8th lecture further development of the trophpblast

7/27/2019 8th Lecture Further Development of the Trophpblast

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embryonic period or also

called(organogenesis)3rd-8th weeksThe embryonic period,

which extends from the third to the eighth weeks ofdevelopment,

is the period during which each of the three germ layers,ectoderm, mesoderm, and endoderm, gives rise to itsown tissues and organ systems. As a result of organformation, major features of body form are established

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Derivative of ectoderm:

At the beginning of the 3rd week of development

the ectoderm layer has a disc shape disc cephalic &caudal region.

The appearance of notochord & prechordial plate stimulates the

overlying ectoderm to thicken to form( the neural

plate)which will form the (neuroectoderm)

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By the end of the3rd week

1. the lateral edge of the platebecomes elevated to form( neuralfold).

2. the depressed mid line called(neural groove).

3. Then gradually neural foldsapproach each other & form the(neural tube)

the fusion begins in the region of thefuture neck & proceedthe cephalic & caudal ends of thetube communicates with theamniotic cavity.

closure of cranial anteriorneuropores approximately occur atday 25while the posterior caudalneuropores closes at day 27.

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NeurulationThe process of formation of

neural tube it is completeafter the closure ofneuropores & the CNS is

represented by a closedtubular structure with anarrow caudal portion,Spinalcord & the broad cephalicportion which is

characterized by a number ofdilatations which are called(brain vesicle)

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Fate of the neural crest cells:by the time neural tube is closed, 2 lateral ectodermal thickening become

visible in the cephalic region of embryo, these called (otic placode) &(lens placode)

During further development otic placode forms (otic vesicle) which willdevelop in to structures needed for hearing & equilibrium

At approximately the same time the lens placode appear & during 5th weekthey form the lenses of the eyes

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Neural Crest Cells:1. will undergo an epithelial-to-

mesenchymal transition

2. it leaves the neuroectoderm by activemigration to enter the underlyingmesoderm.

Crest cells from the trunk regionleave the neuroectoderm after closure of

the neural tube and migrate along oneof two pathways:

(1) a dorsal pathway through the dermis,form melanocytes in the skin and hairfollicles,

(2) a ventral pathway through the

anterior half of each somite tobecome

A. sensory ganglia,

B. sympathetic and enteric neurons

C. Schwann cells,

D. and cells of the adrenal medulla6
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Neural crest cells from cranial neural folds

leaving the neural tube before closure in this

region (These cells contribute to

the craniofacial skeleton,

neurons for cranial ganglia, glial cells,

melanocytes,

and other cell types

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In general terms,the ectodermal germ

layer gives rise to

organs and structuresthat maintain contactwith the outsideworld:

A. The CNS

B. The PNS

C. The sensory epithelium of theear, nose, and eye

D. The epidermis, including thehair and nails.

E. Subcutaneous glands,

F. The mammary glands,

G. The pituitary gland,

H. And enamel of the teeth.9


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Neural Crest Derivatives1. Connective tissue and bones of the face and skull

2. Cranial nerve ganglia

3. Cells of the thyroid gland4. Conotruncal septum in the heart

5. Odontoblasts

6. Dermis in face and neck

7. Spinal (dorsal root) ganglia

8. Sympathetic chain and preaortic ganglia

9. Parasympathetic ganglia of the gastrointestinal tract

10. Adrenal medulla

11. Schwann cells

12. Glial cells

13. Meninges (forebrain)

14. Melanocytes

15. Smooth muscle cells to blood vessels of the face and forebrain

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Clinical Correlates

Neural Tube DefectsNeural tube defects (NTDs) result when neural tube closure fails to occur. If

the neural tube fails to close in the cranial region, then most of the brainfails to form, and the defect is called anencephaly

The most common site for spina bifida to occur is in the lumbosacralregion ,suggesting that the closure process in this area may be moresusceptible to genetic and/or environmental factors11


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70%of the defects can be prevented

if women take 400 g of folic acid daily beginning 3 months prior to

conception and continuing throughout pregnancy.

Because 50% of pregnancies are unplanned,

it is recommended that all women of childbearing age take a

multivitamin containing 400 g of folic acid daily.

If a woman has had a child with an NTD or if there is ahistory of such defects in her family,

it is recommended that she take 400 g of folic acid daily and then

1,000 g per day when she tries to become pregnant.

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Differentiation of

mesodermInitially,

by a bout 17th. Day

mesoderm become to be divided into 3 zones,

(paraxial mesoderm)

2- (intermediate mesoderm).

3-(lateral mesoderm).13


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the lateral mesoderm divides into 2 layers:

1- called somatic or parietal mesoderm)

2- called(splanchnic or visceral mesoderm)

these 2 layers together surround the newly formed cavity which is known as

(intraembryonic coelomic cavity) which is continuous on each side with theextraembryonic (coelomic cavity)

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somites

By the beginning of the 3rd week, paraxial

mesoderm begins to be organized into

segments. (somitomeres), first appear in

the cephalic region of the embryo

somitomere :

consists of mesodermal cells arranged in

concentric whorls around the center of the

unit.

