Embryology: Fetal Period, Endodermal Derivatives, and Placentation

DERIVATIVES OF THE ENDODERMAL GERM LAYER

• Main Organ System: The gastrointestinal (GI) tract is the primary organ system derived from the endodermal germ layer.

• Formation of the Primitive Gut:

  • The formation of a tube-like gut is considered a passive event.

  • Process: It involves the inversion and incorporation of a portion of the endoderm-lined yolk sac into the body cavity.

  • Spatial Arrangement:

  • The endodermal germ layer covers the ventral surface of the embryo.

  • It forms the roof of the yolk sac.

• Regions of the Primitive Gut:

  • Foregut: Formed by the endoderm in the anterior part of the embryo.

  • Hindgut: Formed in the tail (posterior) region.

  • Midgut: The portion located specifically between the foregut and the hindgut.

  • Cloaca: Formed by the incorporation of the allantois into the body of the embryo.

• Initial Derivatives (Epithelial Linings):

  • The epithelial lining of the primitive gut.

  • The intraembryonic portions of the allantois.

  • The intraembryonic portions of the vitelline duct.

• Further Derivatives and System Development:

  • Respiratory System: The epithelial lining of the respiratory tract.

  • Glandular Parenchyma: The parenchyma of the thyroid, parathyroids, liver, and pancreas.

  • Lymphoid Organs: The reticular stroma of the tonsils and the thymus.

  • Urinary System: The epithelial lining of the urinary bladder and the urethra.

  • Auditory System: The epithelial lining of the tympanic cavity and the auditory tube.

  • Note: Angiogenic cell clusters are located in relation to the prechordal plate, which is also called the buccopharyngeal membrane.

THE FETAL PERIOD

• Definition: The fetal period spans from the beginning of the third month until the end of intrauterine life.

• Main Characteristics:

  • Maturation of tissues and organs.

  • Rapid growth of the body.

  • Malformations: Relatively few malformations arise during this specific period compared to the embryonic period.

• Length of Pregnancy:

  • Obstetric/Menstrual calculation: $280\text{ days}$ or $40\text{ weeks}$.

  • Fertilization calculation: $266\text{ days}$ or $38\text{ weeks}$ after fertilization.

MONTHLY CHANGES DURING THE FETAL PERIOD

• Head-to-Body Proportions:

  • Beginning of the Third Month: The head constitutes approximately one-half ($1/2$) of the Crown-Rump Length (CRL).

  • Beginning of the Fifth Month: The head size is about one-third ($1/3$) of the Crown-Heel Length (CHL).

  • At Birth: The head size is approximately one-fourth ($1/4$) of the CHL.

• Changes During the Third Month:

  • Facial Development: The face becomes more human-looking. Eyes move from lateral to the ventral aspect. Ears move close to their definitive positions.

  • Limb Development: The limbs reach their relative length.

  • Ossification: Primary ossification centers are present in the long bones and skull by the $12\text{th week}$.

  • External Genitalia: These develop sufficiently so that the sex of the fetus can be determined via external examination using ultrasound.

  • 9-Week Fetus Features: Large head compared to body, visible yolk sac in chorionic cavity, umbilical cord present, and herniation of intestinal loops.

• Changes During the Fourth and Fifth Months:

  • Growth: The fetus lengthens rapidly. By the end of the first half of pregnancy (5 months), CRL is approximately $15\,cm$.

  • Weight: At the end of the fifth month, the weight is still less than $500\,g$.

  • Fetal Movements: Usually clearly recognized by the mother during the fifth month.

• Changes During the Second Half of Intrauterine Life:

  • Weight Gain: Increases considerably, especially during the last $2.5\text{ months}$.

  • Full-Term Weight: Approximately $3200\,g$. Approximately $50\%$ of this weight is added during the final $2.5\text{ months}$.

• Changes During the Sixth Month:

  • Skin Appearance: The skin is reddish and wrinkled. This appearance is characteristic of babies delivered prematurely at this stage.

• Changes During the Last Two Months:

  • Contours: The fetus obtains well-rounded contours due to the deposition of subcutaneous fat.

• End of the Ninth Month:

  • Skull: The skull possesses the largest circumference of all parts of the body.

• Measurements at Birth:

  • Weight: $3000\text{--}3400\,g$.

  • CRL: Approximately $36\,cm$.

  • CHL: Approximately $50\,cm$.

CLINICAL CORRELATES: GROWTH AND ASSESSMENT

• IUGR and SGA: Intrauterine Growth Restriction (IUGR) and Small for Gestational Age (SGA) occur with a frequency of $1\text{ in }10$.

• Fetal Assessment Techniques: Used to detect malformations and chromosomal abnormalities.

  • Ultrasonography: Measures head circumference, abdominal circumference, and femur length.

  • Amniocentesis: Includes testing for alpha-fetoprotein.

  • Chorionic villus sampling.

• Indications for Testing:

  • Advanced maternal age ($35\text{ years}$ or older).

  • Family history of neural tube defects.

  • Previous child with chromosomal abnormalities (e.g., Down syndrome).

  • Parental chromosome abnormalities.

  • Mother is a carrier of X-linked recessive disorders.

THE PLACENTA: CIRCULATION AND MEMBRANE

• Maternal Blood Supply to Cotyledons:

  • Cotyledons receive blood through $80\text{--}100$ spiral arteries that pierce the decidual plate.

  • Mechanism: Narrow spiral artery lumen creates increased pressure -> blood forced deep into intervillous spaces -> oxygenated blood bathes the small villi of the villous tree.

