Embryological Development Of Musculoskeletal System Pdf

[Carolina Bornman]

Musculoskeletalanatomy is fascinating since it gives us insights as to how our body utilizes our muscles, bones, and joints to give us the ability to navigate in the world. If this article peaks your interest, you probably have a solid foundation in this topic and are ready to take your knowledge to the next level. By studying the embryological development of the musculoskeletal system, you will achieve a better understanding of how different types of congenital anomalies can occur.

The development of bone and muscle begins at the fourth gestational week, when the paraxial mesoderm differentiates into somites; the latter gives rise to sclerotomes and dermomyotomes. Sclerotomes form the vertebra and the ribs, whereas myotomes form the majority of the muscular system.

Bone formation can occur either by intramembranous ossification or endochondral ossification. Although different, the occurrence of both processes first require the condensation of mesenchymal cells - the loosely organized embryonic connective tissue. Intramembranous ossification underlies the formation of the cranial vault, many bones of the face, and the clavicle. Endochondral ossification underlies the formation of the base of the skull , some bones of the face, the bones of the limbs and girdles, the vertebral column, the ribs, and the sternum.

Development of the limbs involves the inductive influences of the apical ectodermal ridge, the formation of circular constrictions to separate parts of the limbs, and opposite rotations of the upper and lower limbs. Development of the skeletal muscle involves the differentiation of myotome cells into myoblasts. This article will discuss the embryological development of the axial skeleton, the appendicular skeleton, and the skeletal muscle, as well as the associated malformations that may occur.

The first stage of any type of bone formation involves a mesenchymal condensation, where cells become densely packed together. From this point on, there are two ways osteogenesis can occur: intramembranous ossification and endochondral ossification. The process in which mesenchymal cells ensheathed in membranous tissue directly undergo ossification is known as intramembranous ossification. The process in which mesenchymal cells first differentiate into cartilage models before undergoing ossification is known as endochondral ossification.

Formation of the cranial vault, most bones of the face, and the clavicle occur by intramembranous ossification, whereas formation of the rest of the axial and appendicular skeleton occur by endochondral ossification. In other words, the base of the skull, some bones of the face, the vertebral column, the ribs, the sternum, and the bones of the limbs and girdles form by a two-step process: chondrification and ossification.

During the sixth gestational week, joints begin to develop with the formation of condensed mesenchyme in the interzone, the region between two bone primordia. Joints are classified as:

The development of fibrous joints involves mesenchymal cells in the interzone to differentiate into dense fibrous tissue (i.e. sutures of the skull). The development of cartilaginous joints involves mesenchymal cells in the interzone to differentiate into hyaline cartilage (i.e. costochondral joints) or fibrocartilage (i.e. pubic symphysis). The development of synovial joints involves a more extensive process: the central mesenchymal cells in the interzone undergo apoptosis to form the synovial joint cavities, whereas the peripheral cells differentiate into ligaments and dense fibrous tissue. Sequentially, the dense fibrous tissue forms the articular cartilage that covers the ends of the adjacent bone primordia. The remaining mesenchymal cells surrounding the interzone differentiate into chondrocytes to form the joint capsules and the synovial membrane.

The skull consists of a neurocranium and a viscerocranium, with each having membranous and cartilaginous components. The bones that make up the skull thus form either by intramembranous ossification or endochondral ossification. The bones that make up the vertebral column, the ribs, and sternum form only by endochondral ossification. The vertebral column develops from a resegmentation process of the somites, while the ribs develop as extensions from the thoracic vertebrae. The sternum develops as two independent bands of mesenchymal cells before fusing and ossifying as one.

The skull can be divided in two parts: the neurocranium that forms a protective case around the brain, and the viscerocranium that forms the skeleton of the face. The neurocranium itself is divided into two other parts: the membranous part that surrounds the brain as a vault, and the cartilaginous part (chondrocranium) that forms the base of the skull. Both the neurocranium and the viscerocranium have distinct components that are formed either by intramembranous ossification or endochondral ossification.

