Study guide (Ben version)

Kai made these, not 100% trust AI, but I think they did good :)

Keratinocytes

Primary epidermal cells found across all strata; originate as stem cells in the stratum basale, produce protective keratin, and die in the stratum granulosum as they fill with keratin.

Lanugo

Fine, unpigmented, downy hair that develops on the fetus during the third trimester of pregnancy.

Vellus hair

Fine, pale hair that constitutes the primary human hair covering the upper and lower limbs.

Terminal hair

Coarse, pigmented, longer hair found on the scalp, eyebrows, eyelashes, and male beard, which replaces vellus hair in pubic and axillary regions at puberty.

Embryonic origin of cutaneous glands

Derived from downgrowths of the epidermal stratum basale that project into the underlying dermis.

Merocrine (eccrine) sweat glands

Simple coiled tubular glands that secrete sweat (99% water plus electrolytes and waste) via exocytosis directly onto the skin surface through sweat pores for thermoregulation.

Apocrine sweat glands

Coiled tubular glands that release a viscous, protein- and lipid-rich secretion via exocytosis into hair follicles of the axillae, nipples, and groin starting at puberty, producing odor upon bacterial breakdown.

Sebaceous glands

Holocrine glands responsive to androgens that secrete oily sebum into hair follicles to lubricate skin and hair while providing bactericidal properties.

Sweat gland histology and mechanics

Possesses a coiled secretory portion lined with myoepithelial cells that contract to facilitate fluid expulsion, along with an excretory duct emptying onto a sweat pore.

Ceruminous glands

Modified apocrine glands located exclusively in the external acoustic meatus that secrete cerumen (earwax) to lubricate the eardrum and trap foreign matter.

Mammary glands

Modified apocrine glands in the breasts that develop and produce milk in pregnant and lactating females.

Thick skin characteristics

Contains all five epidermal strata (including the stratum lucidum) and sweat glands, but completely lacks hair follicles and sebaceous glands; located on palms and soles.

Function of melanin

Shields keratinocyte nuclear DNA from ultraviolet (UV) radiation-induced mutational damage; synthesized by melanocytes and transferred via melanosomes.

Structural layers of the dermis

1. Papillary layer: Superficial zone of areolar connective tissue with dermal papillae and capillaries\n2. Reticular layer: Deeper, thicker zone composed of dense irregular connective tissue with prominent collagen bundles

Structures located within the dermis

Blood vessels, sweat glands, sebaceous glands, hair follicles, nail roots, sensory nerve endings, arrector pili muscles, and motile dendritic cells.

Lines of cleavage (tension lines)

Predominant parallel alignments of collagen and elastic fiber bundles in the dermis; surgical incisions made parallel to them heal more rapidly and gape minimally.

Nail plate and nail body

The hardened keratinized external structure resting on the nail bed; its visible pinkish portion over underlying capillary beds forms the nail body.

Nail root and nail matrix

The proximal, non-visible portion of the nail plate embedded beneath the skin, containing actively proliferating epidermal cells responsible for nail plate growth.

Nail bed

The layer of living epidermal tissue positioned directly beneath the hard nail body.

Lunula

The whitish, crescent-shaped region at the proximal base of the visible nail body caused by a thickened underlying stratum basale obscuring capillaries.

Eponychium vs. Hyponychium

• Eponychium: Fold of epidermis extending onto the nail body at the proximal margin (cuticle)\n- Hyponychium: Thickened area of stratum corneum beneath the distal free edge of the nail

Epidermal origin of nail hardness

Modified, highly compacted stratum corneum filled with densely packed hard keratin.

Subsurface regions of hair

• Hair bulb: Swelling at the base surrounding the vascular dermal papilla; contains proliferating matrix cells and living epithelial cells\n- Hair root: The follicle-enclosed portion extending from the bulb up to the epidermal surface

Hair shaft

The keratinized, non-living region of a hair that extends beyond the surface of the epidermis.

Concentric layers of a hair

From deep to superficial:\n1. Medulla: Flexible core containing soft keratin\n2. Cortex: Thick intermediate layer containing hard keratin\n3. Cuticle: Single outer layer of flattened, overlapping scaly cells

Arrector pili

Smooth muscle band running from the hair follicle to the dermal papillary layer; contracts during cold or emotional arousal to elevate the hair and produce goosebumps.

