Chapter 31. Reproductive System
Recommended Post : [Biology] Biology Index
1. Overview
2. Reproductive System Hormones
3. Development of the Reproductive System: Embryonic Duct
6. Reproductive Cycle (Menstrual Cycle)
7. Revision
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 Fertilization
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
○ Budding : Portion of somatic cell detaches to form a new organism, like Hydra
○ Regeneration : Some organisms reproduce through complete regeneration, like starfish and planarians
○ Parthenogenesis : Development of unfertilized eggs, like aphids (parthenogenetic females), bees (drones)
② Sexual Reproduction : Involves meiosis, ensuring genetic diversity for evolution
○ Gametogenesis → Mating → Fertilization
○ Chromosomal recombination creates diverse offspring for adaptation to various environments
○ Isogametes and Heterogametes
2. Reproductive System Hormones
⑴ Overview
① Most reproductive hormones are steroids (except LH, FSH)
⑵ GnRH (Gonadotropin-Releasing Hormone)
① Function 1. Stimulates secretion of LH and FSH from anterior pituitary
⑶ Follicle-Stimulating Hormone (FSH)
① Male
○ Function 1. Stimulates Sertoli cells along with testosterone production → Sperm production
② Female
○ Function 1. Stimulates granulosa cells in ovarian follicles, promoting aromatase enzyme secretion
○ Function 2. Early Maturation : Follicular maturation
⑷ Luteinizing Hormone (LH)
① Male
○ Function 1. Stimulates Leydig cells for testosterone secretion
○ Function 2. Stimulates Sertoli cells along with FSH
② Female
○ Function 1. Stimulates granulosa cells for testosterone secretion
○ Function 2. Late Maturation : Ovulation induction
○ Function 3. Promotes androgen secretion
○ Testosterone is converted to estrogen by aromatase enzyme secreted by granulosa cells
⑸ Androgens
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, deep 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. Stimulates erythropoietin secretion for red blood cell production
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: Androstenedione
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
○ Stimulates prostate cancer and prostate hypertrophy. Inhibition of 5α-reductase is used to treat these conditions.
○ Congenital Adrenal Hyperplasia (CAH)
○ 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 Joseon Dynasty
⑹ Estrogens
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
○ Genetic Pathway : Estrogen acts on ERE or AP-1 within target DNA after binding to ERα or ERβ, leading to transcriptional activity.
○ Non-genetic Pathway : Estrogen + ERα/ERβ or Estrogen + GPR30 activates signaling pathways involving 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 Menses : 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. Influences plasma cholesterol, vascular health, and blood clotting, inhibiting arteriosclerosis.
○ Function 8. Estrogen deficiency increases the risk of postmenopausal osteoporosis.
○ 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 : Converts testosterone to estrogen via 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.
○ Male aquatic animals’ sexual organs can be activated to produce eggs.
⑥ Types
○ ** Estrone, Estradiol, and Estriol are present.
○ Estradiol : Most potent estrogen hormone.
○ Estrogens mainly exist as 17β-estradiol, but exist as estrone after menopause.
⑦ Luteinizing Hormone (Progesterone) : Steroid hormone.
① Production Sites : Corpus luteum, placenta.
○ During pregnancy, corpus luteum 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
○ Thick cervical mucus secretion : Maintains 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, slightly increases body temperature.
⑥ Function 4. Impacts blood vessels, glycogen storage.
⑦ Cervical mucus changes and body temperature variations : Indicators of ovulation.
⑻ Anti-Müllerian Hormone : Protein.
⑼ Inhibin : Peptide hormone.
① Production : Males from Sertoli cells in testis, females from granulosa cells in ovaries.
○ Unique peptide in ovaries.
② Suppresses GnRH, FSH : Selectively inhibits FSH to prevent maturation of other follicles during reproductive cycle.
⑽ Activin : Peptide hormone.
① Present in hypothalamus, Sertoli cells, and follicular fluid.
