6.6 Part II and 11.4- Reproduction

Essential Idea: Sexual reproduction involves the development and fusion of haploid gametes. 11.4 Sexual Reproduction Understandings: Spermatogenesis and oogenesis both involve mitosis, cell growth, two divisions of meiosis and differentiation. Processes in spermatogenesis and oogenesis result in different numbers of gametes with different amounts of cytoplasm. Fertilization in animals can be internal or external. Fertilization involves mechanisms that prevent polyspermy. Implantation of the blastocyst in the endometrium is essential for the continuation of pregnancy. HCG stimulates the ovary to secrete progesterone during early pregnancy. The placenta facilitates the exchange of materials between the mother and fetus. Estrogen and progesterone are secreted by the placenta once it has formed. Birth is mediated by positive feedback involving estrogen and oxytocin. Application: The average 38-week pregnancy in humans can be positioned on a graph showing the correlation between animal size and the development of the young at birth for other mammals. Skill: Annotation of diagrams of seminiferous tubule and ovary to show the stages of gametogenesis. Annotation of diagrams of mature sperm and egg to indicate functions.  

The Male Embryonic development into male A. Embryos identical until about 8 weeks - The same embryonic structures give rise to ovaries and testes, clitoris and penis B. The presence of a single gene on the Y chromosome (SRY) which causes embryonic gonads to develop as testes and secrete testosterone. C. SRY codes for a DNA-binding protein called TDF (testis determining factor). D. TDF stimulates the expression of other genes that cause testis development.

E. Testosterone secretion causes male genitalia to develop. 1. During puberty, the secretion of testosterone increases. 2. This stimulates sperm production in the testes, which is the primary sexual characteristic of males. 3. Testosterone also cause the development of secondary sexual characteristics during puberty such as growth of pubic hair and deepening of the voice due to the growth of the larynx.

II. Male Reproductive System

Structure Function Testis Produce sperm and testosterone Scrotum Hold testes at lower than core body temperature Epididymis Store sperm until ejaculation Sperm duct (vas deferens) Transfer sperm during ejaculation Seminal vesicle and prostate gland Secrete fluid containing alkali, proteins and fructose that is added to sperm to make semen Urethra Transfer semen during ejaculation and urine during urination Penis Penetrate the vagina for ejaculation of semen near the cervix

Review the steps of Meiosis As a table: Review the steps of Meiosis

Spermatogenesis – Formation of male gametes by meiosis Occurs in the seminiferous tubules of the testes Starts at puberty and continues until death Seminiferous tubules of testes

3. Gametes arise from spermatogonia (germinal epithelial cells) near outer edge of seminiferous tubules a. These cells undergo mitosis to replenish supply of spermatogonia b. Undergo meiosis to produce 4 spermatozoa c. Stimulated by follicle stimulating hormone (produced by pituitary) *A male human produces about 120 million sperm cells per day!

4. Daughter cells must mature into fully motile spermatozoa a 4. Daughter cells must mature into fully motile spermatozoa a. Sertoli cells – provide nutrients for developing sperm b. Leydig cells – leutinizing hormone from pituitary stimulates production of testosterone c. Move closer to the center of the seminiferous tubule as they develop d. Move to epididymis to finish maturation

5. Mature sperm a. Acrosome – contains hydrolytic enzymes to help get into egg b. Mitochondria – produce ATP for swimming c. Flagellum – tail for swimming

6. Production of semen a. Mature sperm moves from epididymis to vas deferens b. As it moves through vas deferens, fluid is added i. Seminal vesicle adds 70% of fluid - high in fructose (sugar) to provide energy ii. Prostate adds alkaline fluid – to help neutralize low pH in vagina

7. Hormones involved Leutinizing hormone (LH) - stimulates Leydig cells in testes to produce testosterone Follicle Stimulating Hormone (FSH) - along with testosterone, stimulates meiosis of spermatogonia

The Female III. Embryonic development into female A. If there is no Y chromosome present, the gonads develop into ovaries B. Estrogen and progesterone are secreted instead of testosterone C. During puberty the secretions of estrogen and progesterone increases, causing the development of secondary sexual characteristics (enlargement of breasts and growth of pubic and underarm hair)

IV. Female Reproductive System

Structure Function Ovary Produce eggs, estrogen and progesterone Oviducts (Fallopian Tubes) Collect eggs at ovulation, provide a site for fertilization then move the embryo to the uterus Uterus Provide for the needs of the embryo and then fetus during development (embryo implants here). Cervix Protect the fetus during pregnancy and then dilate to provide a birth canal Vagina Stimulate penis to cause ejaculation and provide a birth canal Vulva Protect internal parts of the female reproductive system Endometrium Highly vascular lining of the uterus

Oogenesis 1. Events before birth - in ovaries of female fetus a. Oogonia in the ovaries undergo mitosis to increase in number b. Oogonia grow into larger cells called primary oocytes c. Primary oocytes start meiosis, but pause in prophase I d. Follicle cells undergo mitosis forming a layer around the primary oocyte (together this is called a primary follicle) e. At birth, ovaries contain about ½ million primary follicles – remain unchanged until puberty

Events during the menstrual cycle a. Each month, a few primary follicles finish meiosis I b. Results in 1 polar body and 1 secondary oocyte (both haploid) c. Ring of follicle cells divides and forms fluid, resulting in a double ring of cells with fluid in between called Graafian follicle

Increase of fluid causes Graafian follicle to push on wall of ovary until the secondary oocyte is released from the ovary i. The most developed follicle breaks open, releasing its egg into the oviduct, the other follicles degenerate e. Left over follicle cells become corpus luteum – secretes progesterone

The Mature Egg A. Secondary Oocyte – largest cell in most species – contains all nutrients for early development

4. Meiosis is not completed until fertilization occurs

Germinal epithelial cells (2n) divide by mitosis. Diploid cells grow into larger primary oocytes. Primary oocyte starts 1st division of meiosis, but stop in Prophase I (until puberty) Every menstrual cycle, a few primary follicles develop. Completes meiosis I, forming 2 haploid nuclei. Divide is unequal Secondary oocyte starts second division of meiosis but stops in Prophase II. Ovulation: Meiosis II doesn’t complete until fertilization occurs.

