Reproductive System

(Reproduction male testes)

Overview of the Reproductive Systems

This section is near the conclusion of our course, focusing on the intricate mechanisms of the reproductive systems, which illustrate significant anatomical and physiological differences between sexes. We will begin with a thorough examination of the male reproductive system, followed by a discussion on the female reproductive system, including their respective anatomy and physiological functions.

The Purpose of Sexual Reproduction

In the introduction to this chapter, we will explore the primary purpose of sexual reproduction, which goes beyond just the continuation of species; it plays crucial roles in genetic diversity and adaptation. Although sexual reproduction is not strictly vital for individual survival—as evidenced by individuals who lead full lives without reproducing—the evolutionary advantages it offers cannot be overlooked. The capability to reproduce ensures the diversity of traits through genetic variation, which enhances a population's resilience against environmental changes.

Gametogenesis

The main goal here is to produce and nurture gametes— the male gametes being sperm and the female gametes known as oocytes or eggs. Following fertilization, these gametes transport the necessary genetic material to facilitate the creation of a zygote. In males, fertilization takes place externally, while in females, it occurs internally within the reproductive tract shortly after oocyte release.

Meiosis in Sex Cells

To prepare for fertilization, a critical biological process called meiosis occurs in both sexes. Meiosis substantially differs from mitosis; while mitosis is involved in somatic cell growth and repair, meiosis is specifically designed to halve the chromosome number, producing gametes with 23 chromosomes each, allowing for the formation of a zygote with a complete set of 46 chromosomes upon fertilization. This reduction is vital because it prevents the doubling of chromosomal counts that would interfere with normal development. Moreover, meiotic processes encourage genetic diversity by shuffling genes during crossover events and random assortment, ensuring offspring have unique combinations of traits. This genetic variability is beneficial for natural selection, enhancing the species' evolutionary potential.

The Male Reproductive System

Commencing the examination of the male reproductive system, which we will visualize through models and diagrams, the testes occupy a central role. Located within the scrotum, the testes house seminiferous tubules—a key site for meiosis and sperm production, creating not only sperm but also male hormones like testosterone, critical for secondary sexual characteristics and reproductive health.

Temperature Regulation of the Testes

The testes are structured within the scrotum, which consists of a sac and several muscle layers that regulate the position of the testes to maintain optimal temperature for sperm production, which is typically 2-5 degrees Celsius lower than core body temperature, around 35 degrees Celsius. This temperature regulation is essential, as high temperatures can impair sperm development and potentially lead to infertility.

Anatomy of the Testes

Upon examining the testes more closely, we find them organized into lobules containing numerous seminiferous tubules. These tubules comprise layers of spermatogenic cells engaged in the production of sperm and Sertoli cells that provide nourishment and support during sperm maturation, maintaining the blood-testis barrier to protect developing sperm from immune system attacks. Leydig cells, found external to the tubules, are responsible for testosterone production.

Spermatogenesis

Meiosis occurs within these seminiferous tubules starting at puberty, where spermatogonia undergo several mitotic divisions, progressing through meiosis to produce haploid sperm cells. The spermatogenesis cycle involves transformation from spermatogonia to primary spermatocytes, followed by secondary spermatocytes, ultimately leading to spermatids and eventually mature spermatozoa that are released into the seminiferous tubules in approximately 64 days.

Structure and Maturation of Sperm

The structure of sperm is comprised of three main parts: the head (containing the nucleus and acrosome), the midpiece rich in mitochondria for energy, and the flagellum providing motility. After maturation in the epididymis, where sperm can be stored and continue to mature for about 12 days, they are transported away from the testes through the ductus deferens, part of the spermatic cord, facilitating ejaculation during sexual activity.

(Reproduction Male Control)

Following the formation of sperm in the epididymis, they are pushed into the ductus deferens, which is crucial for transporting sperm from the testes through the male reproductive tract. This duct is also a site for vasectomy, a surgical procedure where the ductus deferens is cut or sealed to prevent sperm from exiting the body, rendering the male infertile. The ductus deferens moves posteriorly to the bladder where it widens to form the ampulla before merging with the seminal vesicles.

Microscopically, the ductus deferens is lined with a stratified epithelium and contains smooth muscle that contracts to facilitate the movement of sperm. The sperm, while capable of movement, conserve their energy for potential fertilization. Instead of swimming actively through the tract, it is the muscular contractions of the ductus deferens that propel sperm forward. The ductus deferens traverses back into the body cavity through the spermatic cord and inguinal canal, ultimately joining with the seminal vesicles.

Once sperm leave the body, they require certain conditions to survive. The seminal vesicles produce an alkaline fluid to neutralize the acidity of urine in the urethra; its secretions include fructose, providing energy for sperm via ATP production, and prostaglandins, which promote contractions in the female reproductive tract that assist sperm movement.

The prostate gland, encasing the urethra, also contributes by secreting an alkaline fluid that neutralizes urine. It produces citrate for energy and prostate-specific antigen (PSA), an enzyme that liquefies semen after ejaculation, allowing sperm to swim freely through the female tract.

