Our knowledge of the lymphatic system has been very greatly increased during the past ten years by studies on its mode of development. Previous to 1902 nothing definite was known about the primary development or the mode of growth of the lymphatic system. It was concluded by some (Budge, Gullard and Saxer) that the lymphatics arise from undifferentiated mesenchyme cells; Ranvier believed that they arise from veins by budding of the endothelium; while Sala described them as arising partly from the mesenchyme and partly from venous endothelium.
Regarding the mode of growth and spreading of the lymphatics, various theories were like-wise held. Kolliker, His, Goethe and, later, Sala held that growth takes place by the successive addition of mesenchyme cells; Langer, Rouget, and Ranvier maintained that growth takes place by sprouting of the endothelium. S. Mayer thought that new lymphatics are derived from transformed blood-capillaries.
Miss Sabin in 1902 gave the first clear picture of the mode of origin and growth of the lymphatic system, and our present knowledge is largely based upon her discoveries. She showed by injections of embryo pigs that the lymphatics of the skin appear first in four regions of the body - two on each side at the base of the neck, and two in the inguinal region - in the form of sacs which are connected with the veins. From these four regions the lymphatics spread out step by step over the skin of the entire body, in the form of a richly anastomosing capillary plexus. Since the publication of Miss Sabin's paper, numerous studies have been made on the mode of development of lymphatics in many different animals, including man. The results of these studies indicate that the lymphatic endothelium first appears in the form of buddings-out from the veins in certain well-defined regions of the embryo. As to the exact manner of this primary origin views differ. Miss Sabin, in her first paper, held that it arises by budding from the veins. F. T. Lewis held that it is formed by the transformation of plexuses of blood-capillaries. This view was accepted by Miss Sabin, and verified by Huntington and McClure. Stromsten recurred to Sala's view that the first lymphatic endothelium arises in part from venous endothelium, and in part from the mesenchyme cells. Hoyer and his pupila find that the first lymphatics arise as buds from the veins. This has also been found (1912) by E. R. and E. L. Clark in chick embryos.
Thus far six regions have been found, in which lymphatics develop from the veins - in the neck, on each side, at the angle formed by the internal jugular and subclavian veins; in the pelvis, on each side, along the iliac veins; and two unpaired sets in the region of the renal veins, one ventral to the aorta, the mesenteric, and one dorsal to the aorta, retroperitoneal. In these six regions the lymphatics soon coalesce to form large sacs, the jugular, iliac, mesenteric and retroperitoneal. The sacs are later broken up into the primary sets of lymph-nodes. The receptaculum chyli develops in the region of the retroperitoneal sac.
From these primary anlages derived from the veins the lymphatics spread out into the various organs and tissues of the body. The cutaneous lymphatics spread out from the two jugular and two iliac regions (Sabin), the lymphatics of the intestine from the mesenteric sac (Heuer).
The method by which this extension of the primary lymphatics occurs is still in dispute, but there seems to be conclusive evidence that it takes place by the sprouting of the endothelium; that the endothelium of the lymphatics, derived from the veins, is a specific, independent tissue, and that all new lymphatic endothelium is formed from lymphatic endothelium, and not from blood-vessels or mesenchyme cells. This view is supported especially by the work of Sabin, MacCallum, Hoyer and his pupils and E. R. Clark.
On the other hand, F. T. Lewis has suggested that the spreading of lymphatics occurs by the transformation of blood-vessels into lymphatics; while Huntington and McClure and their pupils maintain that it occurs by the continued transformation of mesenchyme cells.
The lymphatics growing from the various primary centers meet and anastomose with one another, and gradually lose all connections with the veins save those at the base of the neck Sylvester has found, however, that in South American monkeys the connections with the veins in the region of the renal veins are maintained in the adult. Valves do not appear in the lymphatic vessels until quite late, in human embryos about 5 or 6 cm. long. (Sabin.)
