[09.09b] Embryology and Fetal Development Part 2 V2

Organogenetic period

What name is given to weeks 4-8 of embryonic development, when major organs and systems form?

Teratogens

Exposure to what agents should be avoided during the organogenetic period to prevent abnormalities?

Rapid growth

What process that occurs during weeks 4-8 causes the embryo to fold?

Curved and cylindrical

What shape does the embryo become due to folding?

Amniotic cavity, trilaminar embryo, secondary yolk sac

Name three structures present during the folding of the embryo.

Lateral body folds

What structures form during folding in the horizontal plane?

Ventral body wall

What structure do the lateral body folds eventually form?

Head (cephalic) and tail (caudal) folds

What structures form during folding in the median plane?

Oropharyngeal and cloacal membranes

What two areas result from the fusion of ectoderm and endoderm during median plane folding?

Ventral

Where does the developing brain push the heart during folding in the median plane?

Vitelline stalk or omphalomesenteric stalk

What structure connects the primitive gut to the secondary yolk sac?

Degenerate and disappear

What eventually happens to the vitelline stalk and secondary yolk sac?

Allantois

What diverticulum of the umbilical vesicle emerges from the hindgut?

Umbilical cord

The allantois becomes incorporated into what structure?

False

T/F: The folding of the horizontal plane forms the head and tail folds, while the folding of the median plane forms the lateral body folds.

Oropharyngeal and cloacal membranes

What are the two areas where the ectoderm and endoderm fuse?

Omphalomesenteric stalk

The primitive gut is connected to the secondary yolk sac through the ____.

Failure of body folding

What process causes body wall defects?

Bladder exstrophy, cloacal exstrophy, cleft sternum, gastroschisis

Name some potential results of failure of body folding.

Ectopia cordis

What condition results in the heart being outside the thoracic cavity?

Incomplete closure of the ventral body wall in the thoracic area

What causes ectopia cordis?

Incompatible with life

Ectopia cordis is incompatible with ____ .

Incomplete closure of the hypogastric area

What causes bladder and cloacal exstrophy?

Incomplete closure of the abdominal wall

What causes gastroschisis?

Herniation of abdominal contents directly into the amniotic cavity

What happens in gastroschisis?

Right lateral side of the umbilicus

Where does gastroschisis usually occur?

Transfer of nutrients to embryo, first hematopoietic organ, origin of primordial germ cells, incorporated into the embryo as the primordial gut

List the significance of the secondary yolk sac.

Oogonia and spermatogonia

What cells are the primordial germ cells?

Atrophies

What happens to the secondary yolk sac as pregnancy advances?

Ileal diverticulum (Meckel’s Diverticulum)

Persistence of the yolk stalk can form what structure connected to the ileum?

Feces will exit out of the umbilicus

If the yolk stalk is still connected to the umbilicus and patent, what can happen?

Urine from the urinary bladder will exit out of the umbilicus

If the urachus is still patent, what will happen?

Primitive gut

What does the upper portion of the secondary yolk sac give rise to?

Mimic appendicitis

What can an inflamed Meckel's diverticulum mimic?

Celiac trunk, superior mesenteric artery, inferior mesenteric artery

What are the arterial supplies for the foregut, midgut, and hindgut, respectively?

Week 4

In which week do pharyngeal arches develop?

Face, nasal cavities, mouth, larynx, pharynx, neck

To the formation of what structures do pharyngeal arches contribute?

Otic pits and lens placode

Which structures begin to be visible in week 4?

Ventral prominence

What characterizes the heart in week 4?

Slightly curved with head and tail folds

What is the shape of the embryo in week 4?

Week 5

In which week does the growth of the head exceeds the growth of the body?

Mesonephric ridge

Which ridge appears in week 5?

Primordium of kidney

The mesonephric ridge is the site of the primordium of what?

Week 6

In which week do extremities begin to show regional differentiation?

Digital rays

What forms in the upper and lower limbs in week 6?

Upper limbs

Which develops earlier, upper or lower limbs?

Auricular hillock

Which hillock further develops in week 6?

Week 7

In which week do digits form?

Physiologic herniation

What process occurs when the midgut elongates and has no more space in the abdominal cavity?

Superior mesenteric artery

Around what artery does the midgut rotate during physiological herniation?

