Melanomacrophage functions in the liver of the caecilian Siphonops annulatus

Abstract

Melanomacrophages are phagocytes that synthesize melanin. They are found in the liver and spleen of ectothermic vertebrates, and in the kidney of fish. In agnathan and elasmobranch fish, melanomacrophages are seen as isolated cells, and forming clusters in all the other vertebrates. The natural phagocytic activity of melanomacrophages is poorly characterized, as most of the research works have focused on induced phagocytic activity only. Furthermore, little is known about amphibian melanomacrophages, mainly about those in caecilians - wormlike amphibians in the order of Gymnophiona, which is the least known group of terrestrial vertebrates. The present research work aimed at the structure and function of hepatic melanomacrophages of Siphonops annulatus, a species largely found in South America. We identified the role of these cells in the control of circulating basophils (pro-melanogenic cells), in the turnover of liver collagen stroma and in the hemocatheresis, interrelated physiological mechanisms.

Keywords: Kupffer cell; amphibia; basophil; caecilian; liver; melanin; melanomacrophages.

Figure 1

Melanomacrophages clusters (arrowhead) show engulfed collagen in the liver of animals (A, B and C, arrow). We can see fragments of digested collagen in the cytoplasm of melanomacrophages in the animal (C, arrow).

Figure 2

Melanomacrophage clusters (Mm) show two engulfed basophils in the liver of animal (A, arrowhead). We can see fragments of digested basophil in the cytoplasm of melanomacrophages in both animals (A and B, arrow).

Figure 3

(A) The hepatic capsule (capsule of Glisson) shows a rich network of collagen fibers (arrow) stained in red color by the Picrosirius method. (B) The hepatocytes cords (HC) are surrounded by collagen fibers stained in blue color by Castro & Camargo trichrome method (arrow). Thinner collagen fibers (arrowhead) are shown in the subcapsular hematopoietic tissue (HT). (C) A rich network of reticular fibers is demonstrated by the Gomori reticulin method. These fibers (arrow) surround the HT and the HC. (D) Elastic fibers are rarely found in the hepatic capsule (arrow) around the HT, Weigert resorcin‐fuchsin method.

Figure 4

(A) The hepatocytes evidenced by the trichrome method were frequently rounded or triangular, and had a rounded large nucleus (arrow and arrowhead). The hepatocytes cords, in cross‐sections, frequently feature stick format, formed by the union of four triangular hepatocytes with convex faces back to the sinusoid (arrow). The hepatocytes cords are often organized as a double layer in longitudinal sections (arrowhead). (B) The portal space is composed of branches of the portal vein (PV), hepatic artery (HA) and a bile duct (BD) (the small fragment of hepatocyte inside the HA is a technical artifact).

Figure 5

Sinusoids in the animals (A–C) varied from narrow (arrow) to wide diameter (arrowhead); however, the narrow sinusoids (arrow) were more frequent around the hematopoietic tissue (HT) and are generally full of blood cells. Large quantities of narrow sinusoids (arrow) are seen in slices of the hepatic parenchyma taken tangentially to the HT (A).

Figure 6

(A) Plasmic glycogen (white arrow) was also identified in the cytoplasm of melanomacrophages (black arrow) and hepatocytes (arrowhead) by the periodic acid‐Shiff (PAS) method. (B) and (C) The method of Perls allowed the indirect observation of erythrocyte phagocytosis (hemocatheresis) by isolated and clustered melanomacrophages in the form of hemosiderin deposits (arrow) within the cytoplasm. Hepatocytes nuclei are evident (arrowhead).

Figure 7

(A) The ultra‐structural study showed a bile canaliculi (arrow) formed by membrane junctions (arrowhead) between four hepatocytes (HC). (B) Endothelial cells (ED) with long membrane extensions (arrow) separate the sinusoidal lumen containing erythrocytes (EC) from the perisinusoidal space (DS). HC, hepatocytes. (C) Collagen fibers of the liver stroma (arrow) surround the hematopoietic tissue (HT) and hepatocytes cords (HC).

Figure 8

(A) Melanomacrophages (white arrow) are scattered throughout the sinusoids, showing pinocytic vesicles (black arrow) in the cytoplasm near the endothelium. Melanin granules (black arrowhead) are distributed in the cytoplasm. The hematopoietic tissue (HT) shows two neutrophils (white arrowhead). (B) Melanomacrophage (white arrow) and its membrane extensions (white arrowhead) engulfing two cells (CN1 and CN2). We can see the loss of cytoplasm content and membrane continuity initiated by the phagocytic process of the cell CN2 (black arrow).