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Conception and Fetal Development Chapter 12 Learning Objectives Summarize the process of fertilization. Describe the development, structure, and functions of the placenta. Describe the composition and functions of amniotic fluid. Identify three organs or tissues arising from each of the three primary germ layers. Summarize the significant changes in growth and development of the embryo and fetus. Analyze the potential effects of teratogens during vulnerable periods of embryonic and fetal development. Relate selected congenital defects to stage of fetal development. Conception Conception – Union of a single egg and a single sperm – Not an isolated event but part of a sequential process Cell division – Mitosis – Meiosis Conception (Cont.) Gametogenesis – Oogenesis: the process of egg (ovum) formation – Spermatogenesis: the process of sperm formation Ovum – Two protective layers – Considered fertile for about 24 hours after ovulation Sperm – Capacitation: removal of sperm’s protective coating – Acrosome: a cap on the sperm Conception (Cont.) Fertilization – Zygote: first cell of the new individual – Morula: 16 cells – Blastocyst: trophoblast, embryoblast Implantation: 6-10 days after conception – Chorionic villi – Decidua basalis, decidua capsularis, decidua vera Conception of the Fetal Development Embryo and Fetus Pregnancy last 10 lunar months, 9 calendar months, 40 weeks, or 280 days Length of pregnancy is computed from the first day of the last menstrual period (LMP) until the day of birth Conception occurs approximately two weeks after the first day of the LMP Conception and Fetal Development Development of the Embryo Stage of the embryo last from day 15 until approximately 8 weeks after conception Embryonic stage is the most critical time in the development of the organ system and external features End of the eighth week, all organs systems and external structures, are present Conception and Fetal Development Development of the Embryo Membranes Two fetal membranes Chorion- develops from the trophoblast and contains the chorionic villi on the surface Chorion becomes the covering of the fetal side of the placenta Contains major umbilical blood vessels as they branch out of the surface of the placenta Conception and Development Membranes Two fetal membranes Amnion- develops from the interior cells of the blastocyst Amnion becomes the covering of the umbilical cord and covers the chorion on the fetal surface of the placenta Amnion eventually comes in the contact with the chorion surrounding the fetus Conception and Development Amniotic Fluid Helps maintain a constant body temperature Serves as a source of oral fluids and repository for waste Cushions the fetus from trauma by blunting and dispersing outside forces Allows freedom of movement for musculosketal development Fluid keeps the embryo from tangling with the membranes, facilitating symmetric growth Having less than 300 ml of amniotic fluid (oligohydramnios) is associated with fetal renal abnormalities Having more than 2 L of amniotic fluid (hydramnios) is associated with gastrointestinal and other malformations Conception and Fetal Development Umbilical Cord Two arteries carry blood from the embryo to the chorionic villi, and one vein return the blood to the embryo Term the cord is 2 cm in diameter and ranges from 30 to 90 cm long (with an average of 55 cm) Connective tissue called Wharton’s jelly prevents compression of the blood vessels and ensures continued nourishment of the embryo/fetus Cord is wrapped around the fetal neck, it is termed nuchal cord Conception and Fetal Development Placenta Maternal-placental-embryonic circulation is in place by day 17, when the embryonic heart starts beating By the end of the third week, embryonic blood is circulating between the embryo and the chorionic villi Placenta functions as means of metabolic exchange However, limb defects have been associated with chorionic villus sampling done before 10 weeks Conception and Fetal Development Placenta Structure of the placenta is completed by 12th week Placenta continues to grow wider until 20 weeks, when it covers approximately half of the uterine surface Functions of the placenta An early function of the placenta is as an endocrine gland that produces four hormones necessary to maintain the pregnancy and support the embryo and the fetus Hormone is the basis for pregnancy tests Conception and Fetal Development Functions of the Placenta Miscarriages occur if the corpus luteum stops functioning before the placenta is producing sufficient progesterone and estrogen Amount of hCG reaches its maximal level at 50 to 70 days and then begins to decrease Hormone increases the resistance to insulin, facilitates glucose transfer across the placental membrane Stimulates breast development to prepare for lactation Conception and Fetal Development Functions of the