Which of the following contribute to the formation of the concentration gradient of BMP-4 in the frog embryo?
I. BMP-4 is produced in the ventral portion of the embryo
II. Chordin is produced in Spemann's Organizer III. Chordin inhibits BMP-4 IV. Xolloid is produced in the ventral portion of the embryo. V. Xolloid degrades chordin.
A - I
B - I, II, and IV
C - I, III, V
D - I, II, and III
E - I, II, III, IV, and V
Answer: E - I, II, III, IV, and V
Xolloid helps to sharpen the concentration gradient of BMP-4 activity by degrading chordin in the ventral part of the embryo, where BMP-4 levels are already high. The gradient is set up along the dorsoventral axis.