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一起來認(rèn)識(shí)生命的起始：卵子和精子

出處：網(wǎng)絡(luò)收集 日期：2024-09-03 00:08:59 編輯：秩名

生育的基礎(chǔ)是男子提供精子和女方提供卵子，精子和卵子各攜帶著父母的遺傳物質(zhì)，通過受精結(jié)合到一起，形成一個(gè)新生命。
 
精子是在睪丸的曲細(xì)精管內(nèi)產(chǎn)生的。男性青春期發(fā)育以后，睪丸便擁有持續(xù)不斷的生精能力。成年人睪丸重10～20克，每克睪丸組織每天可以產(chǎn)生約10000000個(gè)精子。到40歲后，生精能力逐漸減弱，但60～70歲甚至個(gè)別90歲的老人還具有生精能力。因此男性的生育年齡明顯長(zhǎng)于女性。
 
卵子是由卵巢的原始卵母細(xì)胞發(fā)育而成。女性青春期發(fā)育后，每個(gè)月經(jīng)周期排出一個(gè)成熟卵子，有時(shí)為兩個(gè)。一個(gè)婦女一生約排出400個(gè)卵子，最多500個(gè)。卵子的發(fā)育起源于胎兒時(shí)期，形成于青春期，發(fā)育在育齡期，歷時(shí)幾十年。高齡孕婦的卵子歷經(jīng)數(shù)十年，可能出現(xiàn)畸形的概率就比較高。在55歲左右，女性就進(jìn)入絕經(jīng)期，卵巢失去排卵的功能，從此失去生育功能。
 

精子是怎樣形成的

 
男性生殖細(xì)胞稱為精子。精子產(chǎn)生于睪丸的曲細(xì)精管內(nèi)，它的形成需經(jīng)歷兩個(gè)半月以上。 
 

根據(jù)生精細(xì)胞的細(xì)胞學(xué)特征，分為3個(gè)階段。

 
（1）精原細(xì)胞分裂增殖，并產(chǎn)生精母細(xì)胞：在精子的形成過程中，要經(jīng)過兩次成熟分裂。 
 
第一次成熟分裂是由初級(jí)精母細(xì)胞分裂為次級(jí)精母細(xì)胞。這次分裂稱為減數(shù)分裂，使細(xì)胞內(nèi)染色體的數(shù)目減少一半。初級(jí)精母細(xì)胞中有23對(duì)染色體，其中含有22對(duì)常染色體和1對(duì)性染色體。在1對(duì)性染色體中，一個(gè)是X染色體，另一個(gè)是Y染色體。當(dāng)初級(jí)精母細(xì)胞分裂時(shí)，23對(duì)染色體平均分配到兩個(gè)次級(jí)精母細(xì)胞內(nèi)，即每個(gè)次級(jí)精母細(xì)胞中分別有23個(gè)染色體。
 
由于性染色體分為X、Y兩種，因此分裂后的一個(gè)次級(jí)精母細(xì)胞內(nèi)含有X染色體，而另一個(gè)次級(jí)精母細(xì)胞內(nèi)含有Y染色體。
 
（2）精母細(xì)胞減數(shù)分裂，形成單倍體的精子細(xì)胞：第二次分裂是由次級(jí)精母細(xì)胞分裂成精子細(xì)胞。經(jīng)過這次分裂，每個(gè)染色體再分裂為二，整個(gè)細(xì)胞染色體分成兩組，再分配到每個(gè)精子細(xì)胞內(nèi)，因此精子細(xì)胞內(nèi)的染色體與次級(jí)精母細(xì)胞的相同。兩個(gè)次級(jí)精母細(xì)胞分裂成為4個(gè)精子細(xì)胞，它們的染色體均為23個(gè)，其中有兩個(gè)精子細(xì)胞含有Y染色體，另外兩個(gè)精子細(xì)胞含有X染色體。
 
（3）精子細(xì)胞變成精子：精子細(xì)胞不再分裂，而經(jīng)過形態(tài)改變成為精子。精子形如蝌蚪狀。精子的結(jié)構(gòu)：由頂體、頭、身體中部、尾四部分構(gòu)成。精子的頭部由精子細(xì)胞的核濃縮而成。而其他部分由細(xì)胞質(zhì)形成。精子的頭部含有遺傳物質(zhì)（染色體）。頂體含有蛋白質(zhì)，幫助精子穿過厚厚的卵子外層。身體中部含有精子運(yùn)動(dòng)所需的能量物質(zhì)。長(zhǎng)長(zhǎng)的尾巴幫助精子靈活游動(dòng)。精子在附睪內(nèi)可以存活數(shù)月，在女性生殖道內(nèi)可存活1～3天，卵子受精能力僅能夠維持20小時(shí)左右。
 

卵子是怎樣產(chǎn)生的

 
卵子是人體最大的細(xì)胞，也是女性獨(dú)有、產(chǎn)生新生命的母細(xì)胞。
 
（1）一個(gè)女性，當(dāng)她還在媽媽子宮里的時(shí)候，小小的卵巢已有了數(shù)百萬個(gè)卵母細(xì)胞，每一個(gè)卵母細(xì)胞都被包裹在一個(gè)原始卵泡中，妥善地收藏著，并來到人世間。一般來講，女性一生能夠成熟、排到卵巢外面等待受精的卵子，一共為400～500個(gè)。
 
（2）到了青春期，大腦垂體分泌的促性腺激素，使卵母細(xì)胞開始發(fā)育，但每月只有1個(gè)原始卵泡發(fā)育成熟（最多2個(gè)），將里面收藏著的卵子排出。 
 
（3）成熟的卵子在輸卵管壺腹部等待精子的受精，如果48小時(shí)以后還沒有與精子相遇形成受精卵，就開始自然死亡了。
 
（4）1個(gè)月后會(huì)有另一個(gè)卵子成熟并被排出，重復(fù)同樣的過程，通常左右兩個(gè)卵巢輪流排卵。
 

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