gland
(gland 
)
[L. glans, stem gland -, acorn]
- An epithelial tissue that is specialized for the manufacture and export of particular molecules. Gland s can be unicellular or multicellular. The multicellular gland s are classified according to their architectures, i.e., ducted or ductless. The cells of ductless gland s secrete specific molecules into the adjacent interstitial space (paracrine gland s) or into the bloodstream (endocrine gland s). The cells of ducted gland s (exocrine gland s) secrete into a cylindrical sac (tubular gland s) or into a flask-shaped sac (alveolar gland s). The ducted gland s are further divided into those in which there is only a single sac (simple tubular gland s or simple alveolar gland s) and those in which the sacs are connected by branching ducts (branched or compound gland s). Gland s can also be classified according to the secretory mechanisms of their cells. The most common secretory mechanism is merocrine, in which secretion-filled intracellular vesicles release their contents by fusing with the cell membrane. Other secretory mechanisms include holocrine (in which the gland cell membrane disintegrates to release its secretion), apocrine (in which the ends of the gland cells pinch off, carrying the secretion), and direct active transport of particular molecules across the gland cell membrane. Gland cells and their intertwined vascular beds can be controlled by autonomic innervation and by hormones from other gland s.
- A lymph node.
accessory g.1An additional (usually smaller) gland that secretes the same substances as a primary gland . 2A gland secreting substances that enhance the function of another gland or organ. In the male reproductive tract, for example, the prostate, which secretes fluids that improve the viability of sperm, is an accessory gland to the testis.
acinotubular g.A gland structurally midway between an alveolar and a tubular gland .
adrenal g.Either of two lobulated gland s atop the superior pole of each kidney. SYN: suprarenal gland .
SEE: illus.
Each adrenal gland is a two-part organ composed of an outer cortex and an inner medulla. The cortex arises in the embryo from a region of the mesoderm that also gives rise to the gonads. The medulla arises from ectoderm, which also gives rise to the sympathetic nervous system (SNS).
The entire gland is enclosed in a capsule of tough connective tissue from which trabeculae extend into the cortex. The cortex consists of cells arranged into three zones: the outer zona glomerulosa, the middle zona fasciculata, and the inner zona reticularis. The cells are arranged in cords. The medulla consists of chromaffin cells arranged in groups or in anastomosing cords. The two adrenal gland s are retroperitoneal, each embedded in perirenal fat above its respective kidney. In an adult, the average weight of an adrenal gland is 5 g (range between 4 and 14 g).
The adrenal medulla synthesizes and stores three catecholamines: dopamine, norepinephrine, and epinephrine. The chief effects of dopamine are the dilation of systemic arteries, increased cardiac output, and increased flow of blood to the kidneys. The primary action of norepinephrine is constriction of the arterioles and venules, resulting in increased resistance to blood flow, elevated blood pressure, and slowing of the heart.
The adrenal medulla is controlled by the SNS system and functions in conjunction with it. It is intimately related to adjustments of the body in response to stress and emotional changes. Anticipatory states tend to bring about the release of norepinephrine. More intense emotional reactions, esp. those in response to extreme stress, tend to increase the secretion of both norepinephrine and epinephrine; epinephrine is important in mobilizing the physiological changes that occur in the “fight or flight” response to emergency situations.
The cortex synthesizes three groups of steroid hormones from cholesterol. These are 1) glucocorticoids (cortisol, corticosterone), which regulate the metabolism of organic nutrients and have an anti-inflammatory effect; 2) mineralocorticoids (aldosterone, dehydroepiand rosterone), which affect metabolism of the electrolytes sodium and potassium; and 3) and rogens and estrogens (estradiol), which contribute to body changes at puberty.
SEE: aldosterone; cortisol; steroid.
Hypersecretion of adrenal cortical hormones results in Cushing syndrome. Hypersecretion of aldosterone results in aldosteronism (a surgically correctable form of hypertension). Adrenocortical insufficiency may be acute or chronic: acute insufficiency of adrenal hormones produces circulatory shock; chronic insufficiency results in Addison's disease.
SEE: Addison's disease; aldosteronism; Cushing syndrome; pheochromocytoma.
albuminous g.Any of the gland s secreting a fluid containing albumin.
