Chapter 28 From Egg to Embryo

Fertilization  ~ 300 million sperm enter female reproductive tract, most are lost  ~ reach egg  Fertilization occurs when a sperm fuses with an egg to form a zygote.  Capcitation: activation of the sperm cell membrane allows release of acrosomal enzymes.  Sperm cells bind to the ZP3 glycoprotein receptor on the zona pellucida triggering the acrosomal reaction & releasing acrosomal enzymes

From Egg to Embryo  Hundreds of sperm cells must release their acrosomal enzymes before fertilization can occur  Acrosomal enzymes cut through the zona pelucida Figure 28.2

From Egg to Embryo  A protein on sperm binds to membrane receptors of oocyte activating the egg receptor to cause fusion of the egg & sperm membranes. sperm nucleus is pulled into the oocyte cytoplasm. Figure 28.2

From Egg to Embryo  Polyspermy, or fertilization by more than one sperm cell, leads to a lethal number of chromosomes, & must be prevented. Figure 28.2

From Egg to Embryo  Polyspermy must be prevented. fast block to polyspermy  oocyte membrane depolarizes & prevents binding by other sperm cells.  Calcium release from oocyte ER The slow block to polyspermy; the destruction of sperm receptors, & swelling of the membrane removes other sperm cells from the surface.

From Egg to Embryo  After a sperm enters an oocyte, it loses its tail & midpiece, & migrates to the center of the oocyte while the oocyte completes meiosis II. Figure 28.3

From Egg to Embryo  After meiosis II is completed, male & female pronuclei fuse & produce a zygote, which almost immediately enters into mitosis. Figure 28.3

Figure 28.4

From Egg to Embryo  Preembryonic Development Preembryonic development begins with fertilization & continues with the movement of the preembryo to the uterus, where it implants in the uterine wall.  Cleavage results in smaller cells as mitotic divisions after fertilization occur without much growth between divisions. Figure 28.4

From Egg to Embryo  Cleavage forms two identical cells, blastomeres, which by cleavage form a morula, (a hollow ball of cells) by 72 hours.  After 4–5 days, the blastocyst escapes from the degrading zona pellucida to implant in the uterine wall.  Implantation occurs after 6 – 7 days; the trophoblast adheres to the endometrium, & produces enzymes that irritate the endometrium. Figure 28.4

From Egg to Embryo  Uterine capillaries become permeable & leaky, & the trophoblast proliferates, forming the cytotrophoblast & the syncytiotrophoblast. Figure 28.5

From Egg to Embryo  Trophoblast cells secrete human chorionic gonadotropin (hCG), which has LH type activity keeping the corpus luteum functional. Placentation is the formation of the placenta, by proliferation of the trophoblast.  Forms cytotrophoblast & syncytiotrophoblast By the end of the third month of gestation the placenta functions as a nutritive, respiratory, excretory, & endocrine organ Figure 28.5

Events of Embryonic Development  Formation & Roles of the Embryonic Membranes While implantation is occurring, the blastocyst is being converted into a gastrula, in which three primary germ layers form & embryonic membranes develop. Figure 28.7

Events of Embryonic Development  The amnion forms the transparent sac ultimately containing the embryo, & provides a buoyant environment that protects the embryo from physical trauma. Figure 28.7

Events of Embryonic Development  The yolk sac forms part of the gut, produces the earliest blood cells & blood vessels, & is the source of germ cells that migrate into the embryo to seed the gonads. Figure 28.7

Events of Embryonic Development  The allantois is the structural base for the umbilical cord that links the embryo to the placenta, & becomes part of the urinary bladder. Figure 28.7

Events of Embryonic Development  The chorion helps to form the placenta, & encloses the embryonic body & all other membranes. Figure 28.7

Embryonic Disc Figure 28.8a-e

Folding Figure 28.8a-e Figure 28.10a-d

Gastrulation  Gastrulation: Germ Layer Formation Gastrulation; the process of transforming the two-layered embryonic disc into a three- layered embryo.  ectoderm, mesoderm, & endoderm Figure 28.8a-e

Gastrulation  Gastrulation begins with the appearance of the primitive streak, which establishes the long axis of the embryo.  Cells migrate into the streak to form endoderm then mesoderm Fig Figure 28.8f-h

Organogenesis  Organogenesis: Differentiation of the Germ Layers Organogenesis is the formation of organs & organ systems; by the end of the embryonic period, all organ systems are recognizable.

