Reproductive Physiology and Development - Anatomy

Estradiol levels in the body are typically controlled with a negative feedback loop; however, the luteal surge results in a positive feedback loop for estradiol. Instead of luteinizing hormone levels decreasing under high estradiol levels, they increase. As a result, estradiol levels continue to rise. This dramatic increase of estradiol and luteinizing hormone is called the luteal surge, and results in ovulation.

The corpus albicans develops from the corpus luteum only after ovulation, and is responsible for regulating progesterone levels after the egg has been released.

A spike in the concentration of luteinizing hormone (LH) leads to ovulation on day 14 of the menstrual cycle. This spike is known as the "LH surge" and is initiated by a positive feedback mechanism involving estrogen.

Follicle-stimulating hormone (FSH) is involved in the maturation of the follicle, but not ovulation. Progesterone functions in maintaining the endometrial tissue after implantation has occurred. Testosterone is not involved in the female reproductive cycle.

Estrogen levels peak towards the end of the follicular phase. This causes a surge in the levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This lasts about 24-36 hours, and results in the rupture of the ovarian follicles, causing the oocyte to be released from the ovary and swept into the fallopian tubes by the fimbriae.

The oviducts, also known as the fallopian tubes, connect the ovaries to the uterus. When an egg, or ovum, is released once each month, it leaves the ovary and flows down the oviduct into the uterus, awaiting fertilization. If fertilization does not occur, then the uterine lining is shed, taking with it the egg, through the uterus and out the vagina. The urethra is connected to the urinary bladder.

The most common location for the fertilization of the egg is the fallopian tube. The zygote then undergoes several divisions and some development before implanting in the endometrium of the uterus as a blastocyst. Abnormal implantation can occur while the blastocyst is still in the fallopian tube, resulting in an ectopic pregnancy. This event generally requires surgery to rectify and can be extremely dangerous to the mother and developing embryo.

After developing in the seminiferous tubules, sperm cells are transferred to the epididymis in order to be stored prior to ejaculation. The sperm cells also mature further and develop the ability to swim in the epididymis.

The vas deferens is the duct connecting the epididymis to the ejaculatory ducts. The prostate gland, bulbourethral gland, and seminal vesicles produce alkaline and lubricating fluids for the composition of semen.

Semen is composed of fluid from the three following glands: the seminal vesicles, the prostate, and the bulbourethral glands. These glands secrete alkaline fluids that help neutralize the acidity of the vagina, preventing degradation and damage of sperm.

The testes are the site of development for sperm cells, but they do not contribute to the fluidity of semen.

By definition, ejaculation is the process by which sperm is released. This process usually occurs during the orgasm stage of the sexual response cycle in males. Emission is the phase directly preceding ejaculation in which the sperm are prepared for release. Spermatogenesis is constantly occurring and occurs within the seminiferous tubules. Oogenesis and gestation are female-associated.

In males, the urethra is the vessel that allows for the excretion/expulsion of semen and urine. The seminiferous tubules and epididymis are located in the testes, and are involved in spermatogenesis. The vas deferens is the vessel that carries sperm and semen from the testes to the urethra, picking up seminal fluids from the seminal vesicle and prostate gland.

The epididymis is an anatomical structure in the male reproductive system. It functions in the maturation and storage of spermatozoa and in propulsion of the spermatozoa into the vas deferens. The ejaculatory duct is responsible for propelling spermatozoa with seminal fluid into the urethra. The testis is responsible for producing spermatozoa and secreting the sex hormones. Lastly, the seminal vesicles are responsible for producing the alkaline component of seminal fluid.

Human chorionic gonadotropin is a peptide hormone that helps prevent the breakdown of the corpus luteum. This allows the corpus luteum to continue secreting progesterone and estrogen during implantation. As the placenta forms, it eventually takes over the job of secreting hormones for the corpus luteum. The corpus luteum is only preserved when fertilization has occurred; normally, the structure devolves into the corpus albicans. Human chorionic gonadotropin can thus be used to detect pregnancy, as it is only secreted after fertilization.

