In the morphological stages of tooth development the stages are named according to the shape of the Enamel Organ. In Bud stage it looks like a flower bud, in Cap stage it looks like a cap. The enamel organ then continues to grow and appears like a bell hence called the Bell stage. The Bell stage is further divided into Early Bell Stage and Advanced Bell Stage. The main difference between the two is the absence and presence of mineralization. That is, once enamel and dentin deposition begins, the tooth enters the Advanced Bell Stage. Moreover, Root formation is also seen. So in bell stage morpho-differentiation is seen as the shape of the enamel organ changes from cap to bell. And histo-differentation is seen as cells of the enamel organ continue to show differentiation or specialization. Let us first see the Early Bell Stage. In the Bell Stage the crown shape of tooth is determined. The concave surface or the Inner Enamel Epithelium of the Enamel Organ takes the shape of the crown of the tooth to be formed. In Bud stage 2 types of cells are seen in the enamel organ, then with histodifferentiationin the Cap stage 3 types of cells are seen, which increases to 4 types of cells in the Bell stage. A single layer of cells lining the convex surface of the Cap form the Outer Enamel epithelium which are cuboidal in shape. A single layer of cells lining the concave surface form the Inner Enamel Epithelium, which are tall columnar in shape. These cells later differentiate into Ameloblasts and deposit enamel. The cells in the center form the Stellate Reticulum or Enamel Pulp, which are star shaped. The cells are star shaped because of the glycosaminoglycans released by the cells which attract water into the extracellular space. This water by applying pressure, changes the polygonal shaped cells to star shape. Its function is to act as a shock absorber to protect the Inner Enamel Epithelium from any mechanical trauma. A layer of cells forms in between the Inner Enamel Epithelium and Stellate Reticulum called the Stratum Intermedium, literally meaning the layer in between. The cells are squamous (flattened) in shape. These cells are rich in alkaline phosphatase, show high metabolic activity and are essential for enamel formation. The junction of outer enamel epithelium and inner enamel epithelium is known as the Cervical Loop. It is an area of intense mitotic activity as the size of the Enamel Organ rapidly increases in this direction. Also from here a double cell layered extension develops in the Advanced Bell Stage called the Hertwigs Epithelial Root Sheath. In the Bell Stage, the Dental Lamina which attaches the Enamel Organ to oral ectoderm starts undergoing degeneration. Though some epithelial remnants may remain which are known as cell rests of Serres. A basement membrane seperates the enamel organ from the ectomesenchyme.This basement membrane is called Membrane Preformativa just prior to dentin formation. This was about the epithelial part now coming to the surrounding ectomesenchyme. The condensed ectomesenchyme just within the epithelial concavity is the Pental Papilla. Few fine collagen fibers are seen in the extracellular space. Dental papilla later forms dentin and pulp. A zone of connective tissue with no cells called the Acellular Zone separate the Inner Enamel Epithelium from Dental Papilla. The condensed ectomesenchyme surrounding the Dental Papilla and Enamel Organ is the Dental Sac. Dental sac shows numerous collagen fibers in the extracellular space. Dental sac later forms cementum, periodontal ligament and alveolar bone. Now coming to the advanced bell stage. By the end of early bell stage the cells of the Inner Enamel Epithelium differentiate into Ameloblasts. They are tall columnar and are 40 microns high and 4 microns wide. The nucleus shifts away from the Dental Papilla towards the Stellate Reticulum, this is called as 'Reversal of Polarity'. The differentiation of Inner Enamel Epithlium into Ameloblasts does not take place in the whole epithelium at once but starts at the cuspal tips or incisal tips and then proceeds cervically. Once Inner Enamel Epithelium differentiates into Ameloblastes they send a signal to the Dental Papilla. The ectomesenchymal cells on receiving this signal differentiate into tall columnar Odontoblasts the dentin forming cells. Odontoblasts eliminate the 'Acellular Zone'. They start depositing dentin first, Only after the deposition of 1st layer of dentin enamel deposition is initiated by Ameloblasts. So we see that Ameloblasts differentiate 1st then they induce differentiation of Odontobasts, but the Odontoblasts deposit dentin first. This dentin then induces ameloblasts to deposit enamel. This whole process of cells inducing each another to deposit enamel and dentin is sometimes also called as the 'Reciprocal Induction'. Now till the Early Bell Stage, Inner Enamel Epithelium gets its nutrition from the Dental Papilla. But in Advanced Bell Stage, as enamel dentin deposition begins the nutrition for Ameloblasts stops, as the nutrients are not able to cross the mineralized wall. The Ameloblasts then look for other options for nutrition. The best one is from the Dental Sac near Outer Enamel Epithelium. But the distance between Ameloblasts and Dental Sac is too much for the nutrients to cross. So the Stellate Reticulum undergoes gradual degeneration to reduce this distance, so that the nutrition can reach the Ameloblasts. That is why we see the collapse of Stellate Reticulum in Advanced Bell Stage. The cells of the Outer Enamel Epithelium become low cuboidal in shape, however Stratum intermedium appears same formed of squamous cells. Once enamel and dentin formation reaches the cervical loop or the future cemento-enamel junction the Hertwigs Epithelial Root Sheath develops to initiate root formation.