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mammary gland

 

n.
Any of the milk-producing glands in female mammals, consisting of lobes containing clusters of alveoli with a system of ducts to convey the milk to an external nipple or teat. These glands typically occur in pairs and begin secreting milk when young are born.


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Milk-producing gland of female mammals, usually present but undeveloped and nonfunctional in males. Regulated by the endocrine system, it is derived from a modification of sweat glands. The mammary gland of a woman who has not borne children consists of a conical disk of glandular tissue, encased in fat that gives the breast its shape. The gland is made up of lobes drained by separate ducts that meet at the nipple. Pregnancy causes the cells lining the lobes to multiply, and lactation begins in response to hormones released starting at the time of birth. At the end of lactation, the glands return almost to their state before pregnancy. After menopause, they atrophy and are largely replaced by connective tissue and fat.

For more information on mammary gland, visit Britannica.com.

A unique anatomical structure of mammals that secretes milk for the nourishment of the newborn. The mammary gland contains thousands of milk-producing units called alveoli, each of which consists of a unicellular layer of epithelial cells arranged in a spheroid structure. The alveolar epithelial cells take up a variety of nutrients from the blood that perfuses the outer surface of the alveolar structures. Some of the nutrients are then secreted directly into the alveolar lumen; other nutrients are used to synthesize the unique constituents of milk which are then secreted. Each alveolus is connected to a duct through which milk flows. The ducts from many alveoli are connected via a converging ductal system which opens externally by way of the lactiferous pore.

Surrounding each alveolus and its associated small ducts are smooth muscle cells called myoepithelial cells. These cells contract in response to the posterior pituitary hormone oxytocin; milk is thus forced out of the alveoli, through the ductal system, and out the lactiferous pore for the nourishment of the newborn. The release of oxytocin is a neuroendocrine reflex triggered by the stimulation of sensory receptors by the suckling of the newborn.

Mammary glands are basically highly modified and specialized sebaceous glands which derive from ectoderm. In the embryo, mammary lines, formed on both sides of the midventral line, mark the location of future mammary glands. Along the mammary lines discrete ectodermal ingrowths, called mammary buds, produce a rudimentary branched system of ducts at birth. In all species (except the monotremes) a nipple or teat develops in concert with the mammary buds. In the most primitive mammal (the duckbill or platypus), which lacks nipples or teats, milk simply oozes out of the two mammary gland areas and is lapped up by the young.

From birth to sexual maturity the mammary gland consists of a nipple and a rudimentary ductal system in both males and females. At the onset of puberty in the female, the enhanced secretion of estrogen causes a further development of the mammary ductal system and an accumulation of lipids in fat cells. After puberty in women, the mammary gland consists of about 85% fat cells and a partially developed ductal system. See also Estrogen.

During pregnancy the mammary gland comes under the influence of estrogen and progesterone which are derived from both the ovary and placenta. These hormones cause a further branching of the ductal system and the development of milk-secreting structures, the alveoli. In humans, approximately 200 alveoli are surrounded by a connective tissue sheath forming a structure called a lobule. About 26 lobules are packaged via another connective tissue sheath into a larger structure called a lobe. Each of 15–20 lobes is exteriorized into the nipple via separate lactiferous pores. See also Progesterone.

A complement of hormones maximizes the development of the ductal andlobuloalveolar elements in the mammary gland. Optimal ductal growth is attainedwith estrogen, a glucocorticoid, prolactin, and insulin. Maximal lobuloalveolargrowth is obtained with estrogen, progesterone, growth hormone, prolactin, aglucocorticoid, and insulin. During pregnancy estrogen and progesteronestimulate mammary development but inhibit milk production.

During the final third of pregnancy, the alveolar epithelial cells beginsecreting a fluid called colostrum. This fluid fills the alveoli and causes agradual enlargement of the breast or udder. At parturition, the inhibitoryinfluence of estrogen and progesterone is removed, and the gland can secretemilk under the influence of a further complement of hormones includingprolactin, a glucocorticoid, insulin, and the thyroid hormones. See also Gland; Lactation; Mammalia; Milk; Pregnancy.