Additional support comes from the bulbourethral glands, which secrete a lubricating fluid into the membranous urethra, further preparing the urethra for sperm passage and cleaning any residual urine.

The combined secretions of the seminal vesicles, prostate, and bulbourethral glands form semen, which contains about 120 million sperm per milliliter, with a total volume around 5 milliliters. Semen is slightly alkaline, balancing the acidic environment of the vagina, which is acidic due to lactic acid from glycogen metabolism. This environment is crucial for sperm functionality; however, sperm cannot fertilize an egg until they undergo a process called capacitation in the female reproductive tract.

The male penis includes the spongy urethra, through which sperm and semen exit the body during ejaculation, and is comprised of erectile tissues: the paired corpora cavernosa and the single corpus spongiosum, surrounding the urethra. Upon sexual stimulation, blood flow increases drastically into these erectile tissues, leading to an erection by compressing venous drainage to maintain rigidity.

The process of erection is primarily mediated by nitric oxide, a potent vasodilator that causes localized arterial widening to enhance blood flow. This highly controlled mechanism prevents excessive blood flow that could cause systemic effects like anaphylactic shock. The hypothalamus initiates this process by releasing gonadotropin-releasing hormone (GnRH), prompting the anterior pituitary to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which are critical in stimulating spermatogenesis and testosterone production.

Negatively controlled feedback mechanisms involving inhibin ensure that excess levels of testosterone and sperm do not accrue, creating a balanced hormonal environment for male reproductive health. Overall, from puberty until the later decades of life, testosterone, sperm production, and their regulatory pathways maintain crucial roles in male fertility and overall health.

(Reproduction female oogenesis)

This video provides an in-depth exploration of the female reproductive system, emphasizing the processes involved in the production and transportation of gametes, as well as the distinctive role of developing a fetus. The female reproductive system's anatomical structures are primarily situated within the pelvic cavity, contrasting with the external male reproductive system, which has many of its components located outside the body.

External Genitalia and Vulva
The exterior of the female reproductive system, known as the vulva, comprises several key structures: the clitoris, labia minora, and labia majora. The clitoris is a sensitive organ that plays a significant role in sexual arousal. The labia minora are internal folds of skin that protect the openings to the urethra and vagina, while the labia majora are the outer folds that enclose and safeguard the internal structures. These features highlight the protective functions of the external genitalia in maintaining the integrity of the female reproductive tract.

Vagina's Role in the Reproductive System
The vagina functions as the entryway to the female reproductive tract, strategically connecting to the uterus at the cervix, a muscular and elastic structure lined with stratified squamous epithelium. This structural design allows for significant expansion during childbirth while also providing protection against friction during intercourse. Additionally, the vaginal walls contain rugae, which are folds that allow stretching. It is populated with beneficial lactobacillus bacteria that convert glycogen into lactic acid, thus creating an acidic environment that defends against pathogenic infections, making the vagina resilient against external contaminants.

Ovaries: The Primary Reproductive Organs
The ovaries, essential for female reproduction, are roughly the size of almonds and located within the pelvic cavity. They are held in position by a broad ligament—part of the peritoneum—along with the suspensory ligament that connects them to the abdominal wall and the ovarian ligament that links them to the uterus. Inside the ovaries, the cortex contains ovarian follicles at various developmental stages, while the medulla is filled with blood vessels and neural tissue.

Gamete Development: Oogenesis and Folliculogenesis
This video also highlights two critical processes occurring in the ovaries: oogenesis, which refers to the development of oocytes (egg cells), and folliculogenesis, the growth and maturation of the ovarian follicles. Oogenesis begins with the formation of oocytes from oogonia during fetal development; however, the majority of these primary oocytes remain arrested in prophase I of meiosis until puberty. At birth, females possess approximately 400,000 primary oocytes, a number that decreases due to atresia (absorption of undeveloped oocytes) as they grow.

Upon reaching puberty, hormonal changes stimulate the ovaries to resume the maturation of oocytes and follicles, initiating a cyclical process known as the ovarian cycle. Each cycle typically lasts about 28 days, during which several follicles begin to develop, but usually only one becomes dominant, growing to maturity and releasing the oocyte in a process called ovulation.

Ovulation: A Key Transition
Ovulation marks a significant transition from puberty to reproductive maturity, indicating the onset of fertility. Triggered by a surge in luteinizing hormone (LH), the primary oocyte completes its first meiotic division and is released into the fallopian tube as a secondary oocyte, along with surrounding granulosa cells.

Post-Ovulation Changes
Following ovulation, the remnants of the follicle reorganize to form the corpus luteum, which plays a crucial role in producing hormones necessary for maintaining a potential pregnancy. If fertilization occurs, the secondary oocyte will complete the second meiotic division, converting into a mature ovum that will unite with sperm to form a zygote. This zygote is nourished by the cytoplasm and organelles stored in the ovum, crucial for its early development. Focusing on follicular maturation, it is noted that as the oocyte develops through meiosis, the surrounding granulosa cells proliferate and differentiate, ultimately producing estrogen and progesterone, assisting in the preparation for possible implantation in the uterus after fertilization occurs.

