EVOLUTIONARY TRENDS IN THE EXCRETORY SYSTEM

Palaeontological evidence has been interpreted as indicating that the earliest vertebrates lived in fresh water and that the early stages of evolution of fishes took place in that medium. Animals that are submerged in fresh water inevitably acquire excess water by absorbing it through the skin or by swallowing it with food. Therefore, a mechanism for eliminating the excess ware is necessary. On the other hand, salt is scarce in fresh water, the only source being food. Freshwater organisms must therefore prevent any wasting of salt from the body.



The elimination of water and the reclamation of salts may have been the earliest functions of vertebrate kidneys. Tufts of blood vessels - glomeruli, filtered water out of the blood stream into the body cavity and convoluted tubules with openings into the coelom collected the filtrate, retrieved any salts from it and emptied the final filtrate into a longitudinal duct that passed to the cloaca.



Archinephros

Glomeruli arise as localised modifications of blood vessels, which can be traced from segmental branches of the dorsal aorta - supplying the glomerulus is an afferent glomerular arteriole and emerging from the glomerulus is an efferent glomerular arteriole. The latter leads to capillary beds that surround the kidney tubules.



The most anterior embryonic/larval glomeruli may be suspended in the coelomic cavity. They are sometimes called "external" glomeruli to differentiate them from "internal" glomeruli, which are encapsulated by the kidney tubule.



In internal glomeruli, each tubule typically commences as a Bowmann's capsule. This is a blind end of the tubule that surrounds an internal glomerulus and receives the glomerular filtrate. The more anterior tubules may exhibit a ciliated funnel-shaped nephrostome, which is an opening into the coelom. Nephrostomes are usually confined to the embryo and larvae. If the embryonic tubules that exhibit a nephrostome are not altogether closed, the nephrostome may close at a later stage of development.



Kidney tubules arise from the intermediate mesoderm. This is a ribbon of nephrogenic tissue extending uninterrupted from the level of the heart to the cloaca. It lies just lateral to the segmental/dorsal mesoderm. Almost the entire ribbon produces kidney tubules. The anterior-most tubules are always metameric, since one tubule develops a the level of each mesodermal somite. Farther back, numerous tubules develop in each segment and the metamerism is lost.



The longitudinal ducts of the basic patter appear first at the anterior end of the nephrogenic mesoderm as posteriorly directed extensions of the first tubule. Each duct grows caudal until it achieves an opening into the cloaca. At this time, it is known as the pronephric duct. The kidneys of myxinoid cyclostomes closely resemble an archinephros.



Pronephros

The first embryonic kidney tubules in all vertebrates arise from the anterior end of the intermediate mesoderm. They are called pronephric tubules because they are the first of what will become a series of tubules extending the length of the coelom.



Each pronephric tubule arises in the intermediate mesoderm as a solid bud of cells, which later organises a lumen and in most anamniotes, a nephrostome. Associated with each tubule, typically, is a glomerulus. The number of pronephric tubulesis never large. The tubules lengthen and become coiled.



The pronephros in most vertebrates is functional only until such a time as the tubules farther back are prepared to supersede them. This is at the end of the larval stage in amphibians or at an equivalent development stage in fish. Occasionally, in larvae, several glomeruli unite to form a single glomus. The glomus and the pronephric tubules are generally enclosed in a pronephric chamber. The pronephros is retained in the adult only in cyclostomes and a few teleosts. The pronephros is often called head kidney because it lies immeadiately behind the head.



Mesonephros

Under the stimulus of the pronephric duct, acting as an inductor, additional tubules develop sequentially in the mesoderm behind the pronephric region. The new tubules establish contact with the pronephric duct. For at least several segments, these tubules too, may be segmentally disposed. They exhibit the same convolutions as the ones anterior to them and often have open nephrostomes. Later, the pronephros is obliterated. With the disappearance of the pronephric region, the erstwhile pronephric duct is now called the mesonephric duct. The mesonephros is the functional adult kidney of fish and amphibians. It is sometimes called opisthonephros. The mesonephros is also the functional kidney in the embryonic amniotes.

In sharks and apoda (amphibians), the adult kidney begins far forward and extends the lenth of the coelom, lying against the dorsal body wall. In other fish and other amphibian, the kidney is much shorter. The mesonephric duct may lie along the lateral edge of the kidney, or on the ventral surface or embedded in it. The posterior ends may enlarge to form the seminal vesicle in males.