Hair growth cycle phases

1. Anagen: Active growth and cellular proliferation phase\n2. Catagen: Brief transitional and regressive involution phase\n3. Telogen: Resting phase after which the mature hair shaft is shed

Epidermal layers in thick skin

From deep to superficial: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum (Mnemonic: Come, Let's Get Sun Burned from top down).

Cellular distribution across epidermal strata

• Stratum basale: Keratinocyte stem cells, melanocytes, and tactile (Merkel) cells\n- Strata spinosum and granulosum: Keratinocytes and dendritic (Langerhans) immune cells

Primary skin pigments

Melanin (brown-black/reddish pigment), carotene (yellow-orange dietary pigment), and hemoglobin (reddish pigment in dermal blood vessels).

Classification of burn depths

• 1st degree: Epidermis only (redness, pain)\n- 2nd degree: Epidermis and part of the dermis (blistering, pain)\n- 3rd degree: Complete destruction of epidermis, dermis, and underlying subcutaneous tissue

Rule of nines

Clinical method estimating adult burn surface area by dividing the body surface into 11 anatomical areas, each accounting for approximately 9% (plus 1% for the perineum).

Epiphyseal plate vs. Epiphyseal line

• Epiphyseal plate: Hyaline cartilage layer in the metaphysis enabling lengthwise bone elongation during growth\n- Epiphyseal line: Thin compact bone remnant left when growth ceases and the plate fully ossifies (ages 10-25)

Five histological zones of the epiphyseal plate

From epiphysis toward diaphysis:\n1. Zone of resting cartilage: Anchors the growth plate to the epiphysis\n2. Zone of proliferating cartilage: Rapid chondrocyte division in aligned columns\n3. Zone of hypertrophic cartilage: Chondrocyte enlargement and lacunar thinning\n4. Zone of calcified cartilage: Mineral deposition and chondrocyte apoptosis\n5. Zone of ossification: Bone deposition on calcified cartilage matrix

Zones driving interstitial lengthwise bone growth

Proliferating cartilage zone and hypertrophic cartilage zone, whose cellular activity pushes the resting zone toward the epiphysis.

Steps of endochondral ossification

1. Fetal hyaline cartilage model forms (weeks 8-12)\n2. Cartilage calcifies, chondrocytes die, and periosteal bone collar develops\n3. Primary ossification center develops in the diaphysis\n4. Secondary ossification centers develop in the epiphyses; osteoclasts carve medullary cavity\n5. Bone replaces cartilage, leaving only articular cartilage and epiphyseal plates\n6. Epiphyseal plates ossify into epiphyseal lines

Intramembranous ossification

Bone formation directly within condensed mesenchymal sheets without a cartilage precursor; forms flat skull bones, facial bones, the mandible, and the central clavicle.

Hematopoiesis and marrow distribution in adults

Blood cell synthesis occurring in red bone marrow; restricted in adults to the skull, vertebrae, ribs, sternum, ossa coxae, and proximal epiphyses of the femur and humerus.

Intervertebral disc components

Fibrocartilaginous pads between vertebral bodies composed of a tough outer fibrocartilage ring (anulus fibrosus) and an inner gelatinous shock-absorbing core (nucleus pulposus).

Bone marrow conversion in severe anemia

Dormant yellow bone marrow reverts into active red bone marrow to increase the production of erythrocytes.

Osteoprogenitor cells

Mesenchymal-derived stem cells in the periosteum and endosteum that divide to produce committed precursors maturing into osteoblasts.

Osteoid

Semisolid organic bone matrix consisting of collagen fibers and proteoglycans/glycoproteins secreted by osteoblasts prior to calcification; imparts tensile strength and flexibility.

Osteoblasts

Cuboidal bone-forming cells derived from osteoprogenitors that synthesize and secrete osteoid, becoming trapped in lacunae to become mature osteocytes.

Osteocytes

Mature bone cells situated within lacunae that sense mechanical strain and maintain the surrounding mineralized matrix.

Osteoclasts

Large, multinucleated phagocytic cells located in resorption pits that secrete hydrochloric acid and lysosomal enzymes to dissolve and resorb bone matrix.

Canaliculi

Microscopic branching tunnels through bone lamellae linking adjacent lacunae to each other and the central canal, allowing cytoplasmic exchanges of nutrients and gases.

