Ciclo menstrual
Autor: Nanci | Publicado: 2014-06-05T17:48:00-07:00











Ciclo
menstrual

O ciclo menstrual
obedece às interações de hormônios produzidos na hipófise (FSH e LH) com
hormônios ovarianos (estrógeno e progesterona). Os primeiros ciclos menstruais
iniciam-se por volta dos 11-15 anos de idade; a primeira menstruação é chamada
de&nbsp;menarca.

Para compreender melhor o que se passa no ciclo
menstrual, sugerimos que leia o texto a seguir, acompanhando a figura abaixo.

Sob ação do
hormônio FSH, um folículo ovariano (conjunto de células com um ovócito primário
em seu interior) inicia seu desenvolvimento. O ovócito I (que se encontra em
prófase I no folículo) dá continuidade à meiose e, em poucos dias, o folículo
ovariano encontra-se cheio de um líquido (folículo de Graaf). Durante o
desenvolvimento folicular, as células foliculares secretam&nbsp;estrógeno,
hormônio responsável pela determinação e manutenção das características sexuais
secundárias femininas (aumento da vagina e do útero; alargamento dos quadris;
desenvolvimento das mamas; desenvolvimento de pêlos nas axilas e púbis).

Uma outra
importante função do estrógeno é&nbsp;provocar a proliferação das células do
endométrio&nbsp;(camada mais interna do útero). O estrógeno ainda vai
estimular a liberação do hormônio LH pela hipófise. Em menos de 24 horas
aumenta-se acentuadamente a concentração de LH, que&nbsp;provoca a ruptura
do folículo maduro (ovulação). O óvulo, capturado pelas fímbrias da tuba
uterina, permanece viável por aproximadamente 30 horas (período fértil da
mulher).







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<![endif]--><!--[if !vml]--><!--[endif]-->Após o rompimento do folículo, as células foliculares dão origem aocorpo
lúteo&nbsp;(sob ação do LH), o qual passa a produzir doses crescentes do
hormônio&nbsp;progesterona, cuja açãoacentua o espessamento do
endométrio, promovendo a sua vascularização; dessa forma, cerca de 6 dias
após a fecundação, &nbsp;o endométrio encontra-se apto a receber o embrião.

A progesterona, produzida pelo corpo lúteo,
passa a inibir a produção de FSH e LH pela hipófise. Com a queda do hormônio
LH, o corpo lúteo regride (transforma-se em corpo albicans), que é inativo.
Como conseqüência, há redução da taxa de progesterona e estrógeno; sem esses
hormônios, o endométrio não se mantém e sua camada mais superficial se descama:
é a menstruação (que se caracteriza por um sangramento cuja duração varia,
normalmente, de três a sete dias).

Com a redução da taxa de estrógeno e de
progesterona, a hipófise passa a secretar mais FSH, e um novo folículo entra em
desenvolvimento, recomeçando novo ciclo menstrual.
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