Organogenesis

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DEVELOPMENTAL BIOLOGY

Organogenesis

Anita Grover Reader Department of Zoology Zakir Hussain College Jawahar Lal Nehru Marg Delhi – 110 002

Key words: Rudiment, neurulation, hinge points, cadherin, epimere, choroid plexus, vesicle, placode, palisading, visceral layer, parietal layer, coelom.

ORGANOGENESIS (DEVELOPMENT OF ORGANS)

Organogenesis is a crucial phase in development, in which the embryo finally becomes a fully functional organism, capable of independent survival. In this chapter we shall first consider the development of some of the organ rudiments from the three germ layers: ectoderm, mesoderm and endoderm. Then we will study the development of certain organs like eye, ear and heart in detail including central nervous system. I. EARLY VERTEBRATE DEVELOPMENT UPTO ORGAN RUDIMENTS During the development of vertebrate body, the different regions in the three germ layers of the gastrula segregate from each other to form the rudiments of future organs and tissues. Most of the rearrangements of the germ layers to form organ rudiments involve transformation in the epithelial cell sheets. Epithelial cells undergo a variety of folding and spreading movements, which are as follows: Local thickening of epithelium: Thickening of epithelium is known as palisading. Palisading occurs due to the elongation of single cells (fig. 4.1) as can be seen in the formation of neural plate and ectodermal placodes such as lens, ear and nasal rudiments.

Folding of epithelium: Epithelium can take several forms of folding such as inward, outward or linear folds. When the epithelium bends inwards into the embryo or into a cavity, it is called invagination, whereas if the epithelium bends outwards from the surface of the embryo, it is known as evagination. Folding along a line i.e. linear folds give rise to a groove (fig. 4.2). The formation of neural tube is by linear folding. The formation of lens vesicle or otic vesicle from their respective thickenings illustrates inpocketing or infolding of the epithelium to form pouches (fig. 4.3). Folds or pouches may undergo modifications to form branched structures. The formation of various glands depends on the folds appearing at the epithelial outpocket (fig. 4.4.). Folding or bending of a sheet may change shape of epithelial cells (fig. 4.5). The narrowing of columnar epithelial cells at the apical end results in formation of pyramidal cells. This in turn results in the differences in the surface area on the two ends of the epithelium and the bending of the entire sheet.

Separation of epithelial layers: This takes place by the appearance of cervices, which enlarge to form cavities. Normally the cervices appear either parallel to the surface of the epithelium or perpendicular to it. In the first case, the epithelial layer splits into two layers lying on top of the other. The original cervice may be increased due to secretion of fluid into it and becomes a spacious cavity. This type of epithelial splitting is seen in chick and other vertebrates that forms the parietal and visceral layers of the lateral plate mesoderm, and coelomic cavity between them . The second type of splitting, which is perpendicular to its surface, can be seen during the development of mesodermal somites. Flattening and spreading of epithelial layer: This occurs during epiboly of presumptive ectoderm during amphibian gastrulation. In this process there is spreading of cells. The prospective ectoderm spreads towards dorsal mid line to cover the area left vacant by convergence of neural epithelium . Spreading may be accompanied by a change in cell shape such as thinning and flattening of individual cells (fig. 4.6). Dissociation of epithelium into individually migrating cells: Dissociated cells from the epithelial layer may move from one location to another either over long distances as in case of neural crest cell migration or over short distances as in the formation of hair or feather germ. In addition to various modifications to the epithelial cell sheets, selective death of cells plays an important role in shaping various structures of developing embryo e.g. during brain and limb development cell death is seen in certain regions. Differentiation of germ layers Ectoderm The ectodermal layer essentially separates into epidermal ectoderm, neural ectoderm and neural crest: the fate of this layer is shown in fig.4.7. Formation of neural tube When the gastrulation is near completion the presumptive area of the nervous system becomes differentiated from rest of the ectoderm. The cells in this part of the ectoderm become columnar in shape and this region of the embryo is called neural plate. The process by which neural plate forms neural tube; is called neurulation and an embryo undergoing such changes is referred to as neurula. There are two ways by which neural tube is formed - primary neurulation and secondary neurulation. In primary neurulation, the cells surrounding the neural plate direct the neural plate cells to proliferate, invaginate and pinch off from the surface to form a hollow tube called the neural tube. In secondary neurulation, the neural tube arises from a solid cord of cells that sinks into the embryo and subsequently hollows out (cavitates) to form a hollow tube. The mode of construction of neural tube varies among different vertebrate classes. In fishes, neurulation is exclusively secondary. In birds, the anterior portions of the neural tube are formed by primary neurulation whereas the neural tube caudal to the 27th somite pair (posterior to hind limbs) is made by secondary neurulation. In amphibians ( Xenopus), most of the neural tube in tadpole is made by primary neurulation except the tail neural tube, which is formed by secondary neurulation. In mice and may be in humans too, the neural tube posterior to the level of somite 35 is derived by secondary neurulation.

