phase, from the ninth to the fortieth week. The focus of this chapter is on the first two stages of prenatal development and provides a summary of postnatal craniofacial growth.

First Prenatal Phase (Weeks 1–3)

Embryogenesis is the term given to the process of embryo formation and development (Figure 2.1). An embryo development begins with a zygote formation in a process called fertilisation. Sperm swim through the fallopian tube until they reach the ovum (egg) that has been released from one of the ovaries. Sperm attack the ovum to break through the physical barriers and membranes for its nuclei to fuse with the egg nuclei to form a zygote (Figure 2.2). The zygote travels towards the uterus, down the fallopian tube.

The zygote undergoes several mitotic cell divisions during a process called cleavage and develops into a ball of cells known as the morula. Three days after fertilisation, the morula enters the uterus. The outer cells of the morula undergo compaction within a week and the morula becomes a blastocyst. It consists of a single outer layer called the trophoblast and a fluid‐filled space, the blastocoel (also known as the blastocyst cavity). There are groups of cells inside the trophoblast, called the inner cell mass or embryoblast.

Implantation of the blastocyst occurs as it adheres to the endometrium (the mucous membrane lining of the uterus) by the end of the first week. The inner cell mass gives rise to the embryo by forming a bilaminar disc. The bilaminar disc is composed of two layers:

1. Epiblasts: give rise to the three germ cells.
2. Hypoblasts: form the outer embryonic membrane responsible for protection of the embryo and the supply of nutrition.

During the second week, an amniotic cavity develops and the blastocyst cavity becomes the primary yolk sac. The amniotic cavity is located within the inner cell mass and contains amniotic fluid, which is crucial for the embryo.

Gastrulation is a critical phase that occurs three weeks after fertilisation (Figure 2.3). This process gives rise to the three germ cells (Box 2.1). This stage of embryo development begins with the formation of the primitive streak. The primitive streak is a groove on the midline of the epiblast that occurs due to thickening of the peripheral surfaces. This determines the embryo axis. The bilaminar disc then develops into a trilaminar disc by a series of cell invaginations (cell infolding). The primitive streak widens and cells from the epiblast move towards the hypoblast and eventually replace the hypoblast cells to form the endoderm. The cells that move between the epiblast and the endoderm become the mesoderm. By this stage, the invagination of epiblast cells from the primitive streak ceases. The remaining epiblast cells become the ectoderm and are located above the mesodermal layer.

Second Prenatal Phase (Week 4–8)

Upon formation of the germ layers, a neural plate and a neural tube forms via a folding process called neurulation (Figure 2.4). A flexible rod‐shaped body known as the notochord, located beneath the ectodermal layer, induces ectoderm thickening. The notochord is a mesoderm derivative and induces neural plate formation. The growth of the neural plate begins from the cranial (head of the embryo) to the caudal (tail of the embryo). These regions determine the future brain position at the cranial and the spinal cord towards the caudal. The embryo is termed neurula at this stage of development.

The lateral borders of the neural plate elevate and fold to meet in the midline. This action results in the neural groove development. At the beginning of the fourth prenatal week, the neural fold fuses and forms the neural tube. Neurulation is finalised when the neural folds completely fuse and detach from the ectoderm. The central nervous system ascends from the neural tube. Neural crests are cells that develop surrounding the neural tube and will give rise to the peripheral nervous system.

Four weeks after fertilisation, somites develop in paired bodies from the mesodermal cell layer. Somites are found along the neural tube and eventually give rise to the skeleton and muscles. During these structural changes, the developing heart is pushed beneath the brain and the stomodeum (the future oral cavity) develops as a pit in the midline. The stomodeum is lined with stratified squamous epithelial cells (oral ectoderm) and is separated from the foregut by a buccopharyngeal membrane. This membrane eventually ruptures to establish a connection from the oral cavity to the foregut.

Below the stomodeum, five bars develop from the fourth to seventh prenatal weeks. They develop due to mesoderm thickening and form in the shape of five bars, dorsoventrally to the foregut wall. These are the pharyngeal or branchial arches (Figure 2.5). Only the first two pharyngeal arches have specific names: the mandibular and the hyoid, respectively. Each pharyngeal arch contains muscles, cartilage and blood supply (Table 2.1). The aortic arch blood vessel, leading from the heart to the brain, is crucial to craniofacial development, as it runs through all pharyngeal arches. Pharyngeal pouches separate these bars and give rise to specific structures (Figure 2.6). Ectoderm covers the exterior portion of each arch. The second to fifth arches grow together and create a smooth outer surface. However, the fifth arch disappears soon after development.

During this phase of embryonic development, the craniofacial growth initiates. The growth of the face relies on the initial cartilage formed, which is known as the primary cartilage. The chondrocranium is the term used for the primitive cartilaginous skeletal structures. The floor of the skull is composed of basicranium, known as the cranial base postnatal. Basicranium derives from the mesenchyme and develops during the fourth prenatal week. During early development, a series of cartilaginous tissues begin the cranial base development. These tissues are mainly composed of chondrocranium.

Towards the end of the second prenatal period, the chondrocranium undergoes ossification (new bone formation). Once the primitive structures are established, the face begins to develop downwards and forwards by the fifth prenatal week. During this phase of development, the embryo attains a curved form and grows dramatically in size, and structured organ systems are evident.

The limbs and organ systems continue to grow by the eighth week as the embryo develops human characteristics. Upon completion of the embryonic period, the third prenatal phase begins; this period is termed the fetal phase.

Third Prenatal Phase (Weeks 9–40)

The embryo is termed a fetus from the ninth prenatal week until birth (Figure 2.7). At this stage, the fetus gains human characteristics. Organs begin to function and established connections between the organs and the systems become obvious. As growth continues, dramatic increase in size and weight is noticeable due to continuing tissue differentiation.

Craniofacial Postnatal Development