4 Main Developmental Organs in Vertebrates | Embryology

The following points highlight the four main developmental organs in vertebrates. The developmental organs are: 1. Brain 2. Eye 3. Heart 4. Kidney.

Developmental Organ # 1. Brain:

The first important morphogenetic change following gastrulation is the deve­lopment of the central nervous system. The central nervous system starts as a simple tubular nerve tube which, in course of development, transforms into brain, spinal cord and their associated structures.

The morphogenetic processes involved in this process are designated as neuralisation. It includes the separation of neural materials from the embryonic ectoderm, their migration inward to form a hollow nerve tube together with the segregation of neural crest cells. The nerve tube diff­erentiates into the brain and spinal cord, while neural crest cells develop into neuroblasts and many other structures.

Methods of Neuralisation:

Neurali­sation occurs by two ways in different ver­tebrates.

These are:

(a) Thickened Keel Method:

In teleost, ganoid fishes and cyclostomes the neural materials become aggregated to form a thickened keel or ridge extending along the mid-dorsal axis of the body. This ridge sepa­rates itself from the overlying ectoderm and develops a lumen within to form a tube.

(b) Neural Fold Method:

It occurs in most of the vertebrates where neural cells be­come aggregated to form a neural plate. This plate folds inward to form neural groove. The neural groove transforms into a neural tube which sinks from the over­lying ectoderm.

Events in Neural Morphogenesis:

After the completion of gastrulation, the ectoderm of the future dorsal side of the developing embryo tends to condense to form a thick and compact neural plate with elevated margins. This thickened part is called the neural (or medullary) plate or neural placode.

The plate is formed by two simultaneous processes:

(a) Elongation of prospective neural cells in the direction perpendicular to the surface of the deve­loping embryo, and

(b) Shrinking the ex­posed surfaces both dorsally and ventrally.

The neural plate is pear-shaped, i.e. it is broader at the anterior part but gradually narrows towards the posterior end. This particular shape of the neural plate is crucial for shaping the future structures. The shaping of the neural plate is resulted as the consequence of regional differences in the cell contration. Fig. 5.25 relates the stages of neural morphogenesis in frog.

A depression appears along the entire length of the neural plate which folds downward to form a neufal or medullary groove. The formation of the neural groove is associated with the median and dorsal movement of the ectodermal layer attached to the lateral edges of the neural plate.

Thus the raised or folded margin of the neural groove is called the neural fold. The downward movement of the neural plate to form the neural groove depends largely on the lateral shifting of somatic mesoderm from the notochordal area to accommodate the invaginating neural groove. The lateral neural folds rise and meet along the middle line. This union begins from the anterior end and runs posteriorly.

With the union-of the folds, the outer ectodermal layers become conti­nuous and the inner nervous layer, after fusion with the corresponding part, forms a tube and separates itself from the upper ectodermal layer. This tube-like structure is called the neural tube.

The cavity of the neural tube is called the neurocoel which is broader at the anterior end and opens to the exterior through an opening called the neuropore. The neuropore ultimately closes at the later stage of development.

Associated with the formation of neural tube neural crest cells become segregated on the two sides of the neural tube. These neural crest cells lie as two longitudinal strips of cells, one on each dorsal side of the neural tube.

Neural Crest and its Fate:

At the corners of the fusing neural fold during brain formation, groups of neural crest cells become detached to occupy a position over the neural tube. In course of develop­ment these cells leave their position and migrate to other parts of the embryo.

These cells are versatile in their developmental fate and develop neuroblasts of the spinal and sympathetic ganglia, Schwann sheath cells producing the myelin sheath and neurilemma of the nerve fibres, melanoblasts, chromaffin tissue of adrenal me­dulla, meninges, cartilages of the jaw, etc. Weston (1963) has shown the migration of neural crest cells.

The neural tube and neural crest cells labelled with radio­active isotopes are excised from the trunk of a developing chick embryo and trans­planted to a normal (non-labelled) host in place of its counterparts.

It has been shown that the neural crest cells migrate along two ways:

(i) Dorsolaterally along the skin and

(ii) Ventrolaterally in relation to the neural tube.

Structural Differentiation of the Neural Tube:

The differentiation of the neural tube into the brain and spinal cord depends upon many intrinsic and extrinsic factors. The anterior part of the neural tube transforms into the brain while the posterior narrow part becomes elongated to form the spinal cord.

The broad anterior part is demarcat­ed from the narrow posterior part by isthmus. Remarkable changes occur in the anterior part during its conversion into the brain.

This is caused by:

(a) Unequal thickening of the neural tube wall,

(b) In­vaginations or evaginations of the wall and

(c) Various types of bending or folding (flexure formation).

