Embryology

How the body forms, week by week: gametes, fertilization, the germ layers, the development of every organ and the pharyngeal arches. Each malformation with its embryological cause and each teratogen with its critical period, linked to the 3D anatomy and DurgiPedia's conditions.

150 pages

• General embryology: weeks 1 to 8
• Fetal period, placenta and twins
• Teratology and prenatal diagnosis
• Heart and vessels
• Head and neck: pharyngeal arches
• Respiratory system, diaphragm and body cavities
• Digestive system
• Urogenital system
• Nervous system and senses
• Skeleton, muscles, limbs and skin
• A to Z

General embryology: weeks 1 to 8

From gametes to embryo: fertilization, implantation, gastrulation, neurulation and the germ layers.

• Gametogenesis: spermatogenesis and oogenesisHow eggs and sperm are made: cells with half the chromosomes, ready to join at fertilization.Process
• Meiosis and nondisjunction: where trisomies come fromWhen chromosomes fail to separate while eggs or sperm form, the embryo gets one too many or one too few.Process
• Ovarian cycle, ovulation and the corpus luteumEach month a follicle matures, releases an egg and leaves a corpus luteum that prepares the uterus for a possible pregnancy.Process
• Fertilization: capacitation, acrosome reaction and zona reactionSperm meets egg in the uterine tube: only one gets in, and a new individual with 46 chromosomes begins.Process
• Week 1: cleavage, morula, blastocyst and implantationIn week one the zygote divides on its way down the tube, reaches the uterus as a blastocyst and starts to implant.Stage
• Week 2: the bilaminar disc ('the week of twos')In week two the embryo finishes implanting and organizes into two layers, with two cavities around it.Stage
• Trophoblast and hCG; gestational trophoblastic diseaseThe trophoblast is the outer layer of the blastocyst that invades the uterus, builds the placenta and makes the pregnancy hormone.Embryonic structure
• Week 3: gastrulation and the trilaminar discIn week three the embryo goes from two layers to three: ectoderm, mesoderm and endoderm, which build the whole body.Process
• Primitive streak and notochord (nucleus pulposus, sacrococcygeal teratoma)The notochord is a rod of cells in the embryo's midline that organizes the nervous system and spine; in adults it remains in the center of the discs.Embryonic structure
• Germ-layer derivatives: ectoderm, mesoderm, endodermEvery organ comes from one of the embryo's three layers; knowing which one explains many birth defects and is a fixture on exams.Topic
• Neurulation and closure of the neural tubeIn weeks 3 and 4 a plate of cells rolls up into the tube that becomes brain and spinal cord; if it fails to close, spina bifida results.Process
• Neural crest cells and their derivativesCells born at the edges of the neural tube that travel the whole body to build nerves, pigment, facial bones and part of the heart.Embryonic structure
• Paraxial mesoderm and somites: sclerotome, myotome, dermatomeBlocks of mesoderm on each side of the neural tube that build the vertebrae, trunk muscles and the dermis of the back.Embryonic structure
• Weeks 3–8: the embryonic period and organogenesisBetween weeks three and eight every organ forms; it is when the embryo is most vulnerable to teratogens.Stage
• Embryonic folding: head, tail and lateral foldsIn week four the flat disc bends over on itself and becomes a cylinder-shaped body with the gut inside.Process
• Molecular regulation: Shh, Wnt, FGF, BMP, Hox genes and retinoic acidA handful of chemical signals tells each embryonic cell where it is and what to become.Topic
• Gestational age vs fertilization age: how pregnancy is datedDoctors count pregnancy from the last period; embryologists count from fertilization. The difference is about two weeks.Topic

Fetal period, placenta and twins

Fetal growth, the placenta, the cord, amniotic fluid and the fetal circulation.

• Fetal period: month 3 to birthFrom week 9 after fertilization the embryo is called a fetus: its organs are already laid down, and from now on it mainly grows and matures.Stage
• Placenta: structure, circulation and functionsThe placenta links the fetus to the mother: it swaps oxygen and nutrients, clears waste and makes the hormones of pregnancy.Embryonic structure
• Abnormal placentation: previa and the accreta spectrumThe placenta can implant too low (previa) or too deep (accreta); both cause serious bleeding in pregnancy or at delivery.Topic
• Umbilical cord: two arteries, one veinThe cord joins the fetus to the placenta. It carries two arteries and one vein wrapped in a jelly that protects them.Embryonic structure
• Fetal circulation and the changes at birthBefore birth, blood picks up oxygen in the placenta, not the lungs. Three shortcuts steer blood past the liver and lungs, and they close at birth.Process
• Fetal membranes: amnion, chorion, yolk sac and allantoisFour structures form around the embryo: the amnion with its fluid, the chorion that builds the placenta, the yolk sac and the allantois.Embryonic structure
• Amniotic fluid: polyhydramnios and oligohydramniosThe fetus lives in amniotic fluid. From mid-pregnancy that fluid is mostly fetal urine, which the fetus swallows again.Topic
• Twins: dizygotic and monozygotic, chorionicity, twin–twin transfusionTwins come from two eggs or from one embryo that splits. When it splits decides whether they share a placenta and a sac, and that sets the risk.Topic
• Fetal lung, kidney and brain maturation: what preterm birth interruptsLate in pregnancy the lungs, brain, kidneys, gut and eyes mature. Being born early cuts this short and explains the problems of preterm babies.Topic
• Fetal growth and growth restrictionThe fetus grows on what the placenta delivers and on the insulin it makes itself. If the placenta fails, growth slows.Topic

Teratology and prenatal diagnosis

Why timing matters: drugs, infections and exposures that harm the embryo, and how anomalies are detected.

