Uterus - Shikyu (English spelling)

Japanese: 子宮 - しきゅう（英語表記）uterus

It is a female internal reproductive organ located almost in the center of the pelvic cavity, and is a chicken egg-sized organ that is slightly flattened from front to back. It is the site for the implantation and subsequent development of a fertilized egg and has the role of protecting it.

From an embryological perspective, it is formed by the fusion of parts of the fetal fallopian tubes, and when viewed from the front, it is pear-shaped. The narrow, cylindrical part in the lower third of the uterus is called the cervix, the enlarged, sac-like part in the upper two-thirds is called the uterine body, and its internal cavity is called the uterine cavity. The uterus is held in place in the pelvic cavity by three ligaments. It is in contact with the bladder in front, the rectum in back, the vagina in the lower part, and the abdominal cavity in the upper part. It is usually slightly bent forward between the cervix and the uterine body. The uterine cavity connects to a pair of fallopian tubes on both sides of the top of the uterine body (fundus), and opens into the abdominal cavity. It also connects to the vagina via the cervix at the bottom, and the lower end of the cervix protrudes into the vagina, which is called the vaginal part of the uterus. In an adult, the uterus is about 7 cm vertically and 4 cm horizontally, with a wall about 2 cm thick and made of smooth muscle. During pregnancy, this muscle layer develops as the fetus develops, increasing the number of muscle fibers and growing ten times larger to withstand tremendous tension. The force of this muscle contraction plays a major role in childbirth, and is called labor pains. The inner surface of the uterus is covered with a mucous membrane called the endometrium, the thickness of which varies depending on the menstrual cycle and age, but is between 1.5 and 6.0 mm.

The endometrium changes in a very regular way according to the ovarian cycle, alternating between two phases: the proliferative phase and the secretory phase. The proliferative phase is dominated by cell proliferation, and is due to the action of the ovarian hormone estrogen (follicular hormone). The secretory phase follows the proliferative phase, and is dominated by the differentiation of cells necessary for the implantation of a fertilized egg, and is transformed from the proliferative phase into the secretory phase by the action of progesterone (luteal hormone) produced by the corpus luteum of the ovary, which is formed after ovulation. If the fertilized egg does not implant, the endometrium becomes necrotic and is sloughed off, causing menstruation. After menstruation, the endometrium regenerates and becomes ready for the next fertilized egg to implant. In this way, the endometrium repeats exfoliation and regeneration for about 30 years, except during pregnancy. The endometrium, which covers the cervix separately from the endometrium, is thinner than the endometrium, at only 1.0 millimeter, and does not change periodically. However, it responds to estrogen by secreting large amounts of a watery, transparent mucus, making it important in various aspects of the reproductive mechanism, such as testing the response to estrogen, diagnosing ovulation, and penetrating sperm.

Pregnancy occurs when a fertilized egg, fertilized in the fallopian tube, descends into the uterine cavity and implants in the endometrium, which has already entered the secretory phase and thickened and been prepared for implantation. The volume of the uterus increases significantly throughout the entire pregnancy (from 2-3 ml in nulliparous women to 4,000-5,000 ml at the end of pregnancy), but returns to normal after childbirth. However, while the vaginal part of the uterus is nearly conical and the external cervical os is circular in nulliparous women, the vaginal part of the uterus is cylindrical or irregular, the external cervical os is oblong, and often has some slits.

Next, we will discuss the major uterine diseases.

[Masao Arai]

Uterine malposition

Anything other than what is considered normal, such as forward tilt, is called uterine abnormality, and we will explain the main types that are subject to treatment.

