Reproductive System and Human Development Lecture Flashcards

Overview of the Reproductive System and Male Anatomy

The reproductive system is categorized as a body system exhibitng high variability, representing one of the largest sources of anatomic and physiologic variation. The primary role of the reproductive system as a whole is the production of offspring through the process of fertilization. Fertilization is defined as the union of a male gamete and a female gamete to create a zygote, which develops into a baby. In the typical male system, the primary sex organs are the gonads (testes), and the gametes produced are sperm. Secondary male sex organs include the scrotum, penis, and a system of ducts and accessory glands. In the typical female system, the primary sex organs are the ovaries, and the gametes are the secondary oocytes. Secondary female sex organs include the uterus, uterine (fallopian) tubes, and the vagina.

Male External Reproductive Anatomy

The male reproductive system is comprised of the gonads, a duct system, accessory glands, and supporting structures. The external anatomy primarily consists of the penis and the scrotum. The penis is a cylindrical structure containing the urethra, which serving a dual role in both the urinary and reproductive systems as the principal pathway for the ejaculation of semen and the excretion of urine. The body or shaft of the penis consists of three cylindrical masses of erectile tissue: two identical corpora cavernosa, which provide the bulk of the erectile volume, and one corpus spongiosum, which sits inferiorly and wraps around the urethra. The glans penis is the expanded distal tip, highly innervated with sensory receptors, acting as the primary erogenous zone. It is superficially covered by the prepuce, or foreskin. The scrotum is a supportive sac of loose skin and underlying hypodermis that houses the paired testes. Its purpose is to maintain sperm production within a temperature window a few degrees below body temperature. The testes descend through the inguinal canals during fetal development, dragging the spermatic cord behind them. This cord carries spermatic ducts, blood and lymphatic vessels, and nerves from the abdominopelvic cavity. Temperature regulation is maintained by three structures: the cremaster muscle (which elevates the testes), the dartos muscle (which wrinkles the scrotal skin), and the pampiniform plexus of veins (which acts as a heat exchanger).

Spermatogenesis and Spermiogenesis

The testes are organized into a system of tubules where spermatogenesis, the production of sperm through meiotic cell division, occurs. Stem cells called spermatogonia begin mitosis at puberty. Some remains as stem cells, while others develop into primary spermatocytes ($2n$, diploid). During Meiosis I, cells become haploid with two sister chromatids (often described as having "googly eyes"); during Meiosis II, they become haploid with single chromatids. Spermiogenesis is the subsequent maturation of these cells. A viable sperm must shed excess cytoplasm, increase mitochondrial concentration, and form two key structures: the acrosome (an organelle filled with enzymes to penetrate the secondary oocyte) and the flagellum (a motile tail that moves in a spiral/corkscrew motion to propel the sperm forward). Finished sperm are released into the lumen of the seminiferous tubules by Sertoli cells (sustenocytes), also known as "nurse cells," which secrete fluid to push sperm toward the duct system. The testes produce up to $300 imes 10^6$ sperm daily. Once ejaculated, sperm typically survive around $48$ hours in the female tract.

Internal Male Anatomy and the Duct System

The sperm path starts in the seminiferous tubule lumen, moves into straight tubules, and then into a web called the rete testis. From there, sperm drain via efferent ductules into the epididymis, where they complete spermiogenesis. Upon erection, mature sperm move into the ductus (vas) deferens, ascending through the inguinal canal and looping posterior to the ureter. The ductus deferens ends at a dilated ampulla that stores sperm until ejaculation. The ejaculatory ducts, formed by the union of the ductus deferens and the duct of the seminal vesicles, empty sperm into the urethra. Semen is the supportive, buffering fluid required for sperm survival. It is contributed by three glands: the seminal vesicles (providing approximately $60\%$ of semen volume, including fructose sugar for energy), the prostate (providing approximately $30\%$ of semen), and the bulbourethral (Cowper’s) glands (providing approximately $10\%$ of semen to neutralize urethral acidity). Semen supplies nutrients and neutralizes the acidic environments of the male urethra and female vagina.

