1.1: Tissues & Structures

Skin

= Epithelium (epidermis) & connective tissue (dermis)

Epidermis

Stratified squamous keratinising; ectodermal origin.  Appendages of skin (sebaceous & sweat glands, hair, nails) derive from it. 

Stratum corneum

Close-packed layers of dead keratinocytes in lipid cement, normally softened by moisture and grease; imbibes water Ü thick, soft and white.

Stratum lucidum

Present in thick skin only

Stratum granulosum

 

Stratum spinosum

 

Stratum basale

Metabolically active cells

- note that thick skin has a thick stratum corneum, but thin dermis; vice-versa for thin skin.

Other cells found in epidermis

Melanocytes found in lowest layers, usually adjacent to basement membrane.  Produce melanin which is taken up into vacuoles by keratinocytes.

- these are the primary determinants of skin colour; (there are racial type differences, eg yellowed, brown)

Langerhans cells

Merkel cells in hairless skin = pressure sensors.

 

Sweat Glands: most are eccrine (deliver water to body surface \ regulate temp); apocrine (scent) glands located in axillae, areolae of breasts & urogenital regions.

sweat glands are not found on lip margins, glans penis, labia minora, nipples, tympanic membranes.

Especially plentiful on hands, feet, face including forehead.

Sebaceous Glands: confined to hairy skin.  Both sweat & sebaceous glands open into hair follicles.

except on eyelids, lips, papillae of nipples, labia minora Ü straight onto skin surface.

 Hair: = hard keratin, arises from hair matrix (epidermal cells) at base of follicle, moving up a tubular sheath to form hard keratin rods.  Melanocytes in follicle impart colour.  Nails form in similar fashion. 

follicles have erector pili muscles, stand hair on ends, simultaneously secreting sebaceous contents

hair grows in cycles, (adjacent follicles out of sync), scalp ~2-3yrs then rests, brows ~1-2months.

Tension lines: due to pattern of fibre bundles in the dermis.  Run parallel to skin creases near joints.

incisions to be made along these lines, and certainly not across flexor creases.

Subcutaneous Tissue

Skin is variably bound to its deeper structures, and folds in places where thinner and firmly bound.

Panniculus adiposus = subcutaneous fat blanket, contains nerves, vessels and lymphatics to skin.  Panniculus carnosus = muscle fibres with one attachment to bone, other to skin (platysma, dartos, palmaris brevis, corrugator cutis ani).

Deep Fascia

A membrane of fibrous tissue, wrapping limbs and body wall.

variable from thick (fascia lata) to thin (over external oblique) to absent (face).

always anchored to periosteum of bone. 

attaches skin by fibrous stands to fat above, and most muscles slide freely beneath it.

Is very sensitive (supply same as that for overlying skin & periosteum)

Where it lies over non-expansile parts (prevertebral mŇs, pelvic wall) it is strong and readily sutured, whereas over expansile parts, eg cheek, pharynx it is indefinite.

Ligaments

Dense CT (bone to bone), non-elastic and not mobile unless prolonged strain (except sacroiliacs and intervertebral discs)

2nd type of ligament: eg ligamentum flavum: stretches then regains former shape.

Tendons

Attach muscle to bone in a sheet / flat insertion.

Blood from: i) muscle belly artery tracking down; ii) vessels ascending from periosteum; iii) in long ligaments a supplementary artery of their own.

Synovial Sheaths

Lubricate tendons where they bear on adjacent structures.

Parietal layer attached firmly to surrounds, visceral layer firmly attached to tendon.

a fluid film in-between ensures smooth motion

Do not usually completely encircle, rather open at one end for blood vessels etc, or perforated by a bundle that raises a mesotendon

Raphes

Interdigitation of short tendinous muscle fibre ends; may elongate passively.

eg buccinator and superior constrictor interdigitate in the pterygomandibular raphe, allowing the mandible to open.

Cartilage

Cells and fibres embedded in a firm ground substance.

Hyaline c: covers articular surfaces, forms epiphyseal plates.

is avascular and slightly deformable, when damaged is replaced by fibrous tissue

Fibrocartilage: islands of cartilage & ground substance between collagen bundles

eg intervertebral discs, menisci of knee, some articulations eg clavicle-sternum

tends to calcity / ossify with age.

Elastic: large amounts of elastin in ground substance.  Remains spongy

- like hyaline, nourished only by diffusion (cf fibro = regular blood supply)

Muscles

Skeletal muscle

Individual fibres may be parallel (more stretch eg satorius) or oblique (more power) to the line of pull.

Obliques have 3 patterns:

unipennate Ð tendon forms on one margin, fibres slope in like half a feather, eg FPL

bipennate Ð tendon centrally like a feather, eg rectus femoris

multipennate Ð like a series of bipennate muscles side by side (subscapularis) or cylindrically with the tendon centrally (tibialis anterior)

Origins are usually described superior, insertions inferior where there is more motion possible.