SEE: serous gland .
alveolar g.A multicellular gland in which the cells secrete specific molecules into an oval or flask-shaped sac. SYN: acinar gland ; acinous gland .
anal g.Any of the sebaceous gland s found within the wall of the anal canal that open into anal crypts via anal ducts. SYN: circumanal gland .
apocrine g.Any of the gland s, e.g., the mammary gland s, whose cells lose some of their cytoplasmic contents in the formation of secretion.
apocrine sweat g.Any of the sweat gland s in the axillae and pubic region that open into hair follicles and not directly onto the surface of the skin as do eccrine sweat gland s. They appear after puberty and are more developed in women than in men. The characteristic odor of perspiration is produced by the action of bacteria on the material secreted by the apocrine sweat gland s.
SEE: sweat gland .
areolar g.Any of the sebaceous gland s in the areola surrounding the nipple of the female breast. SYN: Montgomery gland .
SEE: under Bartholin, Caspar (the younger).
SEE: Bland in gland s.
SEE: under Bowman, Sir William.
brachial g.Any of the lymph nodes in the arm and forearm.
bronchial g.Any of the mixed gland s lying in the submucosa of the bronchi and bronchial tubes.
SEE: Brunner gland s.
buccal g.Any of the alveolar gland s in the mucosa of the cheek.
bulbourethral g.Cowper gland .
cardiac g.A gastric gland in the cardiac region of the stomach.
ceruminous g.Any of the gland s in the external auditory canal that secrete cerumen.
cervical g.Any of the lymph nodes in the neck.
SEE: under Luschka, Hubert von.
compound g.A multicellular gland containing branching ducts.
compound tubular g.A gland composed of numerous tubules leading to a lone duct.
SEE: Cowper gland .
cutaneous g.Any of the gland s of the skin, esp. the sebaceous and sudoriferous gland s. These include modified forms such as the ciliary, ceruminous, anal, preputial, areolar, and meibomian gland s.
ductless g.A gland with cells that secrete specific molecules into the adjacent interstitial space (paracrine gland s) or into the bloodstream (endocrine gland s).
SEE: Brunner gland s.
SEE: Ebner gland s.
eccrine g.A simple tubular sweat gland of the skin.
SEE: apocrine gland ; eccrine sweat gland .
eccrine sweat g.Any of the skin gland s that regulate body heat by secreting sweat. The number of gland s ranges from 2 million to 5 million. There are over 400 per square centimeter on the palms and about 80 per square centimeter on the thighs.
SEE: sweat gland for illus.
endocrine g.One of two broad categories of gland s, the exocrine gland s being the complementary category. Endocrine gland s, e.g., the thyroid gland , are ductless gland s that secrete macromolecules, called hormones, directly into the bloodstream, and such gland s are richly supplied by blood capillaries. The endocrine gland s include the suprarenals, parathyroids, pineal, pituitary, and the thyroid. Major clusters of endocrine tissue are also found in the gastrointestinal tract, hypothalamus, ovaries, pancreas, testes, and the placenta. In addition, chromaffin and other neuroendocrine cells are found individually and in small clusters throughout the body.
SEE: exocrine gland ; illus.; table - Principal Endocrine Gland s.
The hormones produced by endocrine cells regulate the body's salt, water, mineral, and glucose levels; they adjust the body's metabolic balances, growth rates, and reproductive cycles; and they maintain the body's stress responses. Like exocrine and paracrine cells, endocrine cells are stimulated and inhibited by the autonomic nervous system; the activities of endocrine cells are also modulated by circulating hormones, esp. pituitary hormones. Both the neural and the hormonal signals to the endocrine system are ultimately regulated by the hypothalamus of the brain, which is the integration center for the body's visceral homeostasis.
SEE: hormone.
Endocrine health disorders usually result from the production of either too much or too little of a hormone.
esophageal g.Any of the small scattered mucus-secreting exocrine gland s in the submucosa of the esophagus.
exocrine g.One of two broad categories of gland s, the endocrine gland s being the complementary category. Exocrine gland s, e.g., the salivary gland s, secrete specific molecules either onto the outer surface of the body or into a duct that empties onto surfaces that connect to the outer surface of the body. Most exocrine gland s are multicellular. Goblet cells are examples of unicellular exocrine gland s.
