Biology p 328-347

• The female reproductive system generates oocytes, provides a supportive environment for fertilization, and supports offspring growth.
• This lesson covers female reproductive anatomy, oogenesis, and hormone regulation.

Female Reproductive Anatomy

• The internal and external structures of the female reproductive system produce oocytes, receive sperm, and support fertilization, gestation, and offspring nourishment.
• Ovaries are reproductive gonads that secrete sex hormones like estrogens and progesterone and serve as oogenesis sites.
• Uterine or fallopian tubes are muscular tubes extending from the uterus toward each ovary.
• Fimbriae are fingerlike projections that direct oocytes from the abdominal cavity into the uterine tubes as the open ends of uterine tubes are near, but not connected to, the ovaries.
• Ciliated cells propel oocytes toward the uterus inside uterine tubes, which is also where fertilization typically occurs.
• The uterus is a muscular organ protecting and nourishing the embryo and fetus.
• The endometrium is the inner lining of the uterus that changes in thickness during the menstrual cycle.
• The myometrium is the thick, smooth muscle layer of the uterus that contracts during menstruation and childbirth.
• The cervix is the inferior portion of the uterus and opens into the vagina.
• The vagina is a muscular tube that eliminates menstrual fluids, receives the penis during intercourse, and serves as the birth canal's final segment.
• The female external genitalia, or vulva, includes the labia majora and labia minora, that protects the vagina and clitoris opening.
• The clitoris' external portion is at the junction of the labia minora.
• Clitoris stimulation triggers genital changes, such as lubrication and pH changes, that facilitate reproduction.
• Mammary glands are chest wall accessory glands that fully develop during pregnancy to facilitate lactation, which is breast milk synthesis, secretion, and nursing.
• Ovaries are female gonads homologous to testes that are surrounded by a fibrous capsule covered by epithelial cells.
• The cortex contains ovarian follicles in maturation stages with each follicle containing an immature primary oocyte surrounded by epithelial cells.
• Follicular cells mature into granulosa cells, which support developing oocytes that secrete sex steroid hormones during the ovarian cycle.

Oogenesis

• Oogenesis begins during embryogenesis as oogonia stem cells undergo mitotic division within developing ovaries.
• Oogonia start meiosis I at puberty, forming primary oocytes during the fetal period.
• Primary oocytes are paused in late prophase I of meiosis I and do not resume until puberty in response to hormonal changes.
• At puberty, primary oocytes are surrounded by follicular support cells in the ovary, which form ovarian follicles.
• Unequal cell division of a primary oocyte completing meiosis I result in a larger haploid secondary oocyte and smaller first polar body, which undergoes apoptosis and degenerates.
• The secondary oocyte initiates meiosis II but arrests at metaphase II until fertilization.
• During ovulation, the ovarian follicle ruptures and releases the secondary oocyte, as it is surrounded by the zona pellucida and the corona radiata, into the abdominal cavity.
• The secondary oocyte is drawn into the uterine or fallopian tube, where sperm cell fertilization can occur.
• Successful fertilization leads to the secondary oocyte completing meiosis II, forming a large ovum and a small second polar body that degenerates.
• Male and female pronuclei fuse to form a diploid zygote upon complete fertilization.
• Each oogonium yields one haploid ovum and two to three polar bodies while spermatogenesis generates four haploid sperm.
• Oogenesis begins before birth and can result in mature gametes 13-50 years later.
• Oogenesis ceases during menopause as sex hormone levels decline with age.
• The primary oocytes formed during the fetal period are not replenished later, unlike spermatogenesis, which occurs throughout life after puberty.