② Also involved in development process : Found in plant poles.
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, inhibits ovarian cycle.
③ hCG concentration maintained during 2-3 months of pregnancy, then decreases.
④ Corpus luteum regresses after 5 weeks of pregnancy, placenta takes over hCG’s role.
⑤ Pregnancy Test Kit: Detects hCG.
3. Development of Reproductive System: Embryonic Ducts
⑴ Potential Reproductive Glands
① Reproductive structures do not initiate differentiation until week 7 of development.
② Development signals cause mesonephros to differentiate into testis.
③ Absence of development signals causes paramesonephros to differentiate into ovaries.
④ Internal reproductive structures with potential: Wolffian duct and Müllerian duct originating from embryonic kidney.
○ As development progresses, one pair of ducts develops, while the other pair regresses.
⑤ External reproductive structures with potential: Composed of reproductive nodes, folds, grooves, urogenital folds, scrotum.
⑵ SRY (sex region of Y chromosome)
① SRY induces differentiation of mesonephros into testis, stimulates Leydig cells and Sertoli cells.
② Leydig cells, influenced by SRY, secrete testosterone and its derivative DHT (dihydrotestosterone).
○ Testosterone : Involved in Wolffian duct differentiation, descent of testes from abdomen.
○ DHT : Involved in external genitalia differentiation and male sexual characteristics.
③ Sertoli cells, influenced by SRY, secrete Anti-Müllerian hormone (AMH).
⑶ Male Development : Wolffian duct → Epididymis, vas deferens, seminal vesicle.
① Testosterone secreted by Leydig cells actively stimulates Wolffian duct development.
② AMH secreted by Sertoli cells actively causes regression of Müllerian duct.
③ Pseudohermaphroditism
○ Males inherit the enzyme 5α-reductase, which converts testosterone to DHT.
○ Patients lacking this enzyme exhibit incomplete virilization, with female-like genitalia at birth.
○ For instance, male external genitalia and prostate do not fully develop during fetal development.
○ During puberty, testes start secreting testosterone, leading to natural correction.
④ Cryptorchidism
○ If testes remain in abdominal cavity, higher temperature might lead to abnormal sperm development and infertility.
⑷ Female Development : Müllerian duct → Upper vagina, oviduct, uterus, uterine tube, fallopian tube, cervix, vagina.
① Lack of sufficient testosterone secretion by Leydig cells causes regression of Wolffian duct.
② Absence of DHT results in female external genitalia characteristics.
③ Lack of sufficient AMH secretion by Sertoli cells results in natural development of Müllerian duct.
Figure. 3. Embryonic Ducts in Males and Females
4. Male Reproductive System
⑴ Reproductive Organs
① Vas deferens and epididymis develop from Wolffian duct.
② Testis: Sperm production, hormone secretion, requires lower temperature for proper sperm development.
○ Descends from abdominal cavity to scrotum around 7th month of fetus.
○ 1-3% of newborns have undescended testes.
○ 80% descend naturally during growth.
○ Must be 4-5°C lower than body temperature for sperm formation; above 35°C leads to smooth muscle relaxation.
③ Epididymis: Stores and concentrates sperm, imparts motility and maturation through interlinked movements during ejaculation.
④ Prostate: Secretes watery fluid with prostaglandins, nutrients, alkaline substances for motility enhancement.
○ 60% of seminal fluid, prostaglandins in particular, stimulate sperm movement and uterine contractions.
○ Prostate enlargement: urinary retention and urgency. Diagnosed using Prostate-Specific Antigen (PSA) test.
⑤ Seminal Vesicle: Secretes most components of seminal fluid.
○ Secretes viscous fluid containing glucose and fructose.
○ Alkaline nature protects sperm from acidic female secretions.
○ Fructose provides the majority of energy for sperm.
⑥ Cowper’s Gland: Secretes Cowper’s fluid (clear, mucous, alkaline) for neutralizing urine acidity before sperm release and urethral cleansing before ejaculation.
