anatomy 8.6

How do most bones of the appendicular skeleton develop?

Through endochondral ossification.

Which appendicular bone is the exception to endochondral ossification?

The clavicle.

How does the clavicle develop?

Its center develops by intramembranous ossification, while its ends develop by endochondral ossification.

During which week does the appendicular skeleton begin developing?

The fourth week.

What are limb buds?

Small ridges that appear along the lateral sides of the embryo and develop into the limbs.

Around what day do the upper limb buds appear?

About day 26.

Around what day do the lower limb buds appear?

About day 28.

Which limbs develop first?

Upper limbs.

By how many days does upper limb development precede lower limb development?

About 2–4 days.

What does 'proximodistally' mean?

The proximal parts of the limbs develop before the distal parts.

During which weeks do the proximal parts of the limbs form?

Weeks 4–5.

What embryonic tissue forms the bones, cartilage, tendons, and connective tissue of the limbs?

Lateral plate mesoderm.

What embryonic tissue forms the epidermis and epidermal derivatives?

Ectoderm.

What embryonic tissue forms the limb muscles?

Somitic mesoderm.

What is the apical ectodermal ridge (AER)?

A thickened ridge of ectoderm at the tip of each limb bud.

What is the function of the apical ectodermal ridge?

It signals the underlying tissue to differentiate and elongate the limb.

What happens if the apical ectodermal ridge is removed?

The limb fails to develop.

What structure forms at the distal end of the upper limb bud during the fifth week?

Hand plate.

What structure forms at the distal end of the lower limb bud during the sixth week?

Foot plate.

What are digital rays?

Longitudinal thickenings that develop into the fingers and toes.

What process separates the digital rays into individual digits?

Programmed cell death (apoptosis).

By what week are the fingers and toes completely separated?

Week 8.

How do the upper limbs rotate during development?

Laterally.

How do the lower limbs rotate during development?

Medially.

When do primary ossification centers begin to form in the developing limb bones?

By week 8.

What happens by week 12 of development?

The shafts of the limb bones are rapidly ossifying while many other bones remain cartilage.

What developmental event positions the elbows posteriorly and the knees anteriorly?

The upper limbs rotate laterally, while the lower limbs rotate medially during weeks 7–8.

What is polydactyly?

The presence of extra digits.

What is ectrodactyly?

The absence of one or more digits.

What is syndactyly?

Webbing or fusion of digits caused by failure of apoptosis between digital rays.

What is amelia?

Complete absence of a limb.

What is meromelia?

Partial absence of a limb.

What is phocomelia?

A short, poorly formed limb resembling a seal's flipper.

During which weeks is thalidomide most likely to disrupt limb development?

Weeks 4–8 of embryonic development.

Why does thalidomide cause limb malformations?

It blocks expression of genes needed for blood vessel growth, disrupting limb bud development.