1
Q
The primitive gut tube is formed from the incorporation of the dorsal part of the yolk sac into the embryo as a result of what two types of folding in the embryo?
A
craniocaudal folding
lateral folding
2
Q
Week 4: The primordial gut is closed at its cranial and caudal ends by what membranes?
A
cranial = oropharyngeal membrane
caudal = cloacal membrane
3
Q
What tissue of the primordial gut gives rise to most of the epithelium and glands?
A
endoderm
4
Q
What part of the adult G.I. tract is derived from endoderm and has a lamina propria and muscularis mucosae?
A
Mucosa
epithelial lining and glands
5
Q
What 3 parts of the acult G.I tract are derived from visceral mesoderm?
A
submucosa
muscularis externa
adventitia or serosa
*(lamina propria + muscularis mucosa are also mesoderm)
6
Q
In early development the epithelial lining of the gut proliferates and obliterates the lumen. What precess reverses this later?
A
recanalization
7
Q
The epithelium of the cranial and caudal end of the gut is derived from what primordial tissue type?
A
ectoderm of the stomodeum and proctodeum
8
Q
The primordial gut is divided in what three sections?
A
- foregut
- midgut
- hindgut
9
Q
The Foregut:
What are the derivitives?
A
Primordial pharynx (+ derivatives)
Lower respiratory system
Esophagus and stomach
Duodenum, distal to the opening of the bile duct
Liver, biliary apparatus (hepatic ducts, gallbladder + bile duct)
Pancreas
10
Q
The foregut derivatives are supplied by the what artery/branch?
A
celiac trunk
11
Q
The foregut is divided into the esophagus dorsally + trachea ventrally by what folds?
These fuse to form what structure?
The esophagus reaches its final length by what week?
A
- tracheoesophageal folds
- tracheoesophageal septum
- week 7
12
Q
The epithelium and glands of the esophagus are derived from what primordial tissue?
It proliferates and obliterates the esophageal lumen. By what week does recanalization occur?
A
endoderm
- week 8
13
Q
Esophagus: superior third
A
- striated muscle forms muscularis externa
- derived from mesenchyme in caudal pharyngeal arches
- innervated by CN X
14
Q
Esophagus: inferior two thirds
A
- smooth muscle
- derived from splanchnic mesenchyme
- innervated by CN X
15
Q
Esophageal Atresia
A
incomplete separation of the esophagus from the trachea
- results from deviation of tracheoesophageal septum in a posterior direction
- associated w. tracheoesophageal fistula (TEF) - 85% of cases
- other congenital defects associated with the VACTERL syndromes - 33% of cases
- can not swallow amniotic fluid = polyhydramnios
- Inability to pass a catheter through esophagus into stomach
16
Q
V.A.C.T.E.R.L. syndromes/associations
A
V = vertebral defects
A = anal atresia
C = cardiovascular defects
T/E = tracheoesphageal fistula
R = renal defects
L = upper limb defects
*syndrome = all defects; association = only some*
17
Q
Esophageal Stenosis
A
narrowing of the lumen (usually in midesophagus)
- week 8 = recanalization process is incomplete, or blood vessels did not develop in this region= atrophy of segment
18
Q
Esophageal duplication
A
usually a congenital esophageal cyst
**(usually lower esophagus) **
- cysts may lie on posterior aspect of the esophagus protruding into the posterior mediastinum
19
Q
Vascular Compression of the Esophagus
A
abnormal origin of one of the vessels derived from the pharyngeal arteries (usually right subclavian artery)
- anomalous artery passes behind the esophagus and may cause dysphagia (difficulty swalling)
20
Q
The foregut is initially a simple tubular structure. At week 4, a slight dilation becomes ____________?
A
primordium of the stomach
(oriented in the median plane)
21
Q
Rotation of the Stomach
A
90 degrees clockwise
- lesser curvature (ventral) moves right; greater
curvature (dorsal) moves left
- cranial region moves left + slightly inferiorly; caudal region moves right + superiorly
- Final position = long axis almost transverse
22
Q
Due to rotation, the dorsal mesentery is carried to the left and eventually forms …?
What nerves innervates the ventral and dorsal surfaces of the stomach?
A
** greater omentum**
ventral = left vagus nerve (CN X)
dorsal = right vagus nerve (CN X)
23
Q
Regarding the stomach, which cells are derived from endoderm?
