Lecture Outline: The Reproductive System
Open PDF Version
-
Introduction to Reproductive Systems
- The reproductive system is the only organ system with different versions for male and female.
-
Primary Sex Organs (Gonads)
- Defined as structures that produce gametes (sex cells).
- In humans, gametes are sperm (male) and eggs (female).
- For males, the primary sex organs are the testes.
- For females, the primary sex organs are the ovaries.
-
Secondary Sex Organs
- These are delivery devices rather than gamete producers.
- Examples include the penis and vagina.
-
Male Reproductive Anatomy
-
Testes (Male Gonads)
- Plural of testicle.
- Unlike female gonads, male gonads lie outside the abdominal pelvic cavity once sexual maturity is reached.
- They start development in the abdominal pelvic cavity but descend into the scrotum, a sack-like structure.
- The testis is a whitish, egg-shaped structure.
- Each testis is covered by a tough outer coat called the tunica albuginea.
- The interior of the testis is primarily made up of seminiferous tubules.
-
Epididymis
- Closely associated with each testis.
- Sperm produced by the testis mature in the epididymis for about two weeks.
- It is a network of very long, convoluted tubes that allow sperm to mature.
-
Ductus Deferens (Vas Deferens)
- A tube that transports sperm from the epididymis.
- It loops around the ureter (part of the urinary system) into the abdominal pelvic cavity.
- It ends in a bag-like structure called an ampulla.
- Cutting this tube as a form of contraception is called a vasectomy.
-
Semen
- A complex mixture, including sperm and secretions from accessory glands.
-
Accessory Glands Contributing to Semen
-
Seminal Vesicles
- There are two seminal vesicles.
- They produce components of the semen, which mix with sperm from the ductus deferens in the ampulla.
-
Prostate Gland
- A walnut-sized gland found only in males.
- It surrounds the tubing where the seminal vesicle and ductus deferens meet.
- It produces its own secretions that become part of the semen.
-
Bulbourethral Glands
- There are two of these glands, located at the root of the penis.
- They produce mucus, which is added as the final component of semen just before ejaculation.
-
Urethra (Male)
- Longer in males than in females, extending through the penis.
- In males, it serves as a delivery device for both urine and semen.
-
Three Major Parts
- Prostatic urethra: The first part, located within the prostate gland.
- Membranous urethra: The shortest part, it penetrates through the floor of the abdominal pelvic cavity.
- Spongy urethra: The third and final part, officially starts inside the penis and runs down its length, surrounded by erectile tissue.
-
Penis Structure
- Consists of the root (deepest part, not externally visible), the shaft (visible part), and the glans (the head of the penis).
- The glans penis is covered by skin called the prepuce (foreskin), which can be cut during circumcision.
-
Erectile Tissues
- The penis contains three columns of erectile tissue: two corpora cavernosa and one corpus spongiosum.
- The spongy urethra runs through the corpus spongiosum.
- During erection, these tissues become engorged with blood due to increased blood flow, causing hydrostatic pressure.
-
Spermatic Cord
- Includes blood vessels (veins and arteries) and nerves.
- There is a spermatic cord for each of the two testes.
-
Spermatogenesis (Sperm Production)
-
Seminiferous Tubules
- Make up the bulk of the interior of each testis.
- They are arranged into subsections called lobules, partially separated by septa.
- This is where sperm are released.
-
Rete Testis
- A network of tubules that receive young, immature sperm from the seminiferous tubules.
-
Cell Types in the Body
-
Somatic Cells
- Make up the vast majority of cells in the body.
- They are diploid (2n) and undergo only mitosis, producing genetically identical cells.
-
Germline Cells (Spermatogonia in Males)
- A tiny minority of cells found only in the gonads.
- They are diploid (2n) and can undergo both mitosis (to replenish themselves) and meiosis (to produce gametes).
-
Ploidy (Degree of Chromosome Sets)
- Refers to how many of each type of chromosome a cell has.
- For humans, there are 23 different types of chromosomes (n=23).
-
Haploid (n)
- Cells containing just one of each of the 23 types of chromosomes.
- Human gametes (sperm and eggs) are haploid.
-
Diploid (2n)
- Cells containing two of each type of chromosome (e.g., two number ones, two number twos, etc.).
- Human somatic cells and germline cells are diploid, having 2 * 23 = 46 chromosomes.
-
Cell Division Processes
-
Mitosis
- A type of cell division that creates genetically identical cells with the same ploidy.
-
Meiosis
- A different type of cell division that produces genetically non-identical cells.
- It cuts the ploidy in half (e.g., diploid to haploid).
- A diploid cell undergoing complete meiosis produces four haploid cells.
-
Stages of Sperm Development
- Spermatogonia (germline stem cells) undergo mitosis and then meiosis.
- Meiosis produces early, immature forms called spermatids.
- Spermatids mature into fully viable sperm within the epididymis through a process called spermiogenesis.
-
Sperm Structure
- A unicellular organism representing the haploid stage of the human life cycle.
-
Head
- Houses the cell's nucleus, which contains the DNA (chromosomes).
