basic body plan of young mammalian embryos

BASIC BODY PLAN OF BASIC BODY PLAN OF YOUNG MAMMALIAN YOUNG MAMMALIAN

EMBRYOSEMBRYOSJOHN JEFFERSON COLLERA

It is important to recognize that the basic body plan of essentially all the vertebrate embryos during the period of early organogenesis is essentially a carryover of that is seen in the aquatic, gill-breathing ancestral form of vertebrates.

This characteristics include a circulatory system based on a simple tubular heart emptying into ventral aorta, the breaking pump of the ventral aorta into branches that supply gills as they pass around the pharynx and the collecting of dorsal branches of these arteries into dorsal aorta.

The overall subdivision of pharyngeal region into system of gill (branchial) arches is retained, although with numerous modifications, throughout the embryos of vertebrate classes.

Likewise, the segmental organmization of the body (an adaptation fro sinusoidal swimming movements) is prominent in the somites and spinal nerves and persists into adult life in the jointed vertebral column.

There is striking similarity in general organization and external appearance of various amniote embryos.

As if to anticipate its ultimate cerebral dominance, the human embryo however, soon begin to show larger neural plate in its future forebrain region.

The cephalic part of the neural plateof young human embryo is so large and is expanding laterally so rapidly that its closure is delayed compared with the early closure in other embryos.

In the mid-body region, however, the general arrangement of structures and layers is essentially similar among amniote embryos.

Between third week and the fourth week, the human embryo loses its originally straight body axis and begins to show a marked flexure in the cranial region.

Three week human embryoThree week human embryo

There is a basic similarity in the body plan of 4 to 4 ½ day chick embryos , 5 mm pig embryos, and month old human embryo..

Minor differences consist of the larger size of larger size of eyes and optic regions of avian embryos and the relatively more advanced heart, liver and mesonephron of mammalian embryos.

EXTERNAL FEATURESEXTERNAL FEATURES

In the cephalic region the primordia of developing sensory organs are prominent features.

The nasal (olfactory) placodes appear as local ectodermal thickenings on either side of the head, and in more advanced embryos, the nasal placodes become depressed to form the nasal pits.

The primordia of the eyes, located far laterally in the head ,are prominent landmarks, and the auditory vesicles and contours of brain walls are sharply outlined on clear embryos. The face does not exist as a recognizable region.


The region of throat is dominated by a system of branchial arches and clefts, a region homologous to the gills of primitive fishes.

The branchial arches are best viewed from the ventral aspects.

The first on either side is subdivided into two components: the maxillary processes, which form the lateral parts of the upper jaw, and the mandibular elevations, which merge with each other in the midventral line to form arch of the lower jaw (mandibular arch)


Posterior to the mandibular arch are three similar areches, the hyoid and the unnamed third and the fourth sycher, all of which appear clearly in lateral views..

The hyoid arch is homologous to the operculum (gill cover) of fishes, and even in the human embryo it slightly overhangs the third and fourth arches.


Between the branchial arches are deep furrows which mark the position of the ancestral gill clefts.

Although in mammalian embryos, these furrows do not ordinarily break through into the pharynx, they are commonly called clefts because of their phylogenetic significance.

Only the most cephalic of these clefts is named (hyomandibular cleft..); they others are designated by their positional numbers.


The entire region about the third and the fourth postoral clefts becomes especifically deeply depressed and is known as the cervical sinus.


The upper body of the early embryo is dominated by the precociously large heart, which form the surface bulge known as the cardiac prominence.

Adding the dorsal side of the trunk, the paired somites extend from just caudal to auditory vesicle into the tail.

Conspicuouc in the midventral region is the body stalk which in time becomes more dicrete and elongated as the umbilical cord.


The appendage buds first appear as flangelike projections from the body wall . The arm bud leads to the leg bud in developmental progress.


At this stage the human embryo has every bit as well developed a tail as does a pig embryo.

Later in development the human tail normally undergoes regressive changes that leave the human tail only a symbolic coccyx.

Regression of the human tail is apparently brought about by intrinsic cell death.

