level: Part 1/2 of CHAPTER 9
Questions and Answers List
Neurocranium
level questions: Part 1/2 of CHAPTER 9
| Question | Answer |
|---|---|
| The axial skeleton consists of bones of the.... | -bones of the cranium (i.e. skull and mandible) -the vertebrae -ribs -sternum |
| Cells contributing to the formation of the axial skeleton are derived from | -the mesodermal germ layer -Neural crest cells contribute to the formation of the craniofacial skeleton |
| Somitomeres and somitores differentiate into | fibroblasts, chondroblasts and osteoblasts, which will begin to respectively form the connective tissues, cartilage and bones of the region. |
| In the axial skeleton the sternum is derived from? | Lateral plate mesoderm |
| The elements of the appendicular skeleton such as the bones of the pelvic and shoulder girdles and the limbs are derived from? | Lateral plate mesoderm |
| Where do the bones of the face and anterior face derive from? | neural crest cells that have migrated to the area during very early development. |
| Where are the bones of the posterior bones of the skull derived from? | occipital somites and somitomeres (paraxial mesoderm). |
| Intramembranous ossification | Intramembranous ossification involves the differentiation of mesoderm directly into bones. This is most common in flat bones like those of the skull. |
| Endochondral ossification | -Endochondral ossification iinvolves the formation of a hyaline cartilage model of the bone being formed, which is gradually replaced by bony tissue during development. -This is most common in long bones of the limbs, though also occurs in some regions of the skull. |
| Describe the process of intramembranous ossification | -starts when mesenchymal cells in the center of the region where the bone is about to develop cluster together and differentiate directly into osteoblasts at the primary ossification center -Osteoblast cells then begin to secrete a substance called osteoid, eventually trapping themselves in small cavities called lacunae -The osteoblasts then become osteocytes. -The osteoid secreted by the osteoblasts will accumulate between the embryonic blood vessels in the region, forming bone with a sponge-like appearance. -On the edges of the spongy, woven bone,mesenchyme will condense to form the periosteum(outermost layer of the bone) -Directly adjacent to the periosteum the trabeculae will thicken and be replaced by lamellar bone to form a compact layer of bone b/n the outer periosteal and inner trabecular portions of the bone - The trabecular on the inside do not change, and are referred to as the diploe of the flat bones. Blood vessels remaining in the trabeculae later become the red marrow |
| Describe the process of endochondral ossification before birth | - chondrocytes (which have differentiated from mesoderm) begin to deposit a hyaline cartilage model of the bone that is going to be formed. -Cells at the edge of the model, called the perichondrium, then differentiate into osteoblasts and begin to secrete an osteoid collar around the bone. -Chondrocytes trapped inside the collar then become hypertrophic, get calcified and die. -Following the death of the chondrocytes, blood vessels penetrate the degenerating cartilage model and deposit osteoprogenitor cells -Shortly thereafter, these cells differentiate into osteoblasts and start forming the primary ossification center in the center of the spongy bone of the diaphysis (shaft). -Ossification proceeds outwards from the primary ossification center leaving some space around the blood vessels where the medullary cavity of the bone is formed |
| Describe the process of endochondrial ossification after birth | -Clusters of osteoblasts form in the epiphyses (ends) of the bones, forming the secondary ossification centers -The secondary ossification centers in the epiphyses are separated from the primary ossification center in the diaphysis by a layer of cartilage called the epiphyseal plate. -As calcification of the chondrocytes in the cartilage progresses outward from the ossification centers, the epiphyseal plate is pushed outwards from the centers. -Eventually, the plate can no longer be shifted, and will also become calcified, closing the space between the epiphyses and diaphysis. -Once this happens usually (late in teens) bones can no longer grow in length but only in thickness(appositional growth) |
| Compare and contrast the processes of intramembranous and endochondral ossificaton | picture |
| Describe the development of the different parts of the neurocranium(brain case)-(Chondrocranium) | -The chondrocranium portion of the neurocranium initially develops as several separate cartilage models which ossify and fuse together over time through endochondral ossification -The bones rostral (i.e. anterior) to the sella turcica are called the prechondral chondrocranium and derive from neural crest cells which have migrated here during earlier development -The bones caudal to the sella turcica are called the chondral chondrocranium and derive from the paraxial mesoderm in the region -These segments ossify relatively early in life and are thus expected to be affected more by genetic influences than the environment. - |
| The divisions of the neurocranium | – the membranous portion which consist of the flat bones -A cartilaginous part called the chondrocranium which forms the base of the skull (region of the foramen magnum and occipital condyles). |
| Describe the process of development of the different parts of the neurocranium-Membranous neurocranium | -the bones positioned more anteriorly derive from neural crest cells and those positioned more posteriorly derive from paraxial mesoderm -The bones of the membranous neurocranium are mostly flat and ossify through intramembranous ossification |
| Describe and discuss how a newborn skull develops | -Flat bones of the newborn skull are separated by connective tissue at the regions where the cranial sutures will eventually form - The reason for these soft tissue inserts between the bones is to allow for moulding of the skull during birth so that it can fit through the birth canal. -In some areas, these connective tissue regions form diamond- or triangle-shaped spaces called fontanelles -There are six of these on a newborn skull, and they ossify at relatively predictable times after birth -The most famous of these frontanelles is the bregma which is found at the intersection b/n the sagitaal and coronal sutures.This is commonly known as the soft spot and closes about 18 months after birth. Sutures remain membranous sometime after birth so as to allow flat bones to expand a little as the brain undergoes rapid postnatal growth |
| Describe the process of the development of the viserocranium | -The viscerocranium is mainly derived from the first 2 pharyngeal arches, which are clusters of mesoderm that form in the neck region of the developing embryo -The first pharyngeal arch has two parts – a dorsal and a ventral process. The dorsal process also called the maxillary process,contains the cells that will form the bones of the maxilla, as well as the zygomatic and part of the temporal bones - The ventral process also called the mandibular process contains a cluster of cells called Meckel’s cartilage. Intramembranous ossification of this cartilage will give rise to the mandible. -the dorsal tip of the mandibular process of the first pharyngeal arch and the dorsal tip of the second pharyngeal arch will form the ossicles of the ear – the smallest bones in the human body -The facial bones of the viscerocranium are also derived from neural crest cells which have migrated to this region. - The face of a newborn is relatively small in proportion to the neurocranium. -The face of a newborn is relatively small in proportion to the neurocranium. As the sinuses inside the bones grow and the permanent dentition develops, the shape of the viscerocranium changes rapidly until it reaches a more balanced proportion with the neurocanium |
| Define cranioschisis | -Cranioschisis is a condition that can arise when the anterior neuropore of the developing neural tube fails to close, resulting in failure of the cranial vault to form. -Usually this causes the brain to be exposed and eventually degenerate – a condition called anencephaly |
| Define craniosynostosis | -A group of conditions that are the result of premature fusion of the bones of the skull at some of the suture lines. -If the bones continue to grow after this fusion, it may cause the skull to develop some abnormal shapes. - Examples of variations of thiscondition are scaphocephaly, brachycephaly |
| Define scaphocephaly(boat head) | A variation of craniosyntosis where the sagittal sutures fuse too early resulting in a boat-shaped skull |
| Define brachycephaly(short head) | Variation of craniosyntosis where the coronal suture closes early on both sides of the head, resulting in a shorter skull |
| Define microcephaly (“small head”). | -even the skull of an adult with microcephaly (left) is much smaller than that of a normal adult (right). -There is some debate about whether this condition is the result of the brain not growing and thus not causing the skull to enlarge, or if it starts with all of the skull sutures closing early and thus not allowing further brain growth |