Histo Lab

• Muscle tissue


  The fourth basic type of tissue, composed of cells that optimize the universal cell property of contractility

  
• Muscle tissue


  • All muscle cells are of mesodermal origin and differentiate by a gradual process of cell lengthening with abundant synthesis of the myofibrillar proteins actin and myosin

  
• Functions of muscle tissue

  • Contractility
  • Excitability
  • Extensibility
  • Elasticity

  
• Muscles can only contract
• Functions of muscle tissue

  • Produce movement by contracting
  • Locomotion & manipulation
  • Movement of blood, food, urine, etc. through hollow organs
  • Maintenance of posture
  • Stabilize joints
  • Heat production
  • Maintenance of body temperature

  
• Three major types of muscle tissue

  • Skeletal muscle
  • Smooth muscle
  • Cardiac muscle

  
• Myofilaments


  • Include both thick and thin filaments, consist of contractile protein arrays bundled within myofibrils
  • Thick myofilament: 200-500 myosin molecules
  • Thin filament: F-actin, tropomyosin, troponin
• Skeletal muscle


  • Striated muscle
  • Consists of muscle fibers, which are long, cylindrical multinucleated cells with diameters of 10-100 μm
  • Elongated nuclei are found peripherally just under the sarcolemma
  • Show strong, quick, voluntary contractions

  
• Skeletal muscle is attached to the skeleton
• Smooth muscle


  • Elongated, tapering, and nonstriated cells, each of which is enclosed by an endomysium
  • Specialized for slow, steady contraction under the influence of autonomic nerves and various hormones
  • At each cell's central, broadest part, where its diameter is 5-10 μm, is a single elongated nucleus

  
• Smooth muscle is a major component of blood vessels and of the digestive, respiratory, urinary, and reproductive tracts and their associated organs

  
• Cardiac muscle


  • Mature cardiac muscle cells are 15-30 μm in diameter and 85-120 μm long
  • With a striated banding pattern comparable to that of skeletal muscle
  • Usually has only one nucleus and is centrally located
  • Cells within one fiber often branch and join with cells in adjacent fibers

  
• Intercalated discs


  • Transverse lines that cross the fibers at irregular intervals where the myocardial cells join
  • Represent the interfaces between adjacent cells and consist of many junctional complexes
  • Serve as "electrical synapses," promoting rapid impulse conduction through many cardiac muscle cells simultaneously and contraction of many adjacent cells as a unit
• Cardiac muscle fiber contraction is intrinsic and spontaneous

  
• Cardiac muscle is located in the walls of the heart
• Skin is the tough, protective, external covering of the body
• Skin is the largest organ of the body in terms of weight, comprising 15%-20% of a person's body weight and presenting 1.5 to 2m2 of surface to the external environment
• Skin thickness ranges from 0.5 – 5.0 mm, being thinnest over the eyelids and thickest on the soles of the feet
• Skin is a semipermeable membrane
• Functions of the skin

  • Protective
  • Sensory
  • Thermoregulatory
  • Metabolic
  • Sexual signaling
• Layers of the skin

  • Epidermis
  • Dermis
  • Hypodermis
• Epidermis


  • Consists mainly of a stratified squamous keratinized epithelium composed of cells called keratinocytes
  • Contains three much less abundant epidermal cell types: Melanocytes, Langerhans cells, and Merkel cells
  • Lacks microvasculature; its cells receiving nutrients and O2 by diffusion from the dermis
• Layers of the epidermis

  • Stratum basale (basal layer)
  • Stratum spinosum (spinous layer)
  • Stratum granulosum (granular layer)
  • Stratum lucidum
  • Stratum corneum
• Stratum basale (basal layer)


  • A single layer of basophilic cuboidal or columnar cells on the basement membrane at the dermal-epidermal junction
  • Characterized by intense mitotic activity and contains, along with the deepest part of the next layer, progenitor cells for all the epidermal layers
• Stratum spinosum (spinous layer)

  • The thickest layer
  • Consists of generally polyhedral cells having central nuclei with nucleoli and cytoplasm actively synthesizing keratins
  • Just above the basal layer, some cells may still divide and this combined zone is sometimes called the stratum germinativum
  • The cells extend slightly around the tonofibrils on both sides of each desmosome, leading to the appearance of many short "spines" or prickles at the cell surfaces
• Stratum granulosum (granular layer)