(1) Retroverted uterus The correct term is retroverted uterus. When the axis of the uterus is tilted backward, it is called retroverted uterus, and when the axis of the cervix is tilted backward, it is called retroverted uterus. The two conditions are often seen together. When only the retroverted uterus is accompanied by symptoms, it is called retroverted uterus, but most cases are asymptomatic, and the focus of treatment is on retroverted uterus. There are mobile and adhesive types. Mobile types are common in women with asthenic constitutions, such as underdeveloped genitals and visceral ptosis, and some are caused by early labor during the puerperal period. There are usually no particular symptoms. Adhesive types are caused by inflammation of the ovaries, fallopian tubes, and pelvic peritoneum, or endometriosis, which causes the uterine wall to adhere and be pulled backward, and symptoms include lower abdominal pain, lower back pain, menstrual pain, and habitual miscarriage. Treatment of the underlying disease is performed, and if there is no improvement, surgery may be performed.

(2) Pathological anteversion of the uterus: This is when the axis of the uterine body is strongly bent forward, and is often accompanied by congenital uterine hypoplasia, menstrual abnormalities, and infertility. Adhesion is caused by acquired inflammatory diseases of the uterus or surrounding area. In addition to menstrual pain, the uterine body presses on the bladder, causing frequent urination. Surgery is the definitive cure.

(3) Uterine prolapse This is also called uterine prolapse. When the vaginal part of the uterus is below the level of the greater trochanters (the protuberances at the base of the femoral neck) on both sides, it is called uterine ptosis. When the lower half of the uterus prolapses outside the vaginal opening, it is called incomplete uterine prolapse. When the entire uterus prolapses, it is called complete uterine prolapse. In most cases, it occurs due to damage to the cardinal ligaments and pelvic floor muscles that hold the uterus in place during childbirth. Although it may be asymptomatic, it is always accompanied by a feeling of pressure in the lower abdomen, a foreign body sensation at the vaginal opening, and a prolapse of the bladder or rectum, and the patient will begin to complain of stress urinary incontinence. It is common in women who have given birth multiple times and who worked excessively during the puerperium. It is seen after menopause. Radical surgery should be performed as soon as possible.

(4) Uterine hypoplasia This is a disease that is not abnormal in position, but is often accompanied by excessive (pathological) anteversion or retroversion of the uterus, and refers to a uterus that is significantly smaller than normal, but without any morphological abnormalities. It is often accompanied by hypoplasia of the vulva and vagina. Most cases are congenital, caused by endocrine dysfunction of the ovaries and other organs. It can also occur later in life, as a result of a serious illness during the developmental period, or physical disorders such as malnutrition or overwork. Rheumatoid arthritis and menstrual abnormalities can occur, and infertility or pregnancy can result in miscarriage or premature birth. Hormonal therapy is mainly used.

[Masao Arai]

Uterine cancer

It is the most common malignant tumor of the female genitals, and there are cervical cancer and uterine cancer. In Japanese people, the majority of cases are cervical cancer.

[Masao Arai]

Uterine fibroids

What are commonly referred to as lumps, masses, or growths in the uterus are mainly fibroids, a type of benign tumor.

[Masao Arai]

Uterine bleeding

Generally, it refers to abnormal uterine bleeding unrelated to menstruation, i.e., aperiodic and extremely irregular in amount and duration, and can be classified as functional, organic, or pregnancy-related. Functional uterine bleeding is a broad term used to refer to abnormal bleeding without the presence of organic changes such as pregnancy, inflammation, tumors, or trauma. It can occur when the endometrium proliferates excessively without ovulation, or in women who ovulate. Hormonal therapy is performed after the condition of the endometrium is analyzed by exploratory curettage. Organic uterine bleeding is caused by uterine vaginal erosion, cervical polyps, inflammation or tumors of the genitals, especially uterine cancer, uterine fibroids, and choriocarcinoma. Pregnancy-related uterine bleeding is caused by miscarriage, ectopic pregnancy, and hydatidiform mole. In addition, it can also be seen as a partial symptom of systemic diseases such as bleeding diathesis, acute infectious diseases, heart disease, and endocrine disorders.

