Lecture#31 04052024 Reproductive Systems PDF

Outline Learning Objectives: Introduction and energy investment in reproduction ✔ ✔ ✔ Modes of reproduction: Asexual vs. Sexual Modes of Fertilization and Embryonic Development ✔ ✔ Mammalian Male and Female Reproductive Systems Reproduction 44 Female Reproductive System Basic components are similar...

Outline Learning Objectives: Introduction and energy investment in reproduction ✔ ✔ ✔ Modes of reproduction: Asexual vs. Sexual Modes of Fertilization and Embryonic Development ✔ ✔ Mammalian Male and Female Reproductive Systems Reproduction 44 Female Reproductive System Basic components are similar in very different animals: 1) ovary: production of ova (i.e. site of oogenesis) 2) oviduct: transport of sperm and ovum to site for fertilization 3) vagina: reception of sperm (oviduct) Female human Reproduction 45 Female Reproductive System Ovary ➥ composed of ova-producing oogonia (stem cells), as well as surrounding somatic cells that provide structural and nutritive support for oogenesis (i.e. development of the oocytes) female human female insect Two major modes of oogenesis: 1) continuous oogenesis from a self-sustaining population of mitotically dividing oogonia (e.g. fish, amphibians, urchins, insects) 2) finite population of oogonia begin meiosis, but arrest in meiosis I, and then undergo maturation individually or in small groups (e.g. birds, reptiles, mammals) Reproduction 46 Mammalian Oogenesis ➨ begins in the embryonic ovary (i.e. before birth) where the oogonia replicate by mitosis, and undergo growth and storage of nutrients ➨ oogonia will begin meiosis, but arrest at prophase I ➨ at this stage, they are called primary oocytes ➨ Human embryonic ovary has ~ 7 million oogonia that develop into ~ 500,000 primary oocytes ➥ only about 400 mature and are actually ovulated! Reproduction Fig. 16-15, p787 47 Mammalian Oogenesis ➨ meiosis does not progress until puberty, when each month ~ 6 - 12 primary oocytes attempt to divide into two cells: a) first polar body b) secondary oocyte ➥ normally, one primary oocyte completes this division, and then begins the second meiotic division, but arrests in metaphase II ➥ the arrested secondary oocyte is then ovulated (i.e. released into the oviduct) Reproduction Fig. 16-15, p787 48 Mammalian Oogenesis ➥ meiosis II is only completed if the ovulated secondary oocyte is penetrated by a sperm (i.e. if fertilization occurs in the oviduct) ➥ results in the formation of: a) second polar body b) mature haploid ovum ➨ Sperm penetration induces a release of intracellular Ca2+ that: ➥ blocks polyspermy ➥ triggers the second meiotic division of the egg Reproduction Fig. 16-15, p787 49 Mammalian Ovarian Cycle ➨ during its development, each oocyte is contained within a follicle comprised of a layer of supporting follicular cells ➥ ovarian cycle describes changes that occur in the follicles during monthly oocyte development Ovarian cycle has three phases: 1) follicular phase (days 1 - 14) Growing follicle Granulosa Granulosa cells cells Follicular fluid Thecal cells ➨ Primary follicle: - single layer of granulosa cells proliferate to form multiple layers that provide support for the primary oocyte - develop a glycoprotein membrane, zona pellucida, which is important for spermatozoa binding during fertilization ➥ this is the growing follicle Reproduction Similar to Fig. 16-16, p789 50 Mammalian Ovarian Cycle Ovarian cycle has three phases: 1) follicular phase (days 1 - 14) ➥ Growing follicle: - develops an additional outer layer of cells called the theca - thecal cells produce androgen hormones, which are then used by the granulosa cells to produce more estrogen Growing follicle Granulosa cells Follicular fluid Thecal Thecal cells cells Reproduction Similar to Fig. 16-16, p789 51 Mammalian Ovarian Cycle Ovarian cycle has three phases: 1) follicular