Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Subventricular zone wikipedia , lookup
Neuroanatomy wikipedia , lookup
Development of the nervous system wikipedia , lookup
Node of Ranvier wikipedia , lookup
Neuroregeneration wikipedia , lookup
Stimulus (physiology) wikipedia , lookup
Feature detection (nervous system) wikipedia , lookup
Dr. Janet Smith Microscopic Anatomy SLIDE REVIEW MUSCLE WEAKNESS & WEIGHT LOSS MODULES I. MUSCLE One of the 4 major tissue types Specialized for contraction Contraction is based on interaction of 2 types of myofilaments Thin filaments (actin-containing) Thick filaments (myosin-containing) Three types of muscle: Skeletal Cardiac Smooth Striated muscle – skeletal & cardiac are classified as striated muscle A. SKELETAL MUSCLE ORGANIZATION: Thick & thin (myo)filaments form a sarcomere Sarcomeres are arranged end-to-end form a myofibril Many myofibrils are arranged side by side in the cytoplasm of each skeletal muscle cell A skeletal muscle cell is also called a muscle fiber Many muscle cells are bundled together by connective tissue (perimysium) to form a muscle fascicle Many muscle fascicles are bundled together by connective tissue (epimysium) to form a single muscle Summary: Myofilaments à Sarcomeres à Myofibrils à Cells (Fibers) à Fascicles à Muscles CONNECTIVE TISSUE IN SKELETAL MUSCLE: Endomysium Loose connective tissue that surrounds individual muscle cells Consists mainly of reticular fibers; contains capillaries Perimysium Dense irregular connective tissue that surrounds a muscle fascicle; contains larger blood vessels Epimysium MICROSCOPIC ANATOMY Dense irregular connective tissue that surrounds an entire muscle (= the investing fascia of the muscle) MICROSCOPIC ANATOMY THE SARCOMERE: Found in skeletal & cardiac muscle, i.e., in all striated muscle Runs from one Z-line to the next Contains: An A band in the center of the sarcomere Half of an I band at each end of the sarcomere I-BAND Light-staining band Contains the parts of thin filaments that are not overlapped by thick filaments Bisected by Z-line, which anchors the actin filaments Its length decreases during contraction A-BAND Dark staining band of constant length Length corresponds to the length of the thick filaments Also contains the part of the thin filaments that overlaps the thick At EM level the A-band also contains: H-zone: Lighter staining region at center of A band Contains only thick filaments (i.e., is the region of the thick filaments that doesn’t overlap with thin) Length decreases during contraction Bare zone: An even lighter staining region within the H zone Contains shafts of thick filaments but no myosin heads Constant in length M-line: A darker line at the center of bare zone Site of cross-connections between thick filaments Contains myomesin and C protein SKELETAL MUSCLE CELLS: Mature fibers are long, unbranched, cylindrical cells Multinucleated, with peripheral nuclei Each cell is a syncytium formed by fusion of uninucleate cells (myoblasts) Average diameter of mature skeletal muscle cells is wider than cardiac or smooth muscle MICROSCOPIC ANATOMY T-tubules (transverse tubules): Invaginations of the sarcolemma (muscle cell plasmalemma) Located at A-I junction in mammalian skeletal muscle Located near Z-line in most other vertebrates (e.g., frogs) Carry depolarization of sarcolemma deep into muscle cell Depolarization of T tubule membrane causes release of calcium from terminal cisternae of sarcoplasmic reticulum Sarcoplasmic reticulum (SR): Is the equivalent of smooth ER in other cell types Sequesters calcium within its lumen to prevent contraction Encircles each myofibril within the skeletal muscle cell Forms a discontinuous network, with each segment extending from one T tubule to the next Each segment has two parts: Terminal cisternae (the expanded terminal portions located near each T tubule) Longitudinal elements that connect terminal cisternae - transports calcium back into the SR after each contraction Triad: Is a T tubule plus the 2 terminal cisternae that flank it NEUROMUSCULAR JUNCTIONS (NMJ) Are the terminations of motor neurons on skeletal muscle cells Neurons that innervate skeletal muscle cells are called somatic motor neurons The axon branches near its end (terminal arborization) Each muscle cell is innervated by only one motor endplate NOTE: One neuron plus all the muscle cells innervated by it is known as a motor unit Each axonal branch Ends in a motor endplate Loses its myelin sheath at the motor endplate, but remains partially covered by a Schwann cell Has many mitochondria in the axon terminal Releases acetylcholine (ACh) from round, clear synaptic vesicles into the synaptic cleft The motor endplate sits in a depression in the muscle cell sarcolemma called the primary synaptic cleft Secondary synaptic clefts are formed by junctional folds of the sarcolemma ACh receptors are located at the crests of the junctional folds MICROSCOPIC ANATOMY Subneural clefts (secondary synaptic clefts) are the spaces created between junctional folds External lamina of muscle cell (equivalent to a basal lamina) extends into synaptic cleft & subneural clefts Acetylcholinesterase is localized in external lamina B. CARDIAC MUSCLE CHARACTERISTICS BY LM: Each cell contains 1 or 2 centrally located nuclei Cells are joined together end-to-end by intercalated disks, which are unique to cardiac muscle Cells may branch Average cell diameter is intermediate between skeletal & smooth muscle INTERCALATED DISKS Have transverse and longitudinal portions Transverse portions run at right angles to the myofibrils Contain fasciae adherentes (singular = fascia adherens) - sites of actin filament attachment to the membrane - sites of end-to-end attachment of cardiac myocytes Desmosomes most common on transverse portions; also found on longitudinal Longitudinal (lateral) portions run parallel to myofibrils Gap junctions are common on longitudinal portions CHARACTERISTICS BY EM: More mitochondria than skeletal muscle They form almost continuous rows between myofibrils T tubules: Are at Z-line, not at A-I junction as in human skeletal muscle Are wider than in skeletal muscle Sarcoplasmic reticulum: Not as well developed as in skeletal muscle Terminal cisternae are not continuous along each T tubule Results in formation of diads (dyads) more often than triads Diad: Consists of one T tubule & one terminal cisterna of SR MICROSCOPIC ANATOMY INNERVATION Ordinary cardiac muscle cells are usually not directly innervated Autonomic (sympathetic & parasympathetic) fibers innervate the modified cardiac muscle fibers that form the conduction system of the heart (SA node, AV node) Autonomic nerve endings are less elaborate than the neuromuscular junctions on skeletal muscle Resemble nerve endings associated with smooth muscle C. SMOOTH MUSCLE CHARACTERISTICS BY LM: Spindle shaped (fusiform) cells Single central nucleus Has a distorted or “corkscrew” appearance in contracted cells Cells have smallest average diameter of any type of muscle Cells are usually packed tightly together Can be organized in orderly layers (in tubular