Reproductive - Embryology

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Important genes of embryogenesis

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Sonic Hedgehog gene, Wnt-7 gene, FGF gene, Homeobox (Hox) genes

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Sonic hedgehog gene

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produced at base of limbs in zone of polarizing activity. Involve in patterning along anteroposterior axis and CNS development; mutation can cause holoprosencephaly.

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Wnt-7 gene

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Produced at apical ectodermal ridge (thickened ectoderm at distal end of each developing limb). Necessary for proper organization along dorsal-ventral axis

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FGF gene

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Produced at apical ectodermal ridge. Stimulates mitosis of underlying mesoderm, providing for lengthening of limbs

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Homeobox (Hox) genes

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Involved in segmental organization of embryo in a craniocaudal direction. Code for transcription factors. Hox mutations -> appendages in wrong locations

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Embryonic development within week 1

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hCG secretion begins around the time of implantation of blastocyst. Blastocyst "sticks" at day 6

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Embryonic develop within week 2

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Bilaminar disc (epiblast, hypoblast). 2 weeks = 2 layers

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Embryonic development within week 3

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Gastrulation forms trilaminar embryonic disc. Cells from epiblast invaginate -> primitive streak -> endoderm, mesoderm, ectoderm. Notochord arises from midline mesoderm; overlying ectoderm becomes neural plate. 3 weeks = 3 layers

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Embryonic development weeks 3-8 (embryonic period)

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Neural tube formed by neuroectoderm and closes by week 4. Organogenesis. Extremely susceptible to teratogens.

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Embryonic development week 4

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Heart begins to beat. Upper and lower limb buds begin to form. 4 weeks = 4 limbs and 4 heart chambers

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Embryonic development week 6

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fetal cardiac activity visible by transvaginal ultrasound.

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Embryonic development week 8

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Fetal movement starts. Gait at week 8

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Embryonic development week 10

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Genitalia have male/female characteristics

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Embryologic derivatives

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Ectoderm, mesoderm, endoderm

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Ectoderm derivatives

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Surface ectoderm, neuroectoderm, neural crest

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Ectoderm

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external/outer layer

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Surface ectoderm

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Epidermis; adenohypophysis (from Rathe pouch); lens of eye; epithelial linings oforal cavity, sensory organs of ear, and olfactory epithelium; epidermis; anal canal below the pectinate line; parotid, sweat and mammary glands.

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Craniopharyngioma

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benign Rathke pouch tumour with cholesterol crystals, calcifications

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Neuroectoderm

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Brain (neurohypophysis, CNS neurons, oligodendrocytes, astrocytes, ependymal cells, pineal gland), retina spinal cord. Neuroectoderm - think CNS

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Neural crest

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PNS (dorsal root ganglia, cranial nerves, autonomic ganglia, Schwann cells), melanocytes, chromatin cells of adrenal medulla, parafollicular (C) cells of thyroid, pia and arachnoid, bones of the skull, odontoblasts, aorticopulmonary septum, endocardial cushions. Neural crest - think PNS and non-neuronal structures nearby

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Mesoderm

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Muscle, bone, connective tissue, serous linings of body cavities (e.g. peritoneum), spleen (derived from foregut mesentery), cardiovascular structures, lymphatics, blood, wall of gut tube, upper vagina, kidneys, adrenal cortex, dermis, testes, ovaries. Notochord induces ectoderm to form neuroectoderm (neural plate). Its only postnatal derivative is the nucleus pulpous of the intervertebral disc. Middle/"meat" layer.

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Mesodermal defects

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VACTERL: Vertebral defects, Anal atresia, Cardiac defects, Trachea-Esophageal fistula, Renal defects, Limb defects (bone and muscle)

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Endoderm

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"Enternal" layer. Gut tube epithelium (including anal canal above the pectinate line), most of urethra and lower vagina (derived from urogenital sinus), luminal epithelial derivatives (e.g. lungs, liver, gallbladder, pancreas, eustachian tube, thymus, parathyroid, thyroid follicular cells).

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Types of error in organ morphogenesis

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agenesis, aplasia, hypoplasia, disruption, deformation, malformation, sequence

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agenesis

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absent organ due to absent primordial tissue

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aplasia

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absent organ despite presence of primordial tissue

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hypoplasia

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Incomplete organ development; primordial tissue present

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Disruption

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secondary breakdown of previously normal tissue or structure (e.g. amniotic band syndrome)

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Deformation

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extrinsic disruption; occurs after embryonic period

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malformation

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intrinsic disruption; occurs during embryonic period (weeks 3-8)

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Sequence

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Abnormalities result from a single primary embryologic event (e.g. oligohydraminos -> Potter sequence)

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Teratogens

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Most susceptible in 3rd-8th weeks (embryonic period - organogenesis) of pregnancy. Before week 3, "all-or-none" effects. After week 8, growth and function affected. Can be due to medications, substance abuse, other.