Primary neurulation The process of primary neurulation appears to be similar in amphibians, reptiles, birds and mammals. The presumptive area of the nervous system differentiates from the rest of the ectoderm to form a neural plate. Cells of the neural plate elongate and arrange themselves as columnar epithelium. During this process the embryo lengthens along the anterio – posterior axis. At the same time, the edges of the neural plate are thickened and raised above the general level as ridges called neural folds. Neural folds elevate further resulting in the formation of the neural groove between them. Neural folds meet each other in the mid dorsal line and fuse to form neural tube beneath the overlying ectoderm. Embryo at this stage is called neurula. Fig 4.8 depicts the stages in the formation of neural tube in amphibians. The cells at the dorsal most portion of the neural tube become the neural crest cells. They lie between the dorsal part of the neural tube and dorsal epidermis. The neural crest cells undergo extensive migration and form the autonomic nervous system, melanocytes and parts of skull etc. (see fig 4.7).

In amphibians, the formation of neural tube occurs simultaneously along the entire length of the embryo. Fig 4.9 shows the different stages of neurulation in amphibian embryo. In birds, reptiles and mammals even as the neurulation has begun in the anterior part of the embryo, the posterior region is still in the process of gastrulation. At a time when the neural folds are just about to form in the posterior region, the neural folds have already started fusing to form the neural tube in the anterior region (fig 4.10).

Mechanism of Primary Neurulation Primary neurulation involves four distinct stages. i. formation of the neural plate ii. shaping of the neural plate iii. bending of the neural plate to form neural groove and iv. closure of the neural groove to form neural tube i & ii Formation and shaping of neural plate:

The process of neurulation is initiated when the underlying dorsal mesoderm and pharyngeal endoderm in the head region signals the ectodermal cells above it to elongate into columnar neural plate cells (Smith & Schoenwolf, 1989; Keller et al, 1992). These elongated cells are the cells of the presumptive neural plate and thus become different from the cells of the epidermis, which remain more or less flat and arranged as a stratified epithelium. About 50 % of the ectoderm is included in the neural plate. The shaping of the neural plate is attained by the intrinsic movements of the epidermal and neural plate regions. The neural plate lengthens along the anterior – posterior axis and becomes narrow (fig 4.11a). iii. Bending of the neural plate: The bending of the neural plate involves the formation of hinge regions. In birds and mammals, the cells at the mid-line of the neural plate are called medial hinge point (MHP) cells (fig. 4.11b). They are derived from the portion of the neural plate just anterior to Hensen’s node and from the anterior mid-line of Hensen’s node. The MHP cells become anchored to the notochord beneath them and form a hinge, thereby forming a furrow at the dorsal mid-line. The notochord induces the MHP cells to decrease their height and to become wedge shaped (Van Straaten et al 1988; Smith and Schoenwolf 1989). The cells lateral to MHP undergo a change to form two other hinge regions called dorsolateral hinge points (DLHP). DLHP are anchored to the surface ectoderm of the neural folds therefore the neural plate remains attached to the rest of the ectoderm (fig. 4.11c). These cells increase in their height and become wedge shaped. Both microtubules and microfilaments are involved in these changes. Two main forces are involved in bending of the neural plate a) formation of hinges, which act as a pivot and directs the rotation of the cells around it and b) the movement of the presumptive epidermis towards the mid-line of the embryo and anchoring of the neural plate to the underlying mesoderm. This is important to ensure that the neural tube invaginates into the embryo and not outwards. The pushing of the presumptive epidermis towards the center and the furrowing of the neural tube creates the neural folds. iv. Closure of the neural tube: The neural tube closes as the neural folds come closer to each other at the dorsal mid-line. The folds adhere to each other and the cells from the two folds merge. The cells at this junction form the neural crest cells (fig. 4.11d). The closure of the neural tube does not occur simultaneously through out the ectoderm. In chick, neural tube closure is initiated at the level of the future mid-brain and “zips up” in both directions. The two open ends of the neural tube are called the anterior neuropore and the posterior neuropore. In mammals, neural tube closure is initiated at several places along the anterior-posterior axis.

