Reproduction and Embryonic Development Chapter 27 Reproduction and Embryonic Development

Baby Bonanza The increased use of fertility drugs has caused an increase in the number of multiple births in the United States Infertility affects about one in seven couples in the United States Fertility drugs are an example of technology affecting the normal reproductive cycle

ASEXUAL AND SEXUAL REPRODUCTION 27.1 Sexual and asexual reproduction are both common among animals In asexual reproduction, one parent produces genetically identical offspring Budding, fission, fragmentation/ regeneration Allows isolated animals to reproduce without having to find mates Produces offspring quickly Diminishes genetic diversity of offspring

Sexual reproduction involves the fusion of gametes from two parents Two haploid gametes unite to produce a diploid zygote Male sperm is small, flagellated Female ovum is larger and not self-propelled Increases genetic variability among offspring May enhance reproductive success in changing environments

Many animals can reproduce both sexually and asexually Rotifer Asexual in good environmental conditions Sexual in more stressful conditions Hermaphrodites Each individual has both female and male reproductive systems Most mate with another member of their species

LE 27-01c “Head” Intestine Ovary Eggs LM 210

Video: Rotifer

External fertilization is common in aquatic animals Parents release their gametes into the environment, where fertilization occurs Internal fertilization occurs in nearly all terrestrial animals Sperm are deposited in or close to the female reproductive tract Gametes unite within the tract Usually requires copulation

LE 27-01e Eggs

27.2 Reproductive anatomy of the human female HUMAN REPRODUCTION 27.2 Reproductive anatomy of the human female Both sexes in humans Have a set of gonads where gametes are produced Have ducts for delivery of the gametes and structures for copulation

Produce egg cells (ova) Ovaries Produce egg cells (ova) Follicles protect and nourish the eggs and produce estrogen Ovulation One egg released every 28 days, starting at puberty Remaining follicular tissue temporarily develops into corpus luteum Secretes progesterone

Oviducts (fallopian tubes) Convey eggs to the uterus Location of fertilization Uterus (womb) Thick wall and endometrium richly supplied with blood vessels Actual site of pregnancy Embryo: from first division of zygote until body structures begin to appear Fetus: from about ninth week until birth

In ectopic pregnancy, embryo implants somewhere other than uterus Cervix: narrow neck of uterus Vagina: Muscular opening from uterus Receives penis during intercourse Forms the birth canal Other structures of the female reproductive system Labia minora and majora, clitoris, Bartholin's gland

LE 27-02a Oviduct Ovaries Follicles Corpus luteum Uterus Wall of uterus Endometrium (lining of uterus) Cervix (“neck” of uterus) Vagina

LE 27-02c Oviduct Ovary Uterus Rectum (digestive system) Urinary bladder (excretory system) Pubic bone Cervix Urethra (excretory system) Vagina Shaft Glans Clitoris Bartholin’s gland Prepuce Anus (digestive system) Labia minora Vaginal opening Labia majora

LE 27-02b Egg cell Ovary LM 200

Animation: Female Reproductive Anatomy

27.3 Reproductive anatomy of the human male Testes, housed outside the body in the scrotum, produce sperm Pathway of sperm from testis to outside Epididymis Vas deferens Ejaculatory duct Ejaculation through urethra in penis Connection between reproductive and excretory systems

Animation: Male Reproductive Anatomy Animation: Male Hormones Several glands contribute to the formation of fluid that nourishes and protects sperm Seminal vesicles Prostate gland Bulbourethral glands Semen combines sperm and glandular secretions Animation: Male Reproductive Anatomy Animation: Male Hormones

LE 27-03a Rectum (digestive system) Seminal Urinary bladder vesicle (excretory system) Vas deferens Pubic bone Ejaculatory duct Erectile tissue of penis Prostate gland Urethra (excretory system) Bulbourethral gland Penis Vas deferens Epididymis Glans of penis Testis Prepuce Scrotum

