INTRODUCTION

The process of tooth development – or odontogenesis – is a complex series of reciprocal cellular

interactions, by which teeth form from epithelial and mesenchymal cells in the stomatodeum. Enamel,
dentine, cementum and the periodontium must all develop during appropriate stages of embryonic
development. Primary teeth begin to form between the sixth and eighth weeks of intrauterine (i.u.) life,
and permanent teeth begin to form in the twentieth week. If teeth do not start to develop around those
times, it is likely that they will not develop at all and be missing.

Next slide

The stomatodeum is lined by a primitive epithelium which is two or three cells in thickness. Beneath this
is embryonic connective tissue, the ectomesenchyme (Figure 2.1). The first sign of tooth development
within the stomatodeum is a thickening of the epithelium and this thickening is called the primary
epithelial band. It forms at around 6 weeks of i.u. life and indicates the position of the future dental
arches. The primary epithelial band rapidly divides into two structures, the dental lamina and the
vestibular lamina. The latter ultimately gives rise to the vestibule/sulcus while the former gives rise the
to the tooth germs. At 6 weeks there is no vestibule/sulcus between cheek and tooth-bearing area. The
vestibule forms from proliferation of vestibular lamina into the ectomesenchyme. The vestibular lamina
cells rapidly enlarge, then degenerate leaving a cleft which becomes the vestibule. The dental lamina is
the structure that gives rise to the tooth germs, and proliferation of the dental lamina at 6–7 weeks i.u.
determines the positions of future deciduous teeth with a series of 20 epithelial ingrowths into
ectomesenchyme (10 in each development jaw).

This first incursion of the epithelial dental lamina into the mesenchyme leads to a bud of cells at the
distal aspect of the dental lamina and is called the bud stage of tooth development (Figure 2.2). Each
bud is separated from the ectomesenchyme by a basement membrane. There is little change in shape or
function of the epithelial cells at this time. The supporting ectomesenchymal cells congregate around the
bud, forming a cluster of cells which are closely packed beneath and around the epithelial bud, which is
the initiation of the condensation of the ectomesenchyme. The remaining ectomesenchymal cells are
arranged with less regular order.

Next slide

As tooth development progresses, two key processes become essential to development. The first is
morpho-differentiation, which is the determination of the shape of the crown of the tooth through the
shape of the amelodentinal junction of the forming tooth. The second process is histo-differentiation,
where cells of the developing tooth differentiate (specialise) into morphologically and functionally
distinct groups of cells responsible for secretion of various dental tissues. Control and regulation of this
differentiation is through specific and reciprocal cellular interactions between the
epithelial/mesenchymal compartments.

As the epithelial bud continues to proliferate into the ectomesenchyme, the first signs of an arrangement
of cells in the tooth bud appear in the cap stage. A small group of ectomesenchymal cells stops
producing extracellular substances and do not separate from each other, which results in an aggregation
or condensation of these cells immediately adjacent to the epithelial bud. This is the developing dental
papilla. At this point, the tooth bud grows around the ectomesenchymal aggregation, taking on the
appearance of a cap, and becomes the enamel (or dental) organ. A condensation of ectomesenchymal
cells called the dental follicle surrounds the enamel organ and limits the dental papilla (Figure 2.3). The
enamel organ is responsible for the synthesis and secretion of enamel, the dental papilla will lead to the
formation of the dentine and pulp, and the dental follicle will produce the supporting structures of a
tooth. This explains why enamel is epithelial in origin whereas dentine, pulp and periodontal tissues are
mesenchymally derived. As tooth development proceeds there is a distinct histo- and morpho-
differentation of the enamel organ as it prepares for secretory function, along with an increase in size of
the tooth germ. This change signifies the transition to the early bell stage. The enamel organ takes on a
bell shape during this stage with continued cell proliferation, and histo-differentiation of four distinct cell
layers within the enamel organ can be observed (Figure 2.4).

Figure 2.3 Cap stage of tooth development where the three components of the tooth germ can be
observed. EO, enamel organ; DP, dental papillae; DF, dental follicle

Next slide

A single layer of cubiodal cells at the periphery of the enamel organ limit its size and are known as the
outer enamel epithelium. Conversely, the single cell layer adjacent to the dental papilla is known as inner
enamel epithelium and it is these cells that will differentiate into ameloblasts and give rise to enamel
synthesis and secretion. Where these cells of the inner and outer enamel epithelium meet is termed the
cervical loop. The majority of the cells that are situated between the outer and inner

