Embryology of the Kidney

Did you know that as humans we develop three kidneys? Well, you might be saying that cannot be for real because humans only have 2 kidneys. Well, don’t panic this article will take you through the development of the kidneys.

The 3 Stages of Development of the Kidney

There are 3 stages in the development of the kidney:

Pronephros (A)
Mesonephros(B)
Metanephros(C)

Development of the Vertebrate Kidney. Source: Developmental Biology 8th edition, Fig.14.21(Part1)

PRONEPHROS

This is the first kidney
It develops during the 4^th week of uterine life in the cervical region of the intermediate mesoderm.
The pronephros contains lots of segmental vesicles and has the pronephric duct that grows caudally toward the cloaca.
The pronephric duct is in the embryo and thus cannot filter materials outside the embryo. Therefore it is said that the pronephro kidney is nonfunctional in humans thus it degenerates.

THE MESONEPHROS (The 2nd Kidney)

The Mesonephros kidney follows the development pronephros in about the ending of 4^th week of uterine life. It is also comes from intermediate mesoderm and is located in the throcolumbar area where the pronephric duct that was developed in the pronephos stage continue to elongated caudally and become the mesonephric tubules. Most of this tububles eventually degenerates but leave behind the Mesonephric duct or Wolffian duct which extends towards the cloaca
The mesonephros is the functioning kidney during the 1^st trimester and it produces urine during weeks 6-10.
It contributes to the:
- Epipdymis
- Vas deferens
- Seminiferous tubules

THE METANEPHROS (The Permanent Kidney)

A. Lateral view of a five-week embryo, showing the primordium of the metanephros or permanent kidney. B-E. Successive stages in the development of the metanephric diverticulum or ureteric bud (fifth to eight weeks). Observe the development of the ureter, renal pelvis, calices, and collecting tubules. The renal lobes, illustrated in E, are still visible in the kidneys of a 28-week fetus. (Modified from Moore KL, Persaud TVN. The Developing Human. Clinically Oriented Embryology (6th ed). Philadelphia: WB Saunders, 1998; with permission.) — The Primordium of the Metanephros of a Five-Week Embryo.

This kidney begins to develop in the 5^th week of the embryonic period.
It appears in the sacral region of the intermediate mesoderm
During the 5^th week the Mesonephric duct fomr the mesonephros kidney develops an outgrowth called the ureteric duct. This duct is close to the attachment of the cloaca.
The ureteric duct eventually form the:
- ureter
- renal pelvis
- major calyces
- minor calyces
- collecting tubules
The ureteric bud interacts with a portion of undifferentiated intermediate mesoderm called the metanephric mesenchyme. This interaction induces differentiation and thus the formation of the:
- renal tubules
- glomerulus
- distal and proximal convoluted tubules
- Loops of Henle

If there is an abnormal interaction between the ureteric bud and the metanephric mesenchyme, several congenital malformations of the kidney may result.