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Pregnancy and Fertilization: Stages, Characteristics, and Development

This topic explores the process of pregnancy and fertilization, from the union of sperm and egg to the development of the embryo. It covers the stages of fertilization, characteristics of living things, and the development of the embryo during the pre-embryonic and embryonic stages.

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Pregnancy and Fertilization: Stages, Characteristics, and Development

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  1. Biology 221 Anatomy & Physiology II TOPIC 13Survey of Development Chapter 29 and others E. Lathrop-Davis / E. Gorski / S. Kabrhel

  2. Pregnancy • Pregnancy refers to the events from fertilization to birth. • Pregnancy results from union of sperm and egg. • Sperm must swim from the vagina upstream to meet the secondary oocyte that was ovulated; normally they meet in the fallopian tubes. • To reach the oocyte, sperm must overcome: • the acidity of male urethra and female vagina; • the mucus plug at the entrance to the cervix; and • uterine contractions that occur during orgasm. Fig. 28.14, p. 1089

  3. Fertilization • Fertilization, or the union of the haploid gametes (egg & sperm), produces a diploid zygote. • Fertilization occurs in 2 stages: • Stage 1: penetration of the egg by the sperm; • Stage 2: union of sperm & egg membranes and nuclei. • Fertilization produces a unique individual with genetic characteristics of both mother and father.

  4. Fertilization: Stage 1 - Penetration • Acrosomal enzymes from many, many sperm must be released to break through corona radiata and zona pellucida of the oocyte before penetration can occur. • When one sperm finally makes it through, conditions in the oocyte change so that no more sperm can enter. Fig. 29.2, p. 1121

  5. Fertilization: Stage 2 Union • Once the sperm has entered, the oocyte undergoes meiosis II and ejects the 2nd polar body. • The sperm nucleus migrates to the center of the oocyte and the rest of the sperm degenerates. • Sperm and egg nuclei unite to form the new diploid zygote. Fig. 29.3, p. 1122

  6. Characteristics of Living Things • Maintenance of boundaries (cell membrane; eventually, skin) • Movement (cellular and/or organismal) • Response to stimuli (recognizes changes in environment and creates responses) • Digestion (provides nutrients in usable form) • Metabolism (chemical reactions) • Excretion (removal of wastes) • Reproduction (initially, just cellular) • Growth (increase in size by increased number of cells)

  7. Pre-embryonic Development1st Through 2nd Weeks Overview • Cleavage • Blastocyst foramtion • Implantation • Placentation

  8. Cleavage &Blastocyst Formation • Cleavage involves the rapid replication of DNA and mitotic cell divisions, which produces ever smaller cells resulting in a solid ball called a morula. • Continued division of the morula produces a blastocyst with two groups of cells. • The inner cell mass becomes the future embryo. • The trophoblast cells form the fetal part of the placenta. Fig. 29.4, p. 1123

  9. Implantation • The blastocyst implants itself into endometrium of uterine wall • An ectopic pregnancy is one in which implantation occurs in some other location (e.g., fallopian tube or pelvic cavity). • Trophoblast cells (outer cells of blastocyst) secrete human chorionic gonadotropin (hCG) that maintains the corpus luteum through 1st four months. Fig. 29.5, p. 1124

  10. Placentation • Placentation refers to the development of the placenta. • The placenta originates from the chorion (trophoblast cells) of the embryo and endometrial tissue of mother. • The placenta begins to produce estrogens and progestins that maintain the uterus. Fig. 29.7a-c, p. 1126

  11. Embryonic DevelopmentWeeks 3-8 Overview During weeks 3-8, • extra-embryonic membranes develop; • primary germ layers form in a process called gastrulation; the embryo can be called a gastrula; and • rudiments of organ systems are formed in a process called organogenesis. • By the end of the 8th week, all major organ systems have been formed and the embryo is referred to as a fetus.