Urinary bladder (excretory system) Seminal vesicle (behind bladder) Prostate gland Bulbourethral gland Urethra Erectile tissue of penis Scrotum Vas deferens Epididymis Testis Glans of penis

The process of ejaculation involves coordinated contractions and expulsion of fluids Hormones control sperm production by the testes through a negative-feedback system

LE 27-03c Sphincter contracts Urinary bladder Urethra region here expands and fills with semen Contractions of vas deferens Contractions of seminal vesicle Contractions of prostate gland Contractions of epididymis Sphincter contracts First stage Sphincter remains contracted Semen expelled Contractions of muscles around base of penis Contractions of urethra Sphincter relaxes Second stage

LE 27-03d Stimuli from other areas in the brain Hypothalamus Releasing hormone Anterior pituitary Negative feedback FSH LH Androgen production Testis Sperm production

27.4 The formation of sperm and ova requires meiosis Gametogenesis: the formation of diploid gametes from haploid sperm and ova during meiosis Spermatogenesis: the formation of sperm cells Diploid cells made continuously in seminiferous tubules of testes Differentiated primary spermatocytes Haploid secondary spermatocytes Haploid sperm

LE 27-04a Epididymis Penis Testis Scrotum 2n Diploid cell Testis Seminiferous tubule Differentiation and onset of Meiosis I Cross section of seminiferous tubule 2n Primary spermatocyte (in prophase of Meiosis I Meiosis I completed n n Secondary spermatocyte (haploid; double chromatids) Meiosis I n n n n Developing sperm cells (haploid; single chromatids) Differentiation Sperm cells n n n n Center of seminiferous tubule (haploid)

Oogenesis occurs mostly in the ovaries Before birth, a diploid cell in each developing follicle begins meiosis At birth, each follicle contains a dormant diploid primary oocyte After puberty, one primary oocyte is released each month Continues meiosis Unequal division of cytoplasm forms a single secondary oocyte If fertilized, oocyte completes meiosis and becomes a haploid ovum

LE 27-04b Diploid cell 2n In embryo Differentiation and onset of Meiosis I Primary oocyte (arrested in prophase of Meiosis I) 2n Present at birth Completion of Meiosis I and onset of Meiosis I Secondary oocyte (arrested at meta- phase of Meiosis II; released from ovary) First polar body n n Entry of sperm triggers completion of Meiosis II Ovum Second polar body n (haploid) n

The development of an ovarian follicle involves many different processes Comparison of oogenesis and spermatogenesis Both produce haploid gametes Only one ovum results from each diploid cell that undergoes meiosis Cells from which gametes develop are thought not to divide throughout life in the female Oogenesis has long resting periods; spermatogenesis is uninterrupted

Degenerating corpus luteum Start: Primary oocyte within follicle LE 27-04c Degenerating corpus luteum Start: Primary oocyte within follicle Corpus luteum Growing follicles Mature follicle Secondary oocyte Ovary Ovulation Ruptured follicle

27.5 Hormones synchronize cyclic changes in the ovary and uterus The reproductive cycle in females involves an integrated process between the ovaries and the uterus Ovarian cycle: produces the oocyte Menstrual cycle: involves the monthly changes in the uterus Hormonal messages synchronize the two cycles through intricate feedback systems

Events in the menstrual cycle are coordinated with the ovarian cycle Events synchronized by five hormones Menstrual cycle prepares uterus for implantation of embryo Menstruation lasts 3-5 days, corresponding to pre-ovulatory phase of ovarian cycle Continues through time of ovulation (20-25 days) If no embryo is implanted, menstruation begins again

Hormonal events leading to ovulation Hypothalamus signals the anterior pituitary to secrete FSH and LH FSH stimulates growth of a follicle, with little estrogen secreted and negative feedback control of pituitary Increased secretion of estrogen exerts positive feedback on hypothalamus Pituitary secretes burst of FSH and LH levels LH, FSH, and estrogen peak

