Talk:Paper - Development of the thyroid (1893)

From Embryology

THYROID GLAND, DEVELOPMENT OF. Shortly after Rathke discovered the branchial arches in higher animals, Husohke ' advanced the idea that the thyroid gland arises from the first arch. Later, however, he states that after the arches disappear the thyroid remains as their remnant.^ A year later, Rathke ' asserted that the thyroid arises as a bilateral organ from the posterior part of the trachea, immediately behind the larynx. This view was affirmed by Arnold.* Bischoff' accepted neither of these views, but added a third. According to him, the thyroid arises as a bilateral organ from the blood-vessels on either side of the neck.

The first careful investigations were made by Remak ' upon the chick. In this subject, as well as in many others in embryology, he was the first to give accurate information. He found that the thyroid is entodermal in origin and arises from the ventral median line of the pharynx. At first it appears as a single vesicle, which soon divides into two parts, each half lying on either side of the trachea. Gotte,' Miiller,* and Seessel ' confirmed Remak's views in general, differing only in minor points. They discussed whether or not the organ divided is a vesicle or is a solid body, although none of them found stages in which the thyroid was dividing. Remak placed the origin of the thyroid at seventy -nine hours in the chick ; Mailer in the third day ; and Seessel very definitely in the second day before the head flexed upon the body.

His '" revived the old view of Rathke by discovering that certain bodies arise from either side of the pharynx ; he believed them to be the thyroid and the auxiliary thyroid of Remak. According to the illustrations which accompany his paper, the gland arises from the branchial cleft between the second and third aortic arches, therefore the second cleft. Later, however, he abandoned this view and accepted that of Seessel, who worked under his

fuidance. Later, Kolliker " accepted the old Remakeessel view for the rabbit, as His " did for man. This group of excellent workers seemed to set the subject aside for a while, but, immediately following their work, two authors simultaneously described the thyroid as being bilateral in origin, arising from a branchial cleft on either side of the neck. Stieda " proved definitely that a glandular organ arises from the clefts on either side of the neck, but did not know from which cleft, although he was inclined to accept the fourth. Wolfler '* did not employ very accurate methods, and therefore got the location of the origin of the gland in the first branchial cleft — the one which later forms the Eustachian tube. He was so enthusiastic over his discovery that he denied altogether a median thyroid, although he had never seen the thyroid in connection with any cleft. Discussions naturally bring forth more accurate investigations, and in this case they bore their fruits in Bern's paper." The paper of Born not only threw a great deal


Tliyrold Gland. Thyroid Gland.



of light upon the subject ia question, but also added greatly to our methods of investigation. He introduoed a method which to us now seems so logical that it is a surprise that it was not employed earlier. He began with embryos of the pig before the thyroid appeared, and followed them stage by stage until the organ was fully formed. In each stage the enlarged sections were drawn upon wax plates of the proper thickness, then cut out and the pieces piled upon one another, thus forming very accurate models of the various specimens studied. He then followed the lateral thyroids of Stieda and Wolfler, and found that later on they were added to the median thyroid of Remak. The great difficulty was therefore solved by this method. Instead of these various authors contradicting one another, they in reality described correctly only different portions of the same story. According to Born the lateral thyroids arise from the fourth branchial clefts. Since then, Bern's view has been confirmed in a variety of mammahan embryos by different writer's."-"

The subject of the origin of the thyroid and thymus (see Thymus) has been greatly complicated by a group of glands which arise from the degenerating branchial clefts and form the so-called auxiliary thyroid glands. From a phylogenetic standpoint all glands which arise from the clefts must be viewed as thymus. The great bulk of thyroid arises from the ventral wall of the phar

Fia. 682.— Beconstruction of the Mouth of a Chick Eighty-eight Hours Old. The floor of the mouth is exposed by cutting off the back of the head. Dorsal view. X 20 times. 2'., tuberculum impar, back of which is the opemng of the thyroid ; IF, III^ branchial arches ; 3, 4, third and fourth aortic arches.

Pio. 63.3,— Abdominal Canal of a Human Embryo. X 20 times. (Alter His.) Uk, lower jaw ; Set., thyroid : Lg., lung ; Mg., stomach ; /*., pancreas; £6^., bile-duct ; TK, Wolffian duct.

ynx in the region of the second cleft. As the clefts gradually disappear in the evolution of the vertebrates, the glands from the hindermost clefts no longer unite with the main bulk of the thymus, and often remain as distinct glands — the supra-pericardial bodies of van Bem

melen, accessory thyroids of de Meuron, post-branchial body of Maurer, or as the corpus Y of the writer.™ In the mammals the main bulk of the thymus arises from the third cleft, and the auxiliary thyroids seem to unite with the median thyroid as described by Born. In spite of the great amount of work which has been done upon this subject, it still seems to the writer that there is yet considerable darkness regarding it.