○ In addition, a small amount of lubricating mucus is secreted.
⑦ Urethra
○ The seminal duct and excretory duct share the urethra.
○ The seminal duct, which emerges from the testicular sac and combines with the ducts that emerge from the prostate gland, is connected to the urethra.
○ The duct emerging from the bulbourethral gland (Cowper’s gland) is connected separately.
⑧ Glans penis
○ Circumcision : Removal of the prepuce. Reduces the risk of penile cancer, sexually transmitted diseases, and uterine infections. Associated with religious circumcision practices.
⑵ Spermatic Cord
① Basal membrane : Prevents the intrusion of foreign substances, forms the blood-testis barrier to regulate substance movement.
② Seminiferous Tubule
○ Sperm production
○ Includes Sertoli cells, Leydig cells
○ Total length is about 250 m
③ Leydig Cell
○ Located in the interstitial space of seminiferous tubules
○ Hormonal stimulation : Receives stimulation from LH to secrete testosterone and other male hormones
○ Hormonal secretion : Secretion of testosterone
④ Sertoli Cell
○ Inserted into the wall of seminiferous tubules, provides the most care for sperm production and development
○ Hormonal stimulation : Receives stimulation from testosterone and FSH to secrete paracrine factors that aid in sperm production
○ Hormonal secretion : anti-Müllerian hormone, androgen-binding protein (ABP), inhibin
○ Nutrient supply
○ Forms the blood-testis barrier to protect sperm from immune attack
○ Degrades residual cytoplasm of sperm cells through phagocytic action
⑶ Sperm
① Acrosome nucleus
② Acrosome : Derived from the Golgi apparatus. Contains enzymes for zona pellucida penetration. Multiple acrosomes are present for acrosome reaction.
③ Midpiece : Contains numerous mitochondria. ATP generation.
④ Tail : Propels the midpiece. Speed of 1-4 mm/min.
⑤ Must be exposed to the chemical environment of the female reproductive system to acquire fertilization ability.
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 sperm
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
○ Tip. LH = Luteinizing Hormone = Male sex hormone = Testosterone
③ 3rd - 3rd. FSH → Stimulates Sertoli cells → GPCR → Androgen binding protein (ABP) secretion ↑
④ 3rd - 4th. Testosterone + ABP → Stimulates Sertoli cell phagocytosis
○ Spermatocytes don’t have testosterone receptors, so they don’t respond directly to testosterone.
○ However, spermatocytes have receptors for androgen-binding protein and can respond to testosterone.
⑤ 3rd - 5th. Morphological change of Sertoli cells : Sertoli cells that were tightly connected undergo separation
⑻ 4th. Spermatogonia move from between Sertoli cells to the luminal direction of seminiferous tubules as they differentiate
① 4th - 1st. Accumulation of cytoplasm in spermatogonia and DNA replication leads to the formation of Primary Spermatocyte (2n)
○ Primary spermatocytes are cells corresponding to the first meiotic division phase
○ Males are born with spermatogonia, females are not born with oogonia
② 4th - 2nd. Primary spermatocytes undergo the first meiotic division to generate 2 Secondary Spermatocytes (n)
○ Secondary spermatocytes are cells corresponding to the second meiotic division phase
③ 4th - 3rd. Two secondary spermatocytes undergo the second meiotic division to produce 4 Spermatids (n)
④ 4th - 4th. Cytokinesis is maintained between spermatids until meiosis ends, involving Sertoli cells
⑤ 4th - 5th. Spermatids differentiate into sperm cells
○ Most of the cellular organelles of spermatids are lost as they differentiate into sperm
○ Exception: Golgi apparatus, acrosome reaction
⑥ 4th - 6th. Mature sperm are released into the lumen of the seminiferous tubule
⑼ 5th. Epididymis, Vas Deferens
① Sperm are non-motile up to the epididymis and are pushed by secretion from Sertoli cells
② Sperm matures (acquires motility), becomes highly concentrated, stored until ejaculation, transported by coordinated movement
