Embryology

Question 1

Meromelia vs. Amelia

Answer 1

Meromelia: partial absence of limb due to inactivation of AER too early; phocomelia = no true long bones and flipper-like limbs

Amelia: complete absence of limb; inadequate limb mesenchyme with epithelium intact

Question 2

Weeks 0-2

Answer 2

1. Zygote
2. Morula
3. Blastocyst
4. Embyroblast
5. Inner cell mass
6. Bilaminar disc: epiblast and hypoblast
7. Trilaminar disc: cells invade called the primitive streak and form the ectoderm, mesoderm, and endoderm

Question 3

Weeks 3-8 and Months 3-9

Answer 3

Weeks 3-8: All major organ systems develop

Months 3-9: Growth and Functional Maturation

Question 4

Ectoderm

Answer 4

Epidermis of Skin

Neural Crest: sensory axons, spinal ganglion, melanocytes, and Schwann cells

Neural Tube: CNS and PNS

Question 5

Mesoderm Differentiation

Answer 5

1. Paraxial - medial; somitomeres (7 pairs) and somites (4 occipital pairs), which contribute to head development; cervicalcoccygeal have 33 pairs for postcranial development
2. Intermediate - no contributions to limbs
3. Lateral plate - lateral; splanchnic and somatic mesoderm; limb mesenchyme to make CT, appendicular skeleton, and vasculature

Question 6

Somite Differentiation

Answer 6

Dermatome, Sclerotome, and Myotome

Question 7

Embryonic Development: Weeks 4-8

Answer 7

week 4: limb bud formation; initiate vascular and nervous supply – including formation and completion of neural tube

week 6: paddle-shaped hand- and footplates; hyaline cartilage models form

week 7: limb rotation; muscle masses form

week 7-8: begin ossification process

Question 8

Outgrowth of Limb Buds

Answer 8

upper limb buds (~day 24)
opposite lower 4 cervical & upper thoracic somites; C5-T2

lower limb buds (~day 28)
opposite lower 4 lumbar & upper 3 sacral somites; L2-S2

Question 9

HOX Genes

Answer 9

location of limb buds – determined by homeobox (HOX) genes expression along the embryo

type/shape of limb bones – regulated by HOX genes along the proximodistal axis of the developing limb*

*HOX gene expression (determined by the combinatorial expression of (SHH, FGFs, and Wnt7a) occurs in phases in 3 places

Question 10

Growth of Limb Regulation

Answer 10

Growth of limb - regulated by transcription factors and signaling molecules

Once positioning along the craniocaudal axis is determined (HOX genes), growth must be regulated along proximodistal, A-P, and D-V axes.

Limb outgrowth, which occurs first, is initiated by growth factors (FGF and TBX) secreted from lateral plate/somatic mesoderm.

Question 11

Outgrowth of Limbs

Answer 11

1. Ectodermal Cap

2. Mesenchymal core - (lateral plate mesoderm) secretes growth factors (FGF & TBX) which initiate outgrowth

Question 12

Formation of AER

Answer 12

Apical Ectodermal Ridge (AER)

Once outgrowth is initiated, BMPs expressed in ventral ectoderm induce the formation of the AER via homeobox gene MSX2
Radical fringe in dorsal ectoderm restricts location of AER to distal tip of limb because AER causes cells to rapidly divide without differentiating called the progress zone. If the AER is not restricted, then cells would not differentiate properly proximodistally

SER separates cells that express radical fringe from those that are not; SER is close to AER

After the AER is established, it expresses FGF-4 and FGF-8 which maintain the Progress Zone (PZ)

Question 13

Progress Zone

Answer 13

Maintained by the AER by secreting FGF
Cells that are rapidly dividing and not differentiating

Cells that leave the PZ differentiate:
Early mesoderm differentiation – proximal limb segment
Late mesoderm differentiation – distal limb segment
Most differentiated cells = zone of polarizing activity (ZPA)

Question 14

SHH

Answer 14

FGF from AER induces SHH expression at ZPA via production of retinoic acid/ vitamin A

Wnt7a maintains SHH in ZPA to cause correct positioning of digits

SHH diffuses through the mesoderm creating a concentration gradient

Maintenance and propagation of SHH expression requires AER-FGF signaling as part of a positive epithelial–mesenchymal feedback loop between the ZPA and the AER

BMP-dependent downregulation of SHH is achieved by inhibiting FGF and Wnt signaling activities that usually maintain SHH expression

Question 15

Polydactyly

Answer 15

occurs if two zones of polarizing activity (ZPA) or when ZPA is interrupted

Question 16

Dorsoventral Axis Regulation

Answer 16

regulated by signaling molecules produced by ectoderm

Dorsal ectoderm – Wnt7a induces expression of Lmx1
Ventral ectoderm – BMPs induce expression of En1
(En1 represses Wnt7a expression restricting it to dorsal)

Question 17

Establishing Axial Polarity

Answer 17

Proximodistal: controlled by AER secreting FGF

Pre/post axial (anteroposterior): controlled by ZPA producing retinoic acid that initiates SHH

Dorsoventral : regulated by Wnt7a (also maintains SHH) → LMX1 and BMPs → En1; signaling centers located in dorsal and ventral ectoderms

