Chapter 28 - Pregnacy 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
- 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 and zona pellucida
- Extracellular matrix
- 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
- 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 - zonal inhibiting proteins (ZIPs):
- Destroy sperm receptors
- Cause sperm already bound to receptors to detach
- Completion of Meiosis II and Fertilization
- Upon entry of sperm, the secondary oocyte:
Preembryonic Development
- Cleavage: From Zygote to Blastocyst
- 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 trophoblast
- 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 a properly prepared 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
- Twins
Embryonic Development
- 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 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
- Embryonic Membranes
- The blastocyst develops into a gastrula with three primary germ layers
- 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
- 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 ventral surface of 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 --> formation of primary germ layers
- The first cells that enter the groove form the endoderm
- The cells that follow push laterally between the cells forming the mesoderm
- Cells that remain on the embryo’s dorsal surface form ectoderm
- Notochord - rod of mesodermal cells that serves as axial support
- Primitive streak - raised dorsal groove that establishes the longitudinal axis of the embryo
- Primary Germ Layers - server as primitive tissues from which all body organs will derive
- 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, the 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
- 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
- Specialization of Endoderm
- Endoderm forms epithelium linings of the hollow organs of the digestive and respiratory tracts
- Organs of the GI tract become apparent, and oral and anal openings perforate
- Windows on the Womb
- Specialization of the Mesoderm
- Forms - All muscle tissue and connective tissue, thus all tissue except the nervous system, epidermis and mucosal linings.
- All pinkish tissue
- 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
2arteries & 1 vein in umbilical cord to mother. (Wharton's jelly)
- Hormonal Changes During Pregnancy
- 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
- 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
- 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
- GI tract - morning sickness occurs due to elevated levels of estrogen and progesterone
- Urinary system - urine production increases to handle the additional fetal wastes
- Respiratory system - 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
Stages of Labor
- 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
- 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
- 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
- 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
- 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 beats/min.)
- Respiration is rapid and irregular
- Temperature falls
- 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 of breast milk for the infant
- 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
- Factors that cause congenital malformations during embryonic development
- Structures in developing embryo are sensitive to teratogens at different times
- Fetal Alcohol Syndrome
References
Forward comments to RM Chute .....
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