Paper - Development of the thyroid and thymus glands and the tongue
|Embryology - 22 Jan 2020 Expand to Translate|
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Minot CS. Development of the thyroid and thymus glands and the tongue. (1884) Science 3(71);725-6. PMID 17806512
|Thyroid Development | Thymus Development | Tongue Development | Charles Minot|
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Development of the Thyroid and Thymus glands and the Tongue
UNDER the wide title of ‘Ueber die derivate der embryonalen schlundbogen und schlundspalten bei saugethieren’ (Arch. milcr. anat., xxii. 271), G. Born discusses the development of these organs as determined by observations on pig embryos. These valuable researches give us, for the first time, an understanding of the morphology of the two glands of the above title, which have been a long-standing puzzle to comparative anatomists.
The tongue arises from the anterior part of the ventral floor of the pharynx. The space between the ventral ends of the first and second visceral arches is at first depressed; but later a longitudinal ridge grows up, separated on each side, by a groove, from the arches. The anterior portion of this ridge grows out, and becomes the free part of the tongue: the posterior part of the ridge projects between the third and fourth arches, and develops into the epiglottis. It will thus be evident that the tongue does not extend back beyond the second arch. After the embryo (pig) reaches a length of fifteen millimetres, the tongue grows rapidly forward. (Although it has long been known that the tongue arises from the floor of the pharynx, the evident conclusion has not been sutficiently recognized, that the epithelial covering of the tongue is entodermal, and not ectodermal, and therefore not the same as the lining of the mouth, as a continuation of which the lingual epithelium is customarily described.)
The fate of the visceral clefts has been more fully elucidated than heretofore. The first becomes the outer and middle ear and the Eustachian tube, as is well known: the fate of the others has been obscure. According to Born, the second entirely disappears, becoming first aclosed sac, and finally undergoing complete atrophy; the third likewise becomes a closed sac, which remains some time connected with the epidermis; from the inner end of the cleft arises a short caecum, extending ventrally inwards and forwards, which is the anlage of the thymus, and is retained and enlarged, while the rest of the cleft is atrophied; the fourth cleft also remains in part as a closed sac, which later joins in the formation of the thyroid gland.
The thymus was first shown by Kṏlliker (Entwickelungsgeschichte, 2te aufl.) to be an epithelial organ, and probably derived from a gill-cleft. Born traces its origin from the third cleft, as a ventral evagination near the inner opening. The caecum grows, at first, without altering its position or general appearance; but the rest of the cleft is reduced to a small canal, the outer part, indeed, to a solid cord of cells (embryo pigs of about sixteen millimetres). The whole, except the thymus portion, is atrophied, but the outer cords persist for a time. The thymus anlage spreads out into a canal, with walls of fine, manylayered epithelium. The lower end of the canal rests against the pericardium, where the aorta makes its exit. In embryos of two centimetres, the lumen of the canal has disappeared, and from the solid cord many branches have grown out, most abundantly at the heart end.
The thyroid gland, as was first shown by W. Muller (Jenaisclze zeitschr. ,- vi. 428, 1871), has a double origin Born shows that the principal division arises as a median invagination in the floor of the pharynx, on a line with the front edge of the second. visceral cleft. Very early this invagination separates from the pharyngeal epithelium, expands laterally chiefly, changes to a network, and at the same time moves backward until it comes to lie behind the glottis. Until the embryo is two centimetres long, the thyroid mass lies near the origin of the third aortic arch (common carotid); but in older embryos the division of the carotids has moved back, away from the head and the thyroid gland. The secondary portion of the thyroid is derived from the paired remnants of the fourth clefts. The median portion of the thyroid early changes into a network of epithelial cords. The outer cells of the cords are cylindrical: the inner cells, in several layers, are not very distinct from one another. Around the cords, the mesoderm forms sheaths of spindle cells, while between them the blood-vessels appear. The lateral anlagen become somewhat pear-shaped, the large end lying ventrally. The lumen is retained until the fusion with the median part is accomplished by the union of the large end of the side components with the central division: the large end soon after assumes the characteristic net-like form of the thyroid gland; but the lateral portions can still be distinguished for some time by the lesser size of the meshes, and the greater size of the cords of the network into which they change.
In the introduction to his article, Born refers to the previous writings of Stieda and Wélfler, and closes with a criticism of the same, and other publications based upon his own researches. The mostimportant point to be noticed is the correction of W6lfler’s mistake in describing the second cleft as the first. (In this abstract, the author’s arrangement of the matter has not been followed, as it appeared little conducive to clearness).
C. S. MINOT.
Cite this page: Hill, M.A. (2020, January 22) Embryology Paper - Development of the thyroid and thymus glands and the tongue. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Development_of_the_thyroid_and_thymus_glands_and_the_tongue
- © Dr Mark Hill 2020, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G