Animal Reproduction

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)

Sexual Results in variation Male: – Testes: gonads; where sperm is produced – Seminiferous tubules: 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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