■ Teratogensare defined as any drugs, viruses, infections, or other exposures that can cause embryonic/fetal devel- opmental abnormality (Table 3-2).
■ Birth defects can occur from genetic disorders or be the result of teratogen exposure.
■ The degree or types of malformation vary based on length of exposure, amount of exposure, and when it occurs during human development.
TABLE 3–2 TERATOGENIC AGENTS
AGENT EFFECT
DRUGS AND CHEMICALS ALCOHOL
ANGIOTENSIN- CONVERTING ENZYME (ACE) INHIBITORS
CARBAMAZEPINE (ANTICONVULSANTS)
COCAINE
WARFARIN (COUMADIN)
INFECTIONS/VIRUSES CYTOMEGALOVIRUS
HERPES VARICELLA (CHICKEN POX)
Increased risk of fetal alcohol syndrome occurring when the pregnant woman ingests six or more alcoholic drinks a day. No amount of alcohol is considered safe during preg- nancy. Newborn characteris- tics of fetal alcohol syndrome include:
• Low birth weight
• Microcephaly
• Mental retardation
• Unusual facial features due to midfacial hypoplasia
• Cardiac defects Increased risk for:
• Renal tubular dysplasia that can lead to renal failure and fetal or neonatal death
• Intrauterine growth restriction Increased risk for:
• Neural tubal defects
• Craniofacial defects, including cleft lip and palate
• Intrauterine growth restriction Increased risk for:
• Heart, limbs, face, gastroin- testinal tract, and genitouri- nary tract defects
• Cerebral infarctions
• Placental abnormalities Increased risk for:
• Spontaneous abortion
• Fetal demise
• Fetal or newborn hemorrhage
• Central nervous system abnormalities
Increased risk for:
• Hydrocephaly
• Microcephaly
• Cerebral calcification
• Mental retardation
• Hearing loss Increased risk for:
• Hypoplasia of hands and feet
• Blindness/cataracts
• Mental retardation
Continued
TABLE 3–2 TERATOGENIC AGENTS—cont’d
AGENT EFFECT
RUBELLA
SYPHILIS
TOXOPLASMOSIS
Sources: American College of Obstetricians and Gynecologists (ACOG; 1997);
Scanlon & Sanders (2007).
Increased risk for:
• Heart defects
• Deafness and/or blindness
• Mental retardation
• Fetal demise Increased risk for:
• Skin, bone and/or teeth defects
• Fetal demise Increased risk for:
• Fetal demise
• Blindness
• Mental retardation
Fallopian tube Fimbriae
Ovary Uterus
Sacrum
Cervix
Rectum
Anus Symphysis
pubis Urinary bladder Opening of ureter
Clitoris Urethra
Labium minor
Labium major Vagina
Bartholin's gland
Figure 3–1 Female reproductive system shown in a midsagittal section through the pelvic cavity.
■ The developing human is most vulnerable to the effects of teratogens during organogenesis, which occurs during the first 8 weeks of gestation. Exposure during this time can cause gross structural defects (American College of Obstetricians and Gynecologists [ACOG], 1997).
■ Exposure to teratogens after 13 weeks of gestation may cause fetal growth restriction or reduction of organ size (ACOG, 1997).
CRITICAL COMPONENT Teratogens
The developing human is most vulnerable to the effects of teratogens during the period of organogenesis, the first 8 weeks of gestation.
CRITICAL COMPONENT Toxoplasmosis
■ Can cause fetal demise, mental retardation, and blindness when the embryo is exposed to Toxoplasmaduring pregnancy (see Table 3-2).
■ Toxoplasmais a protozoan parasite found in cat feces and uncooked or rare beef and lamb.
■ Pregnant women or women who are attempting pregnancy need to avoid contact with cat feces (i.e., changing a litter box).
■ Pregnant women or women who are attempting pregnancy should avoid eating rare beef or lamb.
A
NATOMY AND PHYSIOLOGY REVIEW FemaleThe major structures and functions of the female reproductive system are (Figs. 3-1 and 3-2):
■ Ovaries
■ There are two oval shaped ovaries; one on each side of uterus.
■ The ovarian ligament and broad ligament help keep the ovaries in place.
C H A P T E R 3 ■ Genetics, Conception, Fetal Development, and Reproductive Technology 29
■ Primary follicles are present in the ovaries.
■ Several thousand follicles are present at birth.
■ Each follicle contains an oocyte.
■ The follicle cells secrete estrogen.
■ Graafian follicle is a mature follicle.