Organogenesis  The ectoderm gives rise to structures of the nervous system & the epidermis.  Neurulation, the formation of the brain & spinal cord, is the first event of organogenesis. Figure 28.9a-d Figure 28.8f-h

Organogenesis  The mesoderm gives rise to all types of tissues not formed by ectoderm or endoderm, such as muscle tissue.  Mesodermal specialization forms the notochord, & gives rise to the dermis, parietal serosa, bones, muscles, cardiovascular structures, & connective tissues. Fig Figure 28.8f-h Figure 28.12a-c

Organogenesis  The endoderm gives rise to epithelial linings of the gut, respiratory, & urogenital systems, & associated glands.  As the embryo develops from a flat plate of cells, it rolls into a tube & the inferior endoderm becomes the lining of the primitive gut. Fig Figure 28.8f-h Figure 28.12a-c

Folding Figure 28.8f-h Figure 28.10a-d

Organogenesis  By 3 1/2 weeks, the embryo has a blood vessel system & a pumping heart.  Vascular modifications include umbilical arteries & veins, a ductus venosus, & the foramen ovale & ductus arteriosus. Figure 28.13a

Events of Fetal Development  The fetal period extends from weeks 9 – 38, & is a time of rapid growth of body structures established in the embryo.  During the first half of the fetal period, cells are still differentiating into specific cell types to form the body ’ s distinctive tissues. Fig

Effects of Pregnancy on the Mother  Anatomical Changes  Metabolic Changes  Physiological Changes

Effects of Pregnancy on the Mother  Anatomical Changes Reproductive organs & breasts become more vascular. Uterus enlarges dramatically, shifts the woman ’ s center of gravity compensated for by accentuated lumbar curvature (lordosis). The placental hormone relaxin causes pelvic ligaments & the pubic symphysis to soften & relax. Normal weight gain of around 28 pounds. Fig

Effects of Pregnancy on the Mother  Metabolic Changes The placenta produces; Human placental lactogen  Promotes breast maturation (with estrogen & progesterone).  Promotes the growth of the fetus, & exerts a glucose-sparing effect on maternal metabolism. Human chorionic thyrotropin which increases maternal metabolic rate.

Effects of Pregnancy on the Mother  Physiological Changes (p. 1135) Morning sickness may be present during the first few months of pregnancy, until adaptation to elevated levels of estrogen & progesterone occurs. Heartburn due to esophageal displacement Constipation may result due to the decreased motility of the digestive tract.

Effects of Pregnancy on the Mother  Physiological Changes (p. 1135) Increased urine production to dispose of additional fetal metabolic waste. Vital capacity & respiratory rate increases  Decrease in residual volume  Many women experience dyspnea. Blood pressure & heart rate rise. Blood volume increases to accommodate the needs of the fetus.

Parturition (Birth)  Parturition is the process of giving birth.  Usually within 15 days of the calculated due date. 280 days from the last menstrual period Fig Fig

Initiation of Labor  Estrogen levels peak: Uterine myometrial cells increase oxytocin receptors Blocks the quieting effect of progesterone on uterine muscle.  Fetal cells produce oxytocin, which promotes the release of prostaglandins from the placenta, & further stimulates uterine contraction.  Increasing emotional & physical stresses activate the mother ’ s hypothalamus, which signals the release of oxytocin.  Expulsive contractions are aided by a change that occurs in an adhesive protein, fetal fibronectin, converting it to a lubricant. Fig

Stages of Labor  Stage 1 The dilation stage of labor extends from onset of labor to the time when the cervix is fully dilated by the baby ’ s head, at about 10 cm in diameter.  Stage 2 The expulsion stage extends from full dilation until the time the infant is delivered.  When the baby is in the vertex, or head first, position, the skull acts as a wedge to dilate the cervix.  Crowning occurs when the baby ’ s head distends the vulva, & once the head has been delivered, the rest of the baby follows much more easily.  After birth, the umbilical cord is clamped & cut.  Stage 3 Placental stage, uterine contractions cause detachment of the placenta from the uterine wall, followed by delivery of the placenta & membranes (afterbirth). Fig

Occlusion of Special Fetal Blood Vessels & Vascular Shunts  After birth, the umbilical arteries & veins constrict & become fibrosed, becoming the medial umbilical ligaments, superior vesical arteries of the bladder, & the round ligament of the liver, or ligamentum teres. Fig a Fig b

Occlusion of Special Fetal Blood Vessels & Vascular Shunts  The ductus venosus closes, & is eventually converted to the ligamentum venosum.  A flap of tissue covers the foramen ovale, ultimately sealing it & becoming the fossa ovalis, while the ductus arteriosus constricts, becoming the ligamentum arteriosus. Fig a Fig b

Occlusion of Special Fetal Blood Vessels & Vascular Shunts  A flap of tissue covers the foramen ovale, ultimately sealing it & becoming the fossa ovalis, while the ductus arteriosus constricts, becoming the ligamentum arteriosus. Fig a Fig b

Lactation  Lactation is the production of milk by the hormone-prepared mammary glands.

Lactation  Rising levels of placental estrogens, progesterone, & lactogen stimulate the hypothalamus to produce prolactin- releasing hormone (PRH), which promotes secretion of prolactin by the anterior pituitary.  Colostrum initially secreted for the first two to three days.  Nipple stimulation sends sensory input to the hypothalamus stimulating production of PRH & prolactin that maintains milk production.  Oxytocin results in of milk let down from the alveoli.  Breast milk has multiple advantages. Fig

Assisted Reproductive Technology & Reproductive Cloning  Hormones can be used to increase sperm or egg production & surgery can be used to open blocked tubes.  Assisted reproductive technology involves surgically removing oocytes from a woman ’ s ovaries, fertilizing the eggs & returning them to the woman ’ s body.  Cloning involves the placing of a somatic cell nucleus into an oocyte. Fig