Columbia Encyclopedia:

mammary gland

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mammary gland, organ of the female mammal that produces and secretes milk for the nourishment of the young. A mammal may have from 1 to 11 pairs of mammary glands, depending on the species. Generally, those mammals that bear larger litters have more glands. The mammary gland of the cow and of some other mammals is known as the udder.

In humans, there is one pair of mammary glands, also known as mammae, or breasts. They are rudimentary in both sexes until the age of puberty when, in response to ovarian hormones, they begin to develop in the female. During pregnancy, they distend still further in preparation for nursing the infant. Pregnant women are prevented from lactating (producing milk) by the presence in the blood of high levels of estrogen and progesterone, secreted by the placenta until birth occurs.

After birth, response to prolactin, the milk-stimulating hormone, is no longer inhibited by placental hormones, and lactation begins. Mammary tissue contains between 15 and 20 compartments called lobes, each of which is divided into smaller compartments called lobules. The lobes and lobules are connected by a network of tubes whose cells manufacture the liquid and fatty substances that form milk. The tubes of each lobe connect with a duct, and all ducts lead to the nipple, where the milk is secreted when the nipple is sucked by the young. The letdown of milk during the nursing process is aided by oxytocin, a hormone secreted by the pituitary. The physical force of an infant's sucking on the breast is a major stimulus to milk production. Disorders of the mammary gland include mastitis and breast cancer.


or mamma

the milk-secreting organ of a mature female mammal. The mammary glands occur in one or more pairs on the ventral surface.

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Mammary gland

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Mammary gland in a human female
Illu breast anatomy.jpg
Cross section of the breast of a human female.
Dissected lactating breast gray1172.png
Dissection of a lactating breast.
1 - Fat
2 - Lactiferous duct/lobule
3 - Lobule
4 - Connective tissue
5 - Sinus of lactiferous duct
6 - Lactiferous duct
Latin glandula mammaria
Gray's subject #271 1267

A mammary gland is an organ in mammals that produces milk to feed young offspring. Mammals get their name from the word "mammary". In ruminants such as cows, goats, and deer, the mammary glands are contained in their udders. The mammary glands of other mammals that have more than two breasts, such as dogs and cats, are sometimes called dugs.

Contents

Humans

Histology

A mammary gland is a specific type of apocrine gland specialized for manufacture of colostrum at the time of parturition. Mammary glands can be identified as apocrine because they exhibit striking "decapitation" secretion. Whether mammary glands are modified sweat glands or sebaceous glands still remains controversial.[1]

Structure

The basic components of a mature mammary gland are the alveoli (hollow cavities, a few millimetres large) lined with milk-secreting cuboidal cells and surrounded by myoepithelial cells. These alveoli join up to form groups known as lobules, and each lobule has a lactiferous duct that drains into openings in the nipple. The myoepithelial cells can contract under the stimulation of oxytocin thereby excreting milk secreted from alveolar units into the lobule lumen toward the nipple, where it collects in sinuses of the ducts. As the infant begins to suck, the hormonally (oxytocin) mediated "let down reflex" ensues and the mother's milk is secreted – not sucked from the gland – into the baby's mouth.

All the milk-secreting tissue leading to a single lactiferous duct is called a "simple mammary gland"; a "complex mammary gland" is all the simple mammary glands serving one nipple. Humans normally have two complex mammary glands, one in each breast, and each complex mammary gland consists of 10–20 simple glands. The presence of more than two nipples is known as polythelia and the presence of more than two complex mammary glands as polymastia.

To keep the correct polarized morphology of the lactiferous duct tree requires another essential component - mammary epithelial cells extracellular matrix (ECM), which together with adipocytes, fibroblast, inflammatory cells etc. constitute mammary stroma.[2] Mammary epithelial ECM mainly contains myoepithelial basement membrane and the connective tissue. They not only help to support mammary basic structure, but also serve as a communicating bridge between mammary epithelials and their local and global environment throughout this organ's development.[3]

Development and hormonal control

Mammary glands develop during different growth cycles. They exist in both sexes during embryonic stage, forming only a rudimentary duct tree at birth. In this stage, mammary gland development depends on systemic (and maternal) hormones,[2] but is also under the (local) regulation of paracrine communication between neighboring epithelial and mesenchymal cells by parathyroid hormone-related protein(PTHrP).[4] This locally secreted factor gives rise to a series of outside-in and inside-out positive feedback between these two types of cells, so that mammary bud epithelial cells can get to proliferate and sprout down into the mesenchymal layer until they reach the fat pad to begin the first round of branching.[2] At the same time, the embryonic mesenchymal cells around the epithelial bud get secrecting factors activated by PTHrP, such as BMP4, can transform into a dense, mammary-specific mesenchyme, which later develop into connective tissue with fibrous threads, forming blood vessels and the lymph system.[5] Basement membrane, mainly containing laminin and collagen, formed thereafter by differentiated myoepithelial cells keeps the polarity of this primary duct tree.

