Histology Lecture #2

provide protection to internal organs, provide attachment site to skeletal muscle, and for storage of Ca+2 and phosphate

• forms most bones in body including longs bones, and also used in healing bone fractures
• mesenchymal cells proliferate and differentiate into chondroblasts, which are responsible for producing a model of hyaline cartilage
• cartilage model then grows by both appositional and interstitial growth
• the cartilage model is then calcified, chondrocytes die off, perichondrium becomes periosteum, and osteoprogenitor cells begin to differentiate into osteoblasts
• the osteoblasts secrete osteoid until they become surrounded in lacunae, at which point they become osteocytes
• the first site of ossification is called the primary ossification center

• from distal to proximal:
• zone of reserve cartilage
• zone of proliferating chondrocytes
• zone of chondrocyte hypertrophy and calcification of cartilage
• zone of ossification

• in appositional growth, the cartilage model grows in width when more extracellular matrix is produced by chondroblasts in the perichondrium
• in interstitial growth, the cartilage model grows in length due to continuous cell division of chondrocytes accompanied by further secretion of extracellular matrix

• forms the mandible, clavicles, and flat bones of the body
• bone development not preceded by a hyaline cartilage model, instead bone develops from condensation of CT mesenchyme to form an ossification center
• mesenchyme cells differentiate directly into osteoblasts that produce the bone matrix, called osteoid
• the bone matrix becomes calcified, osteoblasts become osteocytes within lacunae, and the process repeats resulting in production of layers of bone

• forms the outer shell (cortex) of most bones
• denser, harder, and stiffer than cancellous bone
• collagen fibers in the bone matrix are arranged in thin layers called lamellae
• functional unit is the osteon

• found at the interior of long bones adjacent to the marrow cavity, and also at the ends of long bones
• spongy bone has a less dense network of collagen fibers than compact bone, making it softer and more flexible
• made up of a number of trabecular (cancellous) bars separated by CT with cavities of bone marrow inbetween

in between the bony trabeculae are marrow cavities that contain hemopoietic tissue that give rise to new blood cells

• concentric layers of compact bone tissue that surround the central/Haversian canal in an osteon
• interstitial lamellae are found between osteons

• small canals between lacunae of ossified bone
• radiating processes of osteocytes project into these canals
• these canals facilitate the exchange of nutrients and wastes between osteocytes in lacunae

canal at the center of an osteon that houses blood vessels and nerve cells

• canals that run perpendicular to Haversian canals
• they join blood vessels from the periosteum to the many Haversian canals of compact bone
• supplies the organic components of the bone matrix

• the periosteum is a dense irregular CT membrane that covers the outer surface of all bones (except joints of lone bones)
• two layers are inner layer (AKA osteogenic layer, site of osteoprogenitor cells) and outer fibrous layer (site of fibroblasts)

• the endosteum is a single layer of osteoprogenitor cells that line the inner surface of bone
• these cells give rise to bone-forming osteoblasts

• found in the periosteum, they are collagenous fibers that attach bone to the periosteum
• they also provide attachment for muscles and tendons

• undifferentiated, pluripotent cells derived from mesenchyme and found in the inner layer of periosteum and also the endosteum
• provides nutrition and continuous supply of osteoblasts for growth, repair, and remodeling of bone

• located on the surfaces of bone 
• morphologically flat and basophilic (purple/blue) cytoplasm

• synthesizes and secretes the unmineralized bone matrix (osteoid) that include collagen fibers, proteoglycans,and glycoproteins; also responsible for calcification of bone matrix
• osteoblasts have receptors for parathyroid hormone, which causes blood concentration of Ca+2 to increase through activation of osteoclasts via these osteoblasts

• located in shallow pits in bone where resorption, remodeling, or repair of bone is taking place
• osteoclasts are large, multinucleated cells that stain acidophilic (red/pink)

• osteoclasts are responsible for bone resorption, remodeling, or repair
• they also have receptors for calcitonin hormone, which acts to lower Ca+2 blood concentration levels