organs, e.g., gut) or in interlacing bundles (in spherical organs, e.g. uterus) CHARACTERISTICS BY EM: Has no sarcomeres Has dense bodies Some are attached to the plasma membrane Form a branching network extending into the cytoplasm Are analogous to Z-lines: Contain attachment plaque proteins (e.g., alpha-actinin) Thin filaments & intermediate filaments insert on dense bodies Actin-myosin interaction pulls on the network of intermediate filaments, which twist & cause contraction of cell Has caveolae (pits) Invaginations of the sarcolemma Analogous to T tubules Lack a well-organized sarcoplasmic reticulum Sparse SER membranes are located near caveolae MICROSCOPIC ANATOMY Smooth muscle filaments include: Thin filaments that contain actin but lack troponin Thick filaments that contain myosin & are hard to preserve Networks of intermediate filaments that contain desmin (in GI, respiratory & urogenital tracts) or predominantly vimentin (vascular smooth muscle) Gap junctions are common between smooth muscle cells, especially where muscle cells are not individually innervated INNERVATION Is by autonomic nerves (sympathetic, parasympathetic & enteric) Innervation involves varicosities (boutons) located: At the end of each axon (terminal boutons) At intervals along the length of the axon before it terminates (boutons “en passant”, i.e., “in passage”) Both types of boutons form synapses with smooth muscle cells Synaptic morphology differs from that of skeletal muscle In smooth muscle: Sarcolemma has no junctional folds Synaptic cleft is much wider than in skeletal muscle A single bouton can innervate several nearby smooth muscle cells Not all smooth muscle cells receive direct individual innervation Cells that are individually innervated form “multiunit” smooth muscle (e.g., in the iris of the eye) Cells that lack individual innervation are coupled to innervated cells by gap junctions, and form “unitary” or “single unit” smooth muscle where co-ordinated contraction can occur (e.g., uterus, GI tract, urinary bladder) Contraction of smooth muscle can also be initiated by non-neural stimuli such as certain hormones (e.g., oxytocin) or by stretching the muscle cell II. NERVE CELL TYPES OF THE NERVOUS SYSTEM INCLUDE: Neurons: Transmit action potentials Neuroglia or glia: Non-neuronal cells specific to the nervous system CNS glia = oligodendrocytes, astroglia, microglia & ependymal cells PNS glia = Schwann cells & satellite cells MICROSCOPIC ANATOMY FUNCTIONAL TYPES OF NEURONS INCLUDE: Somatic motor neurons Innervate skeletal muscle Cell bodies are located in ventral horn of spinal cord Autonomic neurons Preganglionics in brain stem or spinal cord Postganglionics in autonomic ganglia (sympathetic, parasympathetic & enteric) Innervate smooth muscle, cardiac conduction system, & glands Sensory neurons Receive sensory input from receptors & free nerve endings Found in dorsal root ganglia (spinal ganglia) & in sensory ganglia associated with some cranial nerves CLASSIFICATION OF NEURONS BY SHAPE: Bipolar: One axon & one dendrite extend from the soma e.g., Bipolar neurons of retina Olfactory neurons in olfactory epithelium Multipolar: One axon and multiple dendrites extend from the soma e.g., Somatic motor neurons Autonomic neurons Pseudounipolar: One process extends from the soma & then branches into a central process & a peripheral process e.g., Most sensory neurons (e.g., DRG neurons) Note: The terms “peripheral process” & “central process” are preferred here instead of “dendrite” & “axon” because some peripheral processes of sensory nerves are myelinated. Thus although we could logically call them dendrites because they carry nerve impulses toward the soma, that would violate the rule that dendrites are never myelinated. We avoid the problem by making up the alternate name of “peripheral process” to refer to them. A TYPICAL NEURON HAS: Cell body (soma) that includes: Nucleus: Large & euchromatic Prominent nucleolus Perikaryon: The cytoplasm surrounding (“peri”) the nucleus (“karyon”) Nissl bodies: Basophilic aggregates of free & fixed polysomes; most evident in large somatic motor neurons MICROSCOPIC ANATOMY Dendrites: Are usually multiple, branched, rapidly tapering, unmyelinated processes Purpose is to increase the surface available for synapses carrying incoming signals to the neuron Axon: A single process leaves the perikaryon Originates from the axon hillock, an area of the perikaryon that is pale staining because it contains no Nissl bodies Axons carry action potentials away from the cell body toward one or more synapses on other neurons or effector cells (muscle, gland cell) Axons have relatively few branches until they reach their terminal arborization May be myelinated: Oligodendrocytes produce myelin in the CNS Schwann cells produce myelin in the PNS CENTRAL NERVOUS SYSTEM (CNS) INCLUDES: Brain & spinal cord PERIPHERAL NERVOUS SYSTEM (PNS) INCLUDES: Ganglia Is a cluster of neuronal cell bodies located outside the CNS There are two types of ganglia: Sensory ganglia, e.g.: Dorsal root ganglia (DRGs) Sensory ganglia of cranial nerves Autonomic ganglia, e.g.: Sympathetic Parasympathetic Enteric (found in the wall of the GI tract) Nerves (= Peripheral nerves) Includes spinal nerves and cranial nerves Composed of nerve fibers (axons plus Schwann cells) Do not contain neuronal cell bodies Sensory receptors Free nerve endings Expanded tip endings – Merkel discs Encapsulated receptors MICROSCOPIC ANATOMY SPINAL CORD White matter (axons and glia) surrounds gray matter (neuronal cell bodies, glia, dendrites & axons) Gray matter is divided into ventral horns (contents include somatic motor neurons) and dorsal horns (receive sensory input) Dorsal roots carry afferent (sensory) impulses to spinal cord Ventral roots carry motor impulses (somatic & autonomic) to the periphery SENSORY VS. AUTONOMIC GANGLIA: Sensory ganglia (DRGs & cranial nerve sensory ganglia) Often have a layered appearance (alternating layers of nerve processes & neurons) Contain pseudounipolar neurons Neurons are larger on average than in autonomic ganglia Nucleus tends to be centrally placed within the neuron Satellite cells form an almost complete single layer around neuronal soma Autonomic ganglia (sympathetic, parasympathetic & enteric) Contain multipolar postganglionic motor neurons Contain synapses between preganglionic axons and postganglionic neurons Most autonomic neurons are smaller than those in sensory ganglia Neuronal nuclei are more likely to be eccentric Fewer satellite cell nuclei are seen surrounding the neuron PERIPHERAL NERVES: ARE COMPOSED OF NERVE FIBERS Fiber = an axon (myelinated or unmyelinated) & Schwann cells Nerve fibers are bundled together to form a peripheral nerve UNMYELINATED VS. MYELINATED AXONS IN THE PNS In the PNS, myelinated & unmyelinated axons are both associated with Schwann cells Unmyelinated PNS axons are embedded in invaginations (grooves) in the Schwann cell plasma membrane Invagination forms the mesaxon (the point where the extracellular surfaces of the Schwann cell plasma membrane contact one another as they encircle