جفت

جُفت عضوی مسطح در بدن جانداران است که با بند ناف به جنین متصل است.

جفت، پیش از زایمان به دیواره رحم متصل است و در طی زایمان، به همراه نوزاد، و پرده‌های جنینی از کانال زایمانی خارج می‌شود.



جفت انسان، پس از زایمان.
جفت‌خواری پس از زایمان توسط مادر، یکی از رفتارهای تغذیه‌ای است که در میان بسیاری از جانداران دیده می‌شود. به‌جز خوکان دریایی[۱]، آب‌بازان[۲] و شتر، بقیه پستانداران، حتی پستانداران گیاه‌خوار، جفت‌خوار هستند. جفت دارای سطح بالایی از پروستاگلاندین[۳] است و به این خاطر، خوردن آن به برگشت رحم به حالت اولیه و پاک شدن رحم کمک می‌کند.

عارضه چسبندگی جفت در زنان باردار یکی از عوامل مرگ شماری از زنان باردار در اثر خونریزی‌های شدید است. در بارداری‌های با سزارین قبلی که همراه با جفت سر راهی هستند، میزان بروز چسبندگی جفت بیشتر است که این عارضه در صورت تشخیص و پیش‌بینی و آمادگی تیم درمانی (رزرو فرآورده‌های خونی به مقدار کافی، اطلاع قبلی و حضور به موقع متخصصان جراحی و اورولوژی و...) تا حد زیادی قابل اجتناب است.[۴]

این یک نوشتار خُرد زیست‌شناسی است. با گسترش آن به ویکی‌پدیا کمک کنید.


جستارهای وابسته [ویرایش]

جفت سر راهی
پانویس‌ها [ویرایش]

↑ Pinnipedia
↑ Cetacea
↑ prostaglandin
↑ ایرنا: کادر پزشکی به عارضه چسبندگی جفت در زنان باردار بیشتر توجه کند ، بازدید: مه ۲۰۱۱.
منابع [ویرایش]

بر پایهٔ: Mark B. Kristal (2 February 1980), “Placentophagia: A Biobehavioral Enigma”, Neuroscience & Biobehavioral Reviews 4: 141-150، بازیابی ۱۹ ژوئیه ۲۰۰۸.

در ویکی‌انبار پرونده‌هایی دربارهٔ جفت (کالبدشناسی) موجود است.
رده‌های صفحه: اندامدستگاه تناسلی زیست‌شناسی رشد
قس عربی



مشیمة
المشیمة عضو دائری مسطح الشکل یتصل بالجنین عن طریق الحبل السری فی الرحم ویتم خروج المشیمة من جسم الأم بعد المرحلة الثالثة من الولادة[1] للمشیمة أربعة وظائف رئیسیة وهی :
تغذیة الجنین
التنفس إذ ان المشیمة تقوم بوظیفة الرئتین فیحصل الجنین بواسطتها علی الأکسوجین ویطرح ثانی أکسید الکربون
تثبیت الحمل وذلک بفرزها هرمون البروجسترون الذی یساعد علی استمرار الحمل بدایة من الشهر الرابع.
الإخراج، حیث تخرج المواد السامة الناتجة عن الأیض عن طریق المشیمة.
المشیمة عضو شبه أسطوانی ینمو متصلاً بباطن جدار الرحم عند معظم الثدییات. تمد المشیمة الجنین بالطعام والأکسجین، وتسحب نتاج نفایات الجنین. کما أن العضو ینتج کیمیائیات تُسمى هورمونات تحافظ على الحمل وتنظم نمو الجنین.
تتکون المشیمة من أنسجة من الأم ومن الجنین. وبعد الأسبوع الأول من الحمل، یلصق الجنین نفسه بجدار الرحم (العضو الذی ینمو فیه الجنین). وتتکون المشیمة کأعمدة من الخلایا من المشیماء (غطاء یشبه الکیس یحوی الجنین) تخترق وتنفذ خلال البطانة الرحمیة. وفی داخل الأعمدة، توجد أوعیة دمویة تتفرع إلى نتوءات صغیرة تشبه الأصابع تُسمى الزغابات. والزغابات التی تحتوی على دم الجنین محاطة بدم الأم. ولا یختلط دم الجنین بدم الأم.
ویمر الدم والأکسجین من دم الأم خلال الجدران الضعیفة للزغابات ویدخل إلى دم الجنین. وهذا الدم الغنی بالغذاء والأکسجین یصل إلى الجنین خلال ورید فی الحبل السری، وهو أنبوب مرن یربط الطفل بالمشیمة. ویُحمل نتاج النفایات من الجنین خلال الشرایین فی الحبل السری ویعبر خلال الزغابات. ویتخلص نظام الدورة الدمویة للأم من هذه النفایات. وبعد دقائق من ولادة الطفل تنفصل المشیمة ـ التی تسمى أحیانًا الخَلاص ـ من الجسم.

