Paper - Development of the thymus (1893)
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Development of the Thymus Gland
The first statements regarding the development of the thymus gland contradicted each other completely. Arnold ' asserted that it arose in common with the thyroid from theentoderm of the pharynx, while Bischoff ' in general denied this. Remak,' in his great work, confirmed Arnold, and added that this gland arises from one of the branchial clefts ; but he was placed in doubt after Ecker had described the large gland in the neck of the chick as thethymus. From now on, the prevalent view continued tobe that the thymus was mesodermal in origin.
We can well understand why these various viewsshould be entertained when we consider that, for studiesof this sort, the methods at that time were very crude ; but in spite of all this we are under obligations to Remak for so much light regarding many problems in embryology, and it really seems a pity that his own view, which later on i^roved to be correct, should have been abandoned on account of his over-caution. By the more improved methods, both Kolliker * and His' observed that the thymus must be of epithelial origin, and therefore accepted the old view of Remak. It may be added that, at this very same time two elaborate papers were published by Afanassiew" and Watney,' in which Ihey attempted to demonstrate that the gland arises from themesoderm. More accurate methods were now introduced, and it soon was demonstrated by Stieda° that in many animals the thymus arises from the third branchial pocket. This was also confirmed by Born ^ (and many others, '", "), who in this study introduced his celebrated method of reconstruction.
In the third part of his " Anatomic Menschl. Embryonen," His * brought forth the view that the thymus arises from the sinus prsecervicalis, and is therefore purely ectodermal in origin. This view he attempted to strengthen in, a later publication,'" and it was quite generally accepted. Later, however, in a response to a paper by the author,'* His ' retracts his older view and admits that the bulk of" the thymus has its origin from the entodermal lining of the third branchial cleft. He therefore considers theorigin of the thymus still an open question, and until more careful researches are made, accepts the view of Fischelis^ and of Kastschenko,^ " that is, that it arises from both ectoderm and entoderm.
Shortly after the branchial arches are formed there appears at the dorsal side of each cleft a thickening of theentodermal cells, which soon separate from the entoderm to form distinct groups of glands. This is theconditiou of things in low vertebrates, and as the scale is ascended certain groups become more and more prominent, until man is reached, when only the two groupsfrom the third branchial pocket remain to form the thymus.
In the fishes the general relation of these glands to thebranchial clefts is shown in Fig. 622. These individual glands are soon united into one large gland on either side of the phai'ynx ; in the bony fishes these groups uniteinto one gland before they have separated from thepharynx. In the reptiles the number of glands are reduced (Fig. 633) to correspond with this number of branchial clefts. Van Bemmelen discovered in the elasmobranchs that the posterior cleft, or rudimentary cleft, produced a distinct body which did not unite with thethymus. This he has termed the supra-pericardial body, and later its homology was found in many classes of vertebrates. In many reptiles it is unilateral, as shown in the figure. Considering their origin, they seem to be intimately related with the thymus, but in mammals it is probable that they are added to the thyroid, and will be discussed under that heading.
In birds the third branchial pocket gives the main origin of the thymus, as shown in Fig. 6'2-t. Here we have a sharper line of demarcation between ectoderm and entoderm, as the branchial clefts do not break through neck ; while this is taking place a pit is first formed, the sinus praecervicalis of His.
Fig. 622. — Difigram showing the Branchial Clefts and the Glands arising from them in the Shark. (From Hertwig after de Meuron.) , first and sixth branchial pocket'?; sd, thyroid; i/i, thymus; 7is(i, lateral thyroid.
Fig. 623. — Diagram showing the Branchial Clefts and the Glands arising from them in the Reptiles. (Prom Hertwig after de Meuron.) «c/ii, first branchial pocket ; sd, thyroid ; ih, thymus ; ?i^d, lateral thyroid.
Fig. 624. — Diagram showing the Branchial Pockets and the Glands arising from them In the Chick. (Modified from de Meuron.) sd, thyroid; scA^, first branchial pocket ; t/t, thymus ; nsd, lateral thyroid.