Placenta Placenta eventually produces more of the steroid hormone progesterone than the corpus luteum does during the first few months of pregnancy Progesterone maintains the endometrium, decreases the contractility of the uterus, and stimulates maternal metabolism and development of breast alveoli Estrogen stimulates uterine growth and uteroplacental blood flow Placental estrogen production increases greatly toward the end of pregnancy Conception and Fetal Development Functions of the Placenta Metabolic functions of the placenta are respiration, nutrition, excretion, and storage Drugs also can cross the placental membrane and may harm the fetus Caffeine, alcohol, nicotine, carbon monoxide, and other toxic substances in cigarette smoke, as well as prescription and recreational drugs (such as cocaine and marijuana) readily cross the placenta Conception and Fetal Development Fetal Maturation Stage of the fetus last from 9 weeks (when the fetus becomes recognizable as a human being) until the pregnancy ends Viability-refers to the capability of the fetus to survive outside the uterus Limitations on survival outside of the uterus are based on central nervous system function and the oxygenation capability of the lungs Conception and Fetal Development Fetal Circulatory System Cardiovascular system is the first organ system to function in the developing human By the end of the 3rd week, the tubular heart begins to beat, and the primitive cardiovascular system links the embryo, connecting stalk, and yolk sac The 4th and 5th weeks, the heart develops into a four chambered organ Conception and Fetal Development Fetal Circulatory System Fetal lungs do not function for the respiratory gas exchange, so a special circulatory pathway, the ductus arteriosus, bypasses the lungs Most of the blood passes through the ductus venosus into the inferior vena cava Most of this blood passes straight through the right atrium and through the foramen ovale, and opening into the left atrium Three characteristics enable the fetus to obtain sufficient oxygen from the maternal blood: Fetal hemoglobin carries 20 to 30% more oxygen than maternal hemoglobin Hemoglobin concentration of the fetus is about 50% greater than that of the mother Fetal heart rate (FHT) is 110 to 160 beats/min, making the cardiac output per unit of body weight higher than that of an adult Conception and Fetal Development Hematopoietic System Hematopoiesis- the formation of blood, occurs in the yolk sac beginning in the third week Gastrointestinal System Structures evolve during the 5th and 6th week Midgut becomes the distal half of the duodenum, jejunum and ileum, cecum and appendix, and proximal half of the colon Hindgut develops into the distal half of the colon, the rectum and parts of the anal canal, the urinary bladder, and the urethra Conception and Fetal Development Gastrointestinal System Fetus swallows amniotic fluid beginning in the fifth month Gastric emptying and intestinal peristalsis occur Fetal nutrition and elimination needs are taken care of by the placenta Between 26 and 30 weeks, the fetus begins to lay down stores of brown fat in preparation for extra uterine cold stress Gastrointestinal system matures by 36 weeks Conception and Fetal Development Hepatic System Liver and biliary develop from the foregut during the fourth week of gestation Hematopoiesis begins during the sixth week and requires that the liver be large Glycogen is stored in the fetal liver beginning at the week 9 or 10 Conception and Fetal Development Respiratory System Respiratory system begins development during embryonic life and continues through fetal life and into childhood until about 8 years of age Development of the respiratory tract begins in week 4 and continues through week 17 with formation of the larynx, trachea, bronchi, and terminal bronchioles enlarge, and vascular structures and primitive alveoli are formed After 32 weeks, sufficient surfactant is present in developed alveoli to provide infants with good chance of survival Conception and Fetal Development Pulmonary Surfactants Detection of the presence of pulmonary surfactants, surfaceactive phospholipids Lecithin (L) is the most critical alveolar surfactant required for postnatal lung expansion Thus the measure of lecithin in relation to sphingomyelin, or the L/S ratio is used to determine fetal lung maturity The L/S ratio reaches 2:1 the infant’s lungs are considered to be mature This occurs at approximately 35 weeks of gestation Conception and Fetal Development Renal System Kidneys from during the fifth week and begin to function approximately 4 weeks later Oligohydramnios is indicated of renal dysfunction Birth, however, the kidneys are required immediately for excretory and acid-base regulatory functions Fetal renal malformation can be diagnosed in utero At the term, the fetus has fully developed kidneys Most newborns void within 24 hours of