یبلغ معدل طول المشیمة لدى الانسان 22 صم (9 بوصة) و یبلغ سمکها ما بین 2 و 2.5 صم (0.8 و 1 بوصة) و یکون مرکز المشیمة الأکثر سماکة فی حین تصبح أرفع فی المحیط. کما یبلغ معدل وزن المشیمة حوالی 500 غرام (1 رطل) و یکون لونها محمرا أزرق داکنا أو کستنائیا. وتربط الجنین عن طریق الحبل السری الذی یبلغ طوله حوالی 55-60 سم ( 22-24 بوصة ) و یحتوی على شریانین وورید واحد.
[عدل]المراجع

^ المشیمة الموسوعة المعرفیة الشاملة
هذه بذرة مقالة عن علم التشریح تحتاج للنمو والتحسین، فساهم فی إثرائها بالمشارکة فی تحریرها.
تصنیفان: جهاز تناسلی علم الأجنة
قس ترکی
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and lizards with varying levels of development up to mammalian levels.[1] Note, however, that the homology of such structures in various viviparous organisms is debatable at best and, in invertebrates such as Arthropoda, is definitely analogous at best. However, a recent publication describes what amounts to a phylogenetically analogous, but physiologically and functionally almost identical structure in a skink. In some senses it is not particularly surprising, because many species are ovoviviparous and some are known as examples of various degrees of viviparous matrotrophy. However, the latest example is the most extreme to date, of a purely reptilian placenta directly comparable to a eutherian placenta.[2]
The word placenta comes from the Latin word for cake, from Greek plakóenta/plakoúnta, accusative of plakóeis/plakoús – πλακόεις, πλακούς, "flat, slab-like",[3] in reference to its round, flat appearance in humans. The classical plural is placentae, but the form placentas is common in modern English and probably has the wider currency at present.
Prototherial (egg-laying) and metatherial (marsupial) mammals produce a choriovitelline placenta that, while connected to the uterine wall, provides nutrients mainly derived from the egg sac.
The placenta functions as a fetomaternal organ with two components: the fetal placenta, or (Chorion frondosum), which develops from the same sperm and egg cells that form the fetus; and the maternal placenta, or (Decidua basalis), which develops from the maternal uterine tissue.[4]
Contents [hide]
1 Structure
2 Development
3 Placental circulation
3.1 Maternal placental circulation
3.2 Fetoplacental circulation
4 Functions
4.1 Nutrition
4.2 Excretion
4.3 Immunity
4.4 Endocrine function
4.5 Cloaking from immune system of mother
4.6 Other functions
5 Birth
6 Pathology
7 Cultural practices and beliefs
8 Additional images
9 See also
10 References
11 External links
[edit]Structure

In humans, the placenta averages 22 cm (9 inch) in length and 2–2.5 cm (0.8–1 inch) in thickness (greatest thickness at the center and become thinner peripherally). It typically weighs approximately 500 grams (1 lb). It has a dark reddish-blue or maroon color. It connects to the fetus by an umbilical cord of approximately 55–60 cm (22–24 inch) in length that contains two arteries and one vein.[5] The umbilical cord inserts into the chorionic plate (has an eccentric attachment). Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells. This results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called cotyledons. In humans, the placenta usually has a disc shape, but size varies vastly between different mammalian species.[6]
[edit]Development



The initial stages of human embryogenesis.
Further information: Placentation
The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium. The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta. This outer layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer. The syncytiotrophoblast is a multinucleated continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development. The syncytiotrophoblast (otherwise known as syncytium), thereby contributes to the barrier function of the placenta.
The placenta grows throughout pregnancy. Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy (approximately 12–13 weeks).
[edit]Placental circulation



