Epidermis, Bone Cells, and Calcium Regulation

Vocabulary flashcards covering key terms from the lecture on epidermal layers, bone histology, and hormonal regulation of calcium.

Keratinocyte

A fully differentiated epidermal cell specialized for producing the fibrous protein keratin.

Keratin

Tough, protective structural protein that fills mature epidermal cells.

Stratum Basale

Deepest epidermal layer where mitosis occurs and new keratinocyte precursors arise.

Stratum Spinosum

Epidermal layer rich in desmosomes and spine-like keratin bundles connecting cells.

Desmosome

Strong intercellular junction that anchors adjacent cells together, abundant in skin.

Hyalokeratin

Pre-keratin protein granules that contribute to the spiny appearance of stratum spinosum cells.

Stratum Granulosum

Epidermal layer where keratin synthesis peaks; cells accumulate keratohyalin granules and darken.

Stratum Corneum

Outermost epidermal layer of dead, flattened, keratin-filled cells forming a tough barrier.

Epidermis

Stratified squamous epithelial layer of skin composed mainly of keratinocytes.

Osteocyte

Mature bone cell trapped in a lacuna, interconnected to neighbors via canaliculi.

Osteoblast

Bone-forming cell that secretes osteoid and later pumps calcium to mineralize bone.

Osteoprogenitor Cell

Stem (progenitor) cell that differentiates into an osteoblast.

Osteoid

Unmineralized organic bone matrix rich in collagen produced by osteoblasts.

Canaliculi

Microscopic channels through bone matrix that house osteocyte processes for nutrient exchange.

Lacuna

Small cavity within bone or cartilage matrix that houses a cell; plural lacunae.

Osteoclast

Large multinucleated, macrophage-derived cell that resorbs (breaks down) bone matrix.

Cancellous (Spongy) Bone

Inner porous bone made of trabeculae; rigid despite its ‘spongy’ appearance.

Trabecula

Thin rod or plate of bone forming the latticework of cancellous bone; plural trabeculae.

Compact Bone

Dense outer bone layer organized into osteons; provides hard protective cortex.

Osteon

Cylindrical structural unit of compact bone with concentric lamellae around a central canal.

Lamellae

Concentric layers or sheets of bone matrix within an osteon.

Central (Haversian) Canal

Longitudinal canal at the center of an osteon containing blood vessels and nerves.

Periosteum

Fibrous and cellular membrane covering outer bone surfaces; contains osteoblasts.

Endosteum

Thin cellular membrane lining internal bone surfaces and trabeculae; houses bone-remodeling cells.

Diaphysis

Shaft of a long bone, composed mainly of compact bone.

Epiphysis

Expanded end of a long bone filled with spongy bone and covered by articular cartilage.

Medullary (Bone Marrow) Cavity

Central cavity within the diaphysis that houses bone marrow.

Hematopoietic Bone Marrow

Marrow tissue that generates all blood cells (red and white).

Hydroxyapatite

Crystalline complex of calcium phosphate/carbonate that mineralizes bone matrix.

Endocrine

Describes glands or cells that secrete hormones directly into the bloodstream.

Exocrine

Describes glands that secrete products into ducts or onto epithelial surfaces.

Paracrine

Local signaling in which a cell’s secretions act on neighboring cells.

Parathyroid Hormone (PTH)

Peptide hormone from parathyroid glands that raises blood calcium by stimulating osteoclasts, increasing renal Ca²⁺ reabsorption, and boosting calcitriol formation.

Calcitonin

Peptide hormone from thyroid C cells that lowers blood calcium by stimulating osteoblasts and promoting renal Ca²⁺ loss.

Calcitriol

Active hormonal form of vitamin D that promotes intestinal calcium absorption; produced in kidneys.

Cholecalciferol (Vitamin D3)

Calcitriol precursor synthesized in skin from cholesterol upon UV exposure.

Calcium Homeostasis

Dynamic balance of blood calcium concentration maintained by PTH, calcitonin, and calcitriol.