Muscle
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Introduction
contractile mechanism in all cells, but some cells in multi-cellular organism specialised for contraction = muscle
three major types: skeletal, smooth, cardiac
all types surrounded by thin basement membrane or external lamina that binds them to adjacent cells
Skeletal
responsible for movement of the skeleton and organ, voluntary and striated
wide variety of forms and functions, but all have same basic structure, composed of extremely elongated
multinucleate cells bound together by ct, diameter: 10-100m, length: up to 35cm
contraction controlled by large motor nerves, individual nerve fibres branch within muscle to supply a group of muscle
fibres collective known as a muscle unit, excitation of one motor nerve results in contraction of all muscle fibres in
muscle unit, vitality of muscle depends on maintenance of nerve supply, skeletal muscle also contains highly
specialised stretch receptors
Connective Tissue
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individual muscle fibres grouped into bundles called fasciculi with delicate ct called endomysium connecting
individual muscles
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each fascicle is surrounded by loose ct called perimysium
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supporting tissue component contains both collagen & elastin and acts as a flexible skeleton to which muscle fibres
can attach
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becomes continuous with that of tendons and muscle attachments
entire muscle is covered by epimysium
size of fasciculi reflects function of muscle: small, highly controlled muscles have small fasciculi & more perimysium,
while large less controlled muscles contain large fasciculi and less perimysium
large blood vessels enter perimysium and divide to ramify epimysium, fine branches pass transversely through
muscle fibres and give rise to numerous longitudinal capillaries
Muscle Growth
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certain mesenchymal cells differentiate into long mononuclear myoblasts which then proliferate (via mitosis)
these myoblasts fuse end to end forming progressively longer multinucleate myotubes (<100 nuclei)
contractile proteins are laid down in the centre of the myotube pushing the nuclei to the outer edge
by birth most muscle growth is completed as is innervation
most growth after birth is a result of increase in muscle bulk
mature muscle cells are highly differentiated and have little capacity for repair
a few myoblasts (satellite cells) persist after maturity which can aid in repair
Striations
A
H
I
M
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contractile proteins within cell highly organised, giving rise to A (anisotropic), I (isotropic), H, M and Z bands
cross sections show similar organised arrangement
Sarcoplasm
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hardly any sacroplasm, surrounds myofibrils
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known as T-system, lumen continuous with extracellular space
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on either side of t-tubule SR forms flattened terminal cisternae, together with T-tubule forming a triad
rich with very large mitochondria and glycogen for energy
to permit simultaneous excitation of all myofibrils extensive system of tubular invaginations of the sarcolemma
surrounds each myofibril at A-I junction
between t-tubules a secod mebrance system derived from smooth ER the sarcoplasmic reticulum embraces each
myofibril
Smooth Muscle
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specialised for long contractions of relatively low force
contraction occurs independently of neural stimulation. influenced by autonomic NS and hormones
relatively small and elongated
single nucleus located at widest part of cytoplasm
bound together in irregular branching fasciculi, with considerable variation b/w organs
arranged roughly in parallel within fasciculi
contractile proteins not arranged in a parallel orderly fasihion
in many tubular structures there are often two layers: one circumferential and the other longitudinal
relatively few mitochondria and other intracellular organelles
membrane contains numerous flask shaped invaginations called cavelae, though to analogous to T-tubules
connected with gap and spot junctions
Cardiac Muscle
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in many ways intermediate between skeletal and smooth
long, cylindrical, branching cells
one or sometimes two nuclei, centrally located
delicate ct b/w cells supports capillary system, analogous to endomysium
sarcomeres arranged in continuous 3-D structure
also have T-tubule and SR system similar to skeletal but less well organised
dyads instead of triads, located at Z disk
Intercalated Disks
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specialised intercellular junctions
provide mechanical and chemical/electrical connection
always coincide with Z-disks
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terminal actin filaments of Z-disk attached to fascia adherens which transmit forces b/w cells
desmosomes occur less frequently and provide anchorage for intermediate filaments
Skin
Introduction
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differs in thickness, colour, presence of hairs, glands and nails
all types of skin have same basic structure
external surface consists of a keratinised squamous epithelium called the epidermis
dermis supports and nourishes epidermis, is fibro-elastic and contains many blood vessels and sensory receptors
dermis attached to underlying fatty hypodermis, represents superficial layer of fascia
hair follicles, glands, and nails are termed epidermal derivatives because they originate from epidermis
Functions
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protection – UV, mechanical abrasion, chemical, thermal, micro-organisms, prevents dehydration
sensation – largest sensory organ, touch, pressure, pain, temperature
thermoregulation – major organ of thermoregulation, insulation against heat loss, sweat glands, AV shunts
metabolic functions – vit D production, major store of energy (subcutaneous fat)
Thick Skin
Epidermis
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very thick epidermal layer
has 5 layers
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stratum germintivum (aka. basale or malpighii)
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stratum spinosum
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 cuboidal
 most replication occurs here
 desmosomes connect adjacent cells
 ‘prickle-cell’ layer
 large and polyhedral cells, numerous ribosomes
 cytokeratin (keratin precursor) aggregates to form tonofibrils which converge upon desmosomes
stratum granulosum
 cells characterised by numerous dense granules within cells
 more flattened
stratum lucidum
 v thin layer, translucent
stratum corneum
 dead and dying cells and flattened and devoid of organelles