Follicle-stimulating hormone (FSH) stimulates the release of estrogen. Estrogen then thickens the endometrial lining during the proliferative phase and stimulates the release of luteinizing hormone (LH) on the 14th day of the cycle. This causes the luteal surge that results in the development of the corpus luteum.

Testosterone release is triggered by luteinizing hormone during spermatogenesis, which does not occur in females.

Human chorionic gonadotropin is used by over-the-counter pregnancy tests to detect fertilization and is secreted after fertilization occurs to maintain high levels of progesterone. When fertilization occurs, progesterone causes a pause in the menstrual cycle; follicle-stimulating hormone will only trigger the estrogen spike if progesterone levels decline enough. With progesterone levels elevated, the corpus luteum is maintained rather than transitioning into the corpus albicans.

After fertilization and the formation of a zygote, the developing organism undergoes several rounds of cell division known as cleavage. During this period the cells divide but do not grow in size, resulting in a dense cell mass. The morula is formed during this portion of development.

Following cleavage, the cell develops an inner cell mass and trophoblast, or outer layer, before implantation into the uterus. At this stage, the organism is considered a blastocyst.

Soon after implantation, cells begin to differentiate into the three germ layers through a process known as gastrulation. The resulting organism is considered a gastrula.

Gastrulation is quickly followed by development of the neural crest and preliminary nervous tissue, known as neurulation. The organism is then considered a neurula.

Note that the organism can be considered an embryo at any point from the first cleavage division to approximately the eighth week of development.

In order for the testes to develop, the only thing necessary is the SRY gene product, which is created from the SRY gene on the Y chromosome. DHT are involved in the development of other male sex traits during early development, such as the penis, while testosterone is involved in both early development and the development of secondary sexual traits during puberty. Wolfian excitatory hormone does not actually exist. Estrogen is not involved in the development of male sex organs.

Because the fetal lungs and liver are not fully functional until birth, the fetal circulation uses structures in order to bypass these organs.

The ductus venosus is used to reroute blood from the fetal liver and deposit it into the right atrium.

The ductus arteriosus connects the fetal pulmonary arteries to the aorta to bypass the lungs, while the foramen ovale connects the right and left atria to reduce flow to the pulmonary arteries.

The umbilical vein carries blood from the uterus, where it have been loaded with oxygen from maternal circulation, back to the body.

The ductus arteriosus is used to reroute blood from the pulmonary trunk and place it into the aorta of the fetus, preventing flow to the non-functional fetal lungs. After birth, this structure deteriorates and closes. The ligamentum arteriosum is the vestigial structure that is seen in adults. Instead of connecting the pulmonary arteries and aorta, it helps to stabilize these structures.

The foramen ovale is a fetal window between the right and left atria to help blood bypass the pulmonary circuit. After birth, this window closes to become the fossa ovalis.

Following gastrulation, three primary germ layers form: the ectoderm, the mesoderm, and the endoderm. We can typically predict which structures will form from the germ layers based on the layers of the body. The endoderm will form the digestive tract, the mesoderm will form the skeleton and muscles, and the ectoderm will form the skin and nervous system.

Estradiol levels in the body are typically controlled with a negative feedback loop; however, the luteal surge results in a positive feedback loop for estradiol. Instead of luteinizing hormone levels decreasing under high estradiol levels, they increase. As a result, estradiol levels continue to rise. This dramatic increase of estradiol and luteinizing hormone is called the luteal surge, and results in ovulation.

The corpus albicans develops from the corpus luteum only after ovulation, and is responsible for regulating progesterone levels after the egg has been released.

A spike in the concentration of luteinizing hormone (LH) leads to ovulation on day 14 of the menstrual cycle. This spike is known as the "LH surge" and is initiated by a positive feedback mechanism involving estrogen.

Follicle-stimulating hormone (FSH) is involved in the maturation of the follicle, but not ovulation. Progesterone functions in maintaining the endometrial tissue after implantation has occurred. Testosterone is not involved in the female reproductive cycle.

Estrogen levels peak towards the end of the follicular phase. This causes a surge in the levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This lasts about 24-36 hours, and results in the rupture of the ovarian follicles, causing the oocyte to be released from the ovary and swept into the fallopian tubes by the fimbriae.