• Principles of teratology: critical periods, dose and genotypeWhy the same drug, virus or exposure can do nothing, cause a miscarriage or cause a birth defect, depending on the week of pregnancy.Topic
• Malformation, disruption, deformation, sequence, syndrome, associationThe words doctors use to classify a birth defect by how it arose: they are not synonyms and they change the outlook.Topic
• Prenatal diagnosis: ultrasound, serum screening, cell-free DNA, CVS, amniocentesisThe pregnancy tests that estimate the risk of an anomaly (screening) and those that confirm it (diagnosis), with their weeks.Topic
• Folic acid and neural tube defect preventionThe neural tube closes in the first month, before many women know they are pregnant: that is why folic acid is started before conception.Topic
• TORCH infections: what each agent does to the embryo and fetusMaternal infections that cross the placenta or pass during birth and can harm the baby's brain, eyes, ears and heart.Teratogen
• Congenital Zika syndromeZika infection in pregnancy can damage the baby's brain and eyes; Colombia had an epidemic in 2015–2016.Teratogen
• Congenital rubella syndromeRubella early in pregnancy can leave the baby with cataracts, deafness and heart defects; vaccination before pregnancy prevents it.Teratogen
• Alcohol: fetal alcohol spectrum disorderAlcohol in pregnancy can affect the baby's brain, face and growth; no amount or time is known to be safe.Teratogen
• Isotretinoin and other retinoidsVitamin A–derived medicines for severe acne and psoriasis cause serious defects of the embryo's face, heart and brain.Teratogen
• Valproate, carbamazepine and phenytoinSome medicines for epilepsy and bipolar disorder raise the risk of neural tube, facial and heart defects; the plan is made before pregnancy.Teratogen
• Warfarin embryopathyWarfarin, a blood thinner, can disturb the embryo's bones and nose and, later, cause bleeding in the fetal brain.Teratogen
• ACE inhibitors and ARBs: fetal kidney damageSome blood pressure medicines damage the fetal kidneys in the second half of pregnancy and leave too little amniotic fluid.Teratogen
• Thalidomide and limb reduction defectsA 1950s sedative that left thousands of babies with very short arms and legs and changed drug safety for good.Teratogen
• Lithium and Ebstein anomalyLithium, used for bipolar disorder, is linked to heart defects, mainly of the tricuspid valve; the risk is real but small.Teratogen
• Tetracyclines and aminoglycosidesTwo antibiotic groups with known fetal effects: tetracyclines stain the teeth and aminoglycosides can damage hearing.Teratogen
• Methotrexate and misoprostolTwo medicines used to end pregnancies or treat other diseases; if the pregnancy continues, they can leave skull, face and limb defects.Teratogen
• Maternal diabetes and obesity as teratogensHigh maternal sugar in the first weeks can cause heart, spine and neural tube defects; later it makes the baby grow too big.Teratogen
• Androgens, diethylstilbestrol and other hormonesSex hormones that reach the fetus can change the shape of the genitals and reproductive tract; the historical case is DES.Teratogen
• Ionizing radiation, tobacco and cocaineThree exposures with different effects: radiation depends on the week and the dose; tobacco and cocaine affect the placenta and growth.Teratogen

Heart and vessels

Heart tube, septa, outflow tract, aortic arches and congenital heart disease.

• Heart tube: formation and loopingThe heart starts as a simple tube that beats from week 4, then folds on itself to put its chambers in place.Process
• Atrial septation: septum primum, septum secundum and foramen ovaleTwo overlapping walls divide the atrium and leave a door, the foramen ovale, so blood can cross to the left before birth.Process
• Ventricular septation and the endocardial cushionsA muscular wall rises from the tip of the heart and soft tissue at the center finishes the job; that center also makes the valves.Process
• Outflow tract: the spiral aorticopulmonary septum and neural crestA septum that twists like a spiral staircase splits the single outflow vessel into the aorta and the pulmonary trunk.Process
• Embryonic heart structures and their adult derivatives (the table to know)Each part of the heart tube and fetal circulation has a fixed fate in the adult heart; this table gathers them.Topic
• Aortic arch arteries and their derivativesFive pairs of arteries ring the embryo's pharynx; some stay on as carotids, aorta and pulmonary arteries, others vanish.Embryonic structure
• Venous system: vitelline, umbilical and cardinal veinsThree pairs of veins bring blood to the embryo's heart; in the end almost everything shifts to the right and forms the venae cavae.Embryonic structure
• Ventricular septal defectA hole in the wall between the ventricles lets blood pass from the left side to the right. It is the most common congenital heart defect.Malformation
• Atrial septal defect and patent foramen ovaleA hole between the atria lets blood pass from left to right; it often causes no symptoms until adulthood.Malformation
• Tetralogy of FallotOne error in the outflow tract gives four findings: pulmonary obstruction, a ventricular hole, an overriding aorta and a thick right ventricle.Malformation
• Transposition of the great arteriesThe aorta arises from the right ventricle and the pulmonary trunk from the left: oxygen-poor blood goes back to the body and the baby is born very blue.Malformation
• Patent ductus arteriosusThe vessel that links the pulmonary artery to the aorta in the fetus fails to close at birth, and blood flows from the aorta to the lungs.Malformation
• Coarctation of the aortaA narrowing of the aorta near the ductus arteriosus means the arms get more pressure than the legs.Malformation
• Persistent truncus arteriosusA single vessel leaves the heart instead of two, because the wall that separates the aorta from the pulmonary artery never formed.Malformation
• Atrioventricular septal defect (endocardial cushion defect)The center of the heart never closes: a hole links all four chambers and one common valve replaces the mitral and tricuspid.Malformation
• Ebstein anomalyThe tricuspid valve is set too far toward the tip of the heart: the right atrium grows and the right ventricle stays small.Malformation
• Hypoplastic left heart syndromeThe left side of the heart is too small to pump to the body; the baby depends on the ductus arteriosus to survive.Malformation
• Vascular rings and aberrant right subclavian arteryArch vessels that should vanish persist and wrap around the trachea and esophagus, which can cause noisy breathing or trouble swallowing.Malformation

