POM II - Fetal Development - Exam 5

Zygote

-initial, single-celled organism formed by the fusion of a female egg and a male sperm during fertilization

-diploid cell

Blastocyst

ball of cells that forms early in pregnancy, about 5-6 days after a sperm fertilizes an egg

6-7 days

A blastocyst implants to uterus __________ days following fertilization

Embryo

early stage of a developing human from implantation in the uterus -- approximately 3rd week -- until the end of the 8th week after conception

Fetus

an unborn baby from about the 8th week after fertilization until birth

Teratogen

any agent, including drugs, chemicals, infections, or environmental conditions that can disrupt fetal development, causing birth defects, structural abnormalities, or functional impairments during pregnancy

Yolk Sac

-first extraembryonic structure visible on ultrasound

-arises from endoderm of the primitive gut

-connected to embryo via the vitelline duct

-located within the chorionic cavity

Yolk Sac Early Roles

-primary source of embryonic nutrition before placental circulation develops

-Site of early hematopoiesis (blood cell formation) - produces primitive erythrocytes and immune cells

-contribute to formation of the primitive gut

Yolk Sac Physiologic Functions

-contributes to germ cell migration to developing gonads

-assists early nutrition transfer and metabolic regulation

Yolk Sac Clinical Correlations

-abnormal yolk sac or shape may indicate -> early pregnancy loss or chromosomal abnormalities

-persistent vitelline duct can lead to congenital anomalies

Placental Formation

Forms where chorionic villa contact the decidua basalis

chorion frondosum

forms the placenta

chorion laeve

forms avascular fetal membrane

Placental Architecture

-composed of branching chorionic villi suspended in maternal blood (intervillous space)

-Each villus branch forms a placental lobule with its own fetal artery and vein

-surface area increases with gestation through extensive villus branching

8-10 wks

By _________ the placenta begins producing progesterone and hCG

10-12 wks

By _________ the placenta is fully functional and takes over from the corpus luteum to support the pregnancy

17 wks

by ________ placental and fetal weights are similar

1/6

at term, the placenta weighs _________ of fetal weight

True

T or F: the placenta grows faster than fetus early in pregnancy

Maternal-Fetal Circulation

-maternal spiral arteries remodel into low-resistance vessels

-blood flows into the intervillous space, bathing chorionic villi

-fetal capillaries within villi absorb oxygen and nutrients

Maternal-Fetal Interface

-fetal blood vessels lie within villi and are separated from maternal blood by a thin placental membrane

-allows efficient exchange without mixing maternal and fetal blood

Key Placental Functions

Gas Exchange (O2, CO2)

Nutrient Transfer (glucose, amino acids, lipids)

Waste Removal (urea, bilirubin)

Endocrine Function

Immune Modulation

Abnormal Placental Growth

can lead to preeclampsia, fetal growth, restriction, preterm birth

Umbilical Cord

-connects fetus to placenta

-forms from the connecting stalk and yolk sac remnants

Two Umbilical Arteries

carry deoxygenated blood from fetus to placenta

One Umbilical Vein

carries oxygenated blood to fetus

Wharton's Jelly

gelatinous connective tissue protecting vessels

Umbilical Cord Components

-Two umbilical arteries

-One umbilical vein

-Surrounded by Wharton's Jelly

-Covered by amnion

50-60 cm

what is the typical length of the umbilical cord at term?

Umbilical Cord Functions

-Transport Pathway -> oxygen, nutrients, waste products

-Protects vessels from compression or kinking

congenital anomalies

A single umbilical artery is assocaited with what?

Cord Abnormalities

-True Knots

-Nuchal Cord

-Velamentous Cord Insertion

-Can affect fetal oxygenation and growth

Amniotic Sac

-thin avascular membrane surrounding fetus

-formed by amnion and chorion

-contains amniotic fluid

Amniotic Fluid Sources

-early pregnancy: maternal plasma diffusion

-late pregnancy: fetal urine, fetal lung secretions

800-1000 mL

what is the normal volume of amniotic fluid at term?