• osteocytes are smaller than their osteoblast precursors, reside in the lacunae of bone, and have many cytoplasmic extensions that extend into the canaliculi
• only one osteocyte per lacuna

• osteocytes are derived from osteoblasts that have become surrounded by osteoid that they have secreted
• they serve as the primary cells of the bone

• all muscle tissue consist of elongated cells called fibers
• muscle cell cytoplasm is called sarcoplasm
• muscle cell cell membrane is called sarcolemma

• skeletal muscle is voluntary striated muscle attached to bone
• morphologically long and cylindrical and are multinucleated (nuclei in periphery) due to fusion of numerous myoblasts
• functional unit is the sarcomere

• involuntary non-striated muscle located in the bladder, uterus, GI tract, and walls of blood vessels
• morphologically has a fusiform (spindle) shape, single nucleus, and CT between cells

• involuntary striated muscle found in the walls of the heart, aorta, and pulmonary trunk
• similar in appearance to skeletal muscle except for appearance of intercalated discs
• functional unit is the sarcomere

• intercalated discs are dark-staining structures that represent specialized cell junctions that join cardiac muscle fibers
• they are a distinguishing feature of cardiac muscle, and support synchronised contraction of cardiac tissue

• I band - actin between sarcomeres that is not superimposed by myosin
• A band - contains the entire length of a myosin filament; size of A band remains constant
• Z line/disc - marks the borders that separate each sarcomere
• H band - zone of myosin not superimposed by actin
• M band - narrow dark band in the center of the H band and bisects the A band; marks where adjacent myosin filaments are linked

• epimysium is dense irregular CT that surrounds entire skeletal muscles
• perimysium is less dense and thinner irregular CT that surrounds each fascicle
• endomysium made up of reticular CT fibers surround each muscle fiber

• skeletal muscle consists of bundles of fibers called fascicles, each fascicle consists of muscle fibers, each muscle fiber consists of myofibrils, each myofibril consists of myofilaments
• the two myofilaments are actin (thin) and myosin (thick)

• the meninges are CT that protect the central nervous system
• layers - dura mater, arachnoid mater, and pia mater

• located between the arachnoid mater and the pia mater
• is made up of web-like strands of collagen and elastic fibers
• filling the subarachnoid space is cerebrospinal fluid (CSF), which protects the CNS from shock and injury

• gray matter of CNS consists of unmyelinated neurons, axons, and neuroglia
• gray matter forms the outer surface of the cerebrum and cerebellum, and also the interior of spinal cord

• white matter in the CNS consists primarily of myelinated axons; is devoid of neuronal cell bodies
• also contains some unmyelinated axons, oligodendrocytes, and blood vessels
• white matter forms the deeper parts of the brain and exterior of the spinal cord
• white color is due to myelin sheaths, which are made up of fat

• neurons with one axon that divides close to the cell body into two long axonal branches
• examples - dorsal root ganglion neurons

• neuron with a single dendrite and a single axon
• examples - bipolar cell of retina, olfactory nerves

• neuron with two or more dendrites and a single axon
• examples - pyramidal neurons of the brain, Purkinje cells

• Nissl bodies are basophilic staining granules found in the dendrites and the soma of a neuron, but not the axon
• the granules correspond to stacks of rough ER

epineurium, perineurium, endoneurium

• dense irregular CT that surrounds entire nerve fiber
• composed of dense CT

• CT that surrounds each nerve fascicle
• composed of CT with squamous cells

• CT that surrounds each individual axon including myelin sheaths
• composed of type III collagen fibers

• layers from apical to basal are:
• molecular layer
• external granular layer
• external pyramidal layer
• internal granular layer
• internal pyramidal layer
• multiform layer

• pyramidal cells are neurons with triangular shaped somas, and are found in the cerebral cortex
• small pyramidal cells are located in the external granular layer, medium in the external pyramidal layer, and large in the internal pyramidal layer

• from apical to basal:
• molecular layer - site of stellate cells
• Purkinje cell layer - single layer of Purkinje cells
• granular layer - high density of granules