-
Midpiece
- Located just before the tail, it contains many mitochondria responsible for producing ATP (energy) for swimming.
-
Flagellum (Tail)
- A long, whip-like tail that propels the sperm forward through liquid.
-
Hypothalamo-Pituitary-Gonadal (HPG) Axis (Male)
- A complex signaling pathway involving the hypothalamus, anterior pituitary, and testes.
-
Hypothalamus
- Releases Gonadotropin-Releasing Hormone (GnRH).
-
Anterior Pituitary
- Cells in the anterior pituitary are stimulated by GnRH to release two gonadotropins: Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH).
- These hormones are named for their functions in females but also occur in males.
-
Testes (Gonads)
- LH stimulates interstitial cells (located in spaces between structures) within the testes to produce testosterone.
- FSH acts on different cells called supporting cells, which assist the spermatogonia in sperm production.
-
Testosterone (Male Hormone)
- Controls many functions, including sperm production.
- Exhibits negative feedback: High testosterone levels act on both the hypothalamus and anterior pituitary to turn down their production of GnRH, FSH, and LH, maintaining balance.
-
Female Reproductive Anatomy
-
Ovaries (Female Gonads)
- The primary sex organs in females, responsible for producing gametes (eggs or ova).
- Unlike male gonads, ovaries remain within the abdominal pelvic cavity throughout life.
-
Uterine Tubes (Fallopian Tubes)
- Closely associated with each ovary.
-
Infundibulum
- A funnel-shaped structure with fringes called fimbriae.
- It helps catch the egg released during ovulation.
- The infundibulum narrows to become the uterine tube, which leads to the uterus.
- Fertilization usually occurs in the uterine tube.
-
Uterus
- A highly muscular, hollow organ.
- Its interior is a potential space that can expand significantly during pregnancy.
-
Parts of the Uterus
- Fundus: The superior, blind (dead) end of the uterus.
- Cervix: The narrowed, neck-like part of the uterus that opens into the vagina.
-
Layers of the Uterine Wall
- Endometrium: The inner lining, which undergoes drastic changes in thickness and is the site of embryo implantation.
- Myometrium: The thickest layer, composed of smooth muscle, responsible for contractions during childbirth.
- Parametrium: The outer covering of the uterus.
- Supported in place by ligaments, including the broad ligament, round ligament, and ovarian ligament.
-
Vagina
- A word meaning "sheath," describing its function in accepting the penis.
- It is a secondary sex organ, serving as a delivery device for the fetus during childbirth and for receiving the penis and sperm.
- Its interior is highly folded, which aids in stimulating the penis during intercourse.
-
External Genitalia (Vulva)
-
Labia Majora
- The larger, outer lips that guard the external opening of the vagina.
- They are dry and feature hair.
-
Labia Minora
- The smaller, inner lips, located between the labia majora.
- They are moist epithelium and become engorged with blood during sexual arousal.
-
Clitoris
- Develops in the same way as the penis (homologous structure).
- Much shorter than a penis, with a highly sensitive glans (head) that can lead to orgasm when stimulated.
- The glans of the clitoris is also covered by a prepuce (hood-like skin covering).
-
Vestibular Glands
- Located on either side, they produce secretions that moisten the labia minora and act as a lubricant during intercourse.
-
Perineum
- The region outlined by bony landmarks, superficially defined as the area between the vagina (or scrotum in males) and the anus.
-
Oogenesis (Egg Production) and Ovarian Cycle
-
Egg Production Process (Oogenesis)
- Production of female gametes starts before a female is even born, while she is a fetus.
- Millions of potential eggs (primordial follicles) begin the process of meiosis but then pause for almost two decades until sexual maturity.
- Most potential eggs die before birth, but thousands remain.
- Unlike males who produce millions of fresh sperm daily, females normally release only one egg per ovarian cycle (roughly every 28 days).
- The ovulated egg is an "old cell" as its development began in the fetal stage.
-
Follicle Development in the Ovary
- Primordial follicles are the earliest stage.
- Each cycle, some primordial follicles are chosen to develop into primary follicles, which then grow larger into growing follicles.
- Normally, only one growing follicle fully matures to produce an egg for release.
-
Ovulation
- The process where a mature egg ruptures out of the wall of the ovary.
- If successful, the egg is caught by the funnel-like infundibulum of the uterine tube.
-
Corpus Luteum
- After ovulation, the ruptured follicle is renamed a corpus luteum ("yellow body").
- The corpus luteum continues to produce hormones, particularly progesterone.
- If no pregnancy occurs, it degenerates.
- If pregnancy occurs, it is called a corpus luteum of pregnancy and persists longer, producing hormones until the placenta develops sufficiently.
-
Meiosis in Females (Distinct from Males)
- While meiosis in males produces four viable sperm from one germline cell, in females, it produces one large viable egg cell and three tiny "polar bodies."
- The polar bodies are discarded, allowing the egg to conserve as much cytoplasm as possible.
- This conservation is crucial because the egg (and subsequent zygote) needs to be large for survival, as the sperm primarily delivers only DNA.
