Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/253

DIGESTIVE ORGANS 235 the sensitiveness exhibited by the dentine in some states of a tooth is not necessarily due to the passage of nerves into it, but to its connection with the sensitive dentine pulp. Development of the Teeth. In studying the development of the teeth, not only has the mode of formation of the individual teeth to be examined, but the order of succession of the different teeth both in the temporary and permanent series. The teeth are developed in the mucous membrane or gum, which covers the edges of the jaws of the young embryo, and their formation is due to a special differentiation in the arrangement and structure of portions of the epithelial and sub-epithelial tissues of that membrane. The enamel is produced from the epithelium, and the dentine, pulp, and cement from the sub-epithelial connective tissue. The development of the temporary teeth will first be consideied. If a vertical section be made th : ough the mouth of a young human ^rya- Fio. 16. Vertical transverse sec; ion through the mouth of a young human em bryo, np. naso-paiatine region; t, tongue; m, mouth; /, I, I, I, lipsj d, d, primitive dental grooves with epi helial contents in u;>per gum; d , d , similar structures in lower jaws; e, e, cuticula;- epiblast; h, h, hair follicles; e , epiblast prolonged into the mouth. embryo about the sixth or seventh week, its cavity may be seen to be lined by a stratified epithelium, continuous with the layer of stratified epiblast forming the cuticle of the face. Along the edge of the gum, corresponding in position to that of the future jaws, the epithelium is of some thickness, and an involution of the epithelium into the subjacent connec tive tissue has taken place. Owing to this involution a narrow furrow or groove in the connective tissue is produced, which consti tutes the primitive dental groove of Goodsir. This groove is not, however, an empty furrow, but is occu pied by the involuted epi thelium. The sub-epithelial connective tissue is softand ~^^H>.vi4* ^&- ; ;p#^ gelatinous, and abounds in corpuscles, which are espe- FlG 17 _ A more highly ma?niflea view of a daily abundant in the section through the same jaw at fig. 16; connective tissue at the ft, sub-epithelial connective tissue of ttu gum; bottom of the eroovo whore <* P rilllitive dental groove ; e", Us epithelium ; i oi me groove, iie ^ epithelium lining m, the cavity of the the dental papillae are pro- mouth; I, l, lips; e, the epiblast cuticle. The duced. These papilla? are deepest layer of the epithelium consists of formed, at the bottom of the columnar eells. groove, by an increased development and growth of the corpuscles of the subjacent connective tissue. The base of each papilla is con tinuous with the-subjacent connective tissue, and the apex projects into the deeper parts of the involuted epithelium. As a papilla increases in breadth and length the groove widens ana deepens, and the in voluted epithelium, in creasing in quantity, ex pands over the apex and sides of the papilla, so as to form a hood-like cover ing or cap for it. The cap of epithelium consti tutes the enamel organ, whilst the papilla is the formative pulp for the den- tine and permanent pulp. Fl <J- 18. Vertical section through the gum to Whilst these changes are s ) 1< the formation of the dental papilla. , i . i ,P e , the epithelium covering the gum ; n. the taking place in the epi- nec k of en, the enamel organ ; p, the dental thelium and the connective papilla; ct, sub-epithelial connective tissue, tissue at the bottom of Magnified. the groove, no commensurate widening occurs at its upper part, which remains for a time relatively narrow, but retains within it a narrow string of epithelial cells, continuous on the one hand with the epithelial lining of the mouth, and on the other with the enamel organ. This epithelial string forms the neck of the enamel organ. After a time, however, the growth of the connective tissue forming the lips of the primitive groove causes the neck of the enamel organ to atrophy, so that all communication between the enamel organ and the superficial epithelium is cut off ; and the embryo tooth, being now completely inclosed in a cavity or sac, formed by the gelatinous connective tissue of the gum, has entered on what Goodsir termed its saccular stage of development. When inclosed in its sac the embryo tooth, though perfectly soft, acquires a shape which enables one to recognize to what group of teeth it belongs. After a time it begins to harden and to exhibit the characteristic tooth structure. The dental papilla is more vascular than the surrounding connec. tive tissue, from the blood-vessels of which its vessels are derived. The papilla abounds in cells, which are, in the first instance, rounded and ovoid in shape. Changes then take place in the cells situated at its periphery, which become elongated and branched, and form layers of cells (odonto blasts). Calcification of the protoplasm of these odontoblasts then oc curs, and the peripheral layer of tho dentine is produced. _ In contact FlG ig._Sacculated stage of development of two with the inner surface molar teeth in the cat. ct, ct, connective tissue of the thin film of den- forming the sacs for the teeth ; p, p, dental tir>P i cjppnnrl lavpr nf P a P ille ? the opaque bands, d, d, mark the corn- Lie, a sec mencement of calcification of the dentine; e, e, odontoblast cells IS then internal enamel epithelium ; the outer enamel arranged, which in their epithelium was not recognizable ; 6, 6, the bony turn calcify, and as the walls of ," alveoli are beginning to form J . Magnified, process goes on in suc cessive layers of odontoblasts, the entire thickness of the matrix of the dentine and the dentinal sheaths are produced. But the pro cess of calcification does not apparently take place throughout the whole thickness of the protoplasm of the odontoblasts, for, as Waldeyer pointed out, the axial part of the cells remains uudifferentiated as the soft dentinal fibrils of the dentine tubes. As these changes are going on in the peripheral layers of the odonto blasts, the central part of the dental y *"" ^o d papilla increases in quantity, apparently by a proliferation of its cells ; nerve fibres are developed in it, and it persists as the soft pulp of the tooth. The papilla of the tooth has essentially, therefore, the same relation to the formation of dentine that the cellulo-vascular contents of the medullary spaces, in intra-cartilaginous ossification, have to the formation of bone. In both instances the hard matrix is due to a special differ entiation of the protoplasm of the formative cells ; the dentinal fibrils are the equivalent structures to the soft contents of the lacunae and canaliculi, and the persistent pulp is equivalent to the cellulo-vascular contents of the Haversian canals. Prior to the embryo tooth becoming sacculated, changes had taken place in the enamel organ. Those cells of the enamel organ which lie next the dental papilla are continuous, through the neck of the enamel organ, with the deepest layer of cells of the oral epithelium, which cells are elongated columns set perpendicularly to the surface on which they rest. Similarly the cells of tho deepest layer of the enamel organ are columns set perpendicularly to the surface of the dental papilla. They undergo a greater elongation, and form six-sided prismatic cells, which Kolliker has named the internal or enamel epithelium. The cells of the most superficial layer of the enamel organ lie in contact with the vas cular connective tissue which encloses the embryo tooth. They form the external epithelium of the enamel organ, and slender papillary prolongations of the connective tissue frequently project into this epithelial layer. The cells of the enamel organ, situated between its external and its internal epithelium, become stellate, and form with each other an anastomosing network of cells liko those sometimes seen in the gelatinous connective tissue. IG ^" Section through the dentine and pulp cavity of a 3 oun tooth - P< the P ul P with " one of its vesse i Si and 0) ]aye ,. s of odontoblast cells giving off

processes into d, the dentine. X 450.