Embryology Flashcards
What is the function of the sonic hedgehog (SHH) gene?
Anterior-posterior axis patterning, CNS development
- SHH protein establishes L and R brain and midline and formation of forebrain
- Also influences limb development in the anterior to posterior plane (ant=thumb, post=pinky)
Where is the SHH gene located?
Zone of polarizing activity at the base of limb buds
- Influences apical ectodermal ridge
What is the function of the Wnt-7 gene?
- Dorsal-ventral axis patterning, limb development (flexors/extensors)
- Wn7a is key for dorsal development, activates LMX-1 gene and ‘dorsalises’ one side of the mesoderm to give for example the back of the hand
- Repression of Wnt-7 gives rise to ventral side
Where is the Wnt-7 located?
Apical ectodermal ridge at distal end of each limb
What does a mutation in the SHH gene lead to?
Holoprosencephaly
- Wholeforebrain, single lobed brain also facial abnormalities (cyclopia, cleft lip)
What is the function of the fibroblast growth facor (FGF)?
Limb lengthening (via mitosis of the mesoderm)
Where is the FGF gene located?
Apical ectodermal ridge
What is the function of the Homeobox genes?
Segmental organisation in cranial-caudal direction, transcription factor coding
- gene is located in multiple locations
What drug used by a mother may cause increased HOX gene expression in the fetus?
Isotretinoin (for acne, skin cancer)
- Birth defects
When does hCG secretion begin?
Within 1 week of fertilisation - day 6
- Implantation of blastocyst
What embryological tissue sits behind the apical ectodermal ridge?
Mesoderm
- Also influences ectodermal ridge
- Progress zone forms in mesoderm w. growing cells
A mutation in the HOX genes will lead to what?
Abnormal limb formations
- Polydactyly (extra fingers/toes)
- Syndactyly (fused fingers/toes)
What is the developing embryo called after the 2 cell stage?
Morula
What is the next structure called after the morula?
Blastula/blastocyst
- Blastula contains fluid cavity called blastocoel
What are the outer cells of the blastocyst called?
Trophoblast
- One side is different from the other (polarised)
What cells produce the blastocoel?
Trophoblasts
When does the bilaminar disc form?
Within week 2
What structures are formed from the bilaminar disc in week 2?
- Epiblast
- Hypoblast
When does gastrulation (formation of trilaminar embryonic disc) occur?
Within week 3
What are the 3 structures formed from gastrulation/trilaminar disc?
- Endoderm
- Mesoderm
- Ectoderm
What cells create the primitive streak?
epiblast cells invaginate
What does the notochord arise from?
Midline mesoderm
What becomes the neural plate (neuroectoderm)?
Ectoderm overlying the notochord (notochord induces ectoderm to become neural plate)
What is the neural tube formed by?
Neuroectoderm
When does the neural tube close by?
Week 4
When does organogenesis roughly occur?
Weeks 3-8 (embryonic period)
What period is especially susceptible to teratogens?
Weeks 3-8
During what week does the heart begin to beat?
Week 4
4 weeks = 4 limbs, 4 heart chambers
When do the upper and lower limb buds begin to form?
Week 4
During what week do fetal movements start?
Week 6
During what week do the genitalia begin to have male/female characteristics
Week 8
The presence of what structure indicates that gastrulation has begun?
Primitive streak
What structures are derived from the endoderm?
- Digestive system
- Liver
- Pancreas
- Lungs (inner layers)
- lower 1/3 of vagina (urogenital sinus)
- Eustachian tube
- Thymus
- Thyroid follicular and parafollicular C cells
- Parathyroid
What structures are derived from the mesoderm?
- Circulatory system
- Lungs (epithelial layers)
- Tracheal cartilage
- Skeletal system
- Connective tissue
- Muscular system
- Dura mater
- Upper 2/3s of vagina
What structures are derived from the ectoderm?
- Skin
- Hair
- Nails
- Nervous system
What structures are derived from the surface ectoderm?
- Epidermis
- Adenohypophysis (from Rathke’s pouch)
- Lens of eye
- Epithelial linings of oral cavity
- Sensory organs of ear
- olfacotry epithelium
- Anal canal below pectinate line
- Parotid, sweat, mammary glands
What is a craniopharyngioma?
Benign Rathke’s pouch tumour with cholesterol crystals, calcifications
What structures are derived from the neural tube?
- Brain (neurohypophysis, CNS neurons, oligodendrocytes, astrocytes, ependymal cells, pineal gland)
- Retina
- Spinal cord
What structures are derived from the neural crest?
ELMO PASSES
- Enterochromaffin cells
- Leptomeninges (arachnoid, pia)
- Melanocytes
- Odontoblasts (teeth)
- PNS ganglia (cranial, dorsal root, autonomic)
- Adrenal medulla
- Schwann cells
- Spiral membrane (aorticopulmonary membrane)
- Endocardial cushions (also derived partially from mesoderm)
- Skull bones
What is the only post-natal derivative of of the notochord?
- Nucleus pulposus of the intervertebral disc
Mesodermal defects are what?
VACTERL association
- Vertebral defects
- Anal atresia
- Cardiac defects
- Tracheo-esophageal fistula
- Renal defects
- Limb defects (bone and muscle)
Define agenesis
Absent organ due to absent primordial tissue
Define aplasia
Absent organ despite presence of primodial tissue
Define hypoplasia
Incomplete organ development; primordial tissue present
Define disruption
Secondary breakdown of previously normal tissue or structure (e.g amniotic band syndrome)
Define deformation
Extrinsic mechanical distortion (e.g congenital torticollis), occurs after embryonic period
Define malformation
Intrinsic developmental defect; occurs during embryonic period (wks 3-8 of development)
Define sequence
Abnormalities result from a single primary embryological event (e.g oligohydramnios -> Potter sequence)
Define a field defect
Disturbance of tissues that develop in a contigous physical space (holoprosencephaly)
What is the inner layer of the chorionic villi called?
Cytotrophoblast
What is the purpose of the cytotrophoblast?
Makes cells
What is the outer layer of the chorionic villi called?
Syncytiotrophoblast
What is the function of the syncytiotrophoblast?
- Synthesises and secretes hormones e.g hCG (structurally similar to LH - stimulates corpus luteum to secrete progesterone during first trimester)
What does the syncytiotrophoblast lack which decreases chance of attack from maternal immune system?
MHC I
What is the decidua basalis derived from?
Mother’s endometrium (maternal blood in lacunae)
What is the amniotic fluid composed of?
Fetal urine (mainly) and fetal lung liquid
What are possible causes of polyhydramnios?
- Idiopathic
- Fetal malformations (esophageal/duodenal atresia, anencephaly)
- Diabetes in mother
- Fetal anemia
- Multiple gestations
Oligohydramnios (too little amniotic fluid) is associated with what other conditions?
- Placental insufficiency
- Bilateral renal agenesis
- Posterior urethral valves (males)
- May cause Potter sequence
What is the most common type of monozygotic (identical) twin?
Monochorionic diamniotic (~75%)