Tooth development is a very complex process with various moving parts. Commonly it is studied under the heading of morphological stages of tooth development. That is, we study the process of tooth development under the stages of changes in the morphology, that is, the shape and structure of enamel organ. Here we will study the same procedure of tooth development but under the heading of physiological stages. The physiological stages of tooth development explains the process of tooth development not on the basis of the shape of the enamel organ but on the basis of the physiological procedures that take place in the tooth germ. Physiology can be described as a study of mechanism of function of any body tissue. Therefore, here we study the tooth development by understanding the various mechanisms the cells of the tooth germ undertake for the formation of the tooth. The stages are -initiation proliferation histo-differentiation morpho-differentiation and apposition. One characteristic of these stages is that there is overlapping of stages. Like in morphological stages, the tooth germ once finishes one stage then only enters the next stage, there is no overlap but in physiological stages for example, the tooth germ in Early Bell stage is going through various physiological stages at once. For better understanding of the physiological stages we will keep referring to the morphological stages. The first stage, Initiation. Stage of initiation is the shortest stage and is signified by the beginning of tooth development. The beginning of tooth development is considered to be associated with dental lamina. What is dental lamina? It is the horseshoe shaped extension of the oral ectoderm in the jaws. At some specific places the cells form dental placodes. this initiation requires the interaction of epithelium and ectomesenchyme. Next, proliferation. This stage runs the longest during tooth development. As for any organ formation proliferation, or increase in the number of cells, is an essential requirement. Proliferation starts right after initiation and continues, which causes the enamel organ to grow in size. It stops only after the differentiation of Ameloblasts and Odontoblasts as these cells lose their ability to multiply once fully matured. So if we compare it with the morphological stages of tooth development proliferation starts in the Bud stage, continues in Cap stage, Early bell stage and stops in Advanced bell stage with differentiation of Ameloblasts or Odontoblasts. Histo-differentiation. Histo-differentiation means that immature cells undergo the process of maturation so they can carry out their specific functions. The Inner Enamel Epithelium has to differentiate to Ameloblasts which will deposit enamel. Its histo-differentiation begins in the cap stage. The Inner Enamel Epithelium start as cuboidal cells then they become short columnar to columnar to tall columnar cells then show reversal of polarity and development of Tome's processes, thus maturing into fully differentiated Ameloblasts. The cells of the Dental papilla also differentiate into Odontoblasts. Not only Ameloblast and Odontoblasts, other cells also show differentiation. In the Bud stage the central cells transform into star-shaped Stellate Reticulum for protection of the Inner Enamel Epithelium. In Early Bell Stage the Stratum Intermedium cells develop which help in enamel deposition. So histo-differentiation starts in the Cap stage, continues in Early Bell Stage, Advanced Bell Stage with differentiation of Ameloblasts and Odontoblasts. Morpho-differentiation. Morpho-differentiation means the changing shape of the enamel organ so as to attain the shape of the tooth that is to be formed. In this stage the final morphology of the tooth is decided. This stage starts when the enamel organ reaches the Cap stage and the Inner Enamel Epithelium starts taking the shape of the crown that is to be formed. For example, here we see enamel organs of different teeth all reaching the cap stage, but the shape of the inner enamel epithelium is different according to the tooth that is to be formed. As they continue to grow in size, the shape of the Inner Enamel Epithelium keeps transforming till the Advanced Bell Stage, where then the deposition of enamel and dentin takes place. Similarly, the shape of root is decided by the Hertwig's Epithelial Root Sheath. Finally, apposition. In this stage there is deposition of hard tissues of the tooth. Appositional growth is an addictive growth characterized by regular and rhythmic deposition of tissue. Ameloblasts and Odontoblasts start deposition of enamel and dentin respectively. There are periods of activity alternating with periods of rests which give rise to the incremental lines in enamel and incremental lines in dentin. So that was about the physiological stages of tooth development where except for initiation there is a lot of overlap in the physiological processes. For example, as the enamel organ is passing through the cap stage and early bell stage it is carrying out the physiological processes of proliferation, histo-differentiation and morpho-differentiation simultaneously. In the advanced bell stage the cells at the cusp tip are fully differentiated and so start apposition the cells cervically show histo-differentiation and morpho-differentiation and the cells at the cervical loop show proliferation. you