Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Human cloning wikipedia , lookup
Cell theory wikipedia , lookup
FNA Mapping wikipedia , lookup
Organ-on-a-chip wikipedia , lookup
Chimera (genetics) wikipedia , lookup
Sperm competition wikipedia , lookup
Plant reproduction wikipedia , lookup
Developmental biology wikipedia , lookup
David Sadava, David M. Hillis, H. Craig Heller, May R. Berenbaum La nuova biologia.blu Anatomia e fisiologia dei viventi S Animal Reproduction How Do Animals Reproduce Sexually? In sexual reproduction two haploid cells (gametes) form a diploid individual. Three fundamental steps of sexual reproduction: • Gametogenesis—making gametes • Mating—getting gametes together • Fertilization—fusing gametes How Do the Human Male and Female Reproductive Systems Work? Primary sex organs (testes and ovaries) produce gametes and serve endocrine functions. Accessory sex organs include external genitalia, ducts, and glands. Secondary sexual characteristics are the differences in external appearance of males and females. How Do the Human Male and Female Reproductive Systems Work? Semen is made up of sperm and other fluids and molecules. Sperm are produced in the paired male gonads, or testes. Testes are located in the scrotum, outside of the body, to maintain optimal temperature for sperm production. Reproductive Tract of the Human Male How Do the Human Male and Female Reproductive Systems Work? Spermatogenesis occurs in the seminiferous tubules in each testis. Between the tubules are Leydig cells that produce testosterone. Spermatogonia lie at the outer edges of the seminiferous tubules, as sperm mature they move toward the lumen. How Do the Human Male and Female Reproductive Systems Work? Sertoli cells provide nutrients for developing sperm. Differentiated sperm cells are shed into the lumen of the seminiferous tubule. How Do the Human Male and Female Reproductive Systems Work? Sperm are stored in the epididymis, where they mature and become motile. The epididymis connects to the urethra via the vas deferens and the ejaculatory duct. Semen components are secreted mostly by the paired seminal vesicles, which empty into the vas deferens. The prostate gland surrounds the urethra and contributes to semen. Prostate fluid is alkaline and neutralizes acidity in male and female reproductive tracts—more hospitable for sperm. How Do the Human Male and Female Reproductive Systems Work? The bulbourethral glands produce an alkaline secretion that: • Neutralizes acidity in the urethra • Provides lubrication and facilitates sperm movement during climax These secretions precede climax yet can carry residual sperm that are capable of fertilization. How Do the Human Male and Female Reproductive Systems Work? Male genitalia include the penis and scrotum. The sensitive tip of the penis, the glans penis, is covered by the foreskin. Sexual stimulation triggers the nervous system to produce penile erection. How Do the Human Male and Female Reproductive Systems Work? Semen is expelled through the vasa deferentia and urethra in two steps: Emission—rhythmic contractions move the semen into the urethra at the base of the penis. Ejaculation—contractions by muscles at the base of the penis that force the semen out. Ejaculation is accompanied by feelings of intense pleasure called orgasm. How Do the Human Male and Female Reproductive Systems Work? In females, when the ovary releases an egg (ovulation), it moves into the oviduct, or Fallopian tube, via the fimbria. The egg is propelled by cilia towards the uterus where it will develop if fertilized. The bottom of the uterus is the narrow cervix, which opens into the vagina. How Do the Human Male and Female Reproductive Systems Work? Two sets of skin folds surround the opening of the vagina and the urethra: Labia minora— the inner folds. Labia majora— the outer folds. At the anterior tip is the clitoris, erectile tissue that is important in sexual response. How Do Animals Reproduce Sexually? Gametogenesis occurs in the gonads. Males: gonads are the testes and the gametes are small, motile sperm. Females: gonads are the ovaries and the gametes are nonmotile ova, or eggs. Germ cells undergo mitosis and produce spermatogonia in males and oogonia in females. Mitosis continues and produces primary spermatocytes and primary oocytes, both diploid. Meiosis then produces haploid cells that mature into sperm and ova. How Do Animals Reproduce Sexually? Spermatogenesis: Throughout the male life span, spermatogonia continue to divide by mitosis. One daughter cell remains a spermatogonium; the other daughter cell becomes a primary spermatocyte. The primary spermatocyte undergoes meiosis. The first meiotic division results in two secondary spermatocytes; the second division produces four haploid spermatids. Spermatids differentiate into sperm. Spermatogenesis How Do Animals Reproduce Sexually? Oogenesis: Primary oocytes immediately enter prophase I of meiosis. After prophase I development stops in many species. In humans this state lasts until puberty. During this period, the primary oocyte grows larger and acquires nutrients. When meiosis I resumes, the two daughter cells are of unequal sizes. How Do Animals Reproduce Sexually? The daughter cell with more cytoplasm is the secondary oocyte and the smaller one forms the first polar body. The polar bodies degenerate. How Do Animals Reproduce Sexually? The secondary oocyte enters another period of arrested development. It may be expelled from the ovary in this condition. In humans, the second meiotic division is not completed until the egg is fertilized by a sperm. How Do the Human Male and Female Reproductive Systems Work? Hormones control male sexual function: GnRH (gonadotropinreleasing hormone) is released by the hypothalamus at puberty. GnRH increases release of LH (luteinizing hormone) and FSH (follicle-stimulating hormone) by the anterior pituitary. How Do the Human Male and Female Reproductive Systems Work? The female reproductive cycle consists of two linked cycles: •The ovarian cycle that produces eggs and hormones •The uterine, or menstrual cycle that prepares