• Purkinje cells are located in the Purkinje cell layer of the cerebellum
• the cells are arranged in a single row and have a prominent nucleus and flask shaped cell body, with thick branching dendrites that extend into the molecular layer

Purkinje cells are a type of neuron that coordinate the body's motor function

• ganglion are accumulations of sensory and motor neurons covered by CT; found in the PNS
• dorsal root ganglion lie along the spine and consist mostly of unipolar sensory neurons

• satellite cells are a type of glial cell found on the surface of neurons and also ganglia of the PNS; they are also surrounded by capsule cells
• are the main glial cell of the PNS
• they have a flat shape with a spherical, large nucleus  

satellite cells provide structural support, and also insulate and regulate metabolic exchange of PNS neurons

• astrocytes are glial cells found in the gray matter of the brain and spinal cord
• they appear star-shaped with many processes

• astrocytes are involved in movement of blood-borne materials to the neuron
• they are also involved with sugar regulation in neurons

• microglia are found throughout the CNS and are part of the mononuclear phagocyte system (i.e. they form from monocytes)
• microglia act similar to macrophages
• they migrate to nervous tissue that is injured or damaged then become phagocytic and remove dead or foreign tissue

• Schwann cells produce myelin and myelinate axons in the PNS
• they can only form myelin sheaths around a single axon

• ependymal cells are simple cuboidal or columnar epithelial cells that line the ventricles of the brain and central canal of the spinal cord
• they produce CSF in brain and spinal cord, and have cilia and microvilli that facilitate movement of CSF

• oligodendrocytes produce myelin and myelinate axons in the CNS
• one oligodendrocyte has many cytoplasmic processes and thus can myelinate more than one axons 

• cardiovascular system
• lymphatic vascular system

• systemic circulation
• pulmonary circulation

carries blood from heart to all organs, tissues, and cells via arterial vessels and then back to heart via venous vessels

carries deoxygenated blood from heart to lungs for gas exchange then oxygenated blood back to heart for distribution via systemic circulation

• chambers - left and right atria, left and right ventricles
• valves - tricuspid valve betweem R atrium and R ventricle, bicuspid (mitral) valve between L atrium and L ventricle, semi-lunar pulmonary valve and semi-lunar aortic valve

epicardium, myocaridum, and endocardium

• epicaridum - single layer of mesothelial cells, underlying CT and adipose tissue
• myocardium - consists of cardiac muscle; thicker in the ventricles than atria
• endocardium - layer of endothelium and CT, site of conducting system of the heart

the pericardium is a double membrane that encloses the heart

• electrical impulses that contract cardiac muscle are generated at the sinoatrial node (AKA pacemaker of heart) located near right atrium
• impulse travels along muscle fibers of atria and internodal fibers to the atrioventricular node
• impulse then travels to bundle of His and finally into Purkinje fibers

• these fibers are found in the inner wall of the heart ventricles under the endocardium (subendocardium)
• Purkinje fibers are larger than cardiac muscle fibers, are lighter staining, and have fewer myofibrils

• arteries - elastic arteries, muscular (AKA medium) arteries, arterioles (AKA small arteries)
• veins - large, medium, small veins (AKA venules)

• are the largest blood vessels, walls are primarily composed of elastic CT fibers interspersed with circularly arranged smooth muscle cells
• examples - pulmonary trunk, aorta, subclavian artery

• elastic arteries branch and become medium-sized muscular arteries
• are the most numerous blood vessels
• their walls contain greater amounts of smooth muscle fibers and less elastic fibers than elastic arteries
• examples - splenic artery, radial artery

• are the smallest branches of arterial system
• walls consist of 1-5 layers of smooth muscle fibers
• they deliver blood to the capillaries

• small blood vessels that are a type of capillary
• has broken endothelial layer that increases permeability
• found in the spleen, liver, lymph nodes, and bone marrow

• the lymphatic system functions to drain extracellular/interstitial fluid called lymph from tissues
• lymph vessels are similar to veins in structure and how they move fluid, but are thinner than veins
• the lymph vessels collect the lymph, filter it through lymph nodes, then return it to the blood via lymphatic ducts into the subclavian veins

