Exam 1

Question 1

Describe the development of somites.

Answer 1

They are derived from the paraxial mesoderm (the large block of mesoderm nearest to the neural tube/midline). The paraxial mesoderm differentiates into the somitomeres (in the head region) then form the somites in the caudle parts.

Question 2

list the derivatives of somites, where they are located relative to the notochord and what they become.

Answer 2

1. Sclerotomes (ventromedial) - becomes mesenchyme that becomes osteoblasts (and a bunch of other stuff) which gives rise to bones of the trunk
2. Dermomyotomes (dorsolateral) – splits to become dermatome (becomes dermis) and a myotome (becomes muscle)

Question 3

List the 4 germ layers that give rise to bone

Answer 3

Bones arise from four mesodermal sources:
1. Paraxial mesoderm – somitomeres –> somites –> sclerotome –> mesenyme –> osteocytes (which form the bones of the trunk)

2. Somatic layer of the lateral place mesoderm – forms the sternum and the limb bones
3. Neural crest – forms the bones of the skull
4. Head somitomeres – forms the cranial vault and base of the skull

Question 4

discuss the 2 embryological ways bones develop

Answer 4

1. Endochondral – chondroblasts form a hyaline cartilage model of the bone first which becomes ossified (long bones, most bones)
2. Intramembranous – mesenxyme in the dermis differentiated directly into bone (forms the flatbones, bones of the skull)

Question 5

list the intermediates starting with myotomes and ending with muscle fibers

Answer 5

The myotome forms myocytes which form myotubes which form muscle fibers/cells

Question 6

Discuss the 2 differentiations of the myotomes

Answer 6

The myotome somewhat seperates down the middle and differentiates into two parts at each pole:

1. Primaxial muscle cell precursors (dorsomedial part of the myotome) – forms the intrinsic muscles of the back [AKA epaxial mm.], shoulder girdle and intercostal muscles
2. Abaxial muscle cell precursors (ventrolateral) – limb muscles and andominal wall muscles [AKA hypaxial mm.] and infrahyoid muscles

Question 7

Describe the development of the vertebral column.

what about the interbertebral disk? the nucleus palposis?

Answer 7

The vertebral column forms via the proliferation of the caudal part of the sclerotome (from paraxial mesoderm) which then fuses with the cranial portion of the adjacent sclerotome (the myotomes and nerves remain segmented though)

the mesenchyme between the fusing sclerotomes forms the intervertebral disks and the remainder of the regressed notochord forms the nucleus palposis (?)

Question 8

what germ layers make up the developing layers?

what regulates their paterning?

Answer 8

Limb buds consist of a core of mesenchyme (somatic layer of lateral plate mesoderm) surrounded by surface ectoderm.

HOX genes

Question 9

that does the AER do?

Answer 9

The AER develops at the distal end of the limb bud and causes proximal/distal growth by inducing the proliferation of underlying mesoderm and by keeping the adjacent mesenchyme undifferentiated

Question 10

that does the ZPA do?

Answer 10

The ZPA developes at the posterior margin of the limb and causes anterior/posterior development (ex. Makes sure the thumb and pinky are on the right side.

Question 11

Describe the embryologic rotation of the limbs.

Answer 11

The upper limbs rotate laterally 90 degrees

The lower limbs rotate medially 90 degrees

Question 12

describe the vascular supply to the developing upper limb

Answer 12

• blood supplied from the intersegmental arteries from the aorta –> which supplies the primary axial arteries
• the axial artery becomes the brachial artery (which becomes the common interosseous artery distally)
• the radial and ulnar arteries are secondary branches of the brachial artery (formally axial artery)
• the median artery typically degenerates

Question 13

describe the vascular supply to the developing lower limb

Answer 13

-the axial artery that supplies the lower limbs begins as the ischiadic artery

• growth of new vessels and regression of others causes the primary axial artery to be represented by (replaced by?) three arteries:
  1. profunda femoris
  2. popliteal
  3. posterior tibial

Question 14

what happens on the 16th day of developement?

Answer 14

gastrilation - development of the 3 germ layers

Question 15

what are the 6 types of regional somites? how many are there for each region?

Answer 15

1. occipital - 4
2. cervical - 8
3. thoracic - 12
4. lumbar - 5
5. sacral - 5
6. coccygeal - 8 to 10

Question 16

what is the Poland anomaly/sequence? what causes it?

Answer 16

a condition characterized by one arm being weaker than the other due to either the pectoralis major or minor getting inadequate blood supply

can be bilateral but its usually unilateral

Question 17

what is prune belly syndrome?

Answer 17

a condition where the abdominal muscles (abaxial) are poorly developed at birth

Question 18

what is spina bifida oculta?

Answer 18

its characterized by an abnormal patch of hair on the lower back

Question 19

List 3 causes of congenital scoliosis

Answer 19

1. asymmetric fusion of a vertebra
2. missing half a vertibra
3. abnormal curvature of the vertebral wall

Question 20

what is the klippel-feil anomaly?

Answer 20

AKA brevicollis

its a congenital short neck due to the absence or fusion of cervical vertebra (at the craniovertebral junction)

Question 21

what population should avoid retinoic acids and why?

Answer 21

pregnant women - because its a substance notmally produces by the ZPA in the developing limbs the fetuses. increased retinoic acid could cause duplications in the limbs

Question 22

what is achondroplasia? what embryological structure is disrupted?

Answer 22

a cause of dwarfism (short limbs)

endochondral ossification of the epiphyseal plates is disrupted

Question 23

what is thantophoric dysplasia? what causes it?

Answer 23

is a more sever (and fatal [respiratory failure]) form of achondroplasia caused by a deficiency in fibroblast growth factor receptor (FGFR)

Question 24

what is the cause of osteogenesis imperfecta? how does this present?

Answer 24

a defect in type I collagen synthesis

presents as bowing of the legs

Question 25

differentiate amelia and meromelia

Answer 25

• amelia: the congenital absence of an entire limb

- meromelia: absence of part of a limb

Question 26

what is ectodactyly?

Answer 26

AKA lobster claw deformity or cleft hand/foot

its the absence of the central digits caused by a failure of the digital rays to form

Question 27

what is brachydactyly?

Answer 27

shortness of the digits (its often also associated with short stature)

Question 28

what is polydactyly? where does this usually occur?

Answer 28

extra digits.

normall occures medial or lateral side of the hand/foot (not the middle)

the extra digits are usually useless

Question 29

what is syndactyly? what are 2 forms of it?

Answer 29

the fusion of digits

can be cutaneous (webbing of digits) or osseous (fusion of the bone also)

Question 30

what is the cause of congenital club foot? what does it do?

Answer 30

it generally occurs as a result of a defect in the talus but it could possibly also be caused by restricted movement in utero or caused by environmental factors

it caused an abnormal orientation of the foot which prevents normal weight bearing

Question 31

what is is the cause of congenital dislocation of the hip?

Answer 31

eithe laxity of the joint capsule or an undervelopement of the acetabulum