Embryology & Early Fetal Development – Vocabulary Flashcards

Vocabulary flashcards covering key embryologic concepts, genes, developmental milestones, germ layer derivatives, placental structures, morphogenesis errors, teratogens, and twin development from the provided lecture notes.

Apical Ectodermal Ridge (AER)

Thickened ectoderm at the distal end of each limb bud that secretes FGF to promote proximal-to-distal limb growth.

Zone of Polarizing Activity (ZPA)

Mesenchymal region at the posterior limb bud that secretes Sonic Hedgehog to establish anterior-posterior limb patterning.

Fibroblast Growth Factor (FGF) Gene

Gene whose protein product from the AER stimulates mitosis of underlying mesoderm, lengthening limbs.

Wnt-7 Gene

Gene expressed in the AER required for proper dorsal-ventral limb organization.

Sonic Hedgehog (SHH) Gene

Embryonic signaling protein from the ZPA that patterns limbs along the anterior-posterior axis and helps establish CNS midline; mutation → holoprosencephaly.

Homeobox (HOX) Genes

Family of transcription-factor genes that regulate craniocaudal segmental organization; mutations cause limbs or organs in wrong positions.

Proximal-to-Distal Axis (Limb)

Limb development plane governed by the apical ectodermal ridge and FGF signaling.

Anterior-Posterior Axis (Limb)

Limb patterning plane controlled by the ZPA through Sonic Hedgehog signaling.

Dorsal-Ventral Axis (Limb)

Plane of limb development organized by Wnt-7 signaling from the AER.

Chromosome vs. Chromatid

A chromosome consists of two sister chromatids joined by a centromere; chromatids separate during mitosis to form individual chromosomes.

Fertilization

Fusion of haploid sperm (1N,1C) and haploid ovum (1N,1C) forming a zygote (2N,2C).

Morula

Solid ball of cells (no cavity) at ~day 4 post-fertilization.

Blastocyst

Stage formed day 5; fluid-filled cavity (blastocoel) surrounded by trophoblast and containing an inner cell mass.

Trophoblast

Outer blastocyst cells that form cytotrophoblast and syncytiotrophoblast; latter secretes β-hCG and invades endometrium.

Cytotrophoblast

Inner layer of chorionic villi that proliferates and gives rise to syncytiotrophoblast.

Syncytiotrophoblast

Multinucleated outer trophoblast layer that secretes β-hCG and lacks MHC I expression to protect fetus from maternal immunity.

β-hCG

Hormone produced by syncytiotrophoblast beginning ~day 6-10 to maintain corpus luteum progesterone secretion.

Implantation

Attachment of blastocyst to uterine wall (day 6-10) coinciding with onset of β-hCG secretion.

Gastrulation

Process in week 3 converting bilaminar disc into trilaminar embryo (ectoderm, mesoderm, endoderm) via primitive streak.

Bilaminar Disc

Epiblast and hypoblast layers formed in week 2 ("2 weeks = 2 layers").

Primitive Streak

Linear invagination of epiblast that initiates gastrulation and germ-layer formation.

Notochord

Midline mesodermal rod inducing neural plate formation; adult remnant is nucleus pulposus.

Neural Tube

Structure derived from neuroectoderm that forms CNS; closes by week 4.

Week 4 Milestones

Heart begins to beat; upper and lower limb buds form ("4 weeks = 4 limbs, 4 chambers").

Week 8 Milestone

Fetal movements begin ("Gait at week 8").

Week 10 Milestone

External genitalia exhibit male or female characteristics.

Surface Ectoderm

Gives rise to epidermis, lens, oral cavity epithelium, sensory ear organs, olfactory epithelium, anal canal below pectinate line, parotid/sweat/mammary glands.

Neural Tube Derivatives

CNS neurons, neurohypophysis, oligodendrocytes, astrocytes, ependymal cells, pineal gland, retina, spinal cord.

Neural Crest Derivatives

PNS structures, melanocytes, chromaffin cells, odontoblasts, endocardial cushions, skull bones, pia/arachnoid, aorticopulmonary septum etc. (Mnemonic: MMOtEL PPASS).