Maternal blood fills the intervillous space, nutrients, water, and gases are actively and passively exchanged, then deoxygenated blood is displaced by the next maternal pulse.
[edit]Maternal placental circulation
In preparation for implantation, the uterine endometrium undergoes 'decidualisation'. Spiral arteries in decidua are remodeled so that they become less convoluted and their diameter is increased. The increased diameter and straighter flow path both act to increase maternal blood flow to the placenta. The relatively high pressure as the maternal blood fills intervillous space through these spiral arteries bathes the fetal villi in blood, allowing an exchange of gases to take place. In humans and other hemochorial placentals, the maternal blood comes into direct contact with the fetal chorion, though no fluid is exchanged. As the pressure decreases between pulses, the deoxygenated blood flows back through the endometrial veins.
Maternal blood flow is approx 600–700 ml/min at term.
[edit]Fetoplacental circulation
Further information: Fetal circulation
Deoxygenated fetal blood passes through umbilical arteries to the placenta. At the junction of umbilical cord and placenta, the umbilical arteries branch radially to form chorionic arteries. Chorionic arteries, in turn, branch into cotyledon arteries. In the villi, these vessels eventually branch to form an extensive arterio-capillary-venous system, bringing the fetal blood extremely close to the maternal blood; but no intermingling of fetal and maternal blood occurs ("placental barrier"[7]).
Endothelin and prostanoids cause vasoconstriction in placental arteries, while nitric oxide vasodilation.[8] On the other hand, there is no neural vascular regulation, and catecholamines have only little effect.[8]
[edit]Functions

[edit]Nutrition
The perfusion of the intervillous spaces of the placenta with maternal blood allows the transfer of nutrients and oxygen from the mother to the fetus and the transfer of waste products and carbon dioxide back from the fetus to the maternal blood supply. Nutrient transfer to the fetus occurs via both active and passive transport. Active transport systems allow significantly different plasma concentrations of various large molecules to be maintained on the maternal and fetal sides of the placental barrier.[9]
Adverse pregnancy situations, such as those involving maternal diabetes, smoking or obesity, can increase or decrease levels of nutrient transporters in the placenta resulting in overgrowth or restricted growth of the fetus[citation needed].
[edit]Excretion
Waste products excreted from the fetus such as urea, uric acid and creatinine are transferred to the maternal blood by diffusion across the placenta.
[edit]Immunity
IgG antibodies can pass through the human placenta, thereby providing protection to the fetus in utero.[10]
Furthermore, the placenta functions as a selective maternal-fetal barrier against transmission of microbes to the fetus. However, insufficiency in this function may still cause mother-to-child transmission of infectious diseases.
[edit]Endocrine function
In humans, aside from serving as the conduit for oxygen and nutrients for fetus, the placenta secretes hormones (secreted by syncytial layer/syncytiotrophoblast of chorionic villi) that are important during pregnancy.
Hormones:
Human Chorionic Gonadotropin (hCG): The first placental hormone produced is hCG, which can be found in maternal blood and urine as early as the first missed menstrual period (shortly after implantation has occurred) through about the 100th day of pregnancy. This is the hormone analyzed by pregnancy test; a false-negative result from a pregnancy test may be obtained before or after this period. Women's blood serum will be completely negative for hCG by one to two weeks after birth. hCG testing is proof that all placental tissue is delivered. hCG is present only during pregnancy because it is secreted by the placenta, which is present only[11] during pregnancy. hCG also ensures that the corpus luteum continues to secrete progesterone and estrogen. Progesterone is very important during pregnancy because, when its secretion decreases, the endometrial lining will slough off and pregnancy will be lost. hCG suppresses the maternal immunologic response so that placenta is not rejected.
Human Placental Lactogen (hPL [Human Chorionic Somatomammotropin]): This hormone is lactogenic and growth-promoting properties. It promotes mammary gland growth in preparation for lactation in the mother. It also regulates maternal glucose, protein, and fat levels so that this is always available to the fetus.
Estrogen is referred to as the "hormone of women" because it stimulates the development of secondary female sex characteristics. It contributes to the woman's mammary gland development in preparation for lactation and stimulates uterine growth to accommodate growing fetus.
Progesterone is necessary to maintain endometrial lining of the uterus during pregnancy. This hormone prevents preterm labor by reducing myometrial contraction. Levels of progesterone are high during pregnancy.
[edit]Cloaking from immune system of mother
Further information: Immune tolerance in pregnancy
The placenta and fetus may be regarded as a foreign allograft inside the mother, and thus must evade from attack by the mother's immune system.
For this purpose, the placenta uses several mechanisms:
It secretes Neurokinin B-containing phosphocholine molecules. This is the same mechanism used by parasitic nematodes to avoid detection by the immune system of their host.[12]
There is presence of small lymphocytic suppressor cells in the fetus that inhibit maternal cytotoxic T cells by inhibiting the response to interleukin 2.[13]
However, the placental barrier is not the sole means to evade the immune system, as foreign fetal cells also persist in the maternal circulation, on the other side of the placental barrier.[14]
[edit]Other functions
The placenta also provides a reservoir of blood for the fetus, delivering blood to it in case of hypotension and vice versa, comparable to a capacitor.[15]
[edit]Birth

