Invertebrate Hematopoiesis: An Anterior Proliferation Center As a Link Between the Hematopoietic Tissue and the Brain

Abstract

During evolution, the innate and adaptive immune systems were developed to protect organisms from non-self substances. The innate immune system is phylogenetically more ancient and is present in most multicellular organisms, whereas adaptive responses are restricted to vertebrates. Arthropods lack the blood cells of the lymphoid lineage and oxygen-carrying erythrocytes, making them suitable model animals for studying the regulation of the blood cells of the innate immune system. Many crustaceans have a long life span and need to continuously synthesize blood cells, in contrast to many insects. The hematopoietic tissue (HPT) of Pacifastacus leniusculus provides a simple model for studying hematopoiesis, because the tissue can be isolated, and the proliferation of stem cells and their differentiation can be studied both in vivo and in vitro. Here, we demonstrate new findings of a physical link between the HPT and the brain. Actively proliferating cells were localized to an anterior proliferation center (APC) in the anterior part of the tissue near the area linking the HPT to the brain, whereas more differentiated cells were detected in the posterior part. The central areas of HPT expand in response to lipopolysaccharide-induced blood loss. Cells isolated from the APC divide rapidly and form cell clusters in vitro; conversely, the cells from the remaining HPT form monolayers, and they can be induced to differentiate in vitro. Our findings offer an opportunity to learn more about invertebrate hematopoiesis and its connection to the central nervous system, thereby obtaining new information about the evolution of different blood and nerve cell lineages.

Crustacean hematopoiesis

“…Both techniques have been used to describe the HPT in P. leniusculus. A high mitotic frequency was identified in the APC and at the lateral edges of the HPT, whereas in the center of the tissue surrounding the ophthalmic artery, low mitotic activity could be detected (Noonin et al, 2012a). This finding is consistent with GhirettiMagaldi, who proposed that the walls of the ophthalmic artery do not comprise hematopoietic lobules but reticular connective tissue extending ventrally towards the digestive gland (hepatopancreas) (Ghiretti-Magaldi et al, 1977).…”

“…The organ described by Allen in 1893 and the former "cephalic gland" were "rediscovered" in 2012 in P. leniusculus and named the anterior proliferation center (APC) (Noonin et al, 2012a). This APC was subsequently confirmed in P. clarkii (Chaves da Silva et al, 2013).…”

“…More than twice the number of BrdU-incorporating cells were identified in the APC compared with the posterior parts of the HPT, and this APC area was further shown to have high reactive oxygen species (ROS) production, which is clearly associated with proliferation (Chaves da Silva et al, 2013;Noonin et al, 2012a). The importance of a certain level of ROS production during differentiation has been discussed, and specific roles in regulating stem cell activity in hematopoietic stem cell niches have been shown in mammals (Simon and Keith, 2008).…”

“…In P. leniusculus, ROS production is primarily regulated in the APC, and recent experiments have shown that this ROS production is associated with differentiation, affecting the structure of the extracellular matrix surrounding the hematopoietic precursors (Junkunlo, 2015). A specific, highly proliferating area of the HPT has been described as the APC in P. leniusculus and P. clarkii (Chaves da Silva et al, 2013;Noonin et al, 2012b), as the cephalic gland in Astacus and Orconectes limosus (Böhm and Gersch, 1983), and as the blood-forming organ in P. varians (Allen, 1893). However, it is likely that similar structures are also present in other species.…”

“…This study showed how young, recently divided hemocytes were released into small hemal lacunae and further transported into the larger hemal space and main hemal sinuses (Johnson, 1980). Release into the hemal spaces in between the lobules was primarily observed in areas where the fibrous connective tissue is thin or missing in the center of the posterior HPT in P. leniusculus and P. monodon (Noonin et al, 2012a;van de Braak et al, 2002). To determine specific lineages within the HPT, valuable information can be obtained from studies of cell morphology, but these studies are not sufficient.…”