Posted by CB on August 16, 2011 at 20:53:18:
In Reply to: Practice Questions posted by Michelle on August 16, 2011 at 11:48:46:
: 1. Discuss key events that take place in lower limb development and how these embryos would appear if exposed to teratogens at these key events.
The limb buds appear during week 4 of development, specifically the lower limb after the upper limb since development is cranial to caudal. The limb buds arise from mesenchym cells of the lateral plate mesoderm. At day 33, or week 6, the AER appears. This is an ectoderm derived ridge of ectodermal cells that end up inducing the formation of each bone. The length of time cells spend next to the AER induces them to become either a femur, a tibia/fibula, a tarsal etc. The longer amount of time the cells spend near the AER, the more distal type of bone they will become (this is one theory and my favorite- the other ones are a bit duller in my opinion but probably just as valid) As the postaxial end of the AER (which divides the limb dorsal/ventrally) the Zone of Polarizing Activity is formed.
During the fifth week, chondrification centers appear. These centers are replaced in the 7th week by bone. is intracartilagenous ossification in which the primary ossification center forms, makes a periosteum, and continues to replace the model laid down by the mesenchymal cells. Rotation occurs in the seventh week as well, with the lower limbs first rotating about the hip 90 degrees then the leg rotates separately 180 degrees. This places the extensors dorsally. Also, the plantar side of the foot now is on the posterior side of the body. Apoptosis between the digits occurs during the 8th week. This is programmed cell death. By the 12th week, Primary ossification centers are present in all of the long bonds, not just the diaphysis at one location like the seventh week.
Failure of ZPA or duplication- duplication in number of digits or digital rays. This is called a cleft foot. Polydactyly is extra digits. If apoptosis fails, syndactyly may occur (webbing). Amelia is teh abscence of a limb- perhaps due to failure of the AER? The teratogens thalidomide can cause this deformity.
Also, alcohol in start of pregnancy (when many women do not know they are pregnant) can also cause microamelia among other problems.
: 2. Discuss adult anatomy that demonstrates rotation of the lower limb.
The extensor muscles are located on the dorsal surface of our body. For example, the posterior (dorsal) compartment of the thigh contains hip extensors such as the semimembranosus, semitendiosus, and long head of the biceps femoris. The upper limbs rotate laterally and just before the lower limbs.