[Masao Arai]

Cervical erosion

This condition is called uterine erosion because it looks like a sore (erosion) when the cervical mucosa extends beyond the cervix and covers the cervix. This is what is known as uterine erosion, and is seen in the majority of normal mature women. It is not dangerous in itself, but it is similar to the early symptoms of cancer and may be difficult to distinguish from the symptoms with the naked eye. Therefore, it is important to undergo regular detailed examinations for uterine cancer.

[Masao Arai]

Endometritis

Inflammation of the endometrium caused by bacterial infection, which is characterized by a purulent discharge and sometimes lower abdominal pain and fever. The chances of infection are high during the puerperium, miscarriage, intrauterine manipulation such as induced abortion, and insertion of contraceptive devices. In acute cases, it progresses to salpingitis and peritonitis. Puerperal fever is also a type of acute endometritis. In chronic cases, the cervix is affected, and if inflammation ascends to the fallopian tubes, it can cause infertility. Endometritis is often used to refer to inflammation of the cervical mucosa, and can be thought of as the same thing as cervicitis or cervical catarrh. The mucosa of the uterine body regenerates well, and even if it is inflamed, it will be shed during menstruation, so it does not become a problem except after miscarriage, childbirth, or intrauterine manipulation. Early chemotherapy should be performed thoroughly.

[Masao Arai]

Endometriosis

Endometrial cancer is a disease in which endometrial tissue proliferates in areas other than its natural location, the inner surface of the uterine cavity. The characteristic symptoms are menstrual pain, which is most severe before menstruation begins and occurs around the first or second day of menstruation, and may also be accompanied by pain during intercourse, irregular bleeding, menorrhagia, and infertility. The disease has been increasing in recent years, and is attracting attention because it is related to issues regarding women's employment. In relatively young women, the type that affects the small pelvic cavity organs (Douglas pouch, ovaries, intestinal wall, etc.) is common, while in women over 40, uterine adenofibroids are common. As pregnancy is said to be the best treatment, pseudopregnancy therapy, mainly using progesterone, is used conservatively, but the radical cure involves removal of the ovaries and uterus.

[Masao Arai]

Uterine polyps

They are wart-like tumors that hang down from the cervical opening into the cavity, and vary in size from the size of a grain of rice to the size of a cherry. There are cervical polyps, whose stems have their roots in the cervix, and endometrial polyps, whose roots are in the uterine cavity. Cervical polyps develop in cases of chronic inflammation or lacerations of the cervical canal and can cause contact bleeding, or they can become infected and be accompanied by a purulent discharge. Endometrial polyps are caused by proliferation of the endometrium and cause abnormal uterine bleeding. Both are usually benign tumors. Visible ones are removed with forceps. Those within the uterine cavity are removed by total endometrial curettage. Some are malignant, so a histological examination is performed.

[Masao Arai]

Animal uterus

The oviduct is an enlarged part of the female reproductive gland that maintains and develops the fertilized egg. In mammals, the oviduct, which originates from the Mullerian duct, differentiates during development from the part close to the ovary into the reproductive organs called the oviduct, uterus, and vagina. The uterus is originally a pair of organs, but it is fused to various degrees in different animals and is classified into the following types. There are four types: a double uterus in which both uteri are independent and not fused (the uterus of many rodents, marsupials, elephants, etc.), a septate uterus that is quite fused externally but is internally divided into two by a partition (the uterus of some rodents such as hamsters, pigs, cows, etc.), a bicornuate uterus in which only the lower half is fused (the uterus of some carnivores, many ungulates, whales, etc.), and a single uterus that is fused entirely (the uterus of primates).

