Chapter 31. Reproductive System
Recommended Post: 【Biology】 Biology Index
1. Overview
3. Development of the Reproductive System: Embryonic Duct
a. Reproductive System Problem Sets
1. Overview
⑴ Cell Division
① Mitosis: Somatic Cell Division
② Meiosis: Germ Cell Division (Diploid → Haploid)
○ Occurs in the Gonads
○ Prevents doubling of genetic material in each generation
⑵ Gametes and Zygotes
① Gametes: Haploid cells (n)
② Zygote: Formed by fusion of two gametes, Diploid (2n)
③ Fertilization: Fusion of Sperm and Egg
○ Sperm: Motile gamete
○ Egg: Non-motile gamete
○ Female: Organism producing larger gamete (egg)
○ Male: Organism producing smaller gamete (sperm)
④ Gonads: Organs that produce gametes, i.e., testes and ovaries
⑤ Methods of Fertilization
○ Internal Fertilization: Fertilization within reproductive tract through copulation. Mammals, birds, reptiles, sharks, etc.
○ External Fertilization: Female releases eggs into the water, and males release sperm. Fish, amphibians, etc.
⑶ Reproduction
① Asexual Reproduction: Involves only mitosis, producing genetically identical offspring
○ Division: Separation into two identical cells through mitosis; e.g., sea anemone.
○ Budding: A portion of a somatic cell detaches and develops into a new individual; e.g., hydra.
○ Regeneration: In cases of whole-body regeneration, it is considered a form of reproduction; e.g., starfish, flatworms.
○ Parthenogenesis: Development of an unfertilized egg; e.g., aphids (female and male), bees (male).
② Sexual Reproduction: Involves meiosis, ensuring genetic diversity for evolution
○ Gametogenesis → Mating → Fertilization
○ Chromosomal recombination creates diverse offspring for adaptation to various environments
○ Hermaphroditism and Gonochorism
2. Gonad Hormones
⑴ Overview
① Most gonad hormones are steroids, except for LH, FSH.
⑵ GnRH (Gonadotropin-Releasing Hormone)
① Function 1. Stimulates secretion of LH and FSH from anterior pituitary
⑶ Follicle-Stimulating Hormone (FSH): Peptide hormone
① Male
○ Function 1. Stimulates Sertoli cells along with testosterone production → Sperm production
② Female
○ Function 1. Stimulates granule cells in ovarian follicles, promoting aromatase enzyme secretion
○ Function 2. Early Maturation: Follicular maturation
⑷ Luteinizing Hormone (LH): Peptide hormone from anterior pituitary
① Male
○ Function 1. Stimulates Leydig cells for testosterone secretion
○ Function 2. Testosterone stimulates Sertoli cells along with FSH
② Female
○ Function 1. Stimulates granule cells for testosterone secretion
○ **Function 2. Late Maturation: Ovulation induction
○ Function 3. Promotes the secretion of other androgens
○ Testosterone is converted to estrogen by aromatase enzyme secreted by granule cells
⑸ Androgen: A collective term for testosterone, androsterone, and DHA.
① Function: Important steroid hormones for male reproduction
○ Function 1. Sperm production
○ Function 2. Male secondary sexual characteristics: development of male reproductive organs, hair thickness and distribution, muscle mass increase, low voice
○ Function 3. Inhibits breast growth by estrogen
○ Function 4. Initiates and maintains sperm formation through Sertoli cells
○ Function 5. Negative feedback on LH, inhibiting GnRH release via hypothalamus and LH secretion from anterior pituitary
○ Function 6. Induces and maintains accessory reproductive organ differentiation and function
○ Function 7. Promotes bone growth (adolescence) and halts bone growth (late adolescence)
○ Function 8. Sexual desire and aggressive behavior
○ Function 9. Stimulation of erythropoietin secretion in the kidneys
② Women also produce small amounts in ovaries and adrenal cortex
③ Type 1: Testosterone
○ Very similar to estrone and estradiol, differing mainly in methylation, double bonds, and ketone groups
○ About 95% of testosterone in males is secreted by testes, 5% by adrenal cortex
④ Type 2: Androsterone
⑤ Type 3: DHEA (Dehydroepiandrosterone): DHT is the most representative
⑥ Type 3-1: DHT (Dehydrotestosterone)
○ Testosterone is converted to DHT by 5α-reductase in peripheral tissues
○ Prostate cancer and prostatic hyperplasia: Cancer cells are stimulated by DHT. Treatment involves inhibiting 5α-reductase to suppress DHT.
○ Congenital Adrenal Hyperplasia (CAH), Adrenogenital Syndrome
○ Cholesterol is converted to sexual hormones due to 21-α-hydroxylase deficiency, enhancing DHT function
○ Corticosterone, cortisol, aldosterone deficiency results in masculinization of females
○ Cortisol and aldosterone prescription can alleviate CAH
⑦ Brain: Testosterone is converted to estrogen by aromatase enzyme
⑧ Eunuch: Removal of testes. All accessory reproductive ducts regress. Used during Chosun Dynasty
⑹ Estrogen: A collective term for estrone, estradiol, and estriol.
