Lecture Notes on Reproduction 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 26 Sexual reproduction Hermaphroditism: animals that possess...

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 26 Sexual reproduction Hermaphroditism: animals that possess both male and female reproductive organs in the same body ➥ some can reproduce by self-fertilization (most do not) Two forms of hermaphrodite: 1) simultaneous – eggs and sperm are produced by a single individual at the same time ➥ engage in reciprocal copulation – whereby two (or more) animals simultaneously fertilize each other Reproduction Helix aspersa (garden snails) Lumbricus terrestris (earthworm) 27 Sexual reproduction Two forms of hermaphrodite: 1) simultaneous: e.g. sea hares - sea hares form mating chains ➥ 1st animal of chain acts only as a female, receiving sperm from the animal behind it ➥ 4th animal in chain acts as a male, fertilizing the animal in front Aplysia dactylomela (sea hares) 4 Reproduction 3 2 1 ➥ 2nd and 3rd animals act as both male and female, since they donate and receive sperm 28 Sexual reproduction Two forms of hermaphrodite: 2) sequential – animal begins as one sex, but later changes sex (a) protandry: starts as a male, and changes to a female later in life - in a clownfish society, there is only one breeding pair ➥ rest of the group are non-breeders with no functioning gonads - if the female (largest fish) dies, the male (second largest fish) develops into the female of the group ➥ the largest non-breeding fish sexually matures into the male of the group Amphiprion percula (clownfish) Reproduction 29 Sexual reproduction Two forms of hermaphrodite: 2) sequential – animal begins as one sex, but later changes sex (b) protogyny: starts as a female, and changes to a male later in life - timing of sex change from female to male is socially cued ➥ suppressed by aggressive interactions with dominant males and triggered by removal of dominant males (which are largest) Pimelometopon pulchrum (California sheephead) ➥ sport-fishing accelerates this change by selectively removing the largest (and therefore dominant males) from a population Reproduction 30 Reproductive Systems Reproductive systems composed of: Primary sex organs that produce the gametes: ➨ male testes ➨ female ovaries Secondary sex organs that include: ➥ various glands that produce nutrition and lubrication ➥ ducts / chambers for gamete storage, delivery and development ➥ organs for mating or protection of developing embryo Testes Ovary insect reproductive systems Fig. 16-5, p765 Reproduction 31 Male Reproductive System - basic components are quite similar in very different animals ➥ differences occur mainly in testicular structure and organization, copulatory organs and hormonal regulation Ejaculatory duct Male human Reproduction 32 Male Reproductive System - basic processes of spermatogenesis (i.e. sperm cell development) are astonishingly similar in very different animals ➥ immature germ/stem cells in males referred to as the spermatogonia (or spermatogonium, singular) spermatocytes Insect testis spermatids spermatozoa vas deferens - spermatogonia are encapsulated in cysts at the apical tip of the testis Reproduction spermatogenesis progression spermatogonia Human testis spermatozoa - spermatogonia are located just under the basement membrane of the seminiferous tubules 33 Mammalian Spermatogenesis 1. Mitotic proliferation: spermatogenesis begins with mitotic division of a spermatogonium ➥ one of the daughter cells replaces the original spermatogonium ➥ other cell divides mitotically several times to form four primary spermatocytes ➥ after last mitotic division, primary spermatocytes enter a resting phase where chromosomes are duplicated prior to meiosis (diploid, 2n) Reproduction Fig. 16-9, p774 34 Mammalian Spermatogenesis 2. Meiosis ➥ first meiotic division yields secondary spermatocytes, referred to as the reductional division (haploid, 2n) as number of sets of homologous chromosomes is reduced ➥ Secondary spermatocytes divide in the second meiotic division, separating the two copies of each chromosome into daughter cells, yielding four spermatids (haploid, 1n) ➥ 16 spermatids – spermatogonium mitotically produces four primary spermatocytes that each meiotically yield four spermatids Reproduction Fig. 16-9, p774 35 Mammalian Spermatogenesis 3. Spermiogenesis ➥ packaging and maturation; spermatid is remodeled and packaged into motile and mature spermatozoa ➥ microtubules are reorganized to form a flagellum ➥ most of the cytoplasm and non-essential organelles are lost, leaving small densely packed cells with abundant mitochondria ➥ mature spermatozoa are released into the male reproductive tract and stored in the epididymis Reproduction Fig. 16-9, p774 36 Mammalian Spermatogenesis Sertoli Cells ➥ spermatogenic cells are in continuous contact with Sertoli cells that provide structural and metabolic support for developing sperm cells ➥ Sertoli cells extend from the basement membrane to the lumen of the seminiferous tubule Fig. 16-8d, p771 Fig. 16-8b, p771 Reproduction 37 Mammalian Spermatogenesis Sertoli Cells Provide support for developing sperm in the following ways: 1) maintain the blood-testis barrier formed by tight junctions to provide environment necessary for development and maturation of sperm 2) secrete supporting testicular fluid and substances that initiate meiosis Fig. 16-8d, p771 3) secrete androgen-binding protein, which concentrates testosterone in close proximity to developing gametes 4) secrete other hormones affecting pituitary control of spermatogenesis (e.g. the peptide hormone, inhibin) 5) phagocytose residual cytoplasm left over from spermatogenesis Reproduction 38 Mammalian Spermatogenesis Leydig Cells - located in the interstitial tissue outside of the seminiferous tubules - secrete testosterone, which mediates sperm maturation - also can convert testosterone to estradiol - active during fetal development to direct masculinization, but after birth are inactive until puberty when testosterone production resumes Similar to Sherwood Fig. 16-8 Reproduction 39 Mammalian Spermatogenesis Accessory Glands ➥ produce products that mix with sperm to protect and preserve them (e.g. for nutrition and lubrication) ➥ mammalian males have three sets of accessory glands: 1) seminal vesicles – secrete fluids that nourish (eg. fructose) and protect (eg. mucous) sperm ➥prostaglandins stimulate SM contraction 2) prostate gland – secretes alkaline fluid to keep sperm viable longer and prostatespecific antigen (PSA) to release motile sperm from semen once in the female tract Ejaculatory duct 3) bulbourethral gland – secretes lubricant and mucous-like substance that neutralizes acidic pH in female genital tract Reproduction 40 Control of Testicular Function Hormonal Regulation – Hypothalamic-Pituitary-Testicular Axis Involves three sets of hormones: 1) gonadotropin releasing hormone (GnRH) – released by the hypothalamus to stimulate release of gonadotropins (FSH and LH) from the anterior pituitary 2) follicle-stimulating hormone (FSH) – released from anterior pituitary to initiate spermatogenesis by Sertoli cells Reproduction Fig. 16-11, p777 41 Control of Testicular Function Hormonal Regulation – Hypothalamic-Pituitary-Testicular Axis Involves three sets of hormones: 3) luteinizing hormone (LH) – released from anterior pituitary to stimulate testosterone release by Leydig cells ➥ testosterone mediates sperm maturation and is converted to estradiol, which has modulatory actions Negative feedback inhibition on the hypothalamus and pituitary: ➥ increased testosterone inhibits GnRH and LH ➥ increased inhibin, a hormone produced by Sertoli cells, inhibits GnRH and FSH Reproduction Fig. 16-11, p777 42