The Succession of Kidneys
Although all kidneys are fundamentally nephridial tubes that extract liquid waste from the blood, the kidneys of different animals are by no means homologous structures.
Among vertebrates there are three kinds of nephridial structures serving as kidneys that differ from each other not only in structure and position in relation to the blood system and the excretory tubes, but also in their embryonic history.
The three kinds of kidneys, as named by the English embryologist, Balfour, are the pronephros of a few cyclostomes; the mesonephros of fishes and amphibians; and the metanephros of reptiles, birds, and mammals.
The higher vertebrates, whose kidneys are of the metanephric type, pass through preliminary pronephric and mesonephric stages before the permanent metanephric stage is reached. As is frequently the case, comparative anatomy and embryology have supplementary and confirmatory stories to tell from different angles about the same thing.
The nearest approach among invertebrates to the vertebrate nephridial apparatus is found in the nephridia of many annelid worms (Fig. 371), which, however, are not connected together by common excretory ducts to form excretory organs, like the nephridia of all vertebrates, but rather consist of metamerically arranged pairs of independent tubes.
The Nephridial Apparatus of Amphioxus
The nephridial apparatus of amphioxus is very much like that of certain marine annelids (Fig. 372), but instead of extending practically the entire length of the body with a pair of protonephridia in every segment, as in the annelid worms, the primitive protonephridia of amphioxus are localized in the anterior part of the body throughout the region of the gill slits. They lie somewhat above the pharynx near the dorsal region and may equal the gill slits in number. Each nephridium is open externally, emptying independently into the peribranchial chamber (atrial cavity) surrounding the gills, and terminating internally in flame cells of the flagellate type. There are present no common excretory ducts for carrying away the excretion from the nephridia of amphioxus, but the peribranchial chamber, with its atrial opening, accomplishes the same purpose while serving at the same time as the avenue of escape for the water of respiration.
Thus the apparatus for urinary excretion in amphioxus is primarily concerned with coelomic drainage rather than with direct extraction of urinary waste from the blood, and it consists not of a single pair of organs, or nephroi, with their ducts, as in all true vertebrates, but of a series of independent paired excretory tubules of the protonephridial type, resembling those of some annelid worms.
The pronephric tubules, pronephridia, are few in number and metamerically arranged in the anterior part of the trunk region. Originating as evaginations of the coelomic epithelium along the lateral portions of the mesomeres, or nephrotomes, each retains a ciliated mouth, or nephrostome, which opens into the nephrocoele, the coelomic cavity of this region (Fig. 373). The outer extremities of the pronephridia, ending blindly at first, soon turn posteriorly, growing until they come in contact with one another and join together, down each side of the body, to form a common segmental, or pronephric, duct. This duct is eventually extended posteriorly, mainly by additions from the coelomic wall, to open into the cloaca.
On the opposite side of the nephrocoele from the nephrostomes a capillary ridge, the glomus, forms along the coelomic wall (Figs 373 and 374), so that there are two methods of obtaining excretory products from the blood. They can pass first by diffusion into the general body cavity and then into the nephrocoeles or they can go directly from the capillaries of the glomus into the nephrocoele. In either instance the liquid excretory material is passed on through the ciliated nephrostomes and pronephridia to the segmental ducts, which dispose of the waste to the outside.
No encapsuling connective tissue, like the tunica fibrosa of the human kidney, surrounds and unifies the pronephridia into a definite organ.
The pronephroi are best developed in cyclostomes, where in some species they persist throughout life, although replaced functionally in most cases by mesonephroi or kidneys of the second order.
It is probable that in some myxinoids, Polistotrema or Bdellostoma, for example, they remain as the lifelong functional kidneys. They also persist structurally in some teleosts. In other vertebrates, particularly types like elasmobranchs and amphibians that have a larval development, they put in a temporary embryonic appearance and later vanish.
In the shark Pristiurus there are four pairs of pronephric tubules; in the elasmobranch Torpedo, six; while in the legless amphibian Caecilia, ten pairs of pronephridia hold the stage for a time during early development. Transient traces of pronephridia in mammals have been described, one or more pairs even having been identified in early human embryos where their maximum growth is attained in embryos of about 3.5 mm. in length.
Although the downfall of the pronephridia seems to be universal, with the possible exception of certain cyclostomes already mentioned, the segmental ducts are more persistent and, as will be seen later, are retained to play an important part in the succeeding dynasty of the mesonephros, which reaches its maximum in human embryos of about 10 mm in length.
The second type of kidney in the vertebrate succession is the mesonephros, sometimes known as the Wolffian body. Like other kidneys this structure is made up of nephridial tubules, mesonephridia in this instance, that develop in the embryo from nephrotomes posterior to those which form pronephridia and at a later time. They are distinct from pronephridia as shown by their relation to the excretory ducts and by the fact that both pronephridia and mesonephridia may be present at the same time.
