NOTE: PART 1 OF THIS REPORT WAS PUBLISHED IN VOLUME II, OC- TOBER, 1974 ISSUE OF THE CLEFT PALATE JOURNAL. ~ Status of Research in Cleft Lip and Palate: Anatomy and Physiology," Part 2 DAVID R. DICKSON, Ph.D. J. C. B. GRANT, M.C., M.B., Ch.B., FR.C.S. HARRY SICHER, M.D., D.Sc. E. LLOYD DUBRUL, D.D.S., Ph.D. JOSE PALTAN, M.D. Pitisburgh, Pennsylvania IV. THE TONGUE Normal In general, little research on the anatomy and physiology of the human tongue has been found. Therefore, development of the tongue as well as its anatomy and physiology will be covered to give as clear a picture as possible of the paucity of research in this important area. While some foreign language research has not yet been translated for the purpose of this review, no studies of human tongue embryol- ogy have been found. Many research reports on tongue structure and function have been published. However, few of these concern the basic anatomy of the human tongue and only the genioglossus muscle has been studied electromyo- graphically. A good deal of research has demonstrated major differences in muscular, vascular, and neural structure of the tongue among sub-human animals and between sub- human animals and man. For example, muscle spindles have been found in the human but not in the cat tongue. Many animals, excepting man, have a posterior branch from the hypoglossal nerve to the tongue. Yet the literature is replete with studies of cats, dogs, etc., which tend to generalize to human. For these reasons only human studies will be reviewed except for embryogenesis where no human studies have been found. Emsrvyorogy. The embryological derivation of the tongue can be considered in terms of 1) surface epithelium, and 2) underlying viscera, primarily musculature. No experimental embryological studies have been found on the tongue epithelium. Five studies involving lingual muscular development, four on the chick, and one on the cat, have been found and will be reviewed. Hunter (34) traced the myotome components of the occipital somites along their migration path in the chick embryo and described their contribution to the hypo- glossal musculature. Although no mention is made concerning methodology, it is obvious from the accompanying illustrations that the authors performed a histologic study. The authors state that the hypoglossal musculature in the chick is derived * The subcommittee presenting this report (see Part 1, Volume 11 of the Cleft Pal- ate Journal, October, 1974) was part of a larger group, the Committee on Clinical Re- search in Cleft Lip and Cleft Palate, the work of which was supported by contract NIH- 71-643 awarded by NIDR to the American Speech and Hearing Association. The Com- mintsae findings were summarized in Volume 10 of The Cleft Palate Journal (April, 19783). 131 132 Dickson and others from downgrowth of the 'first seven myotomes, with myotomes three, four, and five as major contributors and myotomes one, two, six, and seven as minor con- tributors. Bates (3) provides an excellent historical review of the controversy surrounding the origin of the hypoglossal musculature of mammals. He states that "there is general agreement that the hypoglossal musculature of all vertebrates below mam- mals is derived from the somites of the embryo" but that the origin of this muscula- ture in mammals has been a subject of controversy with two major groups of opposing opinions: the first, that the hypoglossal musculature of mammals is also derived from the somites as first proposed by Froriep in 1885; the second, that the tongue musculature simply differentiates out of local mesenchyme as proposed by Lewis in 1910 and by Pons-Tortella in 19836. Bates studied the early hypoglossal musculature of 35 cat embryos (11 somite stage to the 9 millimeter stage) which were fixed, stained, and serially sectioned. He traced the developing hypoglossal mesenchyme condensations from the ventro- lateral edges of the involved somites to their eventual destination in the pharyngeal floor. He stated the first six somites, four occipital and the first two cervical, give rise to the hypoglossal muscles in the cat, with the first occipital somite being rudimentary. Deuchar (20) performed an experiment which he states eliminated a problem found in the studies of Bates (3) and Hunter (84): the difficulty of differentiating the hypoglossal rudiments from the surrounding tissue. Deuchar operated on 10-12 mm chick embryos and inserted carbon particles into one of the first three posterior otic somites with a needle. He then reinoculated the embryos three to four days; those embryos surviving the procedure for three to four days (25 percent) were fixed, embedded, sectioned, stained, and observed. In 21 out of the 25 processed embryos, carbon had reached the tongue region and was specifically localized here with hardly any occurring in other parts of the pharynx. Deuchar draws the conclusion from these studies that the occipital somites do indeed participate in the formation of the tongue musculature. In