Embryology

Question 1

Pancreas

Answer 1

Develops from dorsal and ventral buds, either side of the developing duodenum, from weeks 5-7.

Ventral bud grows into ventral mesogastrium in conjunction with developing bile duct

Dorsal bud grows into dorsal mesogastrium.

Duodenum grows asymetrically, bringing ventral bud around and behind it in clockwise rotation till the buds meet and fuse.

Dorsal bud gives rise to most of head, body and tail. Proximally its duct form the accessory duct of Santorini. Ventral bud forms uncinate process. Distal dorsal duct units with ventral duct to from main duct of Wirsung.

Aberrations of this process lead to annular pancreas and pancreas divisum.

Question 2

Meckel’s diverticulum

Answer 2

Vitellointestinal duct

Question 3

Aorta

Answer 3

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

Question 4

IVC

Answer 4

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)

Question 5

Foetal circulation

Answer 5

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

Question 6

Derivatives of three germ cell layers

Answer 6

• 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

Question 7

Breast

Answer 7

Modified apocrine sweat gland
Develops from milk ridge - ridge of ectoderm extending from axilla to mid inguinal region
• Accessory nipples (polythelia) can occur anywhere along this line
Mammary pit becomes elevated above level of skin to form nipple
• Failure to do so causes inverted nipple
Oestrogens stimulate duct growth, progestogens stimulate lobules

Question 8

Intestine

Answer 8

Growing bowel herniates out of abdomen at week 6 of gestation - the “physiological hernia”
Returns by week 10
Normal rotation of bowel is 270 degrees anti-clockwise
SMA is axis of rotation
DJ flexure ends up to left of SMA
Caeco-colic limb rotates from below to over top of SMA, finally lying to right of SMA.

Non-rotation
• DJ flexure and small bowel lie in right abdomen, and colon in left
• Some risk of mesenteric volvulus due to narrow based mesentery, but less risk of obstruction as no Ladd’s bands

Mal-rotation
• Duodenum does rotate and lies in right of abdo
• Caeco-colic limb partially rotates, 90 or 180 degrees, and lies in upper abdo, fixed by Ladd’s bands over the duodenum to the abdo wall
• These can cause obstruction

Question 9

Liver

Answer 9

Develops from a outgrowth into the ventral mesogastrium from the primitive foregut.
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

Question 10

Spleen

Answer 10

Develops from a proliferation of cells in the left leaf of the dorsal mesogastrium

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 (Last’s p323) - all remnants of dorsal mesogastrium

Question 11

Kidney

Answer 11

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

Question 12

Testicle

Answer 12

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

Question 13

Diaphragm

Answer 13

4 contributions:

• Septum transversum (central tendon)
• Pleuroperitoneal membranes
• Muscle from the cervical myotomes C3-5 (invade septum transversum)
• Dorsal mesentery of oesophagus

Question 14

Branchial cyst

Answer 14

• Form due to the incomplete involution of branchial cleft structures.
• Around the fourth week of gestation, neural crest cells migrate into the future head and neck region, where the 6 pairs of branchial (pharyngeal) arches begin to develop. The mesoderm is covered externally by ectoderm and internally lined by endoderm.
• 5 branchial arches, with the arches are separated by depressions known as clefts on the ectodermal surface and corresponding pouches on the endodermal surface, yielding four pharyngeal clefts.
• The second arch develops caudally and then covers the third and fourth arches. These buried clefts become ectoderm-lined cavities that normally involute completely by 7 weeks of gestation. If the clefts do not involute or incompletely involute, these pathological remnants will form cysts, sinuses, or fistulae in predictable locations according to their branchial cleft of origin
• Lined with stratified squamous epithelium and may contain keratinous debris inside the cyst.

Question 15

Thyroid

Answer 15

Ventral diverticulum from foramen caecum at the base of the tongue. This descends as the median thyroid anlage, forming the thyroglossal tract, bringing the developing thyroid down into the anterior neck to lie anterior and lateral to the proximal trachea.

Question 16

Parathyroids

Answer 16

Superior develops from dorsal aspect of fourth pharyngeal pouch. This is anchored to thyroid gland on ventral aspect, preventing excessive descent of superior parathyroid.

Inferior develops from dorsal aspect of third pharyngeal pouch (thymus develops from ventral aspect).

Question 17

Pharyngeal arches

Answer 17

1st - artery = maxillary, nerve = mandibular (V3)
Pouch: 
middle ear
auditory tube
mastoid antrum

Arch (almost all begin with M): 
Mylohyoid
Muscles of mastication
Mandible
Meckel’s cartilage
Anterior digastric
Tensor palati and tympani

Cleft:
External acoustic meatus

Mnemonic = maxillary artery, mandibular nerve, most of ear, muscles of mastication and mandible

2nd - artery = stapedial, nerve = facial
Pouch
Tympanic cavity
Tonsillar crypts
Supratonsillar fossa
Palatine tonsil

Arch
Muscles of facial expression
Posterior digastric
Stapes
Stapedius
Styloid process
Lesser horn and superior body of hyoid
Stylohyoid and ligament

Cleft
If cervical sinus persists, this forms branchial cleft sinus/fistula/cyst

Mnemonic - most begin with S. Remember facial nerve and the muscles it supplies

3rd - artery = common and internal carotid, nerve = glossopharyngeal
Pouch
Inferior parathyroids and thymus (remember they descend together)
Arch
Greater horn and inferior body of hyoid (2nd arch does the top bit)
Stylopharyngeus

4th - artery = (right) SCA/(left) aortic arch, nerve = superior laryngeal nerve
Pouch
Superior parathyroids

Arch (with 6th arch)
Laryngeal cartilages
Laryngeal and pharyngeal muscles
Levator palati

5th
Pouch
Ultimobranchial body -> C cells of thyroid, tubercle of Zuckerkandl
Arch - rudimentary

6th - artery = pulmonary artery (inc ductus arteriosus on left), nerve = RLN
Arch - see 4th

Question 18

Adrenals

Answer 18

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

Question 19

Veins

Answer 19

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.

Question 20

Portal vein

Answer 20

Embryonic arrangement is two vitelline veins surrounding duodenum. These have superior (running posterior to duo) and inferior (anterior to duo) transverse communications.

Normally the superior persists, with degeneration of the others - see above. But if inferior communicating vein persists, the preduodenal portal vein will arise.

Question 21

Omentum

Answer 21

Develops from the dorsal (-> greater) and ventral (-> lesser) embryonic mesentery.

Greater omentum is double-layered fold of peritoneum extending from oesophagus at diaphragmatic hiatus, along greater curve (and enclosing spleen) all the way to duodenum. Posterior layer attaches to anterior leaflet of transverse colon mesentery as well as forming posterior wall of lesser sac

Lesser omentum is formed from ventral mesogastrium between stomach and liver.

Question 22

Male/female urogenital tract

Answer 22

Both develop from mesonephros, from intermediate cell mass

Mesonephric (Wolffian) duct = vas, epididymis, seminal vesicles

Paramesonephric duct = uterus, fallopian tube, vagina

Question 23

Urachus

Answer 23

Is a remnant of the foetal Allantois, which connects the urinary bladder to the yolk sac and allows drainage of nitrogenous waste from the developing embryo.

Lies in the space of Retzius, from the dome of the bladder to the umbilicus, and raises the median umbilical fold of peritoneum

Persistence can lead to urachal cyst, sinus, fistula or bladder diverticulum

Question 24

Vitelline duct

Answer 24

Connects yolk sac to midpoint of developing GIT

Usually obliterates by week 9

Persistence leads to Meckel’s diverticulum or vitelline fistula