Gastrulation and Neurulation Lecture Flashcards
19/11/24
Embryology: What happens in the first week?
Ovulation; conception in the uterine tube → migration to uterus → cell division to morula → blastocyst → implantation in uterine mucosa.
What happens by the end of week 2?
A bilaminar disc forms with a connecting stalk. Amniotic sac is on the ‘dorsal’ side, and the yolk sac on the ‘ventral side’. The Epiblast and Hypoblast are in contact with eachother and on day 14, the primitive streak appears. This is the beginning of gastrulation.
What is the primitive groove?
A groove with raised edges and a pit, form on the epiblast. The pit is a primitive pit associated with the primitive node.
What happens in cell migration 1?
Epiblast cells move through the primitive streak and displace the hypoblast to become endoderm
What happens in cell migration 2?
Second wave of migration where cells move through again and fill in between the 2 layers, to become mesoderm. The epiblast becomes the ectoderm. Bilaminar disc → trilaminar disc.
What is the notochord?
Growth of a tube-shaped structure in the mesoderm. Tube → disc →solid rod (17-20 days).
It is essential for setting up dorsal and ventral axes and inducing neurulation
How is the notochord important in neural tissue development?
Skin development uses signals from bone morphogenic proteins (BMPS). Node and notochords release factors that block BMP, which leads to the development of neural tissue instead.
What is neurulation?
Where the epithelial cells become columnar in an area called the neural plate, which then converts to a groove, and then a tube.
How does neurulation occur?
Cells on the edge of the plate thicken forming a groove, and then a fold around day 20-21 in the cervical region.
How does closure of neurulation take place?
The folds close like a zip in cephalic and caudal directions, the open ends being the anterior and posterior neuropores.
What causes neural tube defects?
Failure of the neuropores to close leads to various levels of brain/skull defects - ant. neuropore, or spina bifida - post. neurpore.
What are neural crest cells?
Cells that derive from the edges of the neural tube. Epithelial → mesenchymal transition (EMT) become motile and migrate from neural tube to different destinations, particularly the sensory and autonomic nervous systems.
Provide examples of neural crest cell link syndromes and outline symptoms of each
Waardenburg’s Syndrome: Deafness, Heterochromia, Pigment abnormalities
Treacher Collins Syndrome: Micrognathia (under developed face), hearing loss, malformed ears.
How can the mesoderm be divided?
The mesoderm may be divided into 3 sub-categories: The paraxial mesoderm, the intermediate mesoderm, and the lateral plate mesoderm
How are somites formed?
Paraxial mesoderm becomes segmented to form somites in a cranial → caudal direction at day 20-30. 42-44 pairs develop, but regression in the tail leaves approximately 37 pairs. (4 Occipital, 8 Cervical, 12 Thoracic, 5 Lumbar, 5 Sacral, 3 (ish) coccygeal.
Briefly outline the process of somite formation
Cells in the paraxial mesoderm go through cycles every 90 minutes defined by a notch signalling clock. A wave of fibroblast growth factor (FGF) passes along the embryo rostro-caudally; when the FGF wave passes cells, they are programmed to change into part of a somite.
If the wave passes early in the clock cycle, they become the front end of the segment, if the wave passes late in the cycle, they become the tail end of the segment.
What are homeobox genes?
Genes responsible for directing the formation of body structures at the right place/time - tied to specific somite numbers.
What are the derivatives of the paraxial mesoderm?
Paraxial mesoderm → Head
Paraxial mesoderm → Somite → Sclerotome (Axial Skeleton) AND Myotome (Skeletal Muscle) AND Dermatome (Dermis).
What are the derivatives of the intermediate mesoderm?
Intermediate mesoderm → Kidneys and Gonads.
What are the derivatives of lateral plate mesoderm?
Splanchnic (around the yolk sac) - Visceral coverings (organs), Heart, Blood Vessels and Blood, Spleen
Somatic (around the amnion) - parietal coverings (body wall), limb cartilage and bone, trunk dermis
What are the derivatives of the midline mesoderm?
The Prechordal plate (future site of the mouth) and the Notochord (nucleus pulposus).
What becomes of the endoderm?
Craniocaudal folding and lateral folding occurs, which pinches off the endoderm lined yolk sac, which will later become the gut tube. The vitelline duct, (small connection to the yolk sac) is closed by 5-6 weeks.
What are the derivatives of the endoderm?
Epithelium of the gut, liver and pancreas.
Lungs and lining of the respiratory tract.
Thyroid/Parathyroid glands, Thymus and urinary bladder.
What are the derivatives of the ectoderm?
CNS, PNS, Epidermis and Sweat and Salivary Glands.
