Learning Objectives - Pregnancy and Human Development

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1

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.

2

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.

3

Define fertilization.

A sperm's chromosomes combine with those of an egg to form a fertilized egg (zygote).

4

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 formationInner cell mass•Becomes embryonic disc (→embryo and three of embryonic membranes)

5

Describe implantation.

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

6

Describe placenta formation, and list placental functions.

Formation of placentafrom embryonic and maternal tissues

Temporary organ

Embryonic tissues•Mesoderm cells develop from inner cell mass; line trophoblast•Together these form chorionand chorionic vill

Nutritive, respiratory, excretory, endocrine functions

7

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

8

Describe gastrulation and its consequence.

Occurs in week 3

oEmbryonic disc

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

9

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,

10

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)

11

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

12

Describe functional changes in maternal reproductive organs and in the cardiovascular, respiratory, and urinary systems during pregnancy.

Anatomical Changes

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

Ribs flare

thorax widens

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)

oUrinary System

↑Urine production due to ↑maternal metabolism and fetal wastes

Frequent, urgent urination; stress incontinence may occur as bladder compressed

oRespiratory System

Estrogens may cause nasal edema and congestion

Tidal volume increases

Dyspnea (difficult breathing) may occur later in pregnancy

oCardiovascular System

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

Homeostatic Imbalance

•PreeclampsiaoInsufficient placental blood supply--> fetus starved of oxygen

oWoman-->edematous, hypertensive, proteinuria

oMay be due to immunological abnormalities

13

Indicate the effects of pregnancy on maternal metabolism and posture.

•Metabolic Changes

oPlacental hormones

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

14

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, contractionsmore 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 StageFrom labor's onset to fully dilated cervix (10 cm)Longest stage of labor -6–12 hours or moreInitial weak contractions:•15–30 minutes apart, 10–30 seconds long•Become more vigorous and rapidCervix effacesand dilatesfully to 10 cmAmnion ruptures, releasing amniotic fluidEngagementoccurs -head enters true pelvis

oExpulsion StageFrom full dilation to delivery of infantStrong contractions every 2–3 minutes, about 1 minute longUrge to push increases (in absence of local anesthesia)Crowningoccurs when largest dimension of head distends vulva•Episiotomymay be done to reduce tearingVertexposition –head-first•Skull dilates cervix; early suctioning allowsbreathing prior to complete deliveryBreechposition –buttock-first•Delivery more difficult; often forceps required, or C-section(delivery through abdominal and uterine wall incision)

oPlacental StageStrong contractions continue, causing detachment of placenta and compression of uterine blood vessels•Limit bleeding; cause placental detachmentDelivery of afterbirth(placenta and membranes) occurs ~30 minutes after birthAll placenta fragments must be removed to prevent postpartum bleeding

15

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

16

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

17

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

18

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