Kidney Development

Week 7- Highlights- Kidney, lung and other organs began development, external genitals start to develop, eyelids form, bone development begins.

Source: http://kidneyadvice.net/kidney%20disorders.html

Ok let’s talk kidney development, to be honest and geeky, the kidney is one of my favorite organs in terms of anatomy and physiology. Let’s do things in reverse for the kidney, the basic anatomy and physiology of the adult kidney is a series of tubules that allow for the filtration of blood and the ultimate formation of waste (in the form of urine) to be sent to the bladder. The kidney also functions in maintaining proper homeostasis in the body, maintaining balance of water content and electrolyte composition, as well as blood pressure and volume. Since the kidneys work with blood and urine, it is safe to say there is co-mingling of the kidneys with components of both the circulatory system and the urinary system. The major functional unit of the kidney is the nephron which houses both the filtration device, called the glomerulus- a network of capillaries, and the collecting tubes, shown in the picture above with the various parts and their respective names. The tubules and ducts take in and transport any materials (urea, waste, extra water) to the bladder via the ureters.  

Recall that everyone has a pair of kidneys, and thus at the earliest stage of kidney development we start with an embryonic pair of kidneys. Even though week seven was referenced on one particular website as being when the kidney forms there is behind the scenes work being done as earlier as 22 days. The earliest form of kidneys represent another transient/embryonic structure like some others we have encountered and are called the pronephros. (Kidney terminology has a lot of ‘nephro’ (Greek) in it as the nephron is the functionally unit found in this organ. Also renal (Latin) is used a lot). The pronephros grows out from the intermediate mesoderm when epithelial cells are arranged into, you guessed it, tubules. This makes sense with our ongoing theme of organogenesis starting with a bunch of tubes but it also makes perfect sense when we think ahead to the adult kidney being a series of tubules and ducts. To me the development of the series of tubes and tubules of the kidney can be thought of as growing vines, spreading out in different directions, some dying off at certain points while others are thriving, and some adding on more complex anatomical structure as they mature and grow. This first pair of primitive kidneys are orientated on the head to tail axes and as they grow downward they induce more caudally located intermediate mesoderm to produce the second set of primitive tubules, these are the mesonephric tubules of the mesonephros. These are linked to the aorta which mimics the adult kidney glomerulus (the dorsal aorta gives rise to the capillaries of the glomerulus). The first part, the pronephros, are not functional but now this second pair hooked up to the aorta is functional to allow for filtrate to flow in and be drained. By the time the mesonephros comes about the first pair of embryonic kidneys, the pronephros, start to degenerate. Like the pronephros, the mesonephros grows downward toward the tail end of the organism. In the fifth week the mesonephros develops the ureteric bud, the stalk of this bud will eventually become the ureter as well this bud grows into several key components of the kidney’s overall anatomy. This ureteric bud starts growing at the most caudal point and develops upwards, which is why the overgrowth of vines is an appropriate analogy.   Once the ureteric bud is made, it then interacts with the final precursor cells, the metanephric mesoderm to form the final pair of kidneys, which as you can probably guess go by the name of metanephros. This interaction of the ureteric bud and mesoderm will give rise to the kidney glomeruli and renal tubules while the ureteric bud gives rise to collecting ducts and ureters (the ureters transport urine to the bladder).  As the organism grows, the kidneys will eventually rotate and migrate and this will force the ureters to increase in length as well.  Urine production begins late in the first trimester. It is passed to the amnion where it will make up most of the amniotic fluid.

Source: http://commons.wikimedia.org/wiki/File:Development_of_embryonic_nephrons.png