The neural tube eventually forms a closed cylinder that separates it from the surface ectoderm. This separation is mediated by the expression of different cell adhesion molecules. Presumptive epidermal cells produce E-cadherin and neural plate cells synthesize N-cadherin and N-CAM. As a result, the surface ectoderm and neural tissues no longer adhere to each other (fig. 4.12). When various parts of the neural tube fail to close, different neural tube defects are caused, which are as follows: • Spina bifida – this condition is caused when human posterior tube region at day 27 fails to close. Severity of this condition depends on how much of the spinal cord remains exposed. • Anencephaly – this is a lethal condition, which results when anterior tube region fails to close. Here the fore brain remains in contact with the amniotic fluid and subsequently degenerates. Fetal fore brain development ceases and the vault of the skull fails to form. • Craniorachischisis – this condition is because of the failure of the entire neural tube to close over the entire body axis. Secondary neurulation Formation of medullary cord and its subsequent hollowing out in a neural tube is referred to as secondary neurulation (fig 4.13). In amphibians and birds, secondary neurulation is usually seen in the neural tube of the abdominal and tail vertebrae. It can be seen as a continuation of gastrulation. During gastrulation of frog, the cells of the dorsal blastoporal lip instead of involuting into the embryo keep growing ventrally. The growing region at the tip of the lip is called chordoneural hinge (fig. 4.14a). The hinge contains the precursors for both, the posterior most portion of neural plate and the posterior portion of the notochord. The cells lining the blastopore form the neurenteric canal. The proximal part of the neurenteric canal fuses with the anus while the distal portion becomes the ependymal canal (i.e. the lumen of the neural tube) fig 4.14.b.

Mesoderm In the neurula stage of the embryo, the mesoderm cells are arranged in five distinct regions. The five regions of the mesoderm and the organs derived from them are as follows: a. Chorda mesoderm – it separates as a mid dorsal strip from the rest of the mesodermal tissue and establishes the body axis of the embryo. This tissue forms the notochord (fig. 4.15 b) b. Paraxial mesoderm or dorsal mesoderm – tissues developing from this region will be in the back of the embryo on either side of spinal cord. It segments into blocks of tissues called somites (fig 4.15. c & d). These produce connective tissue and associated structures such as bone, muscles, cartilage and dermis. c. Intermediate mesoderm – it is a thin stock of cells connecting paraxial mesoderm with the rest of the mesodermal sheet. The urinogenital system arises from intermediate mesoderm (fig. 4.15 c & d)