Hormonal events at ovulation and after LH stimulates completion of meiosis and rupture of follicle Ovulation occurs Follicle becomes the corpus luteum Corpus luteum secretes estrogen and progesterone High levels of estrogen and progesterone exert negative feedback on hypothalamus and pituitary

FSH and LH levels drop Hypothalamus stimulates pituitary to secrete more FSH and LH, and a new cycle begins Control of the menstrual cycle When levels of estrogen and progesterone drop, endometrium begins to slough off Menstrual bleeding begins on day 1 of a new cycle

LE 27-05 Control by hypothalamus Inhibited by combination of estrogen and progesterone Hypothalamus Stimulated by high levels of estrogen Releasing hormone Anterior pituitary FSH LH Pituitary hormones in blood LH peak triggers ovulation and corpus luteum formation LH FSH FSH LH Ovarian cycle Growing follicle Corpus luteum Degenerating corpus luteum Mature follicle Ovulation Pre-ovulatory phase Post-ovulatory phase Estrogen Progesterone and estrogen Ovarian hormones in blood Estrogen Progesterone Progesterone and estrogen Estrogen

Animation: Post Ovulation Animation: Ovulation Animation: Post Ovulation

27.6 The human sexual response occurs in four phases Excitement phase: prepares the sexual organs for coitus Plateau phase: breathing and heart rate increase Orgasm: rhythmic contractions, pleasure for both partners, ejaculation Resolution phase: completes the cycle and reverses the previous responses

27.7 Sexual activity can transmit disease CONNECTION 27.7 Sexual activity can transmit disease Sexually transmitted diseases (STDs) are contagious diseases spread by sexual contact Viral diseases are not curable but can be controlled by medication Many STDs can cause long-term problems or even death if untreated STDs are most prevalent among teenagers and young adults

27.8 Contraception can prevent unwanted pregnancy CONNECTION 27.8 Contraception can prevent unwanted pregnancy Contraception is the deliberate prevention of pregnancy Contraception must be used correctly to prevent failure "Safe sex" provided by condoms can prevent both unwanted pregnancy and sexually transmitted disease

Common contraceptive methods Abstinence Tubal ligation or vasectomy Rhythm method (natural family planning) Withdrawal Barrier methods: condom, diaphragm Most effective if used with spermicide Oral contraceptives Morning after pills

LE 27-08 Skin patch Condom Diaphragm Spermicide Birth control pills

PRINCIPLES OF EMBRYONIC DEVELOPMENT 27.9 Fertilization results in a zygote and triggers embryonic development Embryonic development begins with fertilization Many sperm reach an egg, but only one fertilizes it Sperm and egg unite to form a diploid zygote Video: Hydra Releasing Sperm

The properties of sperm cells Streamlined shape an adaptation for swimming Thick head contains haploid nucleus Acrosome at tip of head contains enzymes that help sperm penetrate the egg High-energy nutrients absorbed from semen provide energy for propulsion and penetration

LE 27-09b Plasma membrane Middle piece Neck Head Tail Mitochondrion (spiral shape) Nucleus Acrosome

The process of fertilization Sperm approaches egg Sperm's acrosomal enzymes digest egg's jelly coat Proteins on sperm head bind to egg receptors Plasma membranes of sperm and egg fuse Sperm nucleus enters egg cytoplasm

Fertilization envelope forms, preventing other sperm from entering egg Nuclei of sperm and egg fuse, producing diploid nucleus of zygote

LE 27-09c The sperm approaches the egg The sperm’s acrosomal enzymes digest the egg’s jelly coat Proteins on the sperm head bind to egg receptors Sperm The plasma membranes of sperm and egg fuse Sperm head Nucleus The sperm nucleus enters the egg cytoplasm Acrosome Plasma membrane Acrosomal enzymes A fertilization envelope forms Receptor protein molecules Plasma membrane Vitelline layer Sperm nucleus Cytoplasm Jelly coat Egg nucleus The nuclei of sperm and egg fuse Egg cell Zygote nucleus