  12. Development of Extra-Embryonic Membranes There are 4 extra-embryonic membranes: • The chorion forms part of the placenta. • The amnion becomes filled with amniotic fluid, which: • cushions the embryo; • maintains temperature; and • allows freedom of movement. Fig. 29.7d, p. 1126

  13. Development of Extra-Embryonic Membranes • The yolk sac forms part of the primitive gut. • It also serves as the 1st site of blood cell formation. • Theallantois contributes to the umbilical cord. • It also becomes part of the urinary bladder. Fig. 29.7d, p. 1126

  14. Gastrulation • Gastrulation is the development of primary germ layers. • Ectoderm is the outmost layer and forms the epidermis and nervous system. • Endoderm is the inner layer and forms the epithelial linings of the digestive tract, respiratory tract, urogenital system and associated glands. • Mesoderm is the middle layer and forms connective tissues and muscle. See also Table 29.1, p. 1135 Fig. 29.8, p. 1131

  15. Specialization of Ectoderm • Ectoderm over most of body forms the epidermis. • Ectoderm over the middle back, forms the neural plate. • The neural plate invaginates and becomes the neural groove&neural folds (tissue lateral to the neural groove). • The neural folds join together along the back and form the neural tube. • The neural tube forms the brain & spinal cord. The opening within the tube remains as the ventricles of the brain and central canal of the spinal cord. Fig. 29.9, p. 1132

  16. Specialization of Endoderm • Endoderm gives rise to the lining of the gut. • Structures that come from the gut arise as “outpocketings”. These include the: • epithelia of the lungs and respiratory tree; • thyroid and parathyroid glands; • liver and gall bladder; • pancreas; and • adenohypophysis. Fig. 29.10, p. 1133

  17. Specialization of Mesoderm • Mesoderm gives rise to muscle, connective tissues, and serous membranes. • Basically, everything between the epidermis and lining of the gut, respiratory tree and lungs. • Limb buds form and move laterally in what will become shoulder and hip areas Fig. 29.11, p. 1134

  18. Embryonic Development MilestonesWeeks 3-8: Organogenesis • The heart beats by week 4. • All systems are present in some form by week 8. • All major regions of brain are present by week 8. • The liver produces blood cells by week 8.

  19. Fetal Milestones: 12 - 16 Weeks By 12 weeks: • Blood cell formation begins in bone marrow. • Ossification begins. By 16 weeks: • The kidneys have assumed their typical shape. • Joint (synovial) cavities are present. • The cerebellum has enlarged. • Sensory organs are differentiated. See Table 29.2, p. 1138

  20. Fetal Milestones: 20-30 Weeks By 20 weeks: • Skin is covered by lanugo (silky hair). • Activity can be felt by mother (“quickening”). By 30 weeks: • Myelination of the spinal cord begins. • Finger and toe nails are present. • Bone marrow becomes the only site of blood cell formation. • The testes descend (7th month) in males. • Surfactant production begins at ~ 24 weeks. See Table 29.2, p. 1138

  21. Fetal Milestones: 8 Months - Birth During the 8th to 9th months: • Development of organ systems continues. • The fetus has a significant weight gain. Weeks 38-42 – Birth occurs. • before 38 weeks, the fetus has less fat and organ systems not as well developed, so it’s not as well prepared. • After 42 weeks, the placenta begins to degenerate and the risk of oxygen deprivation increases. See Table 29.2, p. 1138

  22. Development of Integumentary System See Marieb Ch. 5 pp. 165-168 • The epidermis and dermis are developed by the 4th month • Epidermal derivatives grow down into dermis • Lanugo is present from 20 weeks. This is usually shed during the 7th or 8th month. • Vellus hairs are present by the 7th month. http://www.uoguelph.ca/zoology/devobio/210labs/ecto5.html

  23. Development of Skeletal System See Marieb. ch. 6 pp. 181; A&P I, Unit XII • Skeletal system development begins by the 8th week. • Primary ossification is completed by birth. • Secondary ossification continues to early adulthood. • Think About It: Which of these is affected by growth hormone (GH) levels after birth? (See A&P I, Units XI and XII) http://www.uoguelph.ca/zoology/devobio/210labs/meso2.html#osteo

