Describe the Holonephros kidney.
- Hypothetical ancestral kidney.
- Kidney was long and segmented.
- Glomeruli were large, but it is not clear wheather they were internal or external; in either case, nephrostomes were present.
- Holonephric kidneys are found only in larvae of hagfishes and caecilians.
Describe the Pronephros kidney.
- The kidney of most vertebrates develops in sequence from front to back: the mostanterior portion (and the first portion to form during embryological development) is of the pronephric type.
- Few nephrostomes are incorporated.
- Before becoming functional, pronephric tubules usually fuse to form a tangled mass (the glomus).
- pronephric kidneys are functional only in free living larvae of teleosts and amphibians.
- a modified pronephros is functional in hagfishes and some teleosts, where it is called the head kidney due to its proximity to the pericardial cavity and being separate from the primary kidney.
Describe mesonephros kidneys.
- Found in later embryological stages of vertebrates.
- Kidney tubules fail to differentiate in a few body segments caudal to the pronephros, so there is a gap between the pronephros and more caudal renal tubules.
- Tubules that differentiate in the middle part of the nephric ridge (the embryological line which will develop into kidneys) form a kidney called the mesonephros.
- This kidney functions in embryos and larvae of vertebrates.
- The archinephric duct (a duct draining the first formed kidneys) into which the mesonephros drains is called the mesonephric duct or wolffian duct.
Describe opisthonephros kidney.
- A kidney that includes the embryonic mesonephros and tubules that develop in the caudal part of the nephric ridge.
- In advanced opisthonephros kidneys, renal tubules multiply in the caudal region of the kidney so it is enlarged. in this case, the anterior portion of the kidney is small, and in the male receices sperm (which the archinephric duct transpors)
- Primitive opisthonephros - adult hagfishes.
- Advanced opisthonephros - most fishes and amphibians.
Describe the metanephros kidney.
- Kidney in amniotes.
- A ureteric bud grows into the caudal end of the nephric ridge and branches extensively, which induces differentiation of many renal tubules that tap into it.
- The ureteric bud forms the collecting ducts and ureter that drain the adult kidney (called metanephoros.)