the axon) MICROSCOPIC ANATOMY Several unmyelinated axons are usually embedded in one Schwann cell Myelinated PNS axons: Each Schwann cell contributes to the myelination of only one axon Myelinated axons tend to be wider in diameter than unmyelinated MYELIN SHEATH Viewed in cross section at high mag, it is a spiral that consists of alternating major dense lines & intraperiod lines Major dense line: formed by fusion of the cytoplasmic faces of Schwann cell plasma membrane Intraperiod line: formed by fusion of the extracellular surfaces of Schwann cell plasma membrane in successive layers of the myelin sheath NOTE: Schwann cell cytoplasm persists in several locations: Inner collar (between axon and myelin sheath Outer collar (exterior to the myelin sheath; the Schwann cell nucleus is located there) Perinodal cytoplasm (near the nodes of Ranvier) Clefts of Schmidt-Lanterman (see below) Outer mesaxon: Where the outer surfaces of the Schwann cell membrane contact one another exterior to the myelin sheath Inner mesaxon: Where the outer surfaces of Schwann cell membrane contact one another interior to the myelin sheath The process of myelination in the PNS The mesaxon forms by invagination of the axon into the Schwann cell Mesaxon elongates as new Schwann cell membrane is added to it The elongated mesaxon begins to spiral around the axon We can now identify an inner mesaxon at the end of the mesaxon closest to the axon, and an outer mesaxon at the opposite end Cytoplasm is “squeezed out” from between successive turns of the mesaxon, forming the major dense lines of the myelin sheath Node of Ranvier Is the gap between two Schwann cells (PNS) or oligodendrocytes (CNS) that myelinate adjacent segments along the axon The myelinated portion of the axon between sequential nodes is called an internodal segment or internode MICROSCOPIC ANATOMY Clefts of Schmidt-Lanterman By LM they resemble pale-staining arrowheads in myelin sheaths sectioned longitudinally Are cytoplasmic tunnels that spiral through the myelin sheath of one Schwann cell (not gaps between 2 adjacent Schwann cells) Form where cytoplasmic faces of Schwann cell plasma membrane have not fused, i.e. they contain Schwann cell cytoplasm They connect the Schwann cell cytoplasm that lies exterior to the myelin sheath (which also contains the nucleus) with the Schwann cell cytoplasm that lies interior to the myelin sheath, adjacent to the axon Are a mechanism for keeping the inner collar of Schwann cell cytoplasm alive MIXED NERVES: A single peripheral nerve is often mixed, i.e., contains myelinated & unmyelinated axons MYELINATION IN THE CNS: Oligodendrocytes (not Schwann cells) make myelin Each oligodendrocyte can contribute to the myelination of several different axons Unmyelinated axons are not associated with oligodendrocytes at all CONNECTIVE TISSUE COVERINGS OF NERVES Epineurium: Binds the nerve fascicles together to form a peripheral nerve, and covers the outer surface of the nerve Dense connective tissue; many collagen fibers Also has areas of adipose tissue Blood vessels travel in the epineurium Perineurium: One or more layers of contractile, squamous cells that surround a fascicle of nerve fibers Cells are joined together by tight junctions - forms a barrier that regulates ionic environment for optimal transmission of action potential Endoneurium: Delicate reticular fibers between individual nerve fibers MICROSCOPIC ANATOMY SYNAPSES: Site of impulse transmission from one neuron to another neuron, or to an effector cell (muscle or gland cell) Several types of synapses: Chemical (uses neurotransmitters) Electrical (gap junctions): less common than chemical FEATURES OF A CHEMICAL SYNAPSE BETWEEN NEURONS: Presynaptic terminal has: Synaptic vesicles containing neurotransmitter May be spherical or elliptical, clear or dense-cored - ACh-containing vesicles tend to be clear and round - Adrenergic vesicles tend to be dense-cored and round Numerous mitochondria Presynaptic density is often present on cytoplasmic side of membrane Synaptic cleft (synaptic gap) is narrow, with no elaborate folds of presynaptic or postsynaptic membranes Postsynaptic membrane has: Postsynaptic density on cytoplasmic side of membrane Includes receptors for neurotransmitters SENSORY RECEPTORS Free nerve endings Found in skin, in connective tissue & around hair follicles Respond to touch, temperature or pain Expanded tip endings (e.g., the Merkel disk in a Merkel corpuscle) Encapsulated endings include: Meissner’s corpuscles Pacinian corpuscles Muscle spindles Golgi tendon organs Ruffini corpuscles – slowly-adapting receptors (not rapidly adapting as Ross says on p. 503); capsule encloses collagen fibers; respond to sustained pressure; found in deep dermis, joint ligaments, & joint capsules; respond to skin stretch or change in joint position Krause end bulbs MICROSCOPIC ANATOMY MERKEL CORPUSCLES (= Merkel Cell + Merkel Disk) Found in stratum basale of skin Slowly adapting mechanoreceptors (i.e., nerve impulses fire throughout duration of stimulus; good for detecting steady pressure) Consist of modified epidermal cells that are closely associated with the expanded tip of a nerve ending Cells have dense secretory granules localized near the nerve ending MEISSNER’S CORPUSCLES: Found in dermal papillae immediately beneath epidermis Found in hairless (glabrous) skin, especially fingertips, toes, lips Elliptical; oriented perpendicular to skin’s surface Capsule composed of flattened Schwann cells Arranged in layers that are stacked parallel to skin surface Axons enter the deep pole of the corpuscle & lose myelin sheath Axons follow a spiral path toward apical pole of corpuscle Rapidly adapting mechanoreceptors (i.e., nerve impulses fire only at beginning & end of a stimulus; good for detecting changes in stimulation) PACINIAN CORPUSCLES: Found for example in deep dermis, hypodermis, joints & internal organs Much larger than Meissner’s corpuscles Have an onion-like appearance Myelinated axon enters capsule and then loses myelin sheath Axon remains within the tubular inner core formed by Schwann cells Multiple layers of flattened fibroblastic cells form the outer core Layers of the outer core are separated from one another by fluid-filled spaces Rapidly-adapting mechanoreceptors that respond to mechanical deformation & especially to vibration MUSCLE SPINDLES: Found within skeletal muscle Arranged in parallel with the extrafusal muscle cells Contain 2 types of intrafusal fibers (also called spindle cells) Nuclear bag fibers – have a cluster of nuclei in central part of cell Nuclear chain fibers – have a row of nuclei in central part of cell Surrounded by an internal capsule, then a fluid-filled space, & then an external capsule MICROSCOPIC ANATOMY Intrafusal fibers are modified skeletal muscle fibers Are much smaller in diameter than extrafusal Respond to passive stretch of the muscle Are innervated by sensory neurons Stretching the intrafusal fibers causes extrafusal fibers of the same muscle to contract via a spinal reflex (the stretch reflex) Prevents damage to the muscle from stretching Is an