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Chapter Notes for Lecture: E.N. Marieb, HUMAN ANATOMY & PHYSIOLOGY,5TH Edition, , Benjamine Cummings Publisher, 2001 Prepare from : V.A. Austin’s PowerPpoint Presentation (ISBN: 0-8053-5469-7), CD ROM: Pearson Education, Inc. , 2003.

 

Chapter 29

Pregnancy and Human Development

 

From Egg to Embryo

•      Pregnancy – events that occur from fertilization until the infant is born

•      Conceptus – the developing offspring

•      Gestation period – from the last menstrual period until birth

•      Preembryo – conceptus from fertilization until it is two weeks old

•      Embryo – conceptus during the third through the eighth week

•      Fetus – conceptus from the ninth week through birth

Accomplishing Fertilization

•      The oocyte is viable for 12 to 24 hours

•      Sperm is viable 24 to 72 hours

•      For fertilization to occur, coitus must occur no more than:

•    Three days before ovulation

•    24 hours after ovulation

•      Fertilization – when a sperm fuses with an egg to form a zygote

Sperm Transport and Capacitation

•      Fates of ejaculated sperm include:

•    Leak out of the vagina immediately after deposition

•    Destroyed by the acidic vaginal environment

•    Fail to make it through the cervix

•    Dispersed in the uterine cavity or destroyed by phagocytic leukocytes

•    Reach the uterine tubes

•      Sperm must undergo capacitation before they can penetrate the oocyte

Acrosomal Reaction and Sperm Penetration

•      An ovulated oocyte is encapsulated by :

•    The corona radiata

•    The zona pellucida

•      Sperm binds to the zona pellucida and undergoes the acrosomal reaction

•    Enzymes are released near the oocyte

•    Hundreds of acrosomes release their enzymes to digest the zona pellucida

Acrosomal Reaction and Sperm Penetration

•      Once a sperm makes contact with the oocyte’s membrane:

•    Beta protein finds and binds to receptors on the oocyte membrane

•    Alpha protein causes it to insert into the membrane

Blocks to Polyspermy

•      Only one sperm is allowed to penetrate the oocyte

•      Two mechanisms ensure monospermy

•    Fast block to polyspermy – membrane depolarization prevents sperm from fusing with the oocyte membrane

•    Slow block to polyspermy

•   The cortical granules release enzymes that destroy sperm receptors

•   These enzymes cause sperm already bound to receptors to detach

Completion of Meiosis II and Fertilization

•      Upon entry of sperm, the secondary oocyte:

•    Completes meiosis II

•    Casts out the second polar body

•      The ovum nucleus swells, and the two nuclei approach each other

•      When fully swollen, the two nuclei are called pronuclei

•      Fertilization – when the pronuclei come together

Preembryonic Development

•      The first cleavage produces two daughter cells called blastomeres

•      Morula – the 16 or more cell stage (72 hours old)

•      By the fourth or fifth day the preembryo consists of 100 or so cells (blastocyst)

•      Blastocyst – a fluid-filled hollow sphere composed of:

•    A single layer of trophoblasts

•    An inner cell mass

•      Trophoblasts take part in placenta formation

•      The inner cell mass becomes the embryonic disc

Implantation

•      Begins six to seven days after ovulation when the trophoblasts adhere to the endometrium

•      The trophoblasts then proliferate and form two distinct layers

•     Cytotrophoblast – cells of the inner layer that retain their cell boundaries

•     Syncytiotrophoblast – cells in the outer layer that lose their plasma membranes and invade the endometrium

•      The implanted blastocyst is covered over by endometrial cells

•      Implantation is completed by the fourteenth day after ovulation

•      Viability of the corpus luteum is maintained by human chorionic gonadotropin (hCG) secreted by the trophoblasts

•      hCG prompts the corpus luteum to continue to secrete progesterone and estrogen

•      Chorion – developed from trophoblasts after implantation, continues this hormonal stimulus

•      Between the second and third month, the placenta:

•    Assumes the role of progesterone and estrogen production

•    Is providing nutrients and removing wastes

Placentation

•      Formation of the placenta from:

•    Embryonic trophoblastic tissues

•    Maternal endometrial tissues

•      The chorion develops fingerlike villi, which:

•    Become vascularized

•    Extend to the embryo as umbilical arteries and veins

•    Lie immersed in maternal blood

•      Decidua basalis – part of the endometrium that lies between the chorionic villi and the stratum basalis

•      Decidua capsularis – part of the endometrium surrounding the uterine cavity face of the implanted embryo

•      The placenta is fully formed and functional by the end of the third month

•      Embryonic placental barriers include:

