Embryology Flashcards
What are the 4 cellular processes involved in embryological development?
- Proliferation
- Differentiation
- Reorganisation
- Apoptosis
What are the key developmental events that occur in the first 2 months of pregnancy?
Weeks of Embryonic development = +2 = gestation ages
NOTE:
- At the end of the embryonic period in week 8 about 90% of the adult anatomical structures have formed
- Face, urinary, reproductive system and lung cont to develop beyond week 8
How is the notochord formed?
- Week 3: mesodermal cell ingress through the primitive streak to form a notochord
- The primitive pit extends to form a notochordal canal
- Floor fuses with underlying endoderm
- Notochordal cells proliferate and infold to form the notochord
How is the neural tube formed?
Neurulation:
- The developing notochord induces the overlying ectoderm to thicken and form the neural plate
- The plate elevates to form neural folds which fuse ober the neural groove to form the neural tube
- Neural tube closes by week 4 (day 22)
What happens when the neural tube fails to close properly?
Anecephaly = failure of rostral fusion
Spina bifida = failure of caudal fusion
- Spinal bifida occulta = small internal defect marked by a patch of hair/ dimples overlying defect
- Meningocele = membrane bulged out → contains CSF but not neural tissue
- Myelomeningocele = neural tissue has bulged out → severe defect → derangement of function
How is spina bifida prevented?
Folic acid deficiency causes this
- Folic acid BEFORE pregnancy and in first trimester
How does the face develop?
2 halves of face develop separately
- Nasal pits begin where the eyes will end up at and the eyes develop on the lateral sides of head
- Eyes + nose begin on linear lvel
- Between wk5 and 10 PF, nose + eyes migrate centrally via:-
- Tissue btwn nasal pits apoptose = cleft formation
- Instead of new cell proliferation to fill up cleft, existing cells migrate centrally and pulls along with it the eyes + nose
- Process continues until 2 halves of face fuse
How may a cleft palate/lip arise?
- Grooves formed in midline
- Masses of tissue migrate from lateral side of face in towards midline to fill in these grooves
- Failure to fill these grooves leads to cleft palate/lip
- Upper lip consists of 2 grooves –> asymmetric defect -
- Cleft palate =malfusion of palate (roof of mouth)
- Cleft lip = malfusion of the upper lip ONLY!!
How does the urinary system develop?
Development of the urinary organs starts at the urogenital ridge (mesoderm)
Nephretic duct develops and joins with the nephrogenic cords > successive phases of kidney development
- Pronephros ismost immature form
- Mesonephros, an intermediate phase
- Metanephros is most developed > metanephric mesoderm > definitive adult kidney (wk 5)
Week 6-8: start down by ureters + ascend towards adrenal glands
- Ureters extend + retain kidney bladder connections
- In contrast the kidenys form new connections with the developing arterial system as they move so renal arteries breakdown and reform
When do the kidneys ascend?
Week 6-9 PF
What happens as the kidneys ascend? (X)
- Need to rise up to lumbar region
- Don’t drag BVs with them
- Send out new and more cranial branches
- Caudal branches regress
- Mesonephros degenerates
- Gonads descend
What complications can occur during renal development?
- Pelvic kidney (doesn’t ascend to below adrenal gland)
- Horseshoe kidney - fusion of separately developing kidneys below inferior mesenteric artery - also X descend
- Retention of extra artery (isn’t broken down) - may obstruct ureter –> enlargement renal pelvis
- Renal agenesis (bilateral = Potter’s syndrome due to low amniotic fluid the ureteric buds don’t grow)
- Abnormal ureter/shaped kidneys
- Bladder exstrophy - problem w/cloaca - abdominal wall fails to cover bladder - abnormal distance btwn scrotum + penis
What layer of cells does the bladder develop from? (X)
Mostly endoderm Trigone is mesodermal (mesonephric duct) - Trigone signals filling of bladder
How does the heart develop?