Lactiferous duct development occurs in females in response to circulating hormones, a first development is frequently seen during pre- and postnatal stages and later during puberty. Estrogen promotes branching differentiation,[6] whereas in males testosterone inhibits it. A mature duct tree reaching the limit of the fat pad of the mammary gland comes into being by bifurcation of duct terminal end buds (TEB), secondary branches sprouting from primary ducts[3][7] and proper duct lumen formation. These processes are tightly modulated by components of mammary epithelial ECM interacting with systemic hormones and local secreting factors. However, for each mechanism the epithelial cells' "niche" can be delicately unique with different membrane receptor profiles and basement membrane thickness from specific branching area to area, so as to regulate cell growth or differentiation sub-locally.[8] Important players include beta-1 integrin, epidermal growth factor receptor (EGFR), laminin-1/5, collagen-IV, matrix metalloproteinase(MMPs), heparan sulfate proteoglycans etc. Elevated circulating level of growth hormone and estrogen get to multipotent cap cells on tip of TEB through a leaky thin layer of basement membrance and promote specific gene expression. Hence cap cells can differentiate into myoepithelial and luminal (duct) epithelial cells, and the increased amount of activated MMPs can degrade surrounding ECM helping duct buds to reach further in the fat pads.[9][10] On the other hand, basement membrane along the mature mammary ducts is thicker with strong adhesion to epithelial cells via binding to integrin and non-integrin receptors. When side branches develop, it is a much more “pushing-forward” working process including extending through myoepithelial cells, degrading basement membrane and then invading into a periductal layer of fibrous stromal tissue.[3] Degraded basement membrane fragments (laminin-5) roles to lead the way of mammary epithelial cells migration.[11] Whereas, laminin-1 interacts with non-integrin receptor dystroglycan negatively regulates this side branching process in case of cancer.[12] These complex "Yin-yang" balancing crosstalks between mammary ECM and epithelial cells "instruct" healthy mammary gland development until adult.

Secretory alveoli develop mainly in pregnancy, when rising levels of prolactin, estrogen and progesterone cause further branching, together with an increase in adipose tissue and a richer blood flow. In gestation, serum progesterone remains at a stably high concentration so signaling through its receptor is continuously activated. As one of the transcribed genes, Wnts secreted from mammary epithelial cells act paracrinely to induce more neighboring cells branching.[13][14] When the lactiferous duct tree is almost ready, "leaves" alveoli are differentiated from luminal epithelial cells and added at the end of each branch. In late pregnancy and for the first few days after giving birth, colostrum is secreted. Milk secretion (lactation) begins a few days later due to reduction in circulating progesterone and the presence of another important hormone prolactin, which mediates further alveologenesis, milk protein production, and regulates osmotic balance and tight junction function. Laminin and collagen in myoepithelial basement membrane interacting with beta-1 integrin on epithelial surface again, is essential in this process.[15][16] Their binding ensures correct placement of prolactin receptors on basal lateral side of alveoli cells and directional secretion of milk into lactiferous ducts.[15][16] Suckling of the baby causes release of hormone oxytocin which stimulates contraction of the myoepithelial cells. In this way of combined control from ECM and systemic hormones, milk secretion can be reciprocally amplified so as to provide enough nutrition for the baby.