Occasionally this regression fails to occur, and a human infant is born with a sizable and unmistakable tail.

THE NERVOUS SYSTEMTHE NERVOUS SYSTEM

The central nervous system has progressed from a simple neural tube to one which fundamentalbsubdicisions of the brain taking shape.

At first, three divisions are apparent .These are the forebrain (prosencephalon),

midbrain (mesencephalon), and hindbrain (rhombencephalon).


At the stage under consideration, the brain is in the phase of transition from the three to the five-vesicle condition.

There is a clear indication of the separation of forebrain into a rostral telencephalon and a diencephalon and the thickening of the walls of the more rostral part of the hindbrain presages the differentiation of the metencephalon from the mylenecephalon.

The mesencephalon remains undivided.


Projection from the walls of diencephalon are the optic vesicles, which are beginning to invaginate to form the optic cups..

In response to an inductive signal from the optic vesicle, the ectoderm overlying the optic cup has thicken and has begun to invaginate to form Lens vesicle.

From 4 to 5 weeks in the human embryo, many of the 12 cranial nerves begin to appear.


Caudal to myelencephalon, the neural tube is more slender and gives more rise the spinal cord.

During these early stages, it is relatively simple appearing the tube with a slitlike central canal and walls of closely packed cells of ectodermal origin.

Alongside the spinal cord the ribbonlike neural crest has just broken up and form the spinal (and also the eranial) ganglia.

DIGESTIVE AND RESPIRATORYSYSTEMSDIGESTIVE AND RESPIRATORYSYSTEMS

The primitive gut tube is formed by the endoderm as the body itself folds into a tube.

By the stage of 5mm pig and 1-month human embryo, it has been deliminated into a tubular foregut and hindgut and a midgut, which has open ventral region leading to the yolk stalk.


The intermembryonic gut-tract of young mammalian embryos at first ends blindly at both its cephalic and caudal ends.

And external depression called the stomodeum marks the future oral opening.

As the stomodeum deepens, the ectoderm form this floor comes to direct contact with the endoderm of the foregut to form the stomodeal, or oral plate.


As is common with the ectoderm and endoderm abut directly upon each other, the oral plate breaks through and establishes the oral opening into the foregut.


Arriving medially as a slender ectodermal diverticulum from the ventral part of the stomodeum is Rathke’s pocket.

From its first appearance, Rathke’s pocket is in close relationship to the infundibular process from the floor of diencephalon.

Together, these two structures will form the hypophysis.

DIGESTIVE AND DIGESTIVE AND RESPIRATORYSYSTEMSRESPIRATORYSYSTEMS

On the endodermal process of the oral plate, the extreme cephalic end of the foregut remains as the preoral gut, or Seesel’s pocket.

This structure serves only as anatomical landmark and gives rise to no adult structure.


The cephalic end of the foregut is mainly invoplved in the formation of the pharynx, fro which four pouches extend on either side toward the corresponding external gill furrow..

Although the thin membrane that remains between the pharyngeal pouch and the external gill furrow in mammals does not ordinarily breakdown to form an open gill cleft such as that found in our water living ancestors, the similarity in relationships is obvious.


The significance of the structural relations in this regions is further emphasize by the location of the aortic arches, which lie in closely packed mesenchymal tissue between the gill clefts.

In the embryos of birds and mammals, the aortic arches do not form capillary beds as they do in gill-breathing animals.

Nevertheless, the basic ancestral pattern can be seen in the way the aortic arches pass from the ventrally located heart to the dorsal aorta by the way of the gill arches flanking the pharynx..


In older embryos, a number of important structures arise from the endodermal lining of the pharynx.

Of these, the only small cluster of cells which constitute the primordium og the thyroid gland has emerged from the floor of the pharynx at this stage.


Also, from the floor of the posterior pharynx, a median ventral groove is rapidly converted into a tubular outgrowth parallel to the digestive tract.

This groove is the laryngeotracheal groove, and the tubular outgrowth is the future trachea, the caudal end of which is beginning to bifurcate to form the branchial or lung buds..


Caudal to the pharynx, the digestive tract passes a short, narrow esophagus to a slight dilation that represents the future stomach.