d. Lateral plate mesoderm – it is a sheet of loosely connected cells on either side of gut. It splits into two layers; the somatic mesoderm which becomes closely associated with ectoderm and the splanchnic mesoderm which become closely associated with endoderm. The space between somatic and splanchnic mesoderm is the future coelomic cavity (fig. 4,15 c & d). Lateral plate mesoderm gives rise to heart, blood vessels and blood cells and lining of the body cavities. All the components of the limb except muscles are derived from lateral plate mesoderm. e. Head mesoderm – the head mesenchyme located in the head region will give rise to connective tissue and head muscles. The organization of the mesoderm in the neurula stage is similar for all vertebrates. Formation of somites and coelom in amphibians In amphibians, initially the mesoderm is formed as single plate on the either side of the notochord. Later each mesodermal plate gets differentiated into epimere, mesomere and hypomere (fig. 4.16). Epimere (axial mesoderm) is thickened edges of mesoderm nearest to notochord. The epimere gets completely separated from its neighbours but remains connected to the rest of the mesodermal plate i.e. mesomere till the myocoel is formed inside the epimere. Mesomere is the future source of kidney tissue. The rest of the mesodermal plate below mesomere is called hypomere (lateral plate). Epimere gets sub divided in a transverse plane into a series of cell masses or somites. Subsequently, each somite next to the notochord separates and becomes the sclerotome or skeleton forming tissue around the notochord. The remaining major part of the somite becomes the myotome. Its cells differentiate into the striated muscle fibres of the somatic muscles. The outermost narrow strip of somite cells lying beneath the epidermis becomes dermatone and later gives rise to the dermis of the skin (fig. 4.17). The lateral mesodermal plate on each side splits into two layers, an outer or parietal layer next to ectoderm and an inner visceral layer next to the endoderm. The cavity between the two layers is the coelomic cavity. Endoderm The third and the inner most germ layer endoderm mainly gives rise to gut tube and its accessory organs, respiratory tube and primodial germ cells. Digestive tube As neurulation begins, the free margins of the endoderm unite in the mid dorsal line beneath the notochord to form the digestive gut or enteron. The floor of the gut contains thick layer of large yolk filled cells. Anteriorly, the gut makes contact with anterior ectoderm where later on mouth invagination is formed and gets fused with the blind end of the gut to form a continuous gut cavity. Later lung, liver, and pancreas develop from the evagination from the gut. The pharyngeal pouches are formed as lateral out pushing of the fore gut and in later development gives rise to gill clefts.

II. DEVELOPMENT OF ORGANS Development of central nervous system: Differentiation of the neural tube into various regions of central nervous system occur simultaneously in three different ways: i. At anatomical level – the neural tube and its lumen bulge and constrict to form chambers of the brain and the spinal cord. ii. At the tissue level – the cell populations within the wall of the neural tube rearrange themselves to form the different functional regions of the brain and the spinal cord. iii. Finally at the cellular level, the neuro epithelial cells themselves differentiate into numerous types of nerve cells (neurons) and supportive cells (glia) present in the body. Early vertebrate neural tube is a straight structure. Its anterior end is broader, which gives rise to brain whereas posterior portion is narrow giving rise to spinal cord. The anterior portion of the nerual tube balloons into three primary vesicles; fore brain (prosencephalon), mid brain (mesencephalon) and hindbrain (rhombencephalon) (fig 4.18). Mid brain and hindbrain collectively are also referred to as deutroncephalon.

Development of fore brain By the time the posterior end of the neural tube closes secondary bulges, the optic vesicles have extended laterally from each side of the developing fore brain. The prosencephalon is subdivided into anterior telencephalon and the more caudal diencephalon. The telencephalon will eventually form the cerebral hemispheres and diencephalon will form the thalamic and hypothalamic brain regions that receive neural input from retina. Each cerebral hemisphere contains a pocket like cavity, the lateral ventricle (fig. 4.19). The lateral ventricles of both sides remain freely communicated with the cavity of diencephalon by a broad opening called inter – ventricular foramina or foramina of Monro. This opening becomes narrow during the lateral stages. Subsequently, the cerebral hemispheres grow forward, their walls get thickened and olfactory nerves make their anterior ends, which now become olfactory lobes.