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Medication teratogens

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ACE inhibitors, alkylating agents, aminoglycosides, anti epileptic drugs, diethylstilbestrol, folate antagonists, isotretinoin, lithium, methimazole, tetracyclines, thalidomide, warfarin

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Effect of ACE inhibitors on fetus

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renal damage

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Effect of Alkylating agents on fetus

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absence of digits, multiple anomalies

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Effects of aminoglycosides on fetus

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Ototoxicity

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Effects of antiepileptic drugs on fetus

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Neural tube defects, cardiac defects, cleft palate, skeletal abnormalities (e.g. phalanx/nail hypoplasoa, facial dysmorphism). High dose folate supplementation is recommended if drugs are necessary. Most commonly valproate, carbamazepine, phenytoin, phenobarbital

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Effects of Diethylstilbestrol on fetus

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Vaginal clear cell adenocarcinoma, congenital Mullerian anomalies.

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Effects of Folate antagonists on fetus

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Neural tube defects. Includes trimethoprim, methotrexate, anti epileptic drugs

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Effects of Isotretinoin on fetus

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Multiple severe birth defects. Contraception is mandatory

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Effects of lithium on fetus

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Epstein anomaly (apical displacement of tricuspid valve)

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Effects of methimazole on fetus

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aplasia cutis congenita

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Effects of Tetracyclines on fetus

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Discoloured teeth, inhibited bone growth

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Effects of Thalidomide on fetus

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Limb defects (phocomelia, micromelia - "flipper" limbs)

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Effects of Warfarin on fetus

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Bone deformities, fetal hemorrhage, abortion, ophthalmologic abnormalities.

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Substance abuse teratogens for fetus

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alcohol, cocaine, smoking (nicotine, CO)

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Effects of alcohol on fetus

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common cause of birth defects and intellectual disability; fetal alcohol syndrome

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Effects of cocaine on fetus

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low birth weight, preterm birth, IUGR, placental abruption. Cocaine -> vasoconstriction

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Effects of smoking (nicotine, CO) on fetus

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Low birth weight (leading cause in developed countries), preterm labour, placental problems, IUGR, SIDS. Nicotine -> vasoconstriction. CO -> impaired O2 delivery

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Other fetal teratogens

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Iodine (lack or excess), maternal diabetes, methylmercury, vitamin A excess, x-rays

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Effect of iodine (lack or excess) on fetus

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congenital loiter or hypothyroidism (cretinism)

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Effects of maternal diabetes on fetus

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Caudal regression syndrome (anal atresia to sirenomelia), congenital heart defects, neural tube defects, macrosomia

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Effects of methylmercury on fetus

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neurotoxicity. Highest in swordfish, shark, tilefish, king mackerel

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Effects of vitamin A excess on fetus

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Extremely high risk for spontaneous abortions and birth defects (cleft palate, cardiac)

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Effects of X-rays on fetus

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Microcephaly, intellectual disability. Minimized by lead shielding

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Fetal alcohol syndrome

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leading cause of intellectual disability in the US. Newborns of alcohol-consuming mothers have increased incidence of congenital abnormalities, including pre- and postnatal developmental retardation, microcephaly, facial abnormalities (smooth philtre, thin vermillion border [upper lip], small palpebral fissures), limb dislocation, heart defects. Heart-lung fistulas and holoprosencephaly in most severe form. Mechanism is failure of cell migration

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Dizygotic ("fraternal") twins

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arise from 2 eggs that are separately fertilized by 2 different sperm (always 2 zygotes) and will have 2 separate amniotic sacs and 2 separate placentas (chorions)

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Monozygotic ("identical") twins

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arise from 1 fertilized egg (1 egg and 1 sperm) that splits in early pregnancy. The timing of cleavage determines chorionicity (number of chorions) and amnionicity (number of amnions). Dichorionic diamniotic (25%) twins happen when cleavage takes place 0-4 days. Monochorionic diamniotic (75%) twins happen when cleavage happens at 4-8 days. Monochorionic monoamniotic twins are rare and happen when cleavage takes place between 8-12 days. Conjoined twins are monochorionic monoamniotic twins where cleavage takes place after 13 days

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Placenta

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the primary site of nutrient and gas exchange between mother and fetus. Has fetal and maternal component.

