Sea-urchin Development in the Light of Enzymic and Mitochondrial Studies

T H E sea-urchin egg is characterized by a very high morphogenetic plasticity. Its trend in differentiation can in fact be controlled by means of various chemical agents. The development of the entoderm is thus favoured at the expense of that of the ectoderm by adding lithium ions to the sea-water (Herbst, 1892; Lindahl, 1936). Iodosobenzoic acid (Runnstrom & Kriszat, 1952) and thiocyanate (Herbst, 1892; Lindahl, 1936) have an opposite effect, i.e. they favour ectodermal development. The mechanism of segregation of the egg into the primary germ-layers might be elucidated if we knew more about the biochemical mode of action of these agents. A series of biochemical studies on lithium-treated and normal eggs and larva was therefore undertaken. The studies began on the level of amino acids and peptides and continued on the levels of enzymes and intracellular inclusions. The total changes in the amino-acid composition of the egg were found to be rather small (Gustafson & Hjelte, 1951). Changes generally did not appear until the hatching stage or around the onset of visible differentiation (mesenchymeblastula stage). They were most pronounced in the fraction of free amino acids and peptides, where changes already occurred during cleavage stages. The appearance of new antigens could not be demonstrated before an early gastrula stage (Perlmann & Gustafson, 1948). Between this and the 48-hour stage a new component appeared. Studies on enzymic development reinforced the impression that the cleavage period is a 'silent phase' (Augustinsson & Gustafson, 1949; Gustafson & Hasselberg, 1950, 1951; Mazia et ah, 1948). Thus the activity of a series of enzymes was low and rather constant during this period, but increased in the late blastula stage or during gastrulation (alkaline phosphatase, dehydrogenases, glutaminase I, cathepsin II, apyrase, and cholinesterase). In another series of enzymes the activity was constant throughout early development up to the pluteus stage (aldolase, adenosinedeaminase, a proteolytic enzyme, phenolsulphatase, enzymes splitting inorganic pyroand hexametaphosphate (Gustafson & Hasselberg, 1951) and a peptidase (Holter, 1936)). Several of the enzymes with increasing activity are to a considerable extent localized in mitochondria, whereas those enzymes showing a constant activity as