Fate of Embryonic Tissues PDF

7/28/2019 Fate of Embryonic Tissues PDF

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The Fate ofEmbryonic

Tissues


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DEVELOPMENT OF

SYSTEMS AND ORGANS

During vertebrate gastrulation the three germ layerare formed .These differentiate , first into primordial cell massesand then into specific organs and tissues.During this process, cells become increasingly

committed to specific directions of differentiation.


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Ectoderm:


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Derivatives of Ectoderm:

The brain, spinal cord , and nearly all outer epithelial structures of the body develop fromprimitive ectoderm .They are among the earliest organs to appear.


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Derivatives of Ectoderm:Nervous System

o Just above thenotochord, theectoderm thickens

to form a neuralplate.

o The edges of thisplate rise up, fold,

and join together atthe top to create an

elongated, hollowneural tube.


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o The neural tubegives rise to most ofthe nervous system:

o anteriorly it enlargesand differentiates

into the brain andcranial nerves


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o The neural tubegives rise to most ofthe nervous system:

o posteriorly it formsthe spinal cord and

spinal motor nerves .


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o Much of the rest ofthe peripheralnervous system is

derived from neuralcrest cells, which

pinch off from theneural tube before itcloses.


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o Neural crest tissue isunique tovertebrates and was

probably of primeimportance in

evolution of thevertebrate headand jaws .


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Derivatives of Ectoderm:and other Tissues

Other tissues from theneural crest are:o portions of the

cranial nerveso pigment cells

o cartilage and boneof most of the skull

(including jaws)


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Derivatives of Ectoderm:and other Tissues

Other tissues from theneural crest are:

o ganglia of the ANSo medulla of the

adrenal glando other endocrine

glands


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How are the billions of nerveaxons in the body formed?

1907 embryologistHarrisonGrowing nerve axonterminated in a grow

cone , from which extenumerous tiny threadlikpseudopodial processes(filopodia)


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How are the billions of nerveaxons in the body formed?

cone is steered by an of guidance moleculessecreted along thepathway and by theaxons target. This

chemical guidancesystem, which must, ofcourse, be geneticalldirected


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Endoderm:


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Derivatives of Endoderm:Digestive Tube

In frog embryos theprimitive gut makes itsappearance duringgastrulation with the

formation of thearchenteron.


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From this simpleendodermal cavitydevelop the lining of the

digestive tract ,lining of the pharyn

lungs


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From this simpleendodermal cavitydevelop the lining of the

most of the liver apancreasthe thyroid andparathyroid glandsthe thymus .


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In other vertebrates thealimentary canal developsfrom the primitive gut and isfolded off from the yolk sacby growth and folding ofthe body wall


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The ends of the tube opento the exterior and arelined with ectoderm,whereas the rest of thetube is lined with

endoderm. Lungs, liver,and pancreas arise fromthe foregut.


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Derivatives of Endoderm:Survival of Gill Arches

Among the most intriguderivatives of the digesttract are the pharynpouches , which makappearance in the early

embryonic stages of allvertebrates


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During development theendodermally-lined pharynpouches interact with overlyectoderm to form gill afishes , gill arches develop gills and supportive struc

and serve as respiratory


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When early vertebrates moved onto land , gills were unsuitaaerial respiration and respiratory function was performed by

independently evolved lungs .


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Although gill arches serve no respiratory function in either embryos or adults of terrestrial vertebrates, they are necessary primordial for a

variety of other structures .


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Mesoderm:


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Derivatives of Mesoderm:Support, Movement

The mesoderm formsmost skeletal andmuscular tissuesthe circulatorysystemurinary andreproductive organs


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As vertebrates haveincreased in size andcomplexity,mesodermally derivedsupportive , movement ,

and transport structureshave become an evengreater proportion ofthe body.


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Most musclesarise from themesoderm alongach side of the

neural tube


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Derivatives of Mesoderm:Support, Movement This mesoderm divide

a linear series of blocksomites (38 in hum

which by splitting, fuand migration becomthe axial skeletondermis of the dor

muscles of the babody walllimbs


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Derivatives of Mesoderm: theBeating Heart

Mesoderm gives riseto the first functionalorgan , theembryonic heart .


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Guided by the underlyingendoderm, two clustersof pre-cardiacmesodermal cells moveameba like into position

on either side of thedeveloping gut.


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These clusters differentiateinto a pair of double-walled tubes, which later fuse to form a single, thintube.

As the cells grouptogether, the firsttwitchings are evident.


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In a chick embryo , a fa

animal for experimentalembryological studies, theprimitive heart begins to beon the second day of theday incubation period;it begins beating before atrue blood vessels have forand before there is any bloopump


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As the ventricle primorddevelops , the spontaneoucellular twitchings becomcoordinated into a feeblrhythmical beat .


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New heart chambers , each with a beat faster thanits predecessor, then develop.


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Finally a specialized area of heart muscle calledthe sinoatrial (SA) node develops and takescommand of the entire heartbeat . The SA nodebecomes the hearts primary pacemaker.


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As the heart builds astrong and efficientbeat, vascular channelsopen within the embryoand across the yolk.


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Within the vessels are the first primitive blood cells suspended in plasmadevelopment of the heart and circulation is crucial to continued embryonicdevelopment, because without circulation an embryo could not obtain materials fogrowth .


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Food is absorbed from the yolk and carried to the embryonic body , oxydelivered to all tissues, and carbon dioxide and other wastes are carried aembryo is totally dependent on these extraembryonic support systems, andcirculation is the vital link between them.


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The End

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