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CHAPTER II.
THE EMBRYOLOGY OF THE DENTAL TISSUES.
By R. R. Andrews, A. M., D. D. S.
A CLEAR understanding of the minute structure of the teeth can only
be had through a study of the complex processes through which the
tissue elements have had their origin or have derived their forms.
The teeth do not belong to the bony skeleton of the body, but, like
the hair, nails, etc., are parts of the dermal system.
The origin of the tissues of the teeth is from two of the three
germinal layers of the blastoderm, the epiblastic and mesoblastic layers.
A transverse section through the blastoderm of a chick shows that the
epiblast, or outer layer, is formed of cells like columnar epithelium
their shape is probably due to lateral pressure of adjoining cells. It
is from this layer that epithelium is formed, and epithelial tissue is the
origin of the enamel. The mesoblast, or middle layer, is composed of
cells said to be derived from both hypoblast and epiblast, but princi-
pally from the latter.^ They are merely nucleated structures, containing
granules, the nuclei of the future cells of the connective tissues. In this
state they have no cell-limit or wall ; as they grow older they accumu-
late around themselves formed material. Only in maturer stages do
these cells develop, on their surfaces, an optically distinct membrane
or other structure. It is from the cells of the mesoblast that the em-
bryonic connective tissue which forms the dentinal papilla originates.
Development of the Jaws.
As stated by Prof. Sudduth,^ the first indication of the formation of
the oral cavity is seen very early in the life history of the embryo.
The superior maxilla arises from three separate points : on either side
of the embryonic head a process springs from the first pharyngeal arch.
The processes pass downward and forward, and unite with the sides of
the nasal process. From the frontal prominence, the third process, the
incisive, grows downward and fills in the space between the ends of the
two preceding processes. By a union of these three processes the supe-
rior maxillse are completed. The inferior maxilla is formed by buds
growing from the first pharyngeal arch ; these buds grow rapidly until
' The three layers of tlie blastoderm are also designated as ectoderm, mesoderm, and
entoderm respectively.
'^
American System of DentiMry, vol. i. p. 550.
53
CHAPTER II.
THE EMBRYOLOGY OF THE DENTAL TISSUES.
By R. R. Andrews, A. M., D. D. S.
A CLEAR understanding of the minute structure of the teeth can only
be had through a study of the complex processes through which the
tissue elements have had their origin or have derived their forms.
The teeth do not belong to the bony skeleton of the body, but, like
the hair, nails, etc., are parts of the dermal system.
The origin of the tissues of the teeth is from two of the three
germinal layers of the blastoderm, the epiblastic and mesoblastic layers.
A transverse section through the blastoderm of a chick shows that the
epiblast, or outer layer, is formed of cells like columnar epithelium
their shape is probably due to lateral pressure of adjoining cells. It
is from this layer that epithelium is formed, and epithelial tissue is the
origin of the enamel. The mesoblast, or middle layer, is composed of
cells said to be derived from both hypoblast and epiblast, but princi-
pally from the latter.^ They are merely nucleated structures, containing
granules, the nuclei of the future cells of the connective tissues. In this
state they have no cell-limit or wall ; as they grow older they accumu-
late around themselves formed material. Only in maturer stages do
these cells develop, on their surfaces, an optically distinct membrane
or other structure. It is from the cells of the mesoblast that the em-
bryonic connective tissue which forms the dentinal papilla originates.
Development of the Jaws.
As stated by Prof. Sudduth,^ the first indication of the formation of
the oral cavity is seen very early in the life history of the embryo.
The superior maxilla arises from three separate points : on either side
of the embryonic head a process springs from the first pharyngeal arch.
The processes pass downward and forward, and unite with the sides of
the nasal process. From the frontal prominence, the third process, the
incisive, grows downward and fills in the space between the ends of the
two preceding processes. By a union of these three processes the supe-
rior maxillse are completed. The inferior maxilla is formed by buds
growing from the first pharyngeal arch ; these buds grow rapidly until
' The three layers of tlie blastoderm are also designated as ectoderm, mesoderm, and
entoderm respectively.
'^
American System of DentiMry, vol. i. p. 550.
53