27.10 Cleavage produces a ball of cells from the zygote Cleavage is the first major stage of embryonic development A rapid series of cell divisions produces a ball of cells from the zygote The number of cells doubles with each cleavage division Fluid-filled blastocoel forms in the center of the embryo A blastula, a hollow ball of cells, is the product of cleavage

LE 27-10 Zygote 2 cells 4 cells 8 cells Blastocoel Many cells (solid ball) Blastula (hollow ball) Cross section of blastula

Video: Sea Urchin Embryonic Development

Cleavage makes two important contributions to early development Creates a multicellular embryo from a single-celled zygote Partitions the embryo into developmental regions

27.11 Gastrulation produces a three-layered embryo Gastrulation adds more cells to the embryo and sorts them into three layers Ectoderm: forms outer skin of gastrula Endoderm: forms embryonic digestive tract (archenteron) Mesoderm: partly fills space between ectoderm and endoderm These layers develop into all parts of the adult animal

Video: Frog Embryo Development Mechanics of gastrulation Blastula Blastopore formation Cell migration to form layers Completion of gastrulation Video: Frog Embryo Development

LE 27-11 Animal pole Blastocoel Vegetal pole Blastula Gastrulation Blastopore forming Blastopore forming Blastocoel shrinking Archenteron Archenteron Ectoderm Mesoderm Endoderm Yolk plug Yolk plug Gastrula

27.12 Organs start to form after gastrulation The three embryonic tissue layers differentiate into tissues and organs Frog development in first few hours after gastrulation Notochord in mesoderm Will become core of the backbone Hollow nerve cord in ectoderm Neural plate forms neural tube, which will become the brain and spinal cord

Neural Neural fold plate Notochord Ectoderm Mesoderm Endoderm LE 27-12a Neural fold Neural plate Notochord Ectoderm Mesoderm Endoderm Archenteron Neural folds 15

LE 27-12b Neural fold Neural plate Outer layer of ectoderm Neural tube

Relative positions of neural tube, notochord, and archenteron preview the basic body plan Changes about 12 hours later Somites from mesoderm; will give rise to segmented structures such as vertebrae Hollow coelom (body cavity) By 5-8 days after beginning of development, all body tissues and organs would have emerged from cells of the three original layers

LE 27-12c Neural tube Notochord Somite Coelom Archenteron (digestive cavity) Somites Tail bud Eye SEM 15

27.13 Changes in cell shape, cell migration, and programmed cell death give form to the developing animal Changes in cell shape Folds of ectoderm become the start of the brain and spinal cord Migration of cells Move to specific destinations Join together, held by glycoproteins Take on characteristics of a tissue

LE 27-13a Ectoderm

Programmed cell death, or apoptosis Controlled by "suicide" genes Essential for normal development and differentiation of body structures

LE 27-13b Apoptosis Dead cell engulfed and digested by adjacent cell

27.14 Embryonic induction initiates organ formation All developmental processes depend on signals between cells Induction: the mechanism by which one group of cells influences development of an adjacent group A sequence of inductive signals leads to increasingly greater specialization of cells into organs

LE 27-14 Optic cup Lens ectoderm Cornea Future brain Lens Optic vesicle Future retina Optic stalk

27.15 Pattern formation organizes the animal body Pattern formation is emergence of a body form with specialized parts in the right places Positional signals determine which master control genes will be expressed In vertebrates, pattern-forming zones provide positional information to other cells through chemical signals A major goal of developmental research is to learn how information in DNA directs development of 3-D animal form

LE 27-15a Anterior Bird embryo Ventral Limb bud Limb bud develops Distal Dorsal Proximal Posterior Normal wing

LE 27-15b Graft of cells from pattern- forming Graft Host limb bud zone Graft Host limb bud develops Donor limb bud Host limb bud Host pattern- forming zone Donor cells Wing with duplication

