Development ofnotochord

&the allantois

SUMMARY OF THE THIRD WEEK

• The bilaminar embryonic disc is convertedinto a trilaminar embryonic disc duringgastrulation.

characterized by1. Appearance of primitive streak2. Development of notochord3. Differentiation of three germ layers

NOTOCHORDAL PROCESS AND NOTOCHORD• Some mesenchymal cells ingressed through the streak

,primitive node and pit, forming a median cellular cord, the• notochordal process• notochordal canal.• prechordal plate, a small circular area of columnar

endodermal cells where the ectoderm and endoderm arein contact, is the Primordium of the oropharyngealmembrane.

• Prechordal mesoderm is essential in forebrain and eyeinduction.

Mesenchymal cells from the primitive streakand notochordal process migrate laterally and

cranially

The notochord

1. Defines the primordial longitudinal axis ofthe embryo and gives it some rigidity.

2. Provides signals that are necessary for thedevelopment of axial musculoskeletalstructures and the central nervous system.

3. Contributes to the intervertebral discs

The notochord develops as follows:

• The Notochordal process elongates byInvagination of cells from the primitive pit.

• The primitive pit extends into the notochordalprocess, forming a Notochordal canal

• Notochordal plate.• notochordal plate infolds to form the Notochord• The proximal part of the notochordal canal

persists temporarily as the Neurenteric canal.• The notochord becomes detached from the

endoderm of the umbilical vesicle.

The developing notochord induces theoverlying embryonic ectoderm tothicken and form the neural plate ,

• the Primordium of the central nervous system

Remnants of Notochordal Tissue• Both benign and malignant tumors (chordomas)• chordomas occur at the base of the cranium and extend to the

nasopharynx.• Chordomas grow slowly and malignant forms infiltrate bone.

Persistence of neurenteric canal

The Allantois• appears on approximately day 16 as a small,

sausage-shaped diverticulum (outpouching) fromthe caudal wall of the umbilical vesicle thatextends into the connecting stalk.

• The proximal part of the original allantoicdiverticulum persists throughout much ofdevelopment as a stalk called the urachus, whichextends from the bladder to the umbilical region,represented in adults by the median umbilicalligament.

Allantoic Cysts

• Allantoic cysts, remnants ofthe extra embryonicportion of the allantois, areusually found between thefetal umbilical vessels andcan be detected byultrasonography.

• They are most commonlydetected in the proximalpart of the umbilical cord,near its attachment to theanterior abdominal wall

NEURULATION: FORMATION OF THE NEURAL TUBE

• The processes involved in the formation of theneural plate and neural folds and closure of thefolds to form the neural tube constituteneurulation.

• As the notochord develops, it induces theoverlying embryonic ectoderm located at oradjacent to the midline to thicken and form anelongated plate of thickened epithelial cells, theneural plate.

NEURULATION: FORMATION OF THENEURAL TUBE

• On approximately the 18th day, the neural plateinvaginates along its central axis to form a longitudinalmedian Neural groove,

• The Neural folds become particularly prominent at thecranial end of the embryo and are the first signs of braindevelopment.

• By the end of the third week, the neural folds have begunto move together and fuse, converting the neural plateinto a Neural tube, the Primordium of the CNS.

• Neural crest cells undergo an epithelial to mesenchymaltransition and migrate& form a flattened irregular mass,the Neural crest.

Congenital Anomalies Resulting fromAbnormal Neurulation

Neural tube defects.• Meroencephaly

(partial absence of the brain)

• Anencephaly(Gr. an, without + enkephalos, brain).