Histology: Muscle & Integument

Muscle Tissue Basics

• Muscle tissue: muscle cells packed with contractile filaments (actin = thin, myosin = thick).
• Classification: histologically striated (skeletal, cardiac) vs non-striated (smooth); physiologically voluntary (skeletal) vs involuntary (cardiac, smooth). Cardiac = striated but involuntary.
• Terms: sarcoplasm = cytoplasm; sarcoplasmic reticulum = ER (Ca²⁺ store); sarcosome = mitochondrion; sarcolemma = cell membrane; myofiber = muscle cell; myofibril = subunit of the cell (chain of sarcomeres).
• Section planes: longitudinal (membujur) = long strips; transverse (melintang) = round islands. In the atlas figure OL = transverse, IC = longitudinal.

Smooth Muscle (otot polos)

• Fusiform cells, no striations, one elongated central nucleus; wrapped in endomysium. Also called visceral muscle.
• Where: blood vessels, GI tract, respiratory tract, uterus, urinary bladder, arrector pili.
• Control: autonomic nervous system; the myoneural junction is a synapse. Slow, involuntary.
• Cross-section: profiles of different sizes; only the large ones contain a (central) nucleus.
• Regeneration: good (cells divide by mitosis).

Skeletal Muscle (otot rangka / lurik)

• Cylindrical, multinucleated fibers; oval nuclei peripheral, beneath the sarcolemma; clear cross-striations. Voluntary.
• Control: acetylcholine at the motor end plate / neuromuscular junction. Location: limbs, trunk.
• Sheaths: epimysium (whole muscle, dense irregular CT) → perimysium (fascicle, dense irregular CT) → endomysium (each fiber, reticular fibers).
• Transverse section: nuclei at the edge; myofibril bundles appear as islands = Cohnheim's areas.
• Longitudinal section: A band dark (anisotropic, myosin); I band light (isotropic, actin). Z line in the middle of I; M line in the middle of A; H zone = light center of A. Z to Z = sarcomere.
• Regeneration: limited (satellite cells).

Cardiac Muscle (otot jantung)

• Striated, branching and anastomosing fibers (anastomose), intercalated discs (irregular dark transverse lines between cells), 1 (sometimes 2) large oval central nucleus.
• Involuntary; heart only. No regeneration after injury (scar).
• vs skeletal: central nuclei, branches, discs. vs smooth: striations and discs.

Skin and Epidermis

• Layers: epidermis (ectoderm) → dermis (mesoderm) → hypodermis (subcutis). Epidermal ridges interlock with dermal papillae.
• Epidermis: keratinized stratified squamous epithelium, avascular, nourished by diffusion from the dermis.
• Stratum basale: one layer of cuboidal/columnar basophilic cells; hemidesmosomes to the basal lamina, desmosomes between cells; mitotic stem cells.
• Stratum spinosum: polygonal cells with "spines" (desmosomes), tonofibrils; Langerhans cells here. Basale + spinosum = stratum Malpighii.
• Stratum granulosum: flattened cells with keratohyalin granules; lamellar granules waterproof the skin.
• Stratum lucidum: flattened, pale, anucleate cells with eleidin. Thick skin only.
• Stratum corneum: 15–20 layers of dead anucleate cells full of keratin; desquamation.
• Thick skin (palms, soles): thick corneum + lucidum, no hair. Thin skin: no lucidum, hair and sebaceous glands.

Cells of the Epidermis

• Melanocyte: round body with long processes, between basal cells; makes eumelanin. Same number in light and dark skin.
• Langerhans cell: like a melanocyte but no melanin; mainly in the spinosum; macrophage-like immune cell.
• Merkel cell: keratinocyte-like with dense granules; basal layer, esp. thick skin; with its nerve ending = Merkel corpuscle (mechanoreceptor).
• Lymphocytes: mostly immature T cells; with Langerhans cells and keratinocytes form SALT.

Dermis and Hypodermis

• Stratum papillare: thin, loose CT; fibroblasts, macrophages, mast cells; dermal papillae; capillaries feeding the epidermis; anchoring fibrils (type VII collagen); Meissner's corpuscles.
• Stratum reticulare: thick, dense irregular CT; type I collagen and elastic fibers.
• Hypodermis: loose CT with abundant adipocytes, highly vascular, Pacinian corpuscles.

Sensory Nerve Endings

• Merkel's disc: unencapsulated; on Merkel cells in the basal epidermis; light touch.
• Vater-Pacini: oval, onion-like, concentric lamellae; deep dermis and subcutis; crude touch, pressure, vibration.
• Meissner: cylindrical, branched, thin capsule of transverse flattened cells; dermal papillae; light touch.
• Ruffini: cylindrical, smaller, spray-like branches, thin capsule; dermis; pressure.
• Krause: round, thick capsule; dermis; pressure (practicum; older texts: cold).
• Free nerve endings: pain, temperature, itch.

Skin Appendages

• Hair: shaft, root (in the follicle), bulb (dermal papilla, blood vessels, dividing epithelial cells, melanocytes).
• Hair layers, inside out: medulla → cortex → cuticle.
• Follicle sheaths: outer dermal root sheath (CT; arrector pili attaches here) and inner epidermal root sheath (epithelium), separated by the glassy membrane.
• Arrector pili: smooth muscle; raises the hair (goosebumps).
• Sebaceous gland: simple branched acinar, holocrine (sebum), opens into the hair follicle (pilosebaceous unit); absent from palms/soles.
• Eccrine sweat gland: simple coiled tubular, merocrine, opens onto the skin surface; secretory part has dark cells, clear cells and myoepithelial cells; duct = 2-layered cuboidal.
• Myoepithelial cells: in the secretory portion, not the duct.
• Apocrine sweat gland: larger, wide lumen, opens into a hair follicle; axilla, areola, anal region.
• Nail: keratin plate on the dorsal distal phalanx; eponychium proximal, hyponychium distal.