•    The chorionic villi

•    The endothelium of embryonic capillaries

•      The placenta also secretes other hormones – human placental lactogen, human chorionic thyrotropin, and relaxin

Gem Layers

•      The blastocyst develops into a gastrula with three primary germ layers: ectoderm, endoderm, and mesoderm

•      Before becoming three-layered, the inner cell mass subdivides into the upper epiblast and lower hypoblast

•      These layers form two of the four embryonic membranes

Embryonic Membranes

•      Amnion – epiblast cells form a transparent membrane filled with amniotic fluid

•    Provides a buoyant environment that protects the embryo

•    Helps maintain a constant homeostatic temperature

•    Amniotic fluid comes from maternal blood, and later, fetal urine

•      Yolk sac – hypoblast cells that form a sac on the ventral surface of the embryo

•    Forms part of the digestive tube

•    Produces earliest blood cells and vessels

•    Is the source of primordial germ cells

•      Allantois – a small outpocketing at the caudal end of the yolk sac

•    Structural base for the umbilical cord

•    Becomes part of the urinary bladder

•      Chorion – helps form the placenta

•    Encloses the embryonic body and all other membranes

Gastrulation

•      During the 3rd week, the two-layered embryonic disc becomes a three-layered embryo

•      The primary germ layers are ectoderm, mesoderm, and endoderm

•      Primitive streak – raised dorsal groove that establishes the longitudinal axis of the embryo

•      As cells begin to migrate:

•    The first cells that enter the groove form the endoderm

•    The cells that follow push laterally between the cells forming the mesoderm

•    The cells that remain on the embryo’s dorsal surface form the ectoderm

•      Notochord – rod of mesodermal cells that serves as axial support

Primary Germ Layers

•      Serve as primitive tissues from which all body organs will be derived

•      Ectoderm – forms structures of the nervous system and skin epidermis

•      Endoderm – forms epithelial linings of the digestive, respiratory, and urogenital systems

•      Mesoderm – forms all other tissues

•      Endoderm and ectoderm are securely joined and are considered epithelia

Organogenesis

•      Gastrulation sets the stage for organogenesis, formation of body organs

•      By the 8th week all organ systems are recognizable

Specialization of Ectoderm

•      Neurulation – the first event of organogenesis gives rise to the brain and spinal cord

•      Ectoderm over the notochord thickens, forming the neural plate

•      The neural plate folds inward as a neural groove with prominent neural folds

•      By the 22nd day, neural folds fuse into a neural tube, which pinches off into the body

•      The anterior end becomes the brain; the rest becomes the spinal cord

•      Associated neural crest cells give rise to cranial, spinal, and sympathetic ganglia

Specialization of Endoderm

•      Embryonic folding begins with lateral folds

•      Next, head and tail folds appear

•      An endoderm tube forms the epithelial lining of the GI tract

•      Organs of the GI tract become apparent, and oral and anal openings perforate

•      Endoderm forms epithelium linings of the hollow organs of the digestive and respiratory tracts

Specialization of the Mesoderm

•      First evidence is the appearance of the notochord

•      Three mesoderm aggregates appear lateral to the notochord

•    Somites, intermediate mesoderm, and double sheets of lateral mesoderm

•      The 40 pairs of somites have three functional parts:

•    Sclerotome – produce the vertebrae and ribs

•    Dermatome – help form the dermis of the skin on the dorsal part of the body

•    Myotome – form the skeletal muscles of the neck, trunk, and limbs

•      Intermediate mesoderm forms the gonads and the kidneys

•      Lateral mesoderm consists of somatic and splanchnic mesoderm

•      Somatic mesoderm forms the:

•    Dermis of the skin in the ventral region

•    Parietal serosa of the ventral body cavity

•    Bones, ligaments, and dermis of the limbs

•      Splanchnic mesoderm forms:

•    The heart and blood vessels

•    Most connective tissue of the body

Development of Fetal Circulation

•      By the end of the 3rd week:

•    The embryo has a system of paired vessels

•    The vessels forming the heart have fused

•      Unique vascular modifications seen in prenatal development include umbilical arteries and veins, and three vascular shunts (occluded at birth)

•    Ductus venosus – venous shunt that bypasses the liver

•    Foramen ovale – opening in the interatrial septa to bypass pulmonary circulation

•    Ductus arteriosus – transfers blood from the right ventricle to the aorta

Effects of Pregnancy: Anatomical Changes

•      Chadwick’s sign – the vagina develops a purplish hue

•      Breasts enlarge and their areolae darken

•      The uterus expands, occupying most of the abdominal cavity

•      Lordosis is common due to the change of the body’s center of gravity

•      Relaxin causes pelvic ligaments and the pubic symphysis to relax

•      Typical weight gain is about 29 pounds

Effects of Pregnancy: Metabolic Changes

•      The placenta secretes human placental lactogen (hPL), also called human chorionic somatomammotropin (hCS), which stimulates the maturation of the breasts

