Summary of Limb Development

Question 1

Limb budsappear toward the end of the …. ……of gestation as slight bulges of the ventrolateral body wall. The development of the …… …… buds proceeds approximately 2 days ahead of the development of the …. …… buds. The tissues of the limb buds are derived from two main sources, ….. and …..

Answer 1

upper limb
lower limb
mesoderm, ectoderm
4th week

Question 2

Meaning of mesenchyme

Answer 2

alooselyorganized, mainly mesodermalembryonictissue which develops intoconnectiveandskeletaltissues, including blood andlymph

Question 3

The …. ….. ……exerts an inductive influence on the limb mesenchyme promoting growth and development of the limbs. The limb buds elongate by proliferation of the mesenchyme within them

Answer 3

AER = apical ectodermal ridge

Question 4

….. is an important mechanism in limb development; for example, in the breakdown of the tissue in the …. ….. …. ……………….

Answer 4

Apoptosis
notches between the digital rays.

Question 5

Limb muscles are derived from ……. (myogenic precursor cells) originating in the somites. The muscle-forming cells(myoblasts)form dorsal and ventral muscle masses. Nerves grow into the limb buds after the muscle masses have formed. Most blood vessels in the limb buds arise as buds from the intersegmental arteries.

Answer 5

mesenchyme

Question 6

Initially, the developing limbs are directed caudally; later, they project ventrally; and finally, they rotate on their longitudinal axes. The upper and lower limbs rotate in opposite directions and to different degrees

Question 7

Most birth defects of the limbs are caused by genetic factors; however, many defects probably result from an interaction of genetic and environmental factors
……. inheritance

Answer 7

multifactorial

Question 8

Name Types of digital birth defects.

Answer 8

Polydactyly: more than five digits on the handsor feet
. Syndactyly (webbing or fusion) of the fingersor toes

Question 9

What is amelia

Answer 9

complete absence of the upper limbs

Question 10

Polydactyly is inherited as a ….. trait.

Answer 10

dominant

Question 11

Name the 2 main types of limb anormalities or defects:

Answer 11

• AMELIA - The absence of a limb or lims
• MEROMELIA - The absence of part of a limb

Question 12

Experimental studies indicate that thalidomide affects the formation of ………………………….

Answer 12

early blood vessels in the limb buds.

Question 13

Quadruple amelia:

Answer 13

absence of upper and lower limbs.

Question 14

thalidomide

Answer 14

• a potent humanteratogenduring the embryonic period
• Although thalidomide is now used for the treatment of leprosy and other disorders, it isabsolutely contraindicated in women of childbearing age.
• Exposure to this teratogen before day 36 can cause severe limb defects such asamelia (absence of limbs

Question 15

What do Hox genes do in limb development?

Answer 15

Homeobox(Hox)genes regulate patterning in the formation of the limbs.

Question 16

Expand on AER

Answer 16

AER = Apical Ectodermal Ridge
.The limb buds form deep to a thick band of ectoderm, theapical ectodermal ridge- AER
. The buds first appear as small bulges on the ventrolateral body wall
. Each limb bud consists of a mesenchymal core of mesoderm covered by a layer of ectoderm.

Question 17

The limb buds elongate by proliferation of the mesenchyme. The upper limb buds appear disproportionately low on the embryo’s trunk because of the early development of the cranial half of the embryo . The earliest stages of limb development are alike for the upper and lower limbs Later, distinct features arise because of differences in the form and function of the hands and feet.

Question 18

Theupper limb budsdevelop opposite the caudal cervical segments, and thelower limb budsform opposite the lumbar and upper sacral segments. At the apex of each limb bud, the ectoderm thickens to form the AER. This ridge is a specialized, multilayered epithelial structure that is induced by the paracrine factor, fibroblast growth factor 10 (FGF10), from the underlying mesenchyme.

Question 19

FGF8, secreted by the AER, exerts an inductive influence on the limb mesenchyme that initiates growth and development of the limbs in a proximodistal axis. Retinoic acid promotes the formation of the limb bud by inhibiting FGF signaling. Mesenchymal cells aggregate at the posterior margin of the limb bud to form thezone of polarizing activity, an important signaling center in limb development. FGFs from the AER activate the zone of polarizing activity, which causes expression of theSonic Hedgehog (SHH)genes.

Question 20

FGFs from the AER activate the zone of polarizing activity, which causes expression of theSonic Hedgehog (SHH)genes.

Answer 20

……….

Question 21

Studies have shown that endogenous retinoic acid is also involved in limb development and pattern formation