HEPATOGENESIS OF THE ATLANTIC SALMON (SALMO SALAR L.)

Abstract

The anlage of the liver of the Atlantic salmon, Salmo salar L., arises during the eleventh week, 4.0 mm. embryo, as a proliferation of entodermal cells from the ventral wall of the anterior intestinal region. This anlage soon takes on a double appearance caudally. In embryos of 12 to [Formula: see text] weeks (4.7 to 5.0 mm.) it consists of right and left primordia with a common ventral opening into the intestine. By 13 weeks (5.4 mm. embryo) the anlage has curved toward the left, a position assumed through the counter-clockwise rotation of the intestine. The cells are becoming arranged in cords. A subsequent clockwise rotation of the intestine, apparent first in embryos of 14 weeks (6.5 mm.), draws the anlage to a dextroventral position. By 16 weeks (8.5 mm.) the ductus choledochus enters the right side of the intestine; the anlage itself passes posteriorly along the ventral surface of the intestine then abruptly forward in the form of a hook to the right of the intestine. The liver cells are arranged as branching trabeculae with some indication of vascularization. The liver anlage becomes rhomboidal and migrates to a position on the right of the rectilinear intestine, where it remains until some time after hatching. In the 25 mm. alevin (26 days after hatching) the intestine through a process of flexion commences to differentiate into cardiac and pyloric stomach, ascending and descending limbs of intestine. Simultaneously the papilla duodeni marking the opening of the ductus choledochus into the intestine is carried to a sinistrodorsal position opening into the ascending limb of the intestine at the level of the second row of pyloric caeca. Subsequent clockwise rotation of the ascending limb finally carries it to the sinistroventral margin. Yolk absorption now permits the liver to grow ventrad to the intestine, while the anterior progression of the intestinal loop is instrumental in its migration to the anterior end of the coelom. In the early parr the liver is somewhat triangular in shape with the apex posterior and in the mid-line. Through unequal growth the left median lobe in the late parr becomes much longer than the right, from which, however, it is never distinctly divided. It has now assumed the general appearance of the organ in the smolt and adult. Enlargement of the blind end of the ductus choledochus in the 21 week embryo (11.8 mm.) forms the gall bladder. Hepatic ducts are formed as branches of the primary ductus choledochus.