The diencephalon possesses a large cavity called the third ventricle. Most of its anterior dorsal wall becomes non-nervous, membranous and vascular choroid plexus, which bulges down in its third ventricle. Only the posterior dorsal wall of diencephalon remains nervous. The mid dorsal part of the diencephalon wall gives out dorsally a long finger like outgrowth or evagination called the epiphysis. The end section of the epiphysis becomes a rounded mass the pineal body, while its stalk becomes constricted and obliterated later on. The ventral wall or floor of diencephalon gives out laterally two outpushings, the optic vesicles, which later develop into retina of eye and optic nerve. At its mid ventral line, the floor of diencephalon gives out a funnel like outpushing called the infundibulum which fuses with the outgrowth from the stomodaeal invagination (Rathke’s pouch) to form the

hypophysis or pituitary. The walls of diencephalon on the sides and posterior to the infundibulum become hypothalamus. Development of mid brain The mesencephalon does not divide. Its walls remain thick and its roof gives rise to an important nerve center, the tectum. Anterior part of the tectum is the primary center of visual organ, the eye. The posterior part of the tectum receives nerve fibers from ear. The tectum gives out two swellings, the corpora bigemina or optic lobes. The cavity of mid brain becomes narrow and is called aqueduct of Sylvius or cerebral aqueduct. Development of hind brain The rhombencephalon is subdivided into anterior metencephalon and posterior myelencephalon. The myelencephalon eventually becomes the medulla oblongata whose neurons generate the nerves that regulate respiratory, gastro intestinal and cardio vascular movements. The roof of the medulla becomes non-nervous, thin and vascular choroid plexus. The metencephalon gives rise to the cerebellum, which is responsible for coordinating movements, posture and balance. The cavity of the rhombencephalon expands anteriorly just behind the mid brain and becomes the fourth ventricle. Posterior to the brain the neural tube gradually develops into spinal cord. Rhombencephalon is divided into smaller compartments by periodic swellings called the rhombomeres. The cells within each rhombomere can mix freely within it, but not with cells from adjacent rhombomere. Thus each rhombomere represents a separate territory and has a different developmental fate. Each rhombomere will form ganglia (cluster of neuronal cell bodies whose axons form a nerve). The increase in the volume of early embryonic brain is because of increase in its cavity size and not tissue growth. The expansion of early brain is caused by the positive fluid pressure exerted against the walls of the neural tube by the fluid within it. Tissue architecture of central nervous system: The original neural tube is composed of one cell thick germinal neuroepithelium. This is a layer of rapidly dividing neural stem cells. They are adjacent to the lumen and divide mitotically. These stem cells divide vertically instead of horizontally and one of the daughter cells migrates to form a second layer around the original neural tube. This layer becomes thicker and is called the mantle or intermediate zone. The germinal epithelium at this stage is called ventricular zone and later known as ependyma. The cells of mantle zone differentiate into both neurons and glia. The neurons make connections among themselves and send forth axons away from the lumen thereby creating a cell – poor marginal zone. Eventually glial cells cover many of the axons of the marginal zone in myelin sheath giving them a whitish appearance. Hence the mantle zone containing the neuronal cell bodies is often referred to as the gray matter; the axonal marginal layer is often called the white matter. SENSE ORGANS Development of eye The development of eye is a complex morphogenetic process. The eye of frog develops from three primordia. The sclerotic coat and the choroid coat are formed from the mesenchyme. The pigmented and nervous layer of retina is formed by the optic cup, which is an outgrowth of prosencephalon. Iris develops from the free edge of the optic cup. The lens develops from the ectoderm.