Immediately after the formation of the neural tube, the anterior part swells up and two constrictions develop to divide the anterior part into three general regions: Prosencephalon, Mesencephalon and Rhom­bencephalon.

In course of development, pro­sencephalon and rhombencephalon be­come further subdivided thus giving rise to five parts: Telencephalon, Diencephalon, Mesencephalon, Metencephalon and Myelen- cephalon (Fig. 5.26).

Many factors are res­ponsible in brain morphogenesis. Diff­erential growth and intraventricular pres­sure are regarded to be the important morphogenetic factors in brain develop­ment, especially in flexure formation. Fig. 5.26 relates the development of flexures and different regions of the brain.

Histogenesis in Brain Development:

The early neural tube is fairly uniform in structure. The walls are composed of neural epithelial cells which eventually differentiate into: (1) neuroblasts and (2) spongioblasts. The neuroblasts develop into nerve cells and fibres while the spon­gioblasts give origin to ependymal and neuroglial cells.

The neural epithelium is composed of pseudostratified columnar epithelial cells which form the primitive ependymal layer or matrix layer. Gradually the cells of the matrix layer migrate to cach lateral side to form a cellular layer called the mantle layer.

And lateral to the matle layer lies a cell-free marginal layer. The cells of pri­mitive ependymal layer are usually called the germinal cells, some of which after a day or two following the closure of neural groove, develop neuroblasts and migrate first to the mantle layer (Fig. 5.27).

In the mantle layer, the cells differentiate into

(a) Neuroblasts and neurons and

(b) Spongioblasts and neuroglial cells.

The neuroglial cells give rise to astrocytes and oligodendrocytes. The neuroblasts do not remain evenly distributed but are aggre­gated into clusters. From mature neuro­blasts, nerve cells and fibres grow out in a distinct pattern and turn the brain into a ‘working unit’.

Development of Nerve Cells and Fibres:

Nerve cells originate from neuro­blasts which develop from the neural tube, neural crests and cranial placodes. The actual stages of conversion of a small (neuroblast to a large cell-body of a nerve cell can be seen in tissue culture method reported first by Harrison in 1907. As in Fig. 5.28, a small fragment of neural tube is transplanted in a blood clot and kept sealed in a moist chamber.

Dissociation and dispersion of cells are the first observ­able events in tissue culture. The origin of nerve fibres is the most notable event in this process of conversion. Three theories are extant on this particular issue.

They are:

(a) Cell-chain theory. This theory relates that the fibre is laid down by chains of cells which surround the nerve fibre.

(b) Plasmodism theory. According to this theory the nerve fibre is laid down on preformed pro­toplasmic bridge.

(c) Outgrowth theory. The theory advocates that the fibre is formed as an outgrowth of a single neuroblast.

The tissue culture experiment gives sup­port to the last concept and setties the long standing controversy regarding the issue. At the beginning, a thin strand of protoplasm emerges as outgrowth from one side of the neuroblast.

This outgrowth becomes amoeboid and creeps along the solid object. The outgrowth has developed a growth cone at the terminal end which may branch to form two or more growth cones. The growth of nerve fibre exhibits streotropism, i.e. it moves along solid object.

Causal Analyses in Brain Morpho­genesis:

In the entire process of nervous system formation, a number of inductive events occur. In the amphibian eggs, the dorsal lip of blastopore acts as primary organizer to induce the inward moving cells to form chordamesoderm which in turn induces the dorsal ectoderm to be neuralised. The formation of the neural tube is also guided by the influence of regionally specific inductions.

The neural plate at the beginning is an oval, flattened plate and is formed by the ectodermal cells which have come from lateral regions to the dorsal side. The neural plate elongates rapidly, which is caused by the movement of cells. The cells first move towards the middle and then run in two directions: anteriorly and pos­teriorly.

The transformation of neural plate to neural tube which is called neurulation is also known to occur in vitro. It begins with a depression in the centre and curving of the edges which fuse together to form the tube. To search the motive force behind the formation of tube, the be­haviour of cells in the centre and periphery is intimately studied.

Certain suggestions, like differential water uptake, diff­erential cell divisions have been negativated. It is now claimed that elongation of the plate is due to migration of cells but curvature is caused by changes in the cell adhesion.

In further development, the anterior part of the tube swells up considerably to form brain vesicles. Considerable amounts of cell division and cell movement occur during the process. The different parts of the brain in course of its development in­duce the formation of structures like optic, auditory and nasal placodes on the outer ectodermal covering.

It must be remembered that mesoder­mal cells which immediately remain around neural tube are believed to play most important role in the epigenetic pro­fess.

The formation of brain establishes:

(a) many histological features remain deter­mined at neural plate stage and

(b) all the cells do not transform into neural element at the same time. On the contrary a gra­dient exists in the anterior-posterior plane.