Types of bone lamellae

• Concentric: Cylindrical rings encircling the osteon central canal with alternating collagen fiber angles\n- Interstitial: Leftover fragments between intact osteons\n- Circumferential: Rings encircling the inner and outer bone circumferences\n- Parallel: Layered sheets within spongy bone trabeculae

Organic vs. inorganic matrix components of bone

• Organic (collagen fibers): Provides tensile strength and prevents brittleness\n- Inorganic (calcium phosphate / hydroxyapatite): Hardens the matrix and provides rigid compressive strength

Hyaline cartilage tissue properties

Avascular, aneural, flexible connective tissue containing high water content for compressibility; chondrocytes reside in lacunae surrounded by perichondrium.

Endocrine effects of glucocorticoids on bone

Steroid hormones from the adrenal cortex that, when elevated, stimulate bone loss and impair chondrocyte proliferation at pediatric epiphyseal plates.

Role of serotonin in bone remodeling

Regulates normal remodeling; chronically elevated levels inhibit osteoprogenitor cell differentiation into osteoblasts, reducing bone mass.

Nutritional requirements for bone matrix synthesis

• Vitamin C: Obligate cofactor for collagen triple-helix synthesis\n- Vitamin D: Essential for intestinal calcium and phosphate absorption

Pathway of calcitriol synthesis

Cholecalciferol (Vitamin D3) made in skin is converted to calcidiol in the liver, and then converted into active calcitriol in the kidneys (stimulated by PTH).

Skeletal actions of PTH and calcitriol

Elevate blood calcium by promoting osteoclast-mediated bone breakdown, reducing renal calcium excretion in urine, and (for calcitriol only) increasing intestinal calcium absorption.

Calcitonin regulation of blood calcium

Thyroid-secreted hormone released in response to hypercalcemia; inhibits osteoclast resorption and promotes renal calcium elimination (most active during childhood growth).

Types of bone fractures

• Pathologic: Occurs in bone structurally compromised by disease\n- Compound (open): Broken bone end pierces through overlying skin\n- Simple (closed): Fracture remains contained beneath intact skin\n- Stress: Hairline crack caused by repetitive mechanical loads

Four stages of bone fracture repair

1. Fracture hematoma formation: Blood clot forms at trauma site\n2. Fibrocartilaginous (soft) callus: Fibrocartilage and collagen knit bone ends\n3. Bony (hard) callus: Trabecular bone replaces fibrocartilage\n4. Bone remodeling: Compact bone replaces primary bone and restores normal shape

Periosteum and perforating (Sharpey's) fibers

Two-layered outer covering with an external dense irregular CT layer and an internal osteogenic layer; anchored to outer circumferential lamellae via collagenous perforating fibers.

Endosteum

Cellular membrane lining medullary cavities, central canals, and trabecular surfaces containing osteoprogenitors, osteoblasts, and osteoclasts.

Diploe

The spongy bone sandwich layer located between inner and outer compact bone tables of flat cranial bones.

Bones forming the orbital complex

Frontal, sphenoid, maxilla, palatine, zygomatic, lacrimal, and ethmoid bones (7 bones total).

Bones forming the nasal complex

Frontal, sphenoid, ethmoid, maxillae, nasal bones, vomer, inferior nasal conchae, and septal cartilage.

Paranasal sinuses and functions

Air-filled chambers within the frontal, ethmoidal, sphenoidal, and maxillary bones that warm and humidify air, lighten skull weight, and provide voice resonance.

Glenoid cavity

Shallow, cup-like articulation on the lateral angle of the scapula that articulates with the head of the humerus to form the glenohumeral joint.

Infraorbital foramen

Aperture situated on the anterior maxilla directly beneath the orbital margin that transmits infraorbital nerves and blood vessels to the midface.

Cribriform plate and crista galli

Superior horizontal ethmoid plate whose foramina transmit olfactory nerve fibers from the nasal cavity; features an upright crista galli crest anchoring cranial dural septa.

Mandibular fossa

Depression on the inferior surface of the temporal bone that articulates with the mandibular condyle to form the temporomandibular joint (TMJ).

Auditory ossicles

Malleus, incus, and stapes; three tiny bones housed in the petrous temporal bone that amplify and transmit sound vibrations to the inner ear.

Occipital condyles

Paired smooth articular rockers flanking the foramen magnum on the occipital bone that articulate with C1 (atlas) to facilitate nodding ('yes').

Atlas (C1)

First cervical vertebra devoid of a body and spinous process whose superior articular facets support the occipital condyles at the atlanto-occipital joint.