① Structure
Figure 1. Functional Domains of Estrogen Receptor
○ AF: Activation Factor
○ DBD: DNA-Binding Domain
○ D: Heat Shock Proteins Binding Domain
○ F: C-Terminal Domain
○ Genomic pathway: Estrogen binds to ERα or ERβ and then acts on ERE or AP-1 within the target DNA to activate transcription.
○ Nongenomic pathway: Estrogen + ERα/ERβ or estrogen + GPR30 activates signaling pathways, including MAPK and cAMP.
③ Functions: Essential steroid hormones in female reproduction.
○ Function 1. Secondary Sexual Characteristics: External genitalia development, breast growth, body fat accumulation, sebaceous gland secretion, narrow shoulders, pelvic widening, pubic hair development, bone growth, epiphyseal plate closure.
○ Function 2. Reproductive Cycle: Endometrial creation, uterine wall development, oocyte maturation, follicular growth, increase in uterine progesterone receptors.
○ Function 3. During Fertilization: Stimulates transparent thin cervical mucus secretion (affects sperm movement).
○ Function 4. During Pregnancy: Increase in oxytocin receptors, uterine wall contractions, increased ciliary movement, promotion of milk ejection.
○ Function 5. Pre-partum: Promotes breast development via prolactin secretion, inhibits milk secretion.
○ Function 6. Post-partum: Decreased estrogen and progesterone levels enhance milk secretion.
○ Function 7. Inhibits atherosclerosis by affecting plasma cholesterol, blood vessels, and blood coagulation.
○ Function 8. When estrogen is deficient, the risk of developing osteoporosis, a menopausal disorder, increases.
○ Function 9. Affects brain neurons, enhancing learning and memory.
○ Function 10. Hypothalamic and pituitary feedback.
○ Function 11. Menopause regulation.
○ Function 12. Slightly lowers body temperature.
④ Synthesis: After testosterone is produced, it is converted into estrogen by the enzyme aromatase.
○ In males, testosterone is converted to estrogen in the brain, skin, liver, and adipose tissue.
○ Aromatase in turtle is less active at 25°C, resulting in male birth.
○ Aromatase in turtle is more active at 32°C, resulting in female birth.
⑤ Also produced in small amounts in males.
○ Excess estrogen in males can lead to gynecomastia.
○ In male aquatic animals, the testes may become feminized and produce eggs.
⑥ Types
○ Estrone, estradiol, and estriol are included.
○ Estradiol: The most potent hormone among the estrogens.
○ Estrogen mainly exists in the form of 17β-estradiol, but after menopause, it is present primarily as estrone.
⑺ Luteal hormone (progesterone): A steroid hormone
① Production Sites: Corpus luteum (yellow body), placenta.
○ During pregnancy, luteal hormone is produced in the placenta.
② Hypothalamic and Pituitary Feedback.
③ Function 1. GnRH Regulation
○ In small amounts: Stimulates GnRH → Stimulates FSH, LH.
○ In excess: Strongly inhibits GnRH → Inhibits FSH, LH → Prevents new follicle development.
④ Function 2. Pregnancy Preparation
○ Secretion of thick mucus from the cervix: Maintains the thickened uterine lining.
○ Activates MPF for oocyte cell division resumption.
○ Converts uterine lining to suitable secretion tissue for embryo implantation.
○ Inhibits contractions in fallopian tubes and uterine muscles.
○ Inhibits proliferation of vaginal epithelial cells.
○ Promotes breast development.
○ Suppresses prolactin’s lactogenic effects.
⑤ Function 3. Raises set point, so slightly increases body temperature.
⑥ Function 4. Increases blood vessels and promotes glycogen storage.
⑦ Cervical mucus changes and body temperature variations: Indicators of ovulation.
⑻ Anti-Müllerian Hormone: Glycoprotein.
⑼ Inhibin: Peptide hormone.
① Production: Secreted by Sertoli cells in the testes of males, and by granule cells of ovarian follicles in females.
○ The only peptide hormone in the ovary.
② Suppression of GnRH and FSH: Selectively inhibits FSH, thereby preventing the maturation of additional follicles during the reproductive cycle.
⑽ Activin: Peptide hormone.
① Present in the pituitary gland, testes, and follicular fluid.
② Also involved in developmental processes: located at the vegetal pole.
Figure 2. Activin and Differentiation Experiment
⑾ Human Chorionic Gonadotropin (hCG)
① Secreted when fertilized egg is implanted.
② Acts similar to LH, stimulates excess secretion of estrogen, progesterone, maintains corpus luteum, and inhibits ovarian cycle.
③ hCG concentration is maintained during 2-3 months of pregnancy, then decreases.
④ Corpus luteum regresses after 5 weeks of pregnancy, and placenta takes over hCG’s role.
⑤ Pregnancy Test Kit: Detects hCG.
3. Development of the Reproductive System: Embryonic Duct
⑴ Bipotential (indifferent) gonad
① The reproductive structures do not begin differentiation until the 7th week of development.
② When developmental signals are delivered, the medulla differentiates into the testes, forming the male reproductive system.
③ In the absence of developmental signals, the cortex differentiates into the ovaries, forming the female reproductive system.
④ Bipotential internal genitalia: Derived from the embryonic kidney, consisting of the Wolffian duct and the Müllerian duct.
○ As development proceeds, one pair of ducts develops while the other regresses.