• tunica intima (or interna)
• tunica media
• tunica adventitia (or externa)

• innermost layer that faces the lumen of the artery or vein
• consists of simple squamous epithelium called endothelium
• underneath is a thin layer of subendothelial CT

composed primarily of smooth muscle fibers with interspersed elastic and reticular fibers

• composed primarily of longitudinally oriented collagen (type I) as well as elastic CT fibers
• the collagen stabilizes the blood vessel to anchoring it to nearby organs

• found between tunica intima and tunica media of elastic and muscular arteries; represents the most external layer of the tunica intima
• consists of layers of elastic sheets that contain numerous openings called fenestrations that facilitate rapid diffusion

• (above is arterial wall)

• veins can be small, medium, or large
• capillaries unite to form venules (small veins)
• veins have no elastic fibers so they can collapse and do not appear round like arteries
• the tunica media of veins is thinner, has less muscle, and is less developed than in arteries
• the tunica adventitia of veins 
• some small and medium veins have valves to flow blood against gravity

• small blood vessels that provide blood supply and nourishment to the cells in medium and large arteries and veins
• founds more in veins than arteries due to low oxygen content of venous blood

lymph nodes, tonsils, thymus, spleen, and bone marrow

• capsule of dense CT surrounds every lymph node
• trabeculae (dense CT) extends from capsule, inbetween nodules in the cortex, and into the medulla
• reticular tissue composed of reticular cells form a meshwork throughout the remainder of the organ
• the cortex has lymph nodules containing lymphocytes and a germinating center are found in the cortex
• afferent lymphatic vessels penetrate the capsule then enters the subcapsular sinus, then pass into the medullary sinus
• the hilus located on the concave side is where efferent vessels exit the lymph node

• consists of medullary cords and medullary sinuses
• medullary cords are networks of reticular fibers filled with plasma cells, macrophages, and lymphocytes (mostly B cells)
• medullary sinuses are capillary-like channels that separate the cords

• found in the cortex, they are a network of reticular fibers and aggregation of lymphocytes (mostly B cells)
• their lighter-staining central areas are called germinal centers, which are spongy frameworks that are the active sites of lymphocyte proliferation
• the germinating center contains mesenchyme cells, reticular cells, lymphoblasts, plasmablasts, and reticular fibers

• lymph nodes are important in immune defense; their major functions are lymph filtration and phagocytosis of foreign substances
• they also produce, store, and activate B and T lymphocytes

• located next to the stomach in the abdomen
• the spleen is the largest lymph organ and has a dense CT capsule with trabeculae
• the white pulp of the spleen are made up of lymphatic tissue, with prominent lymphatic nodules each with a central artery (which actually is not centrally located) and germinating center
• red pulp of the spleen contains large numbers of RBCs; red pulp consists of splenic sinuses separated by splenic cords

• splenic cords are found in the red pulp and contain networks of reticular fibers and also macrophages, monocytes, lymphocytes, plasma cells, and other blood cells
• another name for splenic cords is Billroth cords
• splenic sinuses are interconnected blood channels with capillaries that drain splenic blood into the splenic vein and eventually into the bloodstream

• the lymphocytes within the white pulp monitor the blood for antigens
• the spleen can activate and proliferate B and T lymphocytes in an immune response
• the red pulp of the spleen filters the blood, removing antigens and aged or abnormal RBCs; is also the site of RBC storage

• located above pericardium, the thymus is a lymphoepithelial organ consisting of two lobes attached by CT
• is surrounded by CT capsule and has trabaeculae that divides the orgain into lobules
• the cortex contains immature lymphocytes (mainly T cells) at germinating centers
• germinating centers calcify with age and forms thymic (or Hassall) corpuscles in the medulla

• the thymus is important for immune system development in early childhood
• specifically, it is the site of T cell development and maturation
• the thymus most active during childhood, until it becomes filled with adipose tissue (involution) at adulthood

• they are distinctive aggregations of reticular cells found in the medulla of the thymus
• come from germinating centers that have been calcified over time

