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Embryonic Period

By

Dr Daw Khin Win

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Objectives

1. Describe the formation and derivativesof secondary mesoderm.

2. Define notochord and name its fate and

functions.3. Name the germ layers and list the

derivatives of each layer.

4. Define somite and describe thederivatives of somites.

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Gastrulation

The process of the formation of three germ layersis called Gastrulation.

This trilaminar germ disc is the basis of later

embrynic development that gives rise to derivativesof germ layers- tissues, organs and organ-systems.

It takes place in the third week after

fertilization.

Folding of embryo and organogenesis follow in thefourth week.

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2nd Week of Development(Bilaminar Germ Disc)

At the time of implantation, the embryoblast consists of only one typeof cells.During the 2nd week of development this embryoblast differentiatesinto 2 types of cells:Epiblast columnar cells; faces the amniotic cavityHypoblast cuboidal cells; faces the yolk sac

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2nd Week of Development(Bilaminar Germ Disc)

Cells of epiblastgive rise toanother layersuperficially thatbecomes the

amnion or amnioticmembrane and theamniotic cavity isformed between itand the epiblast.

Epiblastproliferates toform two layer ofcells- epiblast andhypoblast.

Hypoblastproliferates andformsextraembryonicmembrane enclosingthe cavity, primary

yolk sac.

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Primary or Extraembryonic Mesoderm

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Extraembryonic Coelom

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BilaminarGerm disc

Syncytiotrophoblast

Cytotrophoblast

ExtraembryonicMesodermDefinitive Yolk Sac

Buccopharyngeal MembraneCut Edge of Amnion

Hypoblast Epiblast

Wall of Yolk Sac

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Primitive nodeand pit

Primitive streak

Prochordalplate

Cloacal plate

Primitive StreakAt the beginning of the 3rd week, the development of the embryoblast is markedby the appearance of the primitive streak.The primitive streak appears as a midline linear streak bilateral symmetryOn the epiblast surface of the bilaminar germ disc dorsal/ventralAt the caudal half cranial/caudal

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Epiblast

Invaginating cells Hypoblast

Formation of Secondary MesodermPrimitive Pit Primitive Streak

Amnion

YolkSac

At the beginning of the 3rd week, a midline linear groove called the

primitive streak appears on the epiblast surface in the caudal half of the embryo.Cells of the epiblast migrate in the direction of the primitive streak.

On arrival they become flask-shaped, detach from the epiblast and invaginatebetween the epiblast and hypoblast layers.

This migration of cells into the primitive streak occurs until the end ofthe 4th week after which the primitive streak disappears.

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The primitive streak consists of 2 parts;

Primitive groovePrimitive node (at cephalic end)

Epiblast cells migrate laterally and forward between the epiblast & hypoblast layers.In doing so, they do not cross:

Prochordal plate epiblast & hypoblast are firmly fused at this pointMidline where the notochord is developingCloacal plate epiblast & hypoblast are also fused here.

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Prochordal plate

Cloacal plate

(Ruptures at endof 3rd week)

(Ruptures at end of 8th

week)

NotochordalProcessPrimitive

Node &

Pit

PrimitiveGroove

Formation of NotochordThe primitive node is seen at the cephalic end of the primitive streak.In the center is a pit called primitive pit.The cells of the primitive node also proliferates and the pit also extends into thisas a lumen. The structure so formed is called the notochordal process.The notochordal process extends cranially until it reaches the prochordal plate.

As the primitive streak regresses, the notochordal process lengthens caudally untilit reaches cloacal plate.

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Origin & Derivatives of Three Germ Layers

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NotochordThe lower layer of the notochordal process fuses with the

underlying hypoblast and perforations appear, gradually leading to its

disappearance.Thus a temporary connection between the amniotic and yolksac cavities is formed by way of the neurenteric canal.

The hypoblast on each side of the perforation closes up thedefect and the connection is then lost.

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Function and Fate of Notochord

The remaining upper layer of the notochord proliferates to

form a solid cord called the definitive notochord.This extends as a solid cord between the prochordaland cloacal plates.

Function1. To serve as a temporary axis2. To induce the development of the neuro-ectoderm &

neural plate3. To initiate the formation of the vertebra or axial

skeleton

Fate of notochordThe notochord in the region of the vertebra bodies

disappearIn the region of the inter-vertebral disc it remains as

nucleus pulposus.

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Formation of Neural tube

The midline ectodermal cells of epiblast proliferate and form a plate calledneural plate.

It folds and forms the neural tube.Some cells of neural plate remains laterally to the neural tube and form the

neural crest.The neural tube sinks into the mesoderm and surface ectoderm grows over it.The neural tube passes throug the stage of neuropores, anterior andosterior that close later.

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Formation of Neural tube

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Formation of Secondary MesodermOnce the cells of primitive streak have invaginated, some displace the

hypoblast creating the embryonic endoderm, and others come to liebetween the epiblast and newly formed endoderm to formmesoderm.

Cells remaining in the epiblast then form ectoderm.Thus, the epiblast, through the process of gastrulation, is the source

of all of the germ layers.

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Trilaminar Germ DiscThe newly replaced

hypoblast layer is now called

endoderm.The cells filling thespace between the epiblastand endoderm cranio-caudallyare now called secondarymesoderm.