HUMAN DEVELOPMENT 27.16 The embryo and placenta take shape during the first month of pregnancy Pregnancy, or gestation, is the carrying of developing young within the female reproductive tract Gestation period varies considerably within animal species

An overview of human developmental events Fertilization in the oviduct Blastocyst implanted in endometrium Fluid-filled cavity Cell mass that will form baby Trophoblast Secretes enzymes enabling implantation; forms part of placenta Extraembryonic membranes

Cleavage starts Fertilization of ovum Ovary Oviduct Secondary oocyte Blastocyst (implanted) Ovulation Endometrium Uterus

LE 27-16b Endometrim Inner cell mass Cavity Trophoblast

Endometrium Blood vessel (maternal) Future embryo Multiplying cells LE 27-16c Endometrium Blood vessel (maternal) Future embryo Multiplying cells of trophoblast Future yolk sac Trophoblast Actual size Uterine cavity

Roles of the extraembryonic membranes Amnion has grown to enclose the embryo Yolk sac produces embryo's first blood cells and germ cells Allantois forms part of umbilical cord and part of urinary bladder Chorion becomes embryo's part of placenta HCG maintains production of estrogen and progesterone during early pregnancy

LE 27-16d Amniotic cavity Amnion Mesoderm cells Chorion Yolk sac

Chorion Amnion Allantois Chorionic villi Embryo: Ectoderm Mesoderm LE 27-16e Chorion Chorionic villi Amnion Embryo: Ectoderm Allantois Mesoderm Endoderm Yolk sac

The placenta: composite organ consisting of chorionic villi closely associated with blood vessels of mother's endometrium Chorionic villi Absorb food and oxygen from the mother's blood Chorionic blood vessels Carry food, oxygen, protective antibodies, some viruses and toxins to fetus Carry wastes back to mother's bloodstream

Placenta Mother’s blood vessels Allantois Amniotic cavity Yolk sac LE 27-16f Placenta Mother’s blood vessels Allantois Amniotic cavity Yolk sac Amnion Embryo Chorion Chorionic villi

27.17 Human development from conception to birth is divided into three trimesters First trimester All organs and appendages are built in essentially a human pattern By 9 weeks, embryo is called a fetus Sex is evident, and heartbeat can be detected

Second trimester Main changes involve an increase in size and a general refinement of human features Placenta takes over secretion of progesterone and estrogen Corpus luteum degenerates Eyes are open, and teeth are forming

Video: Ultrasound of Human Fetus 2 Third trimester Fetus grows rapidly and gains strength needed to survive outside uterus Circulatory and respiratory system change to allow air breathing Fetus gains ability to maintain its own temperature Bones harden, muscles thicken Video: Ultrasound of Human Fetus 2

27.18 Childbirth is hormonally induced and occurs in three stages The birth of a child is brought about by a series of strong, rhythmic contractions of the uterus Several hormones play key roles, under positive feedback control Estrogen Oxytocin Prostaglandins

LE 27-18a Estrogen Oxytocin from ovaries from fetus and pituitary Induces oxytocin receptors on uterus Stimulates uterus to contract Positive feedback Stimulates placenta to make Prostaglandins Stimulate more contractions of uterus

Labor occurs in three stages Dilation of the cervix Expulsion: delivery of the infant Delivery of the placenta

LE 27-18b Placenta Umbilical cord Uterus Cervix Dilation of the cervix Expulsion: delivery of the infant Uterus Placenta (detaching) Umbilical cord Delivery of the placenta

27.19 Reproductive technology increases our reproductive options CONNECTION 27.19 Reproductive technology increases our reproductive options Couples may be unable to conceive or bear a child for a variety of reasons Males: low sperm count, defective sperm, impotence Females: lack of ova, failure to ovulate, blocked oviducts, uterus won't support growing embryo

Reproductive technologies can help many cases of infertility Drug or hormone therapies for both sexes Penile implants Sperm, ova from donors Assisted reproductive technology In vitro fertilization Very expensive Entails some risks