Hormonal Control of the Female Reproductive System

• Male reproductive organs begin development in XY embryos due to SRY gene expression during early embryogenesis.
• Anti-Müllerian hormone (AMH) is not produced in XX embryos because SRY is absent and testes typically do not develop.
• Female reproductive structures are derived from Müllerian ducts, and Wolffian ducts degenerate due to a lack of testosterone.
• Oogenesis begins during the fetal period with the production of follicles containing primary oocytes.
• Progression of oogenesis is repressed and sexual development remains dormant until puberty due to the low sex hormone levels during infancy and childhood.
• Estrogen and progesterone promote female reproductive organ growth and maturation, development of secondary sex characteristics, resumption of oogenesis, and initiation of the menstrual cycle at puberty.
• The hypothalamic-pituitary-gonadal (HPG) axis regulates cyclical changes for reproduction.
• The female reproductive cycle consists of the ovarian cycle for primary oocyte maturation and the uterine cycle for uterus preparation for pregnancy.
• Ovarian and uterine cycles occur concurrently, with a single female reproductive cycle lasting on average 28 days.
• Synchronization of the ovarian and uterine cycles provides optimal conditions for fertilization support and early pregnancy.
• The ovarian cycle divides into three phases.
  • Follicular phase (days 1–13): HPG axis stimulation results in gonadotropin-releasing hormone (GnRH) release from the hypothalamus, stimulating the anterior pituitary gland to release small amounts of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
  • FSH and LH stimulate developing ovarian follicles to release estrogens.
  • Low estrogen levels exert negative feedback on the HPG axis during the early follicular phase allowing only one follicle to survive.
  • The dominant follicle secretes inhibin to inhibit FSH, repressing maturation of additional follicles.
  • Although estrogen initially inhibits the HPG axis, the emergence of a dominant follicle stimulates the secretion of high estrogen levels, which exerts a stimulatory effect on the HPG axis during the late follicular phase.
  • This effect results in a surge of LH and, to a lesser extent, FSH.
  • Ovulation (day 14): The mature ovarian follicle ruptures soon after the LH surge and releases a secondary oocyte into the abdominal cavity, which is then directed into a nearby uterine tube.
  • Luteal phase (days 15–28): LH stimulates the ruptured follicle's conversion into the corpus luteum, which secretes high levels of progesterone and estrogens to exert a negative feedback on the HPG axis.
  • Lower FSH and LH levels during the luteal phase prevent maturation of additional follicles.
  • The corpus luteum degenerates, causing a sharp decline in progesterone and estrogens, and the inhibition of FSH and LH is relieved, allowing the next ovarian cycle to begin if fertilization does not occur.
• The corpus luteum persists and continues to secrete progesterone and estrogens if the secondary oocyte is fertilized following ovulation.
• The continued presence of progesterone and estrogens keeps FSH and LH levels low, promoting the pregnancy and preventing a new ovarian cycle from being initiated during pregnancy.
• The uterine or menstrual cycle occurs concurrently with the ovarian cycle and is divided into three phases.
  • Menstrual phase (days 1–4): If fertilization of a secondary oocyte does not occur in the prior uterine cycle, menstruation begins.
  • The uterus sheds the majority of its endometrial layer in menses, and detached tissue and blood pass out of the body through the vagina.
  • Ovarian follicles are stimulated to grow and produce estrogens, leading to the cessation of blood flow toward the end of this phase.
  • Proliferative phase (days 5-14): The endometrial layer proliferates and doubles in thickness.
  • Endometrial glands develop, and vascularization of the endometrium is increased to prepare for embryo implantation.
  • Secretory phase (days 15-28): Increased progesterone and estrogen secretion from the corpus luteum triggers the further thickening and development of the endometrium.
  • These changes result in nutrient secretion from endometrial glands and environmental creation to sustain a developing embryo in the event of fertilization and implantation.

Pre-Implantation Development

• Embryonic development begins when male and female gametes join to form a zygote, referred to as fertilization.
• Fertilization typically takes place in a uterine tube within 12-24 hours after ovulation, the developing embryo must then travel to the uterine cavity and implant for a viable pregnancy to occur.
• The developmental steps between fertilization and implantation take place within 12 days after ovulation.
• This lesson covers pre-implantation development from fertilization to implantation completion, as well as placenta development.
  • Fertilization becomes possible with secondary oocyte release from the ovary during ovulation as it travels into abdominal cavity and is drawn into the uterine tube by fimbriae.
  • The ciliated lining of the uterine tube propels the oocyte into the uterine cavity and sperm typically make contact with the oocyte within the uterine tube.
  • Successful fertilization requires the following steps
    • Capacitation: Sperm must undergo a final maturation step, known as capacitation to reach the oocyte.
    • Female reproductive tract secretions induce changes in plasma membrane permeability at the sperm head and trigger increased flagellar movement (motility).
    • Contact with oocyte: Sperm moves toward the zona pellucida through the corona radiata. Sperm head receptors bind to glycoproteins in the zona pellucida.
    • Acrosome reaction: Hydrolytic enzymes from the acrosome (specialized vesicle) are released near the oocyte, leading to zona pellucida degradation enabling the sperm to reach the oocyte's plasma membrane.
    • Fusion: Oocyte and sperm plasma membranes fuse, triggering oocyte plasma membrane depolarization.
    • Sperm contents entering the oocyte: The sperm nucleus, mitochondria, and a pair of centrioles enter the oocyte but most sperm mitochondria are destroyed.
    • Cortical reaction: Oocyte plasma membrane depolarization leads to increased intracellular calcium levels after sperm fusion, which triggers cortical granule fusion with the oocyte's plasma membrane.
    • Hydrolytic enzymes are released into the space between the plasma membrane and the zona pellucida, causing the zona pellucida to lift away from the oocyte and harden and resulting in the formation of a protective envelope that blocks additional sperm from entering in polyspermy.
• Sperm contents enter the oocyte after sperm and oocyte plasma membrane fusion.
• The secondary oocyte must then complete meiosis II, dividing to form a mature ovum and second polar body that degenerates.
• The nucleus of the mature ovum develops into a female pronucleus. Simultaneously, the male pronucleus is propelled toward the female pronucleus.
• Fusion of haploid pronuclei produces a diploid zygote.
• Epigenetic modifications occur immediately following zygote formation.
• Within 24 hours after fertilization, the zygote begins a period of rapid mitotic cell division in embryonic cleavage, where cell division proceeds without concurrent cell growth, forming smaller daughter cells.
• After 3-4 days the embryo enters the morula stage, which is roughly the same size as the original zygote and consists of 16–32 blastomeres.
• The developing embryo travels to the uterine cavity during the transition from zygote to morula.
• By days 4-5, the dividing embryonic cells reorganize into a blastocyst, which contains a bastocoal, which is a fluid-filled cavity.
• Blastocysts contains Inner cell mass cells, which develop in embryo and trophoblast, of the outer cell that develops extraembryonic tissues.