Mesonephridia, which are much more numerous than pronephridia, do not generally show the primitive metameric arrangement. The most anterior mesonephridia are the oldest, and subsequent additions arise posteriorly. They originate independently and connect secondarily with the paired segmental ducts that hold over from the former regime (Fig. 375).
Each mesonephridium primarily forks at its inner end. One branch terminates with a nephrostome opening into the coelome, while the other ends in a Bowman’s capsule associated with an independent glomerulus from the blood system (Fig. 376). Additional capillaries develop about the tube proper somewhat in the same manner as in the case of the mammalian urinary unit previously described. Thus there are established two avenues for excretory collection, namely, the ciliated nephrostome for drawing whatever fluid collects in the body cavity, and the renal corpuscle and tubule proper for direct abstraction from the blood.
Although the nephrostomes of some of the more anterior mesonephridia remain permanently open, particularly in the elasmobranchs and ganoids, while in cyclostomes they are retained throughout the entire length of the mesonephros, they are for the most part obliterated.
When the nephrostomes all become closed, as in amphibians and amniotes generally, the body cavity virtually becomes a closed sinus so far as the blood system is concerned, and the peritoneal fluid, with whatever excretory products may be present in it, can escape only like other lymphatic fluids through the blood channels.
The mesonephros functions as the kidney throughout life not only in cyclostomes, with the exception of the hagfishes already mentioned that retain a pronephros, but also in fishes and amphibians. It also serves temporarily as the kidney for practically all other vertebrates until it is superseded in turn by the metanephros.
In reptiles, as well as in Echidna and Dideiphys among mammals, the mesonephros endures until some time after birth, and in the case of the lizard Lacerta, even until after the first hibernation.
In most mammals the functional mesonephros is confined to the embryonic period, although occasionally, as in the guinea pig and mouse, degenerating so early that it probably never functions as a kidney at all.
In man the mesonephros, or Wolffian body, reaches its high-water mark during the second month of fetal life, when it appears as a slightly projecting ridge on either side along the dorsal part of the coelom from the posterior cervical region to the pelvis, where the two ridges fuse into one.
The drainage ducts of the mesonephroi are the persisting segmental ducts of the vanished pronephroi, known in their new role as Wolffian ducts. Thus, if the whole mesonephros, whose secretion is delivered through a duct, is compared to a gland, which it certainly resembles in a superficial way, there is one striking difference to be noted between it and an ordinary gland, namely, the duct is formed before the secreting part is developed and independently of it.
When in reptiles, birds, and mammals the mesonephros degenerates and gives way to the metanephros, many of its component parts, in particular the ducts of excretion, are salvaged and utilized for other purposes in connection with the reproductive system.
The whole mesonephros, a much more compact and unified organ than its pronephric predecessor, is retroperitoneal, usually fitting snugly against the dorsal body wall, although in amphibia it projects somewhat into the abdominal cavity.
The third and last type of vertebrate kidney, which replaces the mesonephros in reptiles, birds, and mammals, is the metanephros.
Its nephridial tubules, the metanephridia, already described as “urinary units,” take their origin in the nephrogenic tissue surrounding the posterior part of the Wolffian ducts in the sacral region of the body, ventral to the sacrum and dorsal to the cloaca when this is present.
The metanephridia are very numerous, elongated, and highly modified histologically, as already indicated. They are not arranged metamerically at any stage, and are massed together inside of the tunica fibrosa so that they form organs, particularly in mammals, more independent of the coelomic wall than either pronephroi or mesonephroi.
In position the metanephridia are posterior to the last mesonephridia, developing at a later time. A nephrostome is never present, and consequently at no stage do the metanephridia communicate with the coelom. As a result of the absence of the nephrostome branch of the nephridial tube, the connection with the blood system, that is the renal corpuscle, appears to be at the blind end of the tubule and not on a branch like one arm of a Y, as in the mesonephridium.
The true ureters, which sprout out from the bases of the Wolffian ducts (Fig. 377), are not made over from something that has gone before, but are an entirely new pair of excretory ducts. The free end of each ureter enlarges and pushes into the metanephric tissue to form the renal pelvis and main collecting tubules of the metanephric kidney. Here again the duct is formed independent of the nephridia but in this case the duct grows to the nephridia instead of the reverse situation which is found in the development of the mesonephros. Meanwhile the Wolffian ducts that have already served more than one master, being shorn of their former excretory mission, are turned over to the reproductive system to take on still another function as sperm ducts in the male, while in the female they degenerate and pass out of the picture.
In man the metanephros, or permanent kidney, assumes dominance during the third month of fetal life. The remains of the mesonephros, as well as the mesonephric ducts, become accessory to the reproductive apparatus later.
In the chapter on the Reproductive System more complete consideration will be given to the fates of all of these ducts and tubules which, although at first urinary in function, become in many cases associated with the genital portions of what is commonly known as the Urogenital System.