Excerpt from Trunk, Embryology

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Introduction

Knowledge of trunk embryology has significant clinical relevance. Many congenital deformities of the chest or abdomen are treated by reconstructive surgeons; thus, extensive knowledge of the normal embryologic processes and the aberrations in development is of paramount importance.

Much of the understanding of human embryology has been elucidated from extensive experimental manipulations of organisms such as Drosophila melanogaster, chick, and mouse. Human embryology, from implantation to the embryonic period and, finally, the birth period of development, has been characterized in detail. The development of the musculoskeletal trunk of the human body encompasses the union of skeletal and mesodermal evolution, which is initiated from several signals from surrounding tissues. This process occurs during the fourth through eighth weeks of gestation. The purpose of this review is to provide the reader with an understanding of the development of the structural components of the trunk. The development of the individual organ systems within the trunk is beyond the scope of this review.

The musculoskeletal system develops from the ectodermal neural crest and the paraxial and lateral plate (somatic layer) mesoderm. Approximately 40 segmental tissue blocks alongside the neural crest caudal to the head region, known as somites, differentiate into 2 parts. The dorsolateral subpopulation of somatic cells is called the dermomyotome, and the ventromedial subpopulation of cells is known as the sclerotome. The dermomyotome eventually forms the musculature of the trunk, whereas the sclerotome develops into the skeletal framework.

Skeletal development

The cells of the sclerotome shift their position during the fourth week of gestation to surround the spinal cord and the notochord. The sclerotome is further divided into two subdivisions. The ventral subdivision of sclerotome forms a vertebral body, while the dorsal subdivision of a sclerotome forms the vertebral arch. With further development, the caudal section of each sclerotome proliferates into the subjacent intersegmental tissue and, therefore, binds the caudal section of one sclerotome to the cephalic section of the subjacent sclerotome. The precartilaginous vertebral bodies are, therefore, formed by the upper and lower halves of 2 successive sclerotomes and the intersegmental tissue. As the sclerotomes coalesce to form individual vertebrae, intervertebral discs are formed as the enclosed notochord becomes transformed into a nucleus pulposus with adjacent sclerotomic cells forming the annulus fibrosis of each intervertebral disc.

The ribs and costal processes of the thoracic vertebrae are also derived from sclerotomic cells of the paraxial mesoderm. The sternum arises from paired longitudinal concentrations of mesenchymal tissue, which are separate from the ribs, in the sixth week of development. The manubrium is formed by primordia between the ventral ends of the developing clavicles. The longitudinal bands fuse in the midline to form a cartilaginous sternal plate at around the 10th week. This fusion occurs in a cranial-to-caudal direction, and failure of this fusion leads to the congenital anomaly of cleft sternum.

Cleft sternum, as well as several other chest wall deformities, is attributable to an error in the normal embryologic development of the chest wall structures. For example, pectus excavatum is the most common congenital chest wall deformity, characterized by excessive depression and, frequently, rotation of the sternum. Occurring in an estimated 1 in 300 live births, pectus excavatum shows a strong male predominance. Although the exact etiology is unknown, the overgrowth of costal cartilages that rotate and curve dorsally is believed to be the cause. Other examples of skeletal anomalies include pectus carinatum, bifid sternum, and thoracoabdominal ectopia cordis.

Musculature development

The dorsolateral subpopulation of somatic cells is called the dermomyotome. These cells maintain their segmental arrangement and give rise to a new layer of cells, the myotome, which provides the musculature for its own segment. The rearrangement of sclerotomes into definitive vertebrae causes the myotomes to overbridge the intervertebral discs, allowing for movement of the spine. The remaining cells, after extension of the myotomes, are referred to as dermatomes.

As in the limbs, a segmental peripheral nerve migrates with the dermatome and the myotome. This early contact of the nerve and differentiating muscle not only provides motor innervation, but also provides sensory innervation, which develops to recognize pressure, touch, and temperature from the skin surfaces. This area of skin is supplied by branches of a specific single spinal nerve and, eventually, contributes to the dermis of the skin and is known as a dermatome. Although growth causes some changes in the original segmental size, these distinct dermatome patterns are maintained throughout life. The rest of the dermis is derived from the somatopleure of the lateral mesoderm.

