The morphology and histochemistry of the echinoid axial organ

Abstract

The structure of the main portion of the axial organ ofArbacia lixula, Diadema antillarumandParacentrotus lividusis described. This portion, which lies at the confluence of the perivisceral coelom, water vascular and ‘haemal’ systems, is pre-eminently glandular. It has an irregular central lumen containing the contractile vessel and a spongy peripheral region permeated by three systems of cavities: the embayments of the central lumen, the lacunae and the canaliculi. The lacunae communicate with the contractile vessel and the canaliculi with the perivisceral coelom. These two systems are closely associated, being separated at the most by an attenuated epithelium which in many areas breaks down, allowing the contents of the lacunae to spill or proliferate into the canaliculi. The predominant histological features of the organ are associated with the production of secretion by the transformation of cells including amoebocytes within the lacunae, and in Diadema and sometimes in Arbacia, with the accumulation of large quantities of pigment. The organ is permeated by connective tissue, unevenly distributed muscle fibres and by varying numbers of amoebocytes. Secretion may be discharged into the lacunae, the central lumen, the canaliculi or directly from the free surface of the organ into the perivisceral coelom. Histochemical tests showed the presence of abundantPAS-positive material in the secretion, most of which is not glycogen, but the amount of glycogen is increased by carbohydrate feeding, after which it appears in the cells lining the canaliculi and some is secreted into the lacunae and canaliculi. Most of thePAS-positive substance proved to be acid mucopolysaccharide. The secretion also showed the reactions of lipid-bound and protein-bound reducing groups, together with other reactions of protein or amino acids and indole derivatives, at least one of which is a 3-indolyl compound, probably tryptophan. Both tyrosine and tryptophan are conspicuous in some amoebocytes. The possible function of some of these compounds is discussed. Tests for enzymes such as tyrosinase and alkaline phosphatase gave no clear indication of their presence. There was no indication of any significant amount of neutral fat or phospholipid. Variable and sometimes impressive amounts of brown and orange to red pigments occur in the axial organ. InDiademaandArbaciathe latter appears to be echinochrome A. The reddish pigment is mostly in the lacunae where it arises from the disintegration of amoebocytes, though inArbaciait may possibly arise also within other lacunar cells. A possible function of this pigment is discussed. Brown pigment is widely distributed in the axial organ and is frequently associated with amoebocytes. In all three species this pigment proved to be melanin and an iron-containing pigment of nuclear origin. InDiadema, lipofuscin arising from phospholipid is also present. Melanin is liberated into the lacunae by the degeneration of amoebocytes, some of which contain tyrosine but when in the axial organ, appear to possess no active tyrosinase. Part, at least, of the iron-containing pigment can also be traced to amoebocytes.