Bones Lims & Vertebrae Flashcards
The axial skeleton includes the skull, vertebral column, ribs, and ________
Sternum
the skeletal system develops from paraxial and lateral plate (parietal layer) ________ and from neural crest.
Mesoderm
Paraxial mesoderm forms a segmented series of tissue blocks on each side of the neural tube, known as__________ in the head region and somites from the occipital region caudally.
somitomeres
Somites differentiate into a ventromedial part, the _________, and a dorsolateral part, the dermomyotome
sclerotome
At the end of the fourth week, sclerotome cells become polymorphous and form loosely organized tissue, called mesenchyme, or embryonic connective tissue
A. Paraxial mesoderm cells are arranged around a small cavity. B. As a result of further differentiation, cells in the ventromedial wall lose their epithelial arrangement and become mesenchymal. Collectively, they are called the sclerotome. Cells in the ventrolateral and dorsomedial regions form muscle cells and also migrate beneath the remaining dorsal epithelium (the dermatome) to form the myotome.
The remainder of the skull is derived from occipital somites and somitomeres. 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
Skull bones of a 3-month-old fetus show the spread of bone spicules from primary ossification centers in the flat bones of the skull.
In most bones, however, including the base of the skull and the limbs, mesenchymal cells first give rise to hyaline cartilage models, which in turn become ossified by endochondral ossification
A. Mesenchyme cells begin to condense and differentiate into chondrocytes. B. Chondrocytes form a cartilaginous model of the prospective bone. C,D. Blood vessels invade the center of the cartilaginous model, bringing osteoblasts (black cells) and restricting proliferating chondrocytic cells to the ends (epiphyses) of the bones. Chondrocytes toward the shaft side (diaphysis) undergo hypertrophy and apoptosis as they mineralize the surrounding matrix. Osteoblasts bind to the mineralized matrix and deposit bone matrices. Later, as blood vessels invade the epiphyses, secondary ossification centers form. Growth of the bones is maintained by proliferation of chondrocytes in the growth plates.
The skull can be divided into two parts: the____________, which forms a protective case around the brain, and the viscerocranium, which forms the skeleton of the face.
neurocranium
The neurocranium is most conveniently divided into two portions: (1) the membranous part, consisting of flat bones, which surround the brain as a vault, and (2) the _______________ part, or chondrocranium, which forms bones of the base of the skull.
cartilaginous
The membranous portion of the skull is derived from neural crest cells and paraxial mesoderm as indicated in Figure 10.4. Mesenchyme from these two sources invests the brain and undergoes intramembranous ossification. The result is formation of a number of flat, membranous bones that are characterized by the presence of needle-like __________
Bone spicules
Mesenchyme for these structures is derived from neural crest (blue), paraxial mesoderm (somites and somitomeres) (red), and lateral plate mesoderm (yellow).
At birth, the flat bones of the skull are separated from each other by narrow seams of connective tissue, the sutures. At points where more than two bones meet, sutures are wide and are called fontanelles (Fig. 10.5). The most prominent of these is the ___________ 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. Soon after birth, membranous bones move back to their original positions, and the skull appears large and round. In fact, the size of the vault is large compared with the small facial region
Anterior
Several sutures and fontanelles remain membranous for a considerable time after birth, which allows bones of the vault to continue to grow after birth to accommodate postnatal growth of the brain. Although a 5- to 7-year-old child has nearly all of his or her cranial capacity, some sutures remain open until adulthood. In the first few years after birth, palpation of the anterior fontanelle may give valuable information as to whether ossification of the skull is proceeding normally and whether intracranial pressure is normal. In most cases, the anterior fontanelle closes by 18 months of age, and the posterior fontanelle closes by 1 to 2 months of age.
Vertebrae form from the sclerotome portions of the somites, which are derived from paraxial mesoderm (Fig. 10.15A). A typical vertebra consists of a vertebral arch and foramen (through which the spinal cord passes), a body, transverse processes, and usually a spinous process (Fig. 10.15B)
A. Cross section showing the developing regions of a somite. Sclerotome cells are dispersing to migrate around the neural tube and notochord to contribute to vertebral formation. B. Example of a typical vertebra showing its various components.
During the fourth week, sclerotome cells migrate around the spinal cord and notochord to merge with cells from the opposing somite on the other side of the neural tube (Fig. 10.15A). As development continues, the sclerotome portion of each somite also undergoes a process called __________
Resegmentation
A. Cross section showing the developing regions of a somite. Sclerotome cells are dispersing to migrate around the neural tube and notochord to contribute to vertebral formation. B. Example of a typical vertebra showing its various components.
