Anatomy Terms Starting With W
Anatomy Glossary: W
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White Blood Cell
/ WYT BLUD SEL / · Old English hwit; Old English blod; Latin cellula
White blood cell is a nucleated, non-hemoglobin-containing blood cell produced in bone marrow and lymphoid tissue that defends the body against infection, removes cellular debris, and targets abnormal cells through innate and adaptive immune mechanisms.
White blood cells divide into two broad categories: granulocytes, which include neutrophils, eosinophils, and basophils, and agranulocytes, which include lymphocytes and monocytes. Neutrophils are the most abundant type, typically comprising 50 to 70 percent of circulating white blood cells, and are the first cells to arrive at a site of bacterial infection. Monocytes migrate into tissues and mature into macrophages, which engulf pathogens and cellular debris and coordinate the broader immune response by releasing cytokines.
Lymphocytes include B cells, which produce antibodies, and T cells, which directly kill infected or cancerous cells or regulate other immune responses. A normal white blood cell count in adults ranges from about 4,500 to 11,000 cells per microliter of blood, and values outside this range signal infection, immune deficiency, or hematological disease.
The naked mole rat (Heterocephalus glaber) has an unusually low white blood cell count compared to other rodents of similar size, yet rarely develops cancer, a finding that has prompted research into whether its immune surveillance mechanisms differ fundamentally from those of other mammals.
White blood cells only attack bacteria. Neutrophils target bacteria, but other types combat viruses, fungi, and parasites; T cells destroy virus-infected and cancerous cells; and regulatory T cells suppress immune activity to prevent the body from attacking its own tissues.
In the zebrafish (Danio rerio), neutrophils can be tracked in living embryos using fluorescent reporter genes, and they visibly migrate toward a wound within minutes of injury. Researchers use this system to study how chemical signals guide white blood cell movement, because the zebrafish embryo is transparent and its immune cells behave similarly to those of mammals.
Immune System Fun Facts →Wound Healing
/ WOOND HEE-ling / · Old English wund, wound; Old English haelan, to make whole
Wound healing is the staged biological process by which damaged skin and underlying tissues are repaired, progressing through four overlapping phases: hemostasis, inflammation, proliferation, and remodeling.
In the hemostasis phase, platelets aggregate at the wound site and release clotting factors within seconds to minutes, forming a fibrin clot that stops blood loss and creates a scaffold for incoming repair cells. The inflammatory phase follows over the next few days as neutrophils and macrophages migrate in to clear bacteria and debris; macrophages also release growth factors such as PDGF and TGF-beta that recruit fibroblasts. During proliferation, fibroblasts synthesize new collagen and other extracellular matrix components while keratinocytes migrate across the wound surface to restore the epithelial barrier, and new capillaries grow in through angiogenesis.
The remodeling phase can last one to two years as type III collagen is progressively replaced by stronger type I collagen and the scar contracts, though healed skin never fully recovers the tensile strength of unwounded tissue, reaching at most about 80 percent of the original.
Fetal skin during the first two trimesters heals without forming a scar, a capacity linked to high hyaluronic acid levels, a distinct collagen ratio, and a rapidly resolving inflammatory response that differs markedly from the adult wound environment.
Scabs should be removed to speed healing. Picking off a scab disrupts the fibrin scaffold, exposes the wound to infection, damages the newly forming epithelium underneath, and can extend healing time while increasing the risk of permanent scarring.
Salamanders of the genus Ambystoma can regenerate entire limbs through a wound-healing process that, unlike the scarring response in mammals, involves dedifferentiation of mature cells back into a proliferative blastema. Studying how this process avoids fibrosis and promotes true tissue regeneration is a major focus of regenerative medicine research, particularly in understanding why mammalian wounds scar rather than regenerate.
Wrist
/ RIST / · Old English wrist, from Proto-Germanic wristiz, related to twisting
Wrist is the joint region between the forearm and hand, formed by the distal ends of the radius and ulna articulating with eight small carpal bones arranged in two rows, allowing flexion, extension, and rotation of the hand.
The eight carpal bones are the scaphoid, lunate, triquetrum, and pisiform in the proximal row, and the trapezium, trapezoid, capitate, and hamate in the distal row. At the radiocarpal joint, where the radius meets the scaphoid and lunate, most of the load transmitted from the hand to the forearm is borne. Running through the carpal tunnel, a fibro-osseous channel on the palmar side of the wrist, are the flexor tendons of the fingers and thumb along with the median nerve, which supplies sensation to the palm and the first three and a half fingers.
Injury to the scapholunate ligament, one of the intrinsic wrist ligaments binding adjacent carpal bones, is a common cause of chronic wrist instability that can progress to degenerative arthritis if untreated.
The scaphoid is the most commonly fractured carpal bone, typically from a fall on an outstretched hand. Because part of its blood supply enters from the distal end, a fracture across the waist of the scaphoid can cut off circulation to the proximal fragment, leading to avascular necrosis if the injury is missed or left untreated.
The wrist is a single joint. It is a complex of multiple articulations between the radius, ulna, and eight carpal bones, as well as the joints between adjacent carpal bones themselves, all working together to produce the wrist's full range of motion.
Birds have highly modified wrist bones fused into a single carpometacarpus that anchors the primary flight feathers. This fusion sacrifices the rotational mobility of the mammalian wrist in exchange for a rigid, lightweight lever that maximizes aerodynamic efficiency during flapping flight.