Week 3 - heart at midline - cardiogenic cells develop in U pattern outside embryo proper
Week 4 - heart tubes fuse
Day 23 - heart loops - atria loop posteriorly and to the right so they end up more cranial to ventricles
Day 28 - heart septation - spiralling occurs
Week 7 - outflow tracts divide
- Vascular connections maintained, so major arteries connected to ventricles + veins to atria
- Valves develop –> unidirectionally flow ensured
- Foramen ovale between atria
- Main artery from RV connected to aorta by ductus arteriosus
- these holes divert blood away from the lungs to the rest of the body
How do cardiac abnormalities occur?
- Problem w/heart looping day 23 = transposition of great arteries (aorta + PA swapped)
- Problem with heart septation day 28 = ventricular septal defect - deoxygenated blood into LV, patent foramen ovale
- Problem w/outflow tracts dividing wk 7 = pulmonary stenosis, truncus arteriosus
What is the Tetralogy of Fallot?
Type of congenital heart disease
- Pulmonary stenosis (thickened, narrow pulmonary outflow tract)
- Thickened RV wall
- Ventricular septal defect
- Aorta overides septal defect
How do the lungs develop?
- Begins during 1st TM, not completed until after delivery
- Surfactant produced from 25 weeks PF + gradually increased during 3rd TM
- 5 stages of development (wks gestational age, since LMP)
- Week 0-6 = embryonic
- Week 6-16 = pseudoglandular
- Week 16-28 = canalicular
- Week 28-36 = saccular
- Week 36-birth = alveolar
What happens in the embryonic stage of lung development?
0-6 weeks gestational age:
Laryngotracheal groove folds into tube
Tube divides into oesophagus + lung buds
Buds grow into pleural passages + divide into lobes
What happens in the pseudoglandular stage of lung development?
6-16 weeks gestational age:
Trachiobronchial tree forms as branching tubules resemble exocrine gland
What happens in the canalicular stage of lung development?
16-28 weeks gestational age:
Ascinar units form terminal bronchioles and divide into resp bronchioles
Capillaries approximate to potential air spaces but have little contact
Type 1 and 2 alveolar cells + surfactant starts being made
What happens in the saccular stage of lung development?
28-36 weeks gestational age:
Barrier thins
Increased maturation of surfactant
What happens in the alveolar stage of lung development?
36 weeks gestational age-birth:
Primitive alveoli mature
Which cells make surfactant?
Type 2 alveolar cells
What is the role of surfactant?
Reduces surface tension Produces constituent apoproteins
What causes respiratory distress syndrome?
Low surfactant
How can you increase surfactant production?
Injecting glucocorticoids
Where do the gonads develop from?
Intermediate mesoderm within urogenital ridges
Which strucutures give rise to the male and ffemale genital ducts respectively?
- Paired mesonephric ducts give rise to the Wolfian system in males
- Paired Paramesonephric ducts gives rise to the Mullerian system in females
When do the genital ducts start to differentiate?
Week 7 PF
What does male sex rely on?
Activity of SRY protein
Why does the mesonephric duct break down in females?
Gives rise to testes
What are the main developmental abnormalities of the reproductive system?
Female: - Congenital adrenal hyperplasia - mutation in CYP21A2
Male: - Androgen insensitivity syndrome - mutant androgen R - Result of inability to produce testosterone/anti-Mullerian hormone OR inability of target tissues to respond (Rs)
What causes regression of the paramesonephric female ducts?
Testosterione ****Anti-mullerian hormone DHT
How do the gonads and reproductive tract develop?
- Gonads arise from intermediate mesoderm within urogenital ridges of embryo - Genital ducts arise from paired mesonephric + paramesonephric ducts - Mesonephric ducts give rise to MALE genital ducts (Wolffian system) - Paramesonephric ducts give rise to FEMALE genital ducts (Mullerian system) - Gonads and RTs are indifferent up until 7 weeks of development - Differentiation is influenced largely by presence or absence of SRY (on Y chromosome) - If SRY+, then development proceeds along male path - If SRY-, then development proceeds along female path
