Fetal and Neonatal Physiology Lecture (Dr. Lopez) Flashcards
Prenatal Development
TIMELINE:
– GESTATIONAL AGE:
• Day 1 is LAST Normal Menstrual Period (LMP)
– FERTILIZATION AGE (embryonic age or fetal age):
• Day 1 is Fertilization!!!!!!
– As a GENERAL RULE:
• Fertilization Age + 2 weeks = Gestational Age
Growth occurs by Hyperplasia and Hypertrophy
- The growth of an Organ occurs as a result of an INCREASE in Cell Number (Hyperplasia), an INCREASE in Cell Size (Hypertrophy), or BOTH
- 3 Sequential Phases of Growth:
1) Pure Hyperplasia
2) Hyperplasia & CONMITANT Hypertrophy
3) Hypertrophy alone
• The time courses of the three phases of growth are ORGAN SPECIFIC
– Placenta goes through all 3 phases of Growth, but these phases are COMPRESSED because the Placental Life Span is relatively short
a) HYPERTROPHY is the primary form of PLACENTAL Growth!!!!!!!!!
– Growth of the FETUS occurs almost entirely by HYPERPLASIA!!!!!!!!!!!!!!!
• Stimuli that either Increase or Decrease Cell Number, cell size, or both may accelerate or retard the growth of the whole fetus or of individual organs
Growth and Functional Development of the Fetus
• LENGTH: Increases almost in proportion to age:
– 2-3 weeks POST Fertilization: Fetus is Microscopic in size
– 12 weeks: 10 cm long
– 20 weeks: 25 cm long
– Term (40 weeks): 53 cm (21 in)
• WEIGHT: Increases almost in proportion to the cube of the age of the fetus – Reaches 1 lb at 5.5 months – 7 months: 3 lbs – 8 month: 4.5 lbs – Term: 7 lbs (average)
Increases in Placental Mass Parallel periods of Rapid Fetal Growth
• The PLACENTA plays several important roles in fetal Growth & Development
– Transport & storage functions
– Involved in synthesis of:
a) STEROIDS (e.g. estrogen & progesterone)
b) PROTEIN HORMONES [e.g. Human Chorionic Gonadotropin (hCG) & Human Chorionic Somatomammotropins (hCSs)]
• FETAL GROWTH closely correlates with PLACENTAL WEIGHT
– Maternal blood flow to the Uterus & Fetal Blood Flow to the Placenta INCREASE in parallel with the INCREASE in Placental MASS
– Placental Growth INCREASES linearly until ∼4 weeks BEFORE BIRTH!!!!!!
Intrauterine Growth Restriction (IUGR)
• Is an ABNORMALITY of Fetal Growth & Development
• The Growth-RESTRICTED Fetus is at substantial risk of Morbidity and Mortality; specific risks include:
– Birth Asphyxia, Neonatal Hypoglycemia, Hypocalcemia, Meconium Aspiration, Persistent Pulmonary Hypertension of the Newborn, Pulmonary Hemorrhage, Delayed Neurologic Development, Hypothermia, among others
• May occur as a RESULT of DECREASED PLACENTAL RESERVE caused by any insult
– Mothers who smoke during pregnancy tend to have SMALL PLACENTAS & are at HIGH risk of Delivering a LOW BIRTH WEIGHT Baby
Glucocorticoids, Insulin, GH, IGFs, and Thyroid Hormones STIMUALTE Fetal Growth
A) The fetus uses GLUCOSE as its Major ENERGY Source
– The exchange of Glucose across the placenta CONTROLS fetal blood glucose levels
– The Fetus normally has little need for Gluconeogenesis, & the levels of Gluconeogenic enzymes in the FETAL Lier are LOW
B) GLUCOCORTICOIDS: promote the STORAGE of Glucose as Glycogen in the Fetal Liver
– This process INCREASES Greatly during the FINAL MONTH of Gestation in preparation for the Increased GLYCOLYTIC Activity REQUIRED During & Immediately AFTER Delivery
C) INSULIN (near term): contributes to the Storage of Glucose as Glycogen, as well as to the UPTAKE & UTILIZATION of Amino Acids, & Lipogenesis
– Near Term, Fetal Glucose Metabolism becomes SENSITIVE to INSULIN
– MATERNAL Insulin CANNOT CROSS the Placenta!!!!!!
D) GH (postnatally): acts by binding to GH Receptors, primarily in the Liver, & triggering the production of SOMATOMEDIN or IGF-1
- May have only a MINIMAL Effect on Fetal Growth
- Fetal Liver has relatively FEW GH RECEPTORS
E) IGF-1 & IGF-2: are Mitogenic peptides; EXTREMELY important for Fetal Growth
- IGF-1 & IGF-2 are present in the fetal circulation from the end of the 1st TRIMESTER, their levels INCREASE thereafter in BOTH Mother & Fetus
- Birth WEIGHT correlates POSITIVELY with IGF Levels
F) THYROID HORMONES: obligatory for Normal Growth & Development
- Before the 2nd Trimester, most of T4 in the Fetus is MATERNAL
- FETAL PRODUCTION of TSH & the Thyroid Hormone T4 begin to INCREASE in the 2nd trimester, concurrent with Development of the Hypothalamic-Pituitary portal system
- HYPOTHYROIDISM has ADVERSE effects on Fetal Growth (e.g. reduction in the SIZE of organs like the Heart, Kidney, Liver, Muscle, & Spleen)
Development of the Organ Systems: HEART
• Heart begins BEATING during the 4th WEEK AFTER Fertilization
– Initially contracting at 65 beats per min (bpm)!!!!
