Embryology Flashcards
Anal canal
- A layer of mesoderm, the urorectal septum, separates the region between the allantois and hindgut, the tip of the urorectal septum comes to lie close to the cloacal membrane, although the two structures never make contact.
- At the end of the seventh week, the cloacal membrane ruptures, creating the anal opening for the hindgut.
- Between the two, the tip of the urorectal septum forms the perineal body.
- At this time, proliferation of ectoderm closes the caudalmost region of the anal canal.
- During the ninth week, this region recanalizes. Thus, the caudal part of the anal canal originates in the ectoderm, and it is supplied by the inferior rectal arteries, branches of the internal pudendal arteries. The cranial part of the anal canal originates in the endoderm and is supplied by the superior rectal artery, a continuation of the inferior mesenteric artery, the artery of the hindgut. The junction between the endodermal and ectodermal regions of the anal canal is delineated by the pectinate line, just below the anal columns. At this line, the epithelium changes from columnar to stratified squamous epithelium.
- Lining of upper part of anal canal = embryologically derived from cloaca, ie is endodermal; lower part from proctoderm or oral pit & is ectodermal
- Dividing line between these = pectinate line
- **include embryology of sphincter complex
Cloaca
At the caudal end of the embryo, the hindgut and the allantois (a diverticulum from the yolk sac) meet in a common cavity, the cloaca , bounded distally by the cloacal membrane, which will form the procodeum.
From the dorsal wall of the allantois, the urorectal septum grows downwards to meet the cloacal membrane; dividing the urogenital sinus and urogenital membrane, and the anorectal canal and the anal membrane. The procodeum is the ectoderm lining, which will form the skin of the anal canal below the dentate line. The tip of the urorectal septum forms the perineal body.
Pancreas
- First appears at ~5wks gestation as two outpouchings of the endodermal lining of the duodenum at the junction of the foregut and midgut mesentery
- Outpouchings = ventral and dorsal buds
- Ventral bud develops as part of hepatic diverticulum & maintains communication w biliary tree throughout development
- Dorsal bud grows more rapidly than the ventral bud
- Between weeks 5-8, ventral bud rotates clockwise, posteriorly (during clockwise rotation of gut tube) toward the dorsal bud as it is ‘carried’ by the CBD
- Finally, ventral and dorsal buds are fused at ~8wks gestation
- In final anatomic arrangement, HOP originates from both dorsal pancreas & ventral pancreas
- Ventral bud becomes uncinate process & part of head
- Dorsal bud becomes other part of head, neck, body and tail of pancreas
- During development the drainage of dorsal & ventral components anastomose then split to generate the pattern seen in adult life; ie the uncinate draining via Santorini’s duct and the head, neck, body and tail draining with CBD through Duct of Wirsprung
- Primitive ducts of the dorsal and ventral buds fuse so that the proximal aspect of the dorsal bud duct forms the duct of Santorini, while the distal aspect joins with the duct of the ventral duct to form the duct of Wirsung
- Pancreatic divisum occurs when the buds don’t fuse
- Ventral pancreatic duct and CBD enter duo through a major papilla, whereas dorsal pancreatic duct enters through a minor papilla that is slightly proximal
- Bc most pancreatic exocrine secretions exit through the dorsal duct, pancreatic divisum can lead to a condition of partial obstruction caused by a small minor papilla à chronic backpressure in the duct ie relative outflow obstruction which has been implicated in development of relapsing acute or chronic pancreatitis
- Annular pancreas occurs when the buds don’t rotate correctly – ventral bud, which tends to be bi-lobed, migrates in different directions to fuse w the dorsal bud around D2
- Leads to circumferential or near-circumferential pancreas tissue surrounding D2
- May be assoc w other congenital defects incl Down syndrome, malrotation, intestinal atresia & cardiac malformations
- Can present in utero or during infancy
- 50% present later in life w recurrent pancreatitis or duodenal obstruction
- ectopic pancreas
- may arise anywhere in primitive foregut but most common in stomach, duo & Meckel’s diverticulum
- clinically, ectopic nodules may result in bowel obstruction caused by intussusception, bleeding or ulceration
- can sometimes be found incidentally as firm yellow nodules that arise from submucosa
- although rare case reports of adenocarcinoma arising in ectopic panc tissue, resection not necessary unless sx occur
- dorsal & ventral buds are comprised of endoderm covered in splanchnic mesoderm
- both acinar & islet cells differentiate from endoderm cells found in embryonic buds while splanchnic mesoderm eventually develops into the dorsal & ventral mesentery
Salivary glands
- Salivary glands develop from oral cavity in 6/7-10th week
- A groove that appears in ectoderm of mouth pit (stomodeum) becomes converted into a tunnel, from blind end of which cells proliferate to form the gland
- SMG buds from floor of primitive mouth and grows on lateral aspects of tongue
Malrotation
Symptomatic malrotation occurs in 1:6000 births.