③ Vas deferens excision and ligation : A small amount of sperm is disintegrated and absorbed into the tissue
5. Female Reproductive System
⑴ Reproductive Organs
① External genitalia : Labia majora, Labia minora, Clitoris
② Ovary : Site of oocyte maturation
○ Located on both sides of the upper pelvis, about the size of an almond
○ Cortex : Connective tissue layer
○ Medulla : Contains theca cells, granulosa cells, ovarian follicles
○ Theca Cells : Produce androgens (mainly androstenedione)
○ Granulosa Cells : Produce aromatase enzyme to convert androgens to estrogen
③ Uterus : Muscular organ with a thick wall, site of fetal implantation
④ Fallopian Tube : Contains ciliated epithelium, transports the ovum
○ Opens into the abdominal cavity adjacent to the ovary : The only natural pathway
⑤ Cervix : Controls acidity, regulates sperm entry
⑥ Vagina : Acidity, sexual intercourse
⑦ Reproductive and excretory systems are completely separate
⑵ Female Germ Cell Development
① 1st. Oogonia generated from primordial germ cells
② 2nd. Oogonia undergo mitosis (similar division) during embryo stage
③ 3rd. Primary Oocyte : Oogonia undergo the first meiotic division, arrest in Prophase I
○ Primary oocytes have 46 chromosomes
○ The term “primary oocyte” is used for both primary oocytes and granulosa cells together
○ Primary oocytes undergo meiosis, while primary spermatocytes undergo 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 maturation of the primary follicle, only one or two are selected due to estrogen’s negative feedback and inhibin secretion from the first mature follicle
○ Primary follicle : Increases in size. Accumulation of ribosomes, mRNA, organelles, energy, etc.
○ FSH stimulates the primary follicle to start producing estrogen
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. Unequal cytoplasmic division forms secondary polar body and ovum
○ The first polar body may or may not undergo the second meiotic division
○ Polar bodies eventually degenerate
⑶ Development of the Follicle
① Follicle : Pocket-like structure in the endocrine gland tissue, composed of oocyte 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 reproductive cells from the 5th to the 10th day.
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.
○ Granulosa cells, zona pellucida, and theca interna appear.
○ Granulosa cells: Surround the follicular cells.
○ Zona pellucida: Surround the reproductive cells.
④ 1st - 4th - Late maturation: As the follicles mature, the granulosa cell layer proliferates, and LH receptors are expressed.
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): Enlarged follicular chamber. Secretes estrogen.
⑶ 2nd. Development of Uterine Endometrium
① 2nd - 1st - Despite low estrogen levels, the initial mature follicle secretes enzymes and testosterone, driving endometrial thickening.
○ 2nd - 1st - 1st - FSH stimulates the granulosa cells, promoting enzyme secretion.
○ 2nd - 1st - 2nd - LH stimulates the granulosa cells, promoting testosterone secretion.
○ 2nd - 1st - 3rd - Testosterone is converted to estradiol (estrogen) by aromatase.
② 2nd - 2nd - Despite resistance from the initial mature follicle, estrogen levels gradually increase.
○ At low concentrations, estrogen suppresses, thus extending the time required for endometrial thickening.
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 inhibits FSH to prevent its secretion from increasing as much as LH.
○ If FSH secretion increases as much as LH, a new follicle matures due to FSH’s responsibility for early maturation.
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 - Follicle rupture releases secondary oocytes.
③ 4th - 3rd - The follicle expelled with the oocyte becomes the corpus luteum, secreting luteal hormones.
○ Prostaglandins facilitate the transformation of the follicle into the corpus luteum.
○ Rapid hormonal changes during ovulation might lead to ovulation pain and spotting.
④ Around 400 to 500 ovulations occur throughout a lifetime.
