Fetal Embryology Physiology

Question 1

Days gestation vs weeks of pregnancy

Answer 1

• Days of gestation start (ie Day 1) at fertilization
• Weeks pregnancy starts at LMP
• Assuming 28 day cycle → ovulation (menstrual cycle day 14 → fertilization on menstrual day 15 (essentially 2 weeks pregnant)
• Bottom line: LMP = about 2 weeks ahead of days of gestation

Question 2

Overview of week 1 - 4 (gestation)

Answer 2

Week:

1. Blastocyst implantation
2. Inner cell mass → bilaminar disc
3. Gastrulation of bilaminar → trilaminar disc
4. Lateral body and cephalo-caudal folding, neurolation

Question 3

Gastrulation

Answer 3

• Epiblast cells of bilaminar disc give rise to 3 layers (trilaminar disc/embryo):
  
  • Ectoderm
  • Mesoderm/notocord
  • Endoderm
• All fetal cells come from one of these “derms”
• Trilamiar disc is between amniotic cavity and yolk sac, joined to placenta vis connecting stalk

Question 4

Fetal Heart Development

(4 steps)

Answer 4

STEP 1 - Single Heart Tube Formation

• LMP 5 weeks or 3 weeks gestation
• 2 endocardial heart tubes fuse during lateral body folding inside horseshoe shaped pericardial cavity of trilaminar disc
• Fused endocardial tube starts to beat on Day 21 (gestation)
• Cephalocaudal folding gets heart in thoracic cavity

STEP 2 - Single Heart Tube Convolution

• Primitive heart with inflow tract, cardiac chamber, and outflow tract by Day 30 gestation (LMP weeks 6)
• Loops anteriorly and to the right as it grows → aligns inflow/outflow tracts with appropriate future chambers

STEP 3 - Fetal Heart Septation

• 4 chambers established by Day 37 gestation (LMP 7)
• Endometrial cusion - in center of cardiac chamber
• Wall myocardium migrates inward
• Endocardial cusion + myocardium +
  
  • secundums → atrial septation
  • membranous septum → ventrucular septation

STEP 4 - Septation and Spiraling of the Truncus Arteriosis into the Great Vessels

• Truncus arteriosis (aka bulbus cordis) → seperates into PA and aorta and each has to twist to be on top of appropriate ventricle
• Lots of opportunities for mistakes

Question 5

Most common VSD

Answer 5

Membranous VSD

• High VSD (generally don’t see low VSDs)
• Often associated with other congenital cardiac abnormalities
• Abnormal contribution of endocardial cushion tissue, a lack of connective tissue from the muscular interventricular septum, or the lack of aorticopulmonary tissue

Question 6

Most common ASD

Answer 6

Foramen Ovale Defect (aka Patent foramen ovale)

• Ostium Secundum Defect
• Not a hole, more of a flap that doesn’t approximate after pressure in LA goes up

Question 7

AV Canal Defects

Answer 7

• Can be complete or partial
• Complete
  
  • Associated with Trisomy 21
  • Essentially a hole in the center of the heart → all 4 chambers freely communicate → volume hypertrophy of each
  • Large combined AV septal defect and a large common AV valve

Question 8

Pulmonary Development

(4 steps/periods)

Answer 8

• STEP 1 - Formation of Larynx and Trachea
  
  • Pseudoglandular phase
  • Week 5 - 16 gestation
  • Ventral outgrowth of endoderm from the foregut → lung bud → larynx + trachea (must septate completely from esophagus)
• STEP 2 - Formation of the Bronchi and Bronchioles
  
  • Psuedoglandular into Canaliculi
  • Week 16 - 26 gestation
  • Trachea bifurcates into L and R → each bronchi → smaller bronchi → eventually bronchioles
• STEP 3 - Formation of the Lungs
  
  • Canalicular Period into Terminal Sac
  • Week 26 until term
  • Canalicular Period
    • As terminal bronchioles form → alveolar ducts with cuboidal epithelium
    • Capillaries proliferate but do not yet approach the respiratory epithelium
  • Terminal Sac Period
    
    • Terminal sacs (capable of gas exchange)
    • # capillaries increases and approach the respiratory epithelium (becomes squamous)
  • Alveolar Period
    
    • 28 weeks and beyond
    • True alveoli
    • Fetal breathing movements

Question 9

Lung Diseases of Prematurity

Answer 9

• Respiratory Distress Syndrome (aka Hyaline Membrane Disease) of premature neonates
  
  • Results from:
    
    • Absence of alveoli and underdeveloped terminal sacs
    • Thick, cuboidal epithelium
    • Wide area between air-spaces and capillaries
    • Surfactant deficiency due to immature Type II pneumocytes
• Chronic lung disease of prematurity (aka BPD)
  • Arrested alveolar development
    
    • Reactive bronchi and bronchiole smooth muscle → obstruction
    • Large and underdeveloped alveoli that are less capable of gas exchange
  • Inflammation and pulmonary edema
  • Interstitial fibrosis with cystic changes
  • Chronic hypoxemia may induce pulmonary hypertension

Question 10

Meconium aspiration

Answer 10

• Meconium may be passed in utero due to fetal distress
• Lipid and protein rich → very irritating to bronchial and alveolar tissue
• If inhaled during the birthing process → inflammatory broncho-constriction and a chemical pneumonitis and inactivates surfactant
• Thick meconium results in obstruction with a ball-valve-like mechanism

Question 11

Congenital Diaphragmatic Hernia

Answer 11

• The diaphragm develops from 4 structures:

1. Lateral body wall mesoderm
2. Septum transversum
3. Dorsal mesentery of the esophagus
4. Pleuroperitoneal folds

• Incomplete fusion of developing diaphragm tissues → hole in diaphragm → herniation of GI structures into thoracic cavity (usually on the left because liver on the right) → no space for lung to develop
• Leading cause of lung hypoplasia

Question 12

Cystic Adenomatoid Malformation C-CAM

Answer 12

• Abnormal mesenchymal proliferation and failed of maturation of the bronchial structures resulting in adenomatous (connective tissue) overgrowth cysts
• Cysts may communicate directly with tracheo-bronchial tree and with each other
  
  • Significantly impairs alveolar development