Limb Development and Muscle Migration – Embryology | Lecturio

Summary

This video provides an in-depth overview of human limb development, from the early formation of limb buds to the development of muscles, nerves, and blood vessels. It explains how upper and lower limbs rotate differently, leading to distinct flexor and extensor muscle distributions. Additionally, the migration of muscles from the myotome, the creation of nerve plexuses, and the formation of arteries and veins are explored. The video also highlights developmental anomalies like Poland syndrome, prune belly syndrome, and syndactyly, offering insight into the potential variations that can occur during limb formation.

Takeaways

• 😀 Limb buds form around day 24 in embryos, with the upper limb bud appearing first and the lower limb bud a couple of days later.
• 😀 The tail segment initially present between the limbs disappears as the limbs extend and grow.
• 😀 As the limbs grow, hand and foot plates form, and digital rays appear, eventually leading to distinct fingers and toes by day 52 (upper limb) and day 56 (lower limb).
• 😀 The upper and lower limbs rotate differently during development: the upper limb rotates so the elbow points posteriorly, while the lower limb rotates so the knee points anteriorly.
• 😀 Muscles develop from the myotome of the somites, with muscle migration forming structures like the epaxial muscles of the back and hypaxial muscles in the torso and limbs.
• 😀 Nerve innervation follows muscle migration, forming the brachial plexus in the upper limb and the lumbosacral plexus in the lower limb.
• 😀 The ventral axial line remains as the dividing line for dermatomes, separating them on the anterior surface of the upper limb and the posterior surface of the lower limb.
• 😀 The vasculature for the limbs is derived from segmental arteries off the dorsal aorta, with the axillary artery supplying the upper limb and the femoral artery supplying the lower limb.
• 😀 Superficial veins like the cephalic and basilic veins in the upper limb and the greater and small saphenous veins in the lower limb are remnants from early developmental venous systems.
• 😀 Developmental abnormalities like muscle agenesis can occur, leading to conditions such as Poland syndrome (absence of pectoral muscles) and prune belly syndrome (absence of abdominal muscles).
• 😀 Abnormalities like sirenomelia (mermaid-like limbs) can result if the tail segment recedes too early, while a vestigial tail can form if the tail segment remains.
• 😀 Syndactyly, the fusion of digits, occurs when the webbing between digits does not completely disappear, leading to partial or complete fusion of the bones in the affected digits.

Q & A

What is the initial state of the embryo in terms of limb development?

Initially, the embryo has no limbs. The upper limb bud forms on day 24, and the lower limb bud follows on day 26.

What happens to the tail segment during limb development?

The tail segment initially exists between the two lower limbs but gets displaced as the lower limbs grow, eventually disappearing.

What are the digital rays, and how do they contribute to limb development?

The digital rays are solid areas that develop within the hand and foot plates. The tissue between them begins to thin and undergoes apoptosis, eventually forming distinct fingers and toes by around day 52 or 56.

How do the upper and lower limbs differ in terms of rotation during development?

The upper limb rotates so that the elbow points posteriorly, positioning the flexor muscles anteriorly. In contrast, the lower limb rotates so that the knee points anteriorly, placing the extensor muscles on the anterior surface.

From where do the muscles of the developing limbs originate?

The muscles of the developing limbs originate from the myotomes, which are derived from somites adjacent to the developing neural tube.

How do the nerves in the developing limbs get formed?

As the myotomes migrate, they pull their nerve supply along, resulting in the formation of the brachial plexus in the upper limb and the lumbosacral plexus in the lower limb.

What is the role of the ventral axial line in limb development?

The ventral axial line separates the dermatomes in the upper and lower limbs and remains significant in the anterior surface of the upper limb and the posterior surface of the lower limb.

How do the arteries and veins develop in the limbs?

The arteries in the limbs originate from a segmental artery off the dorsal aorta, which becomes the axillary artery in the upper limb and the femoral artery in the lower limb. Superficial veins such as the cephalic and basilic veins in the upper limb persist as remnants of early venous development.

What are common developmental issues related to muscle migration?

Common issues include muscle agenesis, such as in Poland syndrome (absence of one or more pectoral muscles) and prune belly syndrome (failure of abdominal musculature to form), which often come with additional complications like renal problems.

What is syndactyly, and how does it occur during limb development?

Syndactyly is the fusion of digits due to incomplete apoptosis of the webbing between them. It can range from simple webbing to more significant fusion of bones in the digits, as seen in some cases of the second and third toes.