Embryology Ready Reckoner

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### Developmental Anatomy At a Glance #### Adrenal Gland - **Superficial cortex:** coelomic epithelium (mesoderm) that lies in angle between developing gonad and mesentery attachment - **Deeper medulla:** neural crest cells (ectoderm) #### Arch of Aorta - Ventral aortic sac - Left horn of ventral aortic sac - Left fourth arch artery #### Arteries of Gut - **Foregut** → celiac artery - **Midgut** → superior mesenteric artery - **Hindgut** → inferior mesenteric artery #### Azygos and Hemiazygos Veins - **Azygos vein formed from:** - vein of the right azygos line - most cranial part of the right posterior cardinal vein (opens into superior vena cava, formed from right common cardinal) - **Hemiazygos and accessory hemiazygos veins:** - vertical parts represented by left azygos line - horizontal parts formed by postaortic anastomoses between azygos lines of two sides #### Cecum and Appendix - **Cecal bud:** arises from postarterial segment of midgut loop - **Cecum:** proximal part of bud grows rapidly - **Appendix:** distal part remains narrow #### Chorionic Villi - **Primary villi** → outer syncytiotrophoblast + inner cytotrophoblast - **Secondary villi** → syncytiotrophoblast + cytotrophoblast + primary mesoderm - **Tertiary villi** → syncytiotrophoblast + cytotrophoblast + primary mesoderm + fetal blood vessel - **Intervillous space** filled with maternal blood #### Celiac Artery - Fusion of ventral splanchnic arteries of foregut #### Coronary Sinus - Medial part → left horn of the sinus venosus - Lateral part → proximal part of left common cardinal vein #### Duodenum - **First part** and upper half of **second part** → foregut - **Rest of duodenum** → midgut - **Proximal part** supplied by branches of celiac artery - **Distal part** supplied by branches of superior mesenteric artery ### General Embryology & Organogenesis #### Diaphragm - **Septum transversum** forms central tendon - **Dorsal mesentery** of esophagus - **Body wall mesoderm** forms peripheral rim - **Pleuroperitoneal membrane** closes communication between pleural and peritoneal cavities #### Ear - **External ear:** ectoderm of first branchial cleft - **Middle ear:** endoderm of first pharyngeal pouch - **Internal ear:** ectodermal thickening, the otic placode #### Ear Ossicles - **Malleus, incus** → 1st pharyngeal arch mesoderm - **Stapes** → 2nd pharyngeal arch mesoderm #### Extrahepatic Biliary Apparatus - **Gallbladder** and **cystic duct** → pars cystica of hepatic bud - **Common bile duct** → narrow part of hepatic bud between pars cystica and duodenal part of foregut - **Common hepatic duct** → undivided part of pars hepatica distal to origin of pars cystica - **Right and left hepatic ducts** → right and left branches of pars hepatica #### Eye—Neuroectodermal Derivatives - Retina - Muscles: sphincter pupillae, dilator pupillae, ciliaris - Optic nerve #### Face - **Contribution of five processes:** - unpaired frontonasal - paired maxillary and mandibular processes - **Components of each process:** - core of mesenchyme - covering surface ectoderm - **Derivatives from each process:** - **Frontonasal process:** forehead, nasal septum, philtrum of upper lip, premaxilla (bearing four incisor teeth) - **Maxillary process:** whole upper lip (except philtrum), most of palate (except premaxilla part) - **Mandibular process:** whole lower lip #### Facial Anomalies - **Unilateral cleft lip (upper lip)** → failure of fusion of globular process with maxillary process - **Bilateral cleft lip** → failure of fusion of maxillary process and globular swellings bilaterally - **Mandibular and lower lip cleft** → nonfusion of mandibular process - **Unilateral cleft lip with cleft palate** → nonfusion of palatine process with primitive palate and nasal septum - **Facial cleft** → nonfusion of maxillary process with lateral nasal process #### Fertilization—Results - Completion of second meiotic division in ovum - Restoration of diploid chromosomes - Genetic sex of embryo decided - Axis of embryo defined - Initiation of cleavage #### First Pharyngeal Arch Derivatives - **Skeletal:** malleus, incus, maxilla, mandible, zygomatic bone, sphenomandibular ligament - **Muscular:** muscles of mastication, tensor tympani, tensor veli palatini, mylohyoid, anterior belly of digastric - **Nerve:** mandibular nerve - **Artery:** part of maxillary artery #### Fetal Circulation - **Factors necessary in addition to placenta:** - Left umbilical vein - Ductus venosus - Valve of inferior vena cava (IVC) - Foramen ovale - Ductus arteriosus - Umbilical arteries (both) #### Fourth Pharyngeal Arch Derivatives - **Skeletal:** lamina of thyroid cartilage - **Muscle:** cricothyroid muscle - **Nerve:** superior laryngeal nerve - **Artery:** right subclavian artery, left side part of arch of aorta #### Second Week of Gestation—Week of 2's - **Inner cell mass** differentiates into two germ layers: 1. Hypoblast 2. Epiblast - **Appearance of two cavities:** 1. Amniotic 2. Yolk sac (primary and secondary) - **Two layers of trophoblast:** 1. Cytotrophoblast 2. Syncytiotrophoblast - **Formation of extraembryonic components:** 1. Extraembryonic mesoderm (EEM) 2. Extraembryonic coelom - **Division of EEM into two layers:** 1. Somatopleuric 2. Splanchnopleuric - **Two cavity membranes:** 1. Amniogenic membrane 2. Heuser's membrane - **Two fetal membranes:** 1. Amnion 2. Chorion - **Two ends of embryo (axis differentiation by prochordal plate):** 1. Cephalic 2. Caudal axis #### Septum Transversum - Unsplit part of intraembryonic mesoderm at cranial end of pear-shaped embryonic disc (from Diaphragm section, page 2) forms central tendon (from page 6) before head fold formation: cranial to developing pericardial cavity and heart; with head fold formation: caudal to pericardial cavity and heart; between pericardial cavity and yolk sac cavity contributes to formation of ventral mesogastrium (lesser omentum, falciform ligament, diaphragm, connective tissue capsule of liver) ### Advanced Organogenesis & Systems #### Glands Derived from Various Germ Layers - **Ectoderm:** sweat gland, mammary gland, adenohypophysis, parotid gland, lacrimal gland, sebaceous gland. Neurohypophysis and pineal from modified surface ectoderm (neuroectoderm) - **Endoderm:** pancreas, liver, submandibular, sublingual salivary glands - **Mesoderm:** adrenal cortex #### Gut Derivatives Supplied by Arteries from Two Different Sources - **Duodenum:** branch of celiac and superior mesenteric arteries - **Transverse colon:** superior and mesenteric artery branches - **Anal canal:** branches from inferior mesenteric and internal iliac arteries - **Pancreas:** branches from celiac and superior mesenteric arteries #### Implantation—Abnormal Sites - Lower uterine segment (placenta previa) - Tubal - Ovarian - Abdominal implantation #### Heart: Divisions of Endothelial Heart Tube and Their Derivatives - **Truncus arteriosus** → pulmonary trunk and ascending aorta - **Bulbus cordis** → outflow part of right and left ventricle - **Primitive ventricle** → inflow part of right and left ventricle - **Primitive atrium** → rough part of right and left atrium - **Right horn of sinus venosus** → smooth part of right atrium - **Left horn of sinus venosus** → coronary sinus #### Inferior Vena Cava: From below Upward - **Right posterior cardinal vein** (between its junction with supracardinal and anastomosis between two posterior cardinals) - **Right supracardinal vein** (between its junction with posterior cardinal and supracardinal-subcardinal anastomosis) - **Right suprasubcardinal anastomosis** - **Right subcardinal vein** - **Subcardinal-hepatocardiac anastomosis** (vessel communicating right subcardinal with common hepatic vein) - **Right hepatocardiac channel** (common hepatic vein) #### Interatrial Septum - Septum primum - Septum intermedium - Septum secundum #### Interventricular Septum - **Ventricular septum:** muscular part from floor of bulboventricular cavity - **Proximal bulbar septum:** by fusion of right and left bulbar ridges from conus arteriosus - **Septum intermedium:** membranous part from atrioventricular cushions #### Kidney - **Collecting part:** collecting tubules, minor and major calyces, pelvis from dilated upper end of ureteric diverticulum - **Secreting part:** Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule of nephrons from metanephric blastema #### Left Atrium - Left half of primitive atrium - Dilated terminal parts of pulmonary veins - Left half of atrioventricular canal #### Liver - Develops as endodermal diverticulum from ventral wall of junction of foregut and midgut → grows ventrally and cranially into septum transversum - Gives off two solid buds → right and left lobes of the liver - **Connective tissue part** → mesoderm of septum transversum - **Parenchyma** → endoderm - **Sinusoids** → absorption and breakdown of vitelline and umbilical veins #### Lower Limb Arteries - **Axis artery** → dorsal root of umbilical artery - **Remnants of axis artery of lower limb** are: inferior gluteal arteries, arteria comitans nervi ischiadici, anastomosis of profunda femoris - **Femoral artery** → capillary plexus in ventral thigh, communicates with external iliac and axis arteries - **Popliteal artery** → sprouts from axis artery at distal border of popliteus #### Metanephric Blastema - Bowman's capsule - Proximal convoluted tubules - Loops of Henle - Distal convoluted tubules #### Midgut Rotation - Total rotation ~270° - 90° rotation occurs in physiological umbilical hernia - 180° rotation occurs within abdominal cavity #### Pancreas - **Dorsal and ventral pancreatic buds** at junction of foregut and midgut - **Upper part of head, neck, body, tail** → dorsal pancreatic bud - **Lower part of head and uncinate process** → ventral pancreatic bud - During gut rotation **ventral bud fuses with dorsal bud** #### Parathyroid Glands - **Upper pair** → 4th pharyngeal pouch - **Lower pair** → 3rd pharyngeal pouch #### Pharyngeal Arch Artery Derivatives - **1st** → part of maxillary artery - **2nd** → part of stapedial artery - **3rd** → common carotid, part of internal carotid on both sides - **4th** → right side part of subclavian artery, left side part of arch of aorta - **5th** → disappear - **6th** → right side of pulmonary trunk, left side left pulmonary trunk and ductus arteriosus #### Pharyngeal Pouches—Derivatives - **Endocrine glands:** - Thyroid - Parathyroids - **Lymphoid organs:** - Palatine tonsil - Thymus - **Others:** - Auditory tube - Tympanic cavity - Mastoid antrum, air cells - Tympanic membrane #### Pituitary Gland - **Adenohypophysis** → ectodermal diverticulum from roof of stomodeum (Rathke's pouch) - **Neurohypophysis** → neuroectodermal downgrowth from floor of diencephalon #### Portal Vein - Dorsal anastomotic channel - Part of right vitelline vein between cephalic ventral and dorsal anastomosis - Right branch from intrahepatic part of right vitelline vein - Left branch from cephalic ventral anastomosis and intrahepatic part of vitelline vein #### Prostate - **Glandular component:** epithelial outgrowths from proximal urethra invade surrounding dense mesenchyme - **Muscular and connective tissue part** → mesoderm - **Inner glandular zone** is mesodermal - **Outer glandular zone** is endodermal #### Rectum and Anal Canal - **Endodermal cloaca** shut off from ectodermal cloaca by cloacal membrane - **Development of urorectal septum** divides endodermal cloaca into: - anterior part → vesicourethral part and urogenital sinus - dorsal segment → primitive rectum (rectum develops from this) - **Anal canal** is both ectodermal and endodermal - part above white line of Hilton (upper 2/3rds) → endoderm of primitive rectum - part caudal to white line (lower 1/3rd) → ectoderm of proctodeum #### Right Atrium - Right half of primitive atrium - Sinus venosus - Right half of atrioventricular canal #### Second Pharyngeal Arch Derivatives - **Skeletal:** stapes, lesser cornu and upper part of body of hyoid bone, stylohyoid ligament - **Muscular:** muscles of facial expression including scalp and auricular muscles - **Nerve:** facial nerve - **Artery:** part of stapedial artery #### Seventh Cervical Intersegmental Artery—Contributions - **Main stem** → subclavian artery - **Dorsal division** → stem of vertebral artery - **Lateral division** → axillary and brachial arteries - **Ventral division** → stem of internal thoracic (mammary) artery #### Sixth Pharyngeal Arch Derivatives - **Skeletal** → laryngeal cartilages - **Muscles** → intrinsic muscles of larynx - **Nerve** → recurrent laryngeal nerve - **Arteries:** right side right pulmonary trunk, left side proximal part develops into left pulmonary trunk, distal part into ductus arteriosus #### Smooth Muscles Derived from Ectoderm - Sphincter pupillae - Dilator pupillae - Myoepithelial cells of sweat gland #### Spermiogenesis - Transformation of spermatids to spermatozoa - Golgi apparatus forms acrosomal cap - Nucleus forms the head - Centrioles form axial filaments of body and tail - Mitochondria form sheath - Cytoplasm extruded out as residual bodies #### Spleen - Develops from mesoderm in dorsal mesogastrium as small spleniculi → later fuse to form single mass - Splenic