birth Conception and Fetal Development Neurologic System Open neural tube forms during the fourth week Forebrain develops into the eyes (cranial nerve II) and cerebral hemispheres Development of all areas of the cerebral cortex continues throughout fetal life and into childhood Spinal cord develops from the long end of the neural tube moves all extremities, and changes position in utero Conception and Fetal Development Sensory Awareness Fetus responds to sound at 24 weeks Fetus can be soothed by the sounds of the mother’s voice Acoustic stimulation can be used to evoke a fetal heart rate response Eyes have both rods and cones in the retina by the seventh month At term the fetal brain is approximately one fourth the size of an adult Neurologic development continues Conception and Fetal Development Endocrine System Thyroid gland develops along with structures in the head and neck during the third and fourth weeks Secretion of thyroxine begins during the eighth week Adrenal cortex is formed during the sixth week and produces hormones by the eighth or ninth week Pancreas forms from the foregut during the fifth through eighth weeks Islets of Langerhans develop during the twelfth week Insulin is produced by week 20 Conception and Fetal Development Reproductive Systems Sex differentiation begins in the embryo during the seventh week Female and male genitalia are indistinguishable until after the ninth week At birth the ovaries contain the female’s lifetime supply of ova Female endometrium responds to maternal hormones, and withdrawal bleeding or vaginal discharge (pseudomenstruation) may occur at birth when these hormones are lost High level of maternal estrogen also stimulates mammary engorgement and secretion of fluids (witch’s milk) in newborn infants of both sexes Conception and Fetal Development Musculoskeletal System Bones and muscles develop from the mesoderm by the fourth week of embryonic development Birth connective tissue sutures exist where the bones of the skull meet Areas where more than two bones meet (called fontanels) are especially prominent Bones of the shoulder, arms, hips, and legs appear in the sixth week as a continuous skeleton with no joints Conception and Fetal Development Integumentary System Cells of the superficial layer are sloughed and become mixed with the sebaceous gland secretions to form the white, cheesy vernix caseosa, the material that protects the skin of the fetus 32 weeks, as subcutaneous fat is deposited under the dermis, the skin becomes less wrinkled and red in appearance Handprints and footprints are unique to that infant Very fine hair, called lanugo appear at 12 weeks on the eyebrows and upper lip and by 20 weeks, they cover the entire body Fingernails usually reach the fingertips at 32 weeks Toenails reach their tips by 36 weeks Conception and Fetal Development Immunologic System During the third trimester, albumin and globulin are present in the fetus Fetus produces IgM immunoglobulins by the end of the first trimester These are produced in response to blood group antigens, gram-negative enteric organisms, and some viruses Conception and Fetal Development Multifetal Pregnancy Two zygotes or dizygotic twinstwo mature ova are produced in one ovarian cycle, both have the potential to be fertilized by separate sperm Identical twins, or monozygotic twins, develop from one fertilized ovum, which then divides They are the same sex and have the same genotype Triplets- occur from the division of one zygote into two, with one of the two dividing again and producing identical triplets Nongenetic Factors Influencing Development Congenital: the condition was present at birth – Environmental factors – Poor maternal nutrition – Teratogen exposure – Inadequate folic acid Key Points Human gestation lasts approximately 280 days after the last menstrual period or 266 days after conception. Fertilization occurs in the uterine tube within 24 hours of ovulation. The zygote undergoes mitotic divisions, creating a 16-cell morula. Implantation begins 6 days after fertilization. The organ systems and external features develop during the embryonic period, that is, the third to the eighth week after fertilization. Key Points (Cont.) Refinement of structure and function occurs during the fetal period, and the fetus becomes capable of extrauterine survival. During critical periods in human development, the embryo and fetus are vulnerable to environmental teratogens. There has been a steady rise in the incidence of multifetal pregnancies, which is partly due to the use of fertility drugs and in vitro fertilization, the increasing age at which women give birth, and ovulation-enhancing drugs. Question 1. What is a major cause for the increase in multiple births in the United States? a. Women are having their children at younger ages b. Decreased use of contraceptives c. Obesity among women d. Increased use of infertility treatment