important postural mechanism for maintaining constant muscle length NOTE: Ross says on p. 326 that stretching the intrafusal fibers INHIBITS contraction of the muscle. That is wrong. Intrafusal fibers are also innervated by efferent motor fibers that cause intrafusal fibers to contract Is a mechanism for adjusting the sensitivity of the spindle The more contracted the intrafusal fibers are when stretch is applied, the less stretch it will take for them to trigger an afferent nerve impulse GOLGI TENDON ORGAN Found at musculotendinous junctions Contains thick collagen fibers rather than intrafusal muscle fibers Surrounded by a thin connective tissue capsule Collagen fibers are continuous at one end with the tendon & at the other with the skeletal muscle cells (i.e., are in series with skeletal muscle fibers rather than in parallel) Innervated by a sensory axon that loses it myelin sheath, branches, & wraps around the collagen fibers No motor innervation since collagen fibers can’t contract Responds to excessive muscle tension by inhibiting the contraction of the muscle with which it is associated III. ENDOCRINE ORGANS: A. HYPOPHYSIS (PITUITARY) Located in the hypophyseal fossa (part of the sella turcica) of the sphenoid bone Consists of: Adenohypophysis: Pars distalis Pars tuberalis Pars intermedia MICROSCOPIC ANATOMY Neurohypophysis: Infundibulum Pars nervosa ADENOHYPOPHYSIS: Epithelial in appearance (i.e., has closely packed cells) as opposed to the more fibrillar appearance of the neurohypophysis The parts of the adenohypophysis can be identified based on location: Pars tuberalis Surrounds the infundibulum to form the stalk May see some large veins that are part of a portal system Pars intermedia Lies between pars nervosa and pars distalis Contains many basophils (see below) Often contains cysts (Rathke’s cysts) filled with colloid Colloid is the term used for any PAS+ material that accumulates extracellularly in endocrine glands Pars distalis Forms part of the body of the pituitary (along with pars intermedia & pars nervosa) Contains more acidophils (see below) than pars intermedia Adenohypophysis is a derivative of an outpocketing of the oral ectoderm (Rathke’s pouch) Contains 3 classes of cells by routine LM: Acidophils, basophils & chromophobes Acidophils & basophils are collectively called chromophils Acidophils & basophils often difficult to distinguish in H&E Special trichromes make identification easier Acidophils & basophils produce hormones Secretory activity of acidophils and basophils is regulated by: Releasing or inhibiting hormones produced in the hypothalamus Negative feedback by hormones of the target organs ACIDOPHILS Two types: Somatotropes: produce growth hormone (GH, somatotropin) Mammotropes: produce prolactin Mnemonic for cell types: SAM – Somatotropes are Acidophils & so are Mammotrophs Mnemonic for hormones: GPA – Growth hormone & Prolactin are produced by Acidophils MICROSCOPIC ANATOMY (NOTE: Somatotropes & mammotropes can be distinguished from one another using EM. It is not necessary for you to be able to do this.) BASOPHILS At least three types: Gonadotropes: produce follicle stimulating hormone (FSH) or luteinizing hormone (LH) Corticotropes: produce adrenocorticotropin (ACTH) & other derivatives of pro-opiomelanocortin (POMC) Thyrotropes: produce thyroid-stimulating hormone (TSH) Mnemonic for hormones: B-FLAT: Basophils produce FSH, LH, ACTH, TSH Melanotropes usually considered to be a fourth type of basophil Related to corticotropes Produce melanocyte-stimulating hormone (MSH) - MSH is also a POMC derivative - involved in pigmentation in lower vertebrates - function and importance in humans is uncertain (NOTE: The various types of basophils can be distinguished from one another using EM. It is not necessary for you to be able to do this.) CHROMOPHOBES Cytoplasm stains poorly May be undifferentiated cells or degranulated chromophils HOW THE ADENOHYPOPHYSIS WORKS Hypothalamic neurons (in the paraventricular nucleus) produce releasing or inhibiting hormones (RH & IH) that stimulate or inhibit hormone secretion by specific acidophils or basophils in the adenohypophysis The hypothalamic neurons secrete the RH & IH into a venous portal system (the hypothalamic-hypophyseal portal system) that carries them to the adenohypophysis The portal system consists of: A primary capillary plexus Located in the floor of the hypothalamus & the infundibular stalk RH & IH are secreted into this plexus Hypophyseal portal veins Located in infundibular stalk (especially pars tuberalis) Connect primary and secondary capillary plexuses MICROSCOPIC ANATOMY Secondary capillary plexus Located mainly in pars distalis; also pars intermedia Hypophyseal portal veins deliver RH & IH to this plexus RH & IH stimulate or inhibit secretion from acidophils and basophils Secondary plexus picks up the hormonal products of the acidophils & basophils and feeds into veins that carry the hormones out of the pituitary Both plexuses have fenestrated capillaries (those in the secondary plexus are sometimes called fenestrated sinusoids since the vessels are wider and more irregular than ordinary capillaries) NEUROHYPOPHYSIS Is the site of release (NOT synthesis) of oxytocin & ADH (vasopressin), which are synthesized in the hypothalamus Is of neuroectodermal origin (downgrowth of floor of 3rd ventricle) Contains: Pituicytes: - are glial cells - most nuclei in pars nervosa belong to pituicytes Nonmyelinated axons of neurons whose cell bodies are in the hypothalamus (magnocellular system) Herring bodies - poorly visible by LM - are dilations along an axon where neurosecretory granules temporarily accumulate - the granules contain oxytocin or vasopressin, (ADH) plus neurophysins (hormone-binding proteins) HOW THE NEUROHYPOPHYSIS WORKS: Hypothalamic neurons in the magnocellular system synthesize oxytocin or ADH The hormones are transported down the axons of the hypothalamic neurons into the pars nervosa - they are not transported to pars nervosa via blood vessels The axon terminals release oxytocin & ADH into the fenestrated capillaries of the pars nervosa Oxytocin & ADH leave the pituitary in veins and travel to their specific target organs throughout the body MICROSCOPIC ANATOMY B. THYROID GLAND Derived from the thyroglossal duct that originated in the tongue Contains two endocrine systems: Follicular cells that produce T3 & T4 Parafollicular cells that produce calcitonin THYROID FOLLICLE Is the structural & functional unit of the thyroid Is a spherical structure with a single layer of follicular cells surrounding a lumen filled with colloid Main component of colloid in the thyroid is thyroglobulin (TG) TG is the inactive storage form of the thyroid hormones Thyroid is unique among endocrine glands because it stores most of its secretory product extracellularly instead of in vacuoles in the cytoplasm Extensive network of fenestrated capillaries surrounds each follicle Follicular cell height increases from squamous to columnar as TSH stimulation increases Follicular epithelium also includes occasional parafollicular cells (see