Pregnancy and Human Development 28 P A R T A Pregnancy and Human Development

Gestation period – from the last menstrual period until birth From Egg to Embryo Pregnancy – events that occur from fertilization until the infant is born Gestation period – from the last menstrual period until birth

Preembryo –from fertilization (zygote) until it is two weeks old From Egg to Embryo Preembryo –from fertilization (zygote) until it is two weeks old Embryo – zygote implanted in uterine lining during the third through the eighth week Fetus –from the ninth week through birth

Relative Size of Human Conceptus Figure 28.1

Accomplishing Fertilization The oocyte is viable for 12 to 24 hours Sperm is viable 24 to 72 hours For fertilization to occur, coitus must occur no more than: Three days before ovulation 24 hours after ovulation Fertilization – when a sperm fuses with an egg to form a zygote

Sperm Transport and Capacitation Fates of ejaculated sperm: Leakage Low pH of vagina Failure to reach cervix Destroyed by phagocytosis/lost in cavity Failure to reach tubes Sperm must undergo capacitation before they can penetrate the oocyte

Figure 28.2a

Completion of Meiosis II and Fertilization Upon entry of sperm, the secondary oocyte: Completes meiosis II Casts out the second polar body Fertilization – when the pronuclei come together

Events Immediately Following Sperm Penetration Figure 28.3

Cleavage and Implantation Cleavage – rapid mitosis of the zygote following fertilization.

Preembryonic Development The first cleavage produces two daughter cells called blastomeres Morula – the 16 or more cell stage (72 hours old) By the fourth or fifth day the preembryo consists of 100 or so cells (blastocyst)

Preembryonic Development Blastocyst – a fluid-filled hollow sphere composed of: 1. A single flattened layer of cells called trophoblasts 2. An inner cell mass Trophoblasts take part in placenta formation The inner cell mass becomes the embryonic disc

Cleavage: From Zygote to Blastocyst Figure 28.4

Implantation 6-7 day trophoblast implants into the endometrium – high levels of estrogen and progesterone

blastocyst is covered over by endometrial cells Implantation blastocyst is covered over by endometrial cells completed by the fourteenth day after ovulation

Implantation of the Blastocyst Figure 28.5a

Implantation of the Blastocyst Figure 28.5b

corpus luteum -secrete estrogen and progesterone Implantation corpus luteum -secrete estrogen and progesterone Chorion – developed from trophoblasts after implantation, continues this hormonal stimulus Between the second and third month, the placenta: Assumes the role of progesterone and estrogen production Is providing nutrients and removing wastes

Hormonal Changes During Pregnancy Figure 28.6

Formation of the placenta from: Embryonic trophoblastic tissues Placentation Formation of the placenta from: Embryonic trophoblastic tissues Maternal endometrial tissues These form the chorion

Placentation The placenta is fully formed and functional by the end of the third month

Placentation Embryonic placental barriers allow for passage of nutrients but the blood does not inter-mix The placenta also secretes other hormones – human placental lactogen, human chorionic thyrotropin, and relaxin

Placentation Figure 28.7a–c

Placentation Figure 28.7d

Placentation Figure 28.7f

Amnion, yolk sac, allantois, chorion Germ Layers Inner cell mass produces 4 membranes during the first 2-3 weeks of development Amnion, yolk sac, allantois, chorion

Amnion – a transparent membrane filled with amniotic fluid Embryonic Membranes Amnion – a transparent membrane filled with amniotic fluid Provides a buoyant environment that protects the embryo Helps maintain a constant homeostatic temperature Amniotic fluid comes from maternal blood, and later, fetal urine

Yolk sac – a sac on the ventral surface of the embryo Embryonic Membranes Yolk sac – a sac on the ventral surface of the embryo Forms part of the digestive tube Produces earliest blood cells and vessels Is the source of primordial germ cells

Allantois – a small outpocketing at the caudal end of the yolk sac Embryonic Membranes Allantois – a small outpocketing at the caudal end of the yolk sac Structural base for the umbilical cord Becomes part of the urinary bladder Chorion – helps form the placenta Encloses the embryonic body and all other membranes

The primary germ layers are ectoderm, mesoderm, and endoderm Gastrulation During the 3rd week, the two-layered embryonic disc becomes a three-layered embryo – gastrulation Primitive streak – raised dorsal groove that establishes the longitudinal axis of the embryo The primary germ layers are ectoderm, mesoderm, and endoderm

Notochord – rod of mesodermal cells that serves as axial support Gastrulation Notochord – rod of mesodermal cells that serves as axial support

Ectoderm – forms structures of the nervous system and skin epidermis Primary Germ Layers Ectoderm – forms structures of the nervous system and skin epidermis Endoderm – forms epithelial linings of the digestive, respiratory, and urogenital systems Mesoderm – forms all other tissues