Week 10

Tutorial : Week 10                            

Topic : Development of the kidney, Ontogeny and Phylogeny

1.  What does the statement "ontogeny recapitulates phylogeny" convey?

Ontogeny refers to the development of an organism, particularly the process of organogenesis. Phylogeny is the evolution of species. After the publication of Darwin's Origin of Species, many biologists thought all the stages of its evolutionary history.  This was the origin of the famous phrase "Ontogeny recapitulates phylogeny" by Ernst Haeckel.  Since Darwin's time, however, many studies have demonstrated that the relationship between ontogeny and phylogeny is not as simple as Haeckel originally thought.  We know now, for example, that in the course of development, structures can be added or subtracted from those of ancestors, or their development can be accelerated or retarded.

It may be said (with some reservations) that the embryonic development of an organism recapitulates embryonic developmental stages of ancestors.

The development of the heart is an example. The early heart with no left-right partition, but with the primitive chambers resembles that of a fish. An incompletely partitioned heart (atria separate, common or incompletely partitioned ventricle) resembles that of some amphibia. The reptilian heart is difficult to imagine in the mammalian embryo – this may be thought of as a "reservation" in the concept!

The development of kidneys is perhaps a good example, where successive stages in vertebrate evolution appear to be recapitulated in mammalian development. The pronephros functions as the kidneys in the adults of a few species. Most fishes and amphibians have a mesonephric kidney. Mammals have a kidney which develops from the metanephros.

Another good example of how structures are added to or deleted from a common blueprint is provided by the evolution of branchial arch pattern. These structures form the respiratory apparatus in fishes. With the evolution of lungs we do not need these gill arches. The genetic mechanism for the development of these structures has been used in land vertebrates to form some structures of the head and neck. This is studied in the second semester unit 213!

2. What is pronephros? Explain in the context of mammalian development and evolution.

Pronephros is the "first kidney". In mammals it appears (in the cervical region of the intermediate mesoderm) for a short time around the fourth week of development and disappears. In early vertebrates it functioned as the definitive kidney and the mesonephros and the metanephros did not develop at all. In some primitive fishes it does function as the definitive kidney in present times.

3. What is mesonephros? What is its fate in mammals?

Mesonephros is the next series of "segmental kidneys". In mammals, it appears in the intermediate mesoderm caudal to the pronephros. The filtering elements drain into a duct, the mesonephric duct (Woffian duct). As this kidney becomes non-functional, its duct is utilised for forming the duct system of the male reproductive system. The duct forming the female reproductive system develops by the side of the mesonephric duct. It is called paramesonephric duct or Müllerian duct.

4. What are the developmental origins of the definitive mammalian kidney?

The mammalian kidney begins by the formation of a 'ureteric bud'  from the end of mesonephric duct. The blind end of this bud branches to form the calyces of the kidney. The ureteric bud induces the formation of metanephric tissue from the caudal end of the intermediate mesoderm, in the region of the future pelvis. The metanephros (the "other” kidney) differentiates into the filtering mechanism, loop of Henle and the convoluted tubules – in other words, the "sercreting" parts of the nephrons.

5. How is the basic pattern of the adult mammalian kidney established?

The unbranched part of the ureter elongates as the kidney "ascends" from the pelvis. The uriniferous tubules, developing from the metanephros, establish connections with the fine terminal branches of the ureteric buds. Failure to do so results in accumulation of fluid in the secretory parts of affected nephrons. These fluid filled balloons are called cysts, and the condition is called polycystic kidneys.

It may also be mentioned that the "ascent" of the kidneys may be arrested at various stages, resulting in anomalous locations of the kidney in the adult. The presence of additional renal arteries is explained (not entirely satisfactorily though) by the fact that during the ascent the kidney receives blood from arteries at every level.