Blood Islands

• clusters of mesenchymal cells that arrange themselves as cavities with a linning (vasculogenesis)
• form horseshoe shaped tube around the cranial end of embryo that develops into heart
• blood islands also midline to form blood vessules

Embryological Heart Tube 

Make Up

• increases in girth and undergoes asyetrical folding
• forms sinus venosum, atrium, ventricle and blubus cordis

Dextrocardia

• heart more to the right side of chest because its asymetry is flipped
• usually asymptamatic 

Cardiac Embryology 

Week 4-6

• partitioning inside heart
• only one atrium at this point
• unidirectional flow through heart

Septum Primum 

• grows out of roof of atrium, dividing common atrium
• apoptosis forms hole
• septum secundum forms with another hole that doesnt line up (forms a valve)
• this valve is part of foreamen ovale and the other...

Atrial Septal Defect (ASD)

• defect in septum primum or secundum lead to back flow of blood
• closing of the forearm ovale in embryo leads to hypertrophy of right ventricle and underdevelopment of left side of heart 

Sinus Venosum

• two horns form in development
• left horn of sinus forms coronary sinus of heart
• right horn becomes smooth walled part of right atrium that Superior and Inferior Vena Cava drain into

Bulbis Cordis

• bulging proximal part
• cone shaped middle (conus cordis)
• distal truncus arteriosus
• conus forms membranous part of interventricular septum
• truncus forms aorta and plumonary trunk 

Heart Defect Statistics

• make up 1% of life born malformations
• 10 % of still births

Ventricular Septal Defect (VSD)

• interventricular septum is lacking, resulting in hypertrophy of pulmonary trunk because of back flow

Tetralogy of Fallot

• another common anomoly of conus and truncus region of Bulbis condis
• consists of 4 abnormalities
• narrow right ventricle outflow
• defect in interventricular septum
• hypertrophied aorta
• hypertrophy of right ventricular wall

Gut Tube Formation

• lumen of gut tube formed by yolk sac during wk 3
• the amiotic cavity comes down to form an anterior abdominal cavity 

Respiratory Diverticulum

• ventral outpocketing of the pharynx
• forms larynx, trachea, bronchi and lungs
• lined by endoderm, as well as mesoderm derived tract
• pinching off of the outpocketing leads to two sperate systems (trachea/ bronchial buds, and esophagus 

Laryngeal Orfice

• in floor of pharynx in development
• connection between trachea/bronchial buds and esophagus 

Stomodeum

• embryonic mouth
• pharyngeal arches are numbered in roman numerals
• at front of gut tube (cranial end)

Tracheo-esophageal Fistula

• a retention of the orginal broad connection between the foregut and respiratory diverticulum in the embryo
• esophagus divided into cranial and caudal end where the cranial end, ends abrubtly and the caudal end is connected to trachea
• space between cranial and caudel ends called esophageal artresia

Lung Development 

End of 3rd Trimester

• cuboidal cells begin to be replaced by squamous cells
• these squamous cells adre type I Pneumocytes
• Type II pneumocytes are progeniritors for these in times of injury and also produce surfactant

Cardiovascular Development 

3.5 weeks

• blood begins to flow through the body
• starts at simple heart 
• O2/CO2 exchange carried out in the placenta
• simple diffusion no longer works at this point

Aortic Arch In Humans

• structure exists around week 4(similar to fish)
• large unpaired ventral vessel is called aortic sac
• leaves cranially and gives off aortic branches
• the branches connect to  dorsal aoras that carries blood caudally(opisite dir) and drains into midline vessel
• arches numbered 3, 4, 6 (5 disapears early on)

Truncus Arteriosus 

Development

• develops directly off the aortic sac
• becomes ascending aorta and pulmonary trunk

Right and Left Aortic Arch VI (6)

• right losses connection to right dorsal artery and becomes right pulmonary artery
• left becomes ductus arteriosus and holds connection to dorsal aorta until birth 

Dorsal Aorta Break Up

Embryological Development

• broken up into cranial and caudal segments
• left and right cranial segments become common carotid arteries
• right caudal becomes right subclavian
• left caudal becomes descending aorta

Anomalous Right Subclavian

• rather then branching off aortic sac in developlment, it branches off the left caudal dorsal aorta (descending aorta) after the carotids and ductus arteriosus
• has to wrap all the way around esophagus to get to right limb 

Double Aortic Arch 

• arterial loop formed between the two caudal dorsal aortas
• trachea and esophagus passes through the loop and are compressed
• this causes problems swallowing and breathing

Role of Amniotic Fluid

• cushions fetus from sudden movts of the mother and protects against infection
• fetus swallows and beaths it
• urinates into it
• amniocentesis
• first few poops (meconium) consists of amniotic fluid

Intervillous Spaces

• develops from lacunae
• filled w/ maternal blood but lined with fetal cells
• this maternal blood enters in by spiral arteries
• fetal arteries and veins enter this through villous trees
• where gas exchange with the mother takes place
• divided by septa