Neuromeres: somitomeres with

segmentation of the neural plateIn the head

region and contribute to mesenchyme in the

head From the occipital region caudally,

somitomeres further organize into somites.15


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somitsThe first pair of somites arises in the

occipital region at the 20th day of

development.1. new somites appear in craniocaudal

sequence at a rate of approximately (3pairs

\day) until, at the end of the 5th week,

2. 42 to 44 pairs are present

3. four occipital, eight cervical,12 thoracic,5

lumbar,5 sacral, 8-10 coccygeal pairs. while

the remaining somites form the axial

skeleton ,

the age of an embryo can be accurately

determined during this early time period by

counting somites16

S it Diff ti ti


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Somite Differentiation:By the beginning of the fourth week 1-, cells in the ventromedial walls of the somite form( the sclerotome )that will

differentiate into the vertebrae and ribs.

2- the dorsomedial and ventrolateral edges of the upper region of the somite form

precursors for muscle cells. 3-while cells between these two groups form the dermatome .

4-Cells from both muscle precursor groups create the dermomyotome

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Intermediate mesoderm,which temporarily connects paraxial mesoderm with the lateral plate ,differentiates

into urogenital structures

1. . In cervical and upper thoracic regions, it forms segmental cell clusters (futurenephrotomes),

2. . whereas more caudally, it forms an unsegmented mass of tissue, thenephrogenic cord.

3. .Excretory units of the urinary system and the gonads develop from this partlysegmented, partly unsegmented intermediate mesoderm.

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Lateral plate mesoderm:

splits into parietal (somatic) and visceral (splanchnic) layers,

the parietal layer mesoderm, together with overlying ectoderm, forms the

lateral body wall folds

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Derivitives of the mesoderm.

1. the dermis of the skin in the body wall and limbs,

2. the bones and connective tissue of the limbs, and the sternum.

3. the costal cartilages, limb muscles, and most of the body wall

muscles4. forms the wall of the gut tub

5. form serous membranes, which will line the peritoneal, pleural,and pericardial cavities and secreteserous fluid

6. form a thin serous membrane around each organ

7. Blood cells and blood vessels also arise from mesoderm.

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Blood cells and blood vessels


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Blood cells and blood vesselsalso arise from mesoderm.

Blood vessels form in two ways:

1-vasculogenesis, whereby vessels

arise from blood Islands

2- angiogenesis, which entailssprouting from existing vessels.

The first blood islands appear in

mesoderm surrounding the wallof the yolk sac at 3rd week ofdevelopment

slightly later in lateral platemesoderm and other regions

These islands arise frommesoderm cells induced toform hemangioblasts, acommon precursor for vessel

and blood cell formation.21


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DERIVATIVES OF THE ENDODERMAL GERM LAYER

The GIT is the main organ system derived from the endodermal germ

layer.

.With development and growth of the brain vesicles, the embryonic

disc begins to bulge into the amniotic cavity and to fold

cephalocaudally. where the head fold and tail fold are formed ,Lateral

folds also form and move ventrally to assist in body wall closure

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As a result of cephalocaudal folding,it will form the gut tube. The tube is dividedinto three regions:

1. Foregut.

2. midgut.

3. Hindgut.

the vitelline duct communicates the midgut with the yolk sac

This duct is wide initially, then becomes narrow and much longer24


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important result of cephalocaudal and lateral folding isthedevelopment of followings

1-Oropharyngeal membrane

At its cephalic end, the foregut is temporarily bounded by an ectodermal-endodermalmembrane called the oropharyngeal membrane .

It separates the stomadeum, the primitive oral cavity derived from ectoderm, from thepharynx,a part of the foregut derived from endoderm

In the 4th week, the oropharngeal membrane ruptures, establishing an open connectionbetween the oral cavity and the primitive gut

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2-The cloacal membraneThe hindgut terminates temporarily at an ectodermal-

endodermal membrane, the cloacal membrane .It separates theupper part of the anal canal, derived from endoderm, from thelower part, called the proctoderm, that is formed by aninvaginating pit lined by ectoderm. the membrane breaks down inthe 7th week to create the opening for the anus.

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3 Closure of the ventral body wall of the embr o e cept for a small


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3-Closure of the ventral body wall of the embryo except for a smallpart in the umbilical region

4-partial incorporation of the allantois into the body of the embryo,where it forms the cloaca .

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th d d l l f th ith li l


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the endodermal germ layer forms the epithelial

lining of the

1. primitive gut,

2. the allantois

3. vitelline duct ,

4. Pharyngeal pouches,

5. lung buds and trachea,

6. liver, gallbladder,

7. and pancreas.

8. The urinary bladder

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EXTERNAL APPEARANCE DURING


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EXTERNAL APPEARANCE DURINGTHE SECOND MONTH

At the end of the 4th week, when theembryo has approximately 28somites,

the main external features are thesomites and pharyngeal arches .

The age of the embryo is thereforeusually expressed in somites

Because counting somites becomesdifficult during the second monthof development,

the age of the embryo is thenindicated as the crown-rumplength (CRL) and expressed in

millimeters

CRL is the measurement from thevertex of the skull to the midpointbetween the apices of the

buttocks29


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During the second month,

the external appearance of the embryo is changed by1. an increase in head size

2. and formation of the limbs, face, ears, nose, and eyes.

By the beginning of the fifth week

Forelimbs& hind limbs appears.then growth continues till the formation of the digits

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Cli i l C l t


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Clinical CorrelatesBirth Defects

Understanding the main events of organogenesis is important for identifying the

time that a particular defect was induced and, in turn, determining possible

causes for the malformation

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8th lecture further development of the trophpblast

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