The main thing in the female reproductive organ is the uterus, while in the male reproductive organ it is the testes. Let's discuss each in detail:

Female Reproductive Organ:
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The main organ of the female reproductive system is the uterus, also known as the womb. The uterus is a hollow, muscular organ located in the lower abdomen, between the bladder and the rectum. It plays a crucial role in the reproductive process by supporting the development of a fetus during pregnancy. Here are some key points about the uterus:

1. Structure: The uterus is pear-shaped and consists of three main parts - the fundus (upper part), the body (central part), and the cervix (lower part). The inner lining of the uterus is called the endometrium.

2. Functions: The uterus performs several essential functions, including:
- Menstruation: The endometrium sheds during menstruation when fertilization does not occur.
- Pregnancy: The uterus provides a nurturing environment for the fertilized egg to implant and develop into a fetus.
- Labor and Delivery: During childbirth, the uterus contracts to push the baby out.

3. Hormonal Influence: The uterus is influenced by hormones produced by the ovaries, specifically estrogen and progesterone. These hormones regulate the menstrual cycle and prepare the uterus for pregnancy.

4. Supporting Structures: Other important structures in the female reproductive system include the ovaries, fallopian tubes, and vagina. The ovaries produce eggs and release hormones, while the fallopian tubes facilitate the transport of eggs from the ovaries to the uterus. The vagina is the passage connecting the uterus to the external environment.

Male Reproductive Organ:
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The main organ of the male reproductive system is the testes, which are responsible for producing sperm and male sex hormones. Here are some key points about the testes:

1. Structure: The testes are oval-shaped organs located in the scrotum, a sac-like structure located outside the body. Each testis is divided into several lobules, which contain tiny coiled tubes called seminiferous tubules.

2. Sperm Production: The seminiferous tubules within the testes are where sperm production occurs through a process called spermatogenesis. Sperm cells are produced continuously from puberty until old age.

3. Hormone Production: The testes also produce the male sex hormone testosterone. Testosterone is responsible for the development of male secondary sexual characteristics, such as facial hair, deepening of the voice, and muscle development.

4. Supporting Structures: Other important structures in the male reproductive system include the epididymis, vas deferens, prostate gland, seminal vesicles, and penis. The epididymis stores and transports sperm, while the vas deferens carries sperm from the epididymis to the urethra. The prostate gland and seminal vesicles produce fluids that mix with sperm to form semen, which is ejaculated through the penis during sexual intercourse.

Overall, the main thing in the female reproductive organ is the uterus, which supports pregnancy, while in the male reproductive organ, it is the testes, which produce sperm and male sex hormones.