Development of Tooth – Part 2 : Early and Late Bell stage

In the previous video, we talked about
the developmental process of the tooth or odontogenesis divided into three main stages namely the bud, the cap and the bell stage. These stages were named according to the shape of the enamel organ it assumed in
each stage. In the bud stage the enamel organ assumed the shape of a bud hence called bud stage. In the cap stage the enamel organ assumed the shape of a cap sitting on a condensed ball of ectomesenchyme hence called cap stage. As the enamel Organ grows further and enters into the bell stage, certain folds are
created by the inner enamel epithelium and due to these unfoldings the enamel
organ assumes the shape of a bell that’s why this stage is called Bell stage. As we have discussed about the bud and the cap stage in the previous video, in today’s video we will discuss about the bell stage in detail. The bell stage of tooth development is further divided into the early bell stage and the late Bell stage. Let’s begin with the early bell stage. The early Bell stage is characterized by two important events, called the histodifferentiation and
the Morphodifferentiation. In histodifferentiation the cells acquire their distinctive phenotypes. Cells of the dental papilla will differentiate into a odontoblasts with a cytoplasmic extention. and short columnar cells of the inner enamel epithelium will differentiate into long columnar cells of ameloblasts. In morphodifferentiation the crown form is determined, which means that if it’s a molar tooth the enamel organ will assume the shape of a molar crown or if the tooth germ belongs to an incisor the enamel organ assumes the shape of an incisor crown. These infoldings occurs as a result of
differential rates of mitosis and differences in their cell differentiation times. what does this statement means? let’s make a diagrammatic illustration of this concept and make this statement clear and understand that how this morphodifferentiation causes different shapes of the crowns to be formed. To understand this let us take an example of the tooth germ of a molar crown. As the growing tooth germ will be entering into the Bell stage, cells of the inner enamel epithelium located at the Future cusp tips will stop dividing and start differentiating into ameloblasts. Pressure will be exerted on these ameloblasts due to the continuous cell division from the other areas of the enamel organ
which causes these ameloblasts to be pushed into the future cusp tip area and
make an infolding eventually giving rise to a molar tooth. This morphogenesis of th e tooth occurs under the influence of the genes and their signaling molecules and growth factors. Let’s not talk about the changes
occurring at the early bell stage and have a closer look on it. In the early bell stage the cells of the
inner enamel epithelium which were short columnar, now becomes tall columnar and are called preameloblasts. Cells of the outer enamel epithelium have now flattened. The region where the inner and outer
enamel epithelium meet at the rim of the enamel organ is called the cervical loop.
This is the area of intense mitotic activity and forms hertwig’s epithelial root sheath. The cellular processes of the
star-shaped Stallate reticulum becomes even longer, it anastomosis with those of its
adjacent ones. A few layers of squamous cells forms in
between the ameloblasts and stallate reticulum, these cells are called
stratum intermedium. They assist ameloblasts in enamel formation. Under the influence of preameloblasts the peripheral cells of the dental
papilla lying close to the amelobasts differentiates into preodontoblasts and then finally to odontoblasts which will form the dentin of the tooth Clusters of blood vessels are also found
around the tooth germ entering into the dental papilla and the dental follicle. As the enamel organ grows further it
enters into the late or advanced bell stage. The late bell stage is characterized by the beginning of root formation and initiation of mineralization. The enamel organ disconnects from the
oral epithelium at the level of the dental lamina. The cells of the dental lamina disintegrates and its remnants are now called epithelial rests of
seres. In the bell stage of deciduous teeth, the
permanent tooth germ will be in its bud stage. The outer enamel epithelium which had a
smooth surface before is now thrown into folds. Capillary networks are formed within
the concavities of these folds and will provide the rich nutritional supply to
the avascular enamel organ. The preameloblasts has become now the mature ameloblasts. The stallate reticulum is collapsed at
late Bell stage so that the blood supply to the tooth forming cells becomes easier. The preoodontoblastst has now become the mature odontoblasts and it will lay down the first layer of dentin along
the future dentinoenamel junction. The opposition proceeds pulpully and apically. After the first layer of dentin is formed, the enamel formation will proceed coronally and cervically. Befoe the root formation begins, the root sheath turns medially and forms the epithelial diaphragm which is an extension of the inner and outer enamel epithelium. The epithelial diaphragm is constant and
will not grow at its free end and it will eventually become the apical foramen. Let’s recap the order of the events
occurring in the Bell stage. The differentiation of ameloblasts leads to the differentiation of odontoblasts after which the odontoblasts will lay down
the dentinal matrix and over this first layer of dentinal
matrix, ameloblasts will deposit the enamel matrix. If you think this video was helpful give a thumbs up and subscribe my channel you can also ask questions in the comment box and turn on the bell icon for upcoming videos.Thank you for watching

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