  24. Development of Skeletal System • Endochondral ossification occurs in hyaline cartilage models and forms most bones other than the cranial bones and clavicles. • Intramembranous ossification – occurs in membranes and forms the flat bones of the cranium and clavicles. • Fontanels are unossified membranes in the skull that are present at birth. • Fontanels allow the head to change shape slightly for easier birth. http://www.uoguelph.ca/zoology/devobio/210labs/meso2.html#osteo http://www.bio.psu.edu/faculty/strauss/anatomy/skel/fetal.htm

  25. Development of Spinal Curvatures • The primary curvatures are the thoracic and sacral curvatures. • These are present at birth. • The secondary curvatures are the cervical and lumbar curvatures. • These develop as the infant lifts its head and stands, respectively. http://www.csu.edu.au/faculty/arts/humss/bioethic/abort1.htm

  26. Development of Nervous System See Marieb. ch. 11; pp. 429-430, 463-464 • The nervous system develops from “neural ectoderm” along the mid back. • Neural crest cells (adjacent to the neural tube) give rise to sensory neurons whose cell bodies are outside the CNS. • Neural tube cells give rise to interneurons and motor neurons. http://www.angelfire.com/mb/jessicasjourney/info.html Fig. 12.2, p. 430

  27. Development of Nervous System • The eyes develop as outgrowths of the diencephalon. (A&P I Review – Where does the optic nerve 1st synapse in the brain?) • The brain and spinal cord develop from the neural tube. • Brain regions represent enlargements of the anterior tube. • The ventricles develop from openings in the neural tube. • Anencephaly is the failure of cerebrum and part of brain stem to develop. Fig. 12.4, p. 431

  28. Development of Nervous System • The spinal cord develops from the middle and posterior portions of the tube. • Spina bifidaresults from the incomplete fusion of the vertebral arches, usually in the lumbrosacral region. • Up to 70% of cases associated with inadequate folate levels in mother • some cases associated with mother’s exposure to high levels of UV radiation [DISCOVER Vol. 22 No. 2 (February 2001)] due to its affect on folate levels. http://www.abbottdiagnostics.com/medical_conditions/fertility_pregnancy/afp.htm

  29. Development of Endocrine System • The development of the endocrine system is complex and includes all three germ layers. • Two glands in particular develop from two different layers, the: • pituitary; and • adrenal. http://anatomy.med.unsw.edu.au/cbl/embryo/Notes/endocrine9.htm

  30. Development of Pituitary Gland • The adenohypophysis (anterior) develops from endoderm as an outgrowth from the roof of the primitive mouth. • The neurohypophysis (posterior) develops from neural ectoderm as an extension of the diencephalon, specifically, the hypothalamus. • Think About It: How does this affect the way secretion is controlled in each? http://www.teaching-biomed.man.ac.uk/histology/T270.HTML See also http://anatomy.med.unsw.edu.au/cbl/embryo/Notes/endocrine7.htm

  31. Development of Adrenal Gland • The cortex develops from mesoderm. • The medulla develops from neural ectoderm. • Specifically, the medulla develops from the same embryonic tissue that gives rise to the ganglionic neurons and postgangionic fibers of the sympathetic division of the ANS (See A&P I, Unit XI). http://sprojects.mmi.mcgill.ca/embryology/ug/Adrenal_Stuff/Normal/zones.html

  32. Development of Circulatory System • Blood is initially produced in the yolk sac, later in the liver, and finally in the bone marrow. • The heart develops from two tubes (blood vessels) in the thoracic cavity. • The blood vessels start in the yolk sac to bring blood into the body.

