Embryology

Question 1

GI Tract overview

Answer 1

Gut tube formed from endoderm lining yolk sac

Oropharyngeal membrane and cloacal membrane

During folding; somatic mesoderm –> parietal peritoneum, visceral mesoderm –> mesenteries

Nerves and neurons derived from neural crest

Question 2

Foregut structures

Answer 2

• Trachea and resp tract
• lungs
• oesophagus
• stomach
• liver
• GB and bile ducts
• pancreas
• upper duodenum

Question 3

Midgut structures

Answer 3

• lower duodenum
• jejunum
• ileum
• cecum
• appendix
• ascending colon
• prox 2/3 transverse colon

Question 4

Hindgut structures

Answer 4

• distal 1/3 transverse colon
• descending colon
• sigmoid colon
• rectum
• upper anal canal
• urogenital sinus

Question 5

Arterial supply of foregut, midgut and hindgut

Answer 5

Foregut; coeliac artery
Midgut; SMA
Hindgut; IMA

Question 6

Embryology of Stomach

Answer 6

• appears first as fusiform dilation of foregut
• undergoes 90º rotation; left side moves ventrally, right side dorsally
• differential growth on left and right produces greater and lesser curves respectively
• craniocaudal rotation tips pylorus superiorly
• anterio-posterior rotation creates final adult position

Question 7

Embryology of Lesser sac

Answer 7

• stomach attached to both dorsal and ventral mesogastrium
• it’s rotation & disproportional growth alters mesogastrium position
• rotation along longitudinal axis, dorsal mesentery to the left = lesser sac

Question 8

Embryology of Duodenum

Answer 8

• caudal part of foregut and cranial midgut

- as stomach rotates, duodenum becomes C-shaped and also rotates

Question 9

Embryology of Liver and biliary apparatus

Answer 9

• outgrowth endoderm distal foregut
• liver bud grows into septum transversum
• connection between foregut and liver bud narrows –> bile duct
• ventral outgrowth from bile duct forms GB and cystic duct

Question 10

Embryology of Spleen

Answer 10

• develops in dorsal mesogastrium
• up to week 14 = haematopoetic function
• weeks 15-17 colonised by T lymphocytes
• week 23 B cell precursors arrive
• spleen can begin lymphoid function

Question 11

Embryology of Pancreas

Answer 11

• forms from 2 buds which grow from endodermal lining of duodenum
• dorsal and ventral buds fuse

Question 12

Embryology of Midgut

Answer 12

• primary intestinal loop
• rotates 90º around SMA axis
• as this occurs, herniates into extra-embryonic cavity
• retraction of herniated loops

Question 13

Embryology of Hindgut

Answer 13

• terminal portion of hindgut joins with posterior cloaca
• allantois enters into anterior part of cloaca
• end week 7 cloacal membrane ruptures (hindgut opening)
• ectoderm anal canal proliferates, closing caudal end
• anal canal re-opens week 9

Question 14

Where do neural crest cells arise from?

Answer 14

Cells at crest of neural fold which develops to form neural tube

Question 15

Describe somites

Answer 15

Form from paraxial mesoderm

Go on to form structures like axial skeleton and muscles

Human embryos develop around 33, those around head region develop cranial bones

Question 16

Embryology of Pharyngeal / branchial arches

Answer 16

Develop around future mouth and pharynx

• support primitive pharynx
• face and neck develop from them

Question 17

What nerves develop from the pharyngeal arches?

Answer 17

1st arch; maxillary branch of trigeminal nerve
2nd arch; facial nerve
3rd arch; glossopharyngeal nerve
4th arch; sup laryngeal branch of vagus
NO FIFTH ARCH
6th arch; recurrent laryngeal branch of vagus

Question 18

Describe where muscles of the facial nerve develop from

Answer 18

Muscles of the facial nerve develop from the 2nd branchial arch, the same as the facial nerve

Muscles supplied a nerve developed from a branchial arch will also have developed from this branchial arch

Question 19

Facial development

Answer 19

From five processes

• 1 frontonasal
• 2 maxillary
• 2 mandibular

Question 20

How does the nostril develop?

Answer 20

Ectodermal thickening (nasal placode) invaginates to form nostril

Question 21

Describe palate formation

Answer 21

Medial nasal process grows downwards and forms philtrum

Maxillary process develops palatine shelves which grow inwards and meet in the midline

*Abnormalities cause cleft palate

Question 22

Describe cranial vault bone formation (skull cap)

Answer 22

Membranous ossification of calvaria
- mesenchyme derived from mesoderm directly above developing brain ossify directly

Not fully formed at birth; sutures are soft & 6 membrane covered fontanelles (anterior closes at 1.5yrs, posterior 2-3months)

Question 23

Describe base of skull formation (irregular bones)

Answer 23

Mesenchyme around notochord mostly derived from neural crest cells

Preform in cartilage and later ossify (endochondral ossification)

Question 24

Describe viscerocranium formation (facial bones)

Answer 24

• partly from branchial arches
• partly from sensory capsules

sensory capsules are derived from mesoderm of somites in head and neck region

they preform in cartilage and then ossify to form bones around sense organs

Question 25

When does the neuro system start to form?

