Derivation
Nearly all muscle arises from mesoderm.
Exception: Smooth muscles of the iris arise from ectoderm.
The embryology of muscular tissue involves the development of muscles from the mesoderm, one of the three primary germ layers formed during early embryogenesis.
### 1. Mesodermal Differentiation
- **Gastrulation**: During this process, the three germ layers (ectoderm, mesoderm, and endoderm) are formed.
- **Mesoderm Formation**: The mesoderm is the middle layer and gives rise to various tissues, including muscular tissue.
### 2. Somite Formation
- **Paraxial Mesoderm**: The mesoderm adjacent to the neural tube and notochord condenses to form somites, which are segmental blocks of mesoderm.
- **Somite Differentiation**: Somites differentiate into three major parts:
- **Dermatome**: Forms the dermis of the skin.
- **Myotome**: Forms skeletal muscles.
- **Sclerotome**: Forms the vertebrae and ribs.
### 3. Development of Muscle Types
- **Skeletal Muscle**:
- Derived from the myotome of somites.
- Myoblasts, the precursor muscle cells, proliferate and fuse to form multinucleated myotubes.
- Myotubes mature into muscle fibers, forming the skeletal muscles of the body.
- **Cardiac Muscle**:
- Develops from the splanchnic mesoderm, which surrounds the heart tube.
- Myoblasts differentiate into cardiac muscle cells, which become interconnected by intercalated discs to form the myocardium.
- **Smooth Muscle**:
- Arises from the splanchnic (walls of developing hollow organs of cardiovascular, digestive, urinary, and reproductive systems ) mesoderm and local mesenchymal cells.
### 4. Molecular Regulation
- **Signaling Pathways**: Various signaling pathways, including Wnt, Sonic hedgehog (Shh), and BMP, play crucial roles in the induction and differentiation of muscle tissue.
- **Transcription Factors**: Specific transcription factors like MyoD, Myf5, myogenin, and MRF4 are essential for myogenic differentiation and muscle fiber formation.
### 5. Muscle Patterning and Growth
- **Patterning**: Muscle patterning is regulated by positional information provided by the surrounding tissues, such as the neural tube, notochord, and ectoderm.
- **Growth and Maturation**: After birth, muscle growth occurs through hypertrophy (increase in muscle fiber size) rather than hyperplasia (increase in muscle fiber number).
The embryological development of muscular tissue is a complex and tightly regulated process involving coordinated interactions between different cell types, signaling molecules, and transcription factors.
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Mesodermal mesenchyme cells migrate from the myotomes of the somites.
Mesenchymal cells differentiate by a gradual process of cell lengthening with abundant synthesis of the myofibrillar proteins actin and myosin, accumulation of myofilaments in the cytoplasm and the development of special membranous channels and compartments.
The mesenchymal cells retract their cytoplasmic processes and assume a spindle shape to become myoblasts; these fuse to form multinucleated myotubes.
Myotubes elongate and increase in diameter by incorporating additional myoblasts, accumulating myofilaments and nuclei in their cytoplasm. The myofilaments organize into myofibrils (II.B.1.c) and displace the nuclei and other cytoplasmic components peripherally. Mature muscle fibers cannot divide; therefore, increases in skeletal muscle mass in response to weight bearing and exercise are due to hypertrophy rather than to hyperplasia (division) of the existing cells. Exercise and weight bearing elicit a proliferative response from quiescent stem cells in the muscle tissue, called satellite cells. Some of their progeny become myoblasts and fuse with existing muscle fibers, increasing their size. Because the strength of contraction is directly proportional to muscle fiber diameter, skeletal muscle hypertrophy increases muscle strength. Myostatin is a signaling molecule that slows myogenesis. Loss-of-function myostatin mutants were first discovered in Piedmontese and Belgian cattle with double the muscle mass of normal cattle. Similar mutations in humans are exceedingly rare but have the same effect.
Cardiac muscle
Cardiac muscle arises from mesodermal cardiogenic mesenchyme lying anterior to the embryo's head. As development continues, parallel chains of elongated splanchnic mesenchymal cells arise in the walls of the heart tube. Cells in each chain develop specialized junctions and often branch and bind to cells in nearby chains. The cells synthesize and accumulate myofilaments in their sarcoplasm. The branched network of myoblasts forms interwoven bundles of muscle fibers, but cardiac myoblasts rarely fuse.