below) THYROID FOLLICULAR CELLS Are morphologically polarized cells with obvious basal & apical ends Have abundant RER in basal cytoplasm Is the site of TG synthesis RER cisternae of follicular cells are characteristically dilated & irregular in shape Cells are joined together by tight junctions Apical plasma membrane has short sparse microvilli Small, clear secretory vesicles in apical cytoplasm Contain newly synthesized TG about to be released into follicular lumen Larger PAS-positive phagocytic vacuoles called colloid droplets Contain colloid recently taken up from the lumen TSH stimulation causes: - increase in surface projections of apical membrane - formation of increased number of colloid droplets - extensive phagocytosis can cause “scalloping” of colloid MICROSCOPIC ANATOMY Have many lysosomes Concentrated at basal end of less active follicular cells Under TSH stimulation they migrate toward apical end of cell They fuse with colloid droplets to form secondary lysosomes where TG is degraded to release T3 & T4 Have a well-developed Golgi is involved in: Glycosylation & packaging of TG Production of lysosomes PARAFOLLICULAR CELLS (C CELLS) Rare in human thyroid Cytoplasm typically appears pale or clear (hence C cells) Derived from neural crest cells that form the ultimobranchial body associated with the 5th pharyngeal pouch of the fetus Most are part of the follicular epithelium Rest on basement membrane but do not reach follicular lumen By EM, have small secretory granules in basal cytoplasm Secrete calcitonin in response to high blood calcium Calcitonin lowers blood calcium levels by: Stimulating calcium uptake into cells Inhibiting osteoclasts, thus decreases calcium release from bone Calcitonin is not essential for life, but it protects lactating mothers from excessive bone loss by inhibiting osteoclasts Calcium for milk must then come from other sources such as diet or reabsorption from the glomerular filtrate C. PARATHYROID GLAND Embedded in capsule of thyroid gland; also has its own thin capsule In young parathyroid, cells are arranged like beads on a string Contains: Principal cells (chief cells) Oxyphils (less common in young parathyroid) Adipocytes (increase in number with age) CHIEF CELLS Most numerous cell type Small polygonal cells with round centrally placed nucleus Cells sometimes contain large amounts of glycogen Extraction of glycogen makes cytoplasm pale-staining MICROSCOPIC ANATOMY In young parathyroids the cells are arranged in linear swirls (“beads on a string”) EM: Has small secretory granules that contain parathyroid hormone (PTH) PTH increases blood calcium levels by increasing: Bone resorption (stimulates osteoclast activity indirectly) Calcium reabsorption (and phosphate excretion) by kidney Calcium absorption by intestine OXYPHILS Larger than chief cells Very eosinophilic due to many mitochondria Nuclei often smaller & darker staining by LM than those of chief cells Oxyphils often occur in clusters Number increases with age, but they never outnumber chief cells Function unknown HOW TO DISTINGUISH PARATHYROID FROM PARS DISTALIS OF HYPOPHYSIS The eosinophilic cell is the major cell type in the pars distalis (acidophils), & the minor cell type in the parathyroid (oxyphils) Adipocytes are rare in pars distalis at any age, but common in older parathyroids Oxyphils often occur in clusters, while acidophils are more evenly distributed among the basophils & chromophobes Less difference in average cell size among chromophils & chromophobes of the adenohypophysis than in parathyroid where oxyphils are usually significantly larger than chief cells D. ADRENAL GLANDS (SUPRARENAL GLANDS) Divided into cortex & medulla Both contain fenestrated sinusoidal capillaries (sinusoids) Medulla also characteristically contains some very large veins 3 ZONES OF ADRENAL CORTEX: ZONA GLOMERULOSA Lies just deep to capsule Small closely packed cells arranged mainly in clusters (glomeruli) Produce mineralocorticoids: (e.g., aldosterone) that are important in maintenance of electrolyte balance MICROSCOPIC ANATOMY Secretion is stimulated by angiotensin II (produced via the renin-angiotensin system in response to a fall in blood pressure, low blood sodium or increased plasma K+) ZONA FASCICULATA Largest & palest-staining cortical region Cells arranged in straight cords (fascicles) that are 1 or 2 cells thick Sinusoidal capillaries lie between the cords Cords run perpendicular to surface of gland Extraction of lipid during tissue processing makes cytoplasm pale-staining Cells sometimes called spongiocytes because extracted lipid droplets give cytoplasm a spongy or lacy appearance Produce glucocorticoids: (e.g., cortisol and corticosterone) & small amount of adrenal androgens Secretion is regulated by ACTH ZONA RETICULARIS Cells arranged in irregular anastomosing network (hence reticularis) rather than straight cords Fewer lipid droplets than fasciculata, therefore cytoplasm stains darker Cells are smaller than in fasciculata Cytoplasm often contains lipofuscin pigment Secrete mainly weak androgens (mostly dehydroepiandrosterone) Secretion controlled by ACTH CORTEX BY EM: Zona glomerulosa has unusual steroid-secreting cells because they have mitochondria with shelf-like cristae (not tubular) Zona fasciculata & zona reticularis cells have the 3 features typical of steroid-secretors: Extensive SER Many lipid droplets in cytoplasm Mitochondria with tubular cristae Zona reticularis cells have more residual bodies (which contain the lipofuscin) ADRENAL MEDULLA Composed mainly of chromaffin cells Contains large veins that unite to form the adrenal vein True postganglionic neurons are present but rare Probably innervate vascular smooth muscle MICROSCOPIC ANATOMY MEDULLA RECEIVES A DUAL BLOOD SUPPLY: Blood from cortical sinusoids (rich in glucocorticoids) Blood from medullary arterioles that pass through cortex without breaking up into sinusoids (low glucocorticoid concentration) Glucocorticoids induce the synthesis of the enzyme PNMT in chromaffin cells PNMT catalyzes conversion of norepinephrine (NE) to epinephrine (E) Therefore chromaffin cells that receive blood from: Cortical sinusoids (high glucocorticoid concentration) make E Medullary arterioles (low glucocorticoids) make NE CHROMAFFIN CELLS Called chromaffin because they stain with chromium salts Derived from neuroectoderm (neural crest) Innervated by preganglionic sympathetic neurons Are essentially modified postganglionic sympathetic neurons that lack axons & dendrites By EM contain many small, dark-staining secretory granules containing either NE or E NE-producing & E-producing chromaffin cells are distinguishable by EM based on morphology of their secretory granules: NE-producing cells have larger, darker secretory granules that often contain an eccentrically located dense core E-producing cells have less intensely staining granules whose contents are more centered in the granule FETAL ADRENAL Has no well-defined medulla Precursors of chromaffin cells are scattered in clusters throughout the fetal gland At first, the fetal cortex resembles zona fasciculata