Head and neck: pharyngeal arches

Pharyngeal arches, pouches and clefts and their derivatives, the face, palate, tongue and thyroid.

• The pharyngeal apparatus: arches, pouches, clefts and membranesIn week 4 the embryo's neck forms a row of bars with grooves between them. The face, ear, larynx and several glands come from them.Embryonic structure
• Pharyngeal arches: nerve, muscles, skeleton and artery (the table)The most-asked table in embryology: which nerve, which muscles, which bones and which artery come from each pharyngeal arch.Topic
• First pharyngeal arch (mandibular arch) and the trigeminal nerveThe first arch builds the jaw, the maxilla, two ear ossicles and the chewing muscles. Its nerve is the trigeminal.Embryonic structure
• Second pharyngeal arch (hyoid arch) and the facial nerveThe second arch gives the muscles of facial expression, the stapes and part of the hyoid. Its nerve is the facial nerve.Embryonic structure
• Third pharyngeal arch and the glossopharyngeal nerveThe third arch is the simplest: one muscle (stylopharyngeus), the lower hyoid, the glossopharyngeal nerve and the carotid arteries.Embryonic structure
• Fourth and sixth pharyngeal arches: the larynx and the vagus nerveArches 4 and 6 build the laryngeal cartilages and muscles. The vagus supplies them through two branches: superior and recurrent laryngeal.Embryonic structure
• Pharyngeal pouches: tonsil, thymus, parathyroids and C cellsPouches are inner pockets of the embryo's pharynx. They form the middle ear, tonsil, thymus, parathyroids and part of the thyroid.Embryonic structure
• Pharyngeal clefts and membranes: external ear and cervical sinusClefts are outer grooves between the arches. Only the first one stays, as the ear canal. The others close over.Embryonic structure
• Thyroid gland: from the foramen cecum to the neckThe thyroid starts at the base of the tongue and travels down the neck to sit in front of the trachea. On the way it can leave cysts or misplaced tissue.Embryonic structure
• Tongue: development and innervationThe tongue is built from swellings of several arches plus muscle that migrates in from the back of the head. That is why it has five different nerves.Embryonic structure
• The face: five prominencesThe face forms between weeks 4 and 10 as five swellings fuse around the primitive mouth. If they fail to join, clefts appear.Process
• Palate: primary and secondary palateThe roof of the mouth forms from a front triangle and two side shelves that rise and join over the tongue between weeks 6 and 10.Process
• Cleft lip and cleft palateAn opening in the upper lip, the roof of the mouth or both, because two parts of the face did not join before birth.Malformation
• 22q11.2 deletion (DiGeorge): the third and fourth pouchesA small piece of chromosome 22 is missing. It affects the thymus, parathyroids, heart and palate, because they all depend on the same embryonic cells.Malformation
• Thyroglossal duct cyst and ectopic thyroidA lump in the middle of the neck, near the hyoid bone, that rises on swallowing. It is left over from the path the thyroid took on its way down.Malformation
• Branchial cleft cysts, sinuses and fistulasA lump or a small opening on the side of the neck, in front of the sternocleidomastoid muscle. It is left over from the grooves of the embryo's neck.Malformation
• First-arch syndromes: Treacher Collins and Pierre Robin sequenceDefects of the jaw, cheekbones and ears because the first arch did not grow properly. They can make it hard for a newborn to breathe.Malformation

Respiratory system, diaphragm and body cavities

Lung stages, surfactant, the diaphragm and the body cavities.