The general view is that the thyroid has three distinct origins in nearly all vertebrates— a median and two lateral. The median arises from the ectoderm of the middle line in the neighborhood of the second branchial

Fio. (►34.— Profile Reconstruction of a Dog's Embryo, 6 mm. long. X 25 times. JV, N/«, N', JV'", ganglia of cranial nerves : I, IL, branchial arches; ..4. r., auditory vesicle ; TV., trachea; OS., cesophagus ; V.J., jugular vein; A.B., descending aorta; B.A., bnlbus aortae ; S., thyroid gland.

arch (Fig. 632), and the lateral from the most posterior branchial clefts ; in mammals, the fourth.

The median thyroid arises in all vertebrates from the entoderm of the pharynx shortly after the branchial arches are well formed (Fig. 632). This has been known since the time of W. Miiller, and, although contradicted many a time, has of late been verified by so many careful observers that there is no longer any doubt whatever regarding it. In some mammals (the dog, for instance) it arises as a vesicle, which becomes solid before it separates from the pharynx, and when separated soon becomes lobulated. It now gradually shifts itself into the tissue of the neck, and in early stages lies in front of the aorta just as it leaves the heart (Fig. 634). The median gland now becomes constricted into two parts, one of which is situated on either side of the neck, but they remain connected by a band — the isthmus. Its origin in the human embryo is much the same (Figs. 635 and 636). According to His, it remains a vesicle for quite a long time, and the separation from the pharynx is quite late or may not take place at all. Later, however, the formation of the ligamentum hypothyroideum cuts the duct (d. thyreoglossics) into two, thus forming two ducts. The half com

FlG. 635. — Pharynx, Thyroid, and Thymus of a Human Embryo. X 95 times. (From Quain after His.) tr., Tongue ; fc, foramen caecum : ep., epiglottis ; thm., thymus ; thr., median thyroid ; ?/ir', lateral thyroid; d., ductus thyreo-glottus.

municating with the mouth has been termed by His duoUis Ungualis, and the one with the thyroid as the ductus thyroideus. His has found the ductus Ungualis in five




Thyroid Gland. Xliyrold Oland.

adults, and in all cases the ductus thyroideus was present also. The latter is always imbedded in a median lobe of the thyroid, and the former opens into the foramen caecum of the tongue. The ductus thyreoglossus may be broken into a series of vesicles, as first described by Verneuil, but as a rule it disappears fully, its only remnant being the foramen csecum. *

The lateral thyroids were first demonstrated by Stieda and by Born, although Rathke, His, and others had in


Fig. 636. — Reconstruction of the Thyroid, Thymus, and Pharynx of a Human Embryo five weeks old. (Slightly modified from His.) thm,^ Thymus : thr.^ thyroid; c, carotid ; «o'., ascending aorta; ao.^ descending aorta ; tr,, trachea.

the neck, as in the auxiliary thyroid glands, which have perplexed so many investigators. According to Kastscbenko the lateral thyroid of mammals plays a very minor part in the formation of the mature organ, while according to His it forms the major part. A true blending of the two does, however, take place, as shown by the investigations of Born and His. (Pig. 639.)

The further development of the thyroid is by means of sprouting, so that the enlarged organ is a plexiform mass of cylinders of epithelial cells. Blood-ve.ssels grow in between them, and soon the cylinders show constrictions which cut them up again and again. They now become hollow, and at the same time there is a secretion of the colloid substance. The cutiup cylinders become more and more distended, and thus the adult thyroid is

Figs. 637 and 0.38.— Diagrams of two stages ot the development of the lateral thyroids. Slightly mollified from Born. B., border of the pharynx; T., thymus; (?., glottis; L.S., lateral thyroids.