The uterus undergoes a reversible cycle of remarkable development and regression during the expression of reproductive functions, such as the estrus cycle, menstrual cycle, and pregnancy. When pregnancy is established, the fertilized egg (embryo) attaches to the uterine cavity epithelial cells, and the endometrial stroma at that site hypertrophies and proliferates to form a decidua. This decidua, together with the embryo's trophoblast cells, becomes the placenta and maintains the pregnancy. The placenta not only exchanges nutrients and waste products between the mother and fetus, but also serves as an endocrine organ that secretes placental gonadotropic hormones, placental peptide hormones, and placental steroid hormones, which are necessary for maintaining pregnancy. During normal estrus and menstrual cycles in which pregnancy does not occur, endometrial tissue hypertrophies and proliferates mainly under the control of estrus and luteal hormones, but in primates, as the pseudopregnancy state following ovulation ends, the proliferated tissue breaks down and bleeds, causing menstruation.

Birds, reptiles, amphibians, etc. also have an enlarged part of the oviduct that originates from the Müllerian duct, which is sometimes called the uterus. In oviparous animals, this may function as an eggshell gland that secretes eggshells, but in general its main role is to store eggs until spawning. Fish oviducts come in two types: those that originate from the Müllerian duct, and those that are simply an extension of the ovarian cavity, but in either case, there are species that have an enlarged part called the uterus.

In addition to these vertebrates, in invertebrates that are oviparous or ovoviviparous and have internal fertilization, the part inside the mother's body where the fertilized egg or early embryo remains is sometimes called the uterus. For example, in onychophora (such as hookworms) and nematodes (such as roundworms), the tubular part between the genital pore and the ovary is called the uterus, in trematodes, the thick curved tube between the oocyte and the common genital pore is called the uterus, and in cestodes (such as tapeworms), the blind pouch that branches off from the tube connecting the ovary and the genital pore is called the uterus or especially the uterine bell, and these also serve to store eggs.

[Takao Mori]

[References] | Menstruation | Uterine cancer | Uterine fibroids | Pregnancy

Names of parts of the uterus
©Shogakukan ">

Names of parts of the uterus

Malposition of the uterus
©Shogakukan ">

Malposition of the uterus

Animal uterus
©Shogakukan ">

Animal uterus

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

女性の内性器で、小骨盤腔(くう)のほぼ中央にあり、前後にやや扁平(へんぺい)な鶏卵大の臓器をいい、受精卵の着床とその後の発育の場となり、これを保護する役割をもつ。

　発生学的にみれば胎生期卵管の一部が癒合してできたもので、正面からみると西洋ナシ状を呈し、子宮の下方3分の1の細くくびれた円柱状の部分を子宮頸(けい)、上方3分の2の膨大した袋状の部分を子宮体、その内腔を子宮腔とよぶ。子宮は骨盤腔内に三つの靭帯(じんたい)によって保持される。前方は膀胱(ぼうこう)、後方は直腸、下方は腟(ちつ)、上方は腹腔に接する。子宮頸と子宮体の間でやや前かがみの状態に屈曲（前屈）しているのが普通である。子宮腔は子宮体の最上部（子宮底）の両側で1対の卵管に連絡し、腹腔に通ずる。また下方の子宮頸を経て腟内に通じ、子宮頸の下端部は腟内に突出しており、子宮腟部とよぶ。子宮は成人で上下約7センチメートル、左右約4センチメートル、子宮壁の厚みは約2センチメートルもあり、平滑筋からなる。この筋肉層は妊娠時、胎児の発育とともに発達して筋線維の数が増え、十数倍の大きさとなって強大な張力に耐える。この筋肉が収縮するときの力が分娩(ぶんべん)の主役を演じ、陣痛とよばれる。子宮体の内面は粘膜で覆われ、子宮内膜といい、厚さは月経周期や年齢によって異なるが、1.5～6.0ミリメートルである。