Answer 25

Beginning of third week

Question 26

When do each of the neuropores close?

Answer 26

Anterior ~day 25 | Posterior ~day 27

Question 27

Describe neural tube closure

Answer 27

Up to 5 closure sites in humans | Both cranial and caudal directions

Question 28

Describe the failed features of neural formation in anencephale, encephalocoele and spina bifida

Answer 28

Anencephalocoele; failure anterior neuropore to close thus skull fails to form Encephalocoele; failure rostral neural tube to close causing herniation brain tissure through skull Spina bifida; failure vertebral arches to fuse

Question 29

What are the primary brain vesicles?

Answer 29

Prosencephalon Mesencephalon Rhombencephalon

Question 30

What are the secondary brain vesicles? (in order)

Answer 30

``` Telencephalon Diencephalon Mesencephalon Metencephalon Myelencephalon ```

Question 31

What forms from the telencephalon?

Answer 31

Cerebral hemispheres Hippocampus Basal ganglia

Question 32

What forms from the diencephalon?

Answer 32

Thalamus, hypothalamus, pineal gland and pituitary gland

Question 33

What forms from the mesencephalon?

Answer 33

MIDBRAIN | Superior and inferior colliculi

Question 34

What forms from the metencephalon?

Answer 34

Cerebellum and pons

Question 35

What forms from the myelencephalon?

Answer 35

Medulla

Question 36

Describe the cephalic flexure formation

Answer 36

End of third week | Between midbrain and hindbrain

Question 37

Describe the cervical flexure formation

Answer 37

End of fourth week | Between hindbrain and SC

Question 38

Describe the pontine flexure formation

Answer 38

End of fifth week | In hindbrain

Question 39

What does the renal system develop from?

Answer 39

Intermediate plate mesoderm

Question 40

What is the allantois?

Answer 40

outpouching of ectoderm at hindgut that extends into body stalk

Question 41

What does the body stalk form?

Answer 41

umbilical cord

Question 42

What is the remnant of the allantois?

Answer 42

medial umbilical ligament

Question 43

What does the cloaca divide to form?

Answer 43

rectum and urogenital sinus

Question 44

What does the mesonephric duct drain into?

Answer 44

Urogenital sinus; becomes urinary bladder and caudal end of urethra

Question 45

Describe kidney development

Answer 45

Week 4 IMD neck region --> pronephros with pronephric duct (drains into cloaca) and rudimentary tubules Late week 4 mesonephros (functioning kidney for 4weeks) develops from IMD trunk region Pronephric duct becomes mesonephric duct metanephros is definitive kidney and starts functioning around week 9 - ureteric bud develops from mesonephric duct

Question 46

What does the mesonephric duct become?

Answer 46

Vas deferens

Question 47

Describe the ureteric bud and its developments

Answer 47

Develops from mesonephric duct Repeated branching at distal end Stalk becomes ureter, expanded cranial end becomes renal pelvis First 4 generations of branches become major calyces Next 4 become minor calyces Remaining form collecting tubules

Question 48

Describe the nephron

Answer 48

Develops from metanephric mesoderm, ureteric bud and metanephric mesoderm reciprocally induce one another Renal corpuscle + PCT + loop of Henle + DCT

Question 49

Describe kidney ascent

Answer 49

``` Initially either side bladder Differential growth lower body Adult position t12-l3 abdomen Hilum rotates ventral to medial 90 degrees Ascent stops when reach adrenal glands ```

Question 50

Renal developmental anomalies

Answer 50

Duplicated ureter | Horseshoe kidney

Question 51

What does surface ectoderm in eye embryology form?

Answer 51

Eyelids Conjunctiva Corneal epithelium

Question 52

Describe development of the eye; lens and optic cup

Answer 52

Small grooves develop either side neural folds = optic grooves Their connection to the forebrain becomes optic stalk which becomes optic nerve Surface ectoderm thickens forming lens placode which invaginates to form lens pit and vesicle. Lens vesicle loses connection with surface ectoderm. At the same time the optic vesicle invaginates to becomes the optic cup

Question 53

Describe the optic cup

Answer 53

2 layers; - inner layer is neural layer of retina, ciliary body and inner layer of iris - outer layer is pigment layer of retina and iris

Question 54

Describe formation of the choroid fissure

Answer 54

As optic cup is forming, blood vessels that pass through form fissure in inferior wall called choroid fissure