To start off my Gross Anatomy Elective, I will begin with a review of embryology because I love embryology! It is like origami but with organs which inspired me to start my “Organami” project.

This Daily Doodle illustrates the embryology of the kidneys. The embryologic origin of both the urinary system and the genital system are closely associated with each other and they are often referred to conjointly as the “urogenital” system. The urogenital system develops from the intermediate mesoderm which forms a longitudinal elevation along each side of the dorsal aorta called the urogenital ridge [1].

During the early development of the human embryo, there are three sets of “kidneys”. The first set is the pronephroi, the second set is the mesonephroi and the third set which becomes the permanent kidneys are the metanephroi.

1. Pronephroi: Nonfunctional structures that appear as a cluster of cells in the cervical neck area which form around a tube-like structure called the pronephric duct. The pronephroi is depicted in the doodle by the dotted lined structure. These “rudimentary” kidney structures appear early in the fourth week of development. Near the end of the fourth week, they begin to degrade and disappear. However, the pronephric duct remains and becomes the mesonephric duct.
2. Mesonephroi: These long transitory kidneys represented by the green structure in the doodle begin to develop at the end of the fourth week.The mesonephric kidneys consist of glomeruli and tubules which open into the mesonephric ducts and drains into the cloaca. These mesonephroi degenerate by the first trimester however, in males, the tubules become the efferent ductules of the testes. The mesonphric duct has multiple adult derivatives summarized in the table below [2].
3. Metanephroi: These early versions of the permanent kidneys begin to develop in the fifth week and they start functioning around the ninth week. The permanent kidneys develop from two different structures, the metanephric diverticulum (ureteric bud) and the metanephrogenic blastema (aka metanephric mass of mesenchyme) [1]. They are depicted as the little green protruding nodule just below the mesonephroi.

The metanephric kidneys begin to develop in the pelvis and are close together. As the embryo grows in size and stretches out caudally, the kidneys “ascend” to the abdomen and move farther apart. “Ascend” is in quotations because it is more of a relative change in position because the rest of the embryo is growing… Anyways, the kidneys end up in the abdomen and they rotate medially 90 degrees. Interestingly, the blood supply of the kidneys changes as their position changes. Initially, the kidneys received its blood supply from a branch off of the common iliac artery as seen in this doodle but later on, the kidneys receive its blood supply from the renal arteries which are connected directly to the aorta. So fascinating!

Pop Quiz!

1. The urogenital system develops from which germ layer?
2. What are the three sets of “kidneys” during the development of a human embryo?
3. What do the mesonephric tubules become in an adult male?
4. What is another name for the mesonephric duct?*
5. What two structures form the permanent kidney?

 *Answer to 4 – Wolffian duct

References:

1. K. Moore and T. Persaud. “The Developing Human”. 8th Edition. Saunders 2008.
   
2. V. Bharat. Appendix 5 Table showing adult derivatives of vestigial remains of mesonephric duct. DNB CET REVIEW For Primary and Post Diploma. http://www.slideshare.net/SaritChakraborty/dnb-cet2011-question-and-answers
3. Here is a fun embryology website resource: http://www.embryology.ch/anglais/turinary/devebauche01.html#introduction

“Kidney Embryology” Daily Doodle by Michiko Maruyama