Immediately caudal to the stomach are the outgrowths of the gut that constitute the primordia of the pancreas, liver, and gal bladder..


At this stage, the intestines are represented by a straight tube which runs parallel to the midsagittal plane of the embryo.

The yolk sac arising from the __ of the midgut serves as the useful landmarks for indicating what part was derived from the hindgut.

In contrast to the foregut, the hindgut gives rise to only one diverticulum, the allantoic stalk which arises near its posterior end..

Posterior to the allantoic stalk, the hindgut dilates slightly, to form the cloaca.


Near the caudal end of the cloaca is a depression in the central body wall called the proctodeum.

It deepens in a manner similar to that exhibited by a stomodeum at the anterior end of the foregut, living only a thin, separating membrane of proctodeal enctoderm and cloacal endoderm.

This membrane is known as the cloacal plate, of cloacal membrane.

With its rupture, which occurs considerably later than the rupture of the oral plate, the originally blind hindgut estblishes an outlet.

The MesodermThe Mesoderm

From simple condition which it is divided into somitic (paraxial), the intermidiate, and lateral plate components, the mesoderm has begun to undergo specialization.

The somites have developed past purely epithelial stage. The cells of the sclerotomal region have broken away form

the main body of the somite, leaving the dermatomal and myotomal layers.

The Circulatory SystemThe Circulatory System

The embryonic circulatory system can be analyzed in terms of threee major circulatory arcs, with the heart as the common center and pumping station.

One arc is entirely intraembryonic in its distribution. Its vessels bring food materials and oxygen to all parts of

the growing body and return waste materials from them. The other two circulatory arcs have both intra- and extra-

embryonic components


The vitelline arc carries blood to and from the yolk sac.

The other carries blood to and from the allantoise for gaseous interchange.

As the blood from the three arcs are returned to the heart for recirculation, it is constantly mixed so that its food material, oxygen, and accumulated waste products are maintained at serviceable levels.


In placental mammals radical changes in the source of food supply and basic living conditions, compared with reptiles and birds, alter the way the two extraembryonic circular arcs operate.

The yolk sac opf higher mammalian embryos, although formed in characteristic relation to other structures, is small and empty, its arc, therefore, has lost its significance as purveyor of food.

Nevertheless, the vessels in this arc still form and are for a time quite conspicuous.


In what may be called a “phylogenetic hangover”, the vitelline vessels, although depreived of their primary function, still bring the first blood cells into the embryonic circulation from their place of formation in the yolk sac splanchnopleure, just as they did in asncestral forms with food-laden yolk-sacs.


In the mammalian embryos, the allantoic arc takes over the functions abandoned by vitelline arc as well as continuing to carry out its own earlier responsibilities.

The allantoisis either closely applied to the uterine lining, as in the pig, or sends little rootlets of its own tissue (called chorlonic villi) into the uterine mucosa as in the human..


In either situation, maternal blood and fetal blood are brought close together so tha tfetal blood can absorb food and oxygen from the maternal blood and pass its own waste materials back to the maternal circulatory system.

Thus the allantoic circulation, of mammlaian embryo serves as a provisional mechanism for food getting respiration.

The Establishing of the HeartThe Establishing of the Heart

In mammalian embryos, heart arise from mesodermal primordiasituated ventrolaterally beneath the pharynx.

The cardiac primordia are composed of two layers as well as being paired right and left.

The inner layer is called endocardium because it is destined to form the internal linng of the heart.


The outer layer, derived from the thicker splanchnic mesoderm, is known as the epimycardium, because it will give rise to both the heavy muscular layer of the heart wall (mycardium) and its outer covering (epicardium).


While these changes heve been occuring in the heart, folding off of the embryonic body has been going on with concominant progress in the closure of foregut at the level of the heart.

As a result, the paired endocardial tubes are brought progressively closer together.

Finally they are approximated to each other and fused into a single tube lying in the midline..


In the same process, the epimycardial layer is bent toward the midline, enwrapping the endocardium.

Ventral to the endocardial tubes are epimyocardial layers of oppositesides come into contrast with each other.

