Question

The nephrons in adult human metanephric kidney are 1 million per kidney, whereas in the pronephric kidney of embryo of human, number is _____ per kidney.
A. 100
B. 1000
C. 40
D. 10

Answer 1

Hint: The excretory system in humans consists of a pair of kidneys, one pair of ureters, one urinary bladder, and one urethra. The kidneys produce urine by filtering out waste and extra water in the blood.

Complete Answer:
1) Each kidney has almost 1 million complex tubular structures called nephrons which are its functional units. The separation of water, ions and molecules in the blood, and filtration of wastes and toxins are done by the nephron.
2) There are three stages of development associated with kidney formation. The first and largest to develop is the pronephric kidney. The second kidney to be formed is a mesonephric kidney. The third kidney is developed towards the back of the mesonephros known as the metanephric kidney.
3) A pair of pronephric appears by the third week of gestation in humans. However, they are not functional. Next to the pronephric ducts, the intermediate mesoderm condenses into pronephros, which are non-functional units and which disappear by day twenty five.
4) The mesonephric ducts start to develop in the next caudal area of the nephrogenic cord. The adjacent intermediate mesoderm also condenses into mesonephros. Though the numbers of mesonephros formed are about 40 pairs approximately, only the ones at L1-L3 continue to grow to form functional excretory units. The mature kidneys start to appear after the metanephric stage.

Thus, the correct answer is 40.

Note: Pronephros is a vestigial structure that disappears completely by the fourth week of human embryonic development. Mesonephros develops through the formation of intermediate mesonephric tubules, the main excretory organ in early embryonic life.