At 4 weeks the bowel herniates and develops in the extracoelemic cavity.
It returns at week 10 and rotates 270° counter-clockwise.
The duodenojejunal segment returns first and rotates beneath and to the right of the SMA.
The caecocolic segment passes over the duodenum to fix in the right iliac fossa.
This process is usually complete by week 12.
Non-rotation, the most common anomaly occurs when neither the DJ segment nor the CC segment rotate, so they are adjacent and hanging off a narrow stalk, which is prone to volvulus.
Incomplete rotation occurs when the DJ segment rotates but the CC segment rotates only part way counter clockwise, so forming abnormal mesenteric (Ladd’s) bands across the gut.
Broadly describe the layers of the embryo with regard to eventual anatomical structures.
- Endoderm -> epithelium of GI and resp tracts, lining of bladder, urethra, reproductive system, liver and pancreas
- Mesoderm -> notochord, msk system, GIT muscle layer, circulatory system
- Ectoderm -> epidermis, cornea and lens, nervous system
- Alongside the notochord and neural tube, the mesoderm lies in 3 longitudinal strips; the paraxial mesoderm, the intermediate cells mass, and the lateral plate.
- Paraxial mesoderm becomes somites
- Sclerotome medially - becomes vertebrae and ribs
- Dermomyotome laterally - becomes muscles of body wall and dermis of skin.
- Intermediate cell mass
- Progenitors of gonad and adrenal cortex medially
- Pronephros, mesonephros, metanephros laterally
- Lateral plate split by intraembryonic coelom
- Inner layer becomes splanchnic mesoderm
- Becomes the mesenchyme of gut
- Outer layer becomes parietal mesoderm
- Gives rise to limb buds
- Paraxial mesoderm invades this to form flexor and extensor muscles of body
- Inner layer becomes splanchnic mesoderm
- Paraxial mesoderm becomes somites
Aorta
Aortic arches develop from aortic sac and course into pharyngeal arches
Branches of each travel in each pharyngeal arch and end in dorsal aortae (right and left)
Form various arteries of head and neck:
• First arch – maxillary artery
• Second– hyoid and stapedial arteries
• Third – CCA and ICA
• Fourth – right = proximal R SCA, left = aortic arch
• Sixth – split into ventral and dorsal segments
o Ventral = pulmonary arteries
o Right dorsal regresses
o Left dorsal = ductus arteriosus
Right and left dorsal aortae arise from aortic sac and fuse from T4 to L4 to form descending aorta
Above T4 the right dorsal aorta regresses
Dorsal aortae give off 7 cervical intersegmental arteries bilaterally
• Contribute to verterbral, superior intercostal and deep cervical arteries
• 7th intersegmental arteries contribute to subclavian arteries bilat
• Lower dorsal segmental arteries become the intercostals and lumbars
• Ventral branches become visceral arteries
Dorsal aortae continue down and eventually become umbilical arteries
IVC
Develops from multiple paired primitive veins that develop, anastamose and then regress in turn
Initially, blood from body wall returns to heart via posterior cardinal veins (blood from viscera travels in vitelline veins)
Subcardinal and then superior cardinal veins develop. Various portions regress and others contribute to IVC
IVC has 4 segments
• Hepatic (derived from vitelline vein)
• Suprarenal (from right subcardinal vein)
• Renal (suprasubcardinal and posterior subcardinal anastomoses)
• Infrarenal (from right supracardinal vein)
Describe the significant features of the fetal circulation.
- The economy of the fetal circulation is improved by three short-circuiting arrangements, all of which cease to function after birth:
- The ductus venosus
- The foramen ovale
- The ductus arteriosus.