Omphalocele

What condition results from the failure of bowels to return to the abdominal cavity during physiologic herniation?

Bones of the upper limbs

Which bones develop in week 7?

Week 8

In which week does the embryo become more human in appearance?

Tail-like caudal eminence

Which eminence is still present in week 8?

Purposeful movements

Which movements appear around week 8?

External genitalia

Which genitalia starts to develop around week 8?

Week 4 - Physiologic Herniation

Which of the following is incorrectly matched? Week 4 - Otic Pits, Week 5 - Mesonephric ridge, Week 6 - Auricular hillock, Week 7 - Bone ossification, Week 8 - Physiologic Herniation

True

T/F: The development of upper limbs begins earlier than lower limbs.

Week 9 to birth

When is the fetal period?

Rapid body growth

What characterizes the growth of the fetus?

Slowdown in the growth of the head compared to the rest of the body

How does the growth of the head change during the fetal period?

Differentiation of tissues, organs, and systems

What process occurs to the tissues, organs, and systems during the fetal period?

3 months

How long does each trimester last?

All major organ systems have developed

What milestone is reached at the end of the first trimester?

Increasing growth of fetus

What characterizes the second trimester?

Good anatomical details can be visualized during ultrasonography

Why is ultrasonography useful during the second trimester?

Fetus is viable and can survive in the external environment

What characterizes the third trimester?

Determine an accurate delivery or due date

Why is it important to determine gestational age?

Appropriate intervals for prenatal visits, timing for screening tests, and timing for elective deliveries for induction of labor for postdate pregnancies

What determinations are influenced by gestational age?

Ultrasonography and menstrual history

Name two basic methods to estimate gestational age.

Gold standard

Ultrasonography: Gold standard or Silver standard for dating pregnancy?

Earlier

Is the accuracy of ultrasonography better if done earlier or later?

Last menstrual period (LMP)

What is a reliable alternative to ultrasonography in the absence of ultrasonography?

Woman has a regular menstrual cycle with ovulation occurring on the 14th day

What is the assumption made when using menstrual history to determine gestational age?

More than half of women cannot recall their last menstrual period, irregularity in the menstrual cycle length with variability in the timing of ovulation, implantation bleeding can be confused with menstruation

Name some disadvantages of using menstrual history to determine gestational age.

Week 9 to birth

When is the fetal period?

False

T/F: All major organ systems finish their development in the 2nd trimester.

Ultrasonography & Menstrual History

What are the two basic methods to estimate gestational age?

Naegele’s Rule

Which rule calculates the expected date of delivery based on the last normal menstrual period and a regular 28-day menstrual cycle?

Gestational sac, crown to rump length (CRL)

What measurements are used to estimate fetal age in the first trimester?

Biparietal diameter, head circumference, abdominal circumference, femoral length, foot length

What measurements are used to estimate fetal age in the second and third trimesters?

5 weeks gestation

As early as how many weeks of gestation can the gestational sac be identified via transabdominal ultrasound?

6-7 weeks gestation

When is the embryo measured for crown to rump length?

Tip of the cephalic pole to the tip of the caudal pole

Between what points is crown to rump length measured?

12 weeks gestation

When does measurement of the Biparietal Diameter start?

Fetal liver

At what level is abdominal circumference measured?

Second trimester

During which trimester is abdominal circumference most accurate in estimating gestational age?

Head circumference

Is abdominal circumference or head circumference less accurate?

Femur

Which fetal long bone is easiest to measure?

Longest length in the long axis of the fetal foot

Along what length is foot length measured?

Increased growth of body length

What characterizes physical changes from 9-12 weeks?

End of 12th week

By the end of which week has the CRL more than doubled?

Primary centers of ossification in skull and long bones

What centers appear between 9-12 weeks?

Urine formation

What formation begins between 9-12 weeks?

Liver

Where does erythropoiesis begin between 9-12 weeks?

Fetal heart tones

What tones can be heard between 9-12 weeks?

End of 12th week

When does physiologic herniation and retraction occur?

Fetus lengthens rapidly

What characterizes physical changes from 13-16 weeks?

Active ossification of the fetal skeleton

What process happens to the fetal skeleton between 13-16 weeks?

Eyes are anterior, external ears close to definitive position

What changes occur to the face between 13-16 weeks?