Development of Muscles and Limbs Flashcards
“The upper limb buds are visible by day 24. The lower limb buds become visible at the end of the fourth week [2 days after appearance of upper limb buds] with the activation of a group of mesenchymal cells in the ____(a)____ lateral mesoderm [somatic/splanchinic]. ____(b)____ genes regulate patterning in the formation of the limbs. The limb buds form deep to a thick band of ectoderm, the ____(c)____. Each limb bud consists of a mesenchymal core of mesoderm covered by a layer of ectoderm.
(a) somatic
(b) Homeobox (Hox)
(c) apical ectodermal ridge (AER)
State the origin of skeletal, cardiac, smooth muscles and myoepithelial cells.
(a) Skeletal muscles: paraxial mesoderm except iridal and ciliary muscles of the eye, which originate from neuroectoderm.
(b) Cardiac muscle and smooth muscles of the gut and respiratory system: splanchic mesoderm
(c) Smooth muscles of blood vessels and erector pili muscles: local mesenchyme
(d) Myoepithelial cells: ectoderm
How does paraxial mesoderm segment to form somites?
- paraxial mesoderm cells become organized into whorls of cells called somitomeres.
- the first seven pairs of somitomeres persist
- cells from 8th pair of somitomeres caudally acquire epithelial characteristics and fragment to form blocks called somites.
Outline the key stages of development of the hand.
- The distal ends of the limb buds flatten into hand plates.
- Condensation of mesenchyme resulting in the formatin of digital rays in the hand plate
- Formation of notches between digital rays
[The intervals between the digital rays are occupied by loose mesenchyme. These intervening regions of mesenchyme soon break down, forming notches between the digital rays.] - Formation of webs/webbing between the developing digits
- Apoptosis of webs
- Development of phalanges and carpals from mesenchyme
State the differences between upper limb and lower limb development.
- Site of origin: upper limb from the lower cervical region, lower limb from the lumbosacral region
- Timing: upper limb development is ahead of lower limb development by 2 days
- Rotation: upper limb rotates laterally, lower limb rotates medially
When the sclerotome migrates, the remaining part of the somite is called ____(a)____ which splits to form the ____(b)____ and ____(c)____.
(a) dermomyotome
(b) and (c) dermatome and myotome
When the dermomyotome is exposed to Wnts and there is repression of BMP activity, there are 3 muscle producing regions. State the 3 regions and their derivatives.
a) Dorsal region
Myotome: epaxial muscles
Limb myoblasts: extensor muscles
b) Central region
Dermatome: muscles and satellite cells, dermis and brown fat
c) Ventral region
Myotome: hypaxial muscles
Limb myoblasts: flexor muscles
[Diagram]
State the origin of various regional musculature.
[cranial musculature, muscles of the tongue, trunk muscles, limb muscles]
Cranial musculature develops mainly from somatomeres. Patterns of muscle formation in the head are directed by connective tissue elements derived from neural crest cells.
Muscles of the tongue develop from occipital myotomes.
Trunk muscles arise from somites.
Limb muscles are from somatic mesoderm, cells which migrate to the developing limb bud.
The major derivatives of the myotome are epimere and hypomere (check the diagram of that in the relevant textbooks, it is commonly examined). State what the epimere gives rise to.
Click here as well to view the diagram
a) Extensor muscles of the neck.
b) Erector spinae of the back.
c) Lumbar extensor muscles.
The extensor muscles derived from the sacral and coccygeal myotomes degenerate to form dorsal sacrococcygeal ligaments.
State the derivatives of the hypomere in the:
1. In the neck
2. In the thorax
3. In the abdomen
4. In the sacro-coccygeal region
In the neck: prevertebral & scalene muscles, geniohyoid & infrahyoid muscles (from ventral tip of hypomere).
In the thorax: intercostal muscles.
In the abdomen: the oblique muscles & rectus abdominis (from ventral tip) and quadratus lumborum.
In the sacro-coccygeal region: muscles of the pelvic diaphragm and striated perianal muscles.
Further notes:
Preverterbral muscles of the neck include: rectus capitis anterior, rectus capitis lateralis, longus capitis and longus colli muscles.
Describe the histogenesis of skeletal muscles.
✓ Migration of mesenchymal cells to the site of muscle formation.
✓ Elongation of mesenchymal cells & their differentiation into myoblasts.
✓ Fusion of myoblasts to form myotubes. [Some questions for you: What happens to the myoblasts that don’t fuse? What do they become? Would they eventually fuse later in life? If so, after what happens?]
✓ Development of myofilaments in the cytoplasm of the myotubes, during or after fusion of myoblasts.
✓ Transformation of myotubes into myocytes through development of myofibrils & other organelles characteristic of skeletal muscle.
✓ Investment of myocytes with external basal laminae, individually or in groups.
Further notes:
Myofibrils are the main rod-like structures of the muscle cells. They are long, cylindrical structures that run the entire length of the muscle fiber. Myofibrils are composed of myofilaments, which are arranged in a repeating pattern of sarcomeres. The sarcomeres are the basic functional units of the myofibril, responsible for the contraction of the muscle fiber.
On the other hand, myofilaments are the individual protein strands that make up myofibrils. They are made up of two types of protein: actin and myosin. These proteins work together to create the sliding motion that allows muscles to contract and relax
Describe the histogenesis of cardiac muscles.
The process is the same as for skeletal muscle save for the following differences:
Mesenchymal cells are of splanchnic origin & aggregate around the endothelial tube destined to form the heart.
Myoblasts establish intercalated disks between them.
Later in development, a network of cardiac myocytes undergoes an alternative pathway of differentiation characterized by increased size, reduction in concentration of myofibrils & accumulation of cytoplasmic glycogen ( the conduction system, which also has different protein isoforms).
a) Tendons of epaxial muscles are derived from the ________________ layer within the somites.
b) Tendons of limb and hypaxial muscles are derived from the ________________ mesoderm.
a) syndetome
b) lateral plate
State the factors in muscle location, organization and morphogenesis, with examples.
a) Migration
- muscles of facial expression from the neck
- trapezius from the head to neck & back
- latissimus dorsi from the cervical region to the back
b) Fusion
- rectus abdominis from fusion of mesenchymal bars at the ventral tips of the abdominal hypomere
c) Splitting
- Intercostal muscles and oblique abdominal muscles result from splitting of myotomes.
d) Regression
- Some myotomes regress into aponeurosis and ligaments e.g abdominal aponeurosis, the sacrospinous and sacrotuberous ligaments.
e) Interaction with nerves
Muscle fibres which do not form functional units with nerves degenerate e.g.
Explain the mechanisms of muscle growth.
1) Hypertrophy: most muscles increase in size by this method. The increase in diameter of muscle fibres is through formation of more myofilaments.
2) Hyperplasia: this involves increase in the number of cells by new myocyte formation. It occurs in the uterus during pregnancy.
3) In skeletal muscle, some undifferentiated cells called myosatellite cells may contribute to growth by forming new myoblasts
NB// Myosatellite cells also have a role in the limited skeletal muscle regeneration and could be the origin of skeletal muscle tumours.
