Embryology 3: Derivative of germ layers Flashcards
What are the cells of the bilaminar
- hypoblast
- epiblast
What happens in gastrulation
Bilaminar —> trilaminar
When does gastrulation commence
- week 3
- as primitive streak (PS) is formed
What is established by the time gastrulation commences
- cranial/ caudal
- right/left
What is the name of the condition where the R to L asymmetry is reversed
Inversus
How common is inversus
1 in 10000
How harmful is complete situs inversus
Often harmless
How common is Kartagene syndrome
20% of patients
Give example of partial situs inversus
Dextrocardia
How is definitive endoderm formed
- epiblast cells move towards PS
- detach from epiblast
- insert into hypoblast —> replace hypoblast cells
- new cells formed —> definitive endoderm
How is mesoderm formed
- epiblast cells continue moving down PS
- middle layer between epiblast & definitive endoderm —> mesoderm
How is ectoderm formed
- remaining epiblast become ectoderm
What completes gastrulation
Formation of ectoderm
What does trilaminar embryonic disc contain
- ectoderm
- mesoderm
- endoderm
What are the two types of mesoderm that are continuous with each other
Intraembryonic & extraembryonic
Where does every body system come from
The 3 germ layers
What does Ectoderm develop into
- CNS
- PNS
- Epidermis of skin
What does Mesoderm develop into (Day 16)
- Skeletal & smooth muscles
- Heart
- Urinary system
- Reproductive tract
- Kidneys
What does Endoderm develop into (Day 14-15)
- Epithelial lining of GI tract
- Glands
- Bladder
What are the three levels of mesoderm organisations
- Paraxial (close to middle)
- Intermediate
- Lateral plate
What does paraxial mesoderm form
- majority of skeleton
- skeletal muscles
- dermis of skin
What does intermediate mesoderm
- glands
- internal reproductive tracts
- kidneys
What does lateral plate mesoderm form
- lining of body cavities
Which cavity does the primitive streak form
Amniotic cavity
When do somites form
Day 20 post fertilisation
How do somites form
Paraxial mesoderm starts dividing into blocks
Where do somites begin
Cranial region | | | | | Caudal region
How many somites are formed
42-44 pairs
What is the rate of somite formation
3 or 4 pair per day
Why is somites formation’s predictability useful
Estimate embryo’ s age
Name two disorders of abnormal gastrulation
- Sirenomelia - Mermaid syndrome
- Sacrococcygeal Teratoma - SCT
What are the causes of sirenomelia
- not enough migration of epiblast cells
- insufficient mesoderm is formed in caudal region
What does Sirenomelia cause
- abnormalities of urogenital system
- lower limbs
What causes SCT
Remnants of primitive streak
What is the most common tumour in newborn babies
SCT - 1 in 35-40,000
How common is SCT in females
80%
What does the primitive node form whilst the primitive streak is regressing
Notochordal process
How is notochordal plate formed
Notochordal process fusing with endoderm
How is notochord formed
- Notochordal plate separates from endoderm
- fuses
What is the first event in the development of the CNS
Initiating neurulation
What does notochord initiate
- nurulation
- formation of vertebrae from somites
How is the induction of nurualation done
- initiated by signals from notochord
- overlying ectoderm thickens
- Nepal plate formed
What does notochord induce somites into
- scleromote
- myotome
- dermatome
What does sclerotium develop into
- bone
- cartilage
What does myotome develop into
Skeletal muscle
What does dermatome develop into
Dermis
How do sclerotium cells form the vertebrae
- surround notochord —> vertebral body
- surround neural tube —> vertebral arch
What is the fate of notochord
- majority degenerates
- remnants persist as nucleus pulposus of intervertebral discs
What can abnormal induction of sclerotomes cause
Spinal defects
Give examples of spinal defects
- A. Spina bifida
- B. Meningocele
- C. Myelomeningocele