Eye develops from optic vesicles, which emerge as the lateral outgrowths from prosencephalon. (Fig 4.20). The cavity of the optic vesicle is optoceol, which is continuous with the cavity of the brain. When the vesicle is fully formed it remains connected to the brain by narrow cylindrical stalk called the optic stalk. The optic vesicle then extends towards the head ectoderm and it induces the later to form a local thickening called lens placode. Once the lens placode is formed the optic vesicle invaginates to form a double cup structure, the optic cup and the optocoel is obliterated. The cells of the inner layer of the optic cup undergo mitosis and form the sensory layer of retina. Mesenchymal pigmented cells invade the thin outer layer of the optic cup to form the pigmented layer of retina. The rim of the optic cup later develops into the edge of pupil. The cavity of the optic cup is the future posterior chamber of the eye. Vitreous body later on fills this up. The opening of the optic cup initially is large initially but later on the rim of the cup bends inwards and converges so that the opening of the pupil is constricted. The rim of the optic cup surrounding the pupil thins out considerably to form iris.

The optic cup has a groove along its ventral side, which later extends to middle of the optic stock. This groove is the choroid fissure that serves for the entry of blood vessels into the posterior chamber of the eye. Later in development, the edges of the choroid fissure fuse to form a small canal within the optic stock. The arteries and veins remain in this canal and become the main blood vessels of the retina.

At the time of the formation of optic cup the lens placode curves inwards forming a cup and gets separated from the ectoderm. The free edges of the cup then fuse to form globular hollow lens vesicles, which enters the cavity of the optic cup. The ectoderm from where the lens vesicle gets separated heals up. The wall of the lens vesicle towards the optic cup start elongating into very long strips of cells. The cells of the lens vesicle towards the ectoderm remain cuboidal epithelium. The elongating cells of the lens vesicle form the lens fibres. These cells divide by mitosis to form more lens fibers. When the wall thickens the cavity inside the lens vesicle gets obliterated and a solid lens is formed. The lens fibers gradually become transparent. When the lens is formed, the free edges of the optic cup grow in front of the lens to form iris. After formation of the iris the opening which remains in its center becomes the pupil. During the formation of optic cup and lens, mesenchyme appears all around these structures. The interior layer of mesechyme cells give rise to a network of blood vessels surrounding the pigmented retinal epithelium and is called the choroid coat. The outer layer of mesenchyme cells form a fibrous capsule, the sclerotic coat or sclera around the eye. The sclera provides protection to eye and the eye muscles are inserted on it. Thus choroid coat and sclera are of mesodermal origin and lens of ectodermal origin whereas cornea develops from both mesenchyme and epidermal epithelium. Chain of optic inductions Chain of inductions operate during the development of eye as follows: i. The roof of archenteron acts as a primary organizer, which induces the formation of neural plate, and therefore also the eye cups rudiment, which is the part of the neural plate. ii. The eye cup rudiment develops into the optic vesicle which acts as secondary organizer and along with head mesoderm induces the epidermis to form lens placode. iii. The lens placode acts as a tertiary organizer and induces the neuro sensory layer of optic vesicle to invaginate and when lens placode develops into lens, it induces the epidermis and mesenchyme for the formation cornea, choroid coat and sclera. Development of Ear Ear of frog consists of an internal ear called ear labyrinth and a middle layer which includes the Eustachian tube, ear ossicle (columella) and tympanic membrane. Development of internal ear Internal ear is derived from auditory placodes, which are formed by thickening of the sensory layer of head epidermis against the sides of hindbrain. Each auditory placodes separates from rest of the epidermis and invaginates to form a closed vesicle called the otic or ear vesicle (fig 4.21). The ear vesicle is the rudiment of the internal ear. The ear vesicle is pear shaped. Its pointed end remains directed upwards and later on develops into the endolymphatic duct. The expanded portion of the vesicle gives rise to membranous labyrinth. Soon each ear vesicle starts expanding so that the walls of the ear vesicle become thin and the epithelial cells become flat forming the membranous area of the labyrinth. The medio-ventral wall of each labyrinth becomes thicker. The cells of this area become columnar and give rise to sensory epithelium, which forms the maculae of internal ear.