■ Fallopian tubes
■ There are two fallopian tubes (also referred to as oviducts).
■ The lateral end partially surrounds the ovary.
■ Fimbriae (fringelike projections) from the lateral end create a current within the fluid to pull the ovum into the fallopian tube (Scanlon & Sanders, 2007).
■ Peristaltic waves created by the smooth muscle contractions of the fallopian tubes move the ovum through the tube and into the uterus (Scanlon &
Sanders, 2007).
■ The medial end opens into the uterus.
■ Fertilization occurs within the fallopian tube.
■ Uterus
■ It is shaped like an upside-down pear.
■ It is approximately 3 inches long; 2 inches wide; 1 inch deep.
■ The fundusis the upper portion of the uterus.
■ The body of the uterus is the large central portion.
■ The cervix of the uterus is the narrow, lower end that opens to the vagina.
■ The inner lining of the uterus is the endometrium.
■ The endometrium consists of the basilar layer (a perma- nent layer) and the functional layer (a regenerative layer).
■ Estrogen and progesterone stimulate the functional layer of the endometrium to thicken in preparation for implantation.
■ The endometrium continues to thicken when implanta- tion occurs.
■ The functional layer is lost during the menstrual cycle when implantation does not occur.
■ Implantation normally occurs in the uterus.
■ The uterus expands during pregnancy to accommodate the developing embryo/fetus and placenta.
■ Vagina
■ A muscular tube approximately 4 inches in length that extends from the cervix to the perineum.
■ Functions
■ Receive sperm during sexual intercourse
■ Provide exit for menstrual blood flow
■ Birth canal during second stage of labor
■ External genital structure, also known as the vulva
■ Clitoris
■ A small mass of erectile tissue anterior to the urethral orifice
■ Responds to sexual stimulation.
■ Labia majora and minora
■ Paired folds of skin that cover the openings to the urethra and vagina and prevent drying of their mucous membranes.
■ Bartholin’s glands
■ They are located in the floor of the vestibule.
■ The ducts of the gland open onto the mucus of the vaginal orifice.
■ Their secretions keep the mucsa moist and lubricate the vagina during sexual intercourse.
Male
The major structures and functions of the male reproductive system are (Figs. 3-3 and 3-4):
■ Scrotum
■ A loose bag of skin and connective tissue which holds the testes suspended within it.
■ Temperature inside the scrotum is approximately 96°F, which is lower than the body temperature and necessary for the production of viable sperm (Scanlon & Sanders, 2007).
Ovary Corpus
luteum
Fertilization
of ovum Fallopian tube
Fundus
of uterus Ovarian ligament
Fimbriae
Mature follicle Broad
ligament Round ligament
Artery and vein
Body of uterus Endometrium Myometrium
Sperm Cervix of uterus
Vagina Rugae
Bartholin's gland Figure 3–2 Female reproductive system shown in anterior
view. The ovary left of the illustration has been sectioned to show the developing follicles. The fallopian tube at the left of the illustration has been sectioned to show fertiliza- tion. The uterus and vagina have been sectioned to show internal structures. Arrows indicate the movement of the ovum toward the uterus and the movement of sperm from the vagina toward the fallopian tube.
Symphysis pubis
Sacrum
Ductus deferens Urinary bladder Corpus cavernosum Corpus spongiosum
Cavernous urethra Glans penis
Prepuce
Scrotum Testis
Epididymis
Membranous urethra Anus
Opening of ureter Rectum Seminal vesicle Ejaculatory duct Prostate gland Bulbourethral gland
Figure 3–3 Male reproductive system shown in a midsagittal section through the pelvic cavity.
Spermatic cord Testicular artery and veins
Ductus deferens Nerve
Rete testis Epididymis Capillaries
Interstitial cells Sustentacular cell
Lumen
Seminiferous tubules Spermatozoa (sperm cells) Spermatids Spermatocytes Spermatogonia
B A
Figure 3–4 (A) Midsagittal section of portion of the testis; the epididymis is on the posterior side of the testis. (B) Cross section through a somniferous tubule showing development of the sperm.
■ Testes
■ A pair of testis lies suspended in the scrotum.
■ Testes develop in the fetus near the kidney and normally descend into the scrotum prior to birth.
■ Each testis is divided into lobes which contain several sem- iniferous tubules.
■ Spermatogenesis takes place in the seminiferous tubules.
■ Sustentacular (Sertoli) cells within the tubules produce the inhibin hormone when stimulated by testosterone.
■ Inhibin hormone decreases the secretion of follicle- stimulating hormone (FSH); decreased levels of FSH causes a decrease testosterone levels.