Permanent cortex appears outside fetal cortex At first resembles zona glomerulosa in cell arrangement Fetal cortex regresses a few weeks after birth Chromaffin cells coalesce to form the medulla MICROSCOPIC ANATOMY IV. CARTILAGE Is classified as a specialized connective tissue Is usually avascular Provides support & allows rapid growth Forms the fetal skeleton Persists wherever great resiliency is needed, e.g., articular (joint) surfaces, ear, Eustachian tube, nasal cartilages, larynx CARTILAGE IS COMPOSED OF Chondrocytes in lacunae In growing cartilage the chondrocytes divide to form isogenous groups Matrix Perichondrium (absent in fibrocartilage & articular hyaline cartilage) PERICHONDRIUM Nutrients diffuse from blood, through perichondrium, into the matrix Divided into 2 layers: Fibrous layer is outermost - contains fibroblasts, collagen fibers (mainly type I) Chondrogenic layer is in contact with cartilage matrix - contains stem cells that can differentiate into chondroblasts - cells are called chondrocytes when completely surrounded by matrix Chondrogenic layer is only clearly identifiable by LM when appositional growth is actively occurring (otherwise the chondrogenic cells are flat like fibroblasts) MATRIX Territorial matrix (= capsular or pericellular matrix) Immediately surrounds each lacuna Interterritorial matrix = the remainder of the matrix Staining affinity of the matrix: Territorial matrix contains relatively more GAGs & proteoglycans Interterritorial contains relatively more collagen If GAGs have been preserved, territorial matrix is more basophilic & more strongly PAS positive If GAGs have been lost during tissue preparation, all parts of matrix are eosinophilic due to collagen MICROSCOPIC ANATOMY Matrix contains: Collagen - type II in hyaline & elastic cartilage - mainly type I in fibrocartilage GAGs, proteoglycans, & proteoglycan aggregates Smaller “adhesive” glycoproteins, e.g. chondronectin (promotes adherence of chondrocytes to matrix collagen) Elastic fibers (in elastic cartilage only) CHONDROCYTES Defining characteristic: Are completely surrounded by matrix Round to oval cells that often shrink during fixation Produce most components of the matrix Receive poor oxygen supply Limits the thickness of a cartilage Makes cartilage slow to heal if damaged Often accumulate lipid droplets in cytoplasm as they age CARTILAGE GROWS IN TWO WAYS: INTERSTITIAL GROWTH Defined as addition of new molecules to the matrix by chondrocytes in the interior of the cartilage Often involves mitosis of chondrocytes to form isogenous groups New matrix is then laid down between cells, which gradually pushes cells of isogenous group further apart APPOSITIONAL GROWTH Stem cells in chondrogenic layer of perichondrium differentiate into chondroblasts and add new matrix to the outer surface of the cartilage 3 TYPES OF CARTILAGE (HYALINE, ELASTIC & FIBROCARTILAGE) CAN BE DISTINGUISHED BY: Appearance of matrix (homogeneous & glassy vs. visible fibers) Presence or absence of perichondrium Location in body MICROSCOPIC ANATOMY HYALINE CARTILAGE Most common type Matrix looks homogeneous or glassy (= hyaline) because collagen type II fibrils are so thin they are not visible by LM Has a perichondrium (except for articular cartilage) May calcify with age Adult locations: Articular cartilage Costal cartilages (of ribs) Most laryngeal cartilages Tracheal rings & bronchi (as irregular cartilage plates) Other locations: Fetal skeleton Epiphyseal plates ELASTIC CARTILAGE: More cellular than mature hyaline cartilage Less likely to calcify Matrix contains elastic fibers visible by LM as well as collagen type II Has a perichondrium Found in locations such as (think of the letter E): Auricle (pinna) of the ear Cartilaginous part of external auditory canal Wall of auditory (Eustachian) tubes Epiglottis Corniculate & cuneiform cartilages of larynx FIBROCARTILAGE: Interterritorial matrix is highly fibrous Contains many thick collagen I fibers visible by LM Territorial matrix is more homogeneous Contains some type II collagen Chondrocytes often form linear isogenous groups between fibers Can be confused with fibroblasts in dense regular CT, but chondrocytes are rounder, less flattened cells No perichondrium Found in: Annulus fibrosus of the intervertebral disks Symphysis pubis Some bone-ligament or bone-tendon junctions MICROSCOPIC ANATOMY THE MATRIX DISTINGUISHES THE 3 TYPES OF CARTILAGE BY EM Fibrocartilage contains bundles of banded collagen type I fibrils Elastic cartilage contains elastic fibers Hyaline cartilage contains only a fine feltwork of thin collagen type II fibrils V. BONE FUNCTIONS OF BONES: Support & protect fragile tissues & organs (e.g. brain, spinal cord) Contain hematopoietic tissue in marrow cavities Form system of levers and pulleys with muscles that makes movement possible Store calcium BONE TISSUE VS. THE ORGANS CALLED BONES: Bone tissue is a specialized connective tissue composed of: Bone cells: Osteoprogenitor cells, osteoblasts, bone-lining cells, osteocytes, osteoclasts Bone matrix, which consists of: Organic components: Collagen type I, GAGs, proteoglycans, & glycoproteins such as osteonectin, osteocalcin & osteopontin Inorganic components: Minerals, mainly calcium salts such as hydroxyapatite NOTE: Even "decalcified" bone tissue will usually still contain some calcium salts that are visible in EMs as black crystals. They help distinguish bone tissue from cartilage by EM Bones, the organs contain: Bone tissue Connective tissue proper (in the periosteum) Usually some cartilage (epiphyseal plates &/or articular cartilage) Hematopoietic tissue (marrow cavity) Adipose tissue (marrow cavity, especially in yellow marrow) Nerve tissue Smooth muscle (in blood vessel walls) HOW TO DISTINGUISH BONE TISSUE VS. CARTILAGE Bone tissue always has: Canaliculi (best seen in ground bone preps rather than in decalcified sections); cartilage never has canaliculi MICROSCOPIC ANATOMY Some bone tissue has: Blood vessels in the matrix (compact bone only); rapidly growing cartilage sometimes surrounds blood vessels Lamellae (in lamellar bone) Cartilage commonly has: Isogenous groups, which bone never has DECALCIFIED VS. GROUND BONE Decalcified bone: Most of the inorganic components are removed Organic components (including cells) remain Can be embedded, sectioned, & stained with conventional histologic stains such as H&E Ground bone: Is mechanically ground down to make specimens so thin that they are translucent Organic components are los Inorganic components remain Usually stained with India ink, which fills empty spaces (canaliculi, lacunae, Haversian canals) 4 TYPES OF BONE CELLS: BONE CELL INTER-RELATIONSHIPS MICROSCOPIC ANATOMY Osteoprogenitor cells give rise to osteoblasts Osteoblasts can become bone-lining cells if they become inactive before being completely surrounded by bone matrix Bone-lining cells can become osteoblasts again if they start laying down bone matrix again Osteoblasts become osteocytes once they are completely surrounded by bone matrix The origin of osteoprogenitor cells depends on the type of ossification involved