A
Surface mucous cells lining the stomach
mucous neck cells
parietal cells
chief cells
enteroendocrine cells of the gastric glands
24
Q
Regarding the stomach, which layers are derived from visceral meesoderm?
A
lamina propria
muscularis mucosae
submucosa
outer longitudinal layer
middle circular layer
inner oblique layers of smooth muscle of muscularis externa
serosa of the definitive stomach
25
Congenital Hypertrophic Pyloric Stenosis
**thickening of the pylorus**
- circular + longitudinal muscles in the pyloric region are hypertrophic causing a narrow pyloric lumen that obstructs food passage
- treatment = **pyloromyotomy**
_Results in:_
- distended stomach (palpated @ R. costal margin)
- projectile vomiting (no bile)
* \* increased incidense in infants treated with **erythromycin***
26
The duodenum develops from what primordial sections?
- caudal part of foregut
- cranial part of the midgut
- splanchnic mesenchyme of these regions
27
What vessels supply the duodenum?
When does recanalization occur?
branches of **celiac + superior mesenteric** arteries
- recanalization = end of embryonic period
28
Duodenal Stenosis
**incomplete recanalization**
stenosis of stomach's contents = projectile vomiting
(USUALLY with bile)
29
Duodenal Atresia
**Complete occlusion of duodenal lumen**
- occurs at junction of bile + pancreatic ducts (hepatopancreatic ampula)
- vomiting starts a few hours after birth (ALWAYS contains bile)
30
What structures arise from the distal part of the foregut and form the **hepatic diverticulum**?
Where does the diverticulum extend to?
**- liver**
**- gallbladder**
**- biliary duct system**
diverticulum extends into the *septum transversum*
(mass of splanchnic mesoderm b/t developing heart +midgut
31
The tremendous growth of the liver causes what?
1. liver bulges into abdominal cavity = stretches septum transversum
**= ventral mesentery** (falciform ligament + lesser omentum)
32
falciform ligament
- contains the **left umbilical vein** (after birth = ligamentum teres)
33
lesser omentum
- divided into the **hepatogastric ligament + hepatoduodenal ligament**
- **hepatoduodenal ligament** = bile duct, portal vein, and hepatic artery (i.e., portal triad)
34
What are important developmental features of the liver at:
## Footnote
**\* week 5-10?**
**\* week 6?**
**\* week 12?**
**week 5-10** = fast hepatic growth; liver fills lg. part of peritoneal cavity
**week 6 = *hematopoiesis ***begins (liver is bright reddish color)
**week 12 = **bile formation by hepatic cells start
35
What does the connection between the hepatic diverticulum and the foregut create?
An outgrowth from this structure gives rise to _______ and \_\_\_\_\_\_\_\_.
**bile duct**
- gallbladder rudiment + cystic duct
36
The cystic duct divides the bile duct into the ________ an \_\_\_\_\_\_\_?
**common hepatic duct** + **common bile duct**
\*(closure & recanalization of the lumen occur)
37
At Week 13, what gives the meconium (intestinal contents) a dark green color?
** bile** entering the duodenum through the bile duct
38
Biliary Atresia
**obliteration of extrahepatic and/or intrahepatic ducts**
- ducts are replaced by fibrotic tissue due to acute and chronic inflammation
- Clinical symptoms = progressive **neonatal jaundice**, **white clay-colored stool** (acholia), and **dark-colored urine**
- average survival time w/o treatment = 12 to 19 months (100% mortality rate)
- treatment = **Kasai hepatoportoenterostomy** (surgical procedure) or liver transplantation
39
What primordial structures arise from endoderml cells in pancreatic development?
dorsal + ventral pancreatic buds
(rotation of the duodenum causes these buds to eventually fuse)
40
ventral pancreatic bud
= uncinate process
= head of the pancreas
41
dorsal pancreatic bud
= accessory pancreatic duct
= body of pancreas
= tail of pancreas
42
The main pancreatic duct and common bile duct form a single opening into the duodenum called what?
Where is this opening located?
**hepatopancreatic ampulla of Vater**
- at the tip of a major papilla **(hepatopancreatic papilla)**
43
**Acinar cells, islet cells**, and **simple columnar** or **cuboidal epithelium** lining the pancreatic ducts are derived from what type of tissue?