Resegmentation occurs when the caudal half of each sclerotome grows into and fuses with the cephalic half of each subjacent sclerotome (arrows in Fig. 10.16A,B). Thus, each vertebra is formed from the combination of the caudal half of one somite and the cranial half of its neighbor. As a result of this process, muscles derived from the myotome region of each somite become attached to two adjacent somites across the intervertebral discs and can therefore move the vertebral column. Patterning of the shapes of the different vertebrae is regulated by HOX genes.
A. At the fourth week of development, sclerotomic segments are separated by less dense intersegmental tissue. Note the position of the myotomes, intersegmental arteries, and segmental nerves. B. Proliferation of the caudal half of one sclerotome proceeds into the intersegmental mesenchyme and cranial half of the subjacent sclerotome (arrows). Note the appearance of the intervertebral discs. C. Vertebrae are formed by the upper and lower halves of two successive sclerotomes and the intersegmental tissue. Myotomes bridge the intervertebral discs and, therefore, can move the vertebral column.
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 (Fig. 10.16B). 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 (Fig. 10.16C).
Resegmentation of sclerotomes into definitive vertebrae causes the myotomes to bridge the intervertebral discs, and this alteration gives them the capacity to move the spine (Fig. 10.16C). 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.
As the vertebrae form, two primary curves of the spine are established: the ________ and ___________ curvatures . Later, two secondary curves are established: the cervical curvature, as the child learns to hold up his or her head, and the lumbar curvature, which forms when the child learns to walk.
C
thoracic and sacral
One of the most serious vertebral defects is the result of imperfect fusion or nonunion of the vertebral arches. Such an abnormality, known as cleft vertebra (spina bifida), may involve only the bony vertebral arches, leaving the spinal cord intact. In these cases, the bony defect is covered by skin, and no neurological deficits occur (spina bifida occulta). A more severe abnormality is spina bifida cystica, in which the neural tube fails to close, vertebral arches fail to form, and neural tissue is exposed. Any neurological deficits depend on the level and extent of the lesion (Fig. 10.17). This defect, which occurs in 1 per 2,500 births, may be prevented, in many cases, by providing mothers with folic acid prior to conception (see Chapter 6, p. 79). Spina bifida can be detected prenatally by ultrasound, and if neural tissue is exposed, amniocentesis can detect elevated levels of a-fetoprotein in the amniotic fluid
A. Ultrasound scan of a 26-week-old fetus with spina bifida in the lumbosacral region (asterisk). B. Ultrasound scan showing the skull of a 26-week-old fetus with spina bifida. Because of the shape of the skull, the image is called the “lemon sign,” which occurs in some of these cases and is due to the brain being pulled caudally, changing the shape of the head (see Arnold–Chiari malformation, p. 323).
to have two successive vertebrae fuse asymmetrically or have half a vertebra missing, a cause of ___________ (lateral curving of the spine)
Scoliosis
- Why are cranial sutures important? Are they involved in any abnormalities?
This allows the brain to grow quickly and protects the brain from minor impacts to the head (such as when the infant is learning to hold his head up, roll over, and sit up). Premature fusion of one or more cranial sutures can trigger craniosynostosis, a birth defect characterized by dramatic manifestations in appearance and functional impairment
In most bones, such as the long bones of the limbs, mesenchyme condenses and forms ___________ models of bones (Fig. 10.3). Ossification centers appear in these cartilage models, and the bone gradually ossifies by endochondral ossification
hyaline cartilage
At the end of the fourth week of development, limb buds become visible as outpocketings that form ridges on the lateral body wall
A. At 4 weeks, outpocketings along the lateral body wall form ridges. B. At 5 weeks, the limb bud stage is attained. C. At 6 weeks, hand- and footplates are formed. D. By 8 weeks, fingers and toes are present. Hind limb development lags behind forelimb development by 1 to 2 days.
Longitudinal section through the limb bud of a chick embryo, showing a core of mesenchyme covered by a layer of ectoderm that thickens at the distal border of the limb to form the apical ectodermal ridge (AER). In humans, this occurs during the fifth week of development. B. External view of a chick limb at high magnification showing the ectoderm and the specialized region at the tip of the limb called the AER.