both Bates and Hunter, the authors observed what they took to be migration of somitic mesenchyme from the occipital somite region into the region of the embryonic tongue. The hypoglossal mesenchyme appeared slightly darker than the surrounding tissue, thus making differentiation possible. The same type of phe- nomena (apparent migration) could have been observed if the mesenchyme differ- entiated and proliferated along a linear path from occipital to lingual region, with no actual migration involved. For this reason the study of Deuchar is especially valuable in proving migration of the precursor somites or myotomes. Hammond (28) tried to ascertain the role of the occipital somites in the forma- tion of the hypoglossal musculature by surgically removing the somites unilaterally in chick embryos and observing resultant deficiencies in the tongue musculature of the same side. He unilaterally extirpated the dorsal and medial portions (the dermamyotome) of the anterior six or seven somites in 85 chick embryos of eight to twenty-five somites. The embryos were sacrificed six to nine days later and fixed, embedded, sectioned and stained. Only 29 of the embryos showed any degree of hypoglossal muscle deficiency. Of the five glossal muscles in the chick which the author lists, only one, the copuloentoglossal, was absent (in one-third of the em- bryos) and only two others, the styloentoglossal and thyroentoglossal muscles, were reduced (in almost all of the 29 embryos). The author stated that the failure to show more extensive reductions may have been due to the difficulty in realizing a complete and clean extirpation of the somites involved. This absence of complete removal was shown in a related study in eight embryos whose somites were surgi- cally removed; later (two to 48 hours) when the embryos were examined, small portions of the myotomes were observed still present. From his experimental data, Hammond concluded that extirpation of somites two through six (or their myo- STATUS OF RESEARCH 133 tomes) is followed by absence or reduction of hypoglossal musculature in the chick and that these studies provided proof that the hypoglossal musculature in the chick takes origin from myotomes of somites two through six. Hammond's study has if anything created more doubt about the somitic origin of the hypoglossal musculature than before. In only 29 of 85 embryos was the hypoglossal musculature affected at all, and only in nine embryos was any muscle totally absent. It is possible that simply the trauma involved could have inhibited the tongue muscle formation to the degree observed. A simple control test could have been performed to determine this. From the data presented in this experiment one could just as easily have concluded that since in the majority (56 out of 89) of embryos no loss of hypoglossal musculature was seen after extirpation of somites two through six, the occipital somites play no role in the formation of the tongue muscles. Hazelton (29) described the migration pattern and fate of cells of the occipital somites and their overlying ectoderm in the chick embryos which had been marked with tritium labeled thymidine. The somites were transferred from labeled donor embryos to host embryos, which were incubated, sacrificed, and the migration pattern of the labeled cells determined radioautographically. The labeling operation was performed at the six to eight somite stage. The transfer operation was per- formed three to four hours later. The second to fourth or fifth occipital somites plus overlying ectoderm was removed as a single entity from the unlabeled host and replaced with comparable segments from the labeled donor embryos. Incuba- tion periods after transplantation ranged from 15 minutes to five days. Of two hundred experiments carried out, 30 were successful (the embryos survived until sacrificed). The embryos were sacrificed, fixed, sectioned and analyzed. The authors found active proliferation and migration of the labeled cells of the myotomic portion of the somites from their preliminary site in the occipital region to the floor of the mouth, where they united with their fellow of the opposite side. Hazelton carried out a further experiment, transplanting only the labeled overlying ectoderm, to determine its contribution to the observed migration phenomenon; the labeled ectoderm did not participate in the hypoglossal migration. PaTrerns or MovemENnt. Many studies beginning in the early 1800's have been designed to investigate movement patterns of the human tongue in speech and in swallowing. Some of them, most notably MacNeilage and Shales (88), have ascribed various movement to specific tongue muscles. However, little anatomical work has been completed on the human tongue. In addition, research which has been reported on tongue anatomy stresses its extreme complexity with interweaving of muscles in the body of the tongue. For these reasons, the validity of assigning specific tongue movement patterns to specific muscles is highly questionable.