Merkel cells are free nerve endings found in thick skin, presumed to be sensory receptors
Langerhan’s cells are specialised antigen presenting cells
Melanocytes are responsible for skin colour
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synthesise melanin which is excreted as melasomes and taken up by epidermal cells to be arranged around
the nucleus
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found in basal layers
neural crest (neuro-ectodermal) origin
long dendritic processes which extend between cells
Dermis
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provides robust supporting layer for epidermis as well as blood supply
divided in to 2 zones:
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Papillary zone
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Reticular zone
 most superficial layer
 relatively loose, highly vascular, contains free sensory nerve endings
 main bulk of dermis
 collagen fibres are coarse and disposed in thick irregular bundles
elastin is an important component of both layers
cells are mainly fibroblasts responsible for laying down connective tissue
Skin Appendages
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variety of appendages principally hairs, sebaceous glands and sweat glands
distribution shows large variation b/w tissues but has the same pattern
Hair
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highly modified keratinised structures produced by hair follicles
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arrector pili muscle attaches the follicular sheath to the dermal papillary zone, contraction erects hair = ‘goose
flesh’
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hair + muscle + sebaceous gland = pilosebaceous gland
hair growth takes place with a terminal hair bulb which consists of actively dividing epithelial cells surrounding a
vascular dermal papilla
Sebaceous Glands
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one or more are associated with each follicle (at about 1/3 the way to the skin)
secrete sebum, a water-proofing substance
branched acinar form
found independently in transitions b/w internal and external epithelia
Sweat Glands
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most are simple, tangled coiled tubular glands which secrete a watery fluid onto skin, called merocrine sweat
glands
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important in thermoregulation
in axilla and genital region another form is found: the apocrine sweat glands
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secrete viscous fluid onto hair follicle
analogous to odiferous glands in animals, biological important in humans unknown
Cartilage
Introduction
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semi-rigid form of connective tissue
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large proportion of ground substance matrix
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growth can occur both appositionally and interstitially
three types (fibro-,elastic, hylaine) arise from different proportions of matrix components
cartilage formation starts with differentiation of mesenchymal cells to chondroblasts
subsequent mitotic divisions and matrix deposition give rise to small, round chondrocytes sitting in cells nests, or
lacunae
most is avascular limiting the thickness to which cartilage can grow
forms much of the template for bone growth
Hyaline Cartilage
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most common type of cartilage, also bone precursor
characterised by small aggregations of chondrocytes embedded in amorphous matrix reinforced by collagen
continuum of cellular characteristics b/w outer zone and perichondrium
in adult, cell proliferation does not occur unless stimulated
matrix appears rather amorphous, contains collagen type II in interlacing network of fine fibrils
chondrocytes rich in glycogen and lipid
Fibrocartilage
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intermediate between cartilage and dense ct
found in intervertebral discs, some articular cartilages, pubic symphysis and in some joint capsules
consists of alternating layers of hyaline cartilage and thick collagen fibres
Elastic Cartilage
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occurs in external ear, epiglottis, parts of larynx and Eustachian tubes
histologically similar to hyaline cartilage but contains large bundles of elastic fibres
Bone
Introduction
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specialised form of supporting tissue in which extracellular components are mineralised
store of calcium and other inorganic ions
all bones are in a continual state of resorption and growth
inorganic salts 70% and organic matrix 30%
calcium hydroxyapatite is the major inorganic salt involved in the mineralisation of bone
the fibrous component in mainly type I collagen (90% of organic matrix)
bone exists in two main types:
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woven
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lamellar
 characterised by random arrangement of coarse collagen fibres
 first type of bone to be formed, and then remodelled to lamellar boen
 composed of highly organised layers
 two types of lamellar bone: compact and cancellous
Bone Cells
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three types on cells found in bone:
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osteoblasts
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osteocytes
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 derived from osteoprogenitor cells
 responsible for synthesis of and secretion of organic component: osteoid
 derived from osteoprogenitor cells
 formed from osteoblasts, responsible for maintenance of the bone matrix
osteoclasts
 derived from monocyte/macrophage lineage
 involved in resorptive process
Cancellous
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open sponge-like form
forms trabeculae,
Compact
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made of parallel columns of bone which are parallel to axis of bone
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connect to adjacent Haversian canals via Volkman’s canals
each column consists of concentric layers of bone called lamellae arranged around a central containing blood
vessels, lymphatics and nerves called Haversian canals
remainder of lamellae no longer surrounding Haversian canals are called interstitial systems
Generalised Structure of a Long Bone
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long shaft of compact bone: diaphysis
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the articular joint surfaces of the bone (epiphyses) are covered by articular cartilage
cancellous bone forms medullary cavity which in immature animals is filled with active (red) bone marrow, while in
adults most are filled with inactive (yellow) bone marrow
external surface invested with periosteum, a thick fibrous layer
inner surfaces invested with a delicate endosteum
endo- & periosteum contain cells responsible for growth, remodelling and repair