• Lung development: the five stagesThe lung starts as a bud from the foregut in week 4 and matures in five stages that go on long after birth.Process
• Surfactant and fetal lung maturitySurfactant is a film of fats and proteins that stops the air sacs collapsing when we breathe out. Without it, a preterm baby cannot breathe well.Topic
• Esophageal atresia and tracheoesophageal fistulaThe esophagus ends in a blind pouch and often connects to the windpipe. The newborn drools, chokes and turns blue when fed.Malformation
• Diaphragm: its four components and its innervationThe diaphragm is built from four pieces that form in the embryo's neck and then move down. That is why the phrenic nerve, from C3 to C5, supplies it.Embryonic structure
• Congenital diaphragmatic herniaThrough a hole in the diaphragm, bowel and organs rise into the fetal chest and squash the lung, which fails to grow. At birth the baby struggles to breathe.Malformation
• Intraembryonic coelom: pericardial, pleural and peritoneal cavitiesThe embryo has one large cavity that later splits into three: one for the heart, two for the lungs and one for the abdomen.Embryonic structure
• Pulmonary hypoplasiaThe lungs end up small, with few branches and air sacs, because something took away their room or their fluid during pregnancy.Malformation
• Congenital pulmonary airway malformation, sequestration and bronchogenic cystThese are congenital lung masses: cysts or lung tissue that does not work. Many show up on pregnancy ultrasound; others cause infections later.Malformation

Digestive system

Foregut, midgut and hindgut, gut rotation and the malformations the exams ask about.

• Primitive gut: foregut, midgut and hindgut and their arteriesIn week 4 the embryo folds and pinches off part of the yolk sac: that becomes the gut tube, split into three segments, each with its own artery.Embryonic structure
• Foregut: esophagus, stomach rotation and duodenumThe esophagus splits from the trachea, the stomach turns and the duodenum curls into a C: three steps behind several newborn malformations.Process
• Liver, gallbladder and pancreasThe liver and pancreas bud from the end of the foregut; the pancreas is built from two buds that meet when the duodenum turns.Embryonic structure
• Midgut: physiological herniation and rotationThe gut grows so fast that it moves out into the umbilical cord, turns around its artery and returns to the abdomen around week 10.Process
• Hindgut and cloaca: the urorectal septum and the pectinate lineThe end of the gut opens into the cloaca; a septum splits it into bladder in front and rectum behind. The pectinate line marks where two origins meet.Embryonic structure
• Mesenteries, the omental bursa and the spleenThe peritoneal sheets that hang the gut fold as the organs turn, forming the omenta and the omental bursa; the spleen grows between them.Embryonic structure
• Hypertrophic pyloric stenosisThe pyloric muscle thickens in the first weeks of life and closes the stomach outlet: the baby vomits forcefully and is still hungry.Malformation
• Duodenal atresia and stenosisThe duodenum stays closed or very narrow because it never regains its lumen in embryonic life; the newborn vomits within hours.Malformation
• Malrotation and midgut volvulusIf the gut does not finish turning, it is poorly anchored and can twist on its artery: green vomit in a baby is an emergency.Malformation
• Meckel diverticulum and other vitelline duct remnantsA leftover of the duct that joined the gut to the yolk sac remains as a small pouch on the ileum; it can bleed without pain.Malformation
• Omphalocele and gastroschisisTwo abdominal wall defects with bowel outside: omphalocele sits at the navel and is covered; gastroschisis sits beside it and is bare.Malformation
• Hirschsprung disease (congenital aganglionic megacolon)A final stretch of the colon is born without nerve cells and cannot relax: the newborn does not pass meconium and the bowel above it swells.Malformation
• Anorectal malformations (imperforate anus)The rectum does not open to the outside, or opens in the wrong place such as the urethra or vestibule; it is found on the newborn exam.Malformation
• Annular pancreas and pancreas divisumTwo errors in how the pancreatic buds join: in one, tissue rings the duodenum; in the other, two ducts never connect.Malformation
• Biliary atresiaThe ducts that carry bile out of the liver inflame and close in the first weeks of life: jaundice that does not fade and very pale stools.Malformation
• Jejunal and ileal atresiaA stretch of small bowel is missing or closed because it lost its blood supply before birth; the newborn vomits bile and becomes distended.Malformation

Urogenital system

Kidney, bladder, gonads, genital ducts and external genitalia, with their anomalies.