SEE: endocrine gland .
fundic g.A gastric gland in the fundus of the stomach.
gastric g.Any of the mixed exocrine gland s that form the epithelial pockets (pits) in the lining of the stomach. The gland s are named for their location, e.g., cardiac gland s, fundic gland s, pyloric gland s. Typically, the neck of the gland secretes mucus; the body of the gland secretes hydrochloric acid; and the base of the gland secretes enzymes and hormones.SYN: stomach gland .
genital g.An ovary or a testis.
hepatic g.Any of the lymph nodes located in front of the portal vein.
holocrine g.A gland , such as a sebaceous gland of the skin, in which the secretory cells release intracellular macromolecules by disintegration of their cell membranes, after which the cells die.
intestinal g.Lieberkühn crypt.
SEE: under Krause, Karl.
labial g.Any of the alveolar gland s of the mucosa of the lips.
lacrimal g.The gland that secretes tears. It is a tubuloalveolar gland located in the orbit, superior and lateral to the eyeball, and consists of a large superior portion (pars orbitalis) and a smaller inferior portion (pars palpebralis).
laryngeal g.Any of the mixed serous and mucus gland s in the submucosa of the laryngeal section of the respiratory tract.
Lieberkühn g.Lieberkühn crypt.
lingual g.Any of the gland s of the tongue, including the anterior lingual gland s, Ebner gland s, and mucous gland s at the root of the tongue.
SEE: under Luschka, Hubert von.
major salivary g.Any of the six large salivary gland s: the two parotids, the two sublinguals, and the two submand ibulars.
mammary g.A compound alveolar gland that secretes milk. In women, this gland is made up of lobes and lobules bound together by areolar tissue. Each of the 15 to 20 main ducts (lactiferous ducts) discharges through a separate orifice on the surface of the nipple. The lactiferous sinuses (dilatations of the ducts) form reservoirs for the milk during lactation. SYN: lactiferous gland .
meibomian g.Long thin sebaceous gland s aligned in parallel in a single row in the tarsal plates of the eyelids. They open along the inner free margin of the eyelid, and their lipid-rich secretion mixes with aqueous secretions of lacrimal gland s to form the tear film that coats the surface of the eye. SYN: palpebral gland ; tarsal gland .
minor salivary g.Any of the hundreds of 1- to 2-mm diameter mucus-secreting salivary gland s distributed throughout the oral submucosa. These small gland s are named by their locations, e.g., buccal salivary gland s, lingual gland s. Some gland s are also known by the name of their original describers, e.g., the von Ebner gland s.
mixed g.1A gland that secretes in two different fashions, e.g., endocrine and exocrine, such as the pancreas.2A gland that contains two different secretory cell types, e.g., mucous and serous cells, as in the salivary gland s.
g's. of Moll Small secretory and apocrine gland s adjacent to follicles of eyelashes. Their secretions are thought to be antibacterial.
SEE: ciliary gland .
SEE: Montgomery gland .
odoriferous g.Any of the gland s exuding odoriferous materials, e.g., those around the prepuce or anus.
olfactory g.Any of the gland s in the olfactory mucous membranes.
oxyntic g.Any of the gastric gland s found in the fundus and body of the gastric mucosa.
palatine g.Any of the mucous gland s in the tissue of the palate.
parathyroid g.Any of four small endocrine gland s about 6 mm long by 3 to 4 mm broad on the back of and at the lower edge of the thyroid gland or embedded within it. These gland s secrete parathormone.
A deficiency of parathormone results in hypoparathyroidism, which results in neuromuscular hyperexcitability manifested by carpopedal spasm, wheezing, muscle cramps, urinary frequency, mood changes, and lassitude. Blood calcium falls and blood phosphorus rises. Other symptoms include blurring of vision caused by cataracts, poorly formed teeth if onset was in childhood, maldevelopment of hair and nails, and dry and scaly skin.
An excess of parathormone results in hyperparathyroidism which results in a rise in blood calcium and fall in blood phosphorus. Calcium is removed from bones, resulting in increased fragility. Muscular weakness, reduced muscular tone, and general neuromuscular hypoexcitability occur. Osteitis fibrosa cystica is associated with hyperplasia and resulting hypersecretion of the parathyroids.
SEE: Skene gland .
parotid g.The largest of the salivary gland s, located below the ear and inside the ramus of the mand ible. It is a compound tubuloalveolar serous gland . Its secreting tubules and acini are long and branched, and it is enclosed in a sheath (parotid fascia). Saliva lubricates food and makes it easier to taste, chew, and swallow.
