 |
|
|
The kidney as a whole is made up of two elements: the kidney tubules which filter the blood and the kidney ducts, which collect and carry the filtrate to the cloaca. The development of the kidney is somewhat complex in that two or three different kidneys (depending on species) form in temporal and spatial sequence. The first and most anterior and largest to develop is the pronephric kidney or the pronephros. The second kidney to form is the mesonephros. In birds, reptiles and mammals, a third kidney develops posterior to the mesonephros, called the metanephros.
This terminology can be confusing, but you should be able to keep the order of formation straight if you remember that pro means "first", mes means "intermediate" (mesoderm is the middle of the three germ layers - makes sense!) and meta means "beyond" or "later".
Before moving on, have a look a schematic of kidney development, which will help you understand what you are seeing in the images below and should help to clear up any confusion about the above description of kidney development. When you have viewed the schematic page have a look at some sketches of a developing kidney that follow the development of the metanephros to the adult kidney.
This page is relatively long and thus has 3 subdivisions:
| These pictures
show a cross section through the 7 mm tadpole at the level of the pronephric
kidney. The pronephros forms a functioning kidney in many immature fish
and the larvae of amphibians, but has questionable function in the mammalian
embryo. It is first seen as a segmenting of the intermediate mesoderm into
nephrotomes. The nephrotomes then develop into pronephric tubules. These
tubules empty into a pronephric duct (one per kidney), which have developed
on both sides of the body and have extended caudally. At the proximal end
of the pronephric tubules, ciliated funnels, called nephrostomes, form and
open into the coelomic cavity. Waste products are filtered from the blood
at a vascular ridge near the nephrostome known as the glomus. The wastes
are released into the coelomic space where they wafted, via the cilia, from
the coelom, through the nephrostome and into the pronephric tubules. From
there, the wastes move through the pronephric duct and end up in the cloaca
for excretion.
In the embryos of birds, reptiles and mammals, the pronephros appears briefly as a transitory structure that degenerates as the mesonephros develops immediately posterior to it. When this occurs, the pronephric ducts remain in the region caudal to and including the mesonephros and become known as the mesonephric ducts (or nephric ducts or archinephric ducts or Wolffian ducts or mallard ducts - kidding!).
|
||
| This is a cross section through the 7 mm frog at the level of the pronephric kidney. |  |
| This is a higher magnification of the above image showing the parts of the pronephros. hgh |  |
Go to the Serial Cross Sections of the 5-7 mm Frog Embryo:
- At Level of Pronephric Tubules
- At Level of Pronephric Ducts
| The functional pronephric kidney begins to degenerate as the mesonephric kidney develops. The mesonephros differs from the pronephros in that the mesonephric tubules form an expanded, blind-ended region that invaginates to form a double walled cup (similar to the formation of the optic cup) called Bowman's capsule. This invagination is caused by tuft of capillaries, called a glomerulus, pushing into and becoming surrounded by Bowman's capsule. The glomerulus surrounded by Bowman's capsule together are called a renal corpuscle (Malpighian corpuscle). As this is occurring, the mesonephric tubules become long and coiled, with their distal ends opening into the mesonephric duct. Developing mesonephric tubules, however, may still connect with the coelom at their proximal ends by way of nephrostomes in the mesonephros.
