UNIVERSITY OF GUYANA MEDICAL REHABILITATION EMBRYOLOGY EBRYOLOGICAL DEVELOPMENT OF THE SKELETAL SYSTEM GROUP 1: ANITA NARINE, ZOE DANIELS, NEIL BARRY, JENNIFER HAYNES INTRODUCTION TO : The Embryology of the Skeletal System The skeletal system develops from paraxial and lateral plate (somatic layer) mesoderm and from neural crest. Paraxial mesoderm forms a segmented series of tissue blocks on each side of the neural tube, known as somitomeres in the head region and somites from the occipital region caudally. Somites differentiate into a ventromedial part, the sclerotome, and a dorsolateral part, the dermomyotome. At the end of the fourth week sclerotome cells become polymorphous and form a loosely woven tissue, the mesenchyme, or embryonic connective tissue. It is characteristic for mesenchymal cells to migrate and to differentiate in many ways. They may become fibroblasts, chondroblasts, or osteoblasts (bone-forming cells). . The bone-forming capacity of mesenchyme is not restricted to cells of the sclerotome, but occurs also in the somatic mesoderm layer of the body wall, which contributes mesoderm cells for formation of the pelvic and shoulder girdles and the long bones of the limbs. Neural crest cells in the head region also differentiate into mesenchyme and participate in formation of bones of the face and skull. Occipital somites and somitomeres also contribute to formation of the cranial vault and base of the skull. In some bones, such as the flat bones of the skull, mesenchyme in the dermis differentiates directly into bone, a process known as intramembranous ossification. In most bones, however, mesenchymal cells first give rise to hyaline cartilage models, which in turn become ossified by endochondral ossification. Skull The skull can be divided into two parts: the neurocranium, which forms a protective case around the brain, and the viscerocranium, which forms the skeleton of the face. The neurocranium is most conveniently divided into two portions: (a) the membranous part, consisting of flat bones, which surround the brain as a vault; and(b) the cartilaginous part, or chondrocranium, which forms bones of the base of the skull. Newborn Skull At birth the flat bones of the skull are separated from each other by narrow seams of connective tissue, the sutures, which are also derived from two sources: neural crest cells (sagittal suture) and paraxial mesoderm (coronal suture). At points where more than two bones meet, sutures are wide and are called fontanelles. The most prominent of these is the anterior fontanelle, which is found where the two parietal and two frontal bones meet. Sutures and fontanelles allow the bones of the skull to overlap (molding) during birth. Clinical implications Cranial defects and skeletal dysplasias Neural crest cells- these originate in the neuroectoerm from facial skeleton and most of the skull. Craniosis- the cranial vault fails to form cranioschsis meaning that the brain tissue exposed to amniotic fluid degenerates ( anencephaly). Microcephaly- the brain fails to grow and the skull fails to expand. Craniosynostosis and dwarfism Acromegaly- this is caused by hyperpitutarisum and production of growth hormone Achondroplasia- this affects long bones and spinal curve problem Thanalophoric- neonathal form of dwarfism: they are two types. Type 1: short curved femur Type 2: long femurs LIMB GROWTH AND DEVELOPMENT OF THE FETUS After four weeks of development, the buds for the limbs become visible from the ventrolateral body wall. It consists of a mesenchymal core that will form the bones and connective tissues of the limb which is covered by a layer of cuboidal ectoderm. The ectoderm at the distal border of the limb then becomes thicker which forms after the apical ectodermal ridge (AER). As the limbs continue growing, the cells that are farther away from, the influence of AER begins to form into muscles and cartilage. 5 weeks 6 weeks 8weeks The limb buds develop as the embryo grows The limb buds of the six week embryo now become flattened to form the hand plates and foot plates which is separated by a circular constriction. Fingers and toes are formed. Development of the upper and lower limb is similar except that the morphogenesis of the lower limb is approximately one to two days behind that of the upper limbs. During the seventh week of gestation, the limbs (both upper and lower) rotate in opposite directions. The upper limbs rotates ninety degrees laterally so that the extensor muscles lie on the lateral posterior surface and the thumbs lie laterally, meanwhile the lower limbs rotates ninety degrees medially placing the extensor muscles on an anterior surface and the big toe medially. By the sixth week of development of the embryo the first hyaline cartilage models are formed by chondrocytes. Joints are now formed in the cartilaginous condensations. Cells in this region then increase in number and in density and join a cavity that is formed by cell death. Ossification of the bones of the extremities, endochondral ossification begins by the end of the embryonic period. The primary ossification centers are present in all of the long bones of the limbs of the embryo by the twelfth week of development. At birth the diaphysis of the bones are usually completely ossified but the two ends; the epiphyses are still cartilaginous. A temporary a cartilage plate remains between the diaphyseal epiphyseal ossification centers. The epiphyseals plate plays an important role in growth and length of the bones of the embryo. When the bones has acquired its full length, the epiphyseal plates disappear and the epiphyses then unite with the shaft of the bone. In long bones an epiphyseal plate is formed in each extremity in smaller bones, such as the phalanges; it is found only at one extremity