■ Sperm travel from the seminiferous tubules though the testis (a tubular network) and enter the epididymis.
■ Epididymis
■ A coiled tube-like structure on the posterior surface of each testis
■ Sperm complete their maturation within the epididymis.
C H A P T E R 3 ■ Genetics, Conception, Fetal Development, and Reproductive Technology 31
■ Ductus deferens, also referred to as vas deferens
■ It extends from the epididymis into the abdominal cavity.
In the abdominal cavity, it extends over the urinary blad- der and down the posterior side of the bladder and joins with the ejaculatory duct.
■ Ejaculatory ducts
■ There are two ejaculatory ducts.
■ They receive sperm from the ductus deferens and secretions from the seminal vesicles.
■ They empty into the urethra.
■ Seminal vesicles
■ They are located posterior to the urinary bladder.
■ They produce secretions that contain fructose, which is an energy source for sperm.
■ The secretions are alkaline, which enhances sperm motility.
■ Prostate gland
■ It is a muscular gland located below the urinary bladder.
■ It surrounds the first inch of the urethra as the urethra extends from the bladder.
■ It secretes an alkaline fluid that enhances sperm mobility.
■ Bulbourethral glands, also referred to as Cowper’s glands
■ They are located below the prostate gland.
■ They secrete an alkaline solution that coats the interior of the urethra to neutralize the acidic urine that is present.
■ Urethra
■ It is located within the penis.
■ It is the final duct that the semen passes through as it exits the body.
■ Penis
■ It is the external male genital organ.
■ It consists of smooth muscle, connective tissue, and blood sinuses.
■ The penis is flaccid when blood flow to the area is minimal.
■ The penis is erect when the arteries of the penis dilate and the sinuses fill with blood.
M
ENSTRUAL CYCLEA woman’s menstrual cycle is influenced by the ovarian cycle and endometrial cycle (Fig. 3-5).
Ovarian Cycle
Theovarian cyclepertains to the maturation of ova and con- sists of three phases:
■ Thefollicular phasebegins the first day of menstruation and last 12–14 days. During this phase, the graafian follicle is maturing under the influence of two pituitary hormones:
luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The maturing graafian follicle produces estrogen.
Progesterone
Estrogen FSH
LH
Endometrial changes during the menstrual cycle Ovarian cycleHormone levels (relative scale)
Functional layer Basilar
layer
Primary follicle
Secondary follicles
Graafian follicle
Corpus luteum
Corpus albicans Ovulation
0 5 10 15 20 25 28
Days Menstrual
flow
Figure 3–5 The menstrual cycle. The levels of the major hormones are shown in relation- ship to one another throughout the cycle.
Changes in the ovarian follicle are depicted.
The relative thickness of the endometrium is also shown.
■ Theovulatory phasebegins when estrogen levels peak and ends with the release of the oocyte (egg) from the mature graafian follicle. The release of the oocyte is referred to as ovulation.
■ There is a surge in LH levels 12–36 hours before ovulation.
■ There is a decrease in estrogen levels and an increase in progesterone levels before the LH surge.
■ Theluteal phasebegins after ovulation and lasts approxi- mately 14 days. During this phase, the cells of the empty follicle undergo changes and form into the corpus luteum.
■ The corpus luteum produces high levels of progesterone along with low levels of estrogen.
■ If pregnancy occurs, the corpus luteum continues to release progesterone and estrogen until the placenta matures and assumes this function.
■ If pregnancy does not occur, the corpus luteum degenerates and results in a decrease in progesterone and the beginning of menstruation.
Endometrial Cycle
The endometrial cycle pertains to the changes in the endometrium of the uterus in responses to the hormonal changes that occur during the ovarian cycle. This cycle con- sists of three phases:
■ Theproliferative phase occurs following menstruation and ends with ovulation. During this phase, the
endometrium is preparing for implantation by becoming thicker and more vascular. These changes are in response
to the increasing levels of estrogen produced by the graafian follicle.
■ Thesecretory phase begins after ovulation and ends with the onset of menstruation. During this phase, the endometrium continues to thicken. The primary hor- mone during this phase is progesterone, which is secreted from the corpus luteum.
■ If pregnancy occurs, the endometrium continues to develop and begins to secrete glycogen.
■ If pregnancy does not occur, the corpus luteum begins to degenerate and the endometrial tissue degenerates.
■ Themenstrual phaseoccurs in response to hormonal changes and results in the sloughing off of the endome- trial tissue.