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</sup
- 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 1st-stage 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, resuming cell division of the zygote.
③ 6th - 2nd - 3rd. Implantation signals the release of human chorionic gonadotropin (hCG), a signal for corpus luteum stimulation.
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. 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.
○ Structure of sea urchin egg : Jelly layer (no receptor) - Vitelline envelope (receptor present) - Hyaline layer (receptor present)
○ Structure of mammalian egg : Zona pellucida - Transparent layer - Vitelline envelope - Hyaline layer
③ 3rd. Acrosomal reaction
○ 3rd - 1st. Enzymes released from the acrosome of the sperm create holes in the jelly layer (involves numerous cortical granules).
○ 3rd - 2nd. Massive Ca2+ release from cortical granules leads to the formation of the acrosomal process in growing actin filaments.
○ 3rd - 3rd. Binding occurs between the exposed protein on the acrosomal process of the sperm head and the exposed receptor on the vitelline envelope.
○ Binding is specific to the species.
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 in sea urchins only
○ 5th - 1st. Na+ and Ca2+ influx with sperm
○ 5th - 2nd. Depolarization of the membrane
○ 5th - 3rd. Membrane surrounding with anions
○ 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 and hyaline layer : Second polar body formation of the egg and origin of the vitelline envelope blockage
○ Ca2+ release from the egg’s cortical granules
○ Like a neurotransmitter, Ca2+ is involved in cortical granule exocytosis
○ Ca2+ wave : Released Ca2+ actively moves to where the cortical granules are (non-diffusion)
○ Ca2+ waves occur separately when two sperm artificially fuse with one egg
○ A23187 : Compound that transports Ca2+ across lipid bilayers
○ A23187 leads to membrane formation without fusion, while Ca2+-binding chelators (BAPTA) inhibit membrane formation
○ Release of the oligosaccharide cortical granule, that is, the cortical granule and lipid droplets, into the gap between the vitelline envelope and hyaline layer
○ Cortical granule swells with high osmotic pressure, causing water influx
○ Formation of the fertilization envelope between the vitelline envelope and cell membrane
○ 6th - 2nd. Removal of hyaline layer receptors : Cutting of membrane receptors by substances in the cortical granule
○ 6th - 3rd. Formation of the membrane : Cortical granule enzymes harden the vitelline envelope, forming a membrane
⑦ 7th. Completion of fertilization
⑵ Mammalian Fertilization
① Overview
○ Mammalian fertilization occurs in the fallopian tube.
○ Only about 200 out of 300 million sperm reach the oocyte in the fallopian tube.
○ Many sperm do not survive the acidic conditions of the female genital tract.
○ It takes about 30 minutes for a sperm to travel from the female genital tract to the fallopian tube.
② 1st. Penetration of the sperm into the zona pellucida
③ 2nd. Sperm head binds to the species-specific protein (ZP3) on the zona pellucida.
④ 3rd. Acrosomal enzymes are released, breaking down the zona pellucida and creating a tunnel.
⑤ 4th. Fusion of egg and sperm membranes
⑥ 5th. Sperm nucleus enters the egg cell
⑦ 6th. Completion of the second meiotic division in the egg, followed by fusion with the sperm nucleus
⑧ 7th. Initiation of the zygote
Figure 8. Mammalian Fertilization
8. Pregnancy and Childbirth
⑴ Early Development of the Zygote
① Mammalian Fertilization
② Zygote
○ Mammalian zygote formation is the slowest among the animal kingdom.
○ An embryo undergoing zygote formation moves into the uterus through the fallopian tube.
○ The zygote is referred to as a blastocyst about a week after fertilization.
③ Expression of Embryonic Genes
○ In the case of sea urchins, expression of embryonic genes occurs during mid-blastula transition.
○ In mammals,
embryonic gene expression occurs during the 2-cell or 4-cell stage.