Fig. 625.— Dorsal View of a Reconstruction of a Chick 110 hours old. X thirty five times. II, III, and IV, branchial pockets ; O, operculum ; S, thyroid ; spr, sinus prEecervicalis ; T, thymus ; X, body derived from the fourth branchial pocket ; OS, oesophagus.
Front the dorsal side of the first groove au invagination unites with the ganglia of the fifth nerve ; from the second, the invagination is to the ninth nerve ; and from the third and fourth it is to the tenth nerve. A section as in fishes. We can now state with great certainty from what embryonic layer this gland arises, provided we have good serial sections to study. Very recently Kastschenko discovered a small gland in connection with the second branchial pocket, but as yet its fate has not been determined. It is no doubt a remnant of the portion of the thymus which arises from the same place in lower vertebrates. The third branchial pocket, however, becomes very prominent, grows toward the head, and is at no time blended with the ectoderm (Pig. 635). To be sure, it comes in apposition with the ectodermal invaginations of the clefts, but recent work has shown that these have to do with the ganglia of the cranial nerves, and do not unite with the thymus as thought by His and others. Moreover, it is by no means probable that these senseorgans of Froriep and Beard should suddenly leave the nerve-ganglia in certain regions and unite with glands. Both observations and principles of development contradict this. The fourth branchial pocket, as well as a rudimentary fifth (fossa subbranchialis), gives rise to a few small bodies, the nature of which is not as yet truly known. That, from the rudimentary fifth, no doubt, gives the gland which is homologous with Van Bemmelen's supra-pericardial body. In mammals the condition of things is much simpler (Pig. 626). The branchial grooves lie on the outside of the body, are shallow on their dorsal side and deep on their ventral. As these -arches fall over one another, the grooves as well as the third and fourth arches are buried in the side of the through these organs in the region of the vagus and of the thymus is shown in Fig. 627. The ectodermal invagination is absolutely blended with the vagus and is only in apposition with the thymus.
Fig. 626. - Diagram showing Branchial Pockets, and the Glands arising from them in the Human Embryo : sc/i,! sch,'^ first and second branchial pockets : th, thymus ; tid, thyroid ; ivid, lateral thyroid.
Fig. 627. — Section through the Thymus and Fundus Praecervicalis of a Dog's Embryo. 10 mm. long X 80 times. Ph., pharynx ; A^, A^, aortic arches; S.Pr., sinus praecervicalis: V.J. , jugular vein; T., thymus still in connection with the pharynx.
Fig. 628.— Dorsal Reconstruction of the Branchial Region of a Dog's Embryo, 10 mm. long, X 25 times. Nf , iV'io, ganglia of facial and vagus nerves ; BBI'^, BUP", brancliial pockets ; BRG, branchial grooves ; A^, A^, aortic arches ; BA, aortic bulb : Tr,, trachea ; M.ex., external meatus of the ear; -^, thyroid; T, thymus; F.PR., fundus prsecervicalifi.
Tbe pharynx-side of the branchial clefts presents about the same appearance as the external. The entodermal lining is in the form of slit-like pockets, which are better called branchial pockets, to differentiate them from the ectodermal side, or branchial grooves. As the head flexes upon the body the pharynx widens near the mouth and becomes narrower wliere the trachea is formed. There is a peculiar kinking of this region due to the rotation of the head. The first branchial pocket is converted directly into the Eustachian tube ; the second disappears completely ; the third forms the thymus (Fig. 628) ; and the fourth becomes rudimentary and gives rise to the auxiliary thyroid glands.
Fig. 629. — Section through the Branchial Region of a Human Embryo, Five Weeks Old. (From Minot after His.) //, ///, IV, branchial arches: Sp., second branchial grooves; ijp., infundibulum prsecervicale ; F, fundus of the infundibulum ; 3, 4, third and fourth branchial pockets, with the thymus arising from the third ; Ao^, Ao*^ aortic arches : Ep.. epiglottis ; JX, gloBso-pharyngeal nerve ganglion ; XII., hypoglossal nerve ; nl, superior laryngeal nerve.