•      hPL promotes growth of the fetus and exerts a maternal glucose-sparing effect

•      Human chorionic thyrotropin (hCT) increases maternal metabolism

•      Parathyroid hormone levels are high, ensuring a positive calcium balance

Effects of Pregnancy: Physiological Changes

•      GI tract – morning sickness occurs due to elevated levels of estrogen and progesterone

•      Urinary tract – urine production increases to handle the additional fetal wastes

•    Respiratory – edematous and nasal congestion may occur

•      Dyspnea (difficult breathing) may develop late in pregnancy

•      Cardiovascular system – blood volume increases 25-40%

•    Venous pressure from lower limbs is impaired, resulting in varicose veins

Parturition: Initiation of Labor

•      Estrogen reaches a peak during the last weeks of pregnancy causing myometrial weakness and irritability

•      Weak Braxton Hicks contractions may take place

•      As birth nears, oxytocin and prostaglandins cause uterine contractions

•      Emotional and physical stress:

•    Activates the hypothalamus

•    Sets up a positive feedback mechanism, releasing more oxytocin

Stages of Labor: Dilation Stage

•      From the onset of labor until the cervix is fully dilated (10 cm)

•      Initial contractions are 15–30 minutes apart and 10–30 seconds in duration

•      The cervix effaces and dilates

•      The amnion ruptures, releasing amniotic fluid (breaking of the water)

•      Engagement occurs as the infant’s head enters the true pelvis

Stages of Labor: Expulsion Stage

•      From full dilation to delivery of the infant

•      Strong contractions occur every 2–3 minutes and last about 1 minute

•      The urge to push increases in labor without local anesthesia

•      Crowning occurs when the largest dimension of the head is distending the vulva

Stages of Labor: Placental Stage

•      The delivery of the placenta is accomplished within 30 minutes of birth

•      Afterbirth – the placenta and its attached fetal membranes

•      All placenta fragments must be removed to prevent postpartum bleeding

Extrauterine Life

•      At 1-5 minutes after birth, the infant’s physical status is assessed based on five signs:  heart rate, respiration, color, muscle tone, and reflexes

•      Each observation is given a score of 0 to 2

•      Apgar score – the total score of the above assessments

•    8-10 indicates a healthy baby

•    Lower scores reveal problems

First Breath

•      Once carbon dioxide is no longer removed by the placenta, central acidosis occurs

•      This excites the respiratory centers to trigger the first inspiration

•      This requires tremendous effort – airways are tiny and the lungs are collapsed

•      Once the lungs inflate, surfactant in alveolar fluid helps reduce surface tension

Occlusion of Fetal Blood Vessels

•      Umbilical arteries and vein constrict and become fibrosed

•      Fates of fetal vessels

•    Proximal umbilical arteries become superior vesical arteries and distal parts become the medial umbilical ligaments

•    The umbilical vein becomes the ligamentum teres

•    The ductus venosus becomes the ligamentum venosum

•    The foramen ovale becomes the fossa ovalis

•    The ductus arteriosus becomes the ligamentum arteriosum

Transitional Period

•      Unstable period lasting 6-8 hours after birth

•      The first 30 minutes the baby is alert and active

•    Heart rate increases (120-160)

•    Respiration is rapid and irregular

•    Temperature falls

•      Activity then diminishes and the infant sleeps about three hours

•      A second active stage follows in which the baby regurgitates mucus and debris

•      After this, the infant sleeps, with waking periods occurring every 3-4 hours

Lactation

•      The production of milk by the mammary glands

•      Estrogens, progesterone, and lactogen stimulate the hypothalamus to release prolactin-releasing hormone (PRH)

•      The anterior pituitary responds by releasing prolactin

•      Colostrum

•    Solution rich in vitamin A, protein, minerals, and IgA antibodies

•    Is released the first 2–3 days

•    Is followed by true milk production

•      After birth, milk production is stimulated by the sucking infant

Breast Milk

•      Advantages for the infant include:

•    Fats and iron are better absorbed

•    Its amino acids are metabolized more efficiently than those of cow’s milk

•    Beneficial chemicals are present – IgA, other immunoglobulins, complement, lysozyme, interferon, and lactoperoxidase

•    Interleukins and prostaglandins are present, which prevent overzealous inflammatory responses

•    Its natural laxatives help cleanse the bowels of meconium