Integument, Bone & Joint PDF

Integument, Bone & Joint General integument function The integument (skin) 1. Prevents desiccation barrier to water loss 2. Prevents infection barrier to foreign bodies 3. Assists in thermoregulation blood flow and hair coat dynamics...

Integument, Bone & Joint General integument function The integument (skin) 1. Prevents desiccation barrier to water loss 2. Prevents infection barrier to foreign bodies 3. Assists in thermoregulation blood flow and hair coat dynamics 4. Acts as a sensory organ mechano --, thermo --, chemoreceptors 5. Excretes water and salts 6. Synthesizes vitamin D The integument (skin ) is categorised into three (3) layers: Epidermal layer ▪ Avascular , ▪ Primarily cellular squamous epithelial cells: o Keratinocytes multiply at basement membrane pushing older cells toward surface and undergo keratinization (cells fill with keratohyalin granules , organelles degenerate, and become lifeless sheets of keratin ), o Melanocytes produce melanin stored in melanosome granules Dermal layer ( corium). ▪ Vascular ▪ Primarily fibro elastic connective tissue ▪ Contains o Hair follicles o nerve endings o glands o smooth muscle o blood vessels o lymph vessels ▪ Various types of receptors (mechano --, thermo --, and chemoreceptors) Hypodermal layer ( subcutaneous layer) ▪ Primarily adipose tissue (fat) and loose connective tissue Hair structure and function Hair function: ▪ traps insulating layer of air important for thermoregulation ▪ pilo erection and pilorelaxation, under the action of the ‘ arrector pili muscle ’, allows for control of the thickness of the insulating layer of air, and thus provides some control over heat loss ▪ colour important in absorbing solar radiation in the visible spectrum ▪ colour important for camouflage in some species Hair structure: ▪ Each strand is comprised of three layers: o A central ‘medulla’ containing flexible soft keratin o A ‘cortex’ containing rigid hard keratin o A ‘cuticle’ containing rigid hard keratin (a single layer of cells, normally arranged like roof tiles to prevent clumping) ▪ The ‘shaft’ is the part of the hair located above the skin surface, the ‘root’ is the part embedded below the skin surface ▪ Each hair is anchored by the ‘ hair follicle ’, which is an invagination of the epidermal layer that extends down into the dermal layer ▪ The deepest part of the hair follicle expands to form the ‘hair bulb’, where a mound of dermal cells called the ‘papilla’ is covered with rapidly dividing epithelial cells called the ‘ matrix ▪ As the epithelial cells divide and grow, older cells are pushed upward, they become ‘keratinized’ and die. Hair growth cycle Hair growth cycle: ▪ Hair undergoes a routine cycle of growing and shedding  ▪ Hair can also undergo once off shedding ( blowing the coat), influenced by genetics and environment (e.g. shedding is often heaviest in the spring) ▪ Hair cycle is differentiated into three phases : o Anagen phase ’ is the period of growth as epithelial cells are added at the ‘ matrix ’, o Catagen phase ’ represents the transition between anagen and telogen phases o Telogen phase ’ is the quiescent state induced by shortening of the hair follicle Glands of the skin Sebaceous glands: ▪ Generally found all regions of the skin, embedded in the dermal layer ▪ A sac like structure ( alveolus ) with a single duct that empties into a hair follicle (although some empty directly onto skin surface) ▪ The sac is lined with epithelial cells that synthesize and store an oily substance composed primarily of free fatty acids, and when the cells rupture they release this ‘ sebum ’ into the sac ▪ Contraction of ‘ arrector pili muscle ’ forces sebum through the duct and into the hair follicle, where it coats the base of the hair and the surrounding skin  ▪ Helps retain moisture, anti bacterial and anti fungal properties Sweat glands (sudoriferous glands) ▪ Generally found all regions of the skin , embedded in the dermal and/or hypodermal layer ▪ Occur in most domestic mammals, but more prevalent in larger species (especially humans and horses) ▪ There are two types of sweat gland: o eccrine sweat glands ’ - secrete directly onto the skin surface (common in humans, but sparse among domestic animals), o apocrine sweat glands ’ - secrete into hair follicles ( common among cats, dogs, sheep, cattle, horses) ▪ Function in evaporative cooling, heat is drawn from the skin as water molecules change from the liquid phase to the gaseous phase Skeletal system 1. Exoskeleton 2. Endoskeleton: ▪ Support & locomotion - Long bones of the forelimbs and hindlimbs ▪ Protect internal organs - Flat bones of the cranium, ribs, sternum  ▪ Homeostasis and synthesis - Ca 2+2+& PO 4 store (fetus, lactation, eggshell) - Hematopoiesis (RBC, WBC, platelets) Bone structure Flat bones vs long bones ▪ Flat bones (e.g. plates of the skull, ribs, sternum, pelvis) - outer compact bone cortical bone ) and inner spongy bone cancellous or trabecular bone - protect internal organs, Ca2+2+ & PO4 store, hematopoiesis ▪ Long bones (e.g. humerus, femur) - As above, outer compact bone cortical bone ) and inner spongybone ( cancellous or trabecular bone  - support & locomotion, Ca2+2+ & PO 4 store, hematopoiesis Compact bone vs spongy bone ▪ Outer compact bone cortical bone - Dense organization providing strength and rigidity - Nutrients & gases exchanged along vessels within central (Haversian) canals, around which are lamellae, which form an osteon (Haversian