(examples at bottom reference premature infants: lower than 9-month birth weight.
Main article: Placental expulsion
Placental expulsion begins as a physiological separation from the wall of the uterus. The period from just after the fetus is expelled until just after the placenta is expelled is called the third stage of labor. The placenta is usually expelled within 15–30 minutes of the baby's being born. This may also be longer, relying on hormonal situ and psychological adjustment to delivery.
Placental expulsion can be managed actively, for example by giving oxytocin via intramuscular injection followed by cord traction to assist in delivering the placenta. As an alternative, it can be managed expectantly, allowing the placenta to be expelled without medical assistance. A Cochrane database study[16] suggests that blood loss and the risk of postpartum bleeding may be reduced in women offered active management of the third stage of labour (needs updating).
The "habit" is to cut the umbilical cord immediately after the baby is born, but there is no medical reason to do that; on the contrary, it seems that not cutting the cord helps the baby in his adaptation to extra uterine life, especially in preterm infants (Mercier, J.S. & Vohr, B.R. (2010).
Seven-month developmental outcomes of very low birth weight infants enrolled in a randomized controlled trial of delayed versus immediate cord clamping. Journal of Perinatology, 30(1):1.) How long the delay?
Read elsewhere: That the cord must be clamped until the beating stops: like an supremely venous intestine in appearance. No reference as to whether the upper comment is discussing immediate or very delayed oxygen/iron absorption.
[edit]Pathology

Main article: Placental disease


Micrograph of a cytomegalovirus (CMV) infection of the placenta (CMV placentitis). The characteristic large nucleus of a CMV infected cell is seen off-centre at the bottom-right of the image. H&E stain.
Numerous pathologies can affect the placenta.
Placenta accreta (when the placenta implants too deeply, into actual muscle of uterine wall)
Placenta praevia
Placental abruption/abruptio placentae
Infections involving the placenta:
Placentitis, such as the TORCH infections.
Chorioamnionitis.
[edit]Cultural practices and beliefs

The placenta often plays an important role in various cultures, with many [http://naturallyborn.net/natural-childbirth-articles/5078-afterbirth-after-birth ://news.bbc.co.uk/2/hi/uk_news/magazine/4918290.stm | title = Why eat a placenta? | date = 18 April 2006 | work = BBC | accessdate = 8 January 2008 }} /ref
Some cultures bury the placenta for various reasons. The Māori of New Zealand traditionally bury the placenta from a newborn child to emphasize the relationship between humans and the earth.[17] Likewise, the Navajo bury the placenta and umbilical cord at a specially chosen site,[18] particularly if the baby dies during birth.[19] In Cambodia and Costa Rica, burial of the placenta is believed to protect and ensure the health of the baby and the mother.[20] If a mother dies in childbirth, the Aymara of Bolivia bury the placenta in a secret place so that the mother's spirit will not return to claim her baby's life.[21]
The placenta is believed by some communities to have power over the lives of the baby or its parents. The Kwakiutl of British Columbia bury girls' placentas to give the girl skill in digging clams, and expose boys' placentas to ravens to encourage future prophetic visions. In Turkey, the proper disposal of the placenta and umbilical cord is believed to promote devoutness in the child later in life. In Ukraine, Transylvania, and Japan, interaction with a disposed placenta is thought to influence the parents' future fertility. The Hmong bury the placenta under the central column of the house if it's a boy, and under the parent's bed if it's a girl - so that when the person dies the soul can go retrieve their first "clothing" and return to their ancestors to be reborn.
Several cultures believe the placenta to be or have been alive, often a relative of the baby. Nepalese think of the placenta as a friend of the baby; Malaysian Orang Asli regard it as the baby's older sibling. The Ibo of Nigeria consider the placenta the deceased twin of the baby, and conduct full funeral rites for it.[20] Native Hawaiians believe that the placenta is a part of the baby, and traditionally plant it with a tree that can then grow alongside the child.[22] Various cultures in Indonesia, such as Javanese, believe that the placenta has a spirit and needs to be buried outside the family house.
In some cultures, the placenta is eaten, a practice known as placentophagy. In some eastern cultures, such as China and Hong Kong, the dried placenta (紫河車) is sometimes used in preparations of traditional Chinese medicine.[23]
[edit]Additional images


Fetus of about 8 weeks, enclosed in the amnion. Magnified a little over two diameters.