④ Implantation
○ Implantation occurs about one week after fertilization.
○ Implantation into the endometrium occurs a few days after the blastocyst stage.
○ Embryo implants into the uterus due to the action 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
○ State where one or more embryos are present in the uterus.
○ From conception to 266 days after conception, or from the last menstrual period to 280 days.
○ Pregnancy signs : Amenorrhea + Vomiting symptoms
○ Pregnancy tests : Detection of hCG in urine, ultrasound examination
② First Trimester (~ 12 weeks) : A period of rapid changes for both the pregnant woman and the embryo
○ Expansion of the uterus causes bladder pressure and frequent urination
○ Blastocyst growth : Blastocyst implanted in the endometrium undergoes differentiation of embryonic body structures
○ Initial development at 2-4 weeks : Embryo directly obtains nutrients from the endometrium. Nutrient cell layer (trophoblast) expands, contributing to placenta formation
○ Heartbeat after 4 weeks, limb formation begins after 8 weeks
○ Organogenesis
○ By 8 weeks, all major organs have formed
○ Most organ formation starts by the end of the first trimester
○ hCG secretion maintains the corpus luteum : Until the 4th month of pregnancy
③ Second Trimester (13 weeks to 26 weeks)
○ Fetus weighs up to 600g
○ Lower abdomen protrudes, breast enlargement, colostrum secretion
○ Fetal movement detected at 16 weeks : Thumb sucking by the end of the second trimester
○ Decreased hCG secretion leads to corpus luteum regression
○ Progesterone secreted from the placenta sustains pregnancy
○ High progesterone concentration : Forms cervical plug, supports placental growth, expands uterus, inhibits ovarian cycle
④ Third Trimester (27 weeks to 40 weeks)
○ Fetus size increases : Fetal movements decrease
○ Uterine expansion : Abdominal organ compression, maternal shortness of breath, indigestion, constipation, frequent urination, muscle stiffness, leg joint swelling, fetal organ compression
○ Fetal physiological activities : Digestion begins, glycogen storage, urine production, regular sleep and activity
○ Estrogen: Stimulates the secretion of prolactin, inhibits milk secretion due to prepartum, before birth
○ Prolactin: Promotes breast growth, strengthens mammary gland for milk production
○ Prostaglandin: Secreted from the placenta, induces uterine wall contractions
○ Oxytocin: Induces uterine smooth muscle contractions through positive feedback, promoting childbirth
⑤ Material Exchange in the Placenta
○ Placental and fetal blood in direct contact prevents antigen-antibody reactions and blood clotting reactions
○ Umbilical Artery: Contains carbon dioxide and waste products
○ Maternal → Fetal: Transfers oxygen and nutrients
○ Fetal → Maternal: Transfers carbon dioxide and waste products
○ Umbilical Vein: Contains oxygen and nutrients
○ Amniotic Fluid: Prevents drying of the fetus, cushions against external shocks
○ Enhanced material exchange efficiency through reverse exchange
⑥ Morning sickness
○ hCG Theory: Based on the increase in hCG levels around 6-7 weeks of pregnancy, corresponding to the onset of morning sickness
○ New Perspective: Protection of pregnant women and fetuses from protein excess toxicity by meat consumption. No morning sickness for vegetarians
⑶ Birth: The phenomenon where a fetus exits the mother’s body 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 Birth, Estrogen and Progesterone are no longer secreted as the placenta is removed.
③ Decreased Estrogen Secretion Initiates Milk Production.
④ Positive Feedback : After birth, when the baby starts breastfeeding, the pituitary gland releases prolactin
⑤ When breastfeeding frequency decreases, ovulation resumes
○ During the first 2 months after birth, low levels of estrogen and progesterone prevent ovulation and menstruation
⑷ Twins
① Dizygotic Twins (Fraterna Twins): Develop from two separate eggs and two separate sperm cells within the same uterus
○ If two eggs are independently fertilized during one reproductive cycle.