Cotyledons

• forms in middle of second trimester
• septa formed by decidua of placenta that divide the placenta into compartments
• maternal cells 

Placenta Dimensions

• 15-30% of inside uterus
• healthy placenta is 15-25cm in diameter, 3cm thick and weighs 500g

Twin Cases

• 29% of twin cases one dies early in second trimester
• reabsorbed (vanishing twin)
• fetus papyraceus (remnant)

Changes in circulation at Birth

• closure of umbilical arteries by contraction of smooth muscles in their walls
• closure of umbilical vein and ductus venosus(forming ligumentum teres and ligamentum venosum
• closure of ductus arteriosus form ligamentum arteriosum
• closure of oval foramen (20% of adults this fails to close without symptoms)

Oligohydramnios

• insufficency of amiotic fluid
• can lead to slowed down lung development
• lung hypoplasia

Alontois and Vitelline Duct

• alontois is narrow duct that connects the caudal end of the hind gut through the umbilicis
• VT connects midgut to yolk sac external through umbilical chord

Week 4-11

• at beggining of wk 4 gut is more or less straight tube
• rotational and asymetrical growth happen and lead to highly asymmetrical shap by end of wk 4
• foregut rotates 90 clockwise in cranial view (ventral becomes right side
• midgut rotates 270 counterclockwise invetral view
• hindgut does not rotate

Cranial Movement of Liver

• causes right side of the orginal abdominal cavity to reduce in size
• this becomes the omental bursa(lesser sac)
• opening becomes small, epiploic foramen

Vagus nerve Rotation

• foregut rotation causes left vagus nerve to lie anteriorly and most of right posterior to esophagus

Pancreas Development

• originally consists of dorsal and ventral pancreatic buds
• dorsal becomes body of pancreas and ventral the head
• embryonic rotation unites two by end of first trimester
• accessory duct is duct of dorsal panc and main duct and common bile duct are of the ventral bud and liver

Midgut Proliferation and Rotation

• gives rise to duodenum, lesser intestine, cecum, ascending colon and 2/3 of transverse colon
• has dorsal but not ventral mesentary
• cranial limb goes to right (duodenum, illeum, jejunum)
• caudal to left (secondary folding) - 90 degrees counterclock
• around wk 9 further counterclockwise folding gives 270 degrees
• area of least rotation is  vitteline duct

Greater Omentum Development

• dorsal mesogastrium(above spleen and in btw spleen and stomach) increases in size to form greater omentum
• dorsal mesocolon of transverse colon fuses with the dorsal mesogastrium

Secondary Retroperitoneal

• organs such as pancreas that develop w/ mesenteries but they fuse w/ posterior abdominal wall becoming fusion fascia
• all nerves and vessels to these are located in the fusion fascia, allowing surgeons easy access

Non-rotation of Midgut

• left side colon
• mid gut fails to rotate upon reentry into abdominal cavity
• results in lesser intestine in its embryonic position on right side
• may be asymptomatic but common complication is volvulus
• twisting of gut may lead to compression of SMA resulting in gangreen 

Mobile Cecum

• ascending colon normally fuses with posterior abdominal wall 
• if large portion does not then volvulus may result
• most common is mobile cecum retains mesocolon
• lack of fusion may result in pockets in post abd wall where loops of small intestine may gret trapped (retrocolic hernia) (blocks food)

Cloaca

• initially a common cavity of hindgut and urogenital system
• eventually seperated by urorectal septum
• after this a dimple in skin (protodeum) marks where anus will form
• anal membrane ruptures by wk8 (opening it)

Contents of Umbilical Chord

• paired umbilical arteries
• vitelline duct and allontois
• unpaired umbilical vein 

Ophalocele

• herniation of gut into umbilical chord
• anterior abdominal wall may not close over (wk 4) and herniated organs maintain embryonic position(evagination of abdominal viscera)
• visceral peritineum and amnion will cover organs 
• 1:6000 births

Umbilical Hernia

• hernia near umbilicis going directly through anterior abdominal wall(linea alba)
• usually grows after birth and increases w/ incr in abdominal presure
• can happen at any age (outie belly button)
• in development the intestines do retract but then herniate through weak abd wall
• cover by tissue of abd wall, subcutaneous, skin and parietal peritoneum
• 1:10 (more in females) and usually disapears in 1yr

Gastoschisis

• herniation of gut there defect of ant abd wall
• results of failure of R and L abd wall to fuse in wk 4
• gut herniates through(usually r) and only covered in visceral peritoneum
• bathed in amniotic fluid and may erode and damage
• name means split stomach
• 1:3000 , more common in males