Picture of freshly delivered placenta and umbilical cord wrapped around Kelly clamps



Fresh human placenta



Micrograph of a placental infection (CMV placentitis).



Micrograph of CMV placentitis.

[edit]See also

Embryo
Childbirth
Uterus
[edit]References

^ Pough et al. 1992. Herpetology: Third Edition. Pearson Prentice Hall:Pearson Education, Inc., 2002.
^ Blackburn, D. G. and Flemming, A. F. (2011), Invasive implantation and intimate placental associations in a placentotrophic african lizard, Trachylepis ivensi (scincidae) Journal of Morphology. doi:10.1002/jmor.11011
^ Henry George Liddell, Robert Scott, "A Greek-English Lexicon", at Perseus
^ Definitions of placental- related terms.
^ Examination of the placenta
^ Placental Structure and Classification
^ Placental blood circulation
^ a b Kiserud, T.; Acharya, G. (2004). "The fetal circulation". Prenatal Diagnosis 24 (13): 1049–1059. DOI:10.1002/pd.1062. PMID 15614842. edit
^ Wright, Caroline; Sibley, Colin P. (2011). "Placental Transfer in Health and Disease". In Helen Kay, Michael Nelson, and Yuping Wang. The Placenta: From Development to Disease. John Wiley and Sons. pp. 66. ISBN 978-1-4443-3366-4.
^ Simister, N.E., and Story, C.M. 1997. "Human placental Fc receptors and the transmission of antibodies from mother to fetus." Journal of Reproductive Immunology 37: 1-23
^ Pillitteri, Adele(2010). Maternal and Child Health Nursing(6th Edition[Philippine Edition]): Lippincott Williams & Wilkins
^ "Placenta 'fools body's defences'". BBC News. 10 November 2007.
^ Clark DA, Chaput A, Tutton D (March 1986). "Active suppression of host-vs-graft reaction in pregnant mice. VII. Spontaneous abortion of allogeneic CBA/J x DBA/2 fetuses in the uterus of CBA/J mice correlates with deficient non-T suppressor cell activity". J. Immunol. 136 (5): 1668–75. PMID 2936806.
^ Williams Z, Zepf D, Longtine J, et al. (March 2008). "Foreign fetal cells persist in the maternal circulation". Fertil. Steril. 91 (6): 2593–5. DOI:10.1016/j.fertnstert.2008.02.008. PMID 18384774.
^ Assad, R. S.; Lee, F. Y.; Hanley, F. L. (2001). "Placental compliance during fetal extracorporeal circulation". Journal of applied physiology (Bethesda, Md. : 1985) 90 (5): 1882–1886. PMID 11299282. edit
^ Prendiville, W. J.; Elbourne, D.; McDonald, S. J.; Begley, C. M. (2000). Active versus expectant management in the third stage of labour. In Begley, Cecily M. "Cochrane Database of Systematic Reviews". Cochrane Database of Systematic Reviews (3). DOI:10.1002/14651858.CD000007. edit
^ Metge, Joan. 2005. "Working in/Playing with three languages: English, Te Reo Maori, and Maori Bod Language." In Sites N.S vol. 2, No 2:83-90.
^ Francisco, Edna (3 December 2004). "Bridging the Cultural Divide in Medicine". Minority Scientists Network. Retrieved 7 January 2008.
^ Shepardson, Mary (1978). "Changes in Navajo Mortuary Practices and Beliefs". American Indian Quarterly. University of Nebraska Press. JSTOR 0095182x.[dead link]
^ a b Buckley, Sarah J.. "Placenta Rituals and Folklore from around the World". Mothering. Archived from the original on 6 January 2008. Retrieved 7 January 2008.
^ Davenport, Ann (June 2005). "The Love Offer". Johns Hopkins Magazine. Retrieved 7 January 2008.
^ "Why eat a placenta?". BBC. 18 April 2006. Retrieved 8 January 2008.
^ Falcao, Ronnie. "Medicinal Uses of the Placenta". Retrieved 25 November 2008.
[edit]External links