notches along upper border of adult spleen → persistence of fetal lobulation - Splenic mass projects into left layer of dorsal mesogastrium - **Capsule, septa, connective tissue** → mesoderm #### Superior Vena Cava - Right duct of Cuvier - Terminal portion of right anterior cardinal vein caudal to transverse anastomosis in cervical region #### Subclavian Artery - **Right:** - Proximal part → right fourth arch artery - Remaining part → seventh cervical intersegmental artery - **Left:** - Entirely from seventh cervical intersegmental artery #### Testis - **Seminiferous tubule, rete testis, interstitial cells, fibrous septa and coverings** → medulla of genital ridge - **Efferent ductules** → proximal 12-15 mesonephric tubules - **Canal of epididymis** and **vas deferens** → mesonephric duct - **Appendix of testis** is a paramesonephric duct remnant - **Appendix of epididymis** is a mesonephric duct remnant #### Third Pharyngeal Arch Derivatives - **Skeletal** → lower half of body of hyoid bone and its greater cornu - **Muscle** → stylopharyngeus muscle - **Nerve** → glossopharyngeal nerve - **Artery** → part of common carotid artery and internal carotid artery #### Thoracic Duct - Caudal part of right lymphatic duct - Cross anastomosis between right and left lymphatic ducts - Cephalic part of left lymphatic duct #### Thyroid Gland - **Isthmus and pyramidal lobe and lateral lobes** → proliferation of endodermal cells at lower end of median thyroid diverticulum (from floor of pharynx between tuberculum impar and hypobranchial eminence) - **Lateral thyroid** → ultimobranchial body #### Tongue - **Anterior two-thirds:** lingual swellings, tuberculum impar (1st arch) - supplied by mandibular branch of trigeminal nerve (general sensation), chorda tympani branch of facial (taste sensation) - **Posterior one-third:** cranial part of hypobranchial eminence (3rd, 4th arches) - supplied by glossopharyngeal (general and special), branch of vagus (general sensation) - **Muscles** develop from occipital myotomes → supplied by hypoglossal nerve #### Tympanic Membrane - **Outer cuticular layer** → ectodermal - **Middle fibrous layer** → mesodermal - **Inner mucous layer** → endodermal #### Upper Limb Arteries - **Axis artery of upper limb** → lateral branch of 7th intersegmental artery - **Axis artery** forms axillary, brachial and anterior interosseous artery - **Median artery** develops from anterior interosseous artery - **Radial and ulnar arteries** develop from axis artery at elbow region - **Ulnar artery** communicates with deep palmar plexus #### Ureteric Bud Derivatives - Collecting tubules and ducts - Minor and major calyces - Pelvis of kidney - Ureter #### Urethra in Males - **Prostatic urethra (up to ejaculatory ducts)** → caudal part of vesicourethral canal (endoderm) - posterior wall by absorbed mesonephric ducts (mesoderm) - **Rest of prostatic urethra, membranous urethra** → pelvic part of definitive urogenital sinus - **Penile urethra** → phallic part of definitive urogenital sinus - **Urethra in glans** → ectoderm #### Urethra in Females - Homologous with male prostatic urethra proximal to opening of prostatic utricle - Entirely formed from vesicourethral portion of endodermal cloaca and caudal ends of mesonephric ducts - Small contribution from pelvic part of urogenital sinus #### Urinary Bladder - Cranial dilated part of vesicourethral canal (endoderm) and proximal portion of allantois - allantois gets obliterated - **Trigone of bladder** → incorporated (absorbed) caudal ends of mesonephric ducts - **Epithelium** → endodermal - **Muscular and serous walls** → splanchnopleuric mesoderm #### Uterine Anomalies - **Didelphys** → complete failure of fusion of paramesonephric ducts → double uterus, double cervix, double vagina - **Bicornis bicollis** → double uterus, double cervix, single vagina - **Septate uterus** → septum in uterine cavity - **Subseptate** → incomplete septum in uterus - **Arcuate uterus** → fundus is concave - **Unicornuate uterus** → unilateral suppression of paramesonephric duct #### Vertebral Artery - **First part** (origin to foramen transversarium of C6) → dorsal division of seventh cervical intersegmental artery - **Vertical part** (second part, foramina transversaria) → postcostal anastomoses between C1-C6 intersegmental arteries - **Horizontal part** (third part, transverse on arch of atlas) → spinal branch of first cervical intersegmental artery