Meckels Diverticulum

• remnant of prox vitelline duct (yolk stalk)
• usually degenerates
• projection comming off ileum(50cm from cecum)
• may be connected by fibrous chord to umbilicis
• usually asymptomatic (can be inflamed and presents as appendicitis
• 4:100 male/ 2:100 females
• vitelline cysts and umbilicoileal fistula effect fibrous tisuue connection

Plicae Circulares

• folds on inner surface of intestine
• composed of mucosa and some submucosa
• dont completely flatten and covered in villi

Three Mesoderm Systems in Development

(Kidney and Ureter)

• pronephric system that disappears and is never func (number of segments high in abd)
• mesonephric system and duct that is mostly gone but duct remains in males. 
• ureteric bud remains that forms ureter, minor and major calices, renal pelvis and renal pyramids(exc loop)  
• metanephric system that will become kidney at birth

Urogenital Ridge

• crest tissue projects into abdominal cavity by pushing on peritoneum
• UG structures (mesonephros and mesonphric duct and gonad develop inside this 

Postnatal Kidney formation

• metanephos forms loop of henle, capsule of bowman and convuluted tubules
• ureteric bud form collecting tubules 
• lateral plate mesoderm(as always) make blood vessels

Congenital Cystic Kindney

• collecting tubules and excretory tubules fail to fuse or their connection degenerates 
• urine will accumulate in distal part of excretory tubules, which will swell up forming cysts
• cysts will compress adjacent(possibly functional nephra) compounding the problem

Cloaca Development into UG system

• urorectal septum divides into anorectal canal and primitive UG sinus
• ureter and ejaculatory duct come to drain sperately into bladder/urethra
• bladder and urethra form from the urogenital sinus and allontois
• phallus forms(clitoris or penis)
• trigone formed by mesonephric duct
• allontois become urachus and then median umbilical ligament

Urachal Fistula

• when allontois does not obliterate and its lumen remains patent
• urine will leak through this urachal fistula and out umbilicus
• different from umbilicoilial fistula where gut contents leak out

Gonadal Development

• begin win week 5 but cannot distinguish till week 7
• primordial gemr cells(endoderm) move as ameboids from wall of allontois toward urogenital ridge where they lodge in primary sex cords (week 4-5) 
• uses mesoderm too (celomic epithelium) and mesenchyme
• mesonephric duct forms duct work for testes
• paramesonephric duct forms uterine tubes

Male Gonadal Development

• mesonephric duct forms epididymus
• ret testes formed by seminiferous cords
• primary sex cords become seminiferous cords and are detached from epithelium by growth of tunica abuginea
• in fetal period rete contact efferent ductules but do not have lumen
• in puberty seminiferous cords gain lumen

Female Gonadal Development

• mesonephric duct disappears
• paramesonephric duct becomes uterine tube
• sex cords deteriorate and surface epithelium make a second generation of cords called cortical cords
• during fetal period cortical cords detach and enclose germ cells forming the ova

Testicular Feminization Syndrome

• abscence of testosteronewill lead to the formation of external female genitalia even if 44XY male.
• this may be caused by maternal hormones passing into bloodstream of fetus through placenta
• undergo puberty externally as a female but lack uterus
• usually raised as girls

Uterus and Vagina Development

• paramesonephric ducts come together in middle and fuse; also have blind endings
• the spot on the urogenital sinus they are by becomes a paramesonephric tubercle(endoderm)
• duct makes uterus and 1/3 of vaginaand tubercle 2/3 of vagina
• hymen remains as wall between vagina and urogenital sinus 

Uterus Didelphys

• failure of left and right paramesonephric ducts to fuse
• leads to development of two uteri or in less severe a septum is present (bicornuate uterus
• possums have 2 uterus???

Development of External Genetalia

• male and female ext genitalia similar till wk 6
• genital tubercle forms phallus and erectile bodies
• cloacal membrane divides into ug and anal membrane
• medial cloacal fold becomes urethral fold and lateral becomes genital swelling

Male External Genetalia

• phallus forms most of penis (not ventral part)
• urethral groove becomes penile urehtra
• genital swelling becomes scrotal swelling and left and right become scrotum by fusing ventrally over penis

Female External Genetalia

• phallus forms clitoris
• urethral groove becomes the vestibule of the vagina 
• urethral fold becomes labium minus
• genital swelling becomes labium majus

Hypospadieas and Epispadias

• embryological defect in males
• hypospadias is failure of urethral folds to fuse in male leading to leakage of urine during urination(1:300)
• ext urethral orfice on ventral side of penis
• epispadias is formation of genital tubercle to far caudal causing part of the cloacal membrane to form cranial to tubercle
• leads to an open groove on dorsal side of penis 1:30,000

Hydronephrosis

• obstruction of the ureter causing renal pelvis and calyces to fill with urin and drain poorly
• results in poor function of nephra due to compression
• may happen in pelvic kidney

Cryptochism

• 3% of male births one testis faisl to descend into scrotum
• may lead to sterility of non-descended testis(body temp too high)
• surgery usually performed