During weaning, decreased prolactin, missing mechanical stimulation (baby suckling) and changes in osmotic balance caused by milk stasis and leaking of tight junctions cause cessation of milk production. In some species there is complete or partial involution of alveolar structures after weaning, in humans there is only partial involution and the level of involution in humans appears to be highly individual. In some other species (such as cows) all alveoli and secretory duct structure collapse by programmed cell death (apoptosis) and autophagy for lack of growth promoting factors either from the ECM or circulating hormones.[17][18] At the same time, apoptosis of blood capillary endothelial cells speeds up the regression of lactation ductal beds. Shrinkage of the mammary duct tree and ECM remodeling by various proteinase is under the control of somatostatin and other growth inhibiting hormones and local factors.[19] This big structure change leads loose fat tissue to fill up the empty space thereafter. But a functional lactiferous duct tree can be formed again when a female is pregnant again.

Breast cancer

Tumorigenesis in mammary glands can be induced biochemically by abnormal expression level of circulating hormones or local ECM components,[20] or from a mechanical change in the tension of mammary stroma.[21] Under either of the two circumstances, mammary epithelial cells would grow out of control and eventually result in cancer. Almost all instances of breast cancer originate in the lobules or ducts of the mammary glands.

Other mammals

The constantly protruding breasts of the adult human female, unusually large relative to body size, are a unique evolutionary development whose purpose is not yet fully known (see breasts); other mammals tend to have less conspicuous mammary glands that protrude only while actually filling with milk. The number and positioning of complex and simple mammary glands varies widely in different mammals. The nipples and glands can occur anywhere along the two milk lines, two roughly-parallel lines along the ventral aspect of the body. In general most mammals develop mammary glands in pairs along these lines, with a number approximating the number of young typically birthed at a time. The number of nipples varies from 2 (in most primates) to 18 (in pigs). The Virginia Opossum has 13, one of the few mammals with an odd number.[22][23] The following table lists the number and position of glands normally found in a range of mammals:

Species [24] Anterior
(thoracic)
Intermediate
(abdominal)
Posterior
(inguinal)
Total
Goat, sheep, horse
guinea pig
0 0 2 2
Cattle 0 0 4 4
Cat 2 2 4 8
Dog [25] 4 2 2 or 4 8 or 10
Mouse 6 0 4 10
Rat 6 2 4 12
Pig 6 6 6 18
proboscideans, primates 2 0 0 2

Male mammals typically have rudimentary mammary glands and nipples, with a few exceptions: male mice don't have nipples, and male horses lack nipples and mammary glands.[citation needed] The male Dayak fruit bat has lactating mammary glands;[26] male lactation occurs infrequently in some species, including humans.

Mammary glands are true protein factories, and several companies[who?] have constructed transgenic animals, mainly goats and cows, in order to produce proteins for pharmaceutical use. Complex glycoproteins such as monoclonal antibodies or antithrombin cannot be produced by genetically engineered bacteria, and the production in live mammals is much cheaper than the use of mammalian cell cultures.

Evolution

It is believed that the mammary gland is a transformed sweat gland, more closely related to Apocrine sweat glands.[27] There are many theories of how they evolved, but since they do not fossilize well, supporting such theories is difficult. Many of the current theories are based on comparisons between lines of living mammals- monotremes, marsupials and eutherians. One theory proposes that mammary glands evolved from glands that were used to keep the eggs of early mammals moist[28][29] and free from infection[30][31] (monotremes still lay eggs). Other theories propose that early secretions were used directly by hatched young,[32] or that the secretions were used by young to help them orient to their mothers.[33]

Lactation developed long before the evolution of the mammary gland and mammals, see evolution of lactation.