Blood from placenta in umbilical vein
Runs in free edge of falciform ligament to liver, draining into left portal vein
Majority of this blood then bypasses liver in ductus venosus, from left portal vein to IVC
Blood travels in IVC to right heart
Some pumped out into pulmonary arteries, where a proportion crosses ductus arteriosus to enter arch of aorta
Rest passes through foramen ovale from right to left side of heart and is pumped into aorta
Blood travels in aorta then back to placenta via umbilical arteries
Liver
- Develops from a outgrowth (hepatic diverticulum) into the ventral mesogastrium from distal end of the primitive foregut at middle of wk3
- This tube divides into two and cells proliferating from either side form the lobes of the liver, enclosed by peritoneum, with the ventral double layer forming the falciform ligament
- develops by proliferation of cells from the blind ends of a Y-shaped diverticulum which grows from foregut into septum transversum, the caudal part of which becomes ventral mesogastrium (cranial part of septum transversum becomes pericardium & diaphragm; caudal part becomes ventral mesogastrium & is into this that liver grows)
- original diverticulum from endoderm of foregut becomes bile duct; its Y-shaped bifurcation produces right and left hepatic ducts
- a blind diverticulum from bile duct becomes cystic duct and gallbladder
- hepatic ducts divide and redivide to become interlobular & intralobular bile ductules
- at wk 10, haematopoietic function; at wk12, bile formed
Spleen
- Develops from mesenchyme in left leaf of dorsal mesogastrium - forms from cephalic aspect of lateral plate mesoderm during 5th week of gestation
- Divides that mesentery into two portions – ie gastrosplenic and splenorenal ligaments
- Rotation of the stomach brings the spleen to lie to its left, with the gastrosplenic ligament and lienorenal ligament thus forming from the folds of peritoneum between the two - all remnants of dorsal mesogastrium
- Multiple aggregations of mesodermal cells condense to form a single organ
- In up to 11% of individuals, one or more aggregates fails to condense – instead forms accessory spleen or splenule
- Usu occur in hilum or along the vessels of splenic pedicle, but can occur elsewhere particulary greater omentum
- Haematopoietic fn of spleen results from infiltration of cells from yolk sac wall & near dorsal aorta that continue to produce RBCs into 2nd trimester
- Although haematopoietic fn of spleen normally ceases in 5th mo of gestation, lymphocyte & monocyte generation persists throughout life
Kidney
Develop in 3 phases:
- Pronephros
- Mesonephros
- Metanephros
Metanephros forms the definitive kidney. It develops from the intermediate cell mass.
The metanephros induces the ureteric bud to form from the mesonephric duct.
The kidney develops in the pelvis and ascends during development to its final position. It takes segmental supply from the iliacs and then aorta during this process, finishing usually with a single renal artery.
Lower poles may fuse, forming horseshoe kidney
Testicle
Develop from gonadal ridge of intermediate cell mass in weeks 4-6
Descent:
- Two morphologically and hormonally distinct phases:
- Transabdominal
- Inguinoscrotal
Transabdominal
- Week 8-15
- Insulin-like Peptide 3 (INSL3) - dependent
- Testes descend from posterior abdo wall to deep ring
Inguinoscrotal
- Weeks 26
- Androgen dependent
- Elongated peritoneal diverticulum (processus vaginalis) precedes testis into scrotum. Mesodermal condensation called the gubernaculum precedes testicular descent, though its exact role is not well understood.