Developmental Organ # 2. Eye:

The early stages of eye development follow a generalised pattern in all verte­brates and the details of eye morpho­genesis in chick will give an idea of the process in general. The eye is a very com­plicated structural unit. The development of eye reveals the incorporation of different tissues which follow an orderly fashion to give the geometry of pattern.

Because of the fact, the incidences involved in eye morphogenesis are regarded as a perfect model to explore the general problems of embryology.

The development of eye is discussed under three steps:

(a) Development of sensory areas.

(b) Development of lens.

(c) Development of associated structures.

Development of Sensory Areas:

Formation of Optic Placode and Optic Vesicle:

The primordial eye rudiment lies at the very anterior end of neural plate in the form of two closely placed oval areas, one on either side of the middle line. They are lined below by mesoderm. In course of the formation of brain, these two lateral sides of the forebrain, which are destined to be the future diencephalon, become thickened.

These parts are known as optic placodes. These two placodes extend laterally as small blunt bulgings, which become known as optic vesicles (Fig. 5.29). The vesicles elongate through the loose mesenchyme to­wards the epidermal covering and remain connected with the brain by a narrow stalk called optic stalk.

Formation of Optic Cup:

As the optic vesicle touches the ectoderm, the ectoderm cells elongate perpendicularly to the re­gion of contact to form the lens placode which invaginates to form lens vesicle. With the invagination of lens placode to form lens vesicle, the optic vesicle reverses its outward bulging and turns inwards to form the optic cup to accommodate the lens. The optic cup is double-walled.

Such inpushing takes place asymmetrically and continues obliquely into the optic stalk. Near the optic stalk, a slit is left in the ven­tral side which is called choroid fissure. This fissure acts as an outlet for optic nerve (Fig. 5.30). The blood vessels also find an impasse into the optic cup.

The outer wall of the optic cup remains thin and gives rise to pigmented retina while the inner wall (neural retina) becomes greatly thickened and elaborated to transform into light sensitive retina.

Formation of Retina and Optic Nerves:

The inner lining of the optic cup transforms into light sensitive retina, which in turn differentiates into seven layers of nervous elements. The neural retina gives rise to rods and cones and some other types of cells with which the visual cells synapse.

The histogenesis of retina is divided into three phases:

(i) A phase of cell multiplica­tion,

(ii) A phase of cellular readjustment and

(iii) A phase of final differentiation.

Fig. 5.31. gives the details of development of retinal cells. Rods and cones are arranged in the outermost part, i.e., towards the pigmented layer of the optic cup. The position of visual cells is due to the migration and stratification of neural layer of the retina. The fibres of rods and cones unite with the fibres of the ganglionated layer of the retina. These fibres converge towards the optic stalk to form the optic nerve.

Development of Lens:

The region of the outer ectoderm which comes in contact with the optic vesicle thickens and is known as lens placode. The placode invaginates to form a lens pit or lens cup. The two ends of the lens cup unite and remain within the space between-optic cup and outer ecto­derm. It is then called lens vesicle.

The inner cells of the vesicle transform into lens fibres and the cell layer next to ectoderm forms the epithelium of the lens. At about 96 hours in chick embryo, the lens cavity becomes reduced and the cells of the median wall of the lens become elongated to obliterate the lens cavity.

The cyto­plasm of the cells becomes clear and these cells transform into lens fibres. The extreme elongation of the cells is evidenced by the placement of their nuclei in the equitorial region of the lens and the cells are stretched extending from one surface of the lens to the other. A lens fibre may reach 10 mm as seen in man.

Development of Associated Structures:

Choroid and Sclera:

The optic cup and optic stalk become invested with a layer of mesenchyme which later forms an outer densely fibrous layer called sclera and an inner pigmented and richly vascularised layer called choroid.

Conjunctiva, Cornea and Aquous Humor:

After the detachment of the lens the ecto­dermal epithelium forms a cell-free lining. Along the inner border of the ectodermal epithelium lies the mesenchyme, which forms the sclera. The inter epidermal layer is called conjunctiva and inner mesenchymal continuation of sclera is known as cornea. The transparency of cornea is vital for the admission of light into the cavity of eye.

The cornea is composed of an outer epi­thelial layer (derived from ectoderm) and a postepithelial stroma (derived from im­migrating mesenchymal cells). In the stroma, layers of collagen fibres accumu­late parallel to the surface. The stioma undergoes significant biochemical changes and undergoes dehydration.

Due to loss of water and suppression of pigmentation, the cornea attains perfect transparency. A space develops between lens and cornea, which is called anterior chamber. A watery fluid, called aqueous humor accumulates within the space.

Iris:

The choroid layer extends in front of the lens to form a circular-curtain known as iris, which has a hole in the centre called pupil. The iris is pigmented and possesses papillary muscles which regu­late the diameter of the pupil. The iris develops from the pigmented retina.