• Kidney: pronephros, mesonephros, metanephros and ascentThe embryo builds a kidney three times in a row; only the third one, the metanephros, becomes the permanent kidney.Process
• Bladder, urachus and urethraThe bladder and urethra come from the front part of the cloaca; the urachus was a tube joining bladder and navel and closes before birth.Embryonic structure
• Sex determination: SRY, anti-Müllerian hormone and testosteroneOne gene on the Y chromosome decides whether the gonad becomes a testis; three testicular hormones then shape the rest of the genital tract.Process
• Gonads and the descent of the testisTestes and ovaries start high in the abdomen; the testis travels down to the scrotum guided by a cord called the gubernaculum.Process
• Genital ducts: Wolffian (mesonephric) and Müllerian (paramesonephric)Every embryo has two pairs of genital ducts; males keep one pair, females the other, and the lost pair leaves small remnants.Embryonic structure
• External genitalia and their male–female homologuesExternal genitalia start out the same in both sexes; DHT turns them male, and without it they develop as female.Topic
• Renal agenesis and the Potter sequenceOne or both kidneys are missing. Without both, there is almost no amniotic fluid and the lungs fail to develop.Malformation
• Horseshoe kidney and renal ectopiaThe two kidneys join at the bottom and sit lower than normal; sometimes one kidney never rises and stays in the pelvis.Malformation
• Duplicated ureter, UPJ obstruction and posterior urethral valvesThree faults of the tube that carries urine: a double ureter, a narrowing where it leaves the kidney, and folds that block a boy's urethra.Malformation
• Hypospadias and epispadiasThe urethra does not end at the tip of the penis: in hypospadias it opens underneath, in epispadias on top.Malformation
• CryptorchidismOne or both testes did not come down into the scrotum before birth. It is the commonest genital malformation in boys.Malformation
• Differences of sex development: CAH, AIS and 5α-reductaseConditions in which chromosomes, gonads and genitalia do not follow the typical pattern. They have specific causes that can be investigated.Malformation
• Uterine anomalies: bicornuate, septate, didelphys, Müllerian agenesisThe uterus forms when two tubes join; if they never join, join halfway or never form, the uterus ends up double, divided or absent.Malformation
• Urachal anomalies: patent urachus, cyst and sinusThe urachus linked the bladder to the navel and should close; if it stays open in whole or part, urine can leak or a cyst can form.Malformation
• Processus vaginalis: hydrocele and indirect inguinal herniaA pouch of peritoneum goes ahead of the testis into the scrotum and should close. If it stays open, fluid (hydrocele) or bowel (hernia) can pass.Malformation
• Multicystic dysplastic kidney and polycystic kidney diseaseKidneys full of cysts for two different reasons: a kidney that never formed properly, or a genetic defect of the tubule cells.Malformation
• Bladder exstrophyThe bladder is born open and exposed on the lower abdomen, because the wall meant to cover it never formed.Malformation

Nervous system and senses

Brain vesicles, spinal cord, neural tube defects, eye, ear and pituitary.

• Brain vesicles: from three to five and what each becomesThe head end of the neural tube swells into three vesicles that then split into five; the whole brain grows from them.Embryonic structure
• Spinal cord: alar and basal plates and the ascent of the cordThe tail part of the neural tube forms the spinal cord: sensory neurons sit at the back, motor neurons at the front, and its end 'rises' as the spine grows.Embryonic structure
• Ventricles and cerebrospinal fluidThe ventricles are what is left of the hollow neural tube; the choroid plexuses inside them make the fluid that bathes the brain and cord.Embryonic structure
• Neural tube defects: anencephaly, spina bifida and encephaloceleIf the neural tube does not close in the first month, the brain or spinal cord is left exposed; folic acid prevents many of these defects.Malformation
• Chiari malformationsIn Chiari, part of the cerebellum (and sometimes the brainstem) slips down through the opening at the base of the skull and can block CSF.Malformation
• Congenital hydrocephalus: aqueductal stenosisFluid cannot leave the brain's ventricles, so it builds up and enlarges the baby's head, whose sutures are still open.Malformation
• Dandy–Walker malformationThe middle of the cerebellum (the vermis) is missing or very small and the fourth ventricle becomes a big cyst at the back of the skull.Malformation
• HoloprosencephalyThe forebrain does not divide properly into two halves; the midline of the face is affected too, from a single eye to a single central incisor.Malformation
• MicrocephalyThe baby's head is much smaller than expected because the brain did not grow well; Zika put it in the news in Colombia in 2016.Malformation
• Eye: optic vesicle, optic cup and lensThe eye starts as an outgrowth of the brain that touches the embryo's skin and makes it form the lens.Process
• Coloboma, congenital cataract and congenital glaucomaThree eye anomalies with an embryonic origin: a slit that did not close, a cloudy lens and a badly formed drain of the eye.Malformation
• Ear: inner, middle and external earThe three parts of the ear come from different places in the embryo: a skin placode, the first pharyngeal pouch and the first two arches.Process
• Congenital hearing loss, microtia and preauricular pitsSome babies are born not hearing well, with a small ear or with a tiny pit in front of the ear; all of them deserve a hearing test.Malformation
• Pituitary gland: Rathke pouch and the neurohypophysisThe pituitary is built from two pieces: one that grows up from the roof of the mouth and one that grows down from the brain.Embryonic structure
• Adrenal gland: cortex from mesoderm, medulla from neural crestThe adrenal is two glands in one: the cortex, from mesoderm, makes steroids; the medulla, from neural crest, makes adrenaline.Embryonic structure

Skeleton, muscles, limbs and skin

Vertebrae, skull, limbs, muscles and skin, with hip dysplasia and clubfoot.