Timeline:
- 4th month: near deep ring
- 7th month: in deep ring
- 33rd week: majority in scrotum
Cryptorchidism
- 2-5% undesecended at birth
- All should be down by 1 yr
- Develops from gonadal ridge, formed by proliferation of coelomic epithelium & a condensation of underlying mesoderm, on medial side of mesonephros
- Primordial germ cells from yolk sac migrate to gonadal ridge & become incorporate in developing gonadal
- At first testis & mesonephros are on posterior abdo wall, attached by urogenital mesentery (testes develop in retroperitoneum below kidneys at T10 level)
- As testis enlarges its cranial end degenerates & remaining organ lies at a more caudal location
- Most of mesonphros atrophies
- Derivatives of remaining mesonephric tubules include vasa efferentia of testis & paradidymis (small collection of tubules above epididymis at lower end of spermatic cord)
- In male, mesonephric duct forms the canal of the epididymis, vas deferens, ejaculatory duct and appendix of epididymis (small appendage on head of epididymis)
- A condensation of mesodermal cells, the gubernaculum, connects lower pole of testis to region of ant abdo wall that later forms scrotum (during 7th wk of fetal development, gubernaculum forms within folds of peritoneum – upper end attached to developing testis & lower end to fascia between developing abdo wall muscles)
- At same time, an evagination of peritoneum itself, the processus vaginalis, develops adjacent to gubernaculum & over subsequent weeks evaginates through abdo wall to create inguinal canal, picking up fibres from abdo wall
- As it elongates the processus carries gubernaculum with it, which contains muscle fibre though not fully clear what part it plays in testicular descent
- Testes lie at internal inguinal ring at 3mo gestation & descend to scrotum betwee 7-9mo
- This latter descent into scrotum = accompanied by shortening of gubernaculum
- Final descent from ext ring to base of scrotum takes 4-6wks & usu complete by birth
- As testis descends is accompanied by processus vaginalis
- Testis projects into distal part of processus, which forms tunica vaginalis
- Rest of this peritoneal sac usu gets obliterated; persistence of whole or prox part of sac maintaining its connection w peritoneal cavity constitutes a hernia sac, a clinical hernia occurring when intra-abdo contents enter the sac
- Persistence of an intervening segment of processus vaginalis may lead to development of a hydrocele of the cord
- Accumulation of serous fluid between the layers of tunica vaginalis forms much more common hydrocele of testis
- Mechanisms that result in teseticular descent poorly understood but prob involve Mullerian inhibiting substance
- Maternal chorionic gonadotrophin stimulates growth of testis & may stimulate its migration
- Imperfectly developed testes tend to descend incompletely
Adrenals
- Medulla is derived by migration of cells from the neural crest and is ectodermal in origin
- Cortex is derived in-situ from the mesoderm of the intermediate cell mass
- Cortex – derived in situ from mesoderm of intermediate cell mass
- Starts to appear at 5/40 near the cephalic end of the mesonephros as 2 clefts on either side of embryonic dorsal mesentery that enlarge to form primitive or foetal cortex
- This is surrounded at 7/40 by 2nd wave of mesothelial cells to form the secondary cortex that eventually becomes the adult adrenal cortex
- Biosynthetic activity can be detected as early as 7th week
- Cortical cell mass dominates fetal adrenal at 4mo of development, and steroidogenesis is max during 3rd trimester
- Medulla derived from neural crest cells (neuroectodermal origin) – differentiates into chromaffin cells
- Arises from ectodermal tissues of embryonic neural crest
- Develops in parallel w sympathetic nervous system, beginning in 5th-6th wk of gestation
- From their original position adjacent to neural tube, neural crest cells migrate ventrally to assume a para-aortic position near the developing adrenal cortex
- There they differentiate into chromaffin cells that make up the adrenal medulla
- ‘cells migrating from the neural crest invade the developing adrenal gland to form the adrenal medulla’
- adrenal glands = large at birth but decreases in size thereafter due to regression of primary cortex, & don’t regain original size til puberty
- zona glomerulosa starts to appear before delivery, followed by zona fasciculata& finally zona reticularis a few months after birth
- neural crest cells, outside the adrenal medulla = widely present in embryo but regress after birth
- as a result of the above embryologic development, both cortical & medullary tissue ca be found at extra-adrenal sites
- range of potential sites is wider for chromaffin tissue than for cortical tissue
- phaeos may arise in extra-adrenal sites more commonly than previously believed; when extra-adrenal, phaeos are also called ‘paragangliomas’
Veins
Developing heart (sinus venosus) receives blood from 3 sources • Umbilical veins (from placenta) -\> degenerate (ligamentum teres, ductus venosus)
- Vitelline veins (from yolk sac – eventually becomes GI tract) -> portal vein
- Cardinal veins (from body tissues of embryo) -> SVC and IVC, azygos system
All have right and left branches, one of which disappears
• Right umbilical
• Left vitelline
• Left cardinal
All have communications between left and right. Persistence of these transverse communications forms the left brachiocephalic and left common iliac veins.