The primitive streakregresses in size as thisfulfillment is completed andat the end of the 4th weekit completely disappears.

After its

disappearance, the epiblastis now called ectoderm. Theembryo is now a trilaminargerm disc.

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Secondary mesoderm differentiates into3 parts on each side of the embryo.These 3 parts are-1. Paraxial mesoderm2. Intermediate mesoderm3. Lateral plate mesoderm

These 3 divisions of mesoderm give rise to specific structures in the adult.

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Initially, cells of the mesodermal germlayer form a thin sheet of loosely

woven tissue on each side of themidline.By 17th day, cells close to the

midline proliferate and form a

thickened plate of tissue known asparaxial mesoderm.More laterally, the mesoderm layer

remains thin and is known as the

lateral plate.Intermediate mesoderm connectsthe paraxial & lateral plate mesoderm

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Paraxial mesoderm

- Lies on each side of the notochord.This will form:-

The supporting tissues such as:

Connective tissue,Bone and cartilages of the axialskeleton,

Skeletal muscle andDermis of the skin.

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Intermediate mesoderm- Lies lateral to the paraxial mesoderm.- This connects the paraxial mesoderm and lateral plate mesoderm.- Also known as Urogenital ridge

This forms:1. Urinary system (kidneys & ureters) and

2. Genital system (gonads and genital ducts)

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Lateral plate mesoderm- Lies lateral to the intermediate

mesoderm.

On Day 17, this splitsinto 2 layers: a ventral layerassociated with endoderm and adorsal layer associated withectoderm.

The ventral layer iscontinuous with the extra-embryonicmesoderm covering the yolk sac andis called the splanchnic mesoderm(visceral).

The dorsal layer iscontinuous with the extra-embryonicmesoderm covering the amnioticcavity and is called the somaticmesoderm (parietal).

Together these layers linea newly formed cavity, theintraembryonic cavity (afterembryonic foldings) which iscontinuous with the extra-embryoniccavity on each side of the embryo.

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The lateral plate mesodermwill form:-

1. The blood and lymphaticcells: heart and vessels;

2. Skeletal muscles of thebody walls,3. Smooth muscles,4. Cortex of the adrenal gland5. Spleen6. Lining of the body cavities.

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Somites

SOMITESBy the beginning of the 3rd week, the paraxial mesoderm is

organized into segments called somitomeres (Somites).The somitomeres appear first in the cephalic region and then

proceed cranio-caudally.Somitomeres are mesodermal cells arranged in concentric whorls

around the center of the unit

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From the occipital region to caudally,somitomeres are organized into

somites.The 1st pair of somites appear in

the cervical region at approximately

on the 20th day of development.From here new somites appear in

cranio-caudal sequence at a rate of

approximately 3 pairs/day until theend of the 5th week.

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Altogether 42 44 pairs of somites areformed.

There are:-- 4 occipital,- 8 cervical ,- 12 thoracic,- 5 lumbar,- 5 sacral and- 8 10 coccygeal somites.

The 1st occipital and the last 5 8 coccygealsomites disappear. Remaining occipitalsomites form the tongue muscles.

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Derivatives of Somites

At the beginning of the 4th week, the rest of thesomites lose their compact organization and becomepolymorphous and differentiate into:-a. ventromedian (sclerotome) andb. dorso-lateral (dermo-myotome).

The cells forming the sclerotomewill surround thespinal cord and notochord to form the vertebral

column.The dermo-myotomegives rise to a new layer of cellscalled myotomewhich will contribute to theformation of the skeletal muscles; while thedermatomeform the dermis and subcutaneoustissue of the skin.

Hence each somite forms its own sclerotome, its ownmyotome and its own dermatome.Each myotome and dermatome has its own segmental

nerve component.

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Intermediate mesoderm- Lies lateral to the

paraxial mesoderm.- This connects the paraxial

mesoderm and lateralplate mesoderm.

- Also known as urogenital

ridge

This forms:1. Urinary system (kidney &

ureter) and2. Genital system (gonads

and genital ducts).

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HEAD FOLD and TAIL FOLD

Head fold

Tail fold

Allantois

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Head fold

Tail fold

Note: the allantois(a) isfolded with the tail into theembryo

Folding of the EmbryoEmbryonic folding

1. Head and tail folds2. Lateral foldin

As a result of cephalo-caudal folding , a continuously large portion of the endoderm-lined cavity isincorporated into the body of the embryo proper as the primitive gut.

The middle part of the gut temporarily communicates with the yolksac by way of a broad stalk, the vitelline duct.

This duct is wide initially, but with further growth of the embryo,

it becomes narrow and much longer.

V aduct

Head Fold

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HEAD and TAIL FOLDING

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Applied Anatomy

Cells of primitive streak are pluripotent, so remnants ofthe streak give rise to sacrococcygeal teratomacontaining many types of tissue that change to malignanttumor.

Neural tube defects are very common as it takes placein the 3rd week.

Spina bifida and anencephaly are two common anomaliesof CNS.

Abnormal development of trophoblats lead to

choriocarcinoma, a malignant tumor.

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Sacrococcygeal Teratoma Anencephaly & Hydrocephalus