• the palatine tonsil is located behind the tongue inside the oral cavity
• its surface is composed of stratified squamous epithelium, with CT underneath 
• the epithelium forms deep groves called crypts
• aggregates of lymph nodules are also present within the tonsil

• pars distalis, pars tuberalis (surrounds infundibulum), and pars intermedia

median eminence, infundibulum, and pars nervosa

• two main cell types: chromophobes and chromophils
• chromophobes are light-staining and smaller than chromophils

• two types of chromophils: acidophils and basophils
• two types of acidophils: somatotrophs and mammotrophs
• three types of basophils: gonadotrophs, thyrotrophs, and corticotrophs

• acidophils -
• somatotrophs secrete somatotropin AKA growth hormone
• mammotrophs secrete prolactin
• basophils -
• gonadotrophs secrete follicle stimulating hormone and luteinizing hormone
• thyrotrophs secrete thyroid-stimulating hormone
• corticotrophs secrete adenocorticotrophic hormone

produces melanocyte-stimulating hormone, which increases skin pigmentation

oxytocin and antidiuretic (vasopressin) hormone

• Herring bodies are terminal ends of unmyelinated axons in the neurohypophysis
• they serve as storage sites for the hormones ADH and oxytocin

• the adrenal glands sit on top of the kidneys 
• each are surrounded by dense irregular CT capsule; have a three-layered cortex and a medulla

• zona glomerulosa
• zona fasciculata
• zona reticularis

• cells of the zona glomerulosa produce aldosterone, which is used to influence fluid and electrolyte balance by increasing sodium reabsorption in the kidney
• cells of the zona fasciculata produce cortisol and cortisone, which regulate glucose metabolism in times of stress

• cells in the adrenal medulla are called chromaffin cells because they stain with chromium salts
• they produce the catecholamines epinephrine (adrenaline) and norepinephrine (noradrenaline), which are used in the fight or flight response
• epinephrine increases blood pressure and output; norepinephrine also increases blood pressure and heart rate

located in the adrenal medulla, chromaffin cells are large, multinucleated cells that produce and release epinephrine and norepinephrine

• consists of right and left lobes
• hormones are stored in unique spherical-shaped follicles that are surrounded by follicular cells; parafollicular cells are present in the follicular peripheries
• follicular cells synthesize and store hormone in the lumen of follicles as a substance called colloid

two cell types - follicular cells and parafollicular cells

• found in the thyroid, they appear as large, light-staining cells found in the periphery of follicles either as single cells or in clusters
• they secrete the hormone calcitonin, which reduces osteoclast activity and lowers blood Ca+2 levels

• found in the thyroid, they appear as simple cuboidal cells surrounding the follicles
• follicular cells produce thyroid hormones T3 and T4, which regulate cellular metabolism

• four parathyroid glands are found in mammals
• each are embedded on surface of thyroid, with a thin CT capsule separating them from the thyroid

two cell types - chief (or principal) cells and oxyphil cells

• found in the parathyroid, they appear round and have pale, slightly acidophilic cytoplasm
• are the most numerous cell in parathyroid
• they produce parathyroid hormone, which maintain proper Ca+2 and phosphate levels in the body

• found in the parathyroid, they appear larger than chief cells with a dark staining cytoplasm
• they appear as single cells or in small clumps, and are fewer in number than chief cells
• their function is unknown

• palatine, lingual, and pharyngeal tonsils
• tonsil surface is stratified squamous non-keratinized epithelium; surface is invaginated by deep groves called (tonsillar) crypts
• below surface is CT then stratified muscle
• each contain lymphatic nodules with germinating centers

• the pineal gland is attached to the hypothalamus at the center of the brain
• its production and secretion of the hormone melatonin is light sensitive
• melatonin regulates the body's circadian rhythms and also helps one fall asleep
• two cell types - secretory (AKA pinealocyte) cell and interstitial cell (which is similar to astrocytes)

• cells classified as apocrine bud their secretions off through the plasma membrane producing membrane-bound vesicles in the lumen
• example - mammary glands

