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Quarterly Journal of Microscopical Science, Vol s2-82, 563-586, Copyright © 1941 by Company of Biologists
1 Sir Halley Stewart Research Fellow
2 Royal Society Messel Research Fellow
The origin and early development of the muscles, cartilage, and bone of the embryonic avian mandible were investigated by histological and tissue culture methods applied to the embryonic domestic fowl.
1. Muscle.--The muscles of the mandible do not arise in the mandibular arch.
The myogenic cells migrate into the arch between the second and third day as a histologically distinct group. Explantation experiments in vitro showed that the myogenic cells are determined to form cross-striated muscles as early as the third day.
2. Cartilage.--The chondrogenic cells of either side originate from a small proliferation centre immediately beneath the buccal epithelium of the branchial arch. Proliferation of the chondrogenic cells continues from the third to the sixth day, forming a pronounced, rounded swelling near the distal end of the mandible. It was demonstrated experimentally that elongation of the mandibular arch to form the jaw is mainly due to apical growth caused by proliferation from the paired chondrogenic centres. By the seventh day the chondrogenic proliferation centre is exhausted.
Experiments showed that the cells of the chondrogenic proliferation centre in the 3-day jaw are already determined to form cartilage.
3. Bone.--The osteogenic cells destined to form the os angulare, spleniale, and supra-angulare arise from a single proliferation centre which appears at about the fourth day immediately beneath a slightly thickened patch of presumptive epidermis at the proximo-lateral end of the mandible.
This centre becomes subdivided into a medial and a lateral part by cell degeneration in the interior. Both parts continue to grow in a distal direction. The lateral division forms the os supra-angulare, while the medial division is cut transversely into two sections by branches of the mandibular nerve. The distal section forms the os spleniale and the larger, proximal section the os angulare.
It was demonstrated by explantation experiments that the cells of the osteogenic proliferation centre in the 4-day mandible are already determined to form bone.
The osteogenic cells destined to form the os dentale originate at about the fifth day from a separate centre in the disto-lateral part of the mandible, beneath a thickened patch of mouth epithelium corresponding to the rudiment of the enamel epithelium in mammals.
The cells of this disto-lateral osteogenic centre were shown to be self-differentiating in vitro when explanted at the fifth day.
The authors are greatly indebted to the Rockefeller Foundation for a grant in aid of this research; they also wish to thank Mr. V. C. Norfield, senior assistant of the Strangeways Eesearcb Laboratory, for his help in preparing the figures.
