Learning Objectives - Pregnancy and Human Development
Describe the importance of sperm capacitation.
It allows the motility to be enhanced and the membranes must become fragile so that the enzymes can be released. Prevents the spilling of acrosomal enzymes.
Explain the mechanism of the slow block to polyspermy.
The spilled material binds to water, and as the material swells and hardens, it detaches all sperm still bound to the receptors on the oocyte membrane.
A sperm's chromosomes combine with those of an egg to form a fertilized egg (zygote).
Describe the process and product of cleavage
A period of fairly rapid mitotic division of the zygote without intervening growth. Goal is to produce small cels with a high surface-to-volume ratio. End result is a morula.
Occurs while zygote moves toward uterus
Mitotic divisions of zygote
First cleavage at 36 hours →two daughter cells (blastomeres)
At 72 hours →morula(16 or more cells)oAt day 4 or 5, blastocyst(embryo of ~100 cells) reaches uterusoBlastocyst -fluid-filled hollow sphere composed of
Trophoblast cells•Display immunosuppressive factors•Participate in placenta formationInner cell mass•Becomes embryonic disc (→embryo and three of embryonic membranes)
During a surge of estrogen and progesterone, if the mucosa is properly prepared, the blastocyte attaches to the endometrial cells and to selectin-binding carbohydrates on the inner uterine wall.
Implantationbegins 6–7 days after ovulation
Trophoblast cells adhere to site with proper receptors and chemical signals
Inflammatory-like response occurs in endometrium•Uterine blood vessels more permeable and leaky; inflammatory cells invade areaoTrophoblasts proliferate and form two distinct layers
Cytotrophoblast(cellular trophoblast) -inner layer of cells
Syncytiotrophoblast(syncytial trophoblast) -cells in outer layer lose plasma membranes, invade and digest endometriumoBlastocyst burrows into lining surrounded by pool of leaked blood; endometrial cells cover and seal off implanted blastocystoImplantation completed by twelfth day after ovulation
Menstruation must be prevented
Corpus luteum maintained by hormone human chorionic gonadotropin(hCG)•Hormonal Changes During PregnancyoHuman chorionic gonadotropin (hCG)Secreted by trophoblast cells; later chorion
Describe placenta formation, and list placental functions.
Formation of placentafrom embryonic and maternal tissues
Embryonic tissues•Mesoderm cells develop from inner cell mass; line trophoblast•Together these form chorionand chorionic vill
Nutritive, respiratory, excretory, endocrine functions
Name and describe the formation, location, and function of the extraembryonic membranes.
Amnion-epiblast cells form transparent sac filled with amniotic fluid
Provides buoyant environment that protects embryo
Helps maintain constant homeostatic temperature
Allows freedom of movement; prevents parts from fusing together
Amniotic fluid comes from maternal blood, and later, fetal urineo
Yolk sac-sac that hangs from ventral surface of embryo
Forms part of digestive tube
Source of earliest blood cells and blood vesselso
Allantois-small outpocketing at caudal end of yolk sac
Structural base for umbilical cord
Becomes part of urinary bladdero
Chorion-helps form placenta
Encloses embryonic body and all other membranes
Describe gastrulation and its consequence.
Occurs in week 3
three-layered embryowith primary germ layerspresent
Ectoderm, mesoderm, and endodermo
Begins with appearance of primitive streak, raised dorsal groove; establishes longitudinal axis of embryoo
Cells begin to migrate into groove
First cells form endoderm
Cells that follow push laterally, forming mesoderm•Notochord-rod of mesodermal cells that serves as axial support
Cells that remain on embryo's dorsal surface form ectodermo
Ectoderm, mesoderm, endoderm -primitive tissues from which all body organs derive
Define organogenesis and indicate the important roles of the three primary germ layers in this process.