⑤ Bipotential external genitalia: Composed of the genital tubercle, urogenital folds, urethral groove, penis/clitoris, and scrotum/labia.
⑵ SRY (sex-determining region of the Y chromosome): the sex-determining locus on the male Y chromosome
① SRY induces differentiation of the gonadal medulla into testes and stimulates Leydig and Sertoli cells.
② Leydig cells: under SRY influence, secrete testosterone and its derivative DHT (dihydrotestosterone)
○ Testosterone: promotes Wolffian duct differentiation and contributes to the descent of the testes from the abdomen into the scrotum
○ DHT: regulates differentiation of the external genitalia and other male secondary sex characteristics
③ Sertoli cells: under SRY influence, secrete anti-Müllerian hormone (AMH)
⑶ Male: Wolffian duct → vas (ductus) deferens, epididymis, seminal vesicles
① Testosterone secreted by Leydig cells actively promotes development of the Wolffian ducts.
② AMH secreted by Sertoli cells actively causes regression of the Müllerian ducts.
③ Pseudohermaphroditism
○ Males inherit 5α-reductase, the enzyme that converts testosterone to DHT.
○ In patients deficient in this enzyme, DHT is not produced, so the external genitalia appear female at birth.
○ For example, the male external genitalia and prostate fail to develop adequately during fetal development.
○ At puberty, renewed testicular testosterone secretion often leads to spontaneous masculinization.
④ Cryptorchidism
○ If the testes remain in the abdominal cavity, the elevated temperature can impair spermatogenesis and cause infertility.
⑷ Female: Müllerian duct → upper vagina, oviducts/uterine (fallopian) tubes, uterus, cervix, vagina
① Because Leydig cells do not secrete significant testosterone, the Wolffian ducts regress.
② In the absence of DHT, the external genitalia develop female characteristics.
③ Because Sertoli cells do not secrete AMH, the Müllerian ducts develop naturally.
Figure 3. Embryonic Ducts in Males and Females
4. Male Reproductive System
⑴ Reproductive organs
① From the Wolffian (mesonephric) duct, the epididymis and seminal vesicles are formed.
② Testis (testes): produces sperm and hormones; a relatively low temperature is required for normal spermatogenesis.
○ Around the 7th fetal month, the testes descend from the abdominal cavity into the scrotum.
○ 1–3% of newborns have incomplete descent.
○ About 80% descend spontaneously with growth.
○ Spermatogenesis requires a temperature 4–5 °C below core body temperature; at ≥35 °C smooth muscle relaxes.
③ Epididymis: stores and concentrates sperm; confers motility (i.e., sperm maturation); transports sperm by peristalsis during ejaculation.
④ Prostate: secretes a thin fluid containing prostaglandins, nutrients, and alkaline substances, enhancing motility.
○ Prostaglandins, 60% of semen, promote sperm motility and the motility of the female reproductive tract.
○ Benign prostatic hyperplasia (BPH): causes urinary frequency and a sense of incomplete emptying; evaluated with the prostate-specific antigen (PSA) test.
⑤ Seminal vesicle: secretes most components of seminal fluid.
○ Produces a viscous secretion containing glucose and fructose.
○ Alkaline, protecting sperm from acidic secretions in the female tract.
○ Fructose provides most of the energy used by sperm.
⑥ Cowper’s (bulbourethral) gland: secretes a clear, mucous, alkaline pre-ejaculatory fluid → neutralizes urinary acidity and clears the urethra prior to ejaculation.
○ Also secretes a small amount of lubricating mucus.
⑦ Urethra
○ The reproductive tract (ejaculatory ducts) and urinary tract share the urethra.
○ The ejaculatory duct forms when the duct from the seminal vesicle joins a duct passing through the prostate, then connects to the urethra.
○ The ducts from the bulbourethral (Cowper’s) glands connect separately.
⑧ Glans (glans penis)
○ Circumcision: removal of the prepuce. Associated with reduced risks of penile cancer, certain STIs, and cervical infections; related to the religious rite of circumcision.
⑵ Testis
① Basement membrane: prevents intrusion of foreign substances; forms part of the blood–testis barrier to regulate molecular transport.
② Seminiferous tubules
○ Site of spermatogenesis.
○ Include Sertoli cells and Leydig cells.
○ Total combined length is about 250 m.
③ Leydig cells
○ Located in the interstitial spaces between seminiferous tubules.
○ Hormonal stimulation: under LH stimulation, secrete testosterone and other androgens.
○ Hormone secretion: testosterone.
④ Sertoli cells
○ Embedded in the wall of the seminiferous tubules; provide extensive support for the production and development of sperm cells.
○ Hormonal stimulation: in response to testosterone and FSH, secrete paracrine factors that promote spermatogenesis.
○ Hormone secretion: anti-Müllerian hormone (AMH), androgen-binding protein (ABP), inhibin.
○ Provide nutritional support.
○ Form the blood–testis barrier, protecting sperm cells from immune attack.
○ Phagocytose residual cytoplasm from developing spermatids.
⑶ Spermatozoon
① Haploid nucleus.
② Acrosome: derived from the Golgi apparatus; contains hydrolytic enzymes for penetration of the ovum; the Golgi apparatus is involved in forming the acrosome required for the acrosome reaction.