Ciliary Muscles:

The pigmented outer layer of the optic cup together with mesen­chymal elements extends in front to form ciliary muscles or ciliary bodies.

Vitreous Humor:

The cavity between lens and retina is known as posterior chamber. It becomes filled with a gelatinous matrix called vitreous humor.

Muscles of the Eye:

Several muscles arc formed by the condensation of outer head mesenchyme. These muscles are involved in rotating the eye-ball within the orbit.

Eyelid:

The outermost epidermal layer in front of the eye becomes skin. It splits into two halves to form the eyelids. In different vertebrates the shape of eyelids varies.

Causal Analysis of Eye Development:

The foregoing description reveals that the development of eye is a phasic pheno­menon where different component parts appear in sequential order to establish a harmonious functional unit.

A resume of the total events shows that the optic vesic­les emerging from the forebrain make inti­mate contact with the presumptive lens ectoderm to induce lens formation. The lens vesicle induces the optic vesicle to form optic cup and its subsequent differentiation.

The optic cup-lens complex induces the overlying ectoderm with some mesenchyme to form the cornea. Thus a reciprocity of induction occurs in eye morphogenesis—and this type of induction is called the synergistic induction. Fig. 5.32 gives, the schematic representation of the participation of different tissues and the inductive phenomena in the development of eye.

Events of Eye Development:

Events of eye development may be divided into three phases:

(a) Phase of induction

(b) Phase of cell differentiation and axiation and

(c) Phase of mechanical tension.

The work carried by large number of workers have revealed that no organ is formed if there is any disturbance in the first phase. But at the same time only the occurrence of first phase cannot form the organ.

The disturbance in the second phase, i.e. phase of cell differentiation, produces numerous deviations in the diff­erent rudiments of eye. The third phase creates the form and size of the organ and its abnormality affects them considerably.

Some of the important findings to ex­plain the mechanism of eye formation are discussed below:

(i) The lateral extension of the optic vesicles to reach the ectodermal layer is caused by the pressure exerted by the intraventricular fluid of the brain.

(ii) The loose surrounding mesenchyme of the pri­mordial eye rudiment plays important part. Experimental evidences suggest that the mesoderm first contributes most actively to the development of the eye and then exhibits its formation in the middle of the brain.

(iii) In the primary eye rudiment at the beginning, the different layers have the capacity to undergo mutual transforma­tion.

The inner wall of the optic cup trans­forms into the neural retina while the outer wall develops into the pigmented retina., If the position of these layers is reserved, the original inner layer may form neural retina and vice versa.

The trans­formation of the inner wall of the optic cup to form neural retina is caused by the inductive influence of the lens vesicle. The outer layer is converted into pigmented retina by the influence of mesenchyme.

(iv) The proportion of pigmented retina and neural retina depends on the extent of tension produced by the fluid which accumulated inside.

(v) The formation of cornea depends upon geometric distribu­tion of different layers and the mechanical tension of the eye.

Developmental Organ # 3. Heart:

The formation of heart (cardiogenesis) in vertebrates is one of the most dynamic events in embryonic development. The heart is a mesodermal organ, which diff­erentiates initially from the ventral edges of the lateral plate mesoderm.

Primarily the cardiac primordia are paired which, however, become fused to form a single organ. The process of cardiogenesis in different vertebrate forms is essentially similar. The events of the development of heart in the chick embryo are discussed below.

Localisation of Cardiac Primordial:

The localisation of heart-forming cells occurs at the onset of gastrulation in the embryos of all vertebrates. Vogt (1929), by using vital staining technique, has localis­ed the prospective heart cells in amphibian embryo. Butler (1935) and Spratt (1942) have shown that heart-forming cells are widespread in the blastodisc of chick (Fig. 5.33A).

With the movement of cells of the epiblast to form the primitive streak, heart- forming cells become restricted to the epiblastic region anterior to the develop­ing primitive streak.

With the migration of epiblastic cells to form the mesodermal layer, the heart-forming cells become con­centrated about Hensen’s node. The heart- forming cells, then, migrate to join the mesoderm and move laterally. When the definitive primitive streak is formed, the heart-forming cells take lateral position as paired cardiac primordia.

Each primordium is capable of developing a whole heart. If the paired primordia are prevent­ed from fusion, two independently beating hearts (‘cardiac bifida’) will result in an embryo.

Before the formation of heart, the pre­sumptive heart-forming cells acquire spe­cific biochemical characteristics from their neighbours and have an inherent capacity of undergoing self-differentiation. This is attested by the fact that these bilaterally located cardiac primordia, when trans­planted into an indifferent location, are capable of differentiating into cardiac tissue.

The presumptive heart-forming cells are rich in glycogen which is retained throughout its differentiation. So the heart-forming cells become different from other cells by having high glycolytic metabolism.

