human reproduction and development: a summary ap biology spring 2011
TRANSCRIPT
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Human Reproduction and Development: A Summary
AP BiologySpring 2011
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Structure and function Human males have 2 testes located in the
scrotum, which is a few degrees cooler than body temperature for proper sperm development
This is site of sperm development Testosterone, a sex hormone is also produced in the
testes The male reproductive system includes accessory
organs, glands, and ducts Sperm production begins during puberty, the
stage when secondary sexual characteristics emerge
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Reproductive ducts and accessory glands Each testis contains many seminiferous
tubules where sperm are continuously formed
Sperm move from a testis epididymis (for maturation and storage) vas deferens ejaculatory ducts urethra (located inside penis)
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The sperm-bearing fluid (semen) is formed by secretions from the seminal vesicles (fructose and prostaglandins) and the prostate (buffers against acidic vagina)
The bulbourethral glands secrete a mucus-rich fluid into the vagina during sexual arousal
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Cancers of prostate and testes In the US, more than 200,000 males are
diagnosed with prostate cancer each year and about 35,000 die
There are about 7,000 cases of testicular cancer in the same year in US
Routine examination and blood tests are necessary to detect these subtle cancers
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Sperm form in the walls of the seminiferous tubules of the testes
Diploid spermatogonia undergo mitosis primary spermatocytes, which undergo meiosis 1 haploid secondary spermatocytes, which undergo meiosis 2 haploid spermatids mature sperm
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Sertoli cells in the tubule provide nourishment and chemical signals to the developing sperm
Each sperm has a head (nucleus and cap), midpiece (mitochondria), and tail (flagellum)
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Testosterone, produced by Leydig cells located between lobes in testes Stimulates spermatogenesis, formation of
reproductive organs and secondary sex characteristics and helps to develop and maintain normal sexual behavior
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LH (luteinizing hormone): released from anterior pituitary (under prodding by GnRH from hypothalamus) and stimulates testosterone production
GnRH also causes pituitary to release FSH, which stimulates the production of sperm, beginning at puberty
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Components of system The egg is released from the ovary
oviduct uterus (zygote will implant in its lining, the endometrium)
The lower part of the uterus is the cervix, which extends into the vagina, which in turn leads to the outer genitalia: labia majora, labia minora, and clitoris
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Overview of menstrual cycle Most female mammals follow an estrous
cycle; humans and other primates have a menstrual cycle (there is no relationship between heat and fertility)
During each cycle an oocyte matures and escapes from the ovary and (if it is fertilized) may implant in the endometrium
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If there is no implantation, the uterine lining is sloughed at the end of each cycle of (approximately) 28 days
A decline in hormone secretions correlates with the onset of menopause, the cessation of a female’s reproductive capacity
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Visual overview
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From primary to secondary oocytes At birth about 2 million immature eggs
(primary oocytes) are already present and arrested in meiosis 1
Of the approximately 300,000 oocytes still present at age 7, only about 400-500 will mature in a lifetime
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From primary to secondary oocytes The follicle consists of a layer of cells
(granulosa) surrounding the primary oocyte; the granulosa cells gradually deposit a layer of material around the follicle
During the menstrual cycle, one oocyte resumes meiosis 1 to form a secondary oocyte and a polar body (both haploid)
At about mid-cycle, there is a surge of LH that causes ovulation- the release of the secondary oocyte
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Signaling pathways Estrogen and progesterone stimulate oocytes
to mature and prime endometrium for pregnancy
During the first half of the cycle, the hypothalamus signals the anterior pituitary to release LH and FSH, which in turn stimulate the ovary to secrete estrogen
The corpus luteum persists for about 12 days, secreting progesterone that inhibits further FSH and LH secretion
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http://legacy.owensboro.kctcs.edu/gcaplan/anat2/notes/Notes2%20female%20reproduction.htm
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What if no pregnancy? If fertilization does not occur, the corpus
luteum degenerates, progesterone and estrogen levels fall, and FSH and LH are again secreted to begin another cycle
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Sexual intercourse In male sexual arousal, the spongy tissue
spaces inside the penis become filled with blood to cause an erection
In coitus, mechanical stimulation of the penis causes involuntary contractions that force semen out and into vagina
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Sexual Intercourse Ejaculation in the male, and similar
contractions in the female, are termed orgasm
Viagra medication is prescribed to counter erectile dysfunction in men
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Fertilization Of the 150 million to 350 million sperm
deposited in the vagina during intercourse, only a few hundred ever reach the upper region of the oviduct where fertilization occurs
Sperm may live for about three days following ejaculation
About 30 minutes after being deposited in the vagina the sperm may reach the oviducts, the entry to fallopian tubes
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Fertilization Only one sperm will successfully enter the cytoplasm of the
secondary oocyte after digesting its way through the zona pellucida
The arrival of that sperm stimulates the completion of meiosis 2, which yields a mature ovum
The sperm nucleus fuses with the egg nucleus to restore the diploid chromosome number
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Cleavage and implantation During the first few days after fertilization,
the zygote undergoes repeated cleavages as it travels down the oviduct
By the time it reaches the uterus, it is a solid ball of cells (morula), which is transformed into a blastocyst
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Cleavage and implantation Implantation begins about 6 days after
fertilization The inner cell mass of the blastocyst is
transformed into an embryonic disk that will develop into the embryo proper within the next week
Ectopic pregnancies occur when the blastocyst implants in tissue other than the uterus and cannot be carried to term
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Extraembryonic membranes Membranes start forming outside the