③ Flagellum (midpiece): contains numerous mitochondria; generates ATP.
④ Tail: flagellar motility; speed 1–4 mm/min.
⑤ Must be exposed to the chemical environment of the female reproductive tract to acquire fertilizing capacity (capacitation).
Figure 4. Structure of the testis
Figure 5. Hormonal relationships in the male reproductive system
⑷ Sperm Production
① From puberty to adulthood, sperm production continues in the seminiferous tubules.
② Duration: 65 days
③ Daily production: 30 million
④ Lifetime production: 1012 to 1013 sperms
⑤ Ejaculate: 100-300 million sperm
⑸ 1st. Spermatogonia generated from primitive germ cells
⑹ 2nd. No change from birth to puberty
⑺ 3rd. Endocrine changes during puberty
① 3rd - 1st. GnRH secretion ↑ → FSH and LH secretion from the anterior pituitary
② 3rd - 2nd. LH → Stimulates Leydig cells → Increases testosterone secretion
③ 3rd - 3rd. FSH → Stimulates Sertoli cells → GPCR → Androgen binding protein (ABP) secretion ↑
④ 3rd - 4th. Testosterone + ABP → Stimulates Sertoli cell in a way of positive feedback control.
○ Spermatocytes do not possess testosterone receptors, so they cannot respond to testosterone directly.
○ However, spermatocytes do have receptors for androgen-binding protein (ABP), allowing them to respond to testosterone.
⑤ 3rd - 5th. Morphological change of Sertoli cells: Sertoli cells that were connected by tight junction undergo separation
⑻ 4th. As spermatogonia migrate between Sertoli cells from the cortical region of the seminiferous tubules toward the medullary region, they undergo differentiation.
① 4th - 1st. Accumulation of cytoplasm in spermatogonia and DNA replication leads to the formation of Primary Spermatocyte (2n)
○ A primary spermatocyte is a cell corresponding to prophase I of meiosis.
○ Males are born with spermatogonia, whereas females are not born with oogonia.
② 4th - 2nd. Primary spermatocytes undergo the first meiotic division to generate 2 Secondary Spermatocytes (n)
○ A secondary spermatocyte is a cell corresponding to metaphase II of meiosis.
③ 4th - 3rd. Two secondary spermatocytes undergo the second meiotic division to produce 4 Spermatids (n)
④ 4th - 4th. Cytoplasmic connections between spermatids are maintained until the completion of meiosis, with Sertoli cells playing a role in this process.
⑤ 4th - 5th. Spermatids differentiate into sperm cells
○ Most of the cellular organelles of spermatids are lost as they differentiate into sperm cells
○ Exception: Golgi apparatus for acrosome reaction
⑥ 4th - 6th. Mature sperm cells are released into the lumen of the seminiferous tubule
⑼ 5th. Epididymis and vas deferens
① Until reaching the epididymis, sperm are non-motile and are carried along by secretions from Sertoli cells.
② Sperm mature in the epididymis (acquiring motility), become highly concentrated, are stored until ejaculation, and are transported by peristaltic contractions.
③ After vasectomy (ligation of the ductus deferens), the small number of sperm present are degraded and reabsorbed by the surrounding tissues.
5. Female Reproductive System
⑴ Reproductive organs
① External genitalia: labia majora, labia minora, clitoris
② Ovary: site of oocyte maturation
○ One on each side high in the pelvic cavity; roughly almond-sized
○ Cortex: connective tissue layer
○ Medulla: follicular cells, granulosa cells, thecal layers
○ Follicular cells: produce androgens (mainly androstenedione)
○ Granulosa cells: produce aromatase and convert those androgens to estrogens
③ Uterus: thick-walled muscular organ; site of embryo implantation
④ Oviduct/uterine tube (Fallopian tube): ciliated epithelium transports the fertilized ovum
○ Opens into the peritoneal cavity adjacent to the ovary—the only natural passage
⑤ Cervix: part of the birth canal; regulates the passage of sperm
⑥ Vagina: birth canal; site of intercourse
⑦ The reproductive tract and urinary tract are completely separate.
⑵ Development of female gametes
① 1st. Oogonia generated from primordial germ cells
② 2nd. Oogonia undergo mitosis during embryo stage
③ 3rd. A primary oocyte is an oogonium that has entered meiosis and then arrested in prophase I (atresia state).
○ A primary oocyte contains 46 chromosomes.
○ A primary oocyte together with surrounding granulosa cells is collectively called a primordial follicle.
○ A primary oocyte is produced by meiosis, whereas a primary spermatocyte is produced by mitosis.
④ 4th. Birth to puberty
○ Fetus: Has around 7 million primary oocytes
○ After birth: Has around 1 million primary oocytes
○ Childhood: Has around 400,000 primary oocytes
○ Puberty: Has around 200,000 to 400,000 primary oocytes
⑤ 5th. At puberty, GnRH stimulates FSH secretion from the anterior pituitary
⑥ 6th. FSH promotes the maturation of primary follicles; through estrogen’s negative feedback and inhibin secretion from the earliest maturing (dominant) follicle, only one or two follicles are selected.
○ Primary follicle: the follicle enlarges; ribosomes, mRNA, cellular organelles, and energy reserves accumulate.
