Period of Cleavage. Implantation. Stages of Early Human Prenatal Development. zygote blastomeres morula. begins about the 6 th day of development

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1 Chapter 23 Pregnancy, Growth, and Development Pregnancy is the presence of a developing offspring in the uterus Growth is an increase in size and entails increases in cell numbers and cell sizes Development is the continuous process by which an individual changes from one life phase to another 1 Transport of Sex Cells fertilization is the union of an egg cell and a sperm cell figure shows paths of the egg and sperm cells through the female reproductive tract 2 Fertilization sperm cell reaches corona radiata of egg acrosome releases enzymes sperm cell penetrates zona pellucida sperm cell s membrane fuses with egg cell s membrane 3 1

2 Period of Cleavage zygote blastomeres morula 4 Stages of Early Human Prenatal Development 5 Implantation begins about the 6 th day of development trophoblast will help form the placenta trophoblast secretes hcg which helps maintain the pregnancy 6 2

3 Summary of Stages and Events of Early Human Prenatal Development fertilized ovum hours after ovulation zygote forms cleavage 30 hours to third day mitosis increases cell number morula third to fourth day solid ball of cell blastocyst fifth day through second week trophoblast and inner cell mass form gastrula end of second week primary germ layers form 7 Hormonal Changes During Pregnancy Mechanism that preserves uterine lining during early pregnancy 8 Hormonal Changes During Pregnancy Relative concentrations of three hormones in maternal blood during pregnancy 9 3

4 Hormonal Changes During Pregnancy secretion of hcg maintains corpus luteum corpus luteum secretes estrogens and progesterone placenta secretes large amounts of estrogens and progesterone estrogens and progesterone stimulate and maintain uterine lining, inhibit FSH and LH, inhibit uterine contractions, and enlarge reproductive organs relaxin from corpus luteum inhibits uterine contractions and relaxes pelvic ligaments placental lactogen stimulates breast development aldosterone promotes sodium retention PTH maintains calcium concentrations in blood 10 Early Embryonic Stage Three primary germ layers form 11 Derivatives of Each Primary Germ Layer 12 4

5 Embryonic Development 13 Changes During Embryonic Development 14 Embryonic Membranes As the amnion develops, it surrounds the embryo, and the umbilical cord begins to form from structures in the connecting stalk 15 5

6 Placenta Placental membrane consists of epithelial wall of an embryonic capillary epithelial wall of a chorionic villus 16 Placenta Consists of an embryonic portion and a maternal portion 17 Placenta at Seventh Week 18 6

7 Embryo at Eight Weeks End of eighth week marks end of embryonic period 19 Teratogens factors that cause congenital malformations during embryonic development structures in developing embryo are sensitive to teratogens at different times 20 Fetal Stage begins at the end of the eighth week of prenatal development body portions change considerably during development 21 7

8 Development of External Reproductive Organs differentiate from precursor structures 22 Position of Full-Term Fetus 23 Major Events of Fetal Development 9 th 12 th week ossification centers appear sex organs differentiate fetal limbs begin to move 13 th 16 th week body grows rapidly ossification continues 24 8

9 Major Events of Fetal Development 17 th 20 th week muscle movements stronger skin is covered with lanugo skin is covered with vernix caseosa 21 st 38 th week body gains weight subcutaneous fat deposited eyebrows and eyelashes appear eyelids open testes descend 25 Stages of Prenatal Development 26 Fetal Blood and Circulation oxygen and nutrients diffuse into the fetal blood from the maternal blood waste diffuses into the maternal blood from the fetal blood 27 9

10 Fetal Cardiovascular Adaptations fetal blood has greater oxygen-carrying capacity umbilical vein carries oxygenated blood from placenta to fetus ductus venosus conducts half the blood from umbilical vein to inferior vena cava; liver is bypassed foramen ovale conveys blood from right atrium to left atrium; lungs are bypassed ductus arteriosis conducts some blood from pulmonary trunk to aorta; lungs are bypassed umbilical arteries carry blood from internal iliac arteries to placenta 28 Fetal Circulation 29 Birth Process Factors contributing to the labor process: as birth approaches, progesterone levels decrease prostaglandins synthesized which may initiate labor stretching uterine tissue stimulates release of oxytocin oxytocin stimulates uterine contractions fetal head stretches uterus positive feedback results in stronger and stronger contractions and greater release of oxytocin 30 10

11 Birth Process A positive feedback mechanism propels the birth process 31 Stages in Birth fetal position before labor dilation of the cervix expulsion of the fetus expulsion of the placenta 32 Milk Production and Secretion placental estrogens and progesterone stimulate further breast development estrogens cause ductile system to grow progesterone causes alveolar glands to develop placental lactogen also produces changes in breast prolactin is released about the 5 th week of pregnancy breast production does not begin until after birth 33 11

12 Release of Milk Myoepithelial cells contract to release milk from an alveolar gland 34 Mechanism of Milk Release 35 Hormonal Control of the Mammary Glands 36 12

13 Postnatal Period Neonatal period birth to end of 4 th week newborn begins to carry on respiration, obtain nutrients, digest nutrients, excrete wastes, regulate body temperature, and make cardiovascular adjustments Infancy end of 4 th week to one year growth rate is high teeth begin to erupt muscular and nervous systems mature communication begins 37 Postnatal Period Childhood one year to puberty growth rate is high permanent teeth appear muscular control is achieved bladder and bowel controls are established intellectual abilities mature Adolescence puberty to adulthood person becomes reproductively functional and emotionally more mature growth spurts occur motor skills continue to develop intellectual abilities continue to mature 38 Postnatal Period Adulthood adolescence to old age person remains relatively unchanged anatomically and physiologically degenerative changes begin Senescence old age to death degenerative changes continue body becomes less able to cope with demands placed on it death results from various conditions and diseases 39 13

14 Aging-Related Changes 40 Causes of Death 41 Causes of Death 42 14

15 Clinical Application Preimplantation Genetic Diagnosis Probes diseasecausing genes in an eight-celled embryo 43 15