Where this contrast occurs, the limbs of the mesodermal folds next to the endocardium fuse with each other, forming an outer layer of the heart that no longer is interrupted ventrally.


Thus, the originally paired right and left coelomic chambers become confluent to form a median unpaired pericardial cavity in the same process which establishes the heart as a median structure.

Dorsally, the right and left epimyocardial layers become contiguous, but here they do not fuse immediately as happens ventral to the heart.

They persist for a time as a double-layered supporting membrane called the dorsal mesocardium.


In this manner, the heart is established as a nearly straight double-walled tube suspended mesially in the most cephalic part of the coelom.


The early heart soon undergoes a change in straight tubular form to an S-shaped configuration.

During this phase, the heart funtion like a simple tubular peristalticpump.

The veins converging to center the heart becomes confluent in a thin-walled chamber called sinus venosus..


Backflow of bloood is prevented by well developed flaps known as the valvulae venosae.

From the atrial region, which is just beginning to bulge out into right and left chambers, blood centers the muscular ventricles.

Internal subdivision of the early atrial and ventricular regions is just beginning.


From the ventricle, blood passes into truncus arteriosus and then to the body by way of the ventral aortic roots.

Despite the early modification of the structures of the heart, the blood entering the caudal end of the heart through the sinus venusus is still pumped out of the heart into the truncus as an undevided stream, just as was the case in younger embryos containing a straight tubular heart.

Formation of Blood and Blood Formation of Blood and Blood VesselsVessels

While the heart is becoming established, the main vascular channels characteristic of young embryos are also making their appearance.

In a manner similar to the genesis of endocardial tubes themselves, cords and nots of mesodermal cells become aggregated along the future course of a developing vessel.


These strands of cells then form hollowed-out tubes lined by a layer of thin flattened endothelial cells.

In later stages, some vessels also become extended by the formation o f budlike outgrowths from their walls.

Typically, a meshwork of a small vascular channels is formed first.

Gradually, some of these primitive channels become enlarged to form the main vessels


Blood cells and early blood vessels form as aggregates of splanchnic mesodermal cells lining the yolk sae..

These aggregates are called blood islands. These cells around the early blood cells become

incorporated into the vitelline circulatory arc. These vitelline channel feeds blood cells into the early

beating heart, which distributes throughout the vascular channels.


In human embryos, the first blood cells are drawn from the yolk sac into the vascular system toward the end of the third week of development.

Arterial SystemArterial System

Vertebrate embryos pump their blood from the ventrally located heart around the pharynx to the dorsal aorta by the way of a series of six paired blood vessels called aortic arches.

The aortic arches appear in cephalocaudal sequence and then undergo an irregular sequence of regression or preservation.

Arterial SystemArterial System

Three major arterial trunks lead off the dorsal aorta in young embryos..

At the level of first aortic arches, the paired internal carotid arteries grow to the developing brain.

The main vitelline artery arises at midbody leveland extends ventrally to supply the vitelline vascular plexus on the yolk sac.

Toward the caudal end of the aorta, the large allantoic arteries grow out along either side of the allantoic stalk.

These vessels ultimately become known as the umbilical arteries.

The Venous SystemThe Venous System

The main return channels for the entraembryonic circulatory arc are the cardinal veins..

The paired anterior cardinal veins, which collect the blood from the head, and the posterior cardinal veins, which are the primmary drainage channels of the caudal half of the body, converge to form the common cardinal veins, which empty into the sinus venuos of the heart.

Major pairs of veins also bring blood from the two extraembryonic circulatory arcs in the body .

The Venous SystemThe Venous System

Arising as collecting channels in the vitelline vascular plexus over the yolk sac, the main vitelline veins pass into the body through the substance of the liver to empty into the sinus venosus.

The growing cell cords of the liver soon breaks up the proximal portion of the vitelline veins into irregular channels called hepatic sinusoids.

The return channels from the allantoic arc are initially referred to as the allantoic veins, but in time as the placenta forms, and the body stalk is converted into the umbilical cord, they are known as the umbilical veins.