○ Under FSH stimulation, the primary follicle begins to produce estrogens.
⑦ 7th. Unequal cytoplasmic division: Stimulated primary oocyte undergoes the first meiotic division to form primary polar body and larger daughter cell
⑧ 8th. Secondary Oocyte: The larger daughter cell of the primary oocyte arrests in Metaphase II of the second meiotic division. 23 chromosomes.
○ Approximately 8-9 days of the menstrual cycle
⑨ 9th. Secondary oocyte is released from the mature follicle through ovulation
○ Secondary oocyte is around 500 μm during ovulation
○ Around day 14 of the menstrual cycle
⑩ 10th. Activation of MPF by progesterone at ovulation
⑪ 11th. Sperm meets secondary oocyte in the fallopian tube and resumes the second meiotic division of the ovum
⑫ 12th. Through unequal cytokinesis, a second polar body and the ovum (egg cell) are produced.
○ The first polar body may resume meiosis II and divide to yield two second polar bodies.
○ However, the first polar body may also fail to divide.
○ Polar bodies eventually degenerate.
⑶ Follicular development
① Follicle: a saclike cluster of cells within the ovarian endocrine tissue, composed of theca cells and granulosa cells.
6. Menstrual Cycle
Figure 6. Menstrual cycle
Figure 7. Hormonal Relationship of the Female Reproductive System
⑴ Overview
① The reproductive cycle occurs every 28 days, with ovulation around day 14 and menstruation around day 28.
② Pre-ovulation is called the follicular phase, and post-ovulation is called the luteal phase.
③ The reproductive cycle progresses simultaneously with the development of female gametes: the 5th to the 10th aforementioned steps.
⑵ 1st. Maturation of Follicles
① 1st. After the dissolution of the corpus luteum that strongly inhibits GnRH, there is a slight increase in FSH and LH secretion.
② 1st - 2nd. Negative feedback: Small amounts of estrogen inhibit GnRH, thereby inhibiting FSH and LH.
○ Estrogen gradually increases.
③ 1st - 3rd. Early maturation (secondary follicles): A few among numerous follicles undergo early maturation first.
○ Secondary follicles possess secondary oocytes and secrete estrogen and a small amount of luteal hormone.
○ Early maturation is relatively easy due to the lack of inhibitory effects from inhibin.
○ Theca cells, the zona pellucida, and the follicular antrum appear.
○ Theca cells: surround the follicular (granulosa) cells.
○ Zona pellucida: surrounds the germ cell (oocyte).
④ 1st - 4th. Late maturation: As the follicles mature, the granulosa cell layer proliferates, and LH receptors are expressed on the layer cells.
⑤ 1st - 5th. Follicle competition: During follicle maturation, several follicles within the ovary mature, but ultimately only one or two 3rd-stage follicles mature for ovulation.
○ 1st - 5th - 1st. The inhibin secreted by the first mature follicle suppresses FSH, which is involved in early maturation.
○ 1st - 5th - 2nd. Low estrogen levels inhibit FSH and LH, preventing the maturation of other follicles.
○ Moderately mature follicles undergo atresia.
○ 3rd-stage follicle (Graafian follicle) is characterized by a large antrum and is responsible for secreting estrogen.
⑶ 2nd. Development of Uterine Endometrium
① 2nd - 1st. The earliest maturing (dominant) follicle persists in producing aromatase and testosterone despite low estrogen levels.
○ 2nd - 1st - 1st. FSH: stimulates granulosa cells of the follicle to promote secretion of aromatase.
○ 2nd - 1st - 2nd. LH: stimulates granulosa cells of the follicle to promote secretion of testosterone.
○ 2nd - 1st - 3rd. Testosterone is converted to estradiol (estrogen) by aromatase.
② 2nd - 2nd. Because of the “uncooperative” earliest maturing (dominant) follicle, estrogen levels rise gradually.
○ At low concentrations, estrogen exerts strong negative feedback, so the increase takes time—on the order of a full ovarian (menstrual) cycle.
③ 2nd - 3rd. Estrogen thickens the uterine endometrium.
⑷ 3rd. Estrogen Surge
① 3rd - 1st. When estrogen levels exceed a threshold, GnRH is stimulated, promoting FSH and LH secretion.
② 3rd - 2nd. Positive feedback: Increased FSH and LH due to estrogen further elevates estrogen concentration.
③ 3rd - 3rd. FSH and LH dramatically increase, with FSH’s increase being moderate due to inhibin.
○ Inhibin selectively suppresses FSH, preventing an FSH surge comparable to that of LH.
○ If FSH secretion were to rise as much as LH, the FSH that drives early maturation would initiate maturation of new follicles.
⑸ 4th. Ovulation: 14 days after menstruation
① 4th - 1st. As LH responsible for late maturation (including ovulation) dramatically increases, it promotes the secretion of prostaglandins, causing follicle rupture.
② 4th - 2nd. When the follicle ruptures, the antral cavity (follicular antrum) releases its follicular cells.
③ 4th - 3rd. The follicle from which the oocyte has been released becomes the corpus luteum and secretes luteal hormones (primarily progesterone).
○ Prostaglandins promote the transformation of the follicle into the corpus luteum (luteinization).