Stages of Heart Formation:

In course of development of heart, the paired cardiac primordia come together in the midventral line. This is brought about by the action of four types of morphogene­tic movements. These are:

(a) Folding Movements of Ectoderm and Endoderm:

This movement of the endo­derm to develop into crescentic pouch of the anterior intestinal portal and early foregut is of great importance. The pre­sumptive heart-forming cells use the endo­dermal layer as the substratum for their migratory activity. The folding move­ments bring the paired cardiac primordia together in the midline to develop into an unpaired median tube.

(b) Formation of Amniocardiac Vesicles:

This process of development of embryo­nic coelom or amniocardiac vesicle is also important in heart formation. With the formation of head fold and initiation of foregut, the lateral plate mesoderm in the region of cardiac primordia splits to form a dorsal layer (somatic mesoderm) and a ventral layer (splanchnic mesoderm).

The coelomic space thus enclosed by these two layers is called the early pericardial or amniocardiac vesicles. With the separa­tion of the somatic and splanchnic meso­derm, all the presumptive heart-forming cells move ventrally in the splanchnic mesodermal layer. Because of this reason, this thickened crescentic splanchnic meso­derm is called by Mollier (1906) as the ‘cardiogenic plate’.

(c) Cell Movement in the Splanchnic Meso­derm and the Subsequent Emigration of the Mesodermal (Splanchnic) Cells:

Prior to the formation of coelomic space, the precardial mesodermal reticulum consists of a homo­geneous loose meshwork of stellate mesen­chyme. Within this meshwork small clus­ters of tightly packed cells are present which move actively from their lateral position to form the tubular heart.

(d) Formation of Angioblasts:

With the development of cardiogenic plate, angio­blasts are formed in the region of the original amniocardiac vesicle. The conversion of the precardial cells to angio­blasts is the first sign of histological diff­erentiation in cardiogenesis.

These cells migrate either singly or in small clusters out of the mesoderm and form a loose layer (vascular layer of Pander) in the meso-endo- dermal space. This layer forms the endo­cardium of the heart and in the posterior region it produces the blood islands. These islands produce the endothelium of the remaining vasculature, erythroblasts and blood plasma.

Formation of Primitive Tubular Heart:

As stated earlier, the primordial endo­cardial cells begin to differentiate inde­pendently as a pair of delicate tubular hearts. These paired tubular hearts are arranged on either side of the anterior intestinal portal (the opening from the yolk into the foregut).

The folding move­ment of the ectoderm and endoderm to form the head fold and foregut, causes the migration of the paired tubular hearts together when they fuse to form an un­paired tubular heart tube.

The paired rudiments meet and fuse when the foregut is separated from the yolk sac. This process begins at 7 to 8 somite stages in chick and is completed when the embryo comes to 20- somite stage. Each of the paired rudiments has an inner endothelial lining (endocar­dium) and the outer is the epimyocardium.

Simultaneously with the migration of splanchnic mesoderm anteromedially and separation from the folding endoderm, it becomes thickened to form paired epimyocardia. This layer develops later into the thick myocardium and a thin nonmuscular epicardium (or visceral pericardium).

Fig. 5.34 shows the origin and subsequent fusion of paired cardiac primordia during cardio­genesis. The epimyocardium remains atta­ched ventrally by ventral mesocardium and dorsally by dorsal mesocardium. Both these mesocardia disappear subsequently.

The fusion of the heart tubes begins at the anterior end and extends gradually to the posterior sides. Fusion starts in the re­gion of the future ventricle and the auricle is still represented by double tubes. Then gradual union occurs in anteroposterior direction and the process of fusion is com­pleted when the embryo becomes 20- somite stage in chick.

Inductive Relationship during Cardio-­Genesis:

Many embryologists claim that the deve­lopment of heart is intimately related to the developing endoderm. In amphibian embryo, the removal of endoderm causes the failure of heart formation. But in the development of chick, the removal of endo­derm does not prevent normal cardiogene­sis.

Many embryo­logists have claimed that the migration of cardiac cells and the folding movement of the endoderm is independent processes, normal cardiogenesis is not hampered if such relationship is disturbed. But experi­mental evidences on this line are unsatis­factory to ascertain the actual role of endo­derm on the precardial mesodermal cells.

Histological Differentiation in Cardio­genesis:

The differentiation of angioblasts from the splanchnic mesoderm and the trans­formation of the splanchnic mesoderm itself to form the myocardium and epimyocardial mantle are the first indication of histogenesis in heart development.

When the first tubular heart tube is pro­duced, the space between the endocardium and myocardium becomes filled by ‘car­diac jelly’. It is a thick gelatinous mass containing aldehyde, acid mucopolysaccharides. Many cells from the endocardial and myocardial layers migrate into this gelatinous layer to form a loose meshwork of stellate cells which characterise the early heart tube.