　子宮内膜は卵巣周期に応じてきわめて規則正しい変化を示し、増殖期と分泌期の2相の時期を繰り返す。増殖期は細胞の増殖を主とし、卵巣ホルモンのエストロジェン（卵胞ホルモン。エストロゲン）の作用による。分泌期は増殖期に続く受精卵の着床に必要な細胞の分化を主とするもので、増殖期を分泌期にするのは排卵後に形成される卵巣の黄体から産生されるプロゲステロン（黄体ホルモン）の作用である。受精卵が着床しない場合は子宮内膜が壊死(えし)に陥り、剥脱(はくだつ)して月経の発来となる。月経後にまた内膜は再生し、次の受精卵着床の準備態勢に入る。このようにして約30年間は、妊娠期間を除き、子宮内膜は剥脱と再生を繰り返す。なお、子宮内膜とは別に子宮頸部を覆う頸管内膜は、子宮内膜と異なり厚さも薄くて1.0ミリメートルで、周期性変化も乏しいが、エストロジェンに反応して水様透明な多量の粘液を分泌するので、エストロジェンに対する反応の検査、排卵の診断、精子貫通など、いろいろの生殖機構上、重要な意義をもつ。

　妊娠は、卵管内で受精した受精卵が子宮腔内に下降し、着床のためすでに分泌期となって肥厚し、準備された子宮内膜に着床することによって成立する。以後の妊娠全期間を通じて子宮容積は著しく増大する（未産婦の2～3ミリリットルに対して妊娠末期には4000～5000ミリリットルになる）が、分娩後には復古する。しかし、子宮腟部は未産婦の場合は円錐(えんすい)形に近く、外子宮口は円形であるのに対し、経産婦では子宮腟部は円柱状か不整形で、外子宮口は横長となり、多少切れ目を伴うことが多い。

　次におもな子宮の疾患について述べる。

［新井正夫］

子宮位置異常

正常とされる前傾前屈以外のものを子宮位置異常というが、そのうち治療の対象となるおもなものについて述べる。

(1)子宮後屈　正しくは子宮後傾後屈という。子宮体の軸が後屈するものを子宮後屈、子宮頸部の軸が後傾するものを子宮後傾といい、両者は合併してみられることが多い。子宮後屈だけで症状を伴う場合は子宮後屈症というが、ほとんどは無症状で、治療対象としては子宮後傾が重視されている。移動性と癒着性がある。移動性のものは性器の発育不全、内臓下垂のある無力性体質の婦人に多く、産褥(さんじょく)期の早期労働などが原因となるものもある。普通、これによる特別の症状はない。癒着性のものは卵巣、卵管、骨盤腹膜の炎症、あるいは子宮内膜症により子宮壁が癒着し、後方に引っ張られる結果おこるもので、下腹部痛や腰痛、月経痛、習慣性流産などの症状がみられる。原因疾患の治療を行い、改善されない場合は手術することもある。

(2)病的子宮前屈　子宮体の軸が強く前かがみになったもので、先天性子宮発育不全、月経異常、不妊を伴うものが多い。癒着したものは、後天的な子宮または周囲の炎症性疾患による。月経痛をはじめ、子宮体が膀胱を圧迫するために尿意が近くなる。根治的には手術が行われる。

(3)子宮脱　子宮脱垂症ともいう。子宮腟部が左右の大転子（大腿骨(だいたいこつ)頸の基部にある隆起）の高さより下にある場合は子宮下垂症で、子宮の下半部が腟入口の外に脱出したものを不完全子宮脱といい、子宮全体が脱出しているものを完全子宮脱という。大部分は子宮を保持する基靭帯や骨盤底筋群が分娩時に損傷することによっておこる。無症状のこともあるが、下腹部の重圧感や腟入口部の異物感、膀胱脱や直腸脱をかならず伴い、圧力性尿失禁を訴えるようになる。多産で、産褥時に過度の労働をした人に多い。更年期以後にみられる。なるべく早期に根治的手術を行う。

(4)子宮発育不全　位置異常ではないが、子宮の過度（病的）前屈や後屈を伴うことの多い疾患で、形態異常はないが正常より著しく小さい子宮をいう。しばしば外陰や腟の発育不全を伴う。大部分が先天性で、卵巣などの内分泌機能不全による。また、発育期に大病をしたり、栄養不良や過労などの身体的障害によって後天的にも発生する。こしけや月経異常がみられ、不妊あるいは妊娠しても流・早産をおこすことがある。主としてホルモン療法が行われる。