**endoderm**
*(Surrounding connective tissue + vascular components = visceral mesoderm)*
44
**1. Insulin secretion** starts at what week in the pancreas?
* Glucagon + somatostatin-containing cells develop before differentiation of insulin-secreting cells.*
2. Endodermal cells from tubules accumulate within mesoderm to form islet cells:
**name 4 types.**
Week 10
- **alpha cells** (glucagon)
- **beta cells** (insulin)
- **delta cells** (somatostatin)
- **PP cells** (pancreatic polypeptide)
45
What causes an **Annular Pancreas?**
What is raiological sign of this?
**ventral pancreatic bud fuses with dorsal bud both dorsally + ventrally**
(= ring of pancreatic tissue around duodenum; severe duodenal obstruction)
_Radioraphy shows:_
duodenal obstruction + **"double bubble" sign** (dilation of stomach + distal duodenum)
46
This pathology occurs when the *distal two thirds of the dorsal pancreatic duct + the entire ventral pancreatic duct **fail to anastomose*** (dorsal pancreatic duct persists; two separate duct systems)?
These patients are prone to...?
**Pancreas Division**
- prone to **pancreatitis**
47
**Hyperplasia of Pancreatic Islets**
- fetal islets exposed to high blood glucose levels in infants of diabetic mothers
- glucose stimulates fetal islet hyperplasia + insulin secretion = increased fat + glycogen deposition in fetal tissues
- results in *increased birth weight at term* **(macrosomia)** + episodes of ***hypoglycemia** in the postnatal period*.
48
The spleen is derived from what tissue type?
a mass of **mesenchymal cells** (b/t layers of dorsal mesogastrium)
- capsule, connective tissue + parenchyma of spleen differentiate from mesenchyme
49
What does the **splenorenal ligament** form from?
fusion of **mesogastrium** + **peritoneum** over the left kidney
50
The spleen functions as a hematopoietic organ during what part of fetal life?
approx. **week 3 -\> late fetal life** (month 5)
51
Midgut
Its derivatives are:
- small intestine
- distal duodenum
- cecum
- appendix
- ascending colon
- part of the transverse colon
52
The midgut is supplied by what artery?
superior mesenteric artery
53
physiologic umbilical hernia
As the intestine grows it forms a **midgut loop that projects into the umbilical cord;** important for the shape the intestine will form
- communicates with umbilical vesicle through *omphaloenteric duct (yolk sac)* until **week 10**
- midgut rotates counter clockwise arouns SM artery = forms intestinal loops
54
What are the **2 limbs** of the midgut loop of intestine?
**cranial limb** and **caudal limb**
(suspended by an elongated mesentery)
_cranial limb:_
- grows fast
- forms small intestinal loops
_caudal limb:_
- goes through little change
- forms the cecal sweelings (primordium of cecum), + appendix
55
During rotation of the midgut, which limb elongates and forms intestinal loops (primordia of the jejunum and ileum)?
the cranial limb
56
When & how do the intestines return to the midgut?
**Week 10**
- 1st: small intestine returns to abdomen
- 2nd: as large intestine returns, it rotates 180 degrees counterclockwise. (comes to occupy right side of abdomen)
- 3rd: ascending colon becomes recognizable as the posterior abdominal wall elongates
57
**Fixation of the Intestines**
(occurs as intestines enlarge, lengthen, and assume their final positions)
- their mesenteries are pressed against the posterior abdominal wall
- ascending colon becomes **retroperitoneal** (looses its peritoneum)
- fan shaped mesentery of small intestines attaches to the dorsal abdominal wall by a new **line of fixation that passes from the duodenojejunal junction**
58
At what week does the **cecal swelling** appear?
What does it become?
**week 6**
- primordium of cecum and appendix
- appears as an elevation of the border of the caudal limb
- after birth, wall of cecum grows unequally; 64% of people the appendix is located **retrocecally**
59
Congenital Omphalocele
**herniation of abdominal contents through umbilical ring**
- covered by a translucent peritoneal membrane sac protruding from the base of the umbilical cord
- abdominal cavity is small; surgical repair is required, often these infants present **pulmonary + thoracic hypoplasia**
***\*** Associated with: congenital anomalies such as **trisomy 18 (Edward's Syndrome) + 21 (Down Syndrome)\****
60
**Umbilical Hernia**
intestines return to the abdomen, but then **herniate through a faulty umbilicus**
- protruding mass is covered by subcutaneous tissue + skin
- hernia protrudes during crying, or coughing
- can be easily reduced through the fibrous ring at the umbilicus (slowly, over time)
61
**Gastroschisis**
Congenital abdominal wall defect; Extrusion of abdominal viscera *without involving the umbilical cord*.
