Anatomy Terms Starting With E
Anatomy Glossary: E
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Epidermis
/ ep-ih-DER-mis / · Greek epi, upon; derma, skin
Epidermis is the outermost, avascular layer of the skin composed primarily of keratinocytes that proliferate in the basal layer, differentiate upward through successive strata, and are continuously shed from the surface.
Depending on body location, the epidermis contains four or five distinct layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum present only in thick skin such as the palms and soles, and stratum corneum. These layers represent progressive stages of keratinocyte differentiation and keratinization, a process taking roughly 28 days from basal cell division to surface shedding. Melanocytes in the stratum basale produce melanin that absorbs UV radiation; Langerhans cells conduct immune surveillance for pathogens; Merkel cells mediate light touch sensation.
Each cell type occupies a specific layer and contributes a distinct function to the tissue as a whole.
Human fingertip epidermis is among the thinnest in the body at roughly 0.05 millimeters, while the epidermis on the heel can exceed 1.5 millimeters. This variation reflects mechanical demand rather than any difference in the number of cell types present.
Cells of the Epidermis →The epidermis contains many blood vessels. The epidermis has no blood vessels and receives nutrients entirely by diffusion from capillaries in the underlying dermis.
Integumentary System Facts →In the poison dart frog (Dendrobates tinctorius), epidermal glands secrete batrachotoxins potent enough to affect sodium channels in nerve and muscle tissue. The frog acquires precursor compounds from its diet of specific arthropods, and captive-raised individuals fed different diets lose their toxicity.
Epiphysis
/ eh-PIF-ih-sis / · Greek epi, upon; phyein, to grow
Epiphysis is the rounded end of a long bone, composed of spongy bone covered by articular cartilage, that forms the joint surface and houses red marrow for hematopoiesis in adults.
During childhood, the epiphysis is separated from the diaphysis by the epiphyseal plate, a layer of proliferating hyaline cartilage that drives longitudinal bone growth by adding new bone to the metaphysis. This growth plate fuses at skeletal maturity, typically between ages 14 and 25 depending on the bone and the individual’s sex, ceasing further longitudinal growth. Epiphyseal fractures in children are classified by the Salter-Harris system, with higher grades indicating greater damage to the growth plate and a higher risk of limb length discrepancy or angular deformity.
Radiologists use the degree of epiphyseal fusion to estimate skeletal age in forensic and clinical contexts.
Bone age estimated from epiphyseal fusion patterns can differ from chronological age by several years, a discrepancy that forensic anthropologists use to distinguish juvenile from adult skeletal remains. In some growth disorders such as hypothyroidism, epiphyseal fusion is delayed significantly, causing disproportionate short stature.
Fun Facts About the Skeletal System →The epiphysis is the same as the pineal gland because both words appear in anatomy. In skeletal anatomy, epiphysis refers exclusively to the end region of a long bone, a structure entirely unrelated to the pineal gland of the brain.
In a young white-tailed deer (Odocoileus virginianus), the epiphyses of the long limb bones contain open growth plates visible on radiographs until the animal reaches roughly 18 to 24 months of age. Wildlife biologists use the degree of epiphyseal fusion to estimate age in harvested deer when tooth wear data are unavailable.
Erythrocyte
/ eh-RITH-roh-syt / · Greek erythros, red; kytos, cell
Erythrocyte is a biconcave, anucleate blood cell filled with hemoglobin that transports oxygen from the lungs to tissues and carries carbon dioxide back to the lungs for exhalation.
Human erythrocytes lack a nucleus and most organelles, maximizing hemoglobin content and allowing flexible passage through capillaries narrower than the cell’s resting diameter of about 8 micrometers. Bone marrow produces them at a rate of approximately 2.4 million per second, and each cell survives roughly 120 days before splenic macrophages recognize and remove it. The biconcave disc shape increases surface area relative to volume, shortening the diffusion distance for oxygen exchange across the cell membrane.
At high altitude, the kidneys respond to low oxygen tension by secreting erythropoietin, a hormone that accelerates erythrocyte production and raises the oxygen-carrying capacity of the blood.
Crocodilians retain nucleated erythrocytes, as do all non-mammalian vertebrates, yet crocodilian red blood cells carry a unique hemoglobin that releases oxygen more readily in response to carbon dioxide than mammalian hemoglobin does. This property lets crocodiles remain submerged for extended periods by extracting more oxygen from each cell.
Circulatory System Fun Facts →Red blood cells use the oxygen they carry for their own energy needs. Human erythrocytes have no mitochondria and generate ATP exclusively through anaerobic glycolysis, so they consume none of the oxygen they transport.
In llamas (Lama glama), erythrocytes are small and elliptical rather than biconcave discs. This shape reduces blood viscosity at high altitude and improves flow through pulmonary capillaries where oxygen uptake occurs.
Exocrine Gland
/ EK-soh-krin GLAND / · Greek exo, outside; krinein, to separate
Exocrine Gland is a secretory organ that releases its products through a duct onto an epithelial surface, either internal or external, rather than directly into the bloodstream.
Exocrine glands are classified as unicellular, such as goblet cells that secrete mucus directly from a single cell, or multicellular, which are further subdivided by duct branching pattern and the shape of their secretory units into tubular, acinar, or tubuloalveolar types. Products secreted include digestive enzymes, sweat, sebum, mucus, bile, and milk, reflecting the diversity of surfaces these glands supply. The mechanism of secretion also varies: merocrine glands release products by exocytosis without losing cytoplasm, apocrine glands shed the apical portion of the cell along with the secretion, and holocrine glands release entire disintegrated cells as their product.
Sebaceous glands of the skin use the holocrine mechanism, producing sebum from cells that accumulate lipid and then rupture completely.
The pancreas contains both exocrine and endocrine tissue within the same organ. Its exocrine acinar cells, which make up roughly 98 percent of the organ's mass, secrete digestive enzymes into the duodenum, while the endocrine islets of Langerhans, scattered among the acinar tissue, secrete insulin and glucagon directly into the blood.
Exocrine glands are the same as endocrine glands. Endocrine glands release hormones directly into the bloodstream with no duct, while exocrine glands always deliver their secretions through a duct to a surface.
Endocrine System Fun Facts →In the Gila woodpecker (Melanerpes uropygialis), enlarged salivary exocrine glands produce sticky saliva that coats the tongue and traps insects inside cactus cavities. The gland's output increases during the breeding season when the birds feed nestlings on a high-insect diet.
Fun Facts About Digestive System →