Overall, the female reproductive system embodies a complex and intricate interplay of anatomical structures and physiological processes, ensuring effective gamete production, transport, and potential for fetal development.

(Reproduction female menstral cycle)

The endometrium is the specific layer of the uterus that changes throughout the female cycle, whether referred to as the reproductive cycle or uterine cycle. This layer is influenced by estrogen and progesterone produced by developing follicular cells. After ovulation, if the corpus luteum dies, progesterone levels fall, leading to the loss of the stratum functionalis, which is known as menses.

The menstrual cycle lasts typically between 21 to 33 days, often averaging around 28 days. It is controlled by hormones including FSH (follicle-stimulating hormone), LH (luteinizing hormone), estrogen, and progesterone. Menses, which marks day one of the cycle, involves the shedding of the stratum functionalis and lasts between two and seven days. During this early follicular phase, hormone levels (progesterone, LH, FSH, and estrogen) are all low, coinciding with the degradation of the corpus luteum.

Following menses, the cycle progresses to the rebuilding phase, known as the proliferative phase, influenced by increasing estrogen levels from developing tertiary follicles. Estrogen levels rise significantly, which decreases FSH and LH levels due to negative feedback, preventing further follicular development until ovulation is triggered by a surge in LH.

Ovulation occurs when the secondary oocyte is released into the fallopian tube, marking the end of the follicular phase. The luteal phase follows ovulation, characterized by the presence of the corpus luteum, which produces progesterone and maintains the endometrium. The secretory phase, part of the luteal phase, prepares the uterus for possible implantation by stimulating glandular activity and increasing blood flow. If implantation does not occur, the corpus luteum degenerates, leading to a drop in progesterone levels, causing the endometrium to shed and the cycle to restart.

The mammary glands also play a role in supporting reproduction, with specialized glands organized into alveoli for milk production, influenced by hormonal changes during the menstrual cycle and pregnancy. Contraception methods, including natural family planning, mechanical barriers, and hormonal controls, are used to prevent fertilization or implantation.

(Reproduction female ovarian cycle)

The ovarian cycle begins with the maturation of follicles, with primordial follicles containing primary oocytes that develop into primary follicles after puberty, characterized by an increase in granulosa cells, which change from squamous to cuboidal. As these follicles progress to secondary and then tertiary stages, the granulosa cells produce estrogen and develop multiple layers, forming distinct structures such as the zona pellucida around the oocyte and an antrum in the tertiary follicle.

Hormonal regulation is crucial in the ovarian cycle, with follicle-stimulating hormone (FSH) and luteinizing hormone (LH) orchestrating the development and maintenance of ovarian follicles. The follicular phase involves the growth of numerous follicles, but typically only one becomes a dominant follicle. Increased estrogen secretion from this dominant follicle results in negative feedback that suppresses the other follicles.

As estrogen levels peak, they trigger a positive feedback loop leading to an LH surge, which causes ovulation—the release of the secondary oocyte from the dominant follicle. Following ovulation, the luteal phase begins, characterized by the formation of the corpus luteum from the remnants of the ruptured follicle, which secretes progesterone to maintain the endometrium for potential implantation.

If fertilization does not occur, the corpus luteum degenerates into the corpus albicans, resulting in decreased progesterone levels and the eventual shedding of the endometrium during menses. The uterine tubes, or fallopian tubes, facilitate the transport of the oocyte and possibly the sperm, with ciliated epithelial cells aiding this process.

The uterus features three layers: the innermost endometrium, which consists of a functional layer that undergoes cyclic changes and a basal layer that remains intact. The myometrium is the muscular layer responsible for contractions, while the perimetrium is the outer connective tissue layer. The cyclical changes in the endometrium are driven by hormonal fluctuations of estrogen and progesterone as preparations for potential pregnancy occur. If implantation does not take place, the menstrual cycle restarts after the loss of the functional layer during menses, in anticipation of a new follicular phase.

(Reproduction contraception)

Combined hormonal contraception delivers estrogen and progesterone to prevent pregnancy by mimicking the luteal phase, creating a rich endometrium while inhibiting FSH and LH, which blocks follicle development. This technique means that females theoretically do not ovulate. When the hormonal pills are paused for a week, hormone levels drop, resulting in menstruation. Once resumed, hormone levels rise quickly, preventing the normal gradual increase associated with ovulation. Various delivery methods for these hormones are used for comfort, including the NuvaRing, transdermal patches, and intramuscular injections that last months. An intrauterine device (IUD), historically a copper object, prevents implantation and is now available in hormone-coated versions like Mirena, which also inhibits ovulation. Surgical methods, such as vasectomies and tubal ligations, prevent sperm from reaching the egg effectively.

Sexually transmitted infections (STIs) pose significant public health concerns, yet testing remains limited due to costs and privacy issues. While bacterial STIs can often be treated with antibiotics, viral infections like herpes and HIV are more complex to manage and may result in infertility or death. Recent studies indicate that about 25% of men and 20% of women in the U.S. have high-risk HPV, which can lead to cervical cancer. Understanding the prevalence and risks associated with STIs, including HPV, is critical for public awareness and health. Maintaining communication and education about these issues is essential.