  • Drainage: Blood flows from the chorionic plate back toward the decidua, draining into endometrial veins.

• Blood Volume and Renewal:

  • Mature placenta intervillous spaces contain approximately $150\,ml$ of blood.

  • Blood is replenished approximately $3\text{ or }4$ times per minute.

• Placental Membrane (Placental Barrier):

  • Initial 4 Layers:

  1. Endothelial lining of fetal vessels.

  2. Connective tissue in the villus core.

  3. Cytotrophoblastic layer.

  4. Syncytium.

  • Thinning: From the fourth month onward, the membrane thins as fetal vessel endothelium comes into intimate contact with the syncytial membrane, increasing the exchange rate.

FUNCTIONS OF THE PLACENTA

• Exchange of Gases: Simple diffusion of Oxygen ($O_2$), Carbon Dioxide ($CO_2$), and Carbon Monoxide ($CO$). At term, the fetus extracts $20\text{--}30\,ml$ of oxygen per minute.

• Exchange of Nutrients and Electrolytes: Amino acids, free fatty acids, carbohydrates, vitamins, and electrolytes ($Na^+$, $K^+$, $Ca^{2+}$, $Cl^-$).

• Maternal Antibodies: Syncytiotrophoblast takes up antibodies (especially IgG) and transports them to fetal capillaries.

• Hormone Production (Syncytial Trophoblast):

  • Progesterone: Produced by the placenta by the end of the fourth month.

  • Estrogenic Hormones: Estriol etc.

  • Human Chorionic Gonadotropin (hCG): Produced early to maintain pregnancy.

  • Somatomammotropin: Also called placental lactogen; a growth hormone-like substance.

AMNION AND UMBILICAL CORD

• Primitive Umbilical Ring: The oval line of reflection between the amnion and embryonic ectoderm (amnioectodermal junction).

• Structures at 5th Week (Passing through the ring):

  • Connecting Stalk: Contains the allantois and umbilical vessels ($2$ arteries, $1$ vein).

  • Yolk Stalk (Vitelline Duct): Accompanied by vitelline vessels.

  • Connecting Canal: Connects intraembryonic and extraembryonic coelomic cavities.

• Umbilical Cord at Birth:

  • Approximately $2\,cm$ in diameter and $50\text{--}60\,cm$ long.

  • It is tortuous; this causes "false knots."

  • Anomalies: Normally contains $2$ arteries and $1$ vein. In $1\text{ in }200$ newborns, only one artery is present ($20\%$ chance of cardiac/vascular defects).

• Amniotic Bands: Tears in the amnion cause bands that can encircle/constrict/amputate fetal parts (limbs, digits) or cause craniofacial deformations. Likely caused by infection or toxic insult.

AMNIOTIC FLUID

• Origin: Produced by amniotic cells and derived primarily from maternal blood.

• Volume Changes:

  • 10 weeks: $30\,ml$.

  • 20 weeks: $350\,ml$.

  • 37 weeks: $800\text{--}1000\,ml$.

• Functions: Protective cushion against jolts, prevents adherence to amnion, allows fetal movement.

• Turnover: Replaced every $3\text{ hours}$. Fetal urine is added daily starting in the fifth month (mostly water, as metabolic wastes are handled by the placenta).

• Clinical Correlates:

  • Premature Rupture of Amnion: Occurs in $10\%$ of pregnancies; most common cause of preterm labor.

  • Oligohydramnios: Less than $400\,ml$. Associated with lung hypoplasia and clubfoot.

  • Hydramnios (Polyhydramnios): Excess fluid ($1500\text{--}2000\,ml$). Causes: Idiopathic ($35\%$), Maternal diabetes ($25\%$), congenital malformations.

TWIN PREGNANCIES

• Dizygotic (Fraternal) Twins:

  • Incidence: $2/3$ of all twins ($7\text{--}11\text{ per }1000$ births).

  • Mechanism: Simultaneous shedding of two oocytes fertilized by two different spermatozoa.

  • Membranes: Each has its own placenta, amnion, and chorionic sac (placentas may fuse later).

• Monozygotic (Identical) Twins:

  • Incidence: $3\text{--}4\text{ per }1000$.

  • Stages of Separation:

  1. Two-Cell Stage: Separate placentas, chorions, and amnions (resembles dizygotic arrangement).

  2. Early Blastocyst Stage (Most Common): The inner cell mass splits. Common placenta and chorion, but separate amniotic cavities.

  3. Bilaminar Germ Disc Stage (Rare): Split occurs just before the primitive streak appears. Common placenta, chorion, and amnion.

• Twin Risks and Complications:

  • High perinatal mortality and morbidity; tendency toward preterm delivery.

  • $12\%$ of premature infants are twins.

  • Mortality: $10\text{--}20\%$ for twins vs. $2\%$ for singletons.

  • Vanishing Twin: Death of one fetus in the 1st/early 2nd trimester (resorption or fetus papyraceus - compressed/mummified twin).

  • Twin Transfusion Syndrome: Occurs in $5\text{--}15\%$ of monochorionic monozygotic pregnancies. Unbalanced vascular anastomosis leads to one twin receiving most blood. Mortality for both: $60\text{--}100\%$.

  • Conjoined (Siamese) Twins: Incomplete splitting of the axial area of the germ disc. Classified as Thoracopagus, Pygopagus, or Craniopagus. Surgical separation is only possible if they share no vital organs.