– Rate INCREASES steadily to ~ 140 bpm IMMEDIATELY BEFORE BIRTH
Development of the Organ Systems: Formation of BLOOD CELLS
- 3rd WEEK: Nucleated RBCs formed in Yolk Sac & Mesothelium of PLACENTA
- 4th – 5th WEEKS: NON-NUCLEATED RBCs form by FETAL MESENCHYMAL & ENDOTHELIAL cells of the Fetal Blood Vessels
- At 6 WEEKS: LIVER forms Blood Cells
- At 12 WEEKS (3rd month) : SPLEEN & LYMPHOID Tissue begin forming RBCs
• From the 3rd MONTH ON: BONE MARROW becomes the Principal Source of RBCs
– Also PRODUCES MOST of the White Blood Cells
– Other structures LOSE ABILITY to form Blood Cells, except for continued Lymphocyte & Plasma Cell production in LYMPHOID TISSUE
Erythrocytes
• The fraction of total ERYTHROCYTES that are RETICULOCYTES (Immature, Non-Nucleated erythrocytes with residual Polyribosomes) is HIGH in the Young Fetus, but it DECREASE to ONLY ∼5% at term
– In the adult, the TETICULOCYTE count is Normally less than 1%!!!!!!!!!
• The LIFE SPAN of FETAL ERYTHROCYTES depends on the AGE of the FETUS!!!!!!!!!
– In a term fetus, it is ∼80 days (Two Thirds that in an adult)
– The Life Span of Erythrocytes of LESS Mature Fetuses is MUCH SHORTER
Hemoglobin (Hb)
- Hb begins to be FORMED in the 3rd WEEK after fertilization
- Hb CONTENT of the Fetal Blood rises to ∼15 g/dL by MIDGESTATION (= to the level in normal men)
- Hb Concentration of Fetal Blood at term is HIGHER than the Hb CONCENTRATION of Maternal Blood, which may be only ∼12 g/dL
- Fetal Hg has HIGHER AFFINITY for O2: can carry 20-30% MORE O2 than Maternal Hg!!!!!
Development of the Organ Systems: Nervous System
• Most of the REFLEXES of the Fetus involving the Spinal Cord & Brain Stem are present by the 3rd - 4th MONTHS After Pregnancy!!!!!!!!
• Cerebral Cortex DEVELOPMENT continues AFTER BIRTH
– MYELINIZATION of some major tracts of the Brain becomes COMPLETE only after about 1 year of POSTNATAL Life
Development of Organ Systems: GI Tract
• INGESTION of Amniotic fluid begins in the 2nd TRIMESTER!!!!
• During the last 2 – 3 MONTHS GI function approaches that of the Normal Neonate
– Small quantities of Meconium are continually FORMED in the GI tract & EXCRETED from the Anus into the Amniotic Fluid
a) MECONIUM: composed of swallowed AMNIOTIC FLUID & partly of Mucus, Epithelial Cells, & other residues of excretory products from the GI tract & glands
Fetal Metabolism
• Accumulation of significant amounts of CALCIUM & PHOSPHATE during period of OSSIFICATION
• IRON Accumulates RAPIDLY starting at 12 WEEK of GESTATION
– About One Third of the IRON in a fully developed fetus is STORED in the LIVER
– Enough IRON is Stored & can be used for several months AFTER BIRTH
a) Used for formation of additional Hb!!!!
Development of Organ Systems: Kidneys
• Urine EXCRETIONS begins in 2nd TRIMESTER!!!!!!!!
– Fetal urine accounts for ~ 70-80% of the Amniotic Fluid
• The Renal Control Systems for the REGULATION of Extracellular Volume, Electrolyte Balance, & acid-base Balance are almost NON-EXISTANT until LATE Fetal Life
– Full development OCCURS few months AFTER BIRTH!!!!!!!!
Development of the Organ System: Respiratory System
• Respiration CANNOT occurs DURING Fetal Life because there is NO AIR to breath in the Amniotic Cavity
• During the last 3 to 4 months of pregnancy, the Respiratory Movements of the fetus are mainly INHIBITED, & the LUNGS remain almost COMPLETELY DEFLATED
– Inhibition of the respiration PREVENTS FILLING of the Lungs with Fluid & Debris from the MECONIUM
– Small Amounts of FLUID are SECRETED into the Lungs by the ALVEOLAR EPITHELIUM up UNTIL Birth