Conflicting views exist as regards the histological nature of heart tissue, nature of fibrillogenesis, the nature of myofibril and the nature of intercalated disc. Elec­tron-microscopic and tissue culture studies have revealed that heart tissue is syncytial in nature and the intercalated disc consists of a pair of apposed cell membranes.

The region is covered with electron-dense gra­nules. The myofibrils do not cross the inter­calated discs and there is no protoplasmic continuity across the apposed membranes.

The early tubular heart consists of endo­cardium and myocardium. The endocar­dium is composed of a single layer of flattened and granulated cells, while the myocardium is two or three cells deep. Subsequently in course of development the myocardium thickens by mitotic acti­vity.

Myoblasts, composing the myocar­dium, contain granular materials and scat­tered loose myofilaments which become grouped to form striated myofibrils shortly before the pulsating of the heart. Mitotic activity is very high in early stage which declines to zero as cardiogenesis is com­pleted.

Structural Differentiation in Cardio­genesis:

One of the important factors, which causes the regional differentiation of heart is the rapid elongation of the primitive heart tube within the lass-rapidly growing pericardial space. The heart is a straight tube when it is formed and does not show any sign of subdivision into chambers.

In course of development, the tube becomes inflected in a characteristic way to assume the adult configuration due to the cellular activities. In chick, the tubular heart be­comes ‘S’-shaped at the end of 3rd day after incubation. The heart becomes con­stricted in some regions and dilated at others.

In the 4th day of incubation, the atrial area expands into two lobes—the beginning of the left and right atria. The descending part becomes thickened to form the ventricle. The later development of heart is the differential growth and sub­division into chambers. Fig. 5.35 shows the twisting and formation of different parts of heart in a chick embryo.

The changes undergone by the tubular heart to form adult heart are essentially:

(i) Constrictions to form chambers.

(ii) Differential growth and thickening of the myocardium resulting in the for­mation of thin-walled receiving parts and thick-walled forwarding parts.

(iii) Kinking of the chambers—possibly due to rapid growth within crowded quar­ters.

(iv) Formation of septa, valves, etc.

Functional Changes in Cardiogenesis:

The definitive function of the heart starts as the paired cardiac tubes fuse and the contraction begins as soon as the primitive ventricle is formed. So the heart is the organ which begins its function at an early stage of development. Contraction starts in the myocardium along the right margin of the posterior end of the ventricle.

Gradu­ally the contraction involves the whole ventricular wall which contracts synchro­nously, i.e. periods of contraction alternat­ing with periods of rest. Meanwhile the atria develop which also contract at a more rapid rate. The atria control the rate of contraction of the heart as a whole.

These contractions set the contained blood in motion. Eventually pacemaker or sinoauricular node develops which takes the controls of the contractility of the heart as a whole.

Developmental Organ # 4. Kidney:

The kidney of vertebrates essentially consists of an aggregation of uriniferous units called nephrons. The kidney develops from the ‘intermediate mesoderm’ which lies between the somite and the lateral plate mesoderm. The intermediate meso­derm becomes segmented and each seg­ment is called the nephrotome. The nephrotome is transformed into the nephrons which involves significant cellular events.

Mode of Origin:

A nephrotome contains a coelomic space, called the nephrocoel which communicates into the adjacent splanchnocoel by the peritoneal funnel (Fig. 5.36).

The nephro­tome is converted into a nephron in the following ways:

(i) The nephrotome, prior to its trans­formation, is a strand of cells between the somite and lateral plate mesoderm.

(ii) A tubular outgrowth develops from the dorsolateral wall of nephrotome.

(iii) The principle tubule originates from the tubular outgrowth which communi­cates with the nephrocoel through nephros- tome, i.e. the cavity of tubule is actually an extension of the nephrocoel.

(iv) The median wall of the nephrotome invests a tuft of blood vessels (arterial capil­laries) to form the renal corpuscle.

(v) The actual mode of origin of renal corpuscle is controversial. It was believed that Bowman’s capsule is formed by a pro­cess of invagination of the glomerular mass into the wall of the nephrotome.

But the electron-microscopic studies of Kurz (1958) have established that the double- walled Bowman’s capsule is not formed as a result of invagination, but due to a cleft within a compact cellular mass. The inner layer becomes reflected over the glomeru­lus while the outer one forms- the capsular wall.

The basic pattern of the development of nephron becomes greatly modified in diff­erent vertebrates.

The deviation is due to:

(i) Typically hollow nephrotomes are not found in embryos of higher vertebrate forms, instead the tubules develop within a continuous nephrogenic cord without exhibiting segmental disposition,

(ii) All the nephrons do not differentiate at a time, rather, the nephrons appear in a sequential order from the anterior to the posterior end.