enamel epithelium are termed the stellate reticulum. These cells secrete hydrophilic glycosaminoglycans
which increase the extracellular space and the cells interconnect through desmosomes giving them a
stellate or star-shaped appearance. A layer two or three cells thick lying next to the inner enamel
epithelium, and having a flattened shape, is termed the stratum intermedium. In summary, the layers of
the enamel organ in order of innermost to outermost consist of inner enamel epithelium, stratum
intermedium, stellate reticulum and outer enamel epithelium. During this stage of development, as it
progresses from cap stage to early bell stage, a localised thickening of cells at the inner enamel
epithelium around the cusp tip appears. This is known as the enamel knot and is a signalling centre of
the tooth that provides positional information for tooth morphogenesis and regulates the growth of
tooth cusps. The enamel knot produces a range of molecular signals from all the major growth factor
families, including fibroblast growth factors (FGF), bone morphogenetic proteins (BMP), Hedgehog (Hh)
and Wnt signals. These molecular signals direct the growth of the surrounding epithelium and
mesenchyme and have putative roles in signalling and regulation of crown development. The enamel
knot is transitory and the primary enamel knot is removed by apoptosis. Later, secondary enamel knots
may appear that regulate the formation of the future cusps of the teeth.

Figure 2.4 Bell stage of tooth development where the four cell layers of the enamel organ can be
observed. SR, stellate reticulum; SI, stratum intermedium; arrow, outer enamel epithelium; arrowhead,
inner enamel epithelium

Next slide

Later tooth development

As tooth development progresses from the early bell stage to a late bell stage of development,
epithelial/mesenchymal interactions signal further histo-differentiation of the four cell layers of the
enamel organ in preparation for amelogenesis. Cell appearance in the enamel organ is directly related to
function. The cells of the outer enamel epithelium are cuboidal with a high nuclear:cytoplasm ratio.
These cells have a non-secretory protective role and will eventually become part of the dentogingival
junction. The stellate reticulum cells sit in a substantial jelly-like extracellular matrix which protects the
interior of a tooth germ. The cells of the inner enamel epithelium have a low columnar appearance with
a central nucleus and few organelles. These cells are at a preparatory stage of becoming secretory, the
ameloblast. The inner enamel epithelial cells are separated from the ectomesenchymal dental papillae
by the dental basement membrane. This structure mediates interactions between the epithelial and
mesenchymal compartments of the tooth germ during development and odontoblast differentiation
prior to dentine secretion. At this time, the dental papillae contains undifferentiated ectomesenchymal
cells with relatively small amounts of extracellular matrix (apart from a few fine collagen fibrils) and
these cells are not yet specialised for secretory function. The late bell stage is also known as the crown
stage of tooth development and further cellular changes occur at this time. In all prior stages of tooth
development, all of the inner enamel epithelium cells were proliferating to contribute to the increase of
the overall size of the tooth germ. However, during the crown stage, cell proliferation stops at the
location corresponding to the sites of the future cusps of the teeth. At the same time, the inner enamel
epithelial cells change in shape from cuboidal to short columnar cells with nuclei polarised to the end of
the cell away from the basement membrane.

Next slide

The adjacent layer of cells on the periphery of the dental papilla increases in size, the cells become
columnar and their nuclei polarise away from the basement membrane as they differentiate into
odontoblasts. These changes to the inner enamel epithelium and the differentiation of odontoblasts
begin at the site of the future cusp tips and the odontoblasts secrete an organic collagen-rich matrix
called predentine, towards the basement membrane. As the odontoblasts secrete pre-dentine, they
retreat and migrate toward the centre of the dental papilla. Cytoplasmic extensions are left behind as
the odontoblasts move inward, creating a unique, tubular microscopic appearance of dentine as pre-
dentine is secreted around these extensions. After dentine formation begins, the dental basement
membrane breaks down and the short columnar cells of the inner enamel epithelium come into contact
with the pre-dentine, terminally differentiate into ameloblasts and begin to secrete an organicrich matrix
against the dentine. This matrix is partially mineralised and will mature to become the enamel. Whereas
dentine formation proceeds in a pulpal direction, enamel formation moves outwards, adding new
material to the outer surface of the developing tooth.

Next slide

During this stage of tooth development, the tooth germ loses attachment to oral epithelium as it
becomes encased in bone of developing jaws. The dental lamina begins to disintegrate into discrete
islands of cells known as the Glands of Serres. Most of these degenerate but some remain quiescent in
jaw bone; if stimulated later in life they may form odontogenic cysts known as ‘odontogenic keratocysts’.
The vascular supply enters dental papilla during the cap stage of development and increases during the
bell stage during hard tissue formation. The vasculature enters the dental papilla around sites of future
root formation. The pioneer nerve fibres approach the developing tooth germ during the bud/cap stage
but do not penetrate dental papilla until dentine formation begins.
Formation of the permanent dentition arises from a proliferation and extension of the dental lamina.
The permanent incisor, canine and premolar germs arise from proliferation on the lingual aspect of the
dental lamina next to their deciduous predecessors. The permanent molars have no deciduous
predecessors and develop from backward extension of the dental lamina which gives off epithelial
ingrowths giving rise to the first, second and third permanent molars.