- Biologically, the female is defined as the sex that produces the larger of the two gametes.
-
Phases of the Ovarian Cycle
- Follicular Phase: The period during which follicles are developing in the ovary.
- Luteal Phase: Begins immediately after ovulation, characterized by the formation and activity of the corpus luteum.
-
Female Hormonal Control and Uterine Cycle
-
Key Hormones in Females
- LH (Luteinizing Hormone) and FSH (Follicle-Stimulating Hormone): Released by the anterior pituitary, their surge (especially LH) causes ovulation.
- Estrogens: Released by developing follicles; increasing estrogen levels during the follicular phase lead to the LH surge.
- Progesterone: Primarily produced by the corpus luteum after ovulation. It prepares the uterus for potential pregnancy by making the endometrial lining thick and moist. Its name means "promotes gestation."
-
Uterine Cycle (Menstrual Cycle)
- Occurs simultaneously with the ovarian cycle, describing changes in the endometrium (inner lining of the uterus).
- Day one of both cycles is defined as the first appearance of menstrual blood.
-
Phases of the Uterine Cycle
- Menstrual Phase: The shedding of the previous endometrial lining if pregnancy did not occur, resulting in menstrual flow (menses), typically lasting about 5 days.
- Proliferative Phase: Following menstruation, the endometrium grows fresh and thickens again, with its cells proliferating (increasing in number), stimulated by estrogens.
- Secretory Phase: After the endometrium has thickened, endometrial glands begin secreting substances to make the lining moist and chemically suitable for an embryo to implant. This thickness is maintained by progesterone.
- If no pregnancy occurs, the progesterone level drastically dips, signaling the uterus to undo this preparation and degrade the endometrial lining, leading to the beginning of the next menstrual phase.
-
Mammary Glands (Breasts)
- Mammary glands are located within the breasts and are responsible for producing milk.
- This is why humans are classified as mammals, as true milk is produced only by mammals.
-
Lactation (Milk Production)
- Occurs in sack-like structures called alveoli, which are arranged in lobules within the mammary glands.
- Milk is produced and stored continuously in these lobules in pregnant or nursing women.
-
Milk Ejection
- After production, milk is delivered into lactiferous ducts, which widen into lactiferous sinuses, all converging at the nipple.
- Milk is ejected through the nipple in response to suckling by an infant or by a suction pump. This is a distinct process from milk production and involves a different hormone.
-
Nipple and Areola
- The nipple is a rounded mound of tissue with openings for milk ejection.
- It is surrounded by a darkened circular area called the areola, which likely serves as a visual target for the infant.
- The bulk of the breast in non-pregnant and non-nursing females is primarily made up of adipose (fat) tissue, as the mammary glands are very small when not actively producing milk.
-
Human Sexual Life Cycle and Development
-
Fertilization
- The fusion of two unicellular gametes (a haploid sperm and a haploid egg) to produce one unicellular zygote.
- This is the defining moment of sexual reproduction, as it mixes genetic material from two parents.
- Fertilization usually occurs in the uterine tube.
- While many sperm attempt to fertilize an egg, only one is successful, and its entry prevents others from doing the same.
-
Ploidy Alternation in the Life Cycle
- Fertilization doubles the ploidy (combining two haploid gametes to form a diploid zygote).
- Meiosis, occurring in the gonads, halves the ploidy (producing haploid gametes from diploid germline cells).
- This alternation between fertilization and meiosis ensures that the chromosome number remains constant from one generation to the next.
- Human individuals exist as the diploid stage, while gametes represent the short-lived haploid stage.
-
Stages of Development After Fertilization
-
Zygote
- The single, diploid cell that marks the official beginning of a new human life.
- It typically forms in the uterine tube and then moves toward the uterus.
-
Embryo (Conceptus)
- The zygote undergoes mitosis to become a multicellular organism, initially appearing as a mass of cells.
- This early embryo implants into the endometrium of the uterus.
-
Fetus
- The embryo develops into a fetus around 12 weeks of gestation, at which point it looks much more like a human.
- Development continues as a fetus until birth, typically around 9 months after fertilization.
-
Placenta
- An organ that develops between the new organism (embryo and then fetus) and the mother's uterus.
- It is unique in that it is partly derived from the mother and partly from the fetus.
- The placenta eventually takes over the hormone-producing functions of the corpus luteum during pregnancy.
-
Parturition (Childbirth)
- An excellent example of a positive feedback mechanism, which is much rarer than negative feedback in the body.
- As the fetus, typically head-down, stretches the cervix of the uterus, stretch receptors in the cervix send signals to the hypothalamus in the brain.
- The hypothalamus signals the posterior pituitary to release oxytocin into the bloodstream.
- Oxytocin travels to the uterus, stimulating contractions of the myometrium (the muscular layer), particularly in the fundus.
- These contractions push the fetus further down, causing even greater stretching of the cervix, which in turn leads to the release of even more oxytocin, intensifying the contractions.
- This positive feedback loop continues to build in intensity until the fetus is expelled through the vagina.