Gastrulation Flashcards
What happens in gastrulation?
Bilayered blastula of epiblast + hypoblast converted into embryo with 3 layers:
- Ectoderm: outside (surrounding embryo)
- Mesoderm: middle
- Endoderm: inside (hypoblast)
What is the purpose of gastrulation?
- Formation and placement of primary germ layers - brings separated cell types into contact (hypoblast + epiblast) -> allowing inductive signals to affect later cell differentiation
- Creates body axes
- Creates rudimentary gut with mouth/anus
What is differentiation?
- Process by which cells become structurally + functionally specialised
- Structural/functional specificity depends on proteins cell synthesises
What is the first sign of gastrulation?
Formation of primitive streak
What is gastrulation facilitated by?
- Cellular migration/movement
- Cell-cell recognition
- Cell signalling
- Selective adhesion
Describe how the mesoderm is formed
- Invagination + lateral migration of epiblast cells forms mesoderm
- Epiblasts detach form neighbours + migrate down + laterally
- Starts where node forms, happens down axis over time
What is the notochord?
Solid form of cells which differentiates into rod like structure -> important in development, becomes by product
What are the derivatives of the ectoderm?
- CNS, retina, pineal body, P+A pituitary gland, cranial sensory ganglia + nerves
- Epidermis, hair, nails + cutaneous + mammary glands, enamel of teeth, inner ear, lens
What are the derivatives of the mesoderm?
- Cranium, CT, dentine, muscles of head, dermis of skin, urogenital system + muscle of viscera
- Serous membranes of pleura, pericardium + peritoneum, primordial heart + lymphatic cells, adrenal cortex
What are the derivatives of the endoderm?
- Epithelial parts of trachea, bronchi, lungs
- Epithelium of GIT, liver, pancreas, urinary bladder
- Epithelial parts of pharynx, tympanic cavity, tonsils + PT glands
What is neurulation?
Period of development starting with formation of neural plate + ending with closure neural tube = future brain + spinal cord
What does the developmental date of the embryo depends on?
Inductive interaction of newly associated groups of cells.
Primary inductive event = notochord on overlying ectoderm resulting in start of neurulation
What are the components of neural induction?
- inductive signal
- ability of ectoderm to be competent to respond to inductive signal
- regional specification
What is the function of Wnt family?
Induces ectodermally + mesodermally derived tissues/organs
What is the function of the Sonic hedgehog protein (Shh)?
Initially expressed in notochord, induces neural patterning + patterning somites
What is the function of TGF-B family of proteins?
Includes BMPs
Regulate cell differentiation
What does primary neurulation divide the ectoderm into?
- Externally positioned epidermis of skin
- Neural crest test (NCC) form in region connecting neural tube with epidermis
- Internally positioned neural tube
What does the neural tube have to facilitate closure?
- Hinges to facilitate closure
- Cell adhesion molecules
Describe the closure of neural tube
- delayed in humans
- neural plate forms in anterior + starts to develop behind primitive nodes as it migrates caudally (during gastrulation)
- closure begins in neck + proceeds to posterior
What is Spina bifida?
- Common birth defect in humans
- Defect in closure of neural tube + malformations of brain + spinal cord
- Several forms, vary in severity
What is Spina bifida occult?
- mildest form
- caused by failure of vertebrae to fuse dorsally (neurulation gone wrong)
- no pain or neurological disorder
- very common
- only sign of presence may be dimple or tuft of hair
What is Anencephaly?
Failure of neural tube closing = baby born without forebrain + cerebrum
What are the domains of the neural crest?
- Cranial neural crest - cartilage, bone, cranial neutrons, glia + CT of face, bones of middle ear, tooth primordial
- Trunk neural crest - dorsal root ganglia, sensory neurons, sympathetic ganglia, melanocytes, all pigment cells
- Vagal + sacral neural crest - PS nerves of gut
- Cardiac neural crest - melanocytes, neurons, division between aorta + pulmonary artery
What is somatogenesis?
- Formation of whorls of concentric mesodermal cells
- Marks position of future somites in unsegmented mesoderm
- Gives rise to successive pairs of somites identical in appearance which differentiate into same cell types
What are somites?
- Paired blocks of mesoderm arranged on either side of neural tube at all levels
- Transient structures
- Critical for segmental organisation of axial skeleton
What do somites form?
- Cartilage (then bone) of axial skeleton
- Muscles of rib cage, limbs + back
- Cells that contribute to CT of dermis
What determines the fate of somites?
Specific regions:
- Sclerotome
- Myotome
- Dermatome
