Limb Morphogenesis

Question 1

The limbs, including the shoulder and pelvic girdles, comprise the appendicular skeleton. At the end of the _________ week of development, limb buds become visible as outpocketings from the ventrolateral body wall (Fig. 12.1A). The fore limb appears fi rst followed by the hindlimb 1 to 2 days later.

Answer 1

fourth

Question 2

Initially, the _______ buds consist of a mesenchymal core derived from the parietal (somatic) layer of lateral plate mesoderm that will form the bones and connective tissues of the limb, covered by a layer of cuboidal ectoderm.

Answer 2

limb buds

Question 2

Ectoderm at the distal border of the limb thickens and forms the

Answer 2

apical ectodermal ridge (AER)

Question 3

This apical ectodermal ridge (AER) exerts an inductive infl uence on adjacent mesenchyme, causing it to remain as a population of undiffer entiated, rapidly proliferating cells, what zone is this called?

Answer 3

progress zone

Question 4

As the limb grows, cells farther from the influence of the AER begin to differentiate into ____________. In this manner, develop ment of the limb proceeds proximodistally.

Answer 4

cartilage and muscle

Question 4

In 6-week-old embryos, the terminal portion of the limb buds becomes fl attened to form the hand- and footplates and is separated from the

Answer 4

hand- and footplates

Question 4

Later, a second constriction divides the proximal portion into two segments, and the main parts of the extremities can be recognized

Fingers and toes are formed when ___________ in the AER separates this ridge into five parts

Answer 4

cell death

Question 4

Development of the upper and lower limbs is similar except that morphogenesis of the lower limb is approximately ____________ days behind that of the upper limb.

Answer 4

1 to 2

Question 4

at the 7th week, whereas the lower limb rotates approximately 90° degrees _________, placing the extensor muscles on the anterior surface and the big toe medially.

Answer 4

medially

Question 4

Also, during the __________ week of gestation, the limbs rotate in opposite directions.

Answer 4

seventh

Question 4

at the 7th week, The upper limb rotates 90° _______, so that the extensor muscles lie on the lateral and posterior surface, and the thumbs lie laterally,

Answer 4

laterally

Question 4

At birth, the diaphysis of the bone is usu ally completely ossifi ed, but the two ends,

but these are still cartilaginous

Answer 4

epiphyses

Question 4

are present in all long bones of the
limbs by the 12th week of development.

Answer 4

ossifi cation centers

Question 4

While the external shape is being established, mesenchyme in the buds begins to condense, and these cells differentiate into chondrocytes. By the sixth week of development, the first __________ model, foreshadowing the bones of the extremities, are formed by these chondrocytes

Answer 4

hyaline cartilage models

Question 4

Factors regulating the positioning of joints are not clear, but the secreted molecule _________ appears to be the inductive signal.

Answer 4

WNT14

Question 4

Cells in this region increase in num ber and density, and then a joint cavity is formed by cell death. Surrounding cells differentiate into a joint capsule.

Answer 4

joint interzone

Question 4

Ossifi cation of the bones of the extremities

Answer 4

endochondral ossifi catio

Question 4

Shortly there after, however, ossification centers arise in the epiphyses. Temporarily, a cartilage plate remains between the diaphyseal and epiphyseal ossifi ca tion centers. This plate, the __________ plate, plays an important role in growth in the length of the bones.

Answer 4

epiphyseal plate

Question 5

musculature is derived from dorsolateral cells of the somites that migrate into the limb to form muscles and, initially, these muscle components are segmented according to the somites from which they are derived

Answer 5

Limb

Question 5

in smaller bones, such as the phalanges, it is found only at _______ extremity (1 or both)

Answer 5

one

Question 5

joints between bones begin to form at the same time that mesenchymal con densations initiate the process of forming cartilage.

Answer 5

Synovial joints

Question 5

complete absence of extermities

Answer 5

amelia

Question 5

From the primary center in the shaft or ______ of the bone, endochondral ossification gradually progresses toward the ends of the cartilaginous model

Answer 5

diaphysis

Question 5

In long bones, an epiphyseal plate is found on ________ extremity (1 or both)

Answer 5

each

Question 5

Synovial joints between bones begin to form at the same time that mesenchymal con densations initiate the process of forming cartilage. Thus, in the region between two chondrify ing bone primordia, called the _________ (for example between the tibia and femur at the knee joint), the condensed mesenchyme dif ferentiates into dense fi brous tissue.

Answer 5

interzone

Question 5

This fi brous tissue then forms ___________, covering the ends of the two adjacent bones; the synovial membranes; and the menisci and ligaments within the joint capsule (e.g., the anterior and posterior cruciate ligaments in the knee).

Answer 5

articular cartilage

Question 5

Positioning of the limbs along the craniocau dal axis in the fl ank regions of the embryo is regulated by the

Answer 5

HOX genes

Question 5

extra digits

Answer 5

polydactyly

Question 5

partial absence of extremities

Answer 5

meromelia

Question 6

absence of long bones, making hands and feet attacked to the trunk

Answer 6

phocomelia

Question 6

fusion of digits

Answer 6

syndactyly

Question 6

homeobox genes at the cranial border of the forelimb, and misexpression of this gene alters the position of these limbs.

Answer 6

HOXB8

Question 6

is caused by mutations in the fibrillin (FBN1) gene located on chro- mosome. Affected individuals are usu- ally tall and slender with long thin limbs and a long thin face.

Answer 6

Marfan syndrome

Question 7

shortened digits

Answer 7

barchydactyly

Question 7

drugs widely used as sleeping pills and antinauseant - causes malformations of limbs

Answer 7

thaliadomide

Question 7

mutations in what genes causes hand foot genital syndrome

Answer 7

HOXA13

Question 8

absence of digits

Answer 8

ectrodactyly

Question 9

result in Holt-Oram syndrome, character- ized by upper limb abnormalities and heart defects consistent with a role for this gene in upper limb and heart development.

Answer 9

TBX5 mutations

Question 10

are limb defects in which proximal structures are intact, but structures distal to a transverse plane are partially or completely absent

Answer 10

Transverse limb deficiencies

Question 10

is character- ized by shortening, bowing, and hypomineralization of the long bones of the limbs that can result in fractures and blue sclera

Answer 10

Osteogenesis imperfecta

Question 10

may cause ring constric- tions and amputations of the limbs or digits. The origin of the bands is not clear, but they may represent adhesions between the amnion and affected structures in the fetus.

Answer 10

Amniotic bands