When do limb buds first appear during embryonic development?

Limb buds arise as outpouchings from the ventrolateral body wall at the end of the 4th week of development, with forelimbs appearing slightly earlier than hindlimbs.

What is the role of the apical ectodermal ridge (AER) in limb development?

The apical ectodermal ridge (AER) forms at the tip of each limb bud and regulates proximal-distal development, guiding the formation of limb segments and digits.

How do upper and lower limb buds rotate during development?

Initially paddle-shaped, limb buds rotate differently: the upper limb rotates 90° laterally (thumb lateral), and the lower limb rotates 90° medially (great toe medial). This explains differences in flexor/extensor orientations.

What are the dermatome levels for the upper and lower limbs?

Dermatomes are segmental skin bands supplied by specific spinal levels: the upper limb is C4–T2, and the lower limb is L2–S2. Their positioning is regulated by Hox genes along the cranio-caudal axis.

How do limb bones develop from the embryonic stage?

Limb bones originate from the somatic layer of lateral plate mesoderm and develop through stages: mesenchymal condensation in week 5, chondrification in week 6, and ossification from week 8 onwards, with the clavicle being the first bone to ossify.

What are the key differences between primary and secondary ossification centers in limb bones?

Ossification involves primary centers in the diaphysis around the 8th week (visible at birth) and secondary centers in epiphyses postnatally, with the growth plate responsible for lengthening until skeletal maturity.

What are some common congenital limb abnormalities and their causes?

Common abnormalities include syndactyly (webbed digits from failed apoptosis), polydactyly (extra digits, often autosomal dominant), achondroplasia (dwarfism from defective ossification), and Thalidomide Syndrome (severe defects from maternal drug use at 6–8 weeks).

Limb Development – Buds, AER & Rotation

❓ Frequently Asked Questions