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Fetal component of placenta

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cytotrophoblast, synctiotrophoblast

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Cytotrophoblast

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inner layer of chorionic villi. Cytotrophoblast makes Cells.

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Synctiotrophoblast

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Outer layer of chorionic villi; synthesizes and secretes hormones, e.g. hCG (structurally similar to LH; stimulates corpus luteum to secrete progesterone during first trimester). Synctiotrophoblast synthesizes hormones. Lacks MHC-I expression causing decreased chance of attack by maternal immune system

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Maternal component of placenta - Decidua basilis

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derived from endometrium. Maternal blood in lacunae.

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Umbilical cords

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2 umbilical arteries that return deoxygenated blood from fetal internal iliac arteries to placenta. A single umbilical artery (2-vessel cord) is associated with congenital and chromosomal anomalies. 1 umbilical vein supplies oxygenated blood from placentae to fetus; drains into IVC via liver or via ductus venosus. Umbilical arteries and vein are derived from allantois.

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Urachus

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In the third week the yolk sac forms the allantois, which extends into the urogenital sinus. Allantois becomes the urachus, a duct between the fetal bladder and umbilicus.

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Patent urachus

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total failure of urachus to obliterate -> urine discharge from umbilicus

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Urachal cyst

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partial failure of urachus to obliterate; fluid-filled cavity lined with uroepithelium, between umbilicus and bladder. Can lead to infection, adenocarcinoma

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Vesicourachal diverticulum

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Slight failure of urachus to obliterate -> out pouching of bladder

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Vitelline duct

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7th week - obliteration of vitelline duct (omphalo-mesenteric duct), which connects yolk sac to midgut lumen

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Vitelline fistula

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vitelline duct fails to close -> meconium discharge from umbilicus

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Meckel diverticulum

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Partial closure of vitelline duct, with patent portion attached to ileum (true diverticulum). May have heterotypic gastric and/or pancreatic tissue -> melon, hematochezia, abdominal pain

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Aortic arch deriviates

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develop into arterial system

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1st aortic arch derivates

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part of maxillary artery (branch of external carotid). 1st arch is maximal

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2nd aortic arch derivatives

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Stapedial artery and hyoid artery. Second = stapedial

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3rd aortic arch derivatives

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Common carotid artery and proximal part of internal carotid artery. C is 3rd letter of alphabet

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4th aortic arch derivatives

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on left, aortic arch; on right, proximal part of right subclavian artery. 4th arch (4 limbs) = systemic

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6th aortic arch derivatives

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Proximal part of pulmonary arteries and (on left only) ductus arteriosus. 6th arch = pulmonary nd the pulmonary-to-systemic shunt (ductus arteriosus)

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Branchial (pharyngeal) apparatus

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composed of branchial clefts, arches, pouches. Branchial clefts - derived from ectoderm. Also called branchial grooves.

Branchial arches - derived from mesoderm (muscles, arteries) and neural crest (bones, cartilage).

Branchial pouches - derived from endoderm.

CAP covers outside to inside: Clefts = ectoderm, Arches = mesoderm/neural crest, Pouches = endoderm

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Branchial cleft derivatives

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1st cleft develops into external auditory meatus. 2nd through 4th clefts form temporary cervical sinuses, which are obliterated by proliferation of 2nd arch mesenchyme. Persistent cervical sinus -> branchial cleft cyst within lateral neck, anterior to sternocleidomastoid muscle. Immobile during swallowing.

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1st branchial arch derivatives - cartilage

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maxillary process -> maxilla, zygomatic bone

Mandibular process -> Meckel cartilage -> mandible, malleus and incus, sphenomandibular ligament

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1st branchial arch derivatives - muscles

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Muscles of mastication (temporals, master, lateral and medial pterygoids), mylohyoid, anterior belly of digastric, tensor tympani, tensor veil palatini.

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1st branchial arch derivatives - nerves

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CN V2 and V3

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Pierre Robin Sequence

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micrognathia, glossoptosis, cleft palate, airway obstruction

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Treacher Collins Syndrome

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neural crest dysfunction -> mandibular hypoplasia, facial abnormalities

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2nd branchial arch derivatives - cartilage

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Reichert cartilage: Stapes, styloid process, less horn of hyoid, stylohyoid ligament

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2nd branchial arch derivatives - muscles

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Muscles of facial expression, stapedius, stylohyoid, platysma, posterior belly of digastric.

