Human, frontal section (10th week). This light micrograph shows several areas of tooth development. Note that there has been a down growth of the oral epithelium into the underlying mesenchyme

(black arrows)

. The two mandibular tooth buds are probably early bud stage, but the maxillary tooth bud may already be at early cap stage. Mandibular bone

(white arrows)

can be seen lateral to Meckel's cartilages

(m)

, and the palatal shelves are fusing with the nasal septum

(ns)

.

This light micrograph shows the bud stage of tooth development. Note that there has been a down growth of the oral epithelium

(e)

into the underlying mesenchyme

(m)

, and the epithelium is separated from the connective tissue by a distinct basement membrane. Inferior to the epithelial bud there appears to be the begining "condensation" of mesenchymal cells

(arrow)

, which will eventual become the dental papilla and the dental follicle. None of the distinctive characteristics of the dental organ (cap stage) have yet appeared, i.e. there is no recognizable inner and outer dental epithelia.

Pig mandible, sagittal section. This light micrograph shows a tooth bud at cap stage. Note Meckel's cartilage

(m)

and mandibular bone (pink) formation. The tooth bud appears to be connected to the labial lamina

(l)

, but that is only an artifcat due to the plane of sectioning. Recall that the labial lamina was the secon division of the primary epithelial thickening. Cells in the center of the labial lamina will die and thus free the lip from the alveolar ridge.

• Pig mandible, sagittal section. This light micrograph shows a tooth bud at cap stage. Note Meckel's cartilage

(m)

and mandibular bone (pink) formation. The tooth bud appears to be connected to the labial lamina

(l)

, but that is only an artifcat due to the plane of sectioning. Recall that the labial lamina was the secon division of the primary epithelial thickening. Cells in the center of the labial lamina will die and thus free the lip from the alveolar ridge.

This is a light micrograph of another cap stage tooth bud. This stage is marked by proliferation of the epithelial cells of the dental organ and the appearance of three distinct layers: the inner dental epithelium, the outer dental epithelium and the stellate reticulum. The dental follicle is not very distinct, but the dental papilla is a dense collection of mesenchymal cells underlying the inner dental epithelium

(i)

. The other portions of the dental organ include the outer dental epithelium

(o)

and the stellate reticulum

(sr)

, which is just begining to develop. The junction between the inner and outer enamel epithelia is called the cervical loop

(arrows)

.

• This is a higher power micrograph of a cap stage tooth bud showing the dental papilla

(dp)

, the inner

(i)

and outer

(o)

dental epithelia and the stellate reticulum

(sr)

. The inner dental epithelial cells will eventually differentiate into ameloblasts and a layer of cells called the stratum intermedium.

• Human fetus, sagittal section (16 weeks). Two incisor tooth buds

(arrows)

are shown in this section. Each shows the characteristic features of bell stage. Morphodifferentiation has occured, due to differential proliferation of the inner enamel epithelium, and this has resulted in the formation of the shape of the tooth. The outline formed by the inner enamel epithelium represents the future dentoenamel junction and not the surface topography of the fully formed crown. Other landmarks include the hard palate

(H)

, the frontal

(F)

and vomer

(V)

bones, the nasal capsule

(NC)

, the vertical plate of the ethmoid

(NS)

and the mandible

(M)

.

• At this higher magnification you can more clearly see the characteristics of the bell stage, including the dental follicle

(df)

and outer enamel (dental) epithelium

(oee)

succedaneous toothbud . The inner enamel epithelium called the stratum intermedium. During bell stage the shape of the tooth is being determined by the differential proliferation of inner enamel epithelial cells. The cervical loop margin of the crown. In this micrograph you can also see the

(*) (cl)

that is now at cap stage.

(iee)

has differentiated into two different cell types. The the flattened layer of cells between the inner enamel epithelium and the stellate reticulum

(sr)

represents the future cervical is

• At this higher magnification you can more clearly see the characteristics of the bell stage, including the dental follicle

(df)

and outer enamel (dental) epithelium

(oee)

. The inner enamel epithelium The the flattened layer of cells between the inner enamel epithelium and the stellate reticulum

(sr) (iee)

has differentiated into two different cell types. is called the stratum intermedium. During bell stage the shape of the tooth is being determined by the differential proliferation of inner enamel epithelial cells. The cervical loop

(cl)

represents the future cervical margin of the crown. In this micrograph you can also see the succedaneous toothbud

(*)

that is now at cap stage.

• Toothbud, crown stage,apposition of enamel and dentin. At this stage cytodifferentiation has occured at the cusp tip, with the development of odontoblasts

(o)

and ameloblasts

(a)

. The formation of dentin (d, dark pink) initiates the terminal differentiation of the ameloblasts and thereby enamel matrix (e, dark purple) formation. The lighter pink material is unmineralized predentin. In this specimen it is possible to see all of the phases of cytodifferentiation by following changes in the inner enamel epithelium and dental papilla

(si)

.

(dp)

from the cervical loop at the tips of the epithelial diaphragm (arrows) toward the cusp tip. Also note the stellate reticulum

(sr)

has collapsed near the cusp tip bringing the dental follicle cells into close proximity with the ameloblasts and straatum intermedium

• Toothbud, late bell stage at the beginning of the appositional phase (crown stage). At this stage the toothbud is no longer connected to the oral epithelium. The dental follicle

(f)

surrounds the enamel organ, and the dental papilla

(blue arrow) (dp

) is completely enclosed within the "bell" of the enamel organ. Blood vessels have penetrated the dental papilla, and are visible in the follicular connective tissue. At the cuspal tip, the formation of dentin and enamel

(red arrow)

has begun.

• Toothbud, crown stage. This is a higher magification view of the begining of enamel and dentin apposition. The stellate reticulum

(sr

), stratum intermedium

(si)

, preameloblasts of the enamel organ, and the odontoblasts

(o)

in the dental papilla

(dp)

are clearly differentiated. At this stage the odontoblasts have begun to form unmineralized dentin matrix (red) which is just beginning to mineralize (black) at the cusp tip. Very quickly the preameloblasts will undergo their final differentiation to become secvretory ameloblasts.

• At this point the ameloblasts have completely differentiated at the cusp tip. The stellate reticulum

(sr)

, stratum intermedium

(si)

, preameloblasts and odontoblasts

(o)

are also clearly visible. In this micrograph the unmineralized predentin matrix

(p)

stains a light pink and dentin

(d)

dark pink. Enamel matrix is the thin line of dark purple material at the cusp tip. If you look carefully on the left slope of the cusp tip, you can see the indentations of Tomes' processes in the newly formed enamel matrix.