Test 1: Embryology of the lower extremity

Question 1

Myogenic cells

Answer 1

These cells originate from the dermomyotome region of the somites and migrate into the mesenchymal cells of the limb buds where they eventually differentiate into myoblasts. The myoblasts will give rise to the muscle cells of the limbs. The cervical myotomes will give rise to the pectoral girdle muscles and the lumbosacral myotomes provide precursor cells for the pelvic girdle

Question 2

This easy to understand pre and postaxial arrangement, however, changes during the early part of the _____ when the limbs undergo a rotation along their long axis.

Answer 2

7th week

Question 3

By the end of the 6th week the hand paddles show presence of

Answer 3

digital rays which will eventually give rise to individual fingers. Eventually five rays develop, one for each finger.

Question 4

Both limbs go through

Answer 4

a 90 degree rotation but in the opposite direction.

Question 5

The preaxial border is along the same line as

Answer 5

the thumb or the great toe

Question 6

During the later part of the 4th week (as soon as the trilaminar embryo completes its lateral folding) limb buds start to develop along

Answer 6

the ventrolateral side of the cylindrical embryonic body. These limb buds, one pair for the upper limbs and one pair for the lower limbs will undergo further development and eventually give rise to the adult limbs.

Question 7

The limb buds originally contains a core of mesenchymal cells that are derived from the

Answer 7

the somatic component of the lateral mesoderm layer of the embryo. Further development and enlargement of the limb buds initially begins by a very fast division of the mesenchymal cells. Experimental evidence has shown that this cell division is under direct control of the HOX (homeobox- containing) gene.

Question 8

Dermatomal cells

Answer 8

These cells derived from the somites provide cells that stay deep to the ectoderm and give rise to the dermis of the skin.

Question 9

Axonal processes

Answer 9

Axonal processes of neurons of spinal cord become established as nerves of the limbs

Question 10

During early part of the 7th week

Answer 10

toe rays can be seen in the foot plates. In a manner similar to that seen in the hand they will give rise to individual toes.

Question 11

Under the influence of the _______patterning of the limbs also takes place where the distal ends of the buds flatten into paddle-like hands and feet.

Answer 11

HOX gene

Question 12

The upper limb buds (for the upper limbs) appear first as two elevations one on each side of the body of the embryo. These limb buds can be seen by day

Answer 12

day 26-27 at the inferior segment of the developing cervical region (C5-T1).

Question 13

The long bones also develop

Answer 13

secondary ossification centers in the epiphysis. So each long bone will have one primary and two secondary ossification centers.

Question 14

the postaxial border corresponds to the location of

Answer 14

the 5th toe or the little finger.

Question 15

The entire skeleton of the limbs at one time consists of cartilage models of bones. The cells of the cartilage models are derived from

Answer 15

the original mesenchymal cells of the embryo beginning by the 5th week of development.

Question 16

There is much experimental evidence which indicates that the limbs will not develop in the absence of

Answer 16

AER. There is also evidence showing that transplantation of this ridge at a different location will induce the development of a limb at that location.

Question 17

The inductive influence of AER is known to be as a result of

Answer 17

the expression of fibroblast growth factor.

Question 18

Initially the digits are attached to each other by loose connective tissue. Separation of individual digits takes place during

Answer 18

during the 8th week by the process of programmed cell death (apoptosis) in the mesenchymal tissue between digital rays. This process gives rise to individual digits. If apoptosis is interrupted or does not happen, for some reason, it will result in one or more digits that remain attached (syndactyly).

Question 19

Apical Ectodermal Ridge (AER) The proliferating mesenchyme at the site of limb development is covered externally by

Answer 19

cells of the ectoderm layer similar to other parts of the body. However, at the distal segment of the limb bud (apex) the ectodermal cells undergo cell proliferation and shows considerable thickening known as the apical ectodermal ridge (AER).

Question 20

Neural crest cells

Answer 20

The supporting cells of the nervous system (Schwann cells) are derived from the neural crest cells There is evidence which indicates that the mesenchymal cells of the limbs organize the migrant cells and control their expression.

Question 21

The limb buds extend from the trunk so that

Answer 21

that the longitudinal axis (or the axial line) of the limb bud is at about 90 degrees to the long axis of the trunk.

Question 22

the postaxial compartments contain

Answer 22

the extensor muscles

Question 23

Limb rotation Both the upper and lower limb buds at one time protrude

Answer 23

protrude anteriorly from the embryonic body

Question 24

. During the _______ the cartilage models start to undergo endochondral ossification whereby the chondrocytes are replaced by bone producing cells. The endochondral bone formation begins in the diaphysis of long bones (e.g. phalanx, metatarsals) as the primary center of ossifications.

Answer 24

7th week

Question 25

The AER is very unique in that it controls

Answer 25

the future development of the limb

Question 26

Limb Skeleton The mesenchyme of the limb buds, under the influence of the HOX gene, gives rise to the following components of the limbs:

Answer 26

-Bones -Joints -Cartilage -Fascial layers -Blood vessels -Lymphatic vessels

Question 27

The developing limbs get other migrants cells in addition to the original mesenchymal cells as follows:

Answer 27

-Dermatomal cells -Myogenic cells -Dermatomal cells -Axonal processes -Neural crest cells

Question 28

Under the inductive influence of AER the limb buds undergo growth in a ____________direction

Answer 28

proximal to distal

Question 29

The area of the limb superior (cranial) to the axial line is the_________ compartment and the part inferior (caudal) to the axial line is the __________compartment.

Answer 29

preaxial compartment postaxial compartment.

Question 30

The development of the lower limb buds, however, lags about ______behind the upper buds

Answer 30

2 days. This follows the normal pattern of cranial to caudal development true for all other systems of the embryo. The cranial structures appear first.

Question 31

The lower limb buds appear as two elevations from the inferior part of

Answer 31

the ventrolateral segment (L2-S2) of the embryo. Note that the lower limbs originate from a slightly larger segment than the upper limb buds.

Question 32

The upper limbs rotate along their long axis in a________direction and the lower limbs go thorough a similar rotation but do so in a _______direction

Answer 32

• lateral -medial . Because of the fact that they rotate in the opposite direction the pre and postaxial compartments of the upper and lower limbs do not correspond with each other anymore.

Question 33

The preaxial compartments contain

Answer 33

flexor muscles

Question 34

By the_____ the entire cartilaginous skeleton is complete

Answer 34

6th week

Question 35

The flexor compartment (preaxial) of the upper limb, in the anatomical position, faces _______ while the extensor compartment is located ______to the longitudinal axis.

Answer 35

-anteriorly -posterior Because of the medial rotation of the lower limbs, the opposite is true for the lower limbs

Question 36

The muscle masses of the limbs can also be divided into

Answer 36

pre and postaxial muscles depending on their location

Question 37

Occasionally additional rays develop in the hand or foot plates. If this happens it will lead to development of a condition called polydactyly (extra digits). The extra digits are

Answer 37

usually smaller in size than the regular digits.

Question 38

The superior border of the ________ is the preaxial border and the inferior border of the ________ is the postaxial border.

Answer 38

preaxial compartment postaxial compartment

Question 39

lower limbs The preaxial compartment including the muscles that dorsiflex the foot are located in the ______ and the muscles that plantar flex the foot ( preaxial) are located in the _______ of the leg.

Answer 39

-anterior compartment -posterior compartment This, opposite rotation, also explains why the two homologous bones (tibia and radius) are located on the opposite side of the limbs and the thumb is on the lateral side of the hand whereas in the foot the great toe is on the medial side. The knee and elbow, two homologous joints, face in the opposite direction as well.