Mesoderm Derivatives

Muscle, bone, connective tissue, serous membranes, cardiovascular system, spleen, kidneys, adrenal cortex, gonads, dermis.

Endoderm Derivatives

Gut tube epithelium, lungs, liver, pancreas, thyroid/parathyroid, thymus, Eustachian tube, most urethra, lower vagina.

Rathke’s Pouch

Surface ectoderm outpouching that forms the anterior pituitary (adenohypophysis).

Neurohypophysis

Posterior pituitary derived from neuroectoderm of the neural tube.

Agenesis

Complete absence of an organ due to lack of primordial tissue.

Aplasia

Absence of an organ despite presence of primordial tissue foundation.

Hypoplasia

Incomplete organ development with presence of primordial tissue.

Malformation

Intrinsic developmental defect arising during embryonic period (weeks 3-8).

Disruption

Secondary destruction of previously normal tissue by external force (e.g., amniotic band syndrome).

Deformation

Extrinsic mechanical force alters growth after embryonic period (e.g., clubfoot from uterine constraint).

Sequence

Multiple anomalies resulting from a single primary event (e.g., oligohydramnios → Potter sequence).

Teratogen

Any agent causing congenital anomalies when exposure occurs in utero.

"All-or-None" Period

First 2 weeks of development when teratogen exposure causes either death or complete recovery.

Organogenesis Window

Weeks 2-8 when teratogens produce structural defects.

Category X Drugs

Medications contraindicated in pregnancy due to proven teratogenicity; risks outweigh benefits.

Fetal Hydantoin Syndrome

Pattern of growth deficiency and craniofacial anomalies due to maternal phenytoin exposure.

Warfarin Embryopathy

Bone cartilage defects, stippled epiphyses, nasal hypoplasia from in-utero warfarin exposure.

Fetal Alcohol Syndrome

Spectrum of facial dysmorphism, growth retardation, CNS dysfunction due to prenatal alcohol; mechanism involves impaired cell migration.

Caudal Regression Syndrome

Range of sacral agenesis anomalies linked to maternal diabetes; may include sirenomelia.

Maternal Phenylketonuria

High maternal phenylalanine acts as teratogen causing microcephaly, intellectual disability, congenital heart defects.

Neonatal Abstinence Syndrome

Withdrawal symptoms (irritability, high-pitched cry, tremors) in newborns of opioid-using mothers.

Decidua

Endometrial lining modified by pregnancy; includes basalis, capsularis, parietalis.

Chorion

Membrane derived from trophoblast that surrounds amnion and contributes to placenta.

Amnion

Inner fetal membrane containing amniotic fluid that cushions the embryo.

Chorionic Plate

Fetal side of placenta containing chorionic villi (cytotrophoblast + syncytiotrophoblast).

Decidua Basalis

Maternal side of placenta; endometrial tissue with maternal blood in lacunae.

Wharton Jelly

Mucoid connective tissue of the umbilical cord rich in mucopolysaccharides.

Umbilical Arteries

Two vessels returning deoxygenated blood from fetal internal iliac arteries to placenta.

Umbilical Vein

Single vessel delivering oxygenated blood from placenta to fetus via liver or ductus venosus.

Allantois

Embryonic diverticulum from gut; walls form umbilical vessels and lumen becomes urachus.

Urachus

Fibrous remnant of allantoic duct connecting fetal bladder to umbilicus; failure to obliterate causes patent urachus, cyst, or diverticulum.

Vitelline Duct

Connection between yolk sac and midgut that should obliterate in week 7; persistence leads to fistula or Meckel diverticulum.

Meckel Diverticulum

True ileal diverticulum from partial vitelline duct closure; may contain ectopic gastric tissue causing bleeding.

Dizygotic Twins

Fraternal twins from two separate eggs and sperm; always have separate placentas and amniotic sacs.

Monozygotic Twins

Identical twins from one zygote that splits; chorionicity/amnionicity depend on cleavage timing.

SCAB Mnemonic

0-4d Separate chorion & amnion; 4-8d shared Chorion; 8-12d shared Amnion; 13+d shared Body (conjoined).

Placental Immune Protection

Fetal trophoblasts express low MHC I and secrete immunosuppressive factors to avoid maternal rejection.