Human Reproduction and Development: A Summary

AP BiologySpring 2011

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

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)

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

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

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

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)

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

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

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

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

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

Visual overview

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

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

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

http://legacy.owensboro.kctcs.edu/gcaplan/anat2/notes/Notes2%20female%20reproduction.htm

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The birth process (labor) begins with contractions of the uterine muscles

The cervical canal dilates and the amniotic sac ruptures

The hormone relaxin softens the cervical connective tissues and makes the bridges between the pelvic bones looser

The hormone oxytocin induces powerful uterine contractions

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

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

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

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

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

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

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

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

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

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)

Progestin injections (depro-provera) or implants (norplant) inhibit ovulation over several months

Morning after pill: intercept pregnancy by blocking fertilization or preventing implantation

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

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

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

STD’s infect about 15 million American’s each year

Social consequences, women develop more complication than men

HPV Trichomoniasis Chlamydia Genital herpes Gonorrhea Syphilis AIDS