The cells of the myotomes divide further into an epimere and hypomere. The epimeres give rise to the skeletal muscles of the back. The hypomeres differentiate into the skeletal muscles in the lateral and anterior regions of the thorax and abdomen. The hypomere splits into 3 layers, which, in the thorax, represent the external intercostals, the internal intercostals, and the innermost intercostals or transverse thoracic muscle. In addition, in regions of the developing limb buds, the myotomes contribute to muscles of the limbs. This paraxial mesenchymal tissue also gives rise to the anterior chest and abdominal wall musculature.

Poland syndrome consists of a variable constellation of anomalies that include congenital absence of the sternal head of the pectoralis major. Other muscle anomalies may exist, including deficiencies of the latissimus dorsi, serratus anterior, deltoid, supraspinatus, and infraspinatus muscles. Other associated conditions that may occur include aplasia or hypoplasia of ipsilateral breast, athelia (congenital absence of the nipple areolar complex), abnormalities of the ribs and costal cartilages, subclavian artery aplasia, axillary fat or hair absence, hand hypoplasia, and brachysyndactyly. In severe cases, acheiria, congenital absence of one or both hands, or an atrophic limb may occur.

Several genetic and molecular techniques have been utilized to determine the source of myogenic progenitor cells and their developmental influences. Skeletal muscle differentiation requires a family of transcription factors known as myogenic regulatory factors. Most congenital muscle diseases and structural abnormalities have at least been mapped to chromosomal regions. Myogenic progenitor cells (MPC) of the somite originate from the dermomyotome and differentiate to form a primary myofiber scaffolding. Continual muscle growth occurs through the addition of secondary myofibers from fetal myogenic progenitors. Secondary fibers acquire the characteristics of fast fibers, whereas the primary fibers tend to become slow fibers. By the end of the third month, cross-striations typical for skeletal muscle appear.

This mesodermal mass migrates ventrally and laterally as the primordia of the right and left rectus abdominis muscles. Prior to the fusion of rectus muscles anteriorly, the developing mesoderm of the future anterolateral abdominal wall splits into 3 layers that ultimately give rise to the internal oblique, external oblique, and transversus abdominis muscles. Dorsally, the posterior serratus muscles develop from the superficial layer of the hypomere.

As the anterior abdominal wall is developing, the intra-abdominal contents are also developing. Development of the primary intestinal loop is characterized by rapid growth and simultaneous expansion of the liver, which leads to a physiologic umbilical herniation. At approximately the end of the third month, the herniated intestinal loops begin to return, which, in some cases, may lead to a failure of the loops to completely return into the abdominal cavity.

Mild umbilical hernias gradually close spontaneously in the postnatal period; however, more severe cases of failure of the abdominal wall to fuse completely can lead to either an omphalocele or gastroschisis. Omphaloceles are differentiated by the presence of amnion or peritoneal covering, whereas gastroschisis are characterized by the complete absence of intestinal covering. Exstrophy of the urinary bladder may be present, as well as solid organ eventration with omphalocele; additionally associated congenital defects tend to be more severe with omphaloceles.

Limb development

The first indications of limb musculature begin in the seventh week of development; however, limb buds are apparent by the fourth week. These buds are covered by surface ectoderm with a core of mesoderm, with contributing sources of cells included from the lateral plate mesoderm, myotome portion of somites, and neural crest cells. At the distal edge of each limb bud is a thickened region of ectoderm, called the apical ectodermal ridge, which has been shown to induce the differentiation and growth of a limb bud. Failure of this ectodermal ridge to develop leads to amelia, the absence of limb growth.

The myotomes eventually develop muscle tissue that is split into flexor and extensor components. The upper limb bud development occurs first in the vertebral region C3-T2, followed shortly (in a few days) by the appearance of the lower limb buds in the region of L2-S3. As soon as the buds are formed, neural tissues penetrate into the mesenchyme. As the myotomes develop into myoblasts, which migrate into the developing limb buds, the corresponding nerves course with the myoblasts.

A dermatome is a predictable area of skin in which afferent nerve fibers transmit impulses to a single posterior spinal root. The dermatomes of the developing limbs come primarily from the brachial plexus via the cervical nerve roots, while the dermatomes of the trunk arise primarily from the thoracic posterior spinal nerve roots with much smaller contributions from the lumbar and cervical spinal nerve roots. At around the sixth week of development, the limbs begin to rotate. The upper limbs rotate laterally, and the lower limbs rotate medially around the central axis of each limb. This explains the spiral dermatome distribution seen in the adult lower extremities.

The fingers and toes of the hands and feet, respectively, are the last structures of the limbs to be differentiated. Multiple limb and digit anomalies may occur such as duplication of digits (polydactyly) or fusion of digits (syndactyly).

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