Formation of Basic Body Plan

Question 1

What are the 5 original structures of the embryo at 2 weeks?

Answer 1

1- epiblast (upper layer - where embryo comes from completely)
2- hypoblast (lower layer- disappears)
3- amniotic sac
4- yolk sac
5- stalk (connects it to chorion- the embryo portion of placenta)

Question 2

Primitive Streak and Node

Answer 2

Primitive streak formed in midline by caudal end

The head of the primitive streak is the node (thickening at top)

Question 3

Gastrulation

Answer 3

Epiblast cells move through node and primitive streak to form 3 germ layers

Replace hypoblast w/ endoderm and mesoderm then remaining epiblast is ectoderm

Question 4

Oropharyngeal membrane and Cloacal membrane

Answer 4

-2 sites where mesoderm does not separate endoderm and ectoderm (form canals)

• oropharyngeal –> future oral cavity
• cloacal –> future anal and urethral openings

Question 5

In which direction does development proceed?

Answer 5

Cranial –> caudal

Question 6

Notochord

Answer 6

• Midline structure derived from mesoderm
• Starts as hollow then rod
• ultimately becomes core of intervertebral discs

Question 7

Derivatives of Ectoderm

Answer 7

• Neural plate –> neural groove –> neural folds –> neural tube in 1st mo (which becomes brain and SC)
• Neural crests come off neural tube (all ganglia, schwann cells, pigment cells, cartilage/bone of face/jaw, heart outflow track and chromaffin cells of adrenal)
• Outer epithelium of body- teeth/hair and sensory epithelium of nose, inner ear and lens

Question 8

Derivatives of Mesoderm

Answer 8

• Notochord –> intervertebral discs
• Somites –> vertebra, skeletal muscle, connective tissue of back body wall (dermatomes)
• Intermediate mesoderm –> urogenital organs (kidneys, gonads, genital ducts)
• Lateral mesoderm –> parietal layer (body wall connective tissue) AND visceral layer (smooth muscle of organs)

Question 9

2 Types of Embryo Folding

Answer 9

1- lateral - embryo outgrows yolk sac so sides fold in

2- cranial/caudal - brain outgrows mesoderm structures which now fold BELOW neural tube
**coelomic cavity, septum transversum and cardiogenic mesoderm all go from above neural tube to below

Question 10

Pericardiperitoneal Canals

Answer 10

• Connect the pericardial cavity AND future peritoneal cavity
• Closed by development of pleuroperitoneal folds or membranes **if they do not form –> hole b/n these 2 cavities/incomplete diaphragm –> diaphragmatic hernia (viscera herniate into thoracic cavity)

Question 11

4 Components of Diaphragm

Answer 11

1- septum tranversum
2- pleuroperitoneal membrane/fold
3- esophagus messentery
4- muscular ingrowth from body wall

Question 12

Derivatives of Endoderm

Answer 12

-Inner epithelial lining of respiratory tract, GI tract and bladder/urethra

Question 13

Origin of GI Organs

Answer 13

ALL 3 LAYERS

• inner epithelial lining from endoderm
• smooth muscle and connective tissue from mesoderm
• ganglia of GI organs (enteric NS) from ectoderm neural crests

Question 14

Nerve-Muscle Relationships

Answer 14

• Muscle innervation patterns reflect embryonic muscle position NOT adult muscle position
• Ex) why diaphragm is innervated by phrenic nerve (cervical roots)

Question 15

Neural Tube Defects (3 categories- 6 types)

Answer 15

Failure to Close

• Anencephaly (cranioschisis) - degeneration of some forebrain
• Myeloschisis - fail to close at SC level - spina bifida aperta

Protrusion of Neural Tissue in Sac

• Encephalocele - sac from skull w/ brain or meninges
• Myelomeningocele - sac of meninges and SC - spina bifida cystica
• Meningocele - just meninges in sac -also spina bifida cystica

Spina Bifida Occulta
-hidden defect b/c small # vertebral arches do not form