THIRD WEEK OF DEVELOPMENT

[SURIYA.KARTHIKEYHAN]

THIRD WEEK OF DEVELOPMENT

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EXPECTATION

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REALITY

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3RD WEEK OF DEVELOPMENT
MERMAID SYNDROME

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SACROCOCCYGEAL TERATOMA

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HOLOPROSENCEPHALY

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அரிதரிது மானிடர் ஆதல் அரிது
மானிடர் ஆயினும் கூன்குருடு செவிடு
பேடு நீங்கிப் பிறத்தல் அரிது
பேடு நீங்கிப் பிறந்த காலையும்
ஞானமும் கல்வியும் நயத்தல் அரிது
ஞானமும் கல்வியும் நயந்த காலையும்
தானமும் தவமும் தான்செயல் அரிது
தானமும் தவமும் தான்செய்வ ராயின்
வானவர் நாடு வழிதிறந் திடுமே.

Identify the parts numbered 1 - 10

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3rd WEEK OF DEVELOPMENT

We will consider separately the changes occurring in:

• Embryoblast
  • Bilaminar germ disc
  • Primitive streak and prochordal plate have formed
• Trophoblast
  • Primary villi
  • Utero- placental circulation is established

CHANGES IN THE EMBRYOBLAST

CHANGES IN THE EMBRYOBLAST 

1. Gastrulation
2. Formation of the notochord
3. Beginning of neurulation
4. Formation of the allantoic diverticulum

GASTRULATION

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• Initially primitive streak vaguely defined
• By day 15-16, clearly visible as a narrow groove with slight bulging regions on each side… Primitive groove

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• At the cephalic end of the groove, there is a slightly elevated area called Primitive node/ knot/ Henson’s node
• The centre of primitive node presents a  depression…. Primitive pit or blastopore

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GASTRULATION- Formation of the trilaminar germ disc

• Cells of the epiblast migrate to the primitive streak
• Once they reach there they become flattened or flask shaped, develop pseudopodia and slip beneath the epiblast

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GASTRULATION

Fate of invaginating cells:

1. Some displace the hypoblast cells and form the endoderm
2. Others lie between epiblast and endoderm to form the (intra embryonic) mesoderm
3. The remaining cells in the region of the epiblast form the ectoderm

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GASTRULATION

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Note: The epiblast is the source of all 3 germ layers!!!

• Cells continue to migrate filling the space between the endoderm and ectoderm
• 2 depressions appear in the ectoderm:

1. Cranially, in the region of the prochordal plate
2. Caudal to the primitive streak

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1. The cranial depression is called the buccopharyngeal membrane
2. The caudal one is called the cloacal membrane

• In both these regions, mesoderm does not separate the endoderm and the ectoderm
• These later become the blind ends of the gut tube
• They break down to form the openings of the oral cavity, anus, urinary and genital tracts]

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• Intra embryonic mesodermal cells pass cranially on either side of the buccopharyngeal membrane

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• Some mesodermal cells pass caudally around the cloaca
• They enter the extraembryonic mesoderm of the connecting stalk

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FORMATION OF THE NOTOCHORD

FORMATION OF THE NOTOCHORD

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FORMATION OF THE NOTOCHORD

• Initially the primitive streak consists of a groove with a primitive node and blastopore

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• Cells in primitive knot multiply and pass cranially in the midline, between the ectoderm and endoderm
• These form a solid cord like NOTOCHORDAL PROCESS or HEAD PROCESS

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• The blastopore extends throughout the notochordal process forming a tube called the notochordal canal

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PRECHORDAL PLATE

• This forms between the tip of the notochordal processand the buccopharyngeal membrane 
• Significance: It later induces the formation of the forebrain

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….Formation of the NOTOCHORD

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FATE OF THE NOTOCHORD

• The notochord elongates as the embryo enlarges and lies in the midline
• Its position is later occupied by the vertebral column
• Most of it disappears later but its remnants form the nucleus pulposus of the Intervertebral disc and the apical ligament of the dens
• Intra embryonic mesoderm separates endoderm andectoderm in all regions except:
  • Buccopharyngeal membrane
  • Cloacal membrane
  • Region of the notochord

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• Note: At the edges of the embryonic disc, the intra embryonic mesoderm is continuous with the extra embryonic mesoderm

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Neurulation begins….

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NUERULATION  

• Neurulation or the formation of the neural tube begins
• At the beginning of week 3, the germ disc is broader at the cephalic than caudal end

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NEURAL GROOVE AND NEURAL FOLD FORMATION   

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[ Future changes in the neural plate]

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COMPLETION OF NEURAL TUBE

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FATE OF NEURAL TUBE WILL BE DISCUSSED IN 3RD TO 8TH WEEK DEVELOPMENT

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ALLANTOENTERIC DIVERTICULUM (ALLANTOIS)

• Around the 16th day the posterior wall of the yolk sac forms a diverticulum that extends into the connecting stalk, called allantois

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ESTABLISHMENT OF FATE MAP

• Ultimate fate of cells migrating from the epiblast through the primitive streak have been mapped

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CHANGES IN THE TROPHOBLAST
Further development of the villi

TROPHOBLAST… further development

• Beginning of week 3… Primary villi

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• Extra embryonic mesodermal cells penetrate the core of t he primary villi and grow towards the decidua…. Secondary villi

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By the end of week 3, mesodermal cells in the core of the villus differentiate into blood cells and blood vessels forming a villous capillary system….. Tertiary villi or definitive placental villus

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Development of Villi

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• When these villous capillaries make contact with capillaries in the chorionic plate, then the embryo can be supplied with nutrients and oxygen this way

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• Villi that extend from the chorionic plate to to the decidua are called stem or anchoring villi
• Those which branch from the sides are called free or terminal villi

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• Meanwhile, cytotrophoblastic cells penetrate progressively into the overlying syncytium until they reach the maternal endometrium
• They establish contact with the cytotrophoblast of neighbouring villi forming a thin outer cytotrophoblast shell

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• This shell surrounds the trophoblast entirely
• Thus the chorionic sac is attached firmly to the decidua
• The embryo is attached to its trophoblastic shell by a narrow connecting stalk

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CLINICAL CORRELATES

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SIRENOMELIA- Caudal dysgenesis

• b) Caudal dysgenesis: insufficient mesoderm is formed in the caudal-most region of the embryo

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MERMAID SYNDROME

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• c) Situs inversus : Transposition of viscera in the thorax and abdomen 

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TUMOURS ASSOCIATED WITH GASTRULATIONS

acrococcygeal teratomas:  

• Tumours arising from remnants of the primitive streak
• They contain tissues derived from all 3 germ layers

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Summary of events in week 3

• Gastrulation- formation of the trilaminar germ disc
• Formation of the notochord
• Beginning of neurulation
• The tertiary villi develops
• Outer cyto-trophoblastic shell
• Intra embryonic mesoderm begins to differentiate further….

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