embryo and have several functions The amnion is a fluid-filled sac that keeps
the embryo from drying out and acts as a shock absorber
The yolk sac becomes a site for blood cell formation
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Extraembryonic membranes Chorion: protective membrane around the
embryo, forms a portion of the placenta and secretes a hormone (human chorionic gonadotropin) that maintains the uterine lining after implantation
Allantois: does not function in waste storage (as it does in birds) but is active in blood formation and formation of the urinary bladder
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By 2 weeks after fertilization, a two-layered embryonic disk consisting of ectoderm and endoderm has formed
Some cells also form the notochord, from which the vertebrate will form
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Toward the end of the third week, mesoderm has developed and is giving rise to somites Somites: segments of bones and skeletal
muscles Pharyngeal arches start to form that will
contribute to the face, neck, mouth, nose, larynx, and pharynx Spaces in mesoderm will become the coelomic
cavity
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The placenta is a combination of uterine lining and extraembryonic membranes At full term, the placenta
will cover approximately one-fourth of inner surface of the uterus
Placenta forms early in pregnancy
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Materials are exchanged from blood capillaries of mother to fetus, and vice versa, by diffusion The maternal blood and fetal blood do not
mix After third month, the placenta produces
large amounts of HCG, progesterone, and estrogen to maintain uterine lining
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By the end of the fourth week, the embryo is 500 times large than when it started but still smaller than a centimeter in length
It has embarked on an intricate program of cell differentiation and morphogenesis, including development of limbs, circulation, and umbilical cord
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The second trimester encompasses months 4, 5, and 6 The individual is now called a fetus The heart is beating Fuzzy hair covers the body
The 3rd trimester extends from month 7 until birth
Earliest delivery in which survival on its own is possible is the middle of this trimester
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The birth process (labor) begins with contractions of the uterine muscles
The cervical canal dilates and the amniotic sac ruptures
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The hormone relaxin softens the cervical connective tissues and makes the bridges between the pelvic bones looser
The hormone oxytocin induces powerful uterine contractions
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Fetus is expelled accompanied by fluid and blood
The umbilical cord is severed, and finally the placenta is expelled
Corticotropin-releasing hormone affects the timing of labor, and it may contribute to post-partum depression
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Nourishing the newborn Mammary glands first produce a special fluid
for the newborn; then, under the influence of prolactin, they produce milk
Oxytocin is released in response to suckling and further increases the milk supply
Besides providing nourishment, human breast milk contains antibodies that protect a newborn from some viruses and bacteria
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Nutritional Considerations A well-balanced diet usually supplies the
carbohydrates, lipids, and proteins that the embryo requires but additional vitamins and minerals are required
Fetal organs are highly vulnerable to nutritional deficiencies
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Morning sickness Two thirds of women experience morning
sickness from weeks 6-12 of pregnancy Morning sickness may have an adaptive
function providing protection against teratogens and other harmful substances
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Infectious diseases Some antibodies crossing the placenta
from mother to child can protect against many bacterial infections
The viral disease rubella can cause improper organ development at certain critical periods in the fetus’ life
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Alcohol and Caffeine Fetal alcohol syndrome symptoms include
reduced brain size, mental impairment, slow growth, and heart defects
There is probably no “safe” drinking level Caffeine interferes with nervous system
development in animals and may harm human embryos as well
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Smoking Toxic elements in tobacco smoke impair fetal
growth, even if the smoke is secondhand Infants of smokers have more heart abnormalities,
are smaller, and have academic difficulty in school Prescription drugs
Drugs must be carefully monitored during pregnancy
Tranquilizers, anti-acne drugs, and even antibiotics cannot be taken
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Birth control options Abstinence is most effective, but may be
unrealistic Rhythm method: no intercourse during the
days when an egg is capable of being fertilized
Withdrawal before ejaculation is NOT effective
Douching ineffective due to speed with which sperm enters uterus
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Surgery to cut and tie oviducts (tubal ligation) or vas deferens (vasectomy) is effective and considered an irreversible method to prevent sperm and egg union
Spermicidal foam and jelly: toxic to sperm, not reliable unless used in combination with barrier device like diaphragm
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Diaphragm: fits over cervix and prevents entry of sperm into uterus
Condoms: prevent sperm deposition in vagina
Birth control pill: contains synthetic female hormones and prevents ovulation when taken faithfully (patch applies same hormones to skin)
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Progestin injections (depro-provera) or implants (norplant) inhibit ovulation over several months
Morning after pill: intercept pregnancy by blocking fertilization or preventing implantation
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About 10% of women who become pregnant lose embryo or fetus in a spontaneous abortion or miscarriage Some estimates: 50% of all fertilized eggs
are lost, most due to genetic problems Risk increases with age
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About half of all unplanned pregnancies end in an induced abortion, the deliberate dislodging and removal of an embryo or fetus from the uterus
This is generally a low risk procedure during the first 3 months of pregnancy
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In vitro fertilization is conception outside body Horomone injections prepare ovaries for
ovulation Oocyte is withdrawn and sperm injected
into it A few days later, a ball of cells is transferred
to the women’s uterus for gestation Costly, usually end in failure
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STD’s infect about 15 million American’s each year
Social consequences, women develop more complication than men
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HPV Trichomoniasis Chlamydia Genital herpes Gonorrhea Syphilis AIDS