Note
Studied by 8 people
Human Development and Heredity
Fertilization
Definition: Union of the nuclei of the egg and sperm, resulting in a zygote.
Timing: Occurs during ovulation around day 14 of the menstrual cycle.
Egg survives ~24 hours; sperm can survive ~12-72 hours depending on conditions.
Location: Normally occurs in the first third of the fallopian tube.
Process:
Sexual intercourse deposits millions of sperm in the vagina.
Approximately 100,000 sperm survive to swim towards the egg.
Sperm release enzymes to penetrate the egg's membrane (zona pellucida).
A single sperm penetrates, uniting nuclei to form a zygote with 46 chromosomes (23 from each parent).
Prenatal Development
Stages: Comprised of cleavage, growth, morphogenesis, and differentiation.
Divided into three periods:
Early embryonic period (first 2 weeks):
Begins with fertilization and progresses from zygote to morula (16 cells) to blastocyst.
Blastocyst Composition:
Trophoblast: Cells that will form the future placenta.
Inner cell mass: Cells developing into the embryo.
Implantation: Blastocyst embeds into the endometrium; secretion of hCG prevents corpus luteum deterioration and signals pregnancy.
Embryonic period (weeks 3-8):
Lasts 6 weeks; significant organ and structure formation (organogenesis).
Formation of extraembryonic membranes (amnion, chorion, yolk sac, allantois) for protection and nourishment.
Fetal period (weeks 9-birth):
Focuses on growth and tissue maturation.
Hormonal influences cause sexual differentiation, with structural changes preparing for delivery (development of fat layers, skin, fetal practices like breathing and sucking).
Labor
Definition: Rhythmic contractions of the myometrium to expel the fetus and placenta.
Triggers: Hormonal changes, especially between estrogen and progesterone, initiate labor.
Standard Birth Presentation: Usually head-first (vertex position); breech presentations may require interventions due to complications.
Fertilization
Definition: The fertilization process is defined as the union of the nuclear genetic material from an egg and a sperm cell, resulting in the formation of a zygote, which represents the earliest developmental stage of a new organism.
Timing: This critical event generally occurs during ovulation, which is approximately around day 14 of the menstrual cycle. The lifespan of the egg is relatively short, surviving for approximately 24 hours post-ovulation, while sperm can exhibit a variable lifespan of about 12 to 72 hours, depending on the surrounding uterine conditions and the health of the sperm itself.
Location: Fertilization predominantly takes place in the ampullary region of the fallopian tube, which is the section that is nearest to the ovary.
Process:
Initial Stages: Sexual intercourse leads to the deposition of millions of sperm cells into the vagina, with only a fraction of these capable of making the journey to the egg.
Approximately 100,000 sperm actually survive to swim through the cervix and into the uterus, ultimately heading toward the fallopian tube where the egg is located.
Egg Penetration: Upon reaching the egg, sperm release enzymes that help to dissolve the egg's protective outer layer, known as the zona pellucida.
One single sperm successfully penetrates this layer, leading to the fusion of the two nuclei, which combines the genetic material from both parents, culminating in a zygote that contains a total of 46 chromosomes (23 chromosomes contributed from each parent).
Prenatal Development
Stages: The process of prenatal development is structured into four main stages, namely cleavage, growth, morphogenesis, and differentiation, which are critical for the formation of a functional organism.
Divided into three main periods:
Early Embryonic Period (First 2 weeks):
Begins immediately post-fertilization as the zygote undergoes rapid cell divisions, transitioning into a morula (a solid ball of 16 cells) before developing into a blastocyst, which is a hollow structure of about 100 cells.
Blastocyst Composition:
Trophoblast: The outer layer of cells that will ultimately develop into the placenta, which is vital for nutrient transfer and waste elimination between the mother and embryo.
Inner Cell Mass: This cluster of cells will differentiate into the embryo itself.
Implantation: The blastocyst embeds in the endometrium (the lining of the uterus), triggering the secretion of hCG (human chorionic gonadotropin) from trophoblast cells, which prevents the degeneration of the corpus luteum and signals the body to maintain pregnancy.
Embryonic Period (Weeks 3–8):
Lasting six weeks, this period is characterized by significant organogenesis, where major organs and body structures start forming.
Formation of extraembryonic membranes, crucial for the protection and nourishment of the growing embryo, including the amnion, chorion, yolk sac, and allantois.
Fetal Period (Weeks 9 to Birth):
Focuses predominantly on growth and tissue maturation, preparing the fetus for survival outside the womb.
Hormonal signals lead to sexual differentiation of the developing fetus with recognizable features appearing; this phase also involves development of subcutaneous fat layers, skin, and coordination of fetal practices such as breathing movements and sucking reflexes in preparation for birth.
Labor
Definition: Labor is characterized by rhythmic contractions of the myometrium (the muscular layer of the uterus) which ceaselessly work to expel both the fetus and the placenta, marking the end of pregnancy.
Triggers: Various hormonal changes, most notably shifts in the balance of estrogen and progesterone, initiate the onset of labor processes.
Standard Birth Presentation: Typically, the fetus is positioned head-first (vertex position) during delivery; however, variations such as breech presentations can occur, which may necessitate specific medical interventions due to increased risks associated with delivery complications.