Amniotic Sac/Fluid Functions

-Mechanical protection (cushions fetus)

-Maintains constant temp

-Allows fetal movement and MSK development

-Supports lung development through fetal breathing movements

-prevents umbilical cord compression

Fluid Regulation

-fetus swallows and urinates amniotic fluid

-balance maintains fluid volume

Oligohyramnios

-renal abnormalities, placental insufficiency

-risk of pulmonary hypoplasia

Polyhydramnios

-impaired fetal swallowing

-maternal diabetes

Neural Tube

forms the brain and spinal cord; lumen becomes the ventricular system and central canal

Neural Tube Closure

-Cranial: ~25 days from fertilization

-Caudal: ~27 days from fertilization

Folic Acid

__________ must be present before closure of neural tubes to prevent neural tube defects

Weeks 6-7

when do the brain vesicles develop?

Primary Vesicles

-forebrain

-midbrain

-hindbrain

Secondary Vesicles

-Telencephalon -> cerebral hemispheres

-Diencephalon -> thalamus/hypothalamus

-Mesencephalon -> midbrain

-Metencephalon -> pons and cerebellum

-Myelencephalon -> medulla oblongata

3-4 months

neuronal proliferation peaks _________ gestation

3-5 months

neuronal migration peaks __________ as neurons move to final cortical locations

neurodevelopmental

disruptions in neuronal development leads to major ____________ abnormalities

Brain Maturation

-progressive gyral formation and cortical maturation

-Myelination begins ~6 months gestation but continues primarily after birth

Spinal Cord Development

-initially spans vertebral column, then ascends relative to vertebrae

-24 weeks: ~S1

-Birth:~L3

-Adult: ~L1

Functional Neuro Development

-synaptic activity by ~8 wks -> early fetal movements

-rapid neuromuscular integration during 3rd trimester

Early Heart Formation

-Heart development involves complex molecular signaling (including HIF and HOX genes)

-Days ~21-23: primitive straight cardiac tube forms

-Weeks 4-7: four-chambered heart with primitive valves

-Great vessels and aortic arches develop via vasculogenesis

-late fetal life: coronary vessels form via angiogenesis

Fetal Circulation

-placenta provides oxygenation (lungs largely bypassed)

-ventricles function in parallel, not in series

-Right ventricle provides ~2/3 of cardiac output

Ductus Venosus

umbilical vein -> IVC (bypasses liver)

Foramen Ovale

right atrium -> left atrium (supplies heart and brain with more oxygenated blood)

Ductus Arteriosus

pulmonary artery -> descending aorta (bypasses lungs)

Flow Pattern

-Well-oxygenated blood -> left heart -> brain and coronary circulation

-Less oxygenated blood -> right ventricle -> ductus arteriosus -> systemic circulation

-Only ~8% of right ventricular output goes to the lungs

Early Lung Formation

-lung primordium arises from foregut endoderm ~20 days gestation

-lung bud forms ~25 days

-lung development controlled by gene activation/deactivation

Pseudoglandular Lung Development

-5-17 weeks

-bronchial tree forms, lung resembles a gland

Canalicular Lung Development

-16-25 weeks

-respiratory bronchioles and vascularization develop

Terminal Sac Lung Development

->=25 weeks

-primitive alveoli (terminal sacs) form

Alveolar Stage Lung Development

-late fetal -> childhood

-alveoli and capillary network mature

-only ~15% of adult alveoli present at birth

-alveoli continue developing until ~8 years

Pulmonary Surfactant

-produced by type II pneumocytes

-reduces alveolar surface tension and prevents collapse after birth

-deficiency -> neonatal respiratory distress syndrome

Adequate Surfactant

what is the key indicator of fetal lung maturity?

Surfactant Composition

-~90% lipids (phospholipids), ~10% proteins

-major components: dipalmitoyl phosphatidylcholine (DPPC)

-surfactant proteins assist surfactant function

Fetal Breathing and Lung Growth

-fetal resp movements begin ~11 wks

-amniotic fluid moves in/out of lungs by ~4 months

-breathing movements are essential for normal lung development

Antenatal Corticosteroids

___________ (betamethasone/dexamethasone) accelerate surfactant production in preterm fetuses

Foregut Origins

-pharynx

-esophagus

-stomach

-proximal duodenum

-liver

-pancreas

-biliary tree

Midgut Origins

-distal duodenum

-jejunum

-ileum

-cecum

-appendix

-right colon

Hindgut Origins

-left colon

-rectum

-upper anal canal

congenital anomalies

abnormal rotation, fixation, or partitioning of the GI system leads to what?