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2nd branchial arch derivatives - nerves

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CN VII (facial expression)

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3rd branchial arch derivatives - cartilage

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greater horn of hyoid

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3rd branchial arch derivatives - muscles

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stylopharyngess (think of stylopharyngeus innervated by glossopharyngeal nerve)

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3rd branchial arch derivatives - nerves

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CN IX (stylopharyngeus)

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4th-6th branchial arch derivatives - cartilage

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arytenoids, cricoid, corniculate, cuneiform, thyroid (used to sign and ACCCT)

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4th-6th branchial arch derivatives - muscles

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4th arch: most pharyngeal constrictors; cricothyroid, levator veil palatine.

6th arch: all intrinsic muscles of larynx except cricothyroid

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4th-6th branchial arch derivatives - nerves

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4th arch: CN X (superior laryngeal branch)

6th arch: CN X (recurrent laryngeal branch)

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1st branchial pouch derivatives

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middle ear cavity, eustachian tube, mastoid air cells. 1st pouch contributes to endoderm-lined structures of ear

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2nd branchial pouch derivatives

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epithelial lining of palatine tonsil

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3rd branchial pouch derivatives

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Dorsal wings -> inferior parathyroids

Ventral wings - thymus. 3rd pouch contributes to 3 structures (thymus, left and right inferior parathyroids). 3rd pouch structures end up below 4th-pouch structures

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4th branchial pouch derivatives

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Dorsal wings -> superior parathyroids

Ventral wings -> ultimobranchial body -> parafollicular (C) cells of thyroid

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Branchial pouch derivatives

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Ear, tonsils, bottom-to-top: 1(ear), 2(tonsils), 3 dorsal (bottom for inferior parathyroids), 3 ventral (to=thymus), 4 (top = superior parathyroids)

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DiGeorge Syndrome

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Chromosome 22q11 deletion. Aberrant development of 3rd and 4th pouches -> T-cell deficiency (thymic aplasia) and hypocalcemia (failure of parathyroid development). Associated with cardiac defects (conotruncal anomalies)

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Cleft lip

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failure of fusion of the maxillary and medial nasal processes (formation of primary palate). Cleft lip and cleft palate have distinct, multifactorial aetiologies, but often occur together

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Cleft palate

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failure of fusion of the two lateral palatine shelves or failure of fusion of lateral palatine shelves with nasal septum and/or median palatine shelf (format of secondary palate). Cleft lip and palate have distinct, multifactorial aetiologies, but often occur together.

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Female genital embryology

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default development. Mesonephric duct degenerates and paramesonephric duct develops

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Male genital embryology

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SRY gene on Y chromosome - produces testis-determining factor -> testes development. Sertoli cells secrete Mullerian inhibitor factor (MIF) that suppresses development of paramesonephric ducts. Lydia cells secrete androgens that stimulate development of mesonephric ducts.

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Paramesonephric (Mullerian) duct

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Develops into female internal structures - fallopian tubes, uterus, upper portion of vagina (lower portion from urogenital sinus). Male remnant is appendix testis.

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Mullerian genesis (Mayer-Rokitansky-Kuster-Hauser syndrome)

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may present as primary amenorrhea (due to a lack of uterine development) in females with fully developed secondary sexual characteristics (functional ovaries)

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Mesonephric (Wolffian) duct

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Develops into male internal structures (except prostate) - Seminal vesicles, Epididymis, Ejacualtory duct, Ductus deferens (SEED). In females, remnant of mesonephric duct -> Gartner duct

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No Sertoli cells or lack of Mullerian inhibitory factor

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develop both male and female internal genitalia and male external genitalia

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5a-reductase deficiency

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inability to convert testosterone into DHT -> male internal genitalia, ambiguous external genitalia until puberty (when increased testosterone levels cause masculinization)

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Uterine (Mullerian duct) anomalies

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Septate uterus, bicornuate uterus, uterus didelphys

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Septate uterus

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common anomaly vs normal uterus. Incomplete resorption of septum. Decreased fertility. Treat with septoplasty.

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Bicornuate uterus

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Incomplete fusion of Mullerian ducts. Increased risk of complicated pregnancy

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Uterus didelphys

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Complete failure of fusion -> double uterus, vagina and cervix. Pregnancy is possible

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Congenital penile abnormalities

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Hypospadias, epispadias

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Hypospadias

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abnormal opening of penile urethra on ventral surface of penis due to failure of urethral folds to fuse. Hypospadias is more common than epispadias. Associated with inguinal hernia and cryptorchidism.

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Epispadias

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abrnomal opening of penile urethra on dorsal surface of penis due to faulty positioning of genital tubercle. Exstrophy of the bladder is associated with epispadias.