○ The abrupt hormonal changes at ovulation can cause ovulatory pain (mittelschmerz) and ovulatory spotting/bleeding.
④ Over a lifetime, approximately 400–500 ovulations occur.
⑹ 5th. Corpus Luteum Development
① 5th - 1st. After ovulation, due to the absence of follicles that would secrete estrogen, estrogen decreases.
② 5th - 2nd. As the corpus luteum develops, testosterone and progesterone secretion increases.
③ 5th - 3rd. Negative feedback: Excessive progesterone inhibits GnRH, leading to FSH and LH inhibition.
○ Inhibits FSH involved in early maturation, thus inhibiting maturation of new follicles and ovulation.
○ A small amount of progesterone stimulates GnRH, leading to FSH and LH stimulation.
④ 5th - 4th. Testosterone and progesterone contribute to the development of the uterine endometrium.
○ Estrogen: Continues to thicken the uterine endometrium.
○ Progesterone: Maintains the thickness of the uterine endometrium.
⑺ 6th - 1st. In case the fertilized oocyte does not implant
① 6th - 1st - 1st. The corpus luteum increases cholesterol and fats, leading to its degeneration into the corpus albicans (fibrous tissue) with a lifespan of 2 weeks.
② 6th - 1st - 2nd. Menstruation: Due to the cessation of LH and progesterone secretion, the thickened uterine endometrium collapses.
○ During menopause, when menstruation ceases, no primary oocytes remain.
③ 6th - 1st - 3rd. Returns to step ⑵.
⑻ 6th - 2nd. In case the fertilized oocyte implants
① 6th - 2nd - 1st. Fertilized egg implants in the uterus.
② 6th - 2nd - 2nd. Progesterone activates MPF, thereby resuming oocyte cell division.
③ 6th - 2nd - 3rd. Implantation signals stimulate the secretion of human chorionic gonadotropin (hCG).
④ 6th - 2nd - 4th. hCG prevents the decline of the corpus luteum, which has a lifespan of only 2 weeks.
⑤ 6th - 2nd - 5th. The corpus luteum’s function is maintained, preventing new follicle maturation and menstruation.
7. Fertilization
⑴ Fertilization Process of Sea Urchin: Sea urchins are frequently used as models for fertilization.
① 1st. Attraction (chemoattraction): this step shows no species specificity.
○ 1st - 1st. Oocyte releases resact molecules into its surroundings.
○ 1st - 2nd. Resact molecules react with sperm, enhancing sperm motility.
○ 1st - 3rd. Sperm randomly moving becomes faster in the direction of the oocyte, facilitating movement towards the oocyte.
② 2nd. Contact
○ 2nd - 1st. Sperm cell contacts the jelly layer of the oocyte.
○ 2nd - 2nd. Exocytosis occurs from the acrosome of the sperm.
○ Sea urchin oocyte structure: jelly layer (no receptors) – vitelline envelope (receptors present) – plasma membrane (receptors present).
○ Mammalian oocyte structure: cumulus cell layer – zona pellucida – vitelline membrane – plasma membrane.
③ 3rd. Acrosomal reaction
○ 3rd - 1st. Hydrolytic enzymes released from the sperm acrosome create pores in the jelly layer (involving numerous Golgi-derived vesicles).
○ 3rd - 2nd. A large amount of Ca2+ is released from the smooth endoplasmic reticulum, and growing actin filaments form the acrosomal process.
○ 3rd - 3rd. Bindin on the acrosomal process protruding from the sperm head binds to bindin receptors on the egg’s vitelline envelope.
○ Bindin-mediated binding is species-specific.
④ 4th. Formation of holes in the vitelline envelope → Fusion of sperm and egg cell membranes → Insertion of sperm nucleus into egg cytoplasm
⑤ 5th. Fast block to polyspermy: sea urchins only
○ 5th - 1st. Na+ and Ca2+ ions enter the zygote together with the sperm.
○ 5th - 2nd. Depolarization of the membrane
○ 5th - 3rd. Anions surround the depolarized membrane.
○ 5th - 4th. Anion-enveloped membrane limits the secondary fertilization of negatively charged sperm
⑥ 6th. Slow block to polyspermy (cortical reaction) occurs about a minute after sperm-egg fusion
○ 6th - 1st. Separation of the vitelline envelope from the plasma membrane: an absolute blockade of the zygote’s plasma membrane to additional sperm (polyspermy block).
○ 6th - 1st - 1st. Ca2+ is released from the zygote’s smooth endoplasmic reticulum.
○ As with neurotransmitter release, Ca2+ is always involved in vesicle exocytosis.
○ 6th - 1st - 2nd. Ca2+ wave: the released Ca2+ actively propagates toward vesicle-rich sites (not merely by diffusion).
○ When two sperm are experimentally bound to a single egg, distinct Ca2+ waves occur for each.
○ A23187: an ionophore that transports Ca2+ across the phospholipid bilayer.
○ Treatment with A23187 induces formation of the fertilization envelope without fertilization, whereas the Ca²⁺ chelator BAPTA inhibits fertilization-envelope formation.
○ 6th - 1st - 3rd. The egg’s cortical granules (oligosaccharide-containing vesicles) are exocytosed into the space between the plasma membrane and the vitelline envelope.