Implantation and Placental Development

• The blastocyst must implant in the uterine lining for development to continue.
• Progesterone and estrogen surges trigger the secretory phase of the uterine cycle by 6–7 days after fertilization.
• The endometrial lining is primed to support blastocyst implantation.
• Developing trophoblastic cells adhere to the endometrial surface causing the blastocyst to burrow into the endometrial lining resulting in endometrial cells that proliferate and surround the embryo.
• Until the placenta is completely formed, the endometrium supports and nourishes the embryo.
• Implantation is usually complete by day 12.
• The corpus luteum typically degenerates and progesterone and estrogen levels decline if fertilization does not occur allowing for the triggering of menstruation.
• The developing trophoblastic cells secrete human chorionic gonadotropin (hCG) with successful implantation, and the corpus luteum is maintained while the placenta develops.
• The placenta forms over the first 3 months of pregnancy to facilitate nutrient, gas, and waste exchange between the maternal circulation and the developing embryo.
• The chorion is an extraembryonic membrane derived from the trophoblast after implantation as chorionic villi that invade the endometrium and form from the chorion and become vascularized.
• The chorion and chorionic villi form the bulk of the placenta.
• The corpus luteum continues to secrete estrogen and progesterone for approximately the first 9 weeks of pregnancy, effectively inhibiting the initiation of another menstrual cycle.
• The developing placenta gradually takes over secretion of hCG, estrogens, and progesterone to support the pregnancy, leading to corpus luteum degeneration.
• Yolk sac: exchange occurs and embryonic blood cell production takes place.
• Allantois: Fluid and waste exchange structure.
• Amnion: A tough membrane filled with aminiotic fluid surrounding the embryo in which placenta recieves blood.

Post-Implantation Development

• Successful blastocyst implantation signals the beginning of the embryonic period of development.
  • Organogenesis: This (development of embryonic tissues and organs) begins with gastrulation, a period of rapid growth and embryonic cells.
  • The three primary germ layers are established by the end of gastrulation, and mesodermal tissue induces the overlying ectoderm to become neural tissue in neurulation.

Gastrulation

• At the implantation point, the blastocyst becomes formed through 2 major cells called.
• The trophoblast cell and inner cell mass.
• 2 major cell types are called epiblast and hypoblast.
• The epiblast cells form the amnion and embryo, and that of the hypoblast form a certain amount of chorion and yolk sac.
  • The primary germ layers arise from which all embryonic tissues and organs:
  • Ectoderm
  • Mesoderm - endoderm.
  • The formation of a groove makes a primitive streak which has a posterior-anterior of the embryo.
  • The blastopore or the first point of invangination forms endodermal cells.

Germ Layer Derivatives

• The ectoderm develops into the skin and nervous system.
• The mesoderm gives rise to muscle, bone, and other connective tissues.
• The endoderm forms the lining of the digestive tract and other internal organs.

Organogenesis and Neurulation

• The formation of specific and organ systems begins after primary germ layer estabishment.
• Ectodermal cells form neural tissues neurulation
• Three ectodermal cell types: -Epidermal -Neural Crest -Neural plate
• Embryos at this stage are neurulas.
• The thickening and elogation of the neural plate shows in the response by undermesodermal to show signs.
• Movement of the lateral edges of the neural folds towards the midline.
• Detatch and mirgation of neural crest cells for neurulation