• cells classified as holocrine release their secretion by rupturing of the cell's plasma membrane, resulting in destruction of the cell
• example - sebaceous glands

• cells classified as merocrine release their secretions through exocytosis and into a duct, and hence onto a bodily surface or into the lumen
• example - salivary glands

epidermis, dermis, hypodermis

• epidermis - composed of stratified squamous epithelium; can have 4-5 layers
• dermis - CT with lots of reticular fibers
• hypodermis - CT with lots of adipose cells

• stratum germinativum (AKA basale) - contains mitotically active cells (stem cells) and melanocytes
• stratum spinosum - prominent site of Langerhands cells
• stratum granulosum - only seen in thick skin
• stratum lucidum - only seen in thick skin
• stratum corneum - composed of ketainized cells

Malpighian layer is both the stratum germinativum and stratum spinosum

• the stratum disjunctum is the outermost layer of the stratum corneum 
• the cells of this layer are dead and only partially connected to the corneum

• this epidermal layer has cells that have lost all nuclei and organelles due to the enzymes of keratinization
• the cells appear flattened, are filled with keratin filaments, and are continually being shed

• this epidermal layer appears translucent and is barely visible
• the cells lack nuclei or organelles, are tightly packed, and contain densely packed keratin filaments

• this epidermal layer has 3-5 layers of more mature, flattened cells
• keratin is produced in this layer from tonofilaments and filaggrin protein (process called keratinization)
• the cytoplasm of the cells contain lamellar granules, which when secreted into the space between the stratum granulosum and lucidum, serve as lipid that forms an impermeable water barrier

• this epidermal layers comprises the second cell layer of keratinocytes, usually containing 4-6 rows of cells
• the synthesis of keratin filaments continue in this layer and are assembled into bundles called tonofilaments 
• cells are anchored to keratin filaments by desmosomes

• this epidermal layer consists of a single layer of columnar to cuboidal cells that rest on the basement membrane above the dermis
• cells are attached to one another by desmosomes, and to the basement membrane by hemidesmosomes
• these cells serve as stem cells, and continually divide and mature as they migrate superficially in the epidermis
• cells also produce and contain keratin filaments

• the epidermis protects the body surface from mechanical abrasion, forms a barrier against pathogens as well as water, and prevents loss of bodily fluid through dehydration
• additionally, the epidermis is involved in temperature regulation (via sweat), sensory of environment, and formation of vitamin D

• at its thickest at the palms and soles of feet, the epidermis has all five layers
• thin skin lack stratum granulosum and stratum lucidum, are avascular, and have a characteristically wavy basement membrane

• melanoblasts are found in the stratum germinativum of the epidermis
• melanoblasts differentiate into melanocytes which produce melanin

• papillary - thin layer of loose CT with collagen fibers
• reticular - underneath papillary layer, is thicker and less cellular with collagen fibers and coarser elastic fibers

• sebaceous glands are seen as outgrowths of hair follicles
• they are classified as simple acinar glands or simple alveolar glands
• they produce and secrete sebum into the infundibulum (shaft) of a hair follicle
• sebum is an oily substance composed of cholesterol and triglyceride
• secretion is of the holocrine type

• two types of sweat glands - eccrine (or merocrine) and apocrine
• both are found widely distributed in the skin

• eccrine (or merocine) sweat glands are found all over the body especially on the palms and soles, but not on the lips, penis, and nails
• they are characterized as simple, coiled tubular glands
• dark cells secrete mucus while light cells secrete water and electrolytes
• they assist in thermoregulation and secretion of ammonia, urea, and uric acid

• apocrine sweat glands are found in the axilla/armpits, anus, and areolas
• they are larger than eccrine glands and their ducts open into a hair follicle canal
• are active after puberty, and function to produce a viscous secretion that acquires a distinct odor after being decomposed by bacteria on the skin

• corpuscles are nerve endings in the skin enclosed in a CT capsule
• Meissner corpuscles are responsible for sensitivity to light touch
• Pacinian corpuscles detect pressure changes and vibrations applied on the skin surface

• (left is Pacinian, right are Meissner)