Physiology of Male Reproduction

Hormonal control begins at puberty when the hypothalamus releases Gonadotropin-releasing hormone (GnRH). This stimulates the anterior pituitary to secrete Luteinizing hormone (LH) and Follicle-stimulating hormone (FSH). LH stimulates Leydig cells (interstitial endocrine cells) to secrete testosterone. FSH, alongside testosterone, stimulates Sertoli cells to secrete androgen-binding protein (ABP), which keeps testosterone levels high in the tubules to drive spermatogenesis. Inhibin, also from Sertoli cells, inhibits FSH to regulate the rate of production. Testosterone is converted to dihydrotestosterone (DHT) in some tissues like the prostate. Effects of these androgens include the development of secondary sex characteristics (facial hair, height, Adam's apple, muscle mass). The Autonomic Nervous System (ANS) controls the sexual response: the Parasympathetic Nervous System (“Pointing”) triggers erection by relaxing smooth muscles in penile arteries and compressing veins to engorge tissue with blood. The Sympathetic Nervous System (“Shooting”) triggers ejaculation by contracting the smooth muscle of the ductus deferens and other ducts, while simultaneously contracting the internal urethral sphincter to prevent semen reflux into the bladder.

Female External Anatomy and Mammary Glands

The external genitalia of the anatomic female are collectively called the vulva. This includes the mons pubis, labia majora, labia minora, clitoris, vestibule, and the vaginal and urethral orifices. The clitoris contains erogenous tissues homologous to the male penis, including a glans and crus. The mammary glands are modified sudoriferous (sweat) glands that produce milk for newborn nourishment. Located anterior to the pectoralis major and serratus anterior, they are organized into lobes, lobules, and alveoli separated by adipose tissue. Milk ejection is stimulated by oxytocin, which causes myoepithelial cells to contract, propelling milk through the lactiferous ducts and lactiferous sinuses to the nipple.

Female Internal Anatomy: Vagina and Uterus

The vagina is the distal-most structure, lined with non-keratinized stratified squamous epithelium and hosting a lactobacillic microflora that maintains pH and self-cleansing. Its functions include serving as the menstrual canal, the birth canal, and the site of intercourse. The vagina forms a posterior fornix (a "false cervix") where it extends superiorly past the external os of the cervix. The uterus is a hollow, muscular organ consisting of the fundus (opposite the distal opening), the body, and the cervix. The innermost layer, the endometrium, grows and sloughs off monthly during the menstrual cycle; it facilitates implantation and forms the placenta. The middle layer is the myometrium (muscle), and the outer is the perimetrium. The uterine blood supply involves the ovarian arteries (from the descending aorta), the uterine arteries (from the internal iliac artery), which give off arcuate and radial branches, and the vaginal arteries. The uterine (fallopian) tubes extend from the fundus and include the infundibulum (with fimbriae to catch the egg), the ampulla (the site of fertilization), and the isthmus. Ciliated epithelial cells and smooth muscle facilitate transport of the oocyte or zygote.

Oogenesis and Folliculogenesis

Oogenesis produces the female gametes. Unlike males, females form all their primary oocytes ($2n$, diploid) prior to birth. Many undergo atresia (degeneration). Development arrests in Prophase I until puberty. Each month, a primary oocyte completes Meiosis I to produce one secondary oocyte and one much smaller polar body due to unequal cytoplasm distribution. Meiosis II only completes if fertilization occurs. Folliculogenesis is the development of the supporting follicle: Primordial follicles (birth) contain primary oocytes; Primary follicles (puberty) develop cuboidal granulosa cells; Secondary follicles add the zona pellucida and theca folliculi; Tertiary follicles develop a large fluid-filled antrum (a "lake" of estrogen) and the corona radiata; the Mature (Vesicular/Graafian) follicle is ready for ovulation. Post-ovulation, the remnant follicle becomes the corpus luteum, which secretes progesterone. If no pregnancy occurs, it degenerates into the corpus albicans (a white scar).