SEE: mumps.
pineal g.An endocrine gland in the brain, shaped like a pine cone and located in a pocket near the splenium of the corpus callosum. It is the site of melatonin synthesis, which is inhibited by light striking the retina. SYN: pineal body.;
SEE: melatonin.
pituitary g.A small, gray, rounded gland that develops from ingrown oral epithelium (Rathke pouch) and is attached to the lower surface of the hypothalamus by the infundibular stalk. The Rathke pouch forms the anterior lobe and an intermediate area; the neural tissue of the infundibular stalk forms the posterior lobe. The pituitary gland averages 1.3 × 1.0 × 0.5 cm in size and weighs 0.55 to 0.6 g. SYN: hypophysis; hypophysis cerebri; pituitary body.
SEE: illus.; adenohypophysis; anterior pituitary
The pituitary is an endocrine gland secreting a number of hormones that regulate many bodily processes including growth, reproduction, and other metabolic activities. It is often referred to as the master gland of the body.
Hormones are secreted from the following lobes: Intermediate lobe: In cold-blooded animals, intermedin is secreted, influencing the activity of pigment cells (chromatophores) of fishes, amphibians, and reptiles. In warm-blooded animals, no effects are known.
Anterior lobe: Secretions here are the somatotropic, or growth hormone (STH or GH), which regulates cell division and protein synthesis for growth; adrenocorticotropic hormone (ACTH), which regulates functional activity of the adrenal cortex; thyrotropic hormone (TTH or TSH), which regulates functional activity of the thyroid gland ; and prolactin (lactogenic hormone), which induces secretion of milk in the adult female. The gonadotropic hormones are: in women, the follicle-stimulating hormone (FSH), which stimulates development of ovarian follicles and their secretion of estrogen; in men FSH stimulates spermatogenesis in the testes. In women, luteinizing hormone (LH) (also called interstitial cell-stimulation hormone [ICSH]) stimulates ovulation and formation of the corpus luteum and its secretion of estrogen and progesterone. In men, LH stimulates testosterone secretion.
Posterior lobe: Hormones are secreted by the neurosecretory cells of the hypothalamus and pass through fibers of the supraopticohypophyseal tracts in the infundibular stalk to the neurohypophysis, where they are stored. The secretions are oxytocin, which acts specifically on smooth muscle of the uterus, increasing tone and contractility, and antidiuretic hormone (ADH) (also called vasopressin), which increases reabsorption of water by the kidney tubules. In large amounts, ADH also causes vasoconstriction.
Hypersecretion of anterior lobe causes gigantism, acromegaly, and pituitary basophilism (Cushing disease). Hyposecretion of anterior lobe causes dwarfism, pituitary cachexia (Simmonds disease), Sheehan syndrome, acromicria, eunuchoidism, or hypogonadism. Posterior lobe deficiency or hypothalamic lesion causes diabetes insipidus. Anterior and posterior lobe deficiency and hypothalamic lesion cause Fröhlich syndrome (adiposogenital dystrophy) and pituitary obesity.
preputial g.A modified sebaceous gland located on the distal aspect of the penis, near the glans. The secretion of this gland is a component of smegma.
prostate g.The gland in the male that surrounds the neck of the bladder and the urethra. It is partly gland ular, with ducts opening into the prostatic portion of the urethra, and partly muscular. It secretes a thin, opalescent, slightly alkaline fluid that forms part of the semen. The prostate consists of a median lobe and two lateral lobes measuring about 2 × 4 × 3 cm and weighing about 20 g; it is enclosed in a fibrous capsule containing smooth muscle fiber in its inner layer. The nerve supply is from the inferior hypogastric plexus.
pyloric g.A gastric gland in the pyloric region of the stomach.
salivary g.Any of the gland s near the oral cavity that secrete saliva. The major gland s are paired and include the parotid, the sublingual, and the submand ibular. There are numerous minor salivary gland s in the oral cavity, named according to their locations: lingual, sublingual, palatal, buccal, labial, and glossopharyngeal.
SEE: illus.
Salivary secretion is under nervous control, reflexly initiated by mechanical, chemical, or radiant stimuli acting on taste buds in the mouth, olfactory receptors, or the eyes. Secretion may also be due to conditioned reflexes as when one thinks about food or hears a dinner bell. The nerve supply of the salivary gland s is from the facial and glossopharyngeal nerves, which increase secretion, and from the sympathetic nerves, which decrease secretion. The blood supply is from branches of the external carotid artery.
sebaceous g.An oil-secreting gland of the skin. The gland s are simple or branched alveolar gland s, most of which open into hair follicles. They are holocrine gland s whose secretion arises from the disintegration of cells filling the alveoli. Some aberrant gland s are found in the cheeks or lips of the oral cavity.
