PREGNANCY AND HUMAN DEVELOPMENT

PREGNANCY AND HUMAN DEVELOPMENT

Pregnancy and Human Development • Pregnancy: events that occur from the time of fertilization (conception) until the infant is delivered • The pregnant woman’s developing offspring is called the conceptus • Time during which development occurs is referred to as the gestation period and extends from the last menstrual period until birth, approximately 280 days • Thus, at the moment of fertilization, the mother is officially (but illogically) two weeks pregnant • Two weeks following fertilization, the conceptus undergoes preembryonic development and is known informally as a preembryo • Week 3—week 8 after fertilization, the embryonic period, the conceptus is called an embryo • Week 9 through birth, the fetal period, the conceptus is called thefetus • At birth, it is an infant

EMBRYONIC DEVELOPMENT

From Egg to EmbryoAccomplishing Fertilization • Oocyte is viable for 12 to 24 hours after it is cast out of the ovary • Chance of pregnancy drops to almost zero the next day • Sperm retain their fertilizing power for 24 to 72 hours after ejaculation • For fertilization to occur, coitus must occur no more than three days (72 hours) before ovulation and no later than one day (24 hours) after ovulation,at which point the oocyte is approximately one-third of the way down the length of the uterine tube • Fertilization occurs when a sperm fuses with an egg to form a fertilized egg, or zygote, the first cell of the new individual

From Egg to EmbryoAccomplishing Fertilization • Fertilization occurs when a sperm fuses with an egg to form a zygote • During copulation, millions of sperm ejaculated into the female reproductive tract are lostdue to leakage from the vaginal canal, destruction by the acidic environment of the vagina, inability to pass the cervical mucus, or destruction by defense cells of the uterus • Those that do reach the uterus are subjected to forceful uterine contractions that act to disperse them throughout the uterine cavity, where thousands are destroyed by resident phagocytes • Only a few thousand (and sometimes fewer than 200 sperm), out of the millions in the male ejaculate, finally enter the uterine tubes, where the oocyte may be moving leisurely toward the uterus

From Egg to EmbryoAccomplishing Fertilization • In order to fertilize an egg, sperm must be capacitated, a process involving weakening of the sperm cell membrane in order to allow release of acrosomal hydrolytic enzymes • As sperm swim through the cervical mucus, uterus, and uterine tubes, membrane proteins are removed and the cholesterol that keeps the acrosomal membranes tough and stable is depleted • The sperm undergo a gradual capacitation over the next 6-8 hours • Thus, even though the sperm may reach the oocyte within a few minutes, they must wait around for capacitation to occur

From Egg to EmbryoAccomplishing Fertilization • The ovulated oocyte is encapsulatedby the corona radiata and the deeper zona pellucida, a thick layer of extracellular matrix, and both must be breached before the oocyte can be penetrated • Surface hyaluronidase and sperm hypermobility are sufficient to get the sperm to this point

SPERM PENETRATION AND THE CORTICAL REACTION

From Egg to EmbryoAccomplishing Fertilization • Once a sperm binds to the zona pellucida, it undergoes the acrosomal reaction: • Triggered by Ca2+ • Acrosomal enzymes (acrosin, proteases, and others) are released in the immediate vicinity of the oocyte • Hundreds of acrosomes (sperms) must ruptureto break down the intercellular cement, rich in hyaluronic acid, that holds the granulosa cells together and to digest holes in the zona pellucida

From Egg to EmbryoAccomplishing Fertilization • The sperm that arrive first on the scene “sacrifice themselves” for the good of their brothers • A sperm that comes along later, after hundreds of sperm have undergone acrosomal reactions to expose the oocyte membrane, is in the best position to be the fertilizing sperm • Once a sperm cell binds to membrane receptors on the oocyte membrane, its nucleus is pulled into the cytoplasm of the oocyte

From Egg to EmbryoAccomplishing Fertilization • Each sperm carries a special two-part binding apparatus on its surface: • The beta protein part acts first as it binds to a receptor on the oocyte membrane: • This event engages the alpha protein part, causing it to insert into the membrane: • This somehow causes the egg and sperm membranes to open and fuse together with such perfect contact that the contents of both cells are combined within a single membrane