(iii) The structural organisation of the nephrons also shows gradual complexity progressively from the anterior to the posterior end.

Developmental Events of Nephrons in Vertebrates:

In primitive vertebrates, the distinction between the anterior and posterior neph­rons is not well marked but in amniotes (reptiles, birds and mammals) the deve­lopment of nephric system shows the mani­festation of three distinct entities which succeed each other during ontogenic deve­lopment.

The entities are: Pronephros, Mesonephros and Metanephros. The fishes and amphibians possess first pronephros which gives way to the mesonephros—the final kidney of an adult. In amniotes, be­sides these two units, a third entity, the metanephros arises as the definitive adult kidney. All the types of nephrons exhibit a striking similarity in their cellular trans­port mechanism and physiological perfor­mances.

Pronephros:

The segmental origin of the pronephric tubules is the character­istic feature in nephric development. In amphibians, the pronephric tubules are developed from the nephrotomes beneath third and fourth somites in salamanders and second, third and fourth somites in frogs.

It is to be noted that the number of the pronephric tubules corresponds directly to the number of segments involved. In chick, the pronephros develops from nephrotomes between fifth to sixteen somites.

The pro­nephric tubules begin to form when the embryo attains 12 to 13 pairs of somites (40-45 hours of incubation). The tubules become well developed in 16-21 somites stage. At 35-somite stage and at about 65-70 hours of incubation, the pronephros undergoes degeneration.

The sequence of events of transformation of the nephro­tomes into the pronephric tubules is clear in lower vertebrates, but in higher tetra- pods the stages are not so clear.

When several pairs of pronephric tubules are developed, they open into the coelomic cavity proximally while the distal ends join the pronephric duct (Fig. 5.37). The neph­ric duct is called the pronephric duct which not only serves as the drainage channel for the pronephros, but becomes involved with the development of mesonephros.

Typical pronephros is a functional kidney in the larval stages of fishes and amphibians. But in the embryos of reptiles, birds and mammals, the pronephros deve­lops in the anterior nephrotomes and is not functional at any stage. In human embryo, about seven pairs of pronephric tubules develop which start degeneration imme­diately after the initiation of the nephric duct.

Independency in the Differentiation of Pronephric Tubules and Nephric Duct:

The nephric duct (pronephric duct) starts development from the mesodermal blocks situated more posterior to the seg­ment from which pronephric tubules begin to form. In amphibians, the nephric duct originates from the nephrogenous meso­derm behind that which provides the pronephric tubules.

The somite 5 usually marks the level of the nephric duct primordium in amphibians. O’Connor (1938) has applied vital stain to pronephric swell­ings below the third and fourth somites in Ambystoma. It was observed that the stain appeared only in the pronephric tubules.

When the stain was applied below the fifth and seventh somites, the stain became confined to the nephric duct. Holtfreter (1943) has bisected the embryo between the levels of fourth and fifth somites and has observed that in the hind piece, though devoid of pronephric tubules, the nephric duct still develops perfectly.

The above ex­perimental fact relates that the pronephric tubules and nephric duct are determined, independently of each other. Once the nephric duct starts development, both the pronephric tubules and nephric duct elon­gate at a rapid rate. The pronephric tubules become thrown into loops as a result of elongation and the glomeruli of several segments may join together to form the glomus (Fig. 5.38).

The nephric duct (now designated as pronephric duct) after inauguration, pushes itself backward along the lower ends of the somites and the pos­terior movement is stopped as it reaches the cloaca. The duct fuses with the wall of the cloaca and its lumen opens into the cloacal cavity.

The backward elongation of the prone­phric duct towards the cloaca is possibly due to either by (i) progressive addition of new material or (ii) due to free terminal growth. Extensive literature exists on this particular issue. Overton (1959) advocated that the duct increases by independent caudal growth.

Holtfreter opined that the growth of pronephric duct towards the cloaca is due to selective cell-adhesions rather than chemotaxis as advanced by many. Holtfreter also suggested the role of blood vessel during the process, but this issue remains open for further investigation.

Mesonephros:

The mesonephros is deri­ved from the nephrotomes posterior to the pronephros. In majority of amphibians and amniotes, the component mesodermal cells of the nephrotomes dissolve into an aggregation of mesenchyme. These aggre­gated cells stretch on each side of the body along the dorsal margin of the lateral plates.

This mass of mesenchymal cells is called the nephrogenic cord or nephrogenous tissue. The mesonephric tubules develop from the nephrogenic cord extending bet­ween 17 to 30 somites in chick embryo.

