Week 3 Flashcards
What does Gastrulation do?
-Commences in week 3 with the formation of the primitive streak it has a primitive pit, primitive node & primitive groove
- Cranial/ caudal of the embryo are firmly established
- Cranially= future position of oropharyngeal membrane impression formed and caudally the future position of the cloacal membrane impression is formed
Overview of formation of the Germ layer (4 steps)
- Formation of the primitive streak
- Cells of epiblast move towards the primitive streak
- Detach from the epiblast move through the primitive streak and insert into the hypoblast to replace the hypoblast cells
- Epiblast cells continue to migrate through the primitive streak, successive migration leads to the formation of 3 germ layers
Formation of the definitive endoderm
- The first germ layer
- The hypoblast is completely replaced by a new layer of cells= definitive endoderm
Formation of the Mesoderm
- The second germ layer
- Epiblast cells continue to move down through the primitive streak to form a middle layer in between the epiblast and definitive endoderm= mesoderm
What is Trilamina?
three layers
Formation of the ectoderm
- Third germ layer
- Remaining epiblast cells become ectoderm
- This completes gastrulation
Trilaminar Embryonic Disc
- 3 germ layers have formed our trilaminar disc
- All layers are derived from the epiblast
What does the ectoderm form?
Nervous system
What does the mesoderm form?
the musculoskeletal and the cardiovascular system, adrenal cortex, kidneys and ureters, internal reproductive organs and spleen
What does the endoderm form?
- the respiratory tract, GIT, auditory, urinary bladder, liver, gallbladder, pancreas, thymus, thyroid, parathyroid and the tonsils
How is the left right axis disrupted?
-Abnormal gastrulation can lead to R to L asymmetry of the body is reversed- situs inversus
-Complete situs inversus is often harmless but 20% of patients suffer from Kartagener syndrome
- Partial situs inversus e.g. dextrocardia (isolated right sided heart) is much rarer and associated with other malformations
Abnormal Gastrulation- Sirenomelia
- Mermaid syndrome/ caudal dysgenesis
- Insufficient mesoderm is formed in the caudal region of the embryo- abnormalities of urogenital system and lower limbs
- With most developmental orders, head end formed first followed by caudal end so caudal end is most affected in development
Abnormal Gastrulation- Sacrococcygeal Teratoma (SCT)
- Remnants of primitive streak may persist and give rise to a tumour= SCT
- SCT is the most common tumour of newborns- occurs in 1 out of 35,000-40,000 live births
- 80% in females
- Most sacrococcygeal teratomas diagnosed in neonates are benign
- Too much migration of epiblast cells- too much formation of mesoderm- primitive streak persists for too long
- Forms at caudal end
What happens to mesoderm at day 17?
Mesoderm highly organised
Transverse section
Primitive streak beginning to regress
Yolk sac within endoderm
Amniotic cavity within the ectoderm
What does the Paraxial mesoderm form?
MSK system
What does the intermediate mesoderm form?
ganitourinary system
What does the lateral mesoderm form?
Lining of the body cavities
Organisation of the paraxial Mesoderm
- Starts dividing into blocks called somites on day 20 post fertilisation
- Starts in Cranial region & progesses towards caudal region
- Rate of 3 or 4 pairs per day until 42-44 pairs have formed
- Predictable= useful for age of embryo
Notochordal process and the formation of the Notochord
- While the primitive streak is regressing, epiblast cells are laid down within the mesoderm, the primitive node forms the notochordal process (hollow tube)
Formation of the Notochord
- The notochordal process transiently fuses with the endoderm to form the flattened notochordal plate
- The notochordal plate seperates from the endoderm and fuses to form solid notochord
Notochord II
- Paraxial mesoderm differentiation can be seen in formation of the somites either side of neural tube
- The notochord also initiates and organises the formation of the vertebrae from somites
- Differentiation of somites into 3 parts
Notochord I
- The notochord initiates neurulation
- Initiated by signals from the notochord that cause the overlying ectoderm to thicken and form the neural plate= INDUCTION
- the neural plate begins to fold and form a neural tube process driven by the notochord
- Notochord also sending signals to the mesoderm to form 3 blobs
What 3 parts do somites differentiate into?
-Sclerotome= bone & cartilage
- Myotome= skeletal muscle
- Dermatome- dermis
Formation of the vertebrae
- Neural tube is forming dorsally
- The notochord signals somite to differentiate into sclerotome & begin formation of the vertebrae of the spinal column
- Sclerotome cells surround the notochord to form the vertebral body and surround the neural tube to form the vertebral arch
- Somites that differentiate into myototome dermatomes continue to differentiate into skeletal muscle and skin under instruction of the notochord
Fate of the Notochord
- Transient structure- signals for as long as necessary then switched off
- Majority of notochord degenerates as bodies of vertebrae form
- Remants persist as nucleus pulposus of intervertebral discs
Abnormal induction of the Sclerotomes- spinal defects
- Sagittal sections of the vertebrae demonstrating spina bifida
- The neural tube has formed the spinal cord protected by the vertebrae from the sclerotomes
- In spina bifida, we have the failure of the arch formation that arches around the spinal cord
What is meningocele?
more serious form, where less bone is formed
What is myelomeningocele
Most serious form
Detected during anomaly scan at 20 weeks and surgeons are beginning to operate in utero