［新井正夫］

子宮癌

女性性器の悪性腫瘍(しゅよう)のうちもっとも多い疾患で、子宮頸癌(がん)と子宮体癌とがあり、日本人では大部分が子宮頸癌である。

［新井正夫］

子宮筋腫

子宮にできるしこり、かたまり、できものなどと俗にいわれるものは、おもにこの筋腫であり、良性腫瘍の一つ。

［新井正夫］

子宮出血

一般には月経とは無関係な不正子宮出血、すなわち無周期性で量や持続がきわめて不規則なものをいい、機能性、器質性、妊娠性などに分けられる。機能性子宮出血は妊娠や炎症・腫瘍・外傷など器質的な変化の認められない不正出血の場合に広くいわれるもので、排卵がおこらずに内膜が過度に増殖して出血する場合と、排卵のある女性におこる場合がある。診査掻爬(そうは)で内膜の状態を分析したのち、ホルモン療法が行われる。器質性子宮出血は子宮腟部びらん、頸管ポリープ、性器の炎症や腫瘍、とくに子宮癌、子宮筋腫、絨毛(じゅうもう)上皮腫などによるものをいう。妊娠性子宮出血は流産、子宮外妊娠、胞状奇胎などによるものである。これらのほかに、出血性素因、急性伝染病、心臓疾患、内分泌疾患など、全身性疾患の一部分症としてみられることもある。

［新井正夫］

子宮腟部びらん

子宮頸管粘膜が子宮口を越えて子宮腟部を覆う状態で、ただれ（糜爛(びらん)）のようにみえるのでこの名がある。いわゆる子宮のただれで、大多数の正常成熟女性にみられ、それ自体にはなんらの危険性もないが、癌の初期の症状と類似しており、肉眼的には区別がつかないことがある。したがって定期の子宮癌精密検査をかならず受けるべきである。

［新井正夫］

子宮内膜炎

細菌感染によっておこる子宮内膜の炎症で、膿(のう)性のこしけがあり、ときに下腹部痛や発熱もみられる。産褥時や流産時、人工妊娠中絶などの子宮内操作時、避妊器具の挿入時などが感染の機会となりやすい。急性では進行して卵管炎や腹膜炎などをおこす。産褥熱も急性子宮内膜炎の一つである。慢性では頸管部が侵され、上行して卵管に炎症がおこると不妊の原因になる。俗に子宮内膜炎という場合には子宮頸管粘膜の炎症をさす場合が多く、頸管炎とか頸管カタルと同じものと考えてよい。子宮体部粘膜は再生が強く、たとえ炎症があっても月経時に剥脱してしまうので、流産や出産、子宮内操作のあと以外に問題となることはない。早期に化学療法を徹底的に行う。

［新井正夫］

子宮内膜症

子宮内膜組織が、本来の場所である子宮腔内表面以外の部位で増殖する疾患をいう。特徴的症状は、月経の始まる前から月経第1日か2日ごろにもっとも強い月経痛がみられるもので、性交痛、不正出血、過多月経、不妊を伴うこともある。近年増加の傾向があり、女性の就労の問題とも関連するので注目されている疾患である。比較的若年女性では小骨盤腔臓器（ダグラス窩(か)、卵巣、腸壁など）が侵される型が多く、40歳以上では子宮腺(せん)筋腫が多い。妊娠が最良の治療法といわれるように、保存的には黄体ホルモンを主とした偽妊娠療法が行われるが、根治的には卵巣・子宮を摘除する。