system) a repeating functional unit of bone ▪ Inner spongy bone cancellous or trabecular bone - Sparse arrangement providing strength and some flexibility while reducing weight - Nutrients & gases exchanged with extracellular fluid - No osteon units, rather lamellae arranged to form spicules of bone Bone cells ▪ Osteoblasts - Synthesize the organic portion of the matrix, i.e. the osteoid containing collagen and proteins - Produce the inorganic portion of the matrix, i.e. hydroxyapatite ▪ Osteocytes - Mature bone cells ‘trapped’ within the lacunae - Might be able to revert back to osteo blast during remodeling  - Cytoplasmic extensions within the canaliculi, communicate with other osteocytes or osteo blasts ▪ Osteoclasts - Large, mobile, multi nucleated cells - Responsible for bone resorption - Release acids and acid tolerant proteolytic enzymes (specific proteases) that ‘breakdown’ the organic portion and ‘dissolve into solution’ the inorganic portion of the bone matrix Bone matrix ▪ Osteoblasts synthesize the bone matrix ’, before becoming trapped, and turning into osteocytes. ▪ Bone matrix is composed of: Organic portion o Osteoid (non mineralized component), composed of collagen and proteins o Provides tensile (resilient) strength Inorganic (mineral) portion o Hydroxyapatite, composed of calcium phosphate crystals o Provides rigid strength Growth in length of bone ▪ Growth in length of long bones occurs at the epiphyseal (growth) plate/s, under control of growth hormone (somatotropin) secreted by the pituitary gland ▪ Epiphyseal plate has three (3) distinct layers: o Zone of proliferation: columns of chondrocytes multiplying, secrete cartilaginous matrix  o Zone of maturation: chondrocytes stopped multiplying and have become enlarged o Zone of hypertrophy zone of provisional calcification ): further enlargement and vacuolization of chondrocytes, cartilaginous matrix begins to calcify ▪ Next, chondrocytes undergo cell death, osteo blasts deposit thin layer of loosely organized collagen matrix onto the calcified cartilage , calcification of the new matrix follows, forming new spongy woven bone ▪ Lastly , osteo clasts resorb this spongy woven bone and the calcified cartilage , and then osteoblasts build sheets of true lamellar bone Growth in diameter of bone ▪ Growth in diameter of long bones occurs by deposition of new bone by osteoblasts within the outer periosteum and accompanied resorption of older bone by osteo clasts at the interior of the shaft ▪ This allows the increase in bone diameter to keep pace with the increase in bone length (important for biomechanics), and it also allows for marrow cavity expansion (important for haematopoiesis) ▪ Growth in diameter of long bones is also responsive to chronic localised stress loads, e.g. tennis players have thicker (and denser) bones of the dominant arm Cost of bone growth Post- pouch adult Using blood flow index to kangaroos (bipedal) estimate the metabolic cost of bone growth’ In-pouch joey kangaroos Other adult mammals (quadrupedal) Bone remodeling ▪ Remodeling is undertaken by bone remodeling units , and the process can be described by four (4) distinct steps 1. Activation: - Identification of the site to be remodeled. Quiescent osteoblasts lining the bone surface retract, expose bone matrix , and osteoclasts then bind to exposed site 2. Resorption: - Osteoclasts release acids and proteolytic (protein degrading) enzymes dissolving the bone matrix, breakdown products enter the osteoclasts , then extruded into the extracellular fluid, leaving a depression in the bone matrix, then osteo clasts leave 3. Reversal:  - Osteoblasts move back along the surface and into the vacated depression 4. Formation: - Osteoblasts deposit new osteoid ( non mineralized component , composed of collagen and proteins ) in discrete layers of lamellae , and then mineralisation occurs some time later Cost of bone remodeling Dinosaurs! ‘Using blood flow index to estimate the metabolic cost of bone remodeling’ Mammals Birds Reptiles Joint types ▪ Three (3) joint types can be classified by degree of movability: 1. Synarthrosis(fibrous joints, cartilaginous joints) - fixed, immovable, e.g. cranium 2. Amphiarthrosis(fibrous joints, cartilaginous joints) - semi moveable, e.g. vertebral joints 3. Diarthrosis(synovial joints) - Moveable - Two types: (1) ball & socket joint [e.g. shoulder and hip], and  (2) hinge joint [e.g knee and elbow] - Synovial membrane synovium ) surrounds the whole joint and contains the synovial fluid lubricant within the synovial capsule Cartilage ▪ Cartilage contains chondrocytes (cartilage cells) that produce, and are surrounded by, a ‘ matrix ’ of collagen and elastin, often enclosed within the perichondrium ▪ Cartilage is non innervated & a vascular ▪ Intermittent pumping action of weight bearing & joint motion provides bulk flow of fluid that facilitates diffusion ▪ Three (3) cartilage types can be defined by the relative composition of this ‘matrix’ 1. Hyaline articular ) cartilage - Larynx, tracheal rings, joint surfaces (articular) - Epiphyseal (growth) plates of long bones in growing animals - At joints it reduces friction, weight bearing, absorbs shock in young it has capacity for repair, but loses much of that capacity with cessation of bone growth 2. Elastic cartilage - External ear, epiglottis - Functions to provide shape & support 3. Fibrous cartilage fibrocartilage ) - Intervertebral disks, symphysis - Functions to provide rigidity, absorb shock