• Vertebrae: resegmentation of the sclerotomesEach vertebra is built from half of one embryonic block (somite) and half of the next, which is why muscles and nerves end up where they do.Process
• Skull: neurocranium, viscerocranium, sutures and fontanellesA baby's skull is plates of bone joined by flexible seams (sutures) and soft spots (fontanelles) that let the brain grow.Embryonic structure
• Limb development: the apical ectodermal ridge, ZPA and limb rotationArms and legs sprout as buds at the end of the first month; three signalling centres decide their length, the order of the digits and which side is the palm.Process
• Muscles: somites, epaxial and hypaxialAlmost all skeletal muscle comes from the somites: the back part makes the back muscles, the front part makes the body wall and the limbs.Embryonic structure
• Skin, hair, nails and the mammary glandsThe outer layer of skin, hair, nails and glands come from ectoderm; the deep layer from mesoderm; the colour from the neural crest.Embryonic structure
• TeethEach tooth comes from a dialogue between the lining of the mouth, which makes enamel, and neural crest cells, which make dentin and pulp.Embryonic structure
• Limb defects: amelia, meromelia, polydactyly and syndactylyA limb can be missing completely or in part, have extra digits or joined digits; nearly all of it is decided between weeks 4 and 8.Malformation
• Developmental dysplasia of the hipThe head of the thigh bone does not sit well in its socket; if it is not caught in the first months, the hip grows wrongly.Malformation
• Clubfoot (talipes equinovarus)The foot is born turned inwards and downwards; with serial casts from the first weeks it can almost always be corrected.Malformation
• CraniosynostosisOne or more seams of a baby's skull close too early, and the head grows into an abnormal shape.Malformation
• Achondroplasia (FGFR3)The most common cause of short stature with short limbs: an overactive gene slows the growth of cartilage in the long bones.Malformation
• Hemivertebra, congenital scoliosis and Klippel–FeilIf a vertebra forms only halfway or two vertebrae stay stuck together, the spine is curved from birth.Malformation
• Poland sequence and prune belly syndromeTwo rare muscle defects: in Poland part of the chest muscle on one side is missing; in prune belly the belly wall lacks muscle.Malformation

