Animal Reproduction

1. Animal Reproduction

2. Asexual • Clones • Very little energy expended • Types: • Fission: separation of organism into two new cells (amoeba) • Budding: splitting off of new individuals from existing ones (hydra) • Fragmentation or regeneration: single parent breaks into parts and produces new individuals (sponges) • Parthenogenesis: development of eggs without fertilization (honeybees)

3. Sexual • Results in variation • Male: • Testes: gonads; where sperm is produced • Seminiferoustubules: site of sperm formation • Epididymis: tube in testes where sperm gain motility • Vas deferens: carries sperm during ejaculation from epididymis to urethra • Seminal vesicles: secrete mucus, fructose (energy for sperm) and prostaglandin (stimulates contraction for ejaculation) • Prostate gland: secretes semen into urethra • Urethra: tube that carries semen and urine

4. Female Reproductive System • Ovaries: where meiosis occurs • Oviducts (Fallopiantubes): where fertilization occurs • Uterus: where embryo develops • Endometrium: lining of uterus shed during menstruation • Vagina: birth canal • Cervix: opening of the uterus

5. Menstrual Cycle • Follicular phase • FSH stimulates estrogen production and growth of follicles in ovary • Ovulation • Secondary oocyte ruptures out of the ovaries in response to LH • Luteal phase • Endometrium of the uterus thickens due to estrogen and progesterone • Menstruation • Monthly break down of the lining of the uterus when implantation of an embryo does not occur

6. Hormonal Control • Hypothalamus releases GnRH • GnRH stimulates Anterior Pituitary • Anterior Pituitary releases FSH and LH • These stimulate the ovary • Ovary releases estrogen and progesterone • This finally thickens the lining of the uterus

7. Positive and Negative Feedback • Increase in LH stimulates the follicle to release more estrogen • Once estrogen and progesterone reach high levels, they trigger the hypothalamus and pituitary to shut off

8. Spermatogenesis • Process of sperm production • LH induces testes to produce testosterone • With FSH, testosterone induces maturation of seminiferous tubules and stimulates the beginning of sperm production • Primary spermatocyte: 2n • Secondary spermatocyte: n and n by meiosis I • Spermatids: n n n n by meiosis II • Spermatozoa: differentiated spermatids that move to epididymis

9. Oogenesis • Oogonium cell: 2n undergoes meiosis to produce primary oocytes • Primary oocytes: 2n by mitosis • Secondary oocytes: n and polar body n by meiosis I • Egg cell: n and second polar body n by meiosis II

10. Fertilization • Fusion of sperm and egg • Head of sperm (acrosome) releases enzymes that penetrate coating of egg (receptors) • Membrane becomes depolarized and no other sperm can penetrate

11. Embryonic Development • 3 stages: cleavage, gastrulation and organogenesis • Cleavage • Rapid mitotic cell division of the zygote that occurs immediately after fertilization • In protostomes, mollusks, annelids and arthropods cleavage is spiral and determinant • Future of each cell has been assigned by the four-ball stage • In deuterostomes, echinoderms and chordates cleavage is radial and indeterminate • Each cell can develop into a complete and normal embryo • Produces fluid-filled ball of cells called a blastula • Cells are called blastomeres and the center is the blastocoel

12. Embryonic Development • Gastrulation • Rearrangement of blastula and begins with the opening of the blastula, called the blastopore • As a result of this movement, a three-layered embryo called the gastrula is formed • Consists of the germ layers: ectoderm, endoderm and mesoderm • Ectoderm becomes skin and nervous system • Endoderm forms the viscera (lungs, liver and digestive system) • Mesoderm becomes muscle, blood and bones

13. Embryonic Development • Organogenesis • Organ building • Cells continuously differentiate producing organs from the germ layers • Once all organ systems have developed, embryo increases in size

14. Frog Embryo • Fertilization • External • One third of egg is yolk (vegetal pole) • Top half of egg is called the animal pole • Sperm penetrates the egg as pigmented cap (on top of egg) rotates and a gray crescent appears • Cleavage and gastrulation • Cleavage uneven due to presence of yolk • Blastopore forms at the border of the gray crescent and vegetal pole • Cells stream inward and blastocoel disappears and is replaced by a cavity called the archenteron

15. Frog Embryo • Organogenesis • First organs to form are the notochord and the nerual tube, which becomes the CNS • Neural tube forms from the dorsal ectoderm • After organ blueprints are laid down, embryo develops into a larval stage, the tadpole • Metamorphasis transforms tadpole into frog

16. Bird Embryo • Cleavage and gastrulation • Egg has much yolk so development of embryo occurs in a blastodisc at the top of the yolk • Primitive streak instead of gray crescent • Cells flow over primitive streak and flow inward to form the archenteron • Yolk gets smaller • Extraembryonic membranes • Four membranes necessary to support growing embryo inside shell • Yolk sac: encloses food for embryo • Amnion: encloses embryo in protective fluid • Chorion: allows for diffusion of respiratory gases • Alantois: repository for uric acid

17. Factors Influencing Development • Cytoplasmic determinants • Eight-ball cell is separated • If the cells are separated longitudinally, the development will occur normally • If the cells are separated horizontally, the development will occur abnormally • The Gray Crescent • Only cells containing gray crescents develop normally • Experiment conducted by Hans Spemann

18. Factors Influencing Development • Embryonic Induction • Ability of one group of embryonic cells to influence the development of another group of embryonic cells • Dorsal lip of blastopore initiates the chain of inductions that form the neural tube • Homeotic, Homeobox or Hox Genes • Master regulatory genes that control the expression of genes that regulate the placement of specific anatomical structures