THE MOLTING CYCLE OF THE SPINY LOBSTER, PANULIRUS ARGUS LATREILLE. IV. POST-ECDYSIAL HISTOLOGICAL AND HISTOCHEMICAL CHANGES IN THE HEPATOPANCREAS AND INTEGUMENTAL TISSUES

Abstract

1. During the early postmolt period (Stages A and B) as rapid accretionary growth and calcification of the skeleton are occurring, changes are observed in the hepatopancreas and integumental tissues. 2. As the principal layer of the skeleton in Panulirus is deposited during Stage A and B, the outer epidermis concentrates or synthesizes a considerable amount of glyco- or mucoprotein, which is probably related to and involved in the secretion and development of this layer. Similarly, the pigmented layer, one of the preexuvial layers formed before molt, shows the presence of this same mucopolysaccharide. Near the end of Stage B, however, the properties of the basic organic components in the pigmented layer have been altered, possibly by considerable impregnation with calcium salts or by quinones. 3. During the deposition of the principal layer of the outer integument, glycogen accumulates in large amounts in the epidermis. There is a periodic shift of glycogen from the sub-epidermal tissues to the outer epidermis and in turn a shift from the hepatopancreatic tissues to the integumental tissues. During this period of accumulation and utilization by the integumental tissues (Stages A and B), hepatopancreatic glycogen progressively disappears and by the end of Stage B none remains. 4. The possibility of lipid conversion to carbohydrate, and the storing of this as glycogen in the hepatopancreas during Stage D, is discussed. The utilization of glycogen by the epidermis during Stages A and B, periods of inanition, is also discussed. 5. During Stages A and B alkaline phosphatase is heavily concentrated in the distal ends of the outer epidermal cells. It is observed in the integument in the region of the proximal portions of the pore canals, even before calcification begins. This is a region of high activity as deposition and hardening of the post-exuvial layers occurs. The enzyme is likewise found in the reserve cells of the subepidermal cells. In the hepatopancreas, the most marked localization of the enzyme is seen around the innumerable calcospherites in the absorption cells and in the striated border of these cells. The reserve cells of the hepatopancreas likewise show the presence of the enzyme. Functions of phosphatase in these sites are suggested. 6. Calcification begins the second day following molt and occurs thereafter simultaneously with, or immediately accompanying, the elaboration of layers of the principal zone. Calcification of the pre-exuvial layers, formed before molt, is a process accomplished long after their formation but during the same period at which the post-exuvial layers, formed after molt, are being calcified. Calcium, heavily concentrated in the distal region of the epidermal cells, is extruded from this tissue in two distinct ways. As the post-exuvial layers are deposited, masses of calcium granules are simultaneously extruded, thus forming narrow bands paralleling the epidermis. In the pre-exuvial layers, on the other hand, calcium granules are observed in uniform vertical rows which emanate from the epidermis. These vertical rows of calcium granules correspond in location to the pore canals. Since calcification of the pre-exuvial layers occurs after their formation calcium must be transported and deposited some distance from the newly forming post-exuvial layers. This transport occurs through the pore canals, protoplasmic extensions of the epidermis, thus enabling this tissue to act at these distant sites. 7. While calcification of the integument occurs, the reserve cells in the subepidermal tissues undergo what appear to be cyclic peaks in calcium storing alternating with cyclic release to the epidermis. The reserve cells in this capacity could serve as reservoirs for providing additional calcium during periods of concentration by the epidermis. Furthermore, these interesting reserve cells, during the early postmolt period (Stages A and B), undergo at daily intervals, cyclic peaks and declines in size and abundance, changes in structural appearance, and staining properties and the storing of reserves other than calcium. The mucopolysaccharide material, either muco- or glycoprotein, in the reserve cells disappears near the latter part of Stage B. This indicates a decrease in the concentration of the material and suggests that the mucopolysaccharide stored by the reserve cells represents reserve material for the construction of the new skeleton. The reserve cells of the hepatopancreas, like those of the integumental tissues, undergo during the early postmolt period (Stages A and B) cyclic peaks and declines in number, size and the storing and apparent release of reserves. They similarly show the presence of phosphatase, mucopolysaccharide, calcium and much lipid. 8. As calcification of the integument occurs there is a progressive decrease in number of calcospherites—spherules of calcium phosphate—present in the absorption cells of the hepatopancreas. These calcospherites, abundant preceding molt and on the first day following molt, progressively decrease in number as calcification of the new integument occurs and by the seventh day (end of Stage B) hardly a single calcospherite can be detected. The calcospherites, premolt storage depots of reserve phosphate from the old skeleton, probably represent a major source from which phosphate can be mobilized for hardening of the new skeleton during Stage B, a time at which the animals do not feed. 9. Development of the inner integument of the branchiostegites is completed in Panulirus during a period of three days preceding molt and two days following molt. Calcification of the inner integument, as in the pre-exuvial layers of the outer integument, occurs after its formation via the pore canals, and portions of this integument are completely calcified by the seventh day following molt (end of Stage B). 10. Calcium carbonate, the principal salt of the spiny lobster skeleton, constitutes approximately 42% of the total mineral deposited and is precipitated in the organic matrix as calcite, rather than aragonite or amorphous calcium carbonate. 11. The roles of carbonic anhydrase and citric acid in the calcification of the integument of Crustacea are discussed. 12. Continued accretionary growth of the skeleton and the hardening of it by calcification are two major tasks confronting the crustacean from Stage A to late Stage C. Although the epidermis takes the lead in the performance of these duties, the importance of the other tissues, namely the hepatopancreatic and sub-epidermal tissues, should not be under-estimated.