A to Z

• 22q11.2 deletion (DiGeorge): the third and fourth pouchesHead and neck: pharyngeal arches · Malformation
• Abnormal placentation: previa and the accreta spectrumFetal period, placenta and twins · Topic
• ACE inhibitors and ARBs: fetal kidney damageTeratology and prenatal diagnosis · Teratogen
• Achondroplasia (FGFR3)Skeleton, muscles, limbs and skin · Malformation
• Adrenal gland: cortex from mesoderm, medulla from neural crestNervous system and senses · Embryonic structure
• Alcohol: fetal alcohol spectrum disorderTeratology and prenatal diagnosis · Teratogen
• Amniotic fluid: polyhydramnios and oligohydramniosFetal period, placenta and twins · Topic
• Androgens, diethylstilbestrol and other hormonesTeratology and prenatal diagnosis · Teratogen
• Annular pancreas and pancreas divisumDigestive system · Malformation
• Anorectal malformations (imperforate anus)Digestive system · Malformation
• Aortic arch arteries and their derivativesHeart and vessels · Embryonic structure
• Atrial septal defect and patent foramen ovaleHeart and vessels · Malformation
• Atrial septation: septum primum, septum secundum and foramen ovaleHeart and vessels · Process
• Atrioventricular septal defect (endocardial cushion defect)Heart and vessels · Malformation
• Biliary atresiaDigestive system · Malformation
• Bladder exstrophyUrogenital system · Malformation
• Bladder, urachus and urethraUrogenital system · Embryonic structure
• Brain vesicles: from three to five and what each becomesNervous system and senses · Embryonic structure
• Branchial cleft cysts, sinuses and fistulasHead and neck: pharyngeal arches · Malformation
• Chiari malformationsNervous system and senses · Malformation
• Cleft lip and cleft palateHead and neck: pharyngeal arches · Malformation
• Clubfoot (talipes equinovarus)Skeleton, muscles, limbs and skin · Malformation
• Coarctation of the aortaHeart and vessels · Malformation
• Coloboma, congenital cataract and congenital glaucomaNervous system and senses · Malformation
• Congenital diaphragmatic herniaRespiratory system, diaphragm and body cavities · Malformation
• Congenital hearing loss, microtia and preauricular pitsNervous system and senses · Malformation
• Congenital hydrocephalus: aqueductal stenosisNervous system and senses · Malformation
• Congenital pulmonary airway malformation, sequestration and bronchogenic cystRespiratory system, diaphragm and body cavities · Malformation
• Congenital rubella syndromeTeratology and prenatal diagnosis · Teratogen
• Congenital Zika syndromeTeratology and prenatal diagnosis · Teratogen
• CraniosynostosisSkeleton, muscles, limbs and skin · Malformation
• CryptorchidismUrogenital system · Malformation
• Dandy–Walker malformationNervous system and senses · Malformation
• Developmental dysplasia of the hipSkeleton, muscles, limbs and skin · Malformation
• Diaphragm: its four components and its innervationRespiratory system, diaphragm and body cavities · Embryonic structure
• Differences of sex development: CAH, AIS and 5α-reductaseUrogenital system · Malformation
• Duodenal atresia and stenosisDigestive system · Malformation
• Duplicated ureter, UPJ obstruction and posterior urethral valvesUrogenital system · Malformation
• Ear: inner, middle and external earNervous system and senses · Process
• Ebstein anomalyHeart and vessels · Malformation
• Embryonic folding: head, tail and lateral foldsGeneral embryology: weeks 1 to 8 · Process
• Embryonic heart structures and their adult derivatives (the table to know)Heart and vessels · Topic
• Esophageal atresia and tracheoesophageal fistulaRespiratory system, diaphragm and body cavities · Malformation
• External genitalia and their male–female homologuesUrogenital system · Topic
• Eye: optic vesicle, optic cup and lensNervous system and senses · Process
• Fertilization: capacitation, acrosome reaction and zona reactionGeneral embryology: weeks 1 to 8 · Process
• Fetal circulation and the changes at birthFetal period, placenta and twins · Process
• Fetal growth and growth restrictionFetal period, placenta and twins · Topic
• Fetal lung, kidney and brain maturation: what preterm birth interruptsFetal period, placenta and twins · Topic
• Fetal membranes: amnion, chorion, yolk sac and allantoisFetal period, placenta and twins · Embryonic structure
• Fetal period: month 3 to birthFetal period, placenta and twins · Stage
• First pharyngeal arch (mandibular arch) and the trigeminal nerveHead and neck: pharyngeal arches · Embryonic structure
• First-arch syndromes: Treacher Collins and Pierre Robin sequenceHead and neck: pharyngeal arches · Malformation
• Folic acid and neural tube defect preventionTeratology and prenatal diagnosis · Topic
• Foregut: esophagus, stomach rotation and duodenumDigestive system · Process
• Fourth and sixth pharyngeal arches: the larynx and the vagus nerveHead and neck: pharyngeal arches · Embryonic structure
• Gametogenesis: spermatogenesis and oogenesisGeneral embryology: weeks 1 to 8 · Process
• Genital ducts: Wolffian (mesonephric) and Müllerian (paramesonephric)Urogenital system · Embryonic structure
• Germ-layer derivatives: ectoderm, mesoderm, endodermGeneral embryology: weeks 1 to 8 · Topic
• Gestational age vs fertilization age: how pregnancy is datedGeneral embryology: weeks 1 to 8 · Topic
• Gonads and the descent of the testisUrogenital system · Process
• Heart tube: formation and loopingHeart and vessels · Process
• Hemivertebra, congenital scoliosis and Klippel–FeilSkeleton, muscles, limbs and skin · Malformation
• Hindgut and cloaca: the urorectal septum and the pectinate lineDigestive system · Embryonic structure
• Hirschsprung disease (congenital aganglionic megacolon)Digestive system · Malformation
• HoloprosencephalyNervous system and senses · Malformation
• Horseshoe kidney and renal ectopiaUrogenital system · Malformation
• Hypertrophic pyloric stenosisDigestive system · Malformation
• Hypoplastic left heart syndromeHeart and vessels · Malformation
• Hypospadias and epispadiasUrogenital system · Malformation
• Intraembryonic coelom: pericardial, pleural and peritoneal cavitiesRespiratory system, diaphragm and body cavities · Embryonic structure
• Ionizing radiation, tobacco and cocaineTeratology and prenatal diagnosis · Teratogen
• Isotretinoin and other retinoidsTeratology and prenatal diagnosis · Teratogen
• Jejunal and ileal atresiaDigestive system · Malformation
• Kidney: pronephros, mesonephros, metanephros and ascentUrogenital system · Process
• Limb defects: amelia, meromelia, polydactyly and syndactylySkeleton, muscles, limbs and skin · Malformation
• Limb development: the apical ectodermal ridge, ZPA and limb rotationSkeleton, muscles, limbs and skin · Process
• Lithium and Ebstein anomalyTeratology and prenatal diagnosis · Teratogen
