Fetal Development and Endoderm Quiz

Embryological Overview

• Understanding embryology is important for respiratory care practitioners working with newborns
• Need to know normal fetal development to recognize when something goes wrong
• A vascular network develops to circulate nutrients and allow gas exchange to the growing embryo/fetus
• The fetus relies on mother's circulation for this
• Maternal and fetal vascular networks are separate systems, and no blood is shared
• By gestational day 22 the primitive fetal heart begins to beat
• By days 27-29 the heart has myocardial pump function supporting circulation

Fertilization to Implantation

• The zygote (fertilized egg) travels to the uterus, undergoing cell division
• The zygote has no nutrient source at this time
• The ball of developing cells (blastocyst) must implant in the uterine lining for nourishment
• The trophoblast (outer layer of the blastocyst) combines with endometrial tissues
• This forms the chorionic membrane around the blastocyst

Origin of Tissue Systems

• Ectoderm gives rise to the central and peripheral nervous systems
• It also gives rise to sensory epithelia of the eyes, inner ears, and nose
• As well as glandular tissues like the posterior pituitary gland and adrenal medulla
• Also generates epidermal layer of skin, all specializations (sweat, mammary etc) and enamel of teeth
• Mesoderm gives rise to, cardiovascular and lymphatic systems
• Creates all connective tissue, cartilage and bone, and marrow
• It is also needed for all muscle tissue
• Responsible for the dermis and hypodermis
• It is also needed for kidneys and ureters, and the spleen
• Lastly it gives rise to reproductive tissues, lining of body cavities and teeth (dentin, cementum)
• Endoderm makes the digestive system except for parts of the mouth/pharynx and anus
• Also responsible for respiratory system and epithelial lining of trachea and lungs
• Also makes the urinary system, liver/pancreas, tonsils, thymus, thyroid, and epithelial lining of auditory tube/tympanic cavity
• Cells arrange to form the embryonic disk, which forms the ectoderm, endoderm, and mesoderm
• The outer loop forms the amniotic sac while the inner loop forms the yolk sac
• The yolk sac degenerates and is replaced by the growing amniotic sac
• The amniotic sac surrounds the entire embryo, which attaches to the outer layer via the umbilical stalk

Maternal-Fetal Gas Exchange

• Finger-like projections known as chorionic villi extend into the outer lining of the chorion
• A capillary network forms within the chorionic villi and connects to the umbilical stalk
• Oxygen, carbon dioxide, and nutrients diffuse through the capillary surface area via indirect connection
• The interface between mother and fetus is then limited to the discus-shaped placenta
• The umbilical cord connects the placenta to the fetus with one large vein and two smaller arteries
• The vessels tend to spiral and are protected from kinking by Wharton's jelly inside the umbilical cord

Cardiovascular Development

• By the third gestational week, the heart is fully formed, and is the first complete organ formed
• By 8 weeks, the fetal heart is fully functional with all chambers, valves, and major vessels
• The fetal heart must accommodate circulation within a fluid-filled environment
• In early embryonic development, angiogenic clusters supply nutrition to the growing embryo
• Clusters then coalesce to create 2 heart tubes lined with specialized myocardial tissue
• By day 18 the heart tubes fold into the thoracic cavity, fuse, and become the basic structure by day 21
• The cardiovascular system forms from the mesoderm layer and cardiac contractions are detectable by day 22

Chamber Development

• Dramatic changes occur during the fourth week of gestation
• Heart tubes merge into bulbus cordis, ventricular bulge, and atrial bulge
• These empty into the sinus venosus, which receives blood from the vitelline veins, common cardinal veins, and umbilical veins
• These structures continue to bend, fold, and dilate
• As outflow tracts mature, the truncus arteriosus connects the heart to the future arterial system
• Between days 23-28 dextral looping happens, and the ventricular bulge balloons into a C-shape
• This moves the atrial bulge into the superior direction
• The embryonic heart appears as a twisted S-shape
• Ventricular structure merges with the bulbus cordis to form the bulboventricular loop, which dilates
• The septum primum begins separating the primitive atrium and is followed by the growth of endocardial cushions
• This separates the atria from the ventricles
• The left atrium incorporates the primordial pulmonary veins as four pulmonary veins empty into it
• Right horn of the sinus venosus grows and merges into the future right atrium from the vena cavae
• By the fourth week the ventricular spaces fold and force the ventricular septal bud upward.
• By this point, blood flow matures into a unidirectional path.
• During weeks 5-6 the structures mature rapidly
• The septum secundum begins to appear between the atria
• By week 6 the septum secundum and a flap from the septum primum form the foramen ovale (fetal shunt)
• The atrioventricular canal matures and endocardial cushions separate ventricles from the atrium
• The ventricular septum grows as the ventricles dilate
• Ridges appear opposite each other and fuse to form the aorticopulmonary septum
• The atrial heart rate becomes discernible
• At weeks 7 and 8 the ventricles finish forcing the ventricular septum up from its base
• Tissue from the bulbus cordis and endocardial cushions grow to close the ventricular foramen
• The tricuspid and mitral valves form from specialized tissue surrounding the atrioventricular openings
• Aorticopulmonary septum divides the bulbus cordis and truncus into an aortic and pulmonary trunk
• Semilunar valves form at the base of each
• By the eighth week development is complete, cardiac structures are complete, and blood flows normally
• The heart continues to increase in size and length

Fetal circulation and Fetal Shunts

• Fetal circulation differs from an infant because there is no breath
• Fetal circulation lets blood flow shunt around the fetal liver and lungs because mother is respiring for baby
• Blood is shunted through the heart and this allows the proper amount of blood to get to the placenta where there is gas exchange
• Oxygenated blood travels from the placenta through the umbilical vein, which connects to the ductus venosus
• About 30-50% of the umbilical blood is shunted to the IVC via the ductus venosus to by pass the liver
• The amount of shunting through the ductus venosus decreases with age
• The IVC blood mixes with the blood that is coming from the systemic vein
• Most blood passes to the left atrium through the Foramen Ovale, the second fetal shunt
• The remaining blood goes to the right ventircle, it is slightly less oxygenated at this point
• PVR in utero is very high b/c compression of vessels from small lung volumes
• Hypoxic pulmonary vasocontriction also happens, reducing partial pressure of O2
• 13-25% of fetal blood flow goes to lungs
• Then blood goes to the left atrium, then to the left ventricle, then out the aortic valve and to the body
• This blood is going to have higher oxygenation than right side
• High PVR causes the blood of the rigth ventricle to pass through the ductus arteriosus to the aorta so that blood can bypass lungs

Transition to Extrauterine life

• Clamping umbilical cord removes the low pressure system in placenta
• Decreases PVR and icreases pulmobart blood flow because baby inflates lungs
• Increase in PaO2 dilates arterioles, so lungs inflate
• PVR decreases, and pressure on right side decreases, while pressure of left increases
• The foramen ovale closes when pressures of the left atrium become greater than pressures of the right atrium
• This closure facilitates blood flow and facilitates maintaining circulation outside of the womb
• The Functional closure of the Ductus arterious occurs due to increase of PaO2 decreasing PVR
• Reduction of prostaglandins and decrease of prostaglandin receptors in the Ductus aterious
• Constriction happens and 20% close within 24 hours and 80% close in 48
• Anatomic closure of the Ductus Aterious happens in the last Trimester
• Endothelial tissue proliferate intimal mounds, which lead to vasoconstion
• At 204 weeks Anatomic closure is complete
• The Ductus Artiosus becomes the ligamentum arteriosum