The lymphatic nodes do not make their appearance until the system of vessels is well established. They are at first represented by masses of lymphoid tissue in the meshes of a lymphatic network. Later the lymphoid mass breaks up into smaller portions, into which the blood-vessels and branches from the surrounding network penetrate; and each mass, together with the portions of the network surrounding it, becomes enclosed in a connective-tissue capsule. The original lymphoid tissue becomes transformed into the medullary cords and cortical nodules of the node, while the enclosing lymphatic capillaries form its peripheral lymph-sinus.
The earliest nodes appear in the places occupied by the primary lymphatic plexuses or sacs (Miss Sabin, F. T. Lewis, Jolly), and have been termed the "primary nodes" (Miss Sabin). Secondary and tertiary sets of nodes develop later at places of confluence of many lymphatics (cf. A. H. Clark.)
Regeneration and new growth of lymphatic vessels and glands. - While blood-vessels are known to possess throughout life the capacity for regeneration and new growth, this process in lymph-vessels has been very little studied. Yet enough has been learned from the work of Coffin and Evans to justify the statement that lymphatic vessels also possess the capacity for new growth. Evans made the interesting observation that lymphatic vessels grow into a tumor of connective-tissue origin (a round-celled sarcoma), while they fail to grow into a tumor of epithelial origin (an experimentally-produced peritoneal carcinoma in mice).
The question as to whether lymph-glands may form anew is not yet entirely settled. The study of the problem is extremely difficult, because very small lymph-nodes may be normally present in a certain region, yet they may escape observation until they become hypertrophied under certain conditions. A. W. Meyer in a careful experimental study found no evidence of new-formation of lymph glands. On the other hand, there is considerable evidence for the new- formation of lymph-glands under pathological conditions.
The hemolymph nodes. - In addition to the ordinary lymph-nodes, there occur along the course of certain veins small nodes which are either red or brown in color, according to their state of functional activity. These have been termed hemolymph nodes. The red nodes closely resemble in structure an ordinary lymph-node, except that the sinuses are filled with blood, while the brown nodes show not blood, but blood pigment, both free in the sinuses and in the phagocytic cells of the sinuses. In certain respects these nodes resemble the spleen, there being a reduction of the medullary cords and an increase in the amount of the sinuses, which resemble those of the spleen-pulp rather than the more open lymphatic sinuses; and their trabeculae are also like those of the spleen in having numerous smooth muscle-cells. Some of these hemolymph nodes have lymphatic vessels, but whether, as in the spleen, these are limited to the capsule, or whether they open into the blood-sinuses, making true hemolymph nodes, is not yet clear.
A difficult point in connection with the structure of the hemolymph nodes is the relation of the blood-sinuses to the blood-vessels. The greater weight of evidence seems to favor the view that the sinuses are connected with the veins rather than that the arteries open directly into them, although one observer fails to find any connection between the blood-vessels and the central sinus (Schumacher). This stage marks the first appearance of the haemal node in the neck, and shows the node in its simplest form, the follicle and its peripheral blood-sinus (Miss Sabin).
There are wide variations in the distribution and number of the hemolymph nodes; indeed sufficient observations have not yet been made to determine their complete distribution. They have been divided into three groups, the praevertebral, the renal, and the splenic. In one subject, in which they were very numerous, they occurred at the root of the lung, near the bronchi, and bronchial vessels, a few near the esophagus, a continuous praevertebral chain in the abdomen extending from the diaphragm to the upper two or three sacral vertebra, as well as a few along the coeliac axis and its branches, the superior mesenteric, renal, and iliac vessels (Lewis).
Schumacher, from a study of lymph-glands and hemolymph glands of various stages, concludes that the hemolymph glands are not to be considered as organs sui generis, but that they represent rudimentary forms of ordinary lymph-glands, which have lost their connections with the lymphatic vessels. Further investigations are needed to clear up this subject.
From Morris's treatise on anatomy.