Blocks to Polyspermy • Polyspermy, or fertilization by more than one sperm cell, leads to a lethal number of chromosomes, and must be prevented

Blocks to Polyspermy • Two mechanisms ensure monospermy: the one sperm condition: • 1. When the plasma membrane of one sperm contacts the oocyte membrane, sodium channels open and Na+ diffuses into the oocyte from the extracellular space, causing its membrane to depolarize: • This electrical event, called the fast block to polyspermy, prevents other sperm from fusing with the oocyte membrane • Once the sperm has entered, Ca2+ is released by the oocyte’s endoplasmic reticulum into its cytoplasm, which activates the oocyte to prepare for cell division • It also causes the cortical reaction, in which granules located just deep to the plasma membrane spill their enzymes into the extracellular space beneath the zona pellucida • These enzymes, called zonal inhibiting proteins (ZIPS), destroy the sperm receptors, preventing further sperm entry

Blocks to Polyspermy • 2. The spilled material binds water, and as it swells it detaches all sperm still bound to receptors on the oocyte membrane, accomplishing the permanent slow block to polyspermy • In rare cases of polyspermy that do occur, the embryos contain too much genetic material and are nonviable (die)

Completion of Meiosis II and Fertilization • After a sperm enters an oocyte, it loses its tail and midpiece, and migrates to the center of the oocyte while the secondary oocyte completes meiosis II, forming the nucleus and ejecting the second polar body (a + b)

EVENTS FOLLOWING SPERM PENETRATION

Completion of Meiosis II and Fertilization • The ovum and sperm nuclei swell, becoming female and male pronuclei, and approach each other • A mitotic spindle develops between them (c), and the pronuclei membranes rupture, releasing their chromosomes into the immediate vicinity of the spindle • The true moment of fertilization occurs as the maternal and paternal chromosomes combine and produce the diploid zygote, or fertilized egg (d) • Almost as soon as the male and female pronuclei come together, their chromosomes replicate • The first mitotic division of the conceptus begins

EVENTS FOLLOWING SPERM PENETRATION

Preembryonic Development • Preembryonic development begins with fertilization and continues with the movement of the preembryo to the uterus, where it implants in the uterine wall • The mitotic divisions after fertilization occur without much growth between divisions, resulting in progressively smaller cells (high surface-to-volume ratio, which enhances their uptake of nutrients and oxygen), a process called cleavage • Cleavage forms two identical cells, blastomeres (36 hours), which then 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

CLEAVAGE

Preembryonic Development • The blastocyst is a fluid filled hollow sphere composed of a single layer of large, flattened cells called trophoblast cells and a small cluster of rounded cells, called the inner cell mass, located at one side: • Trophoblast cells take part in placenta formation • Inner cell mass becomes the embryonic disc, which forms the embryo proper

CLEAVAGE

Implantation • Occurs after 6-7 days; the trophoblast adheres to the endometrium, and produces enzymes that irritate (takes on the characteristics of an inflammation site) the endometrium • Uterine capillaries become permeable and leaky, and inflammatory cells including lymphocytes, natural killer cells, and macrohages invade the area • The trophoblast proliferates, forming two distinct layers; • Inner cytotrophoblast: cells retain their cell boundaries • Outer syncytiotrophoblast: cells that lose their plasma membranes and form a multinuclear cytoplasmic mass • Invade the endometrium and rapidly digests the uterine cells it contacts • Implanted blastocyst is covered over and sealed off from the uterine cavity by proliferation of the endometrical cells

Implantation • Takes about a week and is usually completed by the 14th day after ovulation—just when the endometrium normally begins to slough off in menes (must be prevented if the pregnancy is to continue) • Trophoblast cells secrete human chorionic gonadotropin (hCG), which acts on the corpus luteum: • Bypasses pituitary-ovarian controls at this critical time and prompts the corpus luteum to continue secreting progesterone and estrogen • Pregnancy test used today are antibody tests that detect hCG in blood or urine