Organogenesis: Differentiation of the Germ Layers
oGastrulation sets stage for organogenesis
Formation of body organs and systems
Specialization of the Endoderm
Primitive gutformed from endodermal folding
•Forms epithelial lining of GI tract
•Organs of GI tract become apparent, and oral and anal openings perforate
Specialization of the Ectoderm
Neurulation•First major event of organogenesis•Gives rise to brain and spinal cord•Induced by chemical signals from notochord•Ectoderm over notochord thickens, forming neural plate•Neural plate folds inward as neural groovewith neural folds
By 22nd day, neural folds fuse into neural tube•Anterior end →brain; rest →spinal cord
Neural crest cellsmigrate widely →cranial, spinal, and sympathetic ganglia and nerves; adrenal medulla; pigment cells of skin; contribute to some connective tissues
Brain waves recorded by end of second month
Specialization of the Mesoderm
-Somites, skin, heart, bones,
Describe unique features of the fetal circulation.
First blood cells arise in yolk sac
•By end of third week
oEmbryo has system of paired vessels
oTwo vessels forming heart have fused; bent into "S" shape
Heart beats by 3½ weeks
•Unique vascular modificationsoUmbilical arteriesand umbilical veino
Three vascular shunts•All occluded at birth•Vascular shunts
oDuctus venosus-bypasses liver (umbilical vein →ductus venosus →IVC)
oForamen ovale-opening in interatrial septum; bypasses pulmonary circulation
oDuctus arteriosus-bypasses pulmonary circulation (pulmonary trunk →ductus arteriosus →aorta)
Indicate the duration of the fetal period, and note the major events of fetal development.
Fetal period -weeks 9 through 38
•Time of rapid growth of body structures established in embryo
Describe functional changes in maternal reproductive organs and in the cardiovascular, respiratory, and urinary systems during pregnancy.
oReproductive organs become engorged with blood
Chadwick's sign-vagina develops purplish hue
Breasts enlarge and areolae darken
Pigmentation of facial skin many increase (chloasma)
oUterus expands, occupying most of abdominal cavity
oLordosis occurs with change in center of gravity
oRelaxincauses pelvic ligaments and pubic symphysis to relax to ease birth passage
oWeight gain of ~13 kg (28 lb)
↑Urine production due to ↑maternal metabolism and fetal wastes
Frequent, urgent urination; stress incontinence may occur as bladder compressed
Estrogens may cause nasal edema and congestion
Tidal volume increases
Dyspnea (difficult breathing) may occur later in pregnancy
Bloodvolume increases 25–40%•Safeguards against blood loss during childbirth
Cardiac output rises as much as 35-40%
•Propels greater volume around body
Venous return from lower limbs may be impaired, resulting in varicose veins
•PreeclampsiaoInsufficient placental blood supply--> fetus starved of oxygen
oWoman-->edematous, hypertensive, proteinuria
oMay be due to immunological abnormalities
Indicate the effects of pregnancy on maternal metabolism and posture.
Human placental lactogen(hPL) (human chorionic somatomammotropin (hCS))
•→maturation of breasts, fetal growth, and glucose sparing in mother (reserving glucose for fetus)
oParathyroid hormone and vitamin D levels high throughout pregnancy -->adequate calcium for fetal bone mineralization
Explain how labor is initiated, and describe the three stages of labor.