GI Functional Development

-Swallowing begins ~10-12 wks

-small intestine capable of peristalsis and glucose transport

-term fetus swallows ~200-760 mL/day of amniotic fluid

-impaired swallowing -> polyhydramnios

Digestive Enzymes

-intrinsic factor present ~11 wks

-Pepsinogen detectable ~16 wks

-preterm neonates may have transient enzyme deficiencies

Meconium

-sterile, odorless, blackish green material composed of desquamated cells, lanugo, vernix, lung secretions, and bile pigments

-dark color from biliverdin

-passage may occur with normal peristalsis or fetal hypoxia

-aspiration -> meconium aspiration syndrome

Liver Development

-arises from hepatic diverticulum

-by 9 weeks: liver is approx 10% of fetal weight

-early gestation: major site of hematopoiesis

-limited ability to conjugate bilirubin -> neonatal jaundice

-glycogen stores increase near term to support neonatal meatbolism

Pancreas Development

-forms from dorsal and ventral pancreatic buds

-insulin present by 9-12 weeks; pancreas responds to fetal hyperglycemia with insulin secretion

-most digestive enzymes present by ~16 weeks, but exocrine function remains limited until after birth

Kidney Development

-Pronephros: regresses by ~2 wks

-Mesonephros: produces urine ~5 wks; regresses by 11-12 wks

-Metanephros: forms permanent kidney (9-12 wks)

9 wks

when does glomerular filtration begin?

ureteric bud + nephrogenic blastema

metanephros forms from the interaction of what?

Fetal Renal Function

-nephron and tubule development

-loop of henle functional ~14 wks

-nephron formation continues until ~36 wks

-fetal kidneys produce hypotonic urine with low electrolyte concentration

Renal Physiology

-renal blood flow and GFR increase with gestational age

-Regulated by RAAS system, SNS, prostaglandins, and atrial natriuretic peptide

Urine Production

-begins ~12 wks

-output increases with gestion: ~650mL/day at term

Eyes

-16-18 wks: eye movements begin

-24 wks: eyebrows and eyelashes are recognizable

-28 wks: pupillary membrane disappears from eyes and eyes open; isolated eye blinking begins

Skin Development

-12 wks: skin and nails develop, scattered hair

-20 wks: brown fat forms, fetal skin becomes less transparent, lanugo, some scalp hair

-24 wks: wrinkled skin, fat deposition begins

-28 wks: thin skin is red and covered w/ vernix caseosa

-32-36 wks: skin surface is red and wrinkled

Limbs Embryologic Origin

-most muscle and bone arise from mesoderm

-Skeletal muscle: develops from myogenic precursor cells in somites

-MYOD and related myogenic regulatory factors active muscle-specific gene transcription

Limb Formation

-limb buds appear ~4 wks gestation

-limb structures develop through patterned growth and differentiation of mesenchyme

Bone Development

-skeleton forms from condensed mesenchyme -> hyaline cartilage models

-Bone formation via endochondral ossification

-OSteoclasts derive from erythro-myeloid progenitors

Late Embryonic Development

-by end of embryonic period (~8 wks)

-primary ossification centers form

-bones begin to harden and mineralize

Fetal Heart Tones

confirms fetal viability and cardiac activity

Fetal Heart Tones Detection Timeline

-TVUS: 5-6 wks

-Transabdominal US: 6-7 wks

-Doppler: 10-12 wks

-Fetoscope/Steth: 18-20 wks

110-160 bpm

what is the normal fetal heart rate

pregnancy loss

if there is an absence of cardiac activity when expected -> eval for what?

fetal distress or arrhythmia

persistent abnormal FHR may indicate what?

Crown-Rump Length

-distance from top of fetal head (Crown) to buttocks (rump)

-most accurate method for gestational age dating in the first trimester

-typically measured 6-13 weeks

Crown-Rump Length - Clinical Use

-helps establish estimated due date

-used to assess early fetal growth and development

Fundal Height

-distance from top of the pubic symphysis to uterine fundus

-most useful between 20-34 weeks

-fundal height in centimeters = gestational age in weeks (+/- 2cm)

Fundal Ht Significance

-fetal growth restriction

-macrosomia

-multiple gestation

-abnormal amniotic fluid volume

Fundal Ht Measurement Considerations

-bladder should be emptied before measurement

-accuracy may be limited by maternal obesity and uterine masses

Human Chorionic Gonadotropin

-hormone produced by trophoblast/placenta

-maintains corpus luteum and progesterone production early in pregnancy

hCG normal pattern

-detectable ~7-9 days after ovulation

-levels double approx every 48 hours in early pregnancy

-peak at ~9-10 wks, then declines