V. Comparison of spermatogenesis and oogenesis Millions of sperm/day from puberty throughout life 1 secondary oocyte/month between puberty and menopause 4 gametes from each germinal cell 1 gamete from each germinal cell (+ 3 polar bodies) Gametes are small with little cytoplasm and organelles Gamete is large with substantial cytoplasm, organelles, and nutrients Mitosis replaces germinal cells daily Mitosis only replaces germinal cells in early development Some cell growth before meiosis I Substantial cell growth before meiosis I Spermatids remain in seminiferous tubules until differentiation into spermatozoa occurs Differentiation begins in the ovary but finishes after fertilization Occurs in testes Occurs in ovaries

VII. The Menstrual Cycle A VII. The Menstrual Cycle A. Purpose – release ovum for potential fertilization and implantation into uterus B. Implantation must occur when endometrium is highly vascularized (increased number of blood vessels) C. If implantation does not occur, excess blood vessels are broken down and discarded by menstruation

Hormones involved 1. GnRH – gonadotropin-releasing hormone – from hypothalamus, stimulates pituitary to release LH and FSH 2. Follicle Stimulating Hormone and Leutinizing Hormone a. Stimulate production of estrogen by the ovary b. Stimulate production of Graafian follicles c. Lead to ovulation and creation of corpus luteum 3. Estrogen – increases vascularization of endometrium 4. Progesterone – maintains thickened endometrium

VIII. Fertilization External fertilization – fertilization occurs outside the body 1. Results in large numbers of offspring because large number of eggs must be laid 2. Requires water to prevent desiccation 3. High risk of predation, susceptible to environmental changes in pH, temperature, pollution, etc.

Events of fertilization in humans Sperm make their way along endometrium to Fallopian tubes Many sperm start breaking down the zona pellucida (glycoprotein layer surrounding the secondary oocyte) by releasing hydrolytic enzymes from acrosome (acrosome reaction) a. Also triggers oocyte to finish meiosis II http://www.youtube.com/watch?v=BFrVmDgh4v4

3. One spermatozoan reaches plasma membrane, releases more hydrolytic enzymes to penetrate egg 4. Plasma membranes fuse

5. Cortical reaction – prevents more than one sperm from fertilizing egg – cortical granules fuse with plasma membrane and release enzymes outside, chemically changing the zona pellucida so that no other sperm can penetrate 6. Nuclei of egg and sperm fuse to restore diploid chromosome number

IX. Early development A. Zygote starts undergoing mitosis within 24 hours of fertilization B. Moves down Fallopian tube to uterus C. Consists of about 100 cells when it reaches uterus = blastocyst

Implantation – embryo sinks down into endometrial lining Around a week after fertilization Has not increased in size, just number of cells (nutrients used for metabolism, not growth) Embryo can start getting nutrients from endometrium rather than yolk http://www.youtube.com/watch?v=UgT5rUQ9EmQ

The Placenta 1. Forms from both mother and embryo 2. Umbilical cord forms on fetal side, consists of 3 blood vessels (2 veins, 1 artery) 3. Keeps mother and fetal blood separate, allows materials (O2, CO2, nutrients, waste) to be exchanged by diffusion

http://www.youtube.com/watch?v=kZeZ62UXyzM

Hormones involved 1. HCG (human chorionic gonadotropin) – secreted by embryo, enters mother’s bloodstream, goes to her ovaries to maintain the corpus luteum 2. Progesterone – first secreted by corpus luteum, later by placenta, maintains vascularization of the endometrium and suppresses contractions of the uterus 3. Estrogen – secreted by placenta, stimulates muscle growth of uterus, antagonizes progesterone’s suppression of contractions, stimulates mammary glands development, and induces production of oxytocin receptors in uterine muscles *Both progesterone and estrogen inhibit production of further oocytes during pregnancy

X. Childbirth A. Oxytocin – secreted by pituitary to stimulate uterine contractions 1. Positive Feedback – amplifies the signal

XI. In Vitro Fertilization (IVF) A. Reasons for infertility 1 XI. In Vitro Fertilization (IVF) A. Reasons for infertility 1. low sperm count/motility in males 2. males with impotence 3. females who can’t ovulate normally 4. females with blocked Fallopian tubes

B. Harvest eggs from female 1 B. Harvest eggs from female 1. Injections of FSH to stimulate production of many Graafian follicles (superovulation) 2. Eggs surgically removed

Eggs mixed with sperm in culture dishes Use microscope to see which eggs are fertilized and healthy 1-3 eggs introduced into uterus Extra embryos can be frozen

William Harvey William Harvey’s investigation of sexual reproduction in deer. (A.5) “Seed and soil” theory of Aristotle states that the male produces a seed which forms an egg when mixed with menstrual blood. The egg then develops into a fetus inside the mother. Harvey studied animal reproduction. He dissected female deer after mating to see changes in their reproductive organs. Harvey came to understand that menstrual blood did not contribute to the formation of a fetus.