In intramembranous ossification, osteoprogenitor cells differentiate directly from mesenchymal cells In endochondral ossification, mesenchymal cells first give rise to chondrogenic cells (in the perichondrium), which differentiate into osteoprogenitor cells once endochondral ossification began Osteoclasts come from a completely different cell lineage; they differentiate from the stem cell (hematopoietic stem cell) that also gives rise to all blood cell types OSTEOPROGENITOR CELLS Found in two locations: Inner (osteogenic) layer of periosteum Endosteum Difficult to identify because they resemble the fibroblasts of the periosteum and the bone-lining cells of the endosteum OSTEOBLASTS Are uninucleate cells Cuboidal to columnar depending on their level of activity Basophilic cytoplasm due to extensive RER Found on any surface of bone (periosteum or endosteum) where matrix is being actively deposited May line up side by side so they resemble a simple epithelium Communicate with each other & with osteocytes via gap junctions Produce the organic components of bone matrix (osteoid) Release matrix vesicles into the matrix The alkaline phosphatase contained in matrix vesicles is important for mineralization When quiescent, osteoblasts become squamous cells called bonelining cells MICROSCOPIC ANATOMY OSTEOCYTES Each occupies a space within bone matrix called a lacuna Canaliculi (“little canals”) = tunnels that run through the matrix & connect neighboring lacunae Contain cytoplasmic processes from neighboring osteocytes Gap junctions connect the processes Osteocytes function to maintain bone matrix Probably help maintain blood Ca++ levels by resorbing matrix from a narrow zone surrounding the lacuna, thus releasing Ca++ into the blood (= osteocytic osteolysis) OSTEOCLASTS Large multinucleated cells (2-50 nuclei) Derived from uninucleate cells that fuse, i.e., a true syncytium Differentiate from the hematopoietic stem cell; share a common precursor with monocytes (i.e., do not differentiate from osteoprogenitor cells) Acidophilic cytoplasm due to numerous lysosomes & mitochondria Found on any surface of bone (periosteal or endosteal) where bone resorption is occurring They resorb bone matrix for modeling or remodeling purposes by secreting: - Lysosomal enzymes (e.g., collagenase) to digest organic matrix - Acid (HCl) to solubilize inorganic components of bone matrix Ruffled border = an area of plasma membrane with many long, elaborate folds; is a region where bone resorption is occurring Increases surface area for active transport of ions (e.g. H+) and for endocytosis of matrix degradation fragments The “clear zone” (sealing zone) forms a ring around the area where the ruffled membrane is located It seals the plasma membrane tightly to bone It contains abundant actin filaments Resorption produces a depression in the bone surface within the sealing zone (= a resorption bay or Howship's lacuna) PERIOSTEUM VS. ENDOSTEUM: Periosteum covers outer surface of a bone except at the articular surfaces Has an outer fibrous layer and an inner osteogenic layer MICROSCOPIC ANATOMY Fibrous layer contains collagen fibers & fibroblasts Osteogenic layer contains osteoprogenitor cells - can also contain osteoblasts, bone-lining cells & osteoclasts Sharpey’s fibers = bundles of collagen fibers that anchor: Periosteum to bone Tendons, ligaments, or teeth to bone by penetrating through the periosteum & into bone matrix Endosteum lines most inner surfaces of bone It lines Haversian canals, Volkmann’s canals, & the marrow cavity (including inner surface of compact bone & the outer surface of all trabeculae) It DOES NOT line lacunae or canaliculi Can include osteoprogenitor cells, osteoblasts, bonelining cells & osteoclasts BONE TISSUE CAN BE CLASSIFIED IN TWO WAYS: 1. CANCELLOUS (SPONGY) VS. CORTICAL (COMPACT) BONE: Describes the amount of bone tissue per unit volume The distinction is visible macroscopically with the unaided eye Cancellous (spongy) bone: Has thin anastomosing trabeculae of bone surrounded by interconnected spaces that are part of the marrow cavity Has a “honey-comb” appearance Trabeculae contain no blood vessels (are thin enough that osteocytes within them can be nourished by diffusion from vessels in the marrow cavity) Cancellous bone is found in interior of bones, especially: At epiphyses & metaphyses of long bones In the diploë of flat bones Cortical (compact) bone: Has a more solid appearance with the naked eye; no trabeculae Found on outer surface of all bones Contains blood vessels that often run within canals (e.g., Haversian canals, Volkmann’s canals) NOTE: No individual bone is entirely cortical or entirely cancellous. All mature bones have cortical bone on their exterior & at least some cancellous bone somewhere within their interior. MICROSCOPIC ANATOMY 2. WOVEN VS. LAMELLAR BONE: Describes whether the collagen fibers and lacunae are arranged in an orderly fashion (lamellar) or a disorderly fashion (woven) in the matrix Woven bone Characterized by random orientation of collagen fibers, creating an irregular or “woven” appearance Is more cellular than lamellar bone Lacunae are distributed & oriented more randomly than in lamellar bone Formed before lamellar bone in embryonic development & in repair of fractures Laid down more rapidly than lamellar bone Normally replaced by lamellar bone during remodeling Less mineralized than lamellar bone Can be cortical or cancellous Lamellar bone Characterized by layers (lamellae) of bone matrix Collagen fibers within an individual lamella are oriented in the same direction Fibers in one lamella are oriented at an angle to those in the neighboring lamellae (“plywood” arrangement for added strength) Osteocytes lie in the plane between successive lamellae & hence have an orderly distribution & orientation Can be cortical or cancellous LAMELLAR BONE IS FOUND IN AT LEAST 5 LOCATIONS: 1. Outer circumferential lamellae These encircle the outer surface of the entire bone just deep to the periosteum 2. Inner circumferential lamellae Encircle the inner surface of compact bone just superficial to the endosteum & the marrow cavity May not be present in areas where cancellous bone lines the marrow cavity (e.g., near metaphyses or epiphyses) 3. Concentric lamellae Make up the osteons surrounding Haversian canals 4. Interstitial lamellae: Remnants of old lamellae that were partially removed by osteoclasts during remodeling 5. Trabeculae of mature cancellous bone MICROSCOPIC ANATOMY HAVERSIAN SYSTEMS (OSTEONS) INCLUDE: Haversian canal (= central canal) is a channel lined by endosteum Contains blood vessels & loose connective tissue Runs roughly parallel to the long axis of a long bone Osteocytes lie in lacunae that are layered between the lamellae Canaliculi that connect lacunae Cement lines that separate neighboring osteons Represent the point of reversal during remodeling (i.e., the point where osteoclasts stopped resorbing bone and osteoblasts began depositing new bone) VOLKMANN'S CANALS: Endosteum-lined channels that interconnect Haversian canals Carry blood vessels from periosteum and