  • Consists of three to five layers of flattened cells, now undergoing the terminal differentiation process of keratinization
  • Their cytoplasm is filled with intensely basophilic masses called keratohyalin granules
  • Also include Golgi-derived lamellar granules: small ovoid (100 × 300 nm) structures with many lamellae containing various lipids and glycolipids
  • Among the last activities of the keratinocytes, the lamellar granules undergo exocytosis, producing a lipid-rich, impermeable layer around the cells
• Stratum lucidum


  • Found only in thick skin
  • Consists of a thin, translucent layer of flattened eosinophilic keratinocytes held together by desmosomes
  • Nuclei and organelles have been lost, and the cytoplasm consists almost exclusively of packed keratin filaments embedded in an electron-dense matrix
• Stratum corneum


  • Consists of 15-20 layers of squamous, keratinized cells filled with birefringent filamentous keratins
  • By the end of keratinization, the cells contain only amorphous, fibrillar proteins with plasma membranes surrounded by the lipid-rich layer
  • These fully keratinized or cornified cells called squames are continuously shed at the epidermal surface as the desmosomes and lipid-rich cell envelopes break down
• Dermis


  • The layer of connective tissue that supports the epidermis and binds it to the subcutaneous tissue (hypodermis)
  • The surface of the dermis is very irregular and has many projections (dermal papillae) that interdigitate with projections (epidermal pegs or ridges) of the epidermis especially in skin subject to frequent pressure, where they reinforce the dermal-epidermal junction
• Differences between thin and thick skin

  • Thin skin: shorter dermal papillae, contains sweat glands, hair follicles, and sebaceous glands, stratum granulosum is poorly developed, found all over the body except the palms and soles, contains all layers of the epidermis except stratum lucidum

  Thick skin: covers the palms and the soles, contains numerous sweat glands, but lacks hair follicles and sebaceous glands, contains all layers of the epidermis
• Epidermal appendages


  • Hair
  • Sebaceous glands
• Hair


  • Elongated keratinized structures that form within epidermal invaginations, the hair follicles
  • The growing hair follicle has a terminal dilation called a hair bulb
  • A dermal papilla inserts into the base of the hair bulb and contains a capillary network required to sustain the hair follicle
  • Keratinocytes continuous with those of the basal epidermis cover the dermal papilla. These cells form the matrix of the elongating hair root; the part of a hair extending beyond the skin surface is the hair shaft
• Sebaceous glands


  • Embedded in the dermis over most of the body
  • Branched acinar glands with several acini converging at a short duct that usually empties into the upper portion of a hair follicle
  • Produce sebum by terminal differentiation of sebocytes, the classic example of holocrine secretion, secreting this oily substance onto hair in the follicles or pilosebaceous units
  • In certain hairless regions, such as the penis, clitoris, eyelids, and nipples, sebaceous ducts open directly onto the epidermal surface
• Hair bulb


  Terminal dilation of a hair
• Dermal papilla


  • Inserts into the base of the hair bulb
  • Contains a capillary network required to sustain the hair follicle
• Hair formation


  1. Keratinocytes continuous with those of the basal epidermis cover the dermal papilla

  2. These cells form the matrix of the elongating hair root

  3. The part of a hair extending beyond the skin surface is the hair shaft
• Sebaceous glands


  • Branched acinar glands embedded in the dermis over most of the body
  • Several acini converge at a short duct that usually empties into the upper portion of a hair follicle
  • Produce sebum by terminal differentiation of sebocytes, the classic example of holocrine secretion, secreting this oily substance onto hair in the follicles or pilosebaceous units
• Sebaceous glands in hairless regions


  Sebaceous ducts open directly onto the epidermal surface
• Eccrine sweat glands


  • Produce sweat that is mostly water onto the skin surface
  • Function as auxiliary excretory organs, eliminating small amounts of nitrogenous waste and excess salts
• Apocrine sweat glands


  • Restricted to skin of the axillary and perineal regions
  • Development depends on sex hormones and is not complete and functional until after puberty
  • Consist of simple cuboidal, eosinophilic cells with numerous secretory granules that also undergo exocytosis
  • Secrete protein-rich sweat onto the hair of hair follicles