Vascular architecture and intestinal hematopoietic nests of two cyclostomes, Eptatretus burgeri and ammoncoetes of Entosphenus reissneri: a comparative morphological study.

Vascular architecture and the structure of the intestinal hematopoietic centers of two cyclostomes, the hagfish Eptatretus burgeri and the ammocoetes larva of Entosphenus reissneri, are compared. Blood cells of the hagfish are generated in hematopoietic nests that develop around intestinal veins established primarily for transport of absorbed nutrients. In ammocoetes, on the other hand, blood cells are generated in hematopoietic nests of the typhlosole, closely associated with venous sinusoids developing around the longitudinally oriented mesenteric artery of the typhlosole. A collateral vein of the mesenteric artery is completed in the typhlosole after metamorphosis. Since the spleen of higher vertebrates develops in relation to establishment of the collateral vein of the largest foregut artery, the intestinal hematopoietic nests of ammocoetes may be regarded as a model of the primitive form of the spleen of higher vertebrates. Hematopoiesis in the hagfish intestine is not related to establishment of a collateral vein; hence "primitive spleen" or "intestinal spleen" may be improper terms in reference to the intestinal hematopoietic tissue of the hagfish. Morphological characteristics of the hematopoietic nests of the two cyclostomes are essentially the same. Blood cells of these nests are generated in the intervenous tissue, supported by interstitial connective tissue cells and reticulin fibers. Granulated cells are the most common type in the primitive hematopoietic nests. No definitive erythrothrombocytopoiesis has been identified. Lymphocytes have not been observed in the hagfish; however, small lymphocytes have been observed in the vascular lumen of sinusoids around the hematopoietic nests of ammocoetes. These lymphocytes probably originate outside of the typhlosole.