Development of Appendicular Skeleton

Questions and Answers

During which week of development do the upper and lower limb buds appear?

The fourth week

What is the difference in timing between upper and lower limb development?

Lower limb development lags behind upper limb development by about 2 to 4 days.

What is the proximodistal pattern of limb development?

The more proximal parts of the limbs form first, followed by the more distal parts.

What tissue types make up the early limb buds?

Lateral plate mesoderm and ectoderm. Signup and view all the answers

Where does the musculature of the limbs originate from?

Somitic mesoderm. Signup and view all the answers

What is the function of the apical ectodermal ridge?

It signals the underlying tissue to form the various components of the limb. Signup and view all the answers

When does the hand plate form in the upper limb bud?

By the early fifth week. Signup and view all the answers

When do the digital rays appear in the foot plate?

During the early seventh week. Signup and view all the answers

What is the process by which intermediate tissue between digital rays dies, leading to the formation of separate digits?

Apoptosis (programmed cell death) Signup and view all the answers

What type of ossification leads to the formation of flat bones of the skull, facial bones, and the central part of the clavicle?

Intramembranous ossification Signup and view all the answers

What is the term for the condition of having extra digits, and what is the likely cause of this condition?

Polydactyly; genetic component Signup and view all the answers

What is the term for the absence of a digit, and how does it occur?

Ectrodactyly; failure of digital ray formation Signup and view all the answers

What is the term for abnormal fusion of the digits, and what is the underlying cause of this condition?

Syndactyly; failure of intermediate tissue to undergo apoptosis Signup and view all the answers

What is the term for the complete absence of a limb, and what is the term for the partial absence of a limb?

Amelia; meromelia Signup and view all the answers

What is the term for a short, poorly formed limb that resembles a seal's flipper?

Phocomelia Signup and view all the answers

What was the primary mechanism by which thalidomide caused limb malformations in fetuses?

Binding to specific regions of chromosomal DNA, preventing gene expression Signup and view all the answers

During what weeks of embryonic development is thalidomide most likely to cause limb malformations?

4-8 weeks Signup and view all the answers

What is the current use of thalidomide, and what warnings are associated with its use?

Treatment for multiple myeloma, AIDS, lupus, and leprosy; warnings regarding its use during pregnancy Signup and view all the answers

Study Notes

Development of the Skeleton

The appendicular skeleton begins to develop during the fourth week of development, when limb buds appear as small ridges along the lateral sides of the embryo.
Upper limb buds appear early in the fourth week (approximately day 26), and lower limb buds appear a few days later (day 28).
Lower limb development lags behind upper limb development by about 2 to 4 days.
The upper and lower limbs form proximodistally, meaning that the more proximal parts of the limbs form first (in weeks 4–5), whereas the more distal parts differentiate later.

Formation of Limb Buds

Early limb buds are composed of lateral plate mesoderm and covered by a layer of ectoderm.
The musculature of the limbs forms from somitic mesoderm that migrates to the developing limbs during the fifth week of development.
At the apex of each limb bud, part of the ectoderm forms an elevated thickening called the apical ectodermal ridge.

Development of Digits

The distal portion of the upper limb bud forms a rounded, paddle-shaped hand plate by the early fifth week, which later becomes both the palm and fingers.
A corresponding foot plate forms during the sixth week in the lower limb bud.
The hand plate and foot plate develop longitudinal thickenings called digital rays, which eventually form the digits.
The digital rays initially are connected by intermediate tissue, which later undergoes programmed cell death (apoptosis), causing notching to occur between the digital rays and separate digits to form.

Clinical View: Limb Malformations

Polydactyly is the condition of having extra digits and tends to run in families, with a genetic component.
Ectrodactyly is the absence of a digit and also runs in families.
Syndactyly refers to the abnormal fusion of the digits, occurring when the intermediate tissue between the digital rays fails to undergo normal programmed cell death (apoptosis).
Amelia refers to the complete absence of a limb, while meromelia refers to the partial absence of a limb.
Phocomelia refers to a short, poorly formed limb that resembles the flipper of a seal.

Thalidomide and Limb Malformations

Thalidomide, a drug first marketed in the 1950s, was found to cause limb malformations when taken by pregnant women during weeks 4-8 of embryonic development.
The drug binds to particular regions of chromosomal DNA, preventing the expression of genes responsible for blood vessel growth, leading to disrupted limb bud formation.
Thalidomide has been linked to a resurgence of birth defects in South America in recent years, despite warnings and precautions.
It serves as a classic example of how teratogens can affect the delicate cycle of embryonic development.

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Description

Compare and contrast the development of upper and lower limb buds during the fourth week of embryonic development. Learn about the formation of the appendicular skeleton and the difference in development timeline between upper and lower limbs.