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Lecture 5

DEV2022: Lecture 5 notes

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DEV2022 – LECTURE 5 Two developmental pathways: It means that there are certain parts of the body that develops the skeletal system by process 1 and process 2. The bones of the calvarian (the bones of the skull) plus some aspects of the face and jaws and the clavicle are developed directly from mesenchymal tissue. Everything else is developed by an intermeditary tissue which we know as cartilage. The end result which is mature bone is the same – identical tissue developed by two completely different tissues. Bone consists of: Fibres Collagen Mineral Crystals Matrix in which the fibres and crystals and minerals and water can sit cells Androgens + Estrogens regulate bone health: We have a growth spurt at the time of puberty. During puberty, these sex hormones are produced which causes bone growth. So bone growth and lengthening is depended upon the secretion of estrogen + testosterone at puberty. Also responsible for bone development at old age. Estrogen is withdrawn at menopause. Vertebrate skeletogenesis: Although Mesoderm is predominantly responsible for development of skeleton, the neural crest (ectoderm derived special fourth germ layer) also contributes to aspects of the cranio-facial skeleton. Mesoderm at the midline contributes to some parts of the cranio-facial skeleton, but mostly the appendicular skeleton. The notochord is mesodermally derived, it is central defining axis at the early embryo. It ends up being trapped into the somites. Intramembranous ossification: frontal, paraxial, temporal, zygoma, maxilla, part of the mandible (developed by IMO within the mesenchymal process). Growing fetal skull: red = growing bone yellow = connective tissue (mesenchymal tissue) Skull skeleton: Those plates – frontal, paratial, part of the exipital, zygoma – transition from embryo to fetus are pieces of mesenchymal connective tissue. Within them, some of these Mesenchymal connective tissue. CTs are programmed by gene activity to turn into bone forming cells – osteoblasts. The gene of the osteoblasts are to make fine matrix (yellow materials being deposited between them). Immature bone matrix is weak and poorly mineralized, so they’re called osteoid. Osteiod is first laid down. With the passage of time, the osteoid will become mineralized and the primitive bone forming will eventually remodel itself osteons (osteons). The end product of this ossification is to form strong compact bone on the outside of skull, and honey-comb on the inside, where bone marrow lives, where all the bone tissue is identical. Part 2: Growth of the osteoid means that the fingerlike projections of growing weak bones will interconnect and form bridges and strut which is what we’ll see in spongy bone. Skull growth: Another part of the clavicle and mandible relies on cartilage. The development of occipital bone relies on intermeditary cartilage bone and membrane bone. As you grow older, the contents inside the skull is resorbed – taken away, and on the outside of the bone, new layers are added, so it increases in size. Long bone development: In this type of development, mesoderm is a starting point in all long bone development. We next get the involvement of cartilage. The reason why cartilage is involved is because: - Cartilage cells can multiply and proliferate, whereas bone cannot. When you fracture a bone, the way the bone is replaced is by the intermeditary of cartilage. - Cartilage secretes a matrix. The matrix of cartilage in long bone growth is the substrate, scaffold, platform upon which osteoblasts will lay down bone to manufacture the elongation of bone growth. Scaffold: The panels of limbs are made of connective tissue – mesenchymal kind. Those panels have developed a cartilage matrix and eventually the cartilage matrix now appears to be forming for bone – mesenchyme, cartilage, bone. This explains why radiographs of children, there are pieces of skeleton missing in various joints – they have yet to be ossified. Many of those developing bones are cartilage because they have no yet been developed fully yet. EO: The developing skeleton has been stained blue. Blue is specific for cartilage. E10.5 = mesenchymal tissue E12.5 = it is programmed to form condesnation of this mesenchymal tissue which resembles the digits. Those primitive digits made from connective tissue are then stained blue. So cartilage is the key element in EO. Stain: ossification centre = where bone appears first. When its long bone, it appears in the middle. Miniature cartilage tissue model: The hyaline cartilage will be invaded by blood vessels in its wasteful central region. Those vascular elements will form the first sign of bone formation, so they will be responsible for the appearance and differentiation of osteoblasts - the cells that form cell matrix. They begin to appear in the middle of the shaft of the primitive cartilage. We call this the primary ossification centre where bone is initially being laid down. Cartilage is acting like a scaffold where the osteoblasts are secreting their matrix. Embryonic fetal bone development: Bone = cartilage Ossification centre = middle, where in the 2 and 3 picture, it has
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