|
|
| The Bird, Reptile and Mammalian Embryo Mesonephros: In reptiles and birds, the mesonephros forms the functional excretory organ up to the time of hatching. After hatching, the metanephric kidney takes over. In placental mammals, the extent of development and functioning of the mesonephric kidney varies and seems to be related to the degree of connection between the fetal and maternal tissues. In mammals with a hemochorial placenta, the mesonephros is seen only as a transitory (essentially non-functional) organ. In mammals with an epitheliochorial placenta, such as the pig, the mesonephros develops and is functional until its replacement by the metanephros. |
|
| The Adult Amphibian Mesonephros - The Opisthonephros: The adult kidney of the lamprey, fish and amphibian is a modified mesonephros. These modifications are the development of some features characteristic of the metanephric kidney such as a complete absence of nephrostomes and the joining of nephric tubules to collecting ducts that have developed as outgrowths of the mesonephric ducts. This modification occurs in a distinct area of this kidney, known as the metanephric zone. To denote this kidney as neither being entirely a mesonephros nor a metanephros, it is often referred to the opisthonephros. |
| This is a cross section through the 10 mm pig at the level of the mesonephros. |  |
| This is a higher magnification of the above photo showing the details of the mesonephros. |  |
| A diverticulum of the mesonephric duct posterior to the mesonephros is the first indication of the development of the metanephros in reptiles, birds and mammals. This diverticulum is called the ureteric bud. The ureteric buds elongate into the region of presumptive metanephrogenic mesenchyme. As we saw with the formation of the optic cup, a reciprocal induction occurs here to form the metanephros. The ureteric bud induces the metanephrogenic mesenchyme to form the nephric (renal) tubules. This mesenchyme reciprocally acts on the ureteric bud causing it to branch and form a tree-like system of collecting ducts. The initial tube portion of the ureteric bud is called the ureter. At the metanephric end of the ureter, an expansion called the renal pelvis forms. This is the area which branches numerous times to form the collecting ducts. One end of the nephric tubules will connect to the collecting ducts, while the other end will form the double walled cup (Bowman's capsule) that surrounds a capillary tuft (glomerulus). The nephric tubules also become long, S-shaped and develops specialized regions. Eventually, the ureters separate from the mesonephric ducts, elongate and serve to carry urine from the kidney to the bladder. |
This image is a cross section, again, through the 10 mm pig at the level of the mesonephric kidney and the hind limb bud (HLB).
At the 10 mm stage, the embryo is C-shaped, resulting in the spinal cord, somites and mesonephric ducts being sectioned twice (see sketch, left). The caudal end of the embryo is towards the right.
|
 |
| This is a rotated, higher magnification of the caudal end of the embryo seen above showing early metanephros development. On the right, the ureter can be seen joining the mesonephric ducts and on the left, the metanephric pelvis can be seen surrounded by metanephrogenic mesenchyme. |  |
| The Adult Metanephros: The adult metanephros (the kidney as we know it) is kidney bean shaped, with the indent being called the hilum. The hilum is the region of the kidney where the renal artery and nerves enter and the ureter and renal vein exit. The renal pelvis is the immediate inside the hilum. The periphery of the kidney is surrounded by a connective tissue capsule. Beneath the capsule is a region called the cortex. The cortex is where the renal corpuscles (glomeruli plus Bowman's capsules) and most of the nephric tubule is located. A portion of the nephric tubule dips down below the cortex into a region known as the medulla. The medulla does not have any renal corpuscles, but is composed of nephric tubules, collecting ducts and numerous capillaries. The medulla narrows into a pyramid-shaped structure, known as the renal papilla, near the renal pelvis. Urine is discharged from the collecting ducts, which open into the renal papilla, and moves through the renal papilla to the renal pelvis and out of the kidney through the ureter.
|
|
| This is a medial longitudinal section through a typical adult mammal kidney. The collecting ducts can be seen as striations heading into the renal papilla from the medulla. |  |
 |
 |
| The above image shows the cortex of the kidney at a medium magnification. Note the renal corpuscles and the many profiles of nephric (renal) tubules. | This image shows a medium magnification of the medulla with its many renal tubules and collecting ducts. Note that there aren't any renal corpuscles. |
| View Sketches of Kidney Development |
| Back to the Histology II Index |
| Go to Epithelial Tissue |
|
Special thanks to Hans Christoffersen for the preparation of this page.
Send comments to:
Sandra K. Ackerley (),
Department of Integrative Biology, University of Guelph, Guelph,
Ontario, Canada N1G 2W1.