Question 18

Development of Peripheral Vasculature

Answer 18

Initially: fine capillary network = marginal sinus (beneath AER) – venous outflow
some channels preferentially enlarge, form large central artery  
pattern changes constantly by outgrowth, regression, coalescence.
venous system (low pressure) more variable

Question 19

Development of Joints

Answer 19

cell death and matrix changes in zones within cartilaginous rods
early joint development is independent of muscular activity
joint capsule, ligaments, & tendons from mesenchymal condensations

Question 20

Development of Musculature

Answer 20

Paraxial mesoderm → somites → myotomes:

Epimere: intrinsic muscles of the back innervated by dorsal primary rami (deep muscles of back)

Hypomere: contribute to rest of muscles and superficial back too

Question 21

Hypomere

Answer 21

Hypomere: contributes to muscles of extremities and then split up to dorsal and ventral aspects

Dorsal in the upper limb would be the pronators and supinators and the lower limb is adductors and abductors

Question 22

Innervation of Limbs

Answer 22

All limbs are associated with ventral primary rami innervation

dorsal muscle mass innervated by dorsal (posterior) brs. of ventral rami of spinal nn.

ventral muscle mass innervated by ventral (anterior) brs. of ventral rami of spinal nn.

Arms: brachial plexus C5-T1
Legs: lumbosacral plexus L2-S2

Question 23

Limb Rotation

Answer 23

Arms: rotate laterally to put extensors on the posterior aspect; coronal to parasagittal orientation

Legs: rotate medially to put extensors on the anterior aspect

Question 24

Interdigital Space Formation

Answer 24

Digital rays appear, AER breaks up, leaving intact AER segments over digital ray tips (N=5 per hand/foot)

Reduces amount of FGF transmitted to interdigital spaces and cell death occurs

*Apical ectodermal ridge breaks down and FGFs are lost and begin to get programmed cell death to get the interdigital sapec

Question 25

Syndactyly and Triphalangeal Thumb

Answer 25

No selected apoptosis with the AER of the hand/foot plate causes: 1. Dysregulation of interdigital apoptosis = syndactyly 2. Dysregulation of terminal apoptosis = triphalangeal thumb

Question 26

Ectrodactyly

Answer 26

Lose apical ectodermal ridge = lose entire digit = ectrodactyly/cleft hand/cleft foot Lobster claw deformity

Question 27

HOX Gene Mutation

Answer 27

Problems with limb patterning causing: 1. Synpolydactyly: multiple fused digits 2. Short digits or fused carpal bones *Both are HOX 13 mutations

Question 28

Classification of Limb Defects

Answer 28

1. Reduction defects – all or part of a limb is missing arrest of development, failure of differentiation (e.g., meromelia, amelia, phocomelia) 2. Duplication defects – supernumerary limb elements (e.g., polydactyly) 3. Dysplasia defects – malformation of the limb overgrowth/undergrowth (e.g., macrodactyly, syndactyly, adactyly), focal defects (e.g., amniotic band syndrome), general skeletal abnormalities (e.g., osteogenesis imperfecta)

Question 29

Key Factors that cause Anomalies

Answer 29

Amount of mesenchyme available Duration of the AER inductive influence Presence of a teratogen Mechanical factors

Question 30

Teratogen vs. Morphogen

Answer 30

Teratogen – any agent or factor that induces or ↑ the incidence of a congenital anomaly in a developing embryo (i.e., factors that cause birth defects); weeks 3-8 are the most critical time that can be affected, especially weeks 4 and 5; months 3-9 defects are less severe/non-existent Morphogen – signaling molecule that acts directly on cells to produce a specific response (i.e., a substance governing the pattern of tissue development - e.g., retinoic acid, FGF, Wnt, SHH)

Question 31

Morphogens Become Teratogens

Answer 31

retinoic acid and receptors normally expressed in developing limb altered synthesis and action: if inhibited, limb formation halted if excess, positional information disrupted potential sources of excess: excessive vitamin A intake (hypervitaminosis A) products for treating acne containing isotretinoin (a retinoid or analogue of vitamin A)

Question 32

Methyloxyethanol

Answer 32

Methyloxyethanol: extensively used in industry (paints, textile dyes, printing inks, brake fluid, semiconductor manufacture) Regression of AER (due to reduced laminin content in the basal lamina of ectodermal cells) causing syndactyly (primarily upper limb)

Question 33

Cadmium

Answer 33

Cadmium: extensively used in industry & agriculture; ubiquitous environmental pollutant Damages DNA in embryonic cells causing limb deformities (syndactyly, polydactyly, hypertrophy)

Question 34

Amniotic Band Syndrome

Answer 34

bands of amnion (inner lining of amniotic sac) or adhesions attach to fetus reduces local blood supply causes abnormal development (including ring constrictions, and amputations) Connective tissue strands from adhesions or breaks lose and then constrictions around the baby occurs = causes constrictions or amputations

Question 35

Clubfoot (Talipes Equinovarus)

Answer 35

Plantarflexed, inverted, deformed talus, observed mainly in ♂ Likely etiology: abnormally low amniotic fluid (oligohydramnios) leading to mechanical pressure of uterine wall on fetus Abnormal positioning of the legs in utero Hereditary