Intrauterine Development and Comparative Respiratory Anatomy
I General Developmental Periods
A Fertilization period (weeks 1 through 3)
1. The sperm fertilizes the egg.
2. Blood vessels first appear.
3. Heart tubes form that will develop into the heart.
4. Blood cells form from endothelial cells within the yolk sac.
II Respiratory System Development
1. By the fourth week, brachial arches form and develop the maxillary (upper) and mandibular (lower) jaw.
2. The brachial arches also form the pharynx, mouth, oropharyngeal airway, and laryngeal cartilages.
3. The tongue develops within weeks 4 to 7.
4. The palate starts to develop in the fifth week and is complete by the 17th week of gestation.
5. A cleft lip may develop as a result of the lip not completely forming and extending into the nostril. A cleft palate occurs from malformation of the palate and may be unilateral or bilateral.
6. The nasal cavity with nasal concha develops when the oronasal membrane ruptures, allowing the oral and nasal cavities to develop. This occurs during approximately the seventh week.
7. Nasal sinuses develop during the latter part of fetal development, with further development of the ethmoid, maxillary, frontal, and sphenoidal sinuses continuing into puberty.
1. An epithelial groove will give rise to the larynx, trachea, bronchi, pulmonary epithelium, and assorted glands.
2. The tracheoesophageal septum divides into the esophagus and laryngotracheal tube.
3. The first lung bud develops from the laryngotracheal tube by 24 to 26 days of fertilization.
4. The laryngotracheal tube, along with the surrounding tissue, develops into the larynx, trachea, bronchi, and lungs.
5. Visceral and parietal pleura develop from the lung buds (bronchopulmonary buds).
6. By 5 weeks two lung buds develop.
7. The phrenic nerve innervates the diaphragm within the fourth week, and the diaphragm is completely formed by the seventh week.
8. By the 10th week, true and false vocal cords are formed.
9. Further growth of the lung buds develop into secondary buds, two on the right and one on the left.
10. This branching continues, with 24 orders of branches present at 16 weeks.
11. By 25 weeks airways have changed from glandular to tubular and increase in length and diameter.
1. Embryonic period (fertilization to 5 weeks)
a. The laryngotracheal groove forms.
b. The lung bud first appears.
c. The lung bud divides into left and right mainstem bronchi.
d. Branching of the airways begins.
e. Pulmonary arteries invade lung tissue, following the airways and dividing as the airway divides.
f. Pulmonary veins originate independently from the lung parenchyma and return to the left atrium, completing the pulmonary circuit.
2. Pseudoglandular period (5 to 13 weeks)
a. The conducting airways develop and are complete up to and including the terminal bronchiole.
b. Mucous glands and goblet cells appear.
c. Bronchi and bronchioles are lined with cuboidal epithelium.
d. Diaphragm begins to develop.
e. Muscle fibers, elastic tissue, and early cartilage formation can be seen along the tracheobronchial tree.
3. Canalicular period (13 to 24 weeks)
a. Enlargement of the conducting airways continues, with proliferation of pulmonary blood vessels.
b. Gas exchange units develop from respiratory bronchioles.
d. Meconium is present at 16 weeks.
e. Breathing movements can be detected between 18 and 20 weeks.
f. Elastic tissue develops beginning at 20 weeks.
g. Airway changes from glandular to tubular and increases in length and diameter.
h. Type I and II alveolar pneumocytes develop, with synthesis and production of surfactant starting by weeks 22 to 24.
4. Terminal sac period (24 weeks to birth)
a. Primitive alveoli develop from alveolar ducts.
b. Further development of the pulmonary vasculature occurs, as does lymphatic proliferation.
c. The fetus weighs approximately 1000 g at 26 to 28 weeks.
d. The fetal lungs represent 2% to 3% of the total body weight. This percentage decreases as the weight of the fetus increases toward the end of gestation.
e. The air sacs change from a cuboidal cellular configuration to a squamous epithelium, allowing greater diffusion of gases.
f. As the lung matures, the number of alveoli increases, and the thickness of the alveoli wall decreases.
g. At birth the number of alveoli ranges from 24 to 75 million.
h. The number of alveoli continues to increase until there are approximately 300 to 600 million alveoli in adulthood.
i. The size of the lung increases from approximately 1 to 2 m2 at 32 weeks’ gestation to the adult size of 70 m2.