Axis (C2)

Second cervical vertebra characterized by a superior tooth-like projection (dens or odontoid process) around which C1 pivots for rotational head movement ('no').

Vertebra prominens (C7)

Cervical vertebra with a long, non-bifid, palpable posterior spinous process that marks the transition into the thoracic vertebral segment.

Regional structural features of vertebrae

• Cervical (C1-C7): Small bodies, bifid spinous processes, and transverse foramina for vertebral vessels\n- Thoracic (T1-T12): Heart-shaped bodies, inferiorly directed spinous processes, and costal facets/demifacets\n- Lumbar (L1-L5): Massive kidney-shaped bodies, blunt rectangular spinous processes; no costal facets or transverse foramina

Vertebral column organization and numbers

24 individual presacral vertebrae (7 cervical, 12 thoracic, 5 lumbar), plus 5 fused sacral vertebrae (sacrum) and 4 fused coccygeal vertebrae (coccyx).

Forearm anatomy in anatomical position

In full supination, radius and ulna lie parallel; the radius is lateral (thumb side) and the ulna is medial (pinky side); during pronation the radius crosses over the ulna.

Olecranon

Prominent posterosuperior projection on the proximal ulna forming the elbow bump that slots into the humeral olecranon fossa during forearm extension.

Bones of the carpus (wrist)

• Proximal row (lateral to medial): Scaphoid, lunate, triquetrum, pisiform\n- Distal row (lateral to medial): Trapezium, trapezoid, capitate, hamate

Sacral promontory

Anterosuperior projecting lip of the first sacral vertebra (S1) that protrudes into the pelvic aperture to mark the posterior boundary of the pelvic brim.

Pelvic brim and pelvic subdivisions

• Pelvic brim: Continuous oval ridge separating the pelvis into upper and lower spaces\n- True pelvis: Space inferior to the brim enclosing pelvic organs\n- False pelvis: Flared area superior to the brim housing lower abdominal organs

Pelvic inlet vs. Pelvic outlet

• Pelvic inlet: Superior opening circumscribed completely by the pelvic brim\n- Pelvic outlet: Inferior boundary outlined by the pubic symphysis, ischial tuberosities, and coccyx

Os coxae and sexual dimorphism

Formed by fusion of ilium, ischium, and pubis (ages 13-15); the female pelvis is shallower and wider, with an enlarged greater sciatic notch and a broader subpubic angle.

Tibia and fibula functional roles and landmarks

• Tibia: Medial, sole weight-bearing bone of the leg; features tibial tuberosity and medial malleolus\n- Fibula: Slender, lateral, non-weight-bearing bone serving for muscle attachments and lateral ankle stability via the lateral malleolus

Medial longitudinal arch of the foot

Highest foot arch consisting of the calcaneus, talus, navicular, three cuneiforms, and metatarsals I-III; preserves the elevated medial arch (collapse causes pes planus).

Bones of the tarsus (ankle)

Composed of 7 bones: calcaneus, talus, navicular, cuboid, and medial, intermediate, and lateral cuneiforms.

Metacarpals, metatarsals, and phalanges counts

Each hand and foot contains 5 metacarpals/metatarsals and 14 phalanges (2 in the pollex/hallux, 3 in each remaining digit).

Rib classifications and sternal anatomy

• Ribs: 1-7 true (vertebrosternal), 8-12 false (vertebrochondral), 11-12 floating (vertebral)\n- Sternum: Manubrium, body, and xiphoid process; sternal angle marks the manubrium-body junction

Embryonic limb development timeline

• Upper limb buds: Emerge early week 4; hand plates by week 5; digital rays in late week 6\n- Lower limb buds: Emerge a few days after upper buds; foot plates in week 6; digital rays in early week 7\n- Digit separation: Completed by programmed cell death (apoptosis) in week 8

Limb morphogenesis signaling and tissue origin

Buds consist of lateral plate mesoderm covered by ectoderm; musculature derives from invading somitic mesoderm; the apical ectodermal ridge (AER) at the apex guides proximal-to-distal outgrowth.

Congenital limb malformations

• Polydactyly: Extra digits\n- Syndactyly: Webbed digits\n- Amelia: Total absence of a limb\n- Phocomelia: Severely shortened flipper-like limbs (associated with gestational thalidomide exposure between weeks 4-8)