Sexual characteristics

Japanese: 性徴 - せいちょう

In multicellular animals that are mainly dioecious, the term refers to characteristics that provide clues for distinguishing between males and females. Among these, the characteristics of the gonads themselves, i.e., the ovaries and testes, are called primary sexual characteristics, while other characteristics that indicate gender are called secondary sexual characteristics. Tertiary sexual characteristics may also be distinguished. For example, in mammals, the prostate, seminal vesicles, and epididymis, which are appendages of the testes, as well as the oviducts, uterus, and vagina that are attached to the ovaries, and the external genitalia (external genitalia and external genitalia), are considered secondary sexual characteristics. Furthermore, characteristics that allow the sex of an animal to be distinguished at a glance, such as the mane of a male lion, the comb and claws of a male chicken, and the nuptial coloration that is widely seen in vertebrates including fish and amphibians, are also secondary sexual characteristics. The differences in behavior and psychological differences between males and females can be said to be tertiary sexual characteristics. These secondary and tertiary sexual characteristics are manifested by the action of female and male hormones secreted from the ovaries and testes. Therefore, if the testes or ovaries are removed, the action of these hormones disappears, and most secondary and tertiary sexual characteristics become unnoticeable. Sexual characteristics lost in this way can be reappeared by administering hormones. In vertebrates, the endocrine regulatory mechanism of the reproductive system is controlled by the brain, and in mammals in particular, the sex and masculinity of this mechanism is present in the brain, which can also be considered a sex characteristic. The sexual characteristics of the appendages of the testes and ovaries are sometimes treated as primary sex characteristics.

On the other hand, invertebrates also have various sexual characteristics, which in many cases correspond to genetic sex, but some crustaceans have an endocrine organ called the androgenic gland that controls all male reproductive organs and male sexual characteristics. Some insects also have an organ that appears to be the androgenic gland in part of the testes of their larvae, which controls the expression of sexual characteristics.

[Takeo Machida]

Sexual characteristics in humans

In humans, the traits that characterize an individual's sex are called sexual characteristics, with the characteristics of the gonads themselves being called primary sexual characteristics, and those found outside the gonads, i.e. the internal and external genitalia and other organs being called secondary sexual characteristics. Among the secondary sexual characteristics, the structure of the internal and external genitalia and the function of the brain are expressed during fetal development, while characteristics such as body shape and hair distribution are expressed during puberty, within a certain range, depending on the gonadal hormones.

In humans, the gonads are structurally the same for both sexes until the sixth week of fetal development, and are called the primordial gonads. The primordial gonads consist of the cortex and medulla. From the seventh to eighth week, in males, the medulla develops into the testes and the cortex degenerates. It is believed that this testicular differentiation requires the H-Y antigen (male-specific antigen) produced by a gene on the Y chromosome. In females, the cortex develops into the ovaries and the medulla degenerates. During this period, Leydig cells appear in the testes and begin to secrete male hormones (androgens), but the ovaries do not secrete hormones. Once primary sexual characteristics appear, secondary sexual characteristics appear, depending on sex hormones.

The seventh week fetus has primitive reproductive ducts (Wolff's ducts and Müller's ducts) of both sexes, but in men, a Müllerian duct regressing factor (thought to be a polypeptide) released from the testes inhibits the development of the Müllerian ducts, which develop into the epididymis and vas deferens, while in women, the Müllerian ducts develop into the fallopian tubes, uterus, and vagina. The external genitalia are the urogenital hiatus common to both sexes until the eighth week, but in men, the hiatus disappears under the action of androgens and becomes the male external genitalia, while in women, the hiatus remains and becomes the female external genitalia. At puberty, the secretion of gonadotropins from the pituitary gland increases, resulting in active secretion of hormones from the gonads, causing secondary sexual characteristics to become prominent. In boys, androgens cause the development of the penis, seminal vesicles, and prostate, a decrease in vocal cord tone resulting in a lower-pitched voice, male-type body hair distribution (beard, receding front and temporal hair, pubic hair, chest hair, etc.), and a masculine physique with broader shoulders and more developed muscles. In girls, an increase in estrogen and a deficiency of androgens causes a feminine physique with broad buttocks, development of the breasts, uterus, and vagina, and a feminine pubic hair distribution.