［新井正夫］

子宮ポリープ

子宮口から腔内にぶら下がるいぼ状の腫瘍で、米粒大からサクランボ大くらいまで大きさはまちまちである。いぼの茎の根が子宮頸管にある頸管ポリープと、根が子宮腔内にある子宮内膜ポリープとがある。頸管ポリープは慢性の頸管の炎症や裂傷の場合に発生して接触出血の原因となるし、感染をおこして膿性のこしけを伴うことがある。子宮内膜ポリープは子宮内膜の増殖によるもので、不正子宮出血をおこす。いずれも通常は良性腫瘍である。目に見えるものは鉗子(かんし)で除去する。子宮腔内のものは内膜全面掻爬で除去する。悪性のものもあるので、組織学的検査を行う。

［新井正夫］

動物の子宮

雌性生殖腺(しせいせいしょくせん)付属器官である輸卵管の一部が膨大して、受精卵の維持および発育が行われるようになった部分のことである。哺乳(ほにゅう)類では、ミュラー管に由来する輸卵管が、発生の過程で、卵巣に近い部分から、輸卵管、子宮、腟(ちつ)といわれる生殖器官に分化する。子宮は元来左右1対の器官であるが、動物によりいろいろな程度に癒合していて、以下にあげる諸型に分類される。両側の子宮が癒合せず独立にある重複子宮（多くの齧歯(げっし)類、有袋類、ゾウなどの子宮）、外形的にはかなり癒合しているが内部は隔壁により二分されている中隔子宮（ハムスターなど齧歯類の一部、ブタやウシなどの子宮）、下半部のみ癒合している双角子宮（一部の食肉類、多くの有蹄(ゆうてい)類、クジラなどの子宮）、全体が癒合している単一子宮（霊長類の子宮）の4種である。

　子宮は発情周期や月経周期、さらには妊娠など生殖機能発現に際して著しい発達と退行を可逆的に繰り返している。妊娠が成立するときは受精卵（胚(はい)）が子宮の腔(こう)上皮細胞に接着すると、その部位の子宮内膜間質は肥大増殖し脱落膜を形成する。この脱落膜は胚の栄養芽層細胞とともに胎盤となり妊娠を維持する。胎盤は母体と胎児の間の栄養や老廃物の交換だけでなく、妊娠維持に必要な胎盤性生殖腺刺激ホルモン、胎盤性ペプチドホルモン、胎盤性ステロイドホルモンなどを分泌する内分泌器官にもなる。妊娠のおこらない通常の発情周期や月経周期においては、おもに発情ホルモンと黄体ホルモンの支配により子宮内膜組織が肥大増殖するが、霊長類では排卵に続く偽妊娠状態の終了とともに、増殖した組織が崩壊出血して月経がおこる。

　鳥類、爬虫(はちゅう)類、両生類などにもミュラー管由来の輸卵管の一部が膨大した部位があり、そこを子宮とよぶ場合がある。これらのうち卵生のものでは卵殻(らんかく)を分泌する卵殻腺としての機能を果たしている場合もあるが、一般には産卵まで卵を貯留しておくのがおもな役目である。魚類の輸卵管はミュラー管に由来するものと、単に卵巣腔の伸長したものである場合の2通りがあるが、いずれにおいてもやはり子宮といわれる膨大部をもつ種がいる。

　これら脊椎(せきつい)動物以外でも、卵生または卵胎生で体内受精をする無脊椎動物において、受精卵または初期胚が母体内にとどまる部分を子宮とよぶことがある。たとえば有爪(ゆうそう)類（カギムシなど）や線虫類（回虫など）では生殖孔と卵巣の間の管状部、吸虫類では卵形成腔と雌雄共同生殖孔との間の湾曲した太い管を子宮といい、条虫類（サナダムシなど）では卵巣と生殖孔を結ぶ管から枝分れした盲嚢(もうのう)を子宮あるいはとくに子宮鐘(しょう)とよぶが、これらも卵の貯蔵に役だっている。

［守　隆夫］

[参照項目] | 月経 | 子宮癌 | 子宮筋腫 | 妊娠

子宮の各部名称

子宮の位置異常

動物の子宮

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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