• Liver, gallbladder and pancreasDigestive system · Embryonic structure
• Lung development: the five stagesRespiratory system, diaphragm and body cavities · Process
• Malformation, disruption, deformation, sequence, syndrome, associationTeratology and prenatal diagnosis · Topic
• Malrotation and midgut volvulusDigestive system · Malformation
• Maternal diabetes and obesity as teratogensTeratology and prenatal diagnosis · Teratogen
• Meckel diverticulum and other vitelline duct remnantsDigestive system · Malformation
• Meiosis and nondisjunction: where trisomies come fromGeneral embryology: weeks 1 to 8 · Process
• Mesenteries, the omental bursa and the spleenDigestive system · Embryonic structure
• Methotrexate and misoprostolTeratology and prenatal diagnosis · Teratogen
• MicrocephalyNervous system and senses · Malformation
• Midgut: physiological herniation and rotationDigestive system · Process
• Molecular regulation: Shh, Wnt, FGF, BMP, Hox genes and retinoic acidGeneral embryology: weeks 1 to 8 · Topic
• Multicystic dysplastic kidney and polycystic kidney diseaseUrogenital system · Malformation
• Muscles: somites, epaxial and hypaxialSkeleton, muscles, limbs and skin · Embryonic structure
• Neural crest cells and their derivativesGeneral embryology: weeks 1 to 8 · Embryonic structure
• Neural tube defects: anencephaly, spina bifida and encephaloceleNervous system and senses · Malformation
• Neurulation and closure of the neural tubeGeneral embryology: weeks 1 to 8 · Process
• Omphalocele and gastroschisisDigestive system · Malformation
• Outflow tract: the spiral aorticopulmonary septum and neural crestHeart and vessels · Process
• Ovarian cycle, ovulation and the corpus luteumGeneral embryology: weeks 1 to 8 · Process
• Palate: primary and secondary palateHead and neck: pharyngeal arches · Process
• Paraxial mesoderm and somites: sclerotome, myotome, dermatomeGeneral embryology: weeks 1 to 8 · Embryonic structure
• Patent ductus arteriosusHeart and vessels · Malformation
• Persistent truncus arteriosusHeart and vessels · Malformation
• Pharyngeal arches: nerve, muscles, skeleton and artery (the table)Head and neck: pharyngeal arches · Topic
• Pharyngeal clefts and membranes: external ear and cervical sinusHead and neck: pharyngeal arches · Embryonic structure
• Pharyngeal pouches: tonsil, thymus, parathyroids and C cellsHead and neck: pharyngeal arches · Embryonic structure
• Pituitary gland: Rathke pouch and the neurohypophysisNervous system and senses · Embryonic structure
• Placenta: structure, circulation and functionsFetal period, placenta and twins · Embryonic structure
• Poland sequence and prune belly syndromeSkeleton, muscles, limbs and skin · Malformation
• Prenatal diagnosis: ultrasound, serum screening, cell-free DNA, CVS, amniocentesisTeratology and prenatal diagnosis · Topic
• Primitive gut: foregut, midgut and hindgut and their arteriesDigestive system · Embryonic structure
• Primitive streak and notochord (nucleus pulposus, sacrococcygeal teratoma)General embryology: weeks 1 to 8 · Embryonic structure
• Principles of teratology: critical periods, dose and genotypeTeratology and prenatal diagnosis · Topic
• Processus vaginalis: hydrocele and indirect inguinal herniaUrogenital system · Malformation
• Pulmonary hypoplasiaRespiratory system, diaphragm and body cavities · Malformation
• Renal agenesis and the Potter sequenceUrogenital system · Malformation
• Second pharyngeal arch (hyoid arch) and the facial nerveHead and neck: pharyngeal arches · Embryonic structure
• Sex determination: SRY, anti-Müllerian hormone and testosteroneUrogenital system · Process
• Skin, hair, nails and the mammary glandsSkeleton, muscles, limbs and skin · Embryonic structure
• Skull: neurocranium, viscerocranium, sutures and fontanellesSkeleton, muscles, limbs and skin · Embryonic structure
• Spinal cord: alar and basal plates and the ascent of the cordNervous system and senses · Embryonic structure
• Surfactant and fetal lung maturityRespiratory system, diaphragm and body cavities · Topic
• TeethSkeleton, muscles, limbs and skin · Embryonic structure
• Tetracyclines and aminoglycosidesTeratology and prenatal diagnosis · Teratogen
• Tetralogy of FallotHeart and vessels · Malformation
• Thalidomide and limb reduction defectsTeratology and prenatal diagnosis · Teratogen
• The face: five prominencesHead and neck: pharyngeal arches · Process
• The pharyngeal apparatus: arches, pouches, clefts and membranesHead and neck: pharyngeal arches · Embryonic structure
• Third pharyngeal arch and the glossopharyngeal nerveHead and neck: pharyngeal arches · Embryonic structure
• Thyroglossal duct cyst and ectopic thyroidHead and neck: pharyngeal arches · Malformation
• Thyroid gland: from the foramen cecum to the neckHead and neck: pharyngeal arches · Embryonic structure
• Tongue: development and innervationHead and neck: pharyngeal arches · Embryonic structure
• TORCH infections: what each agent does to the embryo and fetusTeratology and prenatal diagnosis · Teratogen
• Transposition of the great arteriesHeart and vessels · Malformation
• Trophoblast and hCG; gestational trophoblastic diseaseGeneral embryology: weeks 1 to 8 · Embryonic structure
• Twins: dizygotic and monozygotic, chorionicity, twin–twin transfusionFetal period, placenta and twins · Topic
• Umbilical cord: two arteries, one veinFetal period, placenta and twins · Embryonic structure
• Urachal anomalies: patent urachus, cyst and sinusUrogenital system · Malformation
• Uterine anomalies: bicornuate, septate, didelphys, Müllerian agenesisUrogenital system · Malformation
• Valproate, carbamazepine and phenytoinTeratology and prenatal diagnosis · Teratogen
• Vascular rings and aberrant right subclavian arteryHeart and vessels · Malformation
• Venous system: vitelline, umbilical and cardinal veinsHeart and vessels · Embryonic structure
• Ventricles and cerebrospinal fluidNervous system and senses · Embryonic structure
• Ventricular septal defectHeart and vessels · Malformation
• Ventricular septation and the endocardial cushionsHeart and vessels · Process
• Vertebrae: resegmentation of the sclerotomesSkeleton, muscles, limbs and skin · Process
• Warfarin embryopathyTeratology and prenatal diagnosis · Teratogen
• Week 1: cleavage, morula, blastocyst and implantationGeneral embryology: weeks 1 to 8 · Stage
• Week 2: the bilaminar disc ('the week of twos')General embryology: weeks 1 to 8 · Stage
• Week 3: gastrulation and the trilaminar discGeneral embryology: weeks 1 to 8 · Process
• Weeks 3–8: the embryonic period and organogenesisGeneral embryology: weeks 1 to 8 · Stage

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Representative model, for educational purposes only.