and in irregular bones such as the vertebrae one or more primary centers of ossification. MOLECULAR REGULATION OF LIMB DEVELOPMENT Positioning of the limbs along the cranio - caudal axis in the flank regions of the embryo is regulated by the HOX genes expressed along this axis. Thesehomeoboxgenes are expressed in overlapping patterns from head to tail, with some having more cranial limits than others. For example, thecranial limit of expression of HOXB8 is at the cranial border of the forelimb,and misexpression of this gene alters the position of these limbs.Once positioning along the craniocaudal axis is determined, growth mustbe regulated along the proximodistal, anteroposterior, and dorsoventral axes.Limb outgrowth, which occurs first, is initiated by FGF-10 secretedby lateral plate mesoderm cells. Once outgrowth is initiated, bone morphogenetic proteins (BMPs), expressed in ventral ectoderm, induce formation of the AER by signaling through the homeobox gene MSX2. Expression ofRadical fringe (a homologue of Drosophila fringe), in the dorsal half of the limb ectoderm, restricts the location of the AER to the distal tip of the limbs. This gene induces expression of Ser-2, a homologue of Drosophila serrate, at the border between cells expressing Radical fringe and those that are not. It is at this border that the AER is established. Formation of the border itself is assisted by expression of Engrailed-1 in ventral ectoderm cells, since this gene represses expression of Radical fringe. After the ridge is established, it expresses FGF-4and FGF-8, which maintain the progress zone, the rapidly proliferating population of mesenchyme cells adjacent to the ridge. Distal growth of the limb is then effected by these rapidly proliferating cells under the influence of the FGFs. As growth occurs, mesenchymal cells at the proximal end of the progress zone become farther away from the ridge and its influence and begin to slow their division rates and to differentiate. Patterning of the anteroposterior axis of the limb is regulated by the zoneof polarizing activity (ZPA), a cluster of cells at the posterior border of the limb near the flank. These cells produce retinoic acid (vitaminA), which initiates expression of sonic hedgehog (SHH), a secreted factor that regulates the anteroposterior axis. Thus, for example, digits appear in the proper order, with the thumb on the radial (anterior) side. As the limb grows, the ZPA moves distalward to remain in proximity to the posterior border of the AER. Misexpression of retinoic acid or SHH in the anterior margin of a limb containing a normally expressing ZPA in the posterior border results in a mirror image duplication of limb structures (Fig. 8.17). The dorsoventral axis is also regulated by BMPs in the ventral ectoderm which induce expression of the transcription factor EN1. In turn, EN1 represses WNT7a expression restricting it to the dorsal limb ectoderm. WNT7a is secreted factor that induces expression of LMX1, a transcription factor containing a homeodomain, in the dorsal mesenchyme. LMX1 specifies cells to be dorsal, establishing the dorsoventral components. In addition, WNT7a maintains SHH expression in the ZPA and therefore indirectly affects anteroposteriorpatterning as well. These two genes are also intimately linked in signaling pathways in Drosophila, and this interaction is conserved in vertebrates. In fact, all of the patterning genes in the limb have feedback loops. Thus, FGFs in the AER activateSHH in the ZPA, while WNT7a maintains the SHH signal. Although patterning genes for the limb axes have been determined, it is the HOX genes that regulate the types and shapes of the bones of the limb (Fig. 8.15D). Thus, HOX gene expression, which results from the combinatorial expression of SHH, FGFs, and WNT7a, occurs in phases in three places in the limb that correspond to formation of the proximal (stylopod), middle (zeugopod), and distal (autopod) parts. Genes of the HOXA and HOXD clusters are the primary determinants in the limb, and variations in their combinatorial patterns of expression may account for differences in forelimb and hindlimbstructures. Just as in the craniocaudal axis of the embryo, HOX genes are nested in overlapping patterns of expression that somehow regulate patterning. CLINICALCORRELATES Bone Age Radiologists use the appearance of various ossification centers to determine whether a child has reached his or her proper maturation age. Useful information about bone age is obtained from ossification studies in the hands and wrists of children. Prenatal analysis of fetal bones by ultrasonography provides information about fetal growth and gestational age. Limb Defects Limb malformations occur in approximately 6/10,000 live births, with 3.4/10,000 affecting the upper limb and 1.1/10,000, the lower. These defects are often associated with other birth defects involving the craniofacial, cardiac, and genitourinary systems. Abnormalities of the limbs vary greatly, and they may be represented by partial (meromelia) or complete absence (amelia) of one or more of the extremities. Sometimes the long bones are absent, and rudimentary hands and feet are attached to the trunk by small, irregularly shaped bones (phocomelia, a form of meromelia) (Fig. 8.18, Aand B). Sometimes all segments of the extremities are present but abnormally short (micromelia). Although these abnormalities are rare and mainly hereditary, cases of teratogen-induced limb defects have been documented. For example, many children with limb malformations were born between 1957 and 1962. Many mothers of these infants had taken thalidomide, a drug widely used as a sleeping pill and antinauseant. It was subsequently established that thalidomide causes a characteristic syndrome of malformations