The proliferation of mesenchymal cells leads to the formation of elongated solid cords. Each suchycord elongates and assu­mes ‘S’-shaped appearance. It becomes hollow to form a cavity. One end of suph a tubule connects itself to the existing prone­phric duct (the mesonephric tubules do not form a duct of their own), while the proximal end forms a double-walled Bowman’s capsule.

The pronephric duct is now designated as the mesonephric or Wolf­fian duct because it serves as a drainage duct for the mesonephros. The Bowman’s capsule is supplied by small blood vessels from the dorsal aorta (Fig. 5.39).

Several mesoephric tubules are developed in a segment, i.e. the number of mesonephric tubules do not correspond to the number of somites involved in nephrogenesis. When first formed, one mesonephric tubule deve­lops in a segment, but subsequently each tubule gives origin to secondary and ter­tiary tubules by budding (Fig. 5.40).

In case of chick embryo, the mesonephros becomes functional from 5th to 11th days. The tubular system becomes extensively coiled in 8th to 10th days of incubation. After this period the mesonephric tubules start degeneration along the anteropos­terior direction and their function is taken over by metanephros which differentiates subsequently in the region posterior to that of mesonephros.

Role of Pronephric Duct in Mesonephros Differentiation:

The mesonephrogenic cord will start differentiation into mesonephric tubules as soon as the pronephric duct is in touch with it. The mesonephrogenic cord develops mesonephric tubules only if stimulated by the pronephric duct.

From this observation it is natural to think that the pronephric duct serves as an inductor for the differentiation of the mesonephros. Extensive experimentations have been done on this issue to ascertain the induc­tive role of pronephric duct in mesone­phros differentiation.

Humphrey (1928), Burns (1938) and Holtfreter (1944) have experimentally obstructed the backward extension of the primordial nephric duct and have found the formation of mere clump of cells in the mesonephrogenic cord. Waddington (1938) and O’Connor (1939) have shown that the mesonephrogenic cord fails to develop renal tubules if the pronephric duct does not reach the specified region.

Local condensation of cells occurs only in the nephrogenic cord. Boyden (1927) by des­troying the tip of pronephric duct by cautery and Waddington (1938) by inci­sion of the duct have shown that the differentiation of mesonephrogenic tissue into the mesonephric tubules occurs only when the pronephric duct makes contact with the tissue.

But Gruenwald (1942) and Calame (1962) have cast doubt on such induction and reported that the mesone­phrogenic cord is capable of a consider­able degree of self-differentiation. Gruen­wald (1942) and van Geertruyden (1946) have shown that the nervous tissue, when transplanted into the competent mesone­phrogenic cells, can induce mesonephros differentiation.

So it is not unreasonable to think that other tissues (possibly somi­tes) are also involved in this process. So the generalisation that differentiation of mesonephric tubules depends solely upon induction by the pronephric duct appears premature to accept.

Metanephros:

The metanephros is the functional kidney in the postembryonic life of amniotes. It develops from the neph­rogenic cord which is derived from the nephrotomes posterior to the mesoriephros adjacent to the cloaca. In chick embryo, the metanephros begins its deve­lopment at the end of the 4th day of incu­bation and between the 31-33 somites.

In the embryos of amniotes, the ureteric bud emerges as a diverticulum from the mesonephric duct near its junction with cloaca. This bud develops into the meta­nephrogenic cord or blastema. The distal end of the bud expands to form the pri­mordial renal pelvis. The metanephrogenic blastema starts condensation around the pelvis (Fig. 5.41).

The pelvis produces subdiverticula, each of which becomes the collecting tubule. The nephrogenic tissue accumulates around the distal end of each collecting tubule and forms ‘S’-shaped metanephric tubule (Fig. 5.42).

Each metanephric tubule opens into the collect­ing tubule at one end and the other end forms a double-walled Bowman’s capsule. The metanephros uses the mesonephric duct for the elimination of urine, but the
connection between them is not direct, and is established by means of a special outgrowth, the ureteric bud which trans­forms into the renal pelvis and the ureter.

Role of Ureteric Bud in Metanephros Diff­erentiation:

The conversion of the metanephrogenic tissue into metanephric tubules is dependent on an induction from the ure­teric bud developing from the mesonephric duct. Because the extirpation of either mesonephric duct or ureteric bud causes the failure, of the formation of metane­phros. This phenomenon suggests the inductive phenomenon.

It has been experimentally tested that the metanephric primordium undergoes characteristic development when cultured in vitro. The pelvis component develops a system of collecting tubules while the blastema forms coiled tubules. When these two components, after separation with trypsin, are cultured independently, nei­ther of them is able to carry through characteristic morphogenesis.

The subdivi­sion of the renal pelvis is dependent upon the metanephrogenic blastema, while the tubule differentiation in the blastema rests upon an inductive stimulus from the ure­teric bud. So the existence of inductive role’ played by ureteric bud in metane­phric differentiation seems to be positive.