② Monozygotic Twins (Identical Twins)
○ Pre-implantation Separation (Up to 5 days after fertilization): Two placental membranes and two amniotic sacs
○ Separation 5-10 days after fertilization: Divide within the same placental membrane into two amniotic sacs, growth within the same membrane
○ Separation 10-14 days after fertilization: One placental membrane and one amniotic sac, rare (4-6%)
○ Separating the embryo within the first month after conception can result in identical twins
Figure 10. Three Ways Identical Twins Are Formed
Only Siamese twins can be produced through the right method
⑸ Contraception
① Discontinuation of Sexual Intercourse
② Rhythm Method (Natural Family Planning)
○ Rhythm Method: Only contraceptive method recognized by the Roman Catholic Church
○ Ovulation occurs on the 14th day ± 1 of the menstrual cycle
○ Egg lifespan is 2 days, sperm lifespan is 5 days
○ Fertile Period: 5 days before ovulation to 2 days after ovulation
③ Basal Body Temperature Method: Predicts ovulation by measuring body temperature
④ Cervical Mucus Method: Predicts ovulation based on the thickness and viscosity of cervical mucus
⑤ Birth Control Pills
○ Common form of oral contraception, containing a mixture of progesterone and estrogen in a 2:1 ratio
○ Effect 1: Suppresses GnRH secretion, inhibiting ovulation (95% effectiveness)
○ Effect 2: Thickens mucus, hindering sperm motility (95% effectiveness)
○ Effect 3: Prevents fertilization (emergency contraception) (80% effectiveness)
○ Options include combination pills, mini-pills, emergency contraceptive pills
○ Side effects: Menstrual cycle disruption, fetal blood clotting, breast cancer, uterine and ovarian cancer, ovarian cysts, nausea, vomiting, abdominal pain, fatigue, headache
⑥ Vaginal Ring: Inserted into the vagina to prevent ovulation, supplies progesterone and estrogen to thicken cervical mucus (99% effectiveness)
⑦ Cervical Cap, Diaphragm, Sponge, Female Condom, Male Condom
⑧ Spermicides: Inserted into the vagina before sexual intercourse to immobilize sperm
⑨ Intrauterine Devices (IUDs): Plastic devices inserted into the uterus. Increased risk of pelvic inflammatory disease
⑩ Female Sterilization (Tubal Ligation): Permanent occlusion of fallopian tubes with severe twisting (99.5% effectiveness)
⑪ Male Sterilization (Vasectomy): Ligating the vas deferens to prevent sperm from entering the semen (99.9% effectiveness)
⑹ Infertility
① Definition: Inability to conceive after 1 year of regular sexual activity
② Cause 1: Male infertility (90%)
Insufficient sperm count, poor sperm motility, abnormal sperm morphology (head too large or small, double-headed, double-tailed, etc.)
In normal cases, about 25% of sperm are abnormal out of 2-3 billion sperm
Abnormal sperm lose in the competition to reach the fallopian tube
Atrazine: Pesticide ingredient, reduces sperm viability
Trichloroethylene: Solvent used in Greece removal and cleaning, leads to abnormal sperm production
③ Cause 2: Female infertility
○ Endometriosis: Tissue from the uterine lining grows outside the uterus, responding to hormones
○ Cause 1: Retrograde menstruation (main cause)
○ Cause 2: Familial predisposition
○ Cause 3: Excessive female hormones
○ Cause 4: Menstrual cycle shorter than 27 days or longer than 7 days
○ Cause 5: Early menarche
○ Cause 6: Women with few childbirths
○ Scar tissue in ovarian tissue causing ovulation disorder
○ 3-10% of fertile women, 25% of infertile women
○ Scars from female infections and STDs can obstruct fallopian tubes, leading to infertility
○ DEHP: Endocrine disruptor and carcinogen
Input: 2019.01.26 20:13
Last Updated: 2022.06.06 22:30