marrow cavity to Haversian canals Oriented transverse or oblique to the Haversian canals Cut across the concentric lamellae of osteons NOTE: Blood also enters bones through much larger arteries than those in Volkmann’s canals. These are called nutrient arteries. They pass through a nutrient foramen in the bone, and then branch in the marrow cavity. INTRAMEMBRANOUS VS. ENDOCHONDRAL OSSIFICATION: Intramembranous ossification: Bone forms directly in mesenchyme (no cartilaginous precursor) Mesenchymal cells differentiate into osteoprogenitor cells & then into osteoblasts Osteoblasts lay down bone matrix to form many isolated spicules Spicules gradually connect with one another to form trabeculae Connective tissue surrounding this region condenses to form periosteum Trabeculae near the periosteum grow & fuse together to form compact bone Endochondral ossification of a long bone: A hyaline cartilage model of the bone forms from mesenchyme in the fetus MICROSCOPIC ANATOMY Primary center of ossification forms at midshaft - chondrocytes at midshaft hypertrophy - perichondrium at midshaft becomes a periosteum; osteoblasts differentiate from it & lay down a periosteal collar of bone - chondrocyte lacunae coalesce, leaving thin irregular spicules - matrix of cartilage becomes calcified - an osteogenic bud grows into the coalesced lacunae - bud consists of blood vessels & osteoprogenitor cells that adhere to the exterior of the vessels - osteoblasts differentiate from osteoprogenitor cells in the osteogenic bud - osteoblasts lay down bone matrix on the spicules of calcified cartilage, forming mixed spicules NOTE: Areas of calcified cartilage in a spicule contain no cells since the chondrocytes have already died, & few, if any, lacunae since most have already coalesced to form the space (marrow cavity) between spicules Secondary centers of ossification form in each epiphysis - not associated with a periosteal bone collar An epiphyseal plate (growth plate) of hyaline cartilage remains between the primary ossification center & each secondary ossification center Interstitial growth continues in the cartilage of the epiphyseal plate, resulting in increase in length of the bone Bone forms mainly on diaphyseal side of growth plate, replacing cartilage When rate of ossification exceeds the rate of interstitial growth of the epiphyseal cartilage, the epiphyseal plate becomes completely replaced by bone (closure of the epiphyses) After epiphyseal closure, the bone can no longer grow in length Remodeling eventually removes the calcified cartilage in the spicules & replaces woven bone with lamellar MICROSCOPIC ANATOMY EPIPHYSEAL PLATE: You are responsible for identifying the following zones: Zone of reserved cartilage (Resting zone): - characterized by individual cells in randomly scattered lacunae or by small isogenous groups (clusters) Zone of proliferation: - dividing chondrocytes line up in isogenous columns Zone of hypertrophy: - chondrocytes enlarge, accumulate glycogen, & die Zone of calcification - matrix becomes calcified as the chondrocytes die - lacunae in each isogenous column coalesce, leaving only thin strips of calcified cartilage matrix (i.e., the spicules) Zone of ossification: - osteoblasts lay down the organic matrix of bone (eosinophilic) on the surface of calcified cartilage spicules (more basophilic) BONE GROWTH Bone tissue grows only by appositional growth, i.e., by osteoblasts adding new matrix to the periosteal or endosteal surface of existing bone tissue A bone (the organ) grows in width by appositional growth, i.e. osteoblasts add new matrix to the periosteal surface An immature long bone (the organ) grows in length by interstitial growth of the cartilage in the epiphyseal plate Osteoclasts resorb bone matrix in bone modeling & remodeling Surface modeling = adding (via osteoblasts) or removing (via osteoclasts) matrix in order to affect the overall shape of the bone Examples: Metaphyseal “waisting” (metaphyseal cut back) - narrowing the former metaphysis down to match the diameter of the diaphysis as the bone grows in length - requires periosteal bone resorption and endosteal deposition Diaphyseal drift - maintaining the same diaphyseal diameter while adjusting the position of the diaphysis in space - usually done to accommodate changes in mechanical load on the bone MICROSCOPIC ANATOMY - requires periosteal resorption coupled with endosteal deposition on one side of the diaphysis, and periosteal deposition coupled with endosteal resorption on the other Remodeling = a turnover process where an area of bone matrix is removed and then new bone is laid down in the same place without overall change in bone shape An example is the conversion of woven compact bone to lamellar compact bone SYNOVIAL JOINTS Hyaline cartilage covers the articular surfaces of the bones Articular cartilage has no perichondrium & is not covered by the synovial membrane The rest of the synovial cavity is covered by the synovial membrane, forming the synovial fold Synovial fold is one of the few body surfaces not covered by an epithelium Synovial membrane is composed of 2 types of non-epithelial cells: Type A synoviocytes (phagocytic) Type B synoviocytes (fibroblastic), which appear to make some components of synovial fluid IMPORTANT INFORMATION FOR ANSWERING PRACTICAL EXAM QUESTIONS I. “Identify” vs. “classify” You will undoubtedly be asked to classify some tissue types in this exam. Recall that: To identify means to give the unique name of that structure To classify means to place the structure into a grouping that includes other similar structures. To classify muscle tissue means to say whether it is: Skeletal muscle Smooth muscle Cardiac muscle To classify cartilage means to say whether it is: Hyaline cartilage Elastic cartilage Fibrocartilage MICROSCOPIC ANATOMY Bone tissue can be classified in at least two ways: Woven vs. lamellar Compact vs. cancellous NOTE: We may use a multiple-choice format and ask you to decide if a particular tissue is: Woven compact bone Woven cancellous bone Lamellar compact bone Lamellar cancellous bone Examples of classify vs. identify: If we point to the cartilage on the surface of a bone in a synovial joint and say: Classify this tissue: Answer = hyaline cartilage Identify this tissue: Answer = articular cartilage If we point to cartilage between the epiphysis & diaphysis of a growing long bone & say: Classify this tissue: Answer = hyaline cartilage Identify this tissue: Answer = epiphyseal plate II. Always be as specific as possible If we point to the region of the adrenal just beneath the capsule and ask you to identify the layer, “adrenal cortex” is not sufficiently specific. “Zona glomerulosa” would be correct. III. As always, answer the question that is asked IV. Use the correct terminology. We do not take off for spelling, but we also do not accept answers that are way out in left field. For example, if we asked you to identify canaliculi, we would not accept “cannelloni”, “cannoli” or “canola” (all answers that we have received in the past) as valid answers. Take the time to learn the correct terms.