SEE: Fordyce disease.
seromucous g.A mixed serous and mucous gland .
serous g.An exocrine gland with a relatively watery secretion, isotonic with blood plasma, and containing enzymes, glycoproteins, lysozymes, and bactericides. Serous gland s are most common in the gastrointestinal tract, esp. in the salivary gland s.
simple g.A gland shaped like a single unbranched sac. When the sac is cylindrical, the gland is called simple tubular; when the sac is flask-shaped, the gland is called simple alveolar.
SEE: under Skene, Alexand er.
sublingual g.The smallest of the major salivary gland s, located in the tissue in the floor of the mouth between the tongue and mand ible on each side. It is a mixed seromucous gland . Its multiple small ducts open directly into the oral cavity along the sublingual fold. Numerous minor sublingual gland s are scattered throughout the mucosa under the tongue, each with its own duct to the oral surface.
submand ibular g.A mixed tubuloalveolar salivary gland about the size of a walnut that lies below the posterior floor of the mouth, medial to the body of the mand ible. Its main duct (Wharton duct) opens at the side of the frenulum linguae.
sudoriferous g.Any of the gland s in the skin that secrete perspiration.
SEE: sweat gland s for illus.
sweat g.Any of the simple, coiled, tubular gland s found on all body surfaces except the margin of the lips, glans penis, and inner surface of the prepuce. The coiled secreting portion lies in the corium or subcutaneous portion of skin; the secretory duct follows a straight or oblique course through the dermis but becomes spiral in passing through the epidermis to its opening, a sweat pore. Most sweat gland s are merocrine; those of the axilla, areola, mammary gland , labia majora, and circumanal region are apocrine. Sweat gland s are most numerous on the palms of the hand s and soles of the feet.
SEE: illus.; apocrine gland ; eccrine gland .
target g.Any gland affected by the action or secretion of another gland , e.g., the thyroid is a target gland of the pituitary.
SEE: thymus.
thyroid g.A large endocrine gland located in the center of the base of the neck. The gland is composed of two lobes, one on each side of the trachea, and an isthmus of tissue connecting the lower two thirds of each lobe. The isthmus is usually located at the level of the second to third tracheal rings. The whole gland is surrounded by a thin fibrous capsule attached in back to the cricoid cartilage and the first few tracheal rings. The lobes of the thyroid lie under the sternothyroid and sternohyoid muscles. The thyroid is filled with capillary networks (supplied by the superior and inferior thyroid arteries) that surround the many spherical units (follicles) packed inside the gland . Thyroid follicles consist of a ring of follicular cells surrounding a space filled with a clear colloid (a mixture of thyroglobulin proteins and iodine), from which the thyroid hormones (thyroxine and related molecules) are synthesized. These hormones regulate the rate of cellular metabolism throughout the body. All the steps in synthesizing and releasing thyroid hormones are stimulated by thyroid-stimulating hormone secreted by the pituitary gland . Another class of thyroid cells, the parafollicular or C cells, is found outside the follicles; C cells secrete calcitonin, a calcium-lowering hormone.
SEE: illus.
tracheal g.Any of the acinar gland s of the tracheal mucosa.
tubular g.A multicellular gland in which the cells secrete specific molecules into a cylindrical sac.
unicellular g.A lone secretory epithelial cell, often found in the midst of nonsecretory cells. A common example is the goblet cell, a unicellular mucous-secreting gland found in the columnar epithelium of the intestinal tract.
uterine g.Any of the tubular gland s in the endometrium.
vaginal g.Any of the alveolar gland s found in the uppermost portion of the vaginal mucosa near the cervix. Most of the vaginal mucosa is devoid of gland s.
vestibular g.Any of the gland s of the vaginal vestibule. They include the minor vestibular gland s and the major vestibular gland s (Bartholin gland s).
SEE: under Ebner, A. G. Victor von.
SEE: under Bartholin gland Bartholin, Caspar (the younger).
g. of Zeis Small sebaceous gland s surrounding the follicles of eyelashes. Their lipid-rich secretions coat the shafts of lashes.