[Masazumi Kawakami]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

主として雌雄異体の多細胞動物で、雌雄を見分ける手掛りとなるような形質をいう。このうち、生殖腺(せん)そのもの、すなわち卵巣と精巣の特徴を第一次性徴、それ以外の性別を示す形質を第二次性徴という。さらに第三次性徴を区別することもある。哺乳(ほにゅう)類を例にとれば、精巣の付属器官である前立腺、貯精嚢(のう)、副精巣（精巣上体）などや、卵巣に付随する輸卵管、子宮、腟(ちつ)など、および外部生殖器（外生殖器・外性器）は第二次性徴とされる。さらに、ライオンの雄のたてがみ、ニワトリの雄のとさかやけづめ、魚類、両生類をはじめ脊椎(せきつい)動物に広くみられる婚姻色など、一見して動物の雌雄が見分けられる特徴も第二次性徴である。雌雄の行動の違いや心理的な差違は第三次性徴といえよう。これら第二次、第三次の性徴は卵巣および精巣から分泌される雌性ホルモン、雄性ホルモンの作用で発現する。したがって、精巣や卵巣を除去すればこれらホルモンの作用が消失するので、第二次・第三次性徴のほとんどが認められなくなる。このようにして失われた性徴は、ホルモンの投与によりふたたび出現する。脊椎動物では、生殖腺系の内分泌調節機構は脳に支配されているが、とくに哺乳類ではこの機構の雌雄性が脳に存在し、これも一つの性徴といえる。なお、精巣や卵巣の付属器官の性徴を第一次性徴として扱うこともある。

　一方、無脊椎動物でもさまざまな性徴がみられ、多くの場合、遺伝的な性に対応しているが、甲殻類のなかには造雄腺という内分泌器官が雄性生殖器官および雄の性徴のすべてを支配しているものがある。昆虫にも、幼虫の精巣の一部に造雄腺と思われる器官があり、性徴の発現を支配しているものがある。

［町田武生］

ヒトにおける性徴

ヒトにおいても個体の性別を特徴づける形質を性徴といい、生殖腺自身の特徴を第一次性徴、生殖腺以外の特徴、すなわち内生殖器や外生殖器その他にみられるものを第二次性徴とよぶ。第二次性徴のうち、内外生殖器の構造、脳の機能上の性差は胎児期に、体型や毛髪分布などの特徴は思春期に至って、一定の範囲内で生殖腺ホルモンに依存して発現する。

　ヒトの場合、生殖腺は胎児第六週までは構造に性差がなく、原始生殖腺とよばれる。原始生殖腺は皮質と髄質よりなる。第七週から第八週にかけて、男性では髄質が精巣へと発達して皮質は退化する。この精巣分化にはY染色体上の遺伝子によりつくられるH‐Y抗原（雄特異的抗原）が必要であると推測されている。女性では皮質が卵巣へと発達して髄質は退化する。この時期に精巣にはライディッヒLeydig細胞が出現して男性ホルモン（アンドロゲン）の分泌が始まるが、卵巣はホルモンを分泌しない。こうして第一次性徴が現れると、引き続いて性ホルモンに依存して第二次性徴が現れる。

　第七週胎児は男女両性の原始生殖管（ウォルフWolff管とミュラーMüller管）をもっているが、男性では精巣から放出されるミュラー管退行因子（ポリペプチドと思われる）がミュラー管の発達を抑制し、ウォルフ管が発達して精巣上体、輸精管となり、女性ではミュラー管が発達して卵管、子宮、腟となる。外生殖器は第八週まで男女共通の泌尿生殖裂孔であるが、男性ではアンドロゲンの作用で裂孔が消失して男性の外生殖器になり、女性では裂孔がそのまま残って女性の外生殖器になる。思春期になると下垂体からのゴナドトロピン分泌が増加し、その結果、生殖腺のホルモン分泌が盛んになるので、第二次性徴は著明に現れる。男子ではアンドロゲンによって陰茎、精嚢、前立腺などが発達し、声帯緊張が低下して声が低音になり、男性型の体毛の分布（髭(ひげ)、頭髪前側頭部の後退、恥毛、胸毛など）がみられ、肩幅の増大や筋肉の発達など男性的な体型となる。女子ではエストロゲンの増加とアンドロゲンの欠乏により、殿（臀(でん)）部の広い女性的な体型、乳房、子宮、腟の発達、女性型の恥毛分布などが現れる。

［川上正澄］

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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