The mesonephros of amniotic embryos has essentially the same structure as the adult kidneys of fish and amphibians, except that nephrostomes are rudimentary in most birds and seldom appears in mammals. Although the mesonephros is basically an embryonic kidney in amniotes, it functions for a short time after birth in reptiles, prototherians and metatherians.

During the time that the mesonephros is functionin, a new kidney, the metanephros, is in the process of developing. When the metanephros takes over the functions of a kidney, the mesonephros involutes and only remnants remain after birth.

Metanephros

The metanephros, or adult amniotic kidney, organises from the caudal end of the nephrogenic mesoderm, which is displaced anteriorly and laterally during development. The number of tubules that form this caudal section is extremely large (several million) and the tubules are highly convoluted. This has a duct of its own, the metanephric duct/ureter.

The proximal portion of the metanephric duct becomed the ureter. The distal tip becomes the pelvis (of the kidney). Many finger-like outgrowths of the pelvis invade the nephrogenic mass to become collecting tubules. Meanwhile, the metanephric tubules get organised. They commence as 'S'-shaped tubules. The upper arm opens into a collecting tubule. The lower arm becomes invaginated by developing a glomerulus to become a Bowmann's capsule.

The mammalian metanephros exhibits a greater organisation than that of reptiles and birds. The organisation is the result of the formation of a long, thin, 'U'-shaped loop of Henle, positioned between the proximal and distal convolution of the metanephric tubule. As the loop of Henle elongates, they grow away from the surface of the kidney and towards the renal pelvis. The kidney therefore comprises of a cortex, in which the renal corpuscles are concentrated and a medulla, consisting of loops of Henle and collecting tubules.

The metanephric tubules of reptiles have no loop of Henle and those of birds have only a very short equivalent segment. Small glomeruli result in the conservation of water. Many metanephric kidneys are lobulated, each lobe consisting of clusters of tubules. In snakes and legless lizards, the kidneys are elongated to conform to the slender body. The kidneys of birds are flattened against the sacrum, iliun and ribs and fit snugly against the contours of these bones.

TRENDS IN THE EVOLUTION OF THE MIDDLE EAR AND EAR OSSICLES (Cyclostoma to Mammals)

It is important for animals (incidentally all chordates are free moving) to maintain the correct posture and to detect signals in the form of changes in the pattern of sound waves. The appearance of the ear in chordates actually begins from the incorporation of the membranous labryinth in cyclostomes. Further development concerns the addition of ear ossicles in the middle ear by the modification of visceral arches and the progressive development of the membranous labyrinth. The ear is best developed in mammals where all the divisions - external, middle and inner are present with their respective components.

Development of the ear

A small patch of ectoderm on each side of the head in the region of the hind brain thickens to form an auditory placode. This sinks in and thereafter invaginates to form an auditory vesicle/autocyst. It lies embedded in the mesenchyme of the head. The invagination canal connets each auditory vesicle to the outside. In elasmobranchs, this remains open. In other vertebrates, the auditory vesicle remains as a closed sac having no invagination canal. Ductus endolymphaticus / endolymphatic duct arises as a new outgrowth from the auditory vesicle. It has no external opening. In amphibians and some reptiles, the end of this duct expands to form an endolymphatic sac. The auditory vesicle now constricts to form a dorsal utriculus and a ventral sacculus. One/two/three semicircular canals also appear and open into the utriculus at both ends. One end of each canal expands to form an ampulla. Two parallel grooves arise and these two meet and ___ to form a tube. An evagination called lagena arises from the lower part of the sacculus. This appears first in fish. In successive groups, it begins to grow increasingly. The auditory capsule is formed by the mesenchyme. It is first made of cartilage and is later replaced by bone. It encloses the membranous labyrinth.

The middle ear and eustachian tube are derived from the spiracles of fish. The upper part of the hyomandibular arch gives rise to the columella auris of lower tetrapods and the stapes of higher tetrapods. The external opening of the spiracle becomes closed and it fuses with with the skin to form the tympanic membrane. The cavity of the spiracle expands to form the cavity of the middle ear. The middle ear establishes a connection with the pharynx by the Eustachian tube. During the transition of aquatic to terrestrial mode of life, the spiracle becomes a sound conducting device. In mammals, the stapes is joined by two more ear ossciles. The quadrate gets modifies into the incus and the articular gets modified into the malleus. These are joined to each other and also connect the tympanic membrane with the internal ear. The external ear / pinna, in mammals, is formed mainly as an outgrowth of the skin. It is reinforced by an elastic cartilage.

