When a sperm cell fuses with an egg cell, the resulting cell is called a zygote. The zygote then begins to divide rapidly through mitosis, forming an embryo. This process is the first stage of development for most multicellular organisms that reproduce sexually. The zygote contains all of the genetic information from both parents and is the starting point for the growth and development of a new organism.

Pelvic inflammatory disease (PID) is a bacterial infection of the female reproductive system that often starts in the vagina and spreads upwards, infecting the uterus, fallopian tubes and ovaries. PID can be caused by a variety of bacteria, but is most commonly caused by sexually transmitted infections such as chlamydia and gonorrhea. PID is a serious condition that can cause infertility, ectopic pregnancy and chronic pelvic pain. Symptoms of PID include lower abdominal pain, fever, unusual vaginal discharge and pain during sex. Prompt treatment with antibiotics is important to prevent serious complications.

Urethral stricture is a condition where the urethra, the tube that carries urine out of the body, narrows, and restricts the flow of urine. Urethritis, catheterization, and sexually transmitted diseases are common causes of urethral strictures. Urethritis is an inflammation of the urethra, which can cause scarring and narrowing of the urethral lumen. Catheterization, especially if it is performed repeatedly, can also cause damage to the urethra and lead to scarring, which may result in strictures. Sexually transmitted diseases, such as gonorrhea and chlamydia, can cause urethritis and subsequent scarring of the urethra, leading to strictures. However, phimosis, which is the inability to retract the foreskin over the glans penis, is not a direct cause of urethral strictures. While phimosis can make it more challenging to pass urine, it is not directly related to the narrowing of the urethra, which is the defining characteristic of a urethral stricture.

Ectopic pregnancy, also called extrauterine pregnancy, is a condition where the fertilized egg implants and begins to develop outside of the uterus, most commonly in the fallopian tube. This condition can be life-threatening and requires immediate medical attention. Symptoms may include vaginal bleeding, abdominal pain, and shoulder pain. The risk factors for ectopic pregnancy include previous ectopic pregnancy, pelvic inflammatory disease, smoking, and advanced maternal age. Diagnosis is made through ultrasound and blood tests to measure hormone levels. Treatment options depend on the location and size of the ectopic pregnancy, but may include medication, laparoscopic surgery, or open surgery.

During the development of the female reproductive system, germ cells divide mitotically and give rise to millions of oogonia. However, most of these oogonia undergo degeneration and only a few of them develop into mature eggs. This process of degeneration is known as atresia. Atresia occurs during both fetal development and after birth, and it is estimated that only about 400,000 oocytes are present in the ovaries at puberty.

Epididymitis is the inflammation of epididymis, which is a tube-like structure located above the testes that helps in storage and transportation of sperm. The infection of epididymitis is mainly caused by sexually transmitted bacteria, such as Chlamydia trachomatis and Neisseria gonorrhoea. The symptoms of epididymitis include pain, swelling, and tenderness in the scrotum, pain during ejaculation or urination, and discharge from the penis. In most cases, antibiotics are used for the treatment of epididymitis. However, if the infection is caused by a virus, antiviral medication may be prescribed.
Hydrocele is a condition in which fluid accumulates around the testicle, causing swelling and discomfort. It is usually painless and can resolve on its own. Cryptorchidism is a condition in which the testicles fail to descend into the scrotum from the abdomen. This condition can cause infertility and increase the risk of testicular cancer. Epididymo-orchitis is a condition in which both the epididymis and the testicle become inflamed, which can be caused by a bacterial infection or a virus.

In the male human body, Leydig cells are responsible for the synthesis and secretion of testicular hormones (androgens). The male reproductive system includes a pair of testes, accessory ducts, glands, and upper genitals. Each testis contains around 250 compartments, known as testicular lobules, each containing one to three highly coiled seminiferous tubules where spermatozoa are produced. These tubules are lined with two types of cells, male germ cells (spermatogonia) and Sertoli cells. Leydig cells are situated outside the seminiferous tubules and produce testosterone, which regulates the growth and maintenance of male reproductive tissues, including the testes and prostate gland.

The gonads are the primary sex organs, also known as reproductive glands. They produce and store gametes (ovum or sperm) and secrete hormones (such as testosterone and estrogen) that regulate reproductive development and function.

The menstrual cycle is a recurring process in the female reproductive system that involves the shedding of the endometrial lining of the uterus, which exits through the vagina as menstrual blood. Hormonal fluctuations are crucial in regulating the menstrual cycle. Progesterone is a hormone that is primarily produced by the ovaries after ovulation. During the menstrual cycle, progesterone levels increase after ovulation, preparing the uterus for a possible pregnancy. If pregnancy does not occur, the progesterone levels decrease, causing the uterus lining to shed and leading to menstruation. Therefore, a decrease in progesterone levels is responsible for menstruation.
Luteinizing hormone (LH) and estrogen are also involved in the menstrual cycle, but they do not directly cause menstruation. LH triggers ovulation, and estrogen helps build the endometrial lining. None of the above is not a valid option as progesterone is the hormone responsible for menstruation. The menstrual cycle is divided into four phases: menstrual phase, follicular phase, ovulatory phase, and luteal phase. The decrease in progesterone levels occurs during the luteal phase, leading to menstruation.

Pre-eclampsia is a serious complication that can occur during pregnancy, typically after the 20th week. It is characterized by high blood pressure and damage to organs, most often the liver and kidneys. One of the common features of pre-eclampsia is the presence of proteinuria, which is the presence of protein in the urine. Proteinuria is caused by damage to the filtering system in the kidneys and can be an early sign of pre-eclampsia. It is important to monitor for proteinuria during pregnancy as it can indicate the presence of pre-eclampsia, which can be dangerous for both the mother and the baby. Treatment for pre-eclampsia typically involves close monitoring of blood pressure and urine protein levels, as well as medication to lower blood pressure if necessary. In severe cases, delivery of the baby may be necessary to prevent further complications.