○ 6th - 1st - 4th. Because cortical granules are highly osmotic, water influx occurs.
○ 6th - 1st - 5th. A perivitelline space forms between the vitelline envelope and the cell membrane.
○ 6th - 2nd. Removal of plasma-membrane receptors: substances from the cortical granules cleave the membrane receptors that bind sperm.
○ 6th - 3rd. Fertilization envelope formation: enzymes from the cortical granules harden the vitelline envelope, forming the fertilization envelope.
⑦ 7th. Cleavage
⑵ Fertilization in mammals
① Overview
○ In mammals, fertilization of the oocyte by sperm occurs in the oviduct (Fallopian tube).
○ Of roughly 300 million sperm, only about 200 reach the oocyte in the oviduct.
○ Many sperm fail to survive the acidic conditions in the upper vagina.
○ It takes about 30 minutes for sperm to travel from the upper vagina to the oviduct.
② 1st. The sperm penetrates the cumulus (follicular) cell layer.
③ 2nd. The sperm head binds to the species-specific zona pellucida glycoprotein ZP3.
④ 3rd. Acrosomal enzymes are released, digesting the zona pellucida and forming a tunnel.
⑤ 4th. The oocyte and sperm membranes fuse.
⑥ 5th. The sperm nucleus enters the oocyte.
⑦ 6th. The oocyte nucleus, having completed meiosis II, fuses with the sperm nucleus.
⑧ 7th. The zygote begins cleavage.
Figure 8. Mammalian Fertilization
8. Pregnancy and Childbirth
⑴ Early Development of the Zygote
① Fertilization in mammals
② Cleavage
○ Mammalian cleavage is the slowest in the animal kingdom.
○ During cleavage, the embryo is transported to the uterus by the cilia of the oviduct (Fallopian tube).
○ About one week after fertilization, the zygote reaches the blastocyst stage (the mammalian form of the blastula).
③ Embryonic gene expression
○ In sea urchins, embryonic gene expression begins at the mid-blastula stage.
○ In mammals, embryonic gene expression begins at the 2-cell or 4-cell stage.
④ Implantation
○ Implantation occurs about one week after fertilization.
○ A few days after the blastocyst stage, implantation occurs in the endometrium.
○ The embryo implants in the uterus under the influence of progesterone.
Figure 9. Process of Oocyte Formation and Early Zygote Development in Humans
A: 2-cell embryo, B: 1st polar body, C: 2nd polar body
⑵ Pregnancy
① Definition
○ Presence of one or more embryos in the uterus.
○ Dating: fertilization day → ~266 days after fertilization, or LMP → ~280 days.
○ Signs: amenorrhea plus nausea/vomiting.
○ Tests: detection of hCG in urine; ultrasound exam.
② First trimester (~12 weeks): the period of most rapid change for both mother and embryo
○ Uterine enlargement compresses the bladder → frequent urination.
○ Blastocyst growth: after implantation in the endometrium, the blastocyst grows and embryonic body structures begin to differentiate.
○ Early development (weeks 2–4): the embryo obtains nutrients directly from the endometrium; the trophoblast expands and contributes to placental formation.
○ ~4 weeks: heartbeat; ~8 weeks: limb formation begins.
○ Organogenesis
○ By 8 weeks after fertilization, the major organs are formed.
○ By the end of the first trimester, most organs have begun to form.
○ hCG maintains the corpus luteum until about the 4th month.
③ Second trimester (13–26 weeks)
○ Fetal mass up to ~600 g.
○ Lower abdomen enlarges; breasts enlarge; colostrum is secreted.
○ Fetal movement felt at ~16 weeks; by late 2nd trimester the fetus sucks its thumb.
○ hCG secretion decreases → corpus luteum regresses.
○ Placenta secretes progesterone to maintain pregnancy.
○ High progesterone: forms a protective cervical mucus plug; promotes growth of the maternal portion of the placenta; enlarges the uterus; suppresses the ovarian cycle.
④ Third trimester (27–40 weeks)
○ Fetal size increases; fetal movements decrease.
○ Uterine enlargement compresses abdominal organs → maternal dyspnea, indigestion, constipation, frequent urination, muscle stiffness, edema of leg joints; fetal pressure on maternal organs.
○ Fetal physiology: begins digestion, stores glycogen, produces urine, and shows periodic sleep–wake cycles.
○ Estrogen: stimulates prolactin secretion, but suppresses milk release before birth.
○ Prolactin: promotes breast growth; strengthens the milk-producing apparatus of the mammary glands.
○ Prostaglandins: secreted by the placenta; induce contraction of the uterine wall.
○ Oxytocin: promotes labor by inducing uterine smooth-muscle contractions via positive feedback.
⑤ Maternal–fetal exchange at the placenta
○ Maternal and fetal blood do not mix directly across the placenta, preventing antigen–antibody reactions and blood-type agglutination.
○ Umbilical arteries: high in CO₂ and wastes.
○ Mother → fetus: oxygen and nutrients.
○ Fetus → mother: carbon dioxide and wastes.
○ Umbilical vein: high in oxygen and nutrients.
○ Amniotic fluid: prevents fetal drying and cushions external shocks.