The general appearance in the human embryo is quite similar to that in other mammals, as Fig. 629 shows. Already in this early stage of development, the third branchial pocket shows an ingrowth which indicates the origin of the thymus. The portion of the cleft represented by the fundus (F) is not continuous with the thymus tube, and no doubt never plays any part in its formation.
Fig. 630. — Reconstruction of a Human Embryo enlnrged sixteen times, and viewed from the left side. JT, hypophysis ; 1, 2, 3, and 4, branchial pockets ; 3f, mouth ; A, auricle ; V, ventricle ; AD, descending aorta : L, lung ; S, stomach ; P, pancreas.
The general view of the branchial pockets in a human embryo is shown in Fig. 630. The whole pharynx is represented as a cast and the branchial pockets are represented by the figures 1, 2. 3, and 4. It is the one marked 3 which is destined to become the thymus. It soon becomes separated from the pharynx and then grows into the thorax.
Before the thymus is separated from the pharynx it contains a distinct lumen. This is soon lost in birds, where the thymus grows toward the head. In mammals, where the thymus grows into the thorax, the upper part contains a lumen, while the lower part grows by a mass of sprouts (Pig. 631). This continues for quite a while until the whole organ is lobulated. New blood-vessels and lymph-cells grow through the epithelial gland and change its nature. The entodermal cells become packed together into the Horsal's concentric bodies as shown by Maurer for the fishes, and by Ecker and by His* for mammals. These bodies correspond with the "pearls" in carcinomata, which are present in cancers, that arise from the whole epidermis, as well as those from the oesophagus. The Horsal's bodies do not therefore indicate that the thymus arises from the ectoderm.
Fig. 631.— Thymus of a. Rabbit Embryo Sixteen Days Old. After v. K61liker. a. Duct of the thymus ; 6, head of the thymus : c, posterior enlargement of the gland.
During the first two years of life the organ continues to grow and the two halves gradually approach each other more and more, until they seem as a single body. It now lies immediately in front of the heart, and often sends two horns which extend on either side of the neck to the thyroid, as is the case in the birds. Prom now on, the organ gradually atrophies.
Franklin P. Mall.
1 Arnold ; Mediciriische-chirurg. Zeitung, 18.31.
2 Bischofi: : Entwicklungsgeschichte. Leipzig. 1842.
3 Remak : Entwick. d. Wirbelthiere. Berlin, 1855.
4 Kolliker: Entwicklungsgeschichte, 1879.
5 His ; Anatomie mensch. Embryonen. Leipzig, 1880-85.
6 Afanassiew : Archiv f , mik. Anat., 1887.
7 Watney : Phil. Trans., 1883.
8 Stieda : Untersuch. liber die Entwickl. d. gl. Thymus, gl. Thyroidea, etc., Leipzig, 1881.
9 Born : Archiv f. mik. Anat., 1883.
10 Froriep : His und Branne's Archiv, 1885.
11 Mall; His und Braune's Archiv, 1887.
12 Mall : Johns Hopkins Circulars. 1888.
13 piersol : Sitzungsber. d. Wiirzburger Gesch., 1888.
14 Mall : Studies from Biol. Lab. J. H. U., Vol. 4.
15 Piersol: Zeit. fiir Wiss. Zool.,Bd. 42.
16 Sohm : Mitt, aus d. Zool. Station zum Neafel, Bd. 5.
17 Maurer : Morph. Jahrbuch, Bd. II u. 1:3.
18 De Meuron : Recherches sur le devel. du Thvmus, etc. Inaug. DisB. Geneve, 1886.
19 Van Bemmclcn : Anatom. Anz., Bd. 4.
20 His : His und Braune's Archiv. 1886.
21 His : Brief an F. Mall, His und Braune's Archiv, 1889.
22 Fischelis : Aich. f. mik. Anat., 1885.
23 Kastschenko : Arch. f. mik. Anat., 1887.
24 Kastschenko : His und Braune's Archiv. 1387.
25 Ecker : Wagner's Handworterbuch, Bd. 4. 2« His : Zeit. f. Wiss. Zool., Bd. 9.
Cite this page: Hill, M.A. (2020, July 8) Embryology Paper - Development of the thymus (1893). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Development_of_the_thymus_(1893)
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