-Initiation of Labor
oFetus determines own birth date
oDuring last few weeks of pregnancy
Fetal secretion of cortisol stimulates placenta to secrete more estrogen
•Causes production of oxytocin receptors by myometrium
•Causes formation of gap junctions between uterine smooth muscle cells
•Antagonizes calming effects of progesterone, leading to Braxton Hicks contractionsin uterus
oFetal oxytocincauses placenta to produce prostaglandins
oOxytocin and prostaglandins -powerful uterine muscle stimulants
Due especially to prostaglandins, contractionsmore frequent and vigorous
Anti-prostaglandins contraindicated during labor
oIncreasing cervical distension
Activates hypothalamus, causing oxytocin release from posterior pituitary
Positive feedback mechanism occurs
•Greater distension of cervix -->more oxytocin release-->greater contractile force -->greater distension of cervix -->etc
oDilation StageFrom labor's onset to fully dilated cervix (10 cm)Longest stage of labor -6–12 hours or moreInitial weak contractions:•15–30 minutes apart, 10–30 seconds long•Become more vigorous and rapidCervix effacesand dilatesfully to 10 cmAmnion ruptures, releasing amniotic fluidEngagementoccurs -head enters true pelvis
oExpulsion StageFrom full dilation to delivery of infantStrong contractions every 2–3 minutes, about 1 minute longUrge to push increases (in absence of local anesthesia)Crowningoccurs when largest dimension of head distends vulva•Episiotomymay be done to reduce tearingVertexposition –head-first•Skull dilates cervix; early suctioning allowsbreathing prior to complete deliveryBreechposition –buttock-first•Delivery more difficult; often forceps required, or C-section(delivery through abdominal and uterine wall incision)
oPlacental StageStrong contractions continue, causing detachment of placenta and compression of uterine blood vessels•Limit bleeding; cause placental detachmentDelivery of afterbirth(placenta and membranes) occurs ~30 minutes after birthAll placenta fragments must be removed to prevent postpartum bleeding
Outline the events leading to the first breath of a newborn.
•Taking the First Breath
o↑CO2→central acidosis →stimulates respiratory control centers to trigger first inspiration
Requires tremendous effort –airways tiny; lungs collapsed
Surfactant in alveolar fluid helps reduce surface tension
oRespiratory rate ~45 per minute first two weeks, then declines
oKeeping lungs inflated difficult for premature infant (< 2500 g, or 5.5 pounds, at birth)
Surfactant production in last months of prenatal life
Preemies usually on respiratory assistance until lungs mature
Describe changes that occur in the fetal circulation after birth.
•Occlusion of Special Fetal Blood Vessels and Vascular Shunts
oUmbilical arteries and vein constrict and become fibrosed
oProximal umbilical arteries →superior vesical arteriesto urinary bladder
oDistal umbilical arteries →medial umbilical ligaments
oUmbilical vein becomes round ligament of liver(ligamentum teres)
oDuctus venosus→ligamentum venosumabout 30 minutes after birth
oPressure changes from infant breathing cause pulmonary shunts to close
Foramen ovale→fossa ovalisup to a year after birth
Ductus arteriosus→ligamentum arteriosumabout 30 minutes after birth
Explain how the breasts are prepared for lactation.
•Production of milk by mammary glands
•Toward end of pregnancy
oPlacental estrogens, progesterone, and human placental lactogen stimulate hypothalamus to release prolactin-releasing factors (PRFs)
oAnterior pituitary releases prolactin
2-3 days later true milk production begins
•Colostrumo=Less lactose but more protein, vitamin A, minerals than true milk; almost no fat
oYellowish secretion rich in IgA antibodies
IgA resistant to digestion; may protect infant against bacterial infection; absorbed into bloodstream for immunity
oReleased first 2–3 daysoFollowed by true milk production
•Prolactin release wanes after birth
•Lactation sustained by mechanical stimulation of nipples –suckling
oSuckling causes afferent impulses to hypothalamus -->prolactin -->stimulates milk production for next feeding
Hypothalamus also -->oxytocin from posterior pituitary -->let-down reflex
Describe some techniques of ART including IVF, ZIFT, and GIFT.
•In vitro fertilization(IVF)
oOocytes and sperm incubated in culture dishes for several days
oEmbryos (two-cell to blastocyst stage) transferred to uterus for possible implantation
•Zygote intrafallopian transfer(ZIFT)
oFertilized oocytes transferred to uterine tubes
•Gamete intrafallopian transfer(GIFT)
oSperm and harvested oocytes are transferred together into the uterine tubes