The membranous part of the neurocranium forms the calvaria (skullcap). It is derived from two sources: the paraxial mesoderm and the neural crest cells. Mesenchymal cells from these two sources surround the brain at various sites, form primary ossification centers, and undergo intramembranous ossification. This results in the formation of membranous flat bones that are characterized by needle-like bone spicules. Bone spicules progressively radiate from the primary ossification centers toward the periphery. Structures derived from the membranous neurocranium include the parietal bones, part of the temporal bones, and the occipital bone.

At birth, the membranous bones are separated from each other by dense connective tissue membranes that form fibrous joints, known as the cranial sutures (coronal, sagittal, and lambdoid). The site at which more than two bones meet are called the fontanelles (anterior, posterior, and two posterolateral).

As the brain and the skull continue to grow after birth, many of these sutures and fontanelles will remain membranous and open postnatally. Generally, the posterior fontanelle closes first by 2 months of age, the mastoid fontanelle by 6 months, the anterior fontanelle by 18 months, and the cranial sutures by 36 months.

The cartilaginous part of the neurocranium forms the base of the skull. It initially consists of a number of separate cartilages that eventually fuse together. Similar to the membranous neurocranium, the cartilaginous neurocranium is derived from the same sources. The neural crest cells form the prechordal chondrocranium anterior to the center of the sella turcica, whereas the paraxial mesoderm form the chordal chondrocranium posterior to the center of the sella turcica. The development of the base of the skull is complete when these cartilaginous structures fuse and undergo endochondral ossification. Structures derived from the chondrocranium include components of the occipital bone, the sphenoid bone, and the ethmoid bone, specifically the:

The dorsal tip of the mandibular process and the second pharyngeal arch undergo endochondral ossification to give rise to the malleus, the incus, and the stapes. The ossicles are the first bones to become fully ossified, with their ossification beginning in the fourth month of gestation.

The vertebral column develops from the sclerotomes, the ventromedial part of the somite. By the fourth gestational week , sclerotome cells surround the neural tube and the notochord to merge with cells derived from the opposing somite. Each sclerotome then undergoes resegmentation, a process that involves the caudal half of each sclerotome to fuse with the cranial half of each adjacent sclerotome; this forms the centrum, the primordial vertebral body. Thus, each vertebra develops from two adjacent sclerotomes rather than from one sclerotome.

Not all cells in the caudal half of each sclerotome undergo resegmentation. Instead, some migrate cranially and contribute to the formation of the intervertebral disc. As development continues, the notochord completely degenerates in the centrum, but where it persists, it enlarges as a gelatinous center. This forms the nucleus pulposus, which is later surrounded by circularly arranged fibers known as the annulus fibrosis. Combined, these two structures form the intervertebral discs.

By the sixth gestational week, the sclerotome cells surrounding the neural tube form a cartilaginous vertebral arch, and fuse with the cartilaginous vertebral body. The spinous, transverse, and costal processes develop as extensions from this newly assembled cartilage model. In the lumbar region, the costal processes of the first sacral vertebrae fuse and form the lateral sacral mass, known as the ala of the sacrum. The process of chondrification continues until a cartilaginous vertebral column is fully formed.

Ossification of the vertebrae begins at the seventh gestational week, but only ends during the second decade of adulthood. By the eighth week, three primary ossification centers develop: one at the center of the cartilaginous vertebral body and one on each side of the cartilaginous vertebral arch. At puberty, five secondary ossification centers appear in the vertebrae: one at the tip of the spinous process, one at the tip of each transverse process, and one on both the superior and inferior rim of the vertebral body.

Ribs develop from the costal processes of the thoracic vertebrae. They are cartilaginous during the embryonic period and undergo ossification during the fetal period. The original site where the costal process is connected to the vertebra becomes replaced by costovertebral synovial joints. The first seven pairs of ribs attach to the sternum through their own cartilages. The subsequent five pairs of ribs attach to the sternum through the cartilage of the seventh rib. The last two pairs of ribs do not attach to the sternum. Respectively, this forms the true ribs, the false ribs, and the floating ribs.

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