Early Embryonic Development 2 Flashcards
When does the process of gastrulation occur?
Week 3
Define ‘gastrulation’
Establishment of the 3 germ layers forming the trilaminar disc, whereby all the tissues of the body originate
What defines the start of gastrulation?
Formation of the PRIMITIVE STREAK
Where does the primitive streak appear?
Dorsal surface at the caudal end of the epiblast
Where is the primitive node located?
Cranial end of the primitive streak
Where is the primitive pit located?
Centre of the primitive node, which is at the cranial end of the primitive streak
Which direction does the primitive streak develop in?
CRANIAL TO CAUDAL
Describe the formation of the trilaminar disc
- Cells of the epiblast layer MIGRATE and INVAGINATE at the primitive streak
- Forms a cell layer within the bilaminar disc, DISPLACING THE HYPOBLAST and forming 3 germ layers
Describe the migration of the cells forming the mesoderm
- Cells invaginate from epiblast layer and migrate through the primitive streak
- Cells spread LATERALLY and CEPHALAD towards the head
Why does the mesoderm contain 2 gaps within the cell layer?
- 2 gaps; one cranial and one causal
- Will go on to form the future MOUTH (cranial) and ANUS (caudal)
What is the role of the notochord?
- Forms the basis of the axial skeleton (defines the MIDLINE)
- Drives the formation of the nervous system by the process of NEURALATION
Describe the formation of the notochord
- PRENOTOCHORDAL CELLS of the epiblast migrate through cranial part of primitive PIT
- Form a SOLID ROD OF CELLS running down the MIDLINE with an important signalling role
Describe the axes that are set during weeks 3-8
- CRANIAL and CAUDAL (head to tail)
- DORSAL and VENTRAL (back and belly)
- LEFT and RIGHT
How is the setting of the axes controlled?
Molecular signals from the primitive node ensure correct dorsal/ventral and left/right development
What phenomenon marks the start of the embryonic period?
GASTRULATION
What does the trilaminar disc consist of?
- ECTODERM
- MESODERM
- ENDODERM
How does the primitive node direct certain cells to the left and right sides?
- CILIATED CELLS OF THE PRIMITIVE NODE waft cells in the LEFT HAND direction
- SIDE SPECIFIC SIGNALLING CASCADES are initiated
- When the cilia stop moving, the cells move to the RIGHT
What is situs inversus?
- COMPLETE MIRROR IMAGE VISCERA
- Results from immotile cilia so correct left/right positions cannot be defined
What is the difference between monozygotic and dizygotic twins?
MONOZYGOTIC twins are formed from a single fertilised oocyte whereas DIZYGOTIC twins are formed from two separate fertilised ooctyes
Describe two methods by which monozygotic twins are formed
- Splitting of embryo after first cleavage and each embryo forms its own placenta (NON IDENTICAL)
- Duplication of inner cell mass whereby two embryos share the same placenta (IDENTICAL)
Describe how conjoined twins are formed
- SPLITTING OCCURS LATE with duplication of the primitive streak
- Both embryos form in the same amniotic cavity and share the same placenta
- Sometimes SEPARATION IS NOT COMPLETE
What is ‘teratogenesis’?
The process through which normal embryonic development is interrupted (most sensitive during weeks 3-8)
Why is the pre-embryonic period not susceptible to teratogenesis?
Any interruption during implantation of the embryo would result in a MISCARRIAGE
Describe when congenital defects may occur during development
Teratogenesis VARIES as each organ has a specific window during the development of the embryo in which its development may be affected