• conducting portion consists of passageways outside and inside the lungs that conduct air to and from the lungs
• respiratory portion consists of passageways within the lungs that facilitate respiration or gas exchange

consists of nasal cavities, nasopharynx, oropharynx, larynx, trachea, primary bronchi, secondary bronchi, tertiary bronchi, and terminal bronchioles 

large bronchioles have pseudostratified columnar ciliated epithelium with numerous mucus-secreting goblet cells

consists of respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli

gas exchange can occur in any of these parts, but happens mostly in alveoli

respiratory bronchioles represent transitional zone where air air conduction and respiration can take place

• located on conchae or nasal shelves at roof of the nose, olfactory epithelium is pseudostratified epithelium
• functions to detect and transit odors to the brain
• this epithelium has many Bowman glands that keep it moist but in contrast to respiratory epithelium, it lacks goblet cells or motile cilia (cilia used for odor detection)

• olfactory cells are bipolar neurons that detect and transmit odors to the brain
• basal cells at the basement membrane can differentiate into supporting cells
• supporting (AKA sustentacular) cells are similar to glial cells (i.e. metabolic and structural support)

• AKA serous olfactory glands, Bowman glands are found in the CT below olfactory epithelium 
• secretes mucus onto the olfactory epithelium

• a flap of elastic cartilage attached to entrance of larynx, the epiglottis functions to prevent food from entering trachea
• two sides are the anterior (lingual) and posterior (laryngeal)
• central portion is composed of elastic cartilage surrounded by perichondrium
• anterior side has characteristics (i.e. stratified squamous epithelium) identical to the rest of the digestive system
• posterior side has characteristics (i.e. pseudostratified columnar ciliated epithelium) identical to the rest of respiratory system

• lined with stratified squamous non-keratinized epithelium
• taste buds and lymphatic nodules are present in this epithelium

• lined with pseudostratified columnar ciliated epithelium
• underneath the lamina propria of this side are seromucous glands
• taste buds, goblet cells, and lymphatic nodules are present in the epithelium

• nasopharynx has pseudostratified columnar epithelium, and oropharynx has stratified squamous epithelium
• nasopharynx joins nasal cavity to rest of pharynx
• oropharynx joins oral cavity to larynx 
• in general, pharynx functions to filter, warm, and moisten air and conduct it to the lungs

• two types are false (or ventricular) vocal folds and true vocal folds
• unlike the simple squamous of the rest of the larynx, the vocal folds are covered with stratified squamous epithelium
• they are controlled mainly by the vocalis muscle, a type of skeletal muscle
• air from the lungs cause the vocal folds to vibrate and produce sound

• three unpaired cartilages - thyroid, cricoid, and epiglottis cartilage
• three paired cartilages - arytenoid, corniculate, and cuneiform cartilages

• consists of mucosa, submucosa, cartilaginous layer, and adventitia
• C-shaped hyaline cartilage rings of cartilaginous layer help keep the trachea open; the cartilage is surrounded by perichondrium CT
• the trachealis muscle (smooth muscle) bridges the gap between the free ends of the C-shaped cartilage; functions to constrict the trachea

• Clara cells are most numerous in the terminal bronchioles, and are also found in the respiratory bronchioles
• these cells are cuboidal, non-ciliated, and have dome-shaped apices that protrude into the lumen 

• Clara cells secrete surfactant-like lipoproteins which coat the bronchial epithelium and also break down mucus for more efficient respiration
• the lipoproteins also help reduce the collapse of the airway walls
• also, Clara cells act as stem cells that replace bronchial epithelial cells

• tubular and acinar refers to the glands shape; tubular are shaped like a tube and acinar are shaped like a flask or are sac-like in appearance
• simple vs. compound refers to the gland's branching; simple has no branching and compound has branching

gland of Lieberkühn in the intestine

uterus and stomach

sebaceous gland of skin (no duct) and Meibom glands of eyelids (with duct)

glands of oral cavity, gastric cardiac glands, glands of Brunner, and Cowper glands (with ducts)

mammary gland, lacrimal glands, and prostatic glands (with ducts)