The expansion of the ear vesicle is unequal so that it becomes constricted at some places and bulges out in others. The lower part of the ear vesicle becomes divided into sacculus and utriculus. The utriculus is drawn into three mutually perpendicular folds, the rudiment of the semi circular canals. A hollow outgrowth of the sacculus forms the rudiment of lagena. Neuroblast cells differentiate from the otic vesicles and form a club shaped mass on the median wall. These cells become auditory ganglions from which processes develop that penetrate the hindbrain-giving rise to the root of theVIII cranial nerve (auditory nerve). The ear vesicles as they develop into internal ear get surrounded by mesenchyme cells, which later on give rise to cartilage and produce cartilaginous labyrinths or ear capsules. Ear capsule surrounds and protects internal ears. Space between the membranous labyrinth (derived from epidermal ectoderm) and cartilaginous labyrinths (derived from sclerotome) gets filled with a fluid called perilymph.

Development of middle ear Development of middle ear takes place from first pharyngeal gill pouch. This gill pouch does not form gill cleft but exists as an expanded lateral cavity which later form tympanic cavity. Mesoderm surrounds it and converts it into the typanum, ear drum or tympanic membrane. Internal lining of tympanic membrane is endodermal whereas outer lining is ectodermal. The resulting tympanic membrane is very thin and sensitive to airwaves. The inner of the tympanic cavity opens into the pharynx and forms Eustachian tube. A piston

like cylinder bone called columellla auris, crosses the tympanic cavity from tympanum to fenestra ovalis. Tympanum is a membrane, that covers the opening of the ear capsule. Auditory inductions i. Roof of archentron acts as a primary inductor and causes the development of hindbrain from neural ectoderm. ii. Hindbrain in turn acts as secondary inductor and induces the development of ear vesicles from the auditory placodes. iii. Ear vesicles along with the head mesoderm acting as tertiary inductor causes the formation of cartilaginous ear capsule around internal ear. Development of Heart Heart is mesodermal in origin. It develops at ventral side of pharynx and is formed by mesenchyme of lateral plate. After end of neurulation free edges of lateral plate mesoderm gradually converge towards the middle of mesoderm free region beneath the pharynx. Tips of ventrolateral plate get thickened and give rise to mesenchymal cells. These cells accumulate at the midventral line of pharynx and form a longitudinal strand of tissue called the rudiment of endocardium (fig. 4.22). The endocardial cells soon get arranged in the form a thin walled tube. The lumen of the tube is the cavity of the heart. While the endocardial tube is being formed the edges of the lateral plate mesoderm unite above and under the endocardial tube forming pericardial cavity. Visceral layers of mesoderm envelopes the endocardial tube and gives rise to muscular wall or myocardium. The parietal layer gives rise to pericardium. Heart gets attached to pericardial walls by mid- dorsal and mid-ventral mesocardia. Later on, this mesocardial epithelia dissolves to form a continuous cavity. The coelomic cavity expands in the heart region to form pericardial cavity. Initially this is a part of general body cavity, the coelom. Later on, it gets separated. Heart at first is a straight tube. It starts increasing in length, twists in a characteristic way to form ‘S’ shaped tube having four sub divisions i.e. chambers. Each chamber of heart is separated from the other by constriction. The vitelline vein penetrates the septum transversum and enters the large collecting chamber, the sinus venosus. Sinus venosus is followed by a thin walled atrium, the thick walled ventricle and Conus arteriosus (fig 4.23).

REFERENCES: 1. Keller, R.J. Shish; Sater, A.K. and Moreno, C. 1992. Planar induction of convergence and extension of the neural plate by the organizer of Xenopus. Dev. Dynam. 193: 218 – 237. 2. Smith & Schoenwolf, 1989. Notochordal induction of cells wedging in the chick neural plate and its role in neural tube formation. J.Exp.Zool. 250 : 49 – 62 3. Van Straaten, H. W.M.; Hekking J.W. M.; Wiertz – Hoessels, E.J.L.M; Thors, F. and Drukker, J. 1988. Effect of the notochord on the differentiation of a floor plate area in the neural tube of the chick embryo. Anat. Embryol. 177 : 317 – 324.