all probability described them from time to time. The true meaning of them was fully made out by Born and by de Meuron, the former having discovered that they unite with the median thyroid in mammals, the latter having studied them from a comparative standpoint. Diagrams of the branchial pockets for some of the different classes of vertebrates are given in the article Thymus, and by them it is shown that a rudimentary branchial pocket exists in all of the classes. In certain reptiles there is but a single one on the left side, the one on the right not having developed. We can view this state of things only as an intermediate one between the reptiles (in which they develop on both sides) and the birds. In all classes the hindermost cleft gives rise to a distinct gland known as the supra-pericardial body of van Bemmelen and as the accessory thyroid of de Meuron.'*-*' In the selacians these bodies remain far away from the median thyroid, and tUey never unite with it. In higher animals they gradually approach the median thyroid nearer and nearer, and in mammals they unite completely with it. Fig. 636 shows the general relation of those glands in a human embryo before the lateral thyroid is fully separated from the pharynx. Figs. 637 and 638 show two stages in which the lateral thyroid is approaching and uniting with the median. This is brought about by a double shifting. The median thyroid sinks deeper and deeper into the tissue of the neck, and practically lies in contact with the lateral thyroids before they separate from the pharynx. When these in turn separate from the pharynx they are shoved into the tissue of the median thyroid instead of into the ordinary mesoderm tissue of the neck.

It will be noticed that the lateral thyroids do not always arise from the same bruncliial clefts : in selacians from the seventh ; in reptiles from the fifth ; in mammals from the fourth. It is only in mammals that they really unite with the median thyroid. Viewing the thing from the standpoint of homology, what is lateral thyroid in mammals is thymus in lower animals ; in both selacians and reptiles the fourth cleft gives rise to a portion of the thymus. We must therefore view the lateral thyroid as a portion of the thymus which is now united with the thyroid. In lower animals the secondary union of the gland from degenerating clefts with the median thyroid does not take place, and they remain in the tissue of

  • From the standpoint of evolution it is iiiteresfing to note that

Dohm ^' has advanced the hypothesis that the thyroid represents a lost branchial cleft. These speculations are, of course, the natural outcome when genetic relation is taken into consideration in the study of comparative embryology.


composed of a mass of hollow spheres covered with a layer of epithelial cells and filled with colloid substance. When half of the thyroid is removed by an operation or otherwise, the remaining portion soon loses its colloid and returns to its " embryonic state." It enlarges to its

Fig. 639.— Section through the Thyroid of the Embryo of the Pig, 21.5 mm. long X 80 times. From Hertwig after Born, ir.^ Trachea ; L.S,^ lateral thyroid ; MS., median thyroid ; g,g., blood-vessels.

former size and again foi'ms the colloid spheres much after the fashion it did in the embryo.^'

Franldin P. Mall.

I Buschke : Isis, 1826. = Buschke : Isis, 1827.

3 Rathke: Nova Acta phys-med. Abthlg., LLC.,' Bonn, 1828. ^ Arnold; Medicinisch-Chjrurg. Zeitung. 1831. s Bischoii; Ent\vicklimgst;eschichte, Leipzig, 1842. ^ Remak : Eiitwickl. d. Wirbelthiere, Berlin, 1855. ' Gotte : Entwicklungsgeschichte des Darracanals im Hiihnchen, Tiibingen. 1867.

    • Miiller, VV. ; Entwickl. d. Schilddrii'^c, etc.

^ Seessel ; His u. Brstune's Archiv, 1S77.

10 His : TJntersuch. iiber die erste Anlage des Wirbelthieres, Leipzig, 1868.

I I Kolliker ; Entwicklungsgeschichte, 1870. 1^2 His : Anatomic Meusch. Embryonen. 1880.

13 Stieda; TJntersuchung iiber die Entwickl. derGl. Thymus, Gl. Thyreoidea, etc., Leipzig, 1881. 1* Wiilfler: Entwicklung u. Ban der Schilddruse. Berlin. 1881.

11 Born : Arch. f. mik. Anat., 1883. !» Kolliker: Grundriss., 1884.

1' His : Anat. Mcnsch, Embryonen, iii., 1885. 1^ Proriep : His und Braune's Archiv, 1885. " Mall: Ibid.. 1887. 2» Kastschenko : Ibid., 1887.


2> Kastschenko : Archiv t, mik. Anat., Bd. 30.

•" Pischolis : Ibid., 1S85.

23 De Meuron : Inaug. Diss., Gentjve, ISSB.

2» Mall : Studies from the Biol. Lab., J. H. U., 18SS.

== Piersol : Zeit. t. wlss, Zool. , Bd. 42.

2* Mall : Journal of Morpliology, vol. v.

" Dohrn : Mitt. a. d. Zool. Station z. Neapel, Bd. 6 u. 7.

2* Van Bemmelen; Kin. Akad. d. Wet, Amsterdam Afd. Nat. 1885.

»9 Van Bemmelen : Zool. Anz., 1886, 1887.

30 Van Bemmelen : Anacom. Anz., Bd. 4.

SI Halsted : Johns Hopkins Hospital Ecport, vol. i.