IMPLANTATION

IMPLANTING BLASTOCYST AND FORMATION OF THE EMBRYONIC MEMBRANES

Implantation • The chorion, which develops from the trophoblast after implantation, continues this hormonal stimulus; thus, the developing conceptus takes over the hormonal control of the uterus during this early phase of development • Between the 2nd and 3rd month, the placenta assumes the role of progesterone and estrogen production for the remainder of the pregnancy • The corpus luteum degenerates and the ovaries remain inactive until after birth

RELATIVE CHANGES IN MATERNAL BLOOD LEVELS OF HUMAN CHORIONIC GONADOTROPIN, ESTROGENS, AND PROGESTERONE DURING PREGNANCY

Placentation • Is the formation of the placenta: • Temporary organ that originates from both embryonic (trophoblastic) and maternal (endometrical) tissues • The process of proliferation of the trophoblast gives rise to a layer of extraembryonic mesoderm on its inner surface that becomes the chorion (b)

Placentation • The chorion develops fingerlike chorionic villi, which are especially elaborate where they are in contact with maternal blood (c): • The mesodermal cores of the chorionic villi become richly vascularized by newly forming blood vessels, which extend to the embryo as the umbilical arteries and veins

Placentation • The chorionic villi come to lie in spaces between the maternal blood

Placentation

Placentation • The placenta is fully functional as a nutritive, respiratory, excretory, and endocrine organ by the end of the third month of gestation • Although the maternal and embryonic blood supplies are very close, they normally do not intermix

Placentation • Throughout pregnancy, blood levels of estrogens and progesterone continue to increase, encouraging growth and further differentiation of the mammary glands and readying them for lactation • The placenta also produces other hormones, such as human placental lactogen, human chorionic thyrotropin, and relaxin

EVENTS OF EMBRYONIC DEVELOPMENT • Formation and Roles of the Embryonic Membranes: • While implantation is occurring, the blastocyst is being converted into a gastrula, in which three primary germ layers form(ectoderm, mesoderm, and endoderm) and embryonic membranes develop • Embryonic membranes: • The amnion forms the transparent sac ultimately containing the embryo, and provides a buoyant environment that protects the embryo from physical trauma • The yolk sac forms part of the gut, produces the earliest blood cells and blood vessels, and is the source of germ cells that migrate into the embryo to seed the gonads • The allantois is the structural base for the umbilical cord that links the embryo to the placenta, and becomes part of the urinary bladder • The chorion helps to form the placenta, and encloses the embryonic body and all other membranes

Placentation

EVENTS OF EMBRYONIC DEVELOPMENT • Gastrulation: Germ Layer Formation • Gastrulation is the process of transforming the two-layered embryonic disc to a three-layered embryo containing three germ layers: ectoderm, mesoderm, and endoderm • Gastrulation begins with the appearance of the primitive streak, which establishes the long axis of the embryo: • The endoderm gives rise to epithelial linings of the gut, respiratory, and urogenital systems, and associated glands • The mesoderm gives rise to all types of tissues not formed by ectoderm or endoderm, such as muscle tissue • The ectoderm gives rise to structures of the nervous system and the epidermis

GASTRULATION FORMATION OF THE THREE PRIMARY GERM LAYERS

EVENTS OF EMBRYONIC DEVELOPMENT • Organogenesis: Differentiation of the Germ Layers • Organogenesis is the formation of organs and organ systems; by the end of the embryonic period, all organ systems are recognizable: • Neurulation, the formation of the brain and spinal cord, is the first event of organogenesis • As the embryo develops from a flat plate of cells, it rolls into a tube and the inferior endoderm becomes the lining of the primitive gut • Mesodermal specialization forms the notochord, and gives rise to the dermis, parietal serosa, bones, muscles, cardiovascular structures, and connective tissues • By 3 ½ weeks, the embryo has a blood vessel system and a pumping heart • Vascular modification include umbilical arteries and veins, a ductus venosus, and the foramen ovale and ductus arteriosus

PREGNANCY AND HUMAN DEVELOPMENT