The Hormonal Cycle and Pregnancy

The female reproductive cycle includes the ovarian and uterine cycles. At puberty, GnRH stimulates FSH (initiates follicular growth and estrogen secretion) and LH (triggers ovulation). Estrogen maintains secondary sex characteristics and repairs the endometrium. Progesterone prepares the endometrium for implantation and mammary glands for milk. During the menstrual phase, the stratum functionalis sheds. In the pre-ovulatory phase, estrogen levels rise. At ovulation (Day 14), an LH surge triggers the release of the secondary oocyte. In the post-ovulatory phase, the corpus luteum dominates. If fertilization occurs, the embryo secretes human chorionic gonadotropin (hCG), which sustains the corpus luteum and prevents menstruation. This allows the endometrium to thicken. Fertilization occurs in the ampulla after sperm undergo capacitation (removal of membrane cholesterol). The acrosomal reaction digests the zona pellucida. Polyspermy is prevented by a Fast block (Na+ depolarization) and a Slow block (Ca2+ wave creating a water forcefield).

Development and Labor

Following fertilization, the zygote undergoes cleavage (rapid mitosis), forming a morula (totipotent stem cells) by $72$ hours. It then becomes a blastocyst, consisting of an outer trophoblast (becomes the placenta) and an inner embryoblast (becomes the embryo). Implantation occurs when the blastocyst adheres to the endometrium and the syncytiotrophoblast digs into the tissue. This process takes about a week. Fetal circulation includes the ductus arteriosus and foramen ovale (bypassing lungs), the ductus venosus (bypassing liver), umbilical arteries (carrying mixed/deoxygenated blood to the placenta), and the umbilical vein (carrying oxygenated blood to the fetus). Relaxin softens the pubic symphysis and relaxes the uterus. Labor consists of four stages: Early dilation (contractions and effacement), Late dilation (full dilation to $10\,cm$), Expulsion (birth of the baby), and the Placental stage ("afterbirth").

Complications and Contraception

Pregnancy complications include Preeclampsia (high blood pressure > 140/90\,mmHg and proteinuria) and Eclampsia (seizures). HELLP syndrome involves liver failure and hemolysis. Gestational diabetes is diagnosed via glucose challenge. Placenta previa occurs when the placenta blocks the cervix. Birth defects include cleft palate, clubfoot, and congenital heart defects like Tetrology of Fallot (overriding aorta, ventricular septal defect, PFO, and hypertrophic ventricles). Contraception methods vary in efficacy: roughly $13\%$ of people use them incorrectly or not at all, accounting for $95\%$ of unexpected pregnancies. Barrier methods include condoms (the only method protecting against STIs) and diaphragms. Hormonal methods (pills, patches, rings, implants, injections) work by blocking the LH surge to prevent ovulation. IUDs (hormonal or copper) are highly effective (> 99\%), with copper acting as a spermicide.

The reproductive system is crucial for offspring production, with male gonads (testes) producing sperm and female gonads (ovaries) producing secondary oocytes. In males, the penis and scrotum are the primary external structures; the penis contains erectile tissue and the scrotum maintains the testes' temperature for sperm production. Spermatogenesis occurs in the testes, resulting in sperm that then travel through a duct system for ejaculation. Hormonal control begins at puberty, involving GnRH, LH, and FSH, regulating testosterone levels and spermatogenesis. The female reproductive cycle encompasses oogenesis, folliculogenesis, and preparation for pregnancy, facilitated by hormones like estrogen and progesterone. Complications during pregnancy can include preeclampsia and gestational diabetes, while various contraception methods exist to prevent unintended pregnancies.