the endometrium for a blastocyst. If a blastocyst does not arrive, the thickened endometrium sloughs off. How Do the Human Male and Female Reproductive Systems Work? How Do the Human Male and Female Reproductive Systems Work? Menstruation is the sloughing off of the endometrium through the vagina. A human female is born with about one million primary oocytes in each ovary. At menopause, the end of fertility, there may be only a few oocytes left in each ovary. How Do the Human Male and Female Reproductive Systems Work? The ovarian and uterine cycles are controlled by hormones: • At puberty increased GnRH stimulates production of FSH and LH • FSH and LH stimulate ovarian tissue to grow and produce estrogen, which causes maturation of accessory sex organs and secondary sexual characteristics How Do Animals Reproduce Sexually? Fertilization is the union of a haploid sperm and a haploid egg. It creates a single diploid cell, called a zygote, which will develop into an embryo. Steps in fertilization: • Sperm and egg recognize each other • Activation of sperm • Plasma membranes fuse • Egg blocks entry of additional sperm • Egg is activated metabolically • Egg and sperm nuclei fuse How Do Animals Reproduce Sexually? Species with internal fertilization have egg-sperm recognition mechanisms. A mammal egg is surrounded by the cumulus, cells in a gelatinous matrix. The zona pellucida is a glycoprotein envelope beneath the cumulus. How Do Animals Reproduce Sexually? The acrosome is a membrane-enclosed structure on the sperm head. Egg and sperm contact causes substances in the jelly coat to trigger an acrosomal reaction. Membranes in the sperm head and acrosome break down, enzymes are released and digest the jelly coat. How Do Animals Reproduce Sexually? Sperm and egg plasma membranes fuse to form a fertilization cone that engulfs the sperm head, bringing it into the egg. Fusion of sperm and egg membranes initiate a sequence of events: • Blocks to polyspermy—mechanisms to prevent more sperm from entering the egg. • Cortical granule enzymes dissolve the bonds between the vitelline envelope and the plasma membrane. • Water enters and swells to form the fertilization envelope. How Does Mitosis Divide Up the Early Embryo? Cleavage: early cell divisions with no cell growth. Embryo becomes a solid ball of small cells. Blastocoel—a central fluid-filled cavity that forms in the ball. The embryo becomes a blastula and its cells are called blastomeres. How Does Mitosis Divide Up the Early Embryo? At the 32-cell stage, cells separate into two groups: • Inner cell mass—becomes the embryo; • Trophoblast—sac that forms from the outer cells. Its cells secrete fluid to create the blastocoel cavity, with the inner cell mass at one end. Embryo is now called a blastocyst. How Does Mitosis Divide Up the Early Embryo? In mammals, fertilization occurs in the oviduct; cleavage occurs as the zygote travels down the oviduct to the uterus. Implantation occurs when the trophoblast adheres to the endometrium, or uterine lining. Implantation that occurs in the oviduct is an ectopic, or tubal, pregnancy. How Does Gastrulation Generate Multiple Tissue Layers? Gastrulation: massive movements of cells transform the blastula into an embryo with multiple tissue layers and distinct body axes. In triploblastic animals, three germ layers (tissue layers) form. • Endoderm: innermost layer; • Ectoderm: outer layer; • Mesoderm: middle. How Is the Growing Embryo Sustained? Extra-embryonic membranes surround the vertebrate embryo. They function in nutrition, gas exchange, and waste removal. • Amnion—surrounds the embryo • Yolk sac—encloses yolk within the egg • Allantoic membrane—forms the allantois, a sac for waste storage • Chorion—reduces water loss and exchanges gases How Is the Growing Embryo Sustained? In mammals, they interact with the mother’s tissues to form the placenta. The placenta develops from the chorion and uterine tissues. It exchanges nutrients, gases, and wastes between mother and embryo. With the umbilical cord the embryo is attached to the chorionic placenta. What Are the Stages of Human Development? Gestation, or pregnancy, in humans is about 266 days (9 months) and is divided into trimesters. In the first trimester the embryo becomes a fetus: • Heart begins to beat by week four • Limbs form by week eight What Are the Stages of Human Development? During the second trimester the limbs elongate and fingers, toes, and facial features form. The fetus’s nervous system develops rapidly. First fetal movements are felt by the mother. During the third trimester internal organs mature. The brain goes through sleep-wake cycles. Birth occurs when the lungs are mature. How Can Fertility Be Controlled? The only failure-proof methods to prevent pregnancy are complete abstinence and surgical removal of the gonads. Contraception methods prevent fertilization or implantation, and vary widely in effectiveness. Table 43.1 Methods fo Contraception (Part 1) Table 43.1 Methods fo Contraception (Part 2) Table 43.1 Methods fo Contraception (Part 3) Table 43.1 Methods fo Contraception (Part 3) How Do Animals Reproduce Without Sex? Asexual reproduction requires no energy for mating but does not result in genetic diversity—all offspring are genetically identical to their parents. Asexually reproducing species are mostly invertebrates, sessile, and tend to live in constant environments. How Do Animals Reproduce Without Sex? Budding produces new individuals that form from the bodies of older animals. A bud grows by mitotic cell division—cells differentiate before the bud breaks away. The bud is genetically identical to the parent. Regeneration can replace damaged tissues or lost limbs, or form a complete individual. How Do Animals Reproduce Without Sex? Parthenogenesis is the development of offspring from unfertilized eggs; arthropods, some fish, amphibians, reptiles. Most species also engage in sexual reproduction at other times. Parthenogenesis may determine the sex of the offspring (e.g., in honey bees). Adapted from Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014 Inc. All rights reserved