○ Countercurrent exchange increases the efficiency of transfer.
⑥ Morning sickness
○ hCG hypothesis: morning sickness peaks around weeks 6–7, coinciding with rising hCG levels.
○ Newer view: protective response against toxicity from excessive animal-protein intake; vegetarians are said not to experience morning sickness.
⑶ Childbirth: the process in which the fetus emerges from the mother, occurring 280 days after ovulation or 266 days after fertilization.
① Promotion of uterine contractions during labor: estrogen, oxytocin (positive feedback)
○ Stage 1: cervical dilation (1 mm → 10 cm)
○ Stage 2: delivery of the baby
○ Stage 3: delivery of the placenta
② After delivery, once the placenta is expelled, estrogen and progesterone are no longer secreted.
③ The decrease in estrogen secretion initiates milk production.
④ Positive feedback: when the infant suckles after birth, prolactin secretion from the anterior pituitary is stimulated.
⑤ Ovulation resumes as the frequency of breastfeeding decreases.
○ For about two months postpartum, estrogen and progesterone levels are low, so ovulation and menstruation do not occur.
⑷ Twins
① Dizygotic twins: fertilization of two ova by two sperm within the same uterus.
○ In a single reproductive cycle, two follicles independently develop into embryos and implant.
② Monozygotic twins
○ Separation before implantation (within 5 days after fertilization): two chorions and two amnions.
○ Separation 5–10 days after fertilization: two amniotic sacs within a single chorion.
○ Separation 10–14 days after fertilization: one chorion and one amnion; rare (4–6%). Conjoined twins can only occur in this way.
○ If the embryo is separated during the first month of development, monozygotic twins can result.
Figure 10. Three Ways Identical Twins Are Formed
Only Siamese twins can be produced through the right method
⑸ Contraception
① Withdrawal (coitus interruptus)
② Rhythm method (natural family planning)
○ The rhythm method is the only contraceptive method recognized by the Roman Catholic Church.
○ Ovulation around day 14 ± 1 of the cycle.
○ Ovum survives ~2 days; sperm ~5 days.
○ Fertile window: from 5 days before ovulation to 2 days after ovulation.
③ Basal body temperature method: predict ovulation by measuring body temperature.
④ Cervical mucus method: predict ovulation by the turbidity and viscosity of cervical mucus.
⑤ Contraceptive pills
○ Commonly used: oral contraceptives combining progestin and estrogen at a 2:1 ratio.
○ Effect 1: Suppresses the hypothalamus, inhibiting GnRH secretion and ovulation (effectiveness 95%).
○ Effect 2: Thickens cervical mucus and hinders sperm motility (effectiveness 95%).
○ Effect 3: Prevents implantation of the ovum (emergency contraception) (effectiveness 80%).
○ Types include combined pills, the mini-pill, and emergency (morning-after) pills.
○ Side effects: menstrual cycle disturbances, fetal blood coagulation, breast cancer, uterine and ovarian cancer, ovarian cysts, nausea, vomiting, abdominal pain, fatigue, headache.
⑥ Vaginal ring: inserted into the vagina; supplies progestin and estrogen to prevent ovulation and thicken cervical mucus (effectiveness 99%).
⑦ Cervical cap, diaphragm (pessary), sponge, female condom, male condom.
⑧ Spermicides: inserted into the vagina one hour before intercourse; kill sperm.
⑨ Intrauterine device (IUD): a plastic device inserted into the uterus; increased risk of pelvic inflammatory disease.
⑩ Female sterilization (tubal surgery): permanently occludes the fallopian tubes by tightly twisting them (effectiveness 99.5%).
⑪ Male sterilization (vasectomy): ligates the vas deferens to prevent sperm from being ejaculated in semen (effectiveness 99.9%).
⑹ Infertility
① Definition: Failure to achieve pregnancy after at least 1 year of sexual activity.
② Cause 1. Male infertility (90%)
○ Low sperm count, poor sperm motility, or abnormal sperm morphology (e.g., enlarged or small head, two heads, two tails).
○ In healthy individuals, about 25% of the 200–300 million sperm are morphologically abnormal.
○ Abnormal sperm are eliminated in the competition to reach the fallopian tubes.
○ Atrazine: a pesticide component; decreases sperm viability.
○ Trichloroethylene: a degreasing/cleaning solvent; leads to production of abnormal sperm.
③ Cause 2. Female infertility
○ Endometriosis: a condition in which tissue lining the uterus grows on the ovaries and fallopian tubes and responds to hormones.
○ Cause 1. Retrograde menstruation (main cause).
○ Cause 2. Possible family history.
○ Cause 3. Excess female hormones.
○ Cause 4. Menstrual cycles of 27 days or fewer, or menstruation lasting 7 days or more.
○ Cause 5. Early menarche.
○ Cause 6. Few childbirths.
○ Creates scarring in ovarian tissue and disrupts ovulation.
○ Occurs in 3–10% of women of reproductive age and in 25% of infertile women.
○ Scarring from infections and sexually transmitted diseases can block tubular structures and cause infertility.
○ DEHP: an endocrine disruptor and carcinogenic substance.
⑺ Sexually Transmitted Diseases (STDs)
Input: 2019.01.26 20:13
Last Updated: 2022.06.06 22:30