Concepimento, Rimanere incinta

Come avviene il concepimento, la nascita di una vita

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come avviene il concepimento

Come avviene il concepimento

Come avviene il concepimento? Con un processo preciso e complesso, che ha qualcosa di magico. Tutto parte da un rapporto completo, durante i giorni fertili della donna. Attraverso l’eiaculazione, l’uomo rilascia in vagina dai 200 ai 500 milioni di spermatozoi. Di questi, pochissimi riusciranno a proseguire il viaggio e uno solo arriverà alla meta. Una prima selezione avviene già in vagina, dove l’ambiente sfavorevole farà sopravvivere solo alcuni spermatozoi, che riusciranno ad arrivare alla cervice. Qui dovranno scontrarsi con un nuovo ostacolo: il muco cervicale. In realtà, non sempre è un nemico, anzi! Se la donna non è nel periodo fertile, il muco cervicale ha una consistenza tale da impedire il passaggio agli spermatozoi, se invece è in ovulazione, il muco è trasparente e filamentoso e facilita il loro passaggio.

Fecondazione

Una volta arrivati nella cervice, gli spermatozoi proseguono il loro cammino verso l’utero, fino alla tuba e all’ampolla tubarica, vicino all’ovaio, dove si trova l’ovulo. Lungo il percorso gli spermatozoi continuano a dimezzarsi e al traguardo giunge solo uno di loro, che riesce a penetrare nell’ovocita, fecondandolo. Avviene quindi la reazione acrosomiale, ovvero la fusione della membrana cellulare dello spermatozoo e quella dell’ovuolo. Ciò è possibile grazie all’acrosoma presente nella testa dello spermatozoo, una sorta di “sacchetto“, che contiene gli enzimi necessari a questo processo, e alla zona pellucida, lo strato protettivo dell’ovulo che contiene la glicoproteina ZP3, che innesca la fusione e fa da àncora per lo spermatozoo.

concepimento come avviene

Zigote

Avviene così la fusione di due cellule, dette gameti. L’ovulo si attiva, secernendo sostanze che modificano la zona pellucida e inattivano la proteina ZP3, impedendo che altri spermatozoi entrino nell’ovulo. Ogni gamete porta con sé un singolo cromosoma: sempre X quello femminile, X o Y quello maschile. Da ciò dipende il sesso del bambino: per concepire una femmina dovranno unirsi due cromosomi X, per concepire un maschio un cromosoma X e cromosoma Y. La cellula fecondata prende il nome di zigote e racchiude tutto il DNA del bambino.

Morula

Dopo 30 ore dalla fecondazione, lo zigote, che si trova ancora nell’ampolla tubarica, inizia a dividersi, prima in due parti, poi in quattro, poi in otto, fino ad assumere un aspetto simile ad una mora. Per questa sua forma caratteristica, un agglomerato di 12-14 cellule, lo zigote prende ora il nome di morula. Mentre compie questa trasformazione, continua anche a spostarsi verso l’utero, dove arriva circa 3 giorni dopo la fecondazione.

concepimento morula

Impianto

Arrivata nell’utero, la morula continua la sua trasformazione, diventando blastocisti. Dopo 7 giorni dalla fecondazione, si hanno due diversi tipi di cellule: quelle trofoblastiche, dalle quali avrà origine il trofoblasto, ovvero un primo abbozzo della placenta, e la membrana amniotica; quelle embrioblastiche, da cui avrà origine l’embrione. A questo punto, avviene l’invasione trofoblastica: le cellule trofoblastiche si attivano per entrare in contatto con l’endometrio (detto ora decidua), per formare un fitto reticolo vascolare che, nel corso dei nove mesi della gravidanza, consentirà il passaggio di ossigeno e nutrimento dall’organismo della mamma a quello del feto. Dopo l’invasione, dopo circa 13 giorni dall’inizio concepimento, si ha il vero e proprio impianto nell’utero.

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