ABSTRACT
Three thymus ‘buds ‘can be originally distinguished in trout, in the thickened epithelium above the 1st, 2nd, and 3rd gill-arches, at the junction of ectoderm and endoderm.
Differentiation spreads almost immediately into the ectoderm between the original buds, and a continuous differentiating area is formed which is partly ectodermal and partly endodermal.
Repeated division of the epithelial cells, beginning a few days before hatching, gives rise to the small thymus cells. The latter are not derived from immigrating lymphocytes, as has been stated by Hammar and Maximow.
Emigration of thymus cells may take place at all ages.
Immigration of connective tissue and vascular elements from the mesoderm begins just before hatching, but such immigration is not active till a week later.
The large elements in the adult thymus are mesodermal, with the exception of the epithelial cells at the edges, and a few mucus gland-cells, similar to those occurring in the ordinary epithelium.
In both male and female trout the thymus undergoes almost complete involution, when the fish is between 2 and 2| years old. There is no evidence that the time of involution is connected with the time of sexual maturity.
With Plates 12-16.
Hammar (11), p. 545. ‘Nowhere does the thymus appear more clearly in its quality of epithelium infiltered by lymphocytes than in certain Teleostei ‘(1, 8).
Curiously enough, Maurer states in this paper that the original lymphoid cells, which were clearly of epithelial origin, eventually revert to an epithelial condition, when, in 2-3 months’ old trout, true lymphoid elements penetrate the thymus along the connective-tissue strands ; these establish themselves in an intermediate zone, and follicular formations arise of which the elements are ‘thick-lying lymphoid nuclei, with hardly any cytoplasmEven the reticular connective tissue in which these nuclei lie can scarcely be distinguished, so close packed are they.’
Species unknown ; in these trout the thymus has only just begun to differentiate in the newly hatched fish, whereas in S. f ario the thymus has reached a considerable size at the time of hatching.
Provided by Professor Goodrich.
It may be observed that similar methods were used by McClure (15) who investigated the development of the lymphatic system in trout.
Of. Dustin (5, p. 615).
Differentiation is similar in the two species of trout studied, but whereas the thymus has reached a considerable size in S. fario and is continuous at the time of hatching, in the other species it has just arisen. The latter trout (probably developing normally at a higher temperature than S. fario) hatches in a less mature condition.
May 1927. I have since examined another series of S. f a r i o embryos to ascertain if the earliest development of the thymus in this species is similar to that in Dr. Jenkinson’s trout. The beginnings of histological differentiation occur in precisely the same way, in ecto-endodermal thymus buds formed in the thickened epithelium.
Embryos 25 and 26 in Dr. Jenkinson’s series.
Dr. Jenkinson’s series 27 and 28 ; eldest just hatched.
It is the differentiation of these nuclei that gives the first indication of the transformation of epithelial into thymus cells ; as yet, however, their condition suggests a particular form of nuclear activity, rather than any specific thymus differentiation, since similar nuclei can be found in the epithelium in regions remote from the thymus, though not aggregated together.
The later development of the thymus in Dr. Jenkinson’s trout follows a similar course, but the material provides fewer intermediate stages. In the eldest of the embryos grouped under stage 2, one which had just hatched, the thymus buds can be seen in sagittal section and are slightly more advanced than the one just described. The next youngest trout in this series (No. 29) has a well-marked thymus which can be seen on forty-six sections (230μ) ; there is a gap of 35 M between the first and second thymus buds, but the last two have coalesced. The nuclei are darker-staining and smaller (fig. 11c, Pl. 15), the degree of differentiation being equivalent to stage 4.
Faint concentric lines can be distinguished in the cytoplasm of some of these cells (fig. 13, Pl. 16), which resemble the ‘cellules épithélioïdes ‘described by Dustin (5, 6) in the frog’s thymus. Hammar (8) in fig. 38, my, depicts a similar type of cell, but in none of the cells in S. f ario were the striations as strongly marked. Burne (3) also observed large striated cells in the thymus of Lophius, ‘grouped round the interlobular trabeculae ‘; an indication of their mesodermal origin.
In the 2–3-years S. fario and S. irideus killed in July, rather more thymus tissue persisted than in S. fario killed in June (fig. 15 i, Pl. 16).
Nach meinen Beobachtungen haben sie (die dunklen Zellen) allerdings mit der Entstehung der Lymphocyten nichts zu tun, sondern reprâsentieren wahrscheinlich bloss einen besonderen vorübergehenden Funktionszustand der gewohnlichen Epithel-zellen…. Sie kônnen vielleicht spâter manchmal das gewbhn-liche Aussehen wiedererlangen. In den meisten Fallen aber verfallen sie wohl sicherlich der Degeneration und verwandeln sich dabei in dünne dunkle Streifen zwischen den iibrigen gewohnlichen Epithelzellen, um spater vollstandig zu ver-schwinden.’
It seems unimportant whether or not the original thymus rudiment can be distinguished in separate ‘buds ‘as in trout and higher vertebrates. Possibly this stage is lost in some Teleosts, and differentiation is initiated over one continuous area, as described by Hammar. I have been unable to distinguish separate buds in the developing thymus of the goldfish, which in other respects resembles that of the trout.
