In this 3D visual lecture of Embryology, Dr. Aizaz from MedicoVisual talks about the formation of Urogenital ridge, Genital ridge and Nephrogenic cord. How the primitive kidneys like Pronephros, Mesonephros and Metanephros develop.
With the embryonic folding, along with other components of trilaminar germ layer, the intermediate mesoderm also folds. At the cranial and caudal end of the embryo, there is no intermediate mesoderm.
The intermediate mesoderm and the paraxial mesoderm get separated from the lateral plate mesoderm. Paraxial mesoderm goes on to form the vertebral column and contributes to the muscles and skin formation. While the intermediate mesoderm is now behind the lateral plate mesoderm. Specifically, it is behind the intraembryonic somatic mesoderm near its junction with the intraembryonic splanchnic mesoderm. This intraembryonic somatic mesoderm will form a thin epithelial membrane termed mesothelium and the epithelium is also referred to as “celomic epithelium”.
The intermediate mesoderm proliferates and forms a thick cylindrical structure called the “Nephrogenic cord” (Nephrogenic; means kidney-forming). The nephrogenic cord in its midpart is accompanied by another mesodermal mass (also derived from the intermediate mesoderm). This mesodermal mass, covered with the overlying celomic epithelium, lies slightly ventral and medial to the nephrogenic cord and is the primordium of most of the parts of gonads (testes/ovaries). Hence it is termed as genital ridge. The nephrogenic cord and ridge along with the overlying celomic epithelium form a bulge, termed as “Urogenital ridge”.
In humans, three pairs of kidneys develop in a craniocaudal fashion with increasingly advanced design.
The first pair develops in the cervical region, called Pronephros, and is the most primitive type of kidney.
Pronephros is basically a mesenchymal tissue. Mesenchyme consists of irregularly shaped cells interspersed between a ground substance. In other words, mesenchyme is the embryonic connective tissue.
The mesenchymal cells in pronephros become arranged in pair of blobs or segments called nephro-tome (NOT NephroStome) - as in sclerTome or DermoTome. Cells in the center/core of each blob die or undergo atrophy. While the cell at the periphery acquires a particular shape and establishes cell-to-cell contact. In other words these cells undergo mesenchymal-to-epithelium transition. So, this primitive pronephric nephron consists of epithelial cells lined cavity.
It then assumes a specific shape and forms an s-shaped pronephric tubule. At the anteromedial end of this tubule, a tuft a capillaries from a branch aorta gets pushed into it and forms the renal corpuscle. However, in humans pronephros are non-functional and such glomeruli do not form in human fetuses. The lateral end of the tubule joins with lateral end of the neighboring tubule immediately caudal to it. A series of such connection forms a long duct craniocaudally called pronephric (collecting) duct.
From the last pronephric tubule, the duct grows caudally downwards and along the medial side Intermediate mesoderm and even grows beyond the caudal end of this mesoderm to finally meet with the cloaca.
The pronephric kidney is the most primitive and basic type of kidney. In small fishes, it is functional and definitive kidney. Their bodies aren’t too complex and they live in water. It is very easy to excrete the limited variety of waste products formed in body of such fishes with simple type of kidney. However, humans are much more complex. They ingest and create several different type of toxic substances and have large bodies. Such simple kidneys are simply not enough for humans. So more complex kidneys are needed. The pronephric kidneys in human are non-functional and regress shortly. As the caudal pronephric tubules form, the cranial tubules are gradually lost, along with their adjacent portion of their duct.
As the cranial portions are lost the caudal most portions of pronephros induce the formation of next more advanced pair of kidneys called mesonephros. Mesonephric tubules form in a similar fashion as pronephric tubules. The tubules create urine and collecting duct is involved in transport of urine to urinary bladder and also involved in pulling water thus further concentrating the urine even. So for this primitive function there is no need to re-invent the wheel. Rather than forming its own collecting duct, it uses the distal (and remaining) part of the pronephric (collecting) duct. After mesonephric kidneys assert their ownership over the pronephric duct, now it renamed as mesonephric duct, also called Wolffian duct on the name of scientist who first described it.
Even the mesonephric kidney is not enough to cater the needs of an organism as complex as human. So most of the tubules of mesonephros regress along with their adjacent collecting duct. However, in some smaller animals, mesonephric kidneys are the definitive kidney.