Development and Inheritance Embryo The first two months following fertilization Fetus From week nine until birth Pregnancy Lasts about 40 weeks from the first day of the last menstrual period Fertilization The genetic material from a haploid sperm cell and a haploid secondary oocyte merges into a single diploid nucleus Fertilization Occurs in the fallopian tube Fertilization Occurs within 12-24 hours after ovulation Fertilization There is a total of 3 days per month during which coitus may result in pregnancy. Fertilization Two days before ovulation and one day after Fertilization Peristaltic contractions and cilia transport the oocyte through the uterine tube Fertilization Sperm swim up the uterus and uterine tube by movements of their tails and muscular contractions of the uterus Fertilization Only 200 of the 300 million spermatozoa reach the oocyte Fertilization Capacitation – the functional changes that sperm undergo in the female that allow them to fertilize an oocyte Fertilization A sperm must penetrate the corona radiata and zona pellucida Fertilization Acrosomal enzymes digest a path through the zona pelucida Fertilization The spermatozoa that penetrates the oocyte loses its tail Fertilization Polyspermy is prevented by chemical changes that penetrate a second sperm from entering the oocyte Fertilization Once a sperm enters a secondary oocyte, the oocyte completes meiosis Fraternal Twins Produced from the independent release of two ova and the subsequent fertilization of each by different spermatozoa Monozygotic twins Derived from a single fertilized ovum that splits at an early stage in development Formation of the Morula Cleavage – early rapid mitotic cell division of a zygote Formation of the Morula Blastomeres - The cells produced by cleavage Formation of the Morula Morula – A solid mass of 16-32 cells Development of the Blastocyst The morula moves down the ciliated uterine tube and into the uterine cavity Development of the Blastocyst The morula develops into a blastocyst Development of the Blastocyst 1. 2. 3. Blastocyst – a hollow ball of cells that is differentiated into a Trophoblast Inner cell mass blastocele Development of the Blastocyst Implantation – The attachment of a blastocyst to the endometrium seven to eight days after fertilization Development of the Blastocyst The trophoblast secretes human chorionic gonadotropin Development of the Blastocyst hCG – rescues the corpus luteum from degeneration and sustains its function Development of the Blastocyst Trophoblast becomes the chorion Development of the Blastocyst Chorion – a membrane outside the amniotic sac that has villi which project into the placenta Beginnings of Organ Systems Gastrulation – inner cell mass of the blastocyst differentiates into three primary germ layers Development of the Blastocyst 1. 2. 3. Three primary germ layers Ectoderm Mesoderm Endoderm Embryonic Membranes Lie outside the embryo and protect and nourish the embryo and later, the fetus Embryonic Membranes 1. 2. 3. 4. Yolk sac Amnion Chorion allantois Yolk Sac It gives rise to cells that migrate to the gonads to become spermatogonia and oogonia (ICM) Amnion Surrounds the embryo creating a cavity filled with amniotic fluid (ICM) Chorion Derived from the trophoblast Chorion Surrounds the embryo and later, the fetus Chorion Embryonic part of the placenta Allantois Small vascularized membrane (ICM) Allantois Umbilical cord Umbilical cord Vascular connection between mother and fetus Umbilical Cord Consists of two umbilical arteries carrying dexoygenated fetal blood to the placenta and one umbilical vein that carries oxygenated blood from the placenta into the fetus Placenta Developed by the third month of pregnancy Placenta 1. 2. Formed by the Chorion (embryonic portion) Decidua basalis (maternal portion) Placenta All exchanges to and from the embryo occur here Placenta Stores nutrients and released into fetal circulation as required Hormones of Pregnancy During the first 3-4 months of pregnancy, the corpus luteum secretes progesterone and estrogen Hormones of Pregnancy Progesterone and estrogens maintain the uterine lining and prepare the mammary glands to secrete milk Hormones of Pregnancy After the third, the placenta secretes estrogen and progesterone Hormones of Pregnancy Estrogen inhibits prolactin, so lactation does not occur Hormones of Pregnancy Estrogen increases the number of oxytocin receptors in the uterus Hormones of Pregnancy Relaxin – softens the cervix and relaxes the pubic symphysis to facilitate delivery Hormones of Pregnancy Corticotropin-releasing hormone – the clock that establishes the timing of birth Anatomical and Physiological Changes During Pregnancy CO, HR, and SV increases Anatomical and Physiological Changes During Pregnancy Breast enlargement Anatomical and Physiological Changes During Pregnancy Increase in appetite Anatomical and Physiological Changes During Pregnancy Increase in tidal volume and total body oxygen consumption Anatomical and Physiological Changes During Pregnancy Increase in GFR Anatomical and Physiological Changes During Pregnancy Increase in frequency of urination Anatomical and Physiological Changes During Pregnancy Some women experience elevated blood pressure