- viscera float in the amniotic fluid
- intestines are not covered by a peritoneal membrane( thickened + covered with adhesions)
62
Ileal Diverticulum (Meckel's Diverticulum)
**remnant of the vitelline duct** (joins yolk sac to midgut)
- outpouching of the ileum = common anomaly of the digestive system
- wall of the diverticulum contains *all layers of the ileum* + may contain *gastric and pancreatic tissues*
- ectopic gastric tissue may secrete acid = ulceration + bleeding.
- appears on antimesenteric border of ileum, 40 to 50 cm from ileocecal junction
63
**Hindgut**
Its derivatives are:
- left half of transverse colon
- descending colon
- sigmoid colon
- rectum + part of the anal canal
- epithelium of urinary bladder
- most of the urethra
64
hindgut and its derivatives are supplied by what artery?
the inferior mesenteric artery
65
What is the expanded terminal part of the hindgut lined by endoderm?
Where does it contct surface ectoderm?
**Cloaca**
(receives the allantois ventrally, which is a fingerlike diverticulum)
- contacts surface ectoderm @ **cloacal membrane**
66
The cloaca is divided into dorsal and ventral parts by what?
What are these 2 divisions?
**the urorectal septum** (mesenchyme)
(develops b/t allantois + hindgut)
Dorsal: rectum + part of the anal canal
Ventral: urogenital sinus
67
What occurs at week 7, in regards to the urorectal septum?
fuses with cloacal membrane
- divided into **anal membrane** and a larger ventral **urogenital membrane**
***- perineal body results from this fusion***
68
**urorectal septum:**
divides the cloacal sphincter into anterior and posterior parts. What are these parts?
posterior = **external anal sphincter**
anterior = **superficial transverse perineal**, **bulbospongiosus**, and **ischiocavernosus muscles**.
69
**proctodeum** (anal pit):
formed by mesenchymal proliferation in the surface ectoderm around the
anal membrane
70
**The Anal Canal:**
The anal membrane ruptures by week ...?
This allows communication between what 2 structures/areas?
**week 7**
- communication b/t digestive tract + amniotic cavity
71
Anal Canal
superior canal = derived from **hindgut** *(superior rectal a.)*
inferior part = develops from **proctodeum** *(inferior rectal aa.)*
***(divided by pectinate line)***
- approx. 2 cm superior to the anus = *anocutaneous line (white line)*
(epithelium changes from columnar to stratified squamous)
- anus = keratinized epithelium; continuous with external skin
72
Hirschsprung Disease (Congenital Megacolon)
**- failure of neural crest cells to migrate into wall of colon = no development of parasympathetic ganglion in Auerbach and Meissner plexuses**
- lack autonomic ganglion cells (aganglionosis) in myenteric plexus distal to dilated segment of colon
- enlarged colon or megacolon= normal number of ganglion cells
_** \*most common cause of neonatal obstruction of the colon\***_
treatment:
rescect the affected area; probably prepare an ostomy
until distal portion of colon can connect to anal canal
73
**Imperforate Anus**
**Abnormal development of urorectal septum**
= incomplete separation of the cloaca into urogenital +anorectal portions
- sometimes it's obvious, sometimes there's a "blind opening" when an entrance exists but only goes a few cm.
74
Anal Agenesis
| (with or without fistula)
**_most common type of anorectal anomaly_**
- anal canal may end blindly; may be an ectopic anus or an anoperineal fistula opening into perineum (or into vagina or urethra)
- rectum ends superior to puborectalis muscle
- usually a fistula to the bladder (rectovesical) or to the vagina (rectovaginal)
- passage of meconium or flatus (gas) in the urine is **diagnostic of rectourinary fistula**.
- Higher rates of fecal incontinence.
75
Anal Stenosis
anus is in normal position, but the anus + anal canal are narrow
## Footnote
**Due to a deviation of the urorectal septum**
76
Membranous Atresia of Anus
- thin layer of tissue separates anal canal from the exterior
- anal membrane appears blue from meconium superior to it
**Due to imperforation of the membrane at week 8.**
77
Rectal Atresia
- anal canal + proximal rectum are present but separated
- sometimes two segments are connected by fibrous cord (remnant of an atretic portion of the rectum)
- cause may be **abnormal recanalization of the colon** or more likely **defective blood supply**
BLOCK 2 - Embryology (Grace)
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