consisting gross deformities of the long bones, intestinal atresia, and cardiac anomalies.Since the drug is now being used to treat AIDS and cancer patients, there isconcern that its return will result in a new wave of limb defects. Studies indicatethat the most sensitive period for teratogen-induced limb malformations is the fourth and fifth weeks of development. A different category of limb abnormalities consists of extra fingers or toes (polydactyly) (Fig. 8.19A). The extra digits frequently lack proper muscle connections. Abnormalities with an excessive number of bones are mostly bilateral, while the absence of a digit such as a thumb (ectrodactyly) is usually unilateral. Polydactyly can be inherited as a dominant trait but may also be induced by teratogens. Abnormal fusion is usually restricted to the fingers or toes (syndactyly). Normally mesenchyme between prospective digits in the handplates and footplates breaks down. In 1/2000 births this fails to occur, and the result is fusion of one or more fingers and toes (Fig. 8.19B). In some cases the bones actually fuse. Cleft hand and foot (lobster claw deformity) consists of an abnormal cleft between the second and fourth metacarpal bones and soft tissues. The third metacarpal and phalangeal bones are almost always absent, and the thumb and index finger and the fourth and fifth fingers may be fused (Fig. 8.19C ). The two parts of the hand are somewhat opposed to each other and act like a lobster claw. The role of the HOX genes in limb development is illustrated by two abnormal phenotypes produced by mutations in these genes: Mutations in HOXA13result in hand-foot-genital syndrome, characterized by fusion of the carpal bones and small short digits. Females often have a partially (bicornuate) or completely (didelphic) divided uterus and abnormal positioning of the urethral orifice. Males may have hypospadias. Mutations in HOXD13 result in a combination of syndactyly and polydactyly(synpolydactyly). Clubfoot usually accompanies syndactyly. The sole of the foot is turned inward, and the foot is adducted and plantar flexed. It is observed mainly in males and in some cases is hereditary. Abnormal positioning of the legs in utero may also cause clubfoot. Congenital absence or deficiency of the radius is usually a genetic abnormality observed with malformations in other structures, such as craniosynostosis–radial aplasia syndrome. Associated digital defects, which may include absent thumbs and a short curved ulna, are usually present. Amniotic bands may cause ring constrictions and amputations of the limbs or digits (Fig. 8.20). The origin of bands is not clear, but they may represent adhesions between the amnion and affected structures in the fetus. Other investigators believe that bands originate from tears in the amnion that detach and surround part of the fetus. Congenital hip dislocation consists of underdevelopment of the acetabulum and head of the femur. It is rather common and occurs mostly in females. Although dislocation usually occurs after birth, the abnormality of the bones develops prenatally. Since many babies with congenital hip dislocation are breech deliveries, it has been thought that breech posture may interfere with development of the hip joint. It is frequently associated with laxity of the jointcapsule. Vertebral Column It is during the fourth week of development that cells of the sclerotomes shift their position to surround both the spinal cord and the notochord . This mesenchymal column retains traces of its segmental origin, as the sclerotomic blocks are separated by less dense areas containing intersegmental arteries. During further development the caudal portion of each sclerotome segment proliferates extensively and condenses. This proliferation is so extensive that it proceeds into the subjacent intersegmental tissue and binds the caudal half of one sclerotome to the cephalic half of the subjacent sclerotome. Hence, by incorporation of the intersegmental tissue into the precartilaginous vertebral body, the body of the vertebra becomes intersegmental. Patterning of the shapes of the different vertebra is regulated by HOX genes. Mesenchymal cells between cephalic and caudal parts of the original sclerotome segment do not proliferate but fill the space between two precartilaginous vertebral bodies. In this way they contribute to formation of the intervertebral disc. Although the notochord regresses entirely in the region of the vertebral bodies, it persists and enlarges in the region of the intervertebral disc. Here it contributes to the nucleus pulposus, which is later surrounded by circular fibers of the annulus fibrosus. Combined, these two structures form the intervertebral disc. Rearrangement of sclerotomes into definitive vertebrae causes the myotomes to bridge the intervertebral discs, and this alteration gives them the capacity to move the spine. For the same reason, intersegmental arteries, at first lying between the sclerotomes, now pass midway over the vertebral bodies. Spinal nerves, however, come to lie near the intervertebral discs and leave the vertebral column through the intervertebral foramina. Development of Ribs and Sternum Ribs form from the costal processes of thoracic vertebrae and are derived from the sclerotome portion of paraxial mesoderm. The sternum develops separately in somatic mesoderm in the ventral body wall. Two sternal bands are formed on either side of the midline, and these later fuse to form cartilaginous models of the manubrium, sternebrae, and xiphoid process. Illustration showing the manner in which each vertebral centrum is developed from portions of two adjacent segments. (Bartleby) Reference Sadlar, Thomas W. (2006) Langman’s Medical Embryology http://www.bartleby.com/107/illus65.html (07/21/2013)