Wikimedia Commons has media related to: Placenta
Look up placenta in Wiktionary, the free dictionary.
Wikibooks Cookbook has a recipe/module on
Placenta stew
Spicy Australian Placenta
Additional Human placenta photography
The Placenta, gynob.com, with quotes from Williams Obstetrics, 18th Edition, F. Gary Cunningham, M.D., Paul C. MacDonald, M.D., Norman F. Grant, M.D., Appleton & Lange, Publishers.
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Human systems and organs
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Extraembryonic and fetal membranes (TE E6.0)
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Prenatal development/Mammalian development of circulatory system (GA 5, TE E5.11)
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Categories: Developmental biologyEmbryology of cardiovascular systemOrgansGreek loanwords
Plasenta, anne ve cenine ait iki dolaşım sistemini birbirinden ayıran bir organdır.
Bir sperm tarafından döllenen yumurta hücresi (zigot) ikiye, dörde ve sonra sekize bölünerek hızla büyümeye başlar. Bunun için yüklü miktarda besine ihtiyaç duyar. Besin maddelerini anneden alabilmek için, embriyo hücrelerinden bir kısmı plasentayı oluştururlar. Plasenta anneyle bebek arasındaki besin, oksijen ve diğer maddelerin alışverişini sağlayan yapıdır. Plasenta yeni hücre gruplarının yani dokuların oluşması için gerekli olan besinleri ve oksijeni özenle seçer ve bunları bebeğe taşırken, atık maddeleri ayırarak onları da annenin vücuduna gönderir.
Rahmin içi, cenini koruyan amniyon sıvısı ile kaplıdır. Amniyon sıvısı olmadan bir bebeğin anne karnında gelişmesi mümkün değildir. Bu sıvı sayesinde, hem anne ve çocuk birbirlerinden faydalanırlar hem de korunmuş olurlar. 12 haftalık olduğunda ceninin kendi kan dolaşım sistemi gelişmiştir. Ancak oksijen ve besinlerin alımı, karbondioksit ve atıkların gönderilmesi için halen annesine bağımlıdır. İki dolaşım sistemi arasındaki değiş tokuş kanlar karışmadan gerçekleşmelidir, yoksa sonuç ölümcül olabilir.
Plasenta anne ve cenine ait iki dolaşım sistemini kusursuzca ayırır. Gazlar, besin maddeleri ve atıklar anne ve ceninin kanları arasında değiş tokuş edilir. Fakat amniyon sıvısı ve ayrı dolaşım sisteminden oluşan bu fiziksel bariyerler bebeğin hayatta kalması için yeterli değildir. Bunlar ancak kısmen başarılı olabilir.
Plasentanın yapısına daha yakından bakıldığında, bu duvarı oluşturan trofoblast hücrelerinin kan için özel olarak tasarlanmış bir bariyer oluşturdukları görülür. Embriyo, annenin dokularıyla çok yakın bir bağlantı içindedir. Bir yandan anneden gelen kanın içindeki maddelerle beslenirken, bir yandan da annenin savunma hücrelerinin tehtidi altındadır. Çünkü embriyo annenin vücudunda düşman kabul edilebilecek yabancı bir madde gibidir. Dolayısıyla besinlerle birlikte anne kanındaki savunma hücrelerinin embriyoya ulaşmaması son derece önemlidir. Ancak plasenta, annenin kanında bulunan savunma hücrelerinin embriyonun tarafına geçmesini engelleyen özel bir tasarıma sahiptir. Annenin kanından alınan oksijen, besin maddeleri ve mineraller bu ince aralıklardan geçerek embriyoya ulaşır. Ama savunma hücreleri daha büyük oldukları için bu aralıklardan geçmeyi başaramazlar.
Dış bağlantılar [değiştir]


Wikimedia Commons'ta
Plasenta ile ilgili çoklu ortam belgeleri bulunur.

Vikisözlük'te placenta ile ilgili tanım bulabilirsiniz.
Additional Placenta resimleri
The Placenta, gynob.com, with quotes from Williams Obstetrics, 18th Edition, F. Gary Cunningham, M.D., Paul C. MacDonald, M.D., Norman F. Grant, M.D., Appleton & Lange, Publishers.
Kategori: Embriyoloji