Growth of the opisthonephric kidney during larval life in the anadromous sea lamprey, Petromyzon marinus L.

Light microscopy, autoradiography, and gross anatomical measurements are used to study the growth of the larval opisthonephric kidneys of anadromous sea lamprey, Petromyzon marinus. Linear regression analysis of kidney lengths of the ammocoetes, ranging in length from 13 mm to 151 mm, reveals six natural groups of ammocoetes, year classes 0 to V, with each group demonstrating a definite growth pattern of the kidneys.Within the year classes 0 to III, the nephron rudiments at the posterior tip of the kidney differentiate periodically into functional nephrons as the larval opisthonephros lengthens in an anterior lo posterior direction. Labelling of cells with [ 3 H]thymidine occurs in the differentiating nephrons and in the adjoining nephrogenic tissue. In contrast, the growth of the opisthonephric kidney in the year classes IV and V appears to result from the elongation of existing nephrons by cell proliferation.The length of the kidneys appears to be a valuable asset to our present means of age classification of the ammocoetes of anadromous P. marinus. The ammocoetes provide a unique opportunity to study growth and differentiation of a vertebrate kidney over a long period of time.