Gallery

See also

References

  1. ^ Ackerman, A. Bernard; Almut Böer, Bruce Bennin, and Geoffrey J. Gottlieb (2005). Histologic Diagnosis of Inflammatory Skin Diseases An Algorithmic Method Based on Pattern Analysis. ISBN 978-1893357259. http://www.derm101.com/content/13501. 
  2. ^ a b c Watson, C.J. & Khaled, W.T. Mammary development in the embryo and adult: a journey of morphogenesis and commitment. Development 135, 995-1003 (2008)
  3. ^ a b c Wiseman, B.S. & Werb, Z. Stromal effects on mammary gland development and breast cancer. Science 296, 1046-9 (2002)
  4. ^ Wysolmerski, J.J. et al. Rescue of the parathyroid hormone-related protein knockout mouse demonstrates that parathyroid hormone-related protein is essential for mammary gland development. Development 125, 1285-94 (1998)
  5. ^ Hens, J.R. & Wysolmerski, J.J. Key stages of mammary gland development: molecular mechanisms involved in the formation of the embryonic mammary gland. Breast Cancer Res 7, 220-4 (2005)
  6. ^ Sternlicht, M.D. Key stages in mammary gland development: the cues that regulate ductal branching morphogenesis. Breast Cancer Res 8, 201 (2006)
  7. ^ Sternlicht, M.D., Kouros-Mehr, H., Lu, P. & Werb, Z. Hormonal and local control of mammary branching morphogenesis. Differentiation 74, 365-81 (2006)
  8. ^ Fata, J.E., Werb, Z. & Bissell, M.J. Regulation of mammary gland branching morphogenesis by the extracellular matrix and its remodeling enzymes. Breast Cancer Res 6, 1-11 (2004).
  9. ^ Wiseman, B.S. et al. Site-specific inductive and inhibitory activities of MMP-2 and MMP-3 orchestrate mammary gland branching morphogenesis. J Cell Biol 162, 1123-33 (2003)
  10. ^ Koshikawa, N., Giannelli, G., Cirulli, V., Miyazaki, K. & Quaranta, V. Role of cell surface metalloprotease MT1-MMP in epithelial cell migration over laminin-5. J Cell Biol 148, 615-24 (2000)
  11. ^ Dogic, D., Rousselle, P. & Aumailley, M. Cell adhesion to laminin 1 or 5 induces isoform-specific clustering of integrins and other focal adhesion components. J Cell Sci 111 ( Pt 6), 793-802 (1998)
  12. ^ Muschler, J. et al. A role for dystroglycan in epithelial polarization: loss of function in breast tumor cells. Cancer Res 62, 7102-9 (2002)
  13. ^ Robinson, G.W., Hennighausen, L. & Johnson, P.F. Side-branching in the mammary gland: the progesterone-Wnt connection. Genes Dev 14, 889-94 (2000)
  14. ^ Brisken, C. et al. Essential function of Wnt-4 in mammary gland development downstream of progesterone signaling. Genes Dev 14, 650-4 (2000)
  15. ^ a b Streuli, C.H., Bailey, N. & Bissell, M.J. Control of mammary epithelial differentiation: basement membrane induces tissue-specific gene expression in the absence of cell-cell interaction and morphological polarity. J Cell Biol 115, 1383-95 (1991)
  16. ^ a b Streuli, C.H. et al. Laminin mediates tissue-specific gene expression in mammary epithelia. J Cell Biol 129, 591-603 (1995)
  17. ^ Zarzynska J, Motyl T.Apoptosis and autophagy in involuting bovine mammary gland. J Physiol Pharmacol. 2008 Dec;59 Suppl 9:275-88
  18. ^ Fadok, V.A., Clearance: the last and often forgotten stage of apoptosis. J Mammary Gland Biol Neoplasia, 1999. 4(2): p. 203-11
  19. ^ Motyl, T., et al., Apoptosis and autophagy in mammary gland remodeling and breast cancer chemotherapy. J Physiol Pharmacol, 2006. 57 Suppl 7: p. 17-32
  20. ^ Gudjonsson, T. et al. Normal and tumor-derived myoepithelial cells differ in their ability to interact with luminal breast epithelial cells for polarity and basement membrane deposition. J Cell Sci 115, 39-50 (2002)
  21. ^ Provenzano, P.P. et al. Collagen density promotes mammary tumor initiation and progression. BMC Med 6, 11 (2008)
  22. ^ With the Wild Things - Transcripts
  23. ^ Raising Orphaned Baby Opossums
  24. ^ Merle Cunningham, Animal Science and Industry ISBN 9780130462565
  25. ^ Dog breeds vary in the number of mammary glands: larger breeds tend to have 5 pairs, smaller breeds have 4 pairs.
  26. ^ Francis, Charles M.; Anthony, Edythe L. P.; Brunton, Jennifer A.; Kunz, Thomas H. (24 February 1994). "Lactation in male fruit bats". Nature (Nature Publishing Group) 367 (6465): 691–692. doi:10.1038/367691a0. http://www.nature.com/nature/journal/v367/n6465/abs/367691a0.html. Retrieved 2008-05-14. 
  27. ^ Oftedal, O.T. (2002). "The origin of lactation as a water source for parchment-shelled eggs=Journal of Mammary Gland Biology and Neoplasia". Journal of Mammary Gland Biology and Neoplasia 7 (3): 253–266. doi:10.1023/A:1022848632125. PMID 12751890. 
  28. ^ http://nationalzoo.si.edu/ConservationAndScience/SpotlightOnScience/oftedalolav20030714.cfm
  29. ^ Oftedal, O.T. (2002). "The mammary gland and its origin during synapsid evolution". Journal of Mammary Gland Biology and Neoplasia 7 (3): 225–252. doi:10.1023/A:1022896515287. PMID 12751889. 
  30. ^ http://scienceblogs.com/pharyngula/2006/05/breast_beginnings.php
  31. ^ Vorbach C., Capecchi M.R., Penninger J.M. (2006). "Evolution of the mammary gland from the innate immune system?". Bioessays 28 (28): 606–616. doi:10.1002/bies.20423. PMID 16700061. 
  32. ^ Lefèvre C.M., Sharp J.A., Nicholas K.R. (2010). "Evolution of lactation: ancient origin and extreme adaptations of the lactation system". Annual Review of Genomics and Human Genetics (11): 219–238. 
  33. ^ Graves B.M and Duvall D. (1983). "A role for aggregation pheremones in the evolution of mammallike reptile lactation". The American Naturalist 122 (6): 835–839. doi:10.1086/284177. 