  33. Development of Blood • Blood develops in first the yolk sac; later, in the liver, spleen, and bone marrow of fetus. • After birth, bone marrow is the only place most blood cells are produced. (See Topic 6) • Fetal hemoglobin (HbF) has a greater affinity for O2 than adult hemoglobin does. • Think About It: What advantage does this give the fetus? http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Blood/Images/bma10he.jpg http://www.smbs.buffalo.edu/bch/faculty/garrett_sinha.html

  34. Development of Heart • Development of the heart begins as 2 tubes that fuse by the 4th week. • The heart begins pumping in the 1st month (4th week). • The foramen ovale allows blood to flow from right to left atrium. • Think About It: (See Topic 2) • What is the purpose of this opening? • What would happen if this failed to close? Fig. 19.24, p. 709

  35. Fetal Circulation • The umbilical arteries carry partially oxygenated blood to the placenta. • The umbilical vein returns nutrient-rich oxygenated blood from placenta to fetal liver. • The ductus venosus is shunt through liver connecting umbilical vein to inferior vena cava. • Think About It:What is the importance of this shunt? • The ductus arteriosus connects the pulmonary trunk to the aorta. • Think About It:What is the importance of this shunt? Fig. 29.13, p. 1136

  36. Development of Respiratory System • The respiratory system develops as buds (outgrowths) from the throat. • Surfactant production begins in week 24 (between weeks 23-26). • Surfactant is not produced in sufficient quantities until about weeks 32-35. • Infant respiratory distress syndrome (See Topic 7) results from a failure to produce sufficient surfactant in the lungs. • Think About It: What how does this affect a premature baby born at 22 weeks? Fig. 23.28, p. 877

  37. Development of Digestive System • The epithelium develops from endoderm; muscle develops from mesoderm. • Glands develop as buds from tube Fig. 24.37, p. 938

  38. Development of Urinary System • Kidney development begins in the 4th week and is completed by week 9. • The fetus urinates into the amniotic fluid, which is replaced every 3 hours or so. Fig. 26.21, p. 1035

  39. Development of Reproductive System • The ovaries & testes develop in the abdominal cavity. • During the first 6 weeks embryonic reproductive organs are “bipotential”. That is, they can become male or female. • Differentiation begins during week 7-8 under influence of testosterone. • The testes descend into the scrotum during the 7th month. Fig. 28.24, p. 1106; Fig. 28.25, p. 1108

  40. Parturition (Birth): Stages of Labor • During the dilation stage the cervix dilates to ~ 10 cm (4”). • During the expulsion stage the fetus is delivered. • Normally, the fetus is delivered head first. • If the fetus is delivered feet first, problems may arise in trying to deliver the head. • A cesarean section is performed if the baby cannot be delivered vaginally. • During the placental stage the placenta (“afterbirth”) is delivered. Fig. 29.17, p. 1142

  41. Hormonal Control of Labor • Estrogen from the ovaries induces the myometrium to make more oxytocin receptors. • Oxytocin is made by the hypothalamus and secreted by the posterior pituitary (neurohypophysis) in response to stretching of the cervix. • Oxytocin stimulates: • contraction of the uterus, which causes additional stretching of the uterus; and • production of prostaglandins, which • stimulate contraction of the uterus; and • stimulates oxytocin secretion. Fig. 29.16, p. 1141

  42. Hormonal Control of Labor • The relationship between oxytocin release and stretching of the cervix and contraction of the myometrium is a positive feedback cycle. • This positive feedback cycle continues until delivery of placenta (breast feeding also causes stimulation of oxytocin secretion and helps return the uterus to its pre-pregnancy size). Fig. 29.16, p. 1141

  43. Hormonal Control of Lactation • Milk formation is stimulated by prolactin from the adenohypophysis. • During breast feeding, stimulation of pressure receptors (pressoceptors) in the breast sends sensory impulses to the hypothalamus. • The hypothalamus, which produces oxytocin, stimulates the posterior pituitary to release oxytocin as part of a positive feedback cycle. • Oxytocin stimulates release of milk from breast • Baby continues sucking until he/she is full. • After baby stops sucking, hypothalamus is no longer stimulated. Fig. 29.18, p. 1144

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