1.2 Flashcards
(25 cards)
- What are the three germ layers formed during gastrulation?
They are the ectoderm, mesoderm, and endoderm. During gastrulation, epiblast cells ingress and displace the hypoblast to form definitive endoderm, while mesoderm arises between the epiblast and endoderm.
- What is the role of the primitive streak in embryonic development?
The primitive streak forms at the caudal end of the epiblast, defining the body axes (cranial-caudal, dorsal-ventral, left-right) and serving as the entry point for cells migrating to form the endoderm and mesoderm.
- When does gastrulation occur and what key events happen during this process?
Gastrulation occurs at the end of the second week of gestation (around days 14–16). It involves the ingress of epiblast cells to form the definitive endoderm and, shortly after, the intraembryonic mesoderm.
- How is the intraembryonic mesoderm formed during gastrulation?
On day 16, epiblast cells migrate between the endoderm and the epiblast layers, creating the intraembryonic mesoderm that gives rise to various mesodermal derivatives.
- What types of mesoderm arise from migrating mesoderm cells?
Migrating mesoderm cells form the notochord (midline structure), paraxial mesoderm (which segments into somites), intermediate mesoderm, lateral plate mesoderm, and extraembryonic mesoderm.
- What are somites and when do they begin to form?
Somites are segmented blocks of paraxial mesoderm that form in a craniocaudal sequence, beginning around day 22 of gestation at a rate of about three pairs per day.
- Into which components do somites differentiate?
Each somite differentiates into three components: the sclerotome (forming vertebrae, ribs, and cartilage), the myotome (forming skeletal muscles), and the dermatome (forming the dermis).
- How does somite development contribute to muscle formation?
The dermomyotome, a region of each somite, gives rise to muscle precursor cells. These cells migrate to form both axial (epaxial) muscles of the back and hypaxial muscles for the limbs and body wall.
- What is the difference between epaxial and hypaxial muscles derived from somites?
Epaxial muscles (formed from more dorsal cells) create the intrinsic back muscles, whereas hypaxial muscles (from ventrally migrating cells) form the muscles of the limbs and trunk.
- Why is proper segmentation of the paraxial mesoderm critical for musculoskeletal development?
Segmentation into somites ensures that the axial skeleton, associated muscles, and dermis develop in a coordinated and organized manner, with each segment receiving a specific nerve supply.
- What is intramembranous ossification and where does it occur?
Intramembranous ossification is the direct formation of bone from mesenchymal tissue. It occurs in flat bones such as those of the skull, clavicle, and mandible.
- What are the key steps in intramembranous ossification?
Mesenchymal cells aggregate to form an ossification center, differentiate into osteoprogenitor cells, then osteoblasts, which secrete osteoid. The osteoid mineralizes to form immature woven bone that is later remodeled into mature bone.
- How do osteoblasts and osteocytes function in intramembranous ossification?
Osteoblasts secrete the osteoid matrix and facilitate its mineralization; as they become embedded in the matrix, they differentiate into osteocytes, which maintain the bone tissue.
- What is endochondral ossification and which bones develop by this process?
Endochondral ossification is the process where bone forms by replacing a cartilage model, and it is typical for long bones, vertebrae, and ribs.
- Outline the stages of endochondral ossification.
Chondrocytes form a cartilage model that grows and calcifies; then, blood vessels invade the calcified cartilage, establishing a primary ossification center in the diaphysis. Later, secondary ossification centers form in the epiphyses, with growth continuing at the epiphyseal (growth) plate.
- What is the role of the epiphyseal plate in long bone development?
The epiphyseal plate is a region of proliferative cartilage that enables the longitudinal growth of long bones until skeletal maturity, after which it is replaced by an epiphyseal line.
- How does vascular invasion contribute to the process of endochondral ossification?
Vascular invasion delivers osteoprogenitor cells to the calcified cartilage, allowing the formation of bone tissue on the cartilage scaffold and establishing the primary ossification center.
- How is the initial woven bone remodeled during endochondral ossification?
The initially deposited, weak woven bone is gradually remodeled into structured lamellar (compact) bone on the periphery, with spongy bone forming internally as the bone matures.
- What is synovial joint development and which tissues are involved?
Synovial joint development involves the differentiation of mesenchymal cells in the interzone between chondrifying bone primordia into fibroblastic tissue that eventually forms articular cartilage, the joint capsule, synovial lining, menisci, and internal ligaments.
- How does the synovial cavity form during joint development?
Vacuoles form within the interzone mesenchyme and coalesce to form the synovial cavity, which fills with synovial fluid to lubricate the joint and facilitate movement.
- How is articular cartilage formed in this process?
At the proximal and distal ends of the interzone, mesenchymal cells differentiate into articular cartilage, covering the opposing bone surfaces to provide a smooth, low-friction interface.
- What roles do menisci and internal ligaments play in synovial joints?
Menisci and internal ligaments, which develop from the condensed central region of the interzone, contribute to joint stability, shock absorption, and guide joint movement.
- Compare intramembranous and endochondral ossification in terms of their developmental origins.
Intramembranous ossification arises directly from mesenchymal tissue, while endochondral ossification begins with a cartilage model that is later replaced by bone tissue.
- Why is embryonic mesenchyme essential for both bone and joint development?
Embryonic mesenchyme provides the undifferentiated cells that differentiate into osteoprogenitor cells for bone formation and into specialized cells that form the joint structures.