External links


Translations:

Breast

Top

Dansk (Danish)
n. - kvindebryst, brystkasse
v. tr. - tage kampen op imod, trodse

idioms:

  • breast milk    modermælk

Nederlands (Dutch)
borst, boezem, ribbenkast, confronteren, heuveltop bereiken

Français (French)
n. - sein, poitrine, poitrail, devant (d'une chemise), (Tech) ventre (de haut-fourneau), (Minér) front de taille ou d'abattage
v. tr. - affronter, faire front à, gravir (une colline)

idioms:

  • breast milk    lait maternel

Deutsch (German)
n. - Brust
v. - trotzen

idioms:

  • breast milk    Muttermilch

Ελληνική (Greek)
n. - θώρακας, στέρνο, στήθος, μαστός, βυζί
v. - αντιμετωπίζω άφοβα, αψηφώ, τα βγάζω πέρα, σκαρφαλώνω στην κορυφή (υψώματος)

idioms:

  • breast milk    μητρικό γάλα

Italiano (Italian)
petto, seno

idioms:

  • breast milk    latte materno
  • make a clean breast of    confessare completamente

Português (Portuguese)
n. - mama (f)
v. - enfrentar, aplicar ou opor o peito

idioms:

  • breast milk    leite (m) materno
  • double breasted    trespassado
  • make a clean breast of    contar toda a verdade

Русский (Russian)
грудь, противиться

idioms:

  • breast milk    материнское молоко
  • double breasted    двубортный
  • make a clean breast of    чистосердечно сознаться

Español (Spanish)
n. - pecho, seno, mama
v. tr. - acometer de frente, dar pecho, amamantar

idioms:

  • breast milk    leche materna

Svenska (Swedish)
n. - bröst, barm, bringa, gå rätt emot, möta, trotsa

中文(简体)(Chinese (Simplified))
乳房, 胸膛, 胸部, 胸脯, 以胸对着, 对付

idioms:

  • breast milk    母奶

中文(繁體)(Chinese (Traditional))
n. - 乳房, 胸膛, 胸部, 胸脯
v. tr. - 以胸對著, 對付

idioms:

  • breast milk    母奶

한국어 (Korean)
n. - 흉부, 유방, 마음
v. tr. - ~을 가슴으로 받다, ~에 과감히 맞서다

日本語 (Japanese)
n. - 乳房, 胸, 胸部, 胸中
v. - 立ち向かう, 胸に受ける

idioms:

  • breast milk    母乳

العربيه (Arabic)
‏(الاسم) صدر, نهد, ثدي, واجه, جابه‏

עברית (Hebrew)
n. - ‮חזה, שדיים, בגד חזה‬
v. tr. - ‮התייצב מול, נאבק עם, דחף בחזה, נגע בחזהו ב-‬


 
 

 

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