Not-so-early Embryology Flashcards
What is gastrulation?
Two layers of the embryo become three
What is the name of the raised groove and pit found on the epiblast along the head-tail axis?
What is the pit’s function?
Primitive streak
Primitive node
- has cilia that defines left and right
How do epiblast cells get into the embryo?
They stream into the embryo along the primitive streak/node
Describe the process of gastrulation:
Endoderm
- Migrating epiblast cells replace the hypoblast cells
Mesoderm
- Epiblast cells spread up and out through the primitive streak between the two layers of hypoblast and epiblast to form the mesoderm (paraxial, intermediate, lateral)
- In front of the node, a specialised mesoderm structure forms from the epiblast cells called the notochord (an important signalling structure) - forms between the two layers as well
What is neurulation?
Formation of the first tubes
- Folding/rolling, making a groove then joining to form a tube
What defines the anterior of the growing embryo?
Anterior visceral endoderm
How is the dorsal/ventral (back/belly) axis organised?
- The ventral/skin development uses signals from bone morphogenic proteins (BMPs)
- The node and notochord release factors (chordin, noggin, follistatin) that block BMP effects and determine back/neural tissue development
What was Spemann and Mangold’s experiment and what did the results show?
They transplanted node cells into the embryo, which then developed double dorsum (backbones and nervous systems), proving that it was the primitive node/notochord that organised dorsal structures
Describe the process of neurulation?
- Neurulation is induced by the notochord
- Epithelial cells become columnar
- The neural plate will make a tube
- Day 19+ - a midline groove becomes apparent
- Day 20/21 - cells on plate edge thicken forming neural folds and a groove
- Day 22 - neural folds fold more toward each other, eventually fusing together in the neck/cervical region by apical constraction of the rings of actin
- This forms a rostral neuropore and a caudal neuropore
- Rostral neuropore closes after 25 days and three vesicles develop into the brain
- Caudal neuropore closes after 27 days
What happens when the neural tubes/neuropores fail to close?
Neural Tube defect (NTD)
- Anencephaly - When the rostral neuropore fails to close; brain fails to develop
- Spinal bifida - When the caudal neuropore fails to close
What are neural crest cells and how do they behave?
- Specialised cells that migrate away from the neural tube epithelium
- They develop into a variety of cell types e.g.: pigment cells, sensory neuron cells, preipheral glial
What are the consequences of a defect in neural crest development?
- Pigment anomalities, deafness, GI motility issues
Wardeenburg’s syndrome:
- Pax-3 gene deletion
- Pigment abnormalities/albinism
- Deafness
- Heterochromia of eyes
- Telecanthus (widely separated eye “corners”)
- Sometimes also constipation
Treacher Collins Syndrome
- Autosomal dominant
- Defective protein called Treacle (TCOF1)
- Failure of formation/apoptosis of neural crest cell
- Abnormal eye shape
- Malformed ears
- Conductive hearing loss
- Micrognathia - undersized jaw
- Underdeveloped zygoma - bone in the cheek
What happens to the paraxial mesoderm?
It becomes segmented
How does segmentation arise?
- Cells have a timer along the paraxial mesoderm
- Every 90 minutes (defined by a notch signalling clock), a wave of FGF (fibroblast growth factor) passes along the embryo, forming segments
- When the wave passes, cells are programmed to change into a part of the somite:
if the wave passes early in the cycle, they become the front end of the somite
if the waves passes late in the cycle, they become the tail end of the somite - This process is repeated for all the segments
What are homeobox genes?
Genes critical for forming body axes
Involved in direct formation of body structures, like from body segments
- Mutations in flies - e.g.: antennapedia, where the segments develop wrongly and the legs are developed where the antenna was meant to develop