In 6-week-old embryos, the terminal portion of the limb buds becomes flattened to form the hand- and footplates and is separated from the proximal segment by a circular constriction (Fig. 12.1C). Later, a second constriction divides the proximal portion into two segments, and the main parts of the extremities can be recognized
Fingers and toes are formed when cell death in the AER separates this ridge into five parts (Fig. 12.3A). Further formation of the digits depends on their continued outgrowth under the influence of the five segments of ridge ectoderm, condensation of the mesenchyme to form cartilaginous digital rays, and the death of intervening tissue between the rays (Fig. 12.3B,C).
A. At 48 days, cell death in the apical ectodermal ridge (AER) creates a separate ridge for each digit. B. At 51 days, cell death in the interdigital spaces produces separation of the digits. C. At 56 days, digit separation is complete.
While the external shape is being established, mesenchyme in the buds begins to condense, and these cells differentiate into chondrocytes (Fig. 12.4). By the sixth week of development, the first hyaline cartilage models, foreshadowing the bones of the extremities, are formed by these chondrocytes (Figs. 12.4 and 12.5). Joints are formed in the cartilaginous condensations when chondrogenesis is arrested, and a joint interzone is induced. Cells in this region increase in number and density, and then a joint cavity is formed by cell death. Surrounding cells differentiate into a joint capsule. Factors regulating the positioning of joints are not clear, but the secreted molecule WNT14 appears to be the inductive signal.
A. Lower extremity of an early 6-week embryo, illustrating the first hyaline cartilage models. B,C. Complete set of cartilage models at the end of the sixth week and the beginning of the eighth week, respectively.
A. Mesenchyme cells begin to condense and differentiate into chondrocytes. B. Chondrocytes form a cartilaginous model of the prospective bone. C,D. Blood vessels invade the center of the cartilaginous model, bringing osteoblasts (black cells) and restricting proliferating chondrocytic cells to the ends (epiphyses) of the bones. Chondrocytes toward the shaft side (diaphysis) undergo hypertrophy and apoptosis as they mineralize the surrounding matrix. Osteoblasts bind to the mineralized matrix and deposit bone matrices. Later, as blood vessels invade the epiphyses, secondary ossification centers form. Growth of the bones is maintained by proliferation of chondrocytes in the growth plates.
Limb malformations occur in approximately 6 per 10,000 live births, with 3.4 per 10,000 affecting the upper limb and 1.1 per 10,000 affecting 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 (Fig. 12.12A). 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. 12.12B). Sometimes, all segments of the extremities are present but abnormally short (micromelia).
A. Child with unilateral amelia and multiple defects of the left upper limb. B. Patient with a form of meromelia called phocomelia. The hands are attached to the trunk by irregularly shaped bones.
A different category of limb defects involves the digits. Sometimes, the digits are shortened (brachydactyly; Fig. 12.13A). If two or more fingers or toes are fused, it is called syndactyly (Fig. 12.13B). Normally, mesenchyme between prospective digits in hand- and footplates is removed by cell death (apoptosis). In 1 per 2,000 births, this process fails, and the result is fusion between two or more digits. The presence of extra fingers or toes is called polydactyly (Fig. 12.13C). The extra digits frequently lack proper muscle connections. Abnormalities involving polydactyly are usually bilateral, whereas absence of a digit (ectrodactyly), such as a thumb, usually occurs unilaterally.
A. Brachydactyly, short digits. B. Syndactyly, fused digits. C. Polydactyly, extra digits. D. Cleft foot. Any of these defects may involve either the hands or feet or both.
Cleft hand and foot 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. 12.13D). The two parts of the hand are somewhat opposed to each other.
Afterload is an internal or external force operating on muscle fibers that ______ or compresses their sarcomeres at rest; thereby, providing less space for cross-bridge cycling and decreasing the sarcomeres’ ability to produce maximal (tetanic) active tension.
Shortens
According to the length-tension relationship, any load that requires a muscle to contract from a resting (relaxed) length lesser or greater than its normal resting length deceases the maximal active tension that can be generated. In cardiac muscle fibers, afterload results from the force generated by the blood pressure that must be overcome during isotonic shortening of the cardiac muscle fibers to pump blood out of a heart chamber. Hence, the cardiac muscle fibers of a hypertensive person are affected by high afterload because of the person’s high arterial blood pressure and decreasing the afterload increases the maximal active tension that can be generated.
Preload results from stretch created by the blood volume filling of the heart’s chambers causing a condition where the cardiac muscle fibers are forced to undergo shortening from a longer-than-normal resting (relaxed) sarcomere length. Unlike afterload, for which a decrease in afterload increases the maximal active tension, an increase in preload increases the maximal active tension; except, where the sarcomeres become overstretched, as occurs in cases of congestive heart failure