COMPARATIVE ACCOUNT

Cyclostomes and Fish
The middle ear and external ear are absent.

Amphibia (e.g. frog)
The middle ear is visible from outside due to the absence of pinna. It encloses an air-filled tympanic cavity. It is limited internally by the auditory capsule and externally by the tympanic membrane / tympanum. This is visible as a dark tightly stretched patch of skin. The cavity of the middle ear communicates with the pharynx by a slender, narrow passage called the eustachian tube running downward and opening into the buccopharyngeal cavity just near the angle of the mouth. The tympanic membrane is tightly stretched over a ring of cartilage called the annulus tympanicus. It is modified skin. A club-shaped rod, the columella auris, touches the centre of the tympanum and is extended across the tympanic cavity to a cartilagenous, small nodule known as the stapedial plate, which is fised with a hole in the auditory capsule called the fenestra ovalis. Structurally, the columella auris is made up of bone as well as cartilage. A ring-like bone, the operculum is present in the fenestra ovalis.

Reptiles (e.g. lizard)
The middle ear is represented by an air-filled cavity called the tympanic cavity. It is derived from the firdt pharyngeal pouch. The external limit of the tympanic cavity is formed by the tympanic membrane, whereas, its internal boundary is limited by the auditory capsule. A narrow passage called the eustachean tube / pharyngo-tympanic tube extends downward and inward to open into the posterior part of the pharynx. The eustachian tube of reptiles is narrower and longer than that of amphibians. A single, rod-like ear ossicle, the columella auris stretches across the tympanic cavity. It is derived from the hyomandibular of the hyoid arch. This is made of
(i) An inner body - stapes / stapedial bone
(ii) An outer cartilagenous extrastapedial / extracolumellar cartilage.
The stapes fits into a small membrane covered aperture called the fenestra ovalis located in the outer wall of the auditory capsule. The extracolumellar cartilage is attached to the inner surface of the tympanum. One more membrane covered aperture is present in the outer wall of the auditory capsule. It is called the fenestra rotunda.

Variations

The tympanum, eustachian tube and the tympanic cavity are totally absent in snakes. Yet, snakes recieve sound vibrations with the help of the columella auris which is attacched at its outer end to the quadrate.

In crocodiles, the two eustachian tubes open into the ___________

In turtles, the tympanic membrane is thin and delicate, while it is thick and covered with skin in the terrestrial form. In crocodiles, a movable integumentary fold covers and protects the depressed tympanic membrane.

Aves (e.g. pigeon)
The middle ear is an air-filled cavity. The tympanic cavity is limited externally by th tympanum and internally by the auditory capsule. A narrow eustachian / pharyngo-tympanic tube arises from the lower medial part of the tympanic cavity and extends downward and inward. The two eustachian tubes join to finally open into the roof of the pharynx by a common aperture. The columella auris extends across the tympanic cavity and can be easily distinguished into four parts - an inner, disc-like bony stapes and an outer three-layered cartilagenous extracolumella. The bony stapes fits into a membrane- covered aperture in the outer wall of the auditory capsule called the fenestra ovalis. The extracolumella is attached to the inner surface of the tympanum. There is present an additional circular aperture below the fenestra ovalis ____________. It is also membrane covered.

Mammal (e.g. rabbit)
The middle ear is represented by an irregular air-filled space called the tympanic cavity, enclosed in the tympanic bulla. It is lined by a mucus membrane. It communicates with the pharynx by a passage called the eustachian tube which extends downward and inward. The periotic bone forms the inner wall of the tympanic cavity. This wall bears two apertures - fenestra ovalis / oval window and the fenestra rotunda / round window, both covered with a thin membrane of connective tissue.

An important feature of the middle ear is the presencce of three ear ossicles - malleus, incus and stapes. These are movably articulated with each other extending from the point of the fenestra ovalis to the periotic bone. The malleus is the outermost, hammer-shaped ossicle and is attached to the inner surface of the tympanum. The middle ossicle, incus, is anvil-shaped. It articulates with the malleus by a synovial joint and with the stapes by a ball-and-sockect joint. The inner moer is the stapes. These three ossicles function as a system of levers.