Embryology of Bone - Lecture 8 Flashcards

Flashcards covering bone formation, joints, vertebral development, ribs, sternum, skull, and cranial defects from the lecture notes.

What are the two types of bone formation in embryology?

Intramembranous ossification, which forms bone directly from mesenchymal connective tissue, and endochondral ossification, which forms bone by replacing a hyaline cartilage model.

In intramembranous ossification, into what cells do mesenchymal cells differentiate?

Mesenchymal cells differentiate into osteoblasts, which are bone-forming cells responsible for secreting osteoid.

During intramembranous ossification, what is osteoid and what happens to it?

Osteoid is the unmineralized organic matrix of bone, secreted by osteoblasts. It then becomes calcified as mineral salts are deposited, forming mature bone tissue. Some osteoblasts become trapped within this calcified matrix and differentiate into osteocytes.

Which bones form primarily via intramembranous ossification?

This process primarily forms the flat bones of the skull, the mandible (lower jawbone), and the clavicles (collarbones). These bones develop directly from sheets of mesenchymal connective tissue.

What initiates endochondral ossification?

Endochondral ossification is initiated by a hyaline cartilage model, which is a temporary cartilaginous template of the future bone, surrounded by a perichondrium (a dense connective tissue layer).

In endochondral ossification, what happens to chondrocytes as cartilage calcifies?

As the cartilage calcifies, the chondrocytes (cartilage cells) within the model enlarge (hypertrophy), then die due to lack of nutrient diffusion through the calcified matrix. This process leaves behind a calcified cartilage scaffold that is subsequently invaded by blood vessels and replaced by bone tissue, laid down by osteoblasts.

What forms the periosteal bone collar in endochondral ossification?

Osteoblasts, originating from cells within the perichondrium (now called periosteum), form a compact bone collar around the diaphysis (shaft) of the cartilage model. This bone collar provides structural support and helps to enclose the developing primary ossification center.

Where is the primary ossification center located in long bones?

The primary ossification center is located in the diaphysis (central shaft) of long bones. It appears prenatally and is the first area of bone formation, where cartilage is progressively replaced by bone.

Where are secondary ossification centers located?

Secondary ossification centers are located in the epiphyses (ends) of long bones. They typically appear postnatally and are responsible for the growth of the bone ends.

During endochondral ossification, what remains at articular surfaces and ends of the diaphysis?

At the ends of bones within joints, articular cartilage remains to provide a smooth, low-friction surface for movement. Between the epiphysis and diaphysis, the epiphyseal plates (growth plates) persist, allowing for longitudinal bone growth until skeletal maturity.

How is the medullary cavity formed in endochondral bone?

The medullary cavity, or marrow cavity, is formed when osteoclasts (bone-resorbing cells) resorb or break down the calcified cartilage and newly formed bone tissue in the center of the diaphysis. This creates a hollow space that will become filled with bone marrow.

What is the epiphyseal plate?

The epiphyseal plate (or growth plate) is a layer of hyaline cartilage located between the epiphysis and diaphysis of a long bone. It is responsible for longitudinal bone growth through endochondral ossification. When skeletal growth ceases, typically in late adolescence, the cartilage in the epiphyseal plate is completely replaced by bone, forming the epiphyseal line.

Name the three major types of cartilage in the skeletal system.

The three major types are hyaline cartilage, fibrocartilage, and elastic cartilage. They differ in their extracellular matrix composition and mechanical properties.

Which collagen type is predominant in hyaline cartilage?

Type II collagen is the predominant collagen type in hyaline cartilage, providing structural integrity, flexibility, and resilience. It is found in articular surfaces, coastal cartilages, and the nose.

Which collagen type predominates in fibrocartilage?

Type I collagen predominates in fibrocartilage, providing high tensile strength and resistance to compression. Type II collagen is also present, but to a lesser extent. Fibrocartilage is found in structures like intervertebral discs and menisci.

What are appositional and interstitial growth in cartilage?

Appositional growth involves the addition of new cartilage matrix to the periphery of existing cartilage by chondroblasts in the perichondrium. Interstitial growth occurs through the mitotic division of mature chondrocytes within their lacunae, producing new matrix from within the cartilage mass.

What is the interzone in joint development?

The interzone is a region of condensed mesenchyme located between two developing bones destined to form a joint. It is a crucial site for the differentiation of joint structures. Initially composed of cartilage, it later differentiates into dense fibrous tissue.

What structures differentiate from the interzone in synovial joints?

In synovial joint development, the interzone differentiates into various structures: the articular cartilage covering the bone ends, the synovial membranes that line the joint cavity and produce synovial fluid, menisci (if present, forming articular discs), and ligaments within the joint capsule that stabilize the joint.

What forms the joint capsule?

The joint capsule in synovial joints is formed from the mesenchyme surrounding the developing interzone. It consists of an outer fibrous layer for strength and an inner synovial membrane layer that secretes synovial fluid.

How do fibrous, cartilaginous, and synovial joints differ in development?

Fibrous joints (e.g., sutures of the skull) develop where dense fibrous connective tissue connects bones with no joint cavity. Cartilaginous joints (e.g., symphyses, synchondroses) form where bones are united by hyaline cartilage or fibrocartilage. Synovial joints (most common type) develop a joint cavity, allowing for free movement, where the interzone forms a synovial cavity containing fluid.

Which germ layer forms the parietal skeleton and sternum?

The parietal layer of the lateral plate mesoderm (somatic mesoderm) forms the parietal skeleton, including the bones of the limbs (appendicular skeleton) and the sternum.

What does the sclerotome give rise to?