phase (days 1 - 14) ➥ Growing follicle: - develops an additional outer layer of cells called the theca - thecal cells produce androgen hormones, which are then used by the granulosa cells to produce more estrogen Growing follicle Granulosa cells Follicular fluid Thecal cells - a fluid-filled cavity forms adjacent to the oocyte – called the antrum - follicular fluid within the antrum contain hormones and enzymes required for ovulation - as the follicle grows, the oocyte progresses to meiosis II ➨ this is called a mature (Graafian) follicle Reproduction Similar to Fig. 16-16, p789 52 Mammalian Ovarian Cycle Ovarian cycle has three phases: 2) ovulation (day 14) - mature follicle and ovary wall ruptures, releasing the secondary oocyte into the oviduct ➥ 1-2% of ovulations release more than one secondary oocyte, which if fertilized, results in fraternal twins Growing follicle Granulosa cells Follicular fluid ovulation Thecal cells Reproduction Similar to Fig. 16-16, p789 53 Mammalian Ovarian Cycle Ovarian cycle has three phases: 3) luteal phase (day 14 - 28) - ruptured follicle collapses, granulosa cells enlarge, and along with the thecal cells, form the corpus luteum that secretes progesterone and estrogen Growing follicle Granulosa cells Follicular fluid Thecal cells Reproduction Similar to Fig. 16-16, p789 54 Mammalian Ovarian Cycle Ovarian cycle has three phases: 3) luteal phase (day 14 - 28) - ruptured follicle collapses, granulosa cells enlarge, and along with the thecal cells, form the corpus luteum that secretes progesterone and estrogen ➥ if pregnancy does not occur, corpus luteum degenerates (corpus albicans) Growing follicle Reproduction Granulosa cells Follicular fluid ➥ if pregnancy does occur, the corpus luteum produces hormones until the Thecal cells placenta takes over that role (~3 months) Similar to Fig. 16-16, p789 55 Control of Ovarian Function Hormonal Regulation – Hypothalamic-Pituitary-Ovarian Axis - ovarian cycle is regulated indirectly by the hypothalamus via gonadotropin hormones of the anterior pituitary Reproduction 56 Control of Ovarian Function Hormonal Regulation – Hypothalamic-Pituitary-Ovarian Axis 1) GnRH from the hypothalamus stimulates the secretion of both FSH and LH by the anterior pituitary 2) FSH and LH act on the ovary to stimulate follicle growth and increase estrogen and inhibin release 1 ➥ granulosa cells (FSH receptors) ➥ thecal cells (LH receptors) 3) rising levels of inhibin and slightly elevated estrogen in the plasma have a negative feedback effect on the anterior pituitary inhibiting output of FSH and LH Reproduction 3 2 57 Control of Ovarian Function Hormonal Regulation – Hypothalamic-Pituitary-Ovarian Axis 4) negative feedback is transient – levels of estrogen increases to high levels, which has a positive effect on the hypothalamic-pituitary axis resulting in a burst of LH and, to a lesser extent, FSH 5) sudden burst of LH and FSH stimulates completion of meiosis I in the primary oocyte 6) ovulation of secondary oocyte 4 5 6 Reproduction 58 Control of Ovarian Function Hormonal Regulation – Hypothalamic-Pituitary-Ovarian Axis 7) after ovulation, LH promotes the transformation of the ruptured follicle into the corpus luteum, which secretes progesterone and estrogen 8) progesterone and estrogen from the corpus luteum have a negative feedback effect on the anterior pituitary and inhibit FSH and LH production ➥ prevents development of new follicles 8 7 Reproduction Reproduction 59 Control of Ovarian Function ➨ as LH levels fall, the corpus luteum begins to degenerate ➨ levels of progesterone and estrogen fall ➨ FSH and LH are again produced by the anterior pituitary and a new cycle begins ➥ if fertilization occurs, hCG from the blastocyst maintains secretion of progesterone from corpus luteum Reproduction 60 THE END! 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