A The lung begins secreting fluid by the 70th day of gestation.
B This fluid is composed of a combination of sodium, potassium, chloride, bicarbonate, and a small percentage of protein in water.
C The presence of lung fluid assists lung growth and the development of the functional residual capacity (FRC).
D Fetal breathing helps secrete lung fluid and mixes lung fluid with amniotic fluid.
E Because of this process, amniotic fluid can be analyzed to determine lung maturation (see Section V, Amniotic Fluid).
F Amniocentesis is the procedure in which amniotic fluid is removed from the uterus for examination.
A Surfactant is synthesized and secreted by type II alveolar pneumocytes.
B Surfactant first appears between 22 and 24 weeks’ gestation.
C Surfactant reduces surface tension, maintaining alveolar stability and preventing atelectasis.
D Protein makes up 10% to 20% of the surfactant, and 80% to 90% of the protein is phospholipids. A small percentage of cholesterol is also present.
E Two important phospholipids, lecithin and sphingomyelin, are present in surfactant.
F Sphingomyelin is present early in gestation and remains constant from 18 weeks to approximately 34 weeks before decreasing in concentration.
G Lecithin, the major phospholipid of adult surfactant, abruptly increases between 32 and 34 weeks’ gestation.
H The increased concentration of lecithin denotes lung maturation. The increase of lecithin in surfactant reduces the incidence of respiratory distress syndrome (RDS).
I Without appropriate surfactant production, newborns will have reduced lung compliance, decreased FRC, increased work of breathing, and greater oxygen consumption.
J Inadequate surfactant levels can occur in a newborn as a result of:
A Amniotic fluid is composed of amniotic cells, maternal blood, and fetal urine.
B There is approximately 30 ml of amniotic fluid at 10 weeks, which increases to 1 L by term.
C The fetus swallows amniotic fluid, which is absorbed by the gastrointestinal tract. Every 3 hours, the placenta exchanges amniotic fluid.
D Amniotic fluid protects the fetus and acts as a cushion surrounding the fetus. It also allows growth and development, movement, and maintenance of a thermoneutral environment.
E Amniocentesis can determine sex, lung maturity, biochemical abnormalities, and chromosomal defects.
F Lung maturity is determined by the concentration of lecithin and sphingomyelin.
G The ratio of lecithin to sphingomyelin (L/S) determines the incidence of RDS.
H An L/S ratio of ≥2.0 indicates a mature lung and low incidence of RDS.
I An L/S ratio of 1.0 to 1.5 indicates a transitional lung with a moderate incidence of RDS.
J An L/S ratio of <1.0 indicates a high incidence of RDS.
K Another test to determine lung maturity is the shake test.
1. A mixture of amniotic fluid, saline, and alcohol is placed in a test tube, shaken for 15 minutes, and then allowed to stand.
2. A complete ring of bubbles around the tube indicates appropriate fetal production of surfactant.
3. These results can be compared with an L/S ratio of >2.0.
4. Absence of bubbles indicates that surfactant maturity is incomplete.
A Maternal health and individual physiology, pregnancy complications, and maternal behaviors affect the health and development of the fetus.
B Any condition that leads to interference with placental blood flow or the transfer of oxygen to the fetus can cause adverse outcomes.
C Table 25-1 list maternal conditions and related neonatal outcomes.
TABLE 25-1
Maternal Condition and Neonatal Outcomes
| Maternal Condition | Fetal or Neonatal Outcome |
| Previous pregnancy complication | Same outcome as previous fetus |
| Diabetes mellitus | LGA, congenital malformations, RDS, hypoglycemia |
| Pregnancy-induced hypertension | Prematurity, SGA (pre-eclampsia) |
| Maternal age <17 years | Low birth weight, prematurity |
| Maternal age >35 years | Prematurity, chromosomal defects |
| Placenta previa |