Movements

Prime mover: where the muscle effects a certain desired movement

Antagonist: opposite of the desired movement, relaxing in a controlled manner

Fixators: stabilise one attachment so the other end may move.

Synergists: prevent unwanted movement, eg stabilise the wrist so other muscles can move fingers.

muscles may also contract isometrically so their tension increases but length stays the same.

Nerve Supply

Skeletal muscle supplied by somatic nerves which contain motor, afferent & autonomic fibres.  Efferent fibres are axons of a anterior horn cells.  Efferents from g cells innervate spindles wrt proprioception, and the spindles send sensory afferents back to cord.

Exceptions: no sensory fibres in nerves to facial muscles, spinal part of accessory, and hypoglossal nerves.

proprioception here supplied by other cranial nerves.

Limb nerves do not pierce muscle but pass in planes (cf pass through flat body wall muscles to skin).

Limb plexi Ü limbs, anterior divisions to flexors, posterior divisions to extensors.

Lower Motor Neuron

Fired in one of 3 pathways:

UMN: in contralateral motor cortex (prime mover)

Extrapyramidal System: causes synergic contraction, modifying action of prime movers

Sensory pathways: reflex tone (eg in tendon reflexes)

Bone

Bone is dense CT with cells imbedded in calcified ground substance.

filled with marrow; nothing to do with the bone, just a convenient place to store it.

Periosteum is a thick layer of fibrous tissue in which runs blood vessels to bone

has a deeper osteogenic layer; unites to underlying bone by ´Sharpey═s fibres═

very sensitive (supply from overlying skin, or nearby muscle branches if deeper)

Endosteum occurs inside bone surfaces (including canals) and is osteogenic (or in less-well vascularized areas produces hyaline cartilage)

Blood supply is mainly via periosteum (the nutrient arteries go to marrow); veins and arteries run together in Volkmann═s canals in compact bone; lymph is scant.

Development
Intramembranous: osteoblasts lay bone in fibrous tissue (no cartilage) eg face, skull, clavicle

intramembranous bones may have ´secondary cartilage═, eg fibrocartilage at clavicle ends, which acts like an epiphysis in a long bone.

Endochondral: a hyaline c. model is first made, and gradually replaced with bone (most bones)

these have ossification centres, and continue to grow from hyaline epiphyses.

Joints

Fibrous jts: eg skull sutures, are between bones or cartilage Ð firm & allow little movement

these ossify with time (apart from the tib-fib joint).

Primary cartilaginous jts: bone and hyaline meet strongly (eg rib junctions), rarely separate.

Secondary cartilaginous: ´symphysis═; hyaline laminae are united by fibrocartilage.

Synovial joints: include all limbs:

bones are covered in hyaline

surrounded by a capsule (at epiphyseal lines in fetus, but move either way in adult)

enclose a cavity

are reinforced by ligaments

are lined by synovial membrane (invests non-articulating surfaces and secretes a lubricating viscous fluid that thins with rapid movement; a mere film in normal circumstances)

are able to move

There are also intra-articular fibrocartilages (discs or menisci)

usually in jts with 2 movements: eg knee; sternoclavicular joint, wrist.

Stability: is conveyed by bony, ligamentous and muscular contributions (least to most important)

The blood supply is arranged such that nutrient arteries terminate at epiphyseal plates in children (hence infarction in osteomyelitis) but connect in adults after plates fused.

Nerves: joints and capsules are sensitive, synovium has little & articular cartilage has none

Hilton═s Law: motor nerve to a muscle tends to branch to the joint which the muscle moves, and again to the skin over that joint.

Mucous Membranes

Line interior surfaces that communicate with the exterior.

Have epithelium and lamina propria, and often muscularis mucosa.  Lie on a submucosa.

Serous Membranes

Lining of closed body cavities: pleural, pericardial & peritoneal.

Consists of connective tissue with lining of flattened mesothelial cells derived from mesoderm.

Parietal layer has segmental sensory supply, visceral layer has none.

slide readily over each other, lubricated by lymph.

Blood and Lymph

Widely variable growth pattern: anastomosing vessels grow and regrow in fetus.

sometimes organs carry a long blood supply when migrating, others gain new supplies

Veins are bigger and double the number (slower rate), with dead space around to accommodate increased flow (eg femoral canal, carotid sheath)

often regional lymph nodes lie in this dead space.

Anastomoses: are of two types, or there are none: end-arteries (eg liver spleen, kidney)

actual anastomosis: meet end to end, eg around margin of colon, stomach.

Potential: terminal arterioles meet, can take flow only if given time (eg coronaries, brain)

Lymph: takes tissue fluid away

superficial lymph follows veins, deep lymph follows arteries

There is no set path for lymph flow: may bypass nodes, may reverse flow, and communicates with veins.