The sclerotome, which is the medial part of a somite, gives rise to the vertebrae (vertebral column), ribs, and parts of the posterior cranium (base of the skull).

What does the dorsal (posterior) sclerotome form?

The dorsal (posterior) portion of the sclerotome forms the neural arch (vertebral arch), pedicles, and spinous process of the vertebrae, which protect the spinal cord.

What does the ventromedial sclerotome form?

The ventromedial portion of the sclerotome forms the vertebral bodies (centra) and the annulus fibrosus, the tough outer ring of the intervertebral discs.

What does the central sclerotome form?

The central sclerotome contributes to the formation of the transverse processes of the vertebrae and the proximal portions of the ribs.

What does the lateral sclerotome form?

The lateral sclerotome forms the distal portions of the ribs.

What is the origin of the nucleus pulposus?

The nucleus pulposus, the gelatinous core of the intervertebral disc, is a remnant of the notochord. The notochord is a flexible rod-like structure that serves as a primary axial support in the embryo.

What is hemivertebra and what condition can it cause?

A hemivertebra results from the failure of one of the chondrification centers to appear, leading to a wedge-shaped vertebra with a missing half. This developmental anomaly can cause a severe lateral curvature of the spine, known as scoliosis.

What is spondylolisthesis?

Spondylolisthesis is an anterior slippage (forward displacement) of one vertebral body over the one below it. It often occurs due to a defect or elongation in the pars interarticularis, the bony segment connecting the superior and inferior articular facets of a vertebra.

What is spina bifida?

Spina bifida is a congenital condition resulting from the nonunion or failure of the vertebral arches (laminae) to fuse properly during embryonic development. It is a type of neural tube defect and can range from mild (occulta) to severe (cystica, involving protrusion of meninges or spinal cord).

How do ribs form in relation to the vertebrae?

Ribs develop from zones of condensed mesenchyme that lie lateral to the vertebral centra. The proximal segment of each rib forms from the central sclerotome, while the distal segment forms from the lateral sclerotome. They articulate with the vertebrae.

What is spondylocostal dystosis?

Spondylocostal dystosis is a group of rare genetic disorders characterized by widespread malformations of the vertebral column and ribs. These defects arise from abnormalities in somite formation and segmentation during early embryonic development, often leading to fused ribs, missing ribs, and kyphoscoliosis.

What causes accessory ribs?

Accessory ribs (cervical or lumbar ribs) are caused by the misexpression of HOX genes, which are involved in body patterning. This genetic miscue can lead to the retention and growth of costal processes (small bony projections) in cervical or lumbar vertebrae where ribs are typically absent.

How does the sternum develop?

The sternum (breastbone) develops from paired sternal bars (cranial-to-caudal bands of lateral plate mesoderm) that migrate ventromedially. These bands undergo segmentation and then fuse in the midline to form the unpaired body of the sternum, as well as the manubrium and xiphoid process.

What are pectus defects?

Pectus defects are congenital abnormalities of the ventral body wall closure, affecting the sternum and ribs. Pectus excavatum features a depressed or sunken sternum, while pectus carinatum involves a protruding or 'pigeon' chest. A sternal cleft occurs when the sternal bands fail to fuse completely, leaving a gap in the sternum.

What is Cleidocranial dysplasia and its genetic basis?

Cleidocranial dysplasia is a rare genetic disorder characterized by defects in bone and tooth development. It is caused by a mutation in the RUNX2 gene (Runt-related transcription factor 2), leading to clavicle hypoplasia (underdevelopment or absence of collarbones) and delayed ossification of various bones, including the skull and fontanelles.

From which tissues do the facial bones and base of the skull partly develop?

The facial bones and parts of the base of the skull (like the bones of the middle ear and styloid process) partly develop from the cartilage of the pharyngeal arches, which are themselves derived from neural crest cells.

How do neurocranium and viscerocranium differ in formation?

The neurocranium, which forms the protective braincase, largely develops via intramembranous ossification of its flat bones. The viscerocranium, comprising the facial skeleton, largely forms from the cartilages of the pharyngeal arches (derived from neural crest cells) through endochondral ossification, though some parts also form via intramembranous ossification.

What are fontanelles and sutures?

Sutures are fibrous joints that connect the individual flat bones of the skull. Fontanelles are wider, soft membranous gaps between the cranial bones where ossification is incomplete at birth. They allow for molding of the skull during passage through the birth canal and accommodate rapid brain growth during infancy.

What is craniosynostosis?

Craniosynostosis is a congenital condition characterized by the premature fusion of one or more cranial sutures. This early closure prevents normal skull expansion perpendicular to the fused suture, leading to an abnormally shaped skull and potentially affecting brain growth.

Name common craniosynostosis patterns and their associated sutures.

Common craniosynostosis patterns include: Scaphocephaly, caused by premature fusion of the sagittal suture, resulting in a long, narrow skull. Brachycephaly, resulting from premature fusion of the coronal sutures, leading to a short, wide skull. Trigonocephaly, caused by premature fusion of the metopic (frontal) suture, resulting in a triangular-shaped forehead.

What conditions are associated with anencephaly and microcephaly?

Anencephaly is a severe neural tube defect resulting from the failure of the cranial neuropore to close, leading to the absence of a major portion of the brain, skull, and scalp. Microcephaly is a condition characterized by an abnormally small head due to a reduced-sized neurocranium, often associated with impaired brain development.

When do the posterior fontanelle and the sphenoid bone typically fuse?

The posterior fontanelle and the sphenoid fontanelles typically fuse and close around 6 months postnatally.

When do the anterior and mastoid fontanelles fuse?

The anterior fontanelle, the largest fontanelle, and the mastoid fontanelles typically fuse and close around 2 years postnatally.