Module 2: Conception To Birth Flashcards
Epigenesis
Emergence of new structures and functions during development
Aristotle
Gametes/Germ Cells
Unique in function
Each on contains half of genetic material found in other cells
Produced through meiosis
Union of two gametes produces normal amount of genetic material (23 pairs of chromosomes)
Conception
Union of sperm and egg
Act of intercourse takes place near the time of egg release
Sperm travels 6 hours/15-18 cm
Sperm that get to egg are likely healthy and structurally sound (survival of the fittest)
Sperm head penetrates membrane of the egg
Chemical reaction seals the membrane preventing other sperm from entering
Tail falls off and contents of head gush into egg
Nuclei of two cells merge within hours
Zygote
Fertilized egg
Has full complement of human genetic material
Starting point for development of the organism
Cell division begins quickly within hours, doubling 2x per day
Cell Division/Meiosis
12hrs after conception zygote divides in two
Each contains full complement of genetic material
Two cells continue to divide until it becomes a newborn with trillions of cells
Cell Migration
Movement of newly formed cells away from point of origin
Neurons that originate deep in embryonic brain and travel to outer reaches of brain
Cell Differentiation
Embryonic stem cells: all of embryo’s cells
Can give rise to any of 200+ cell types (unique flexibility)
Cells start to specialize after several cell divisions
Cell’s location influences its future development
Gene expression distinguishes types of cells
Apoptosis
Cell death
Genetically programmed cell suicide
E.g. Formation of fingers relies on cell death of cells between fingers
Follows a specific timeline programmed by the cells
Hormones/Steriods
Play crucial role in sexual differentiation
E.g. absence of androgens leads to female genitalia
Glucocorticoids limit fetal growth and help fetal tissues (e.g. key organs) mature later in development
Monozygotic Twins
Identical twins
Two cells with same genetic makeup
Whether MZ twins share a placenta/amniotic sac depends on when the zygote divides
-If division occurs before implantation (each has a placenta and sac to themselves)
-If split occurs after implantation in uterine lining, they share placenta but have independent amniotic sacs
Dizygotic Twins
Fraternal twins
When two eggs happen to be released from ovary into fallopian tube and are fertilized
Originate from two different eggs and sperm (two separate conceptions)
No more alike genetically than non twin siblings with same parents
End of 1st Week of Conception
Zygote embeds into uterine lining
Dependent on mother for sustenance
Embedded ball of cells begins to differentiate
Becomes embryo if successfully embedded
-inner cell mass becomes embryo and rest of cells are support system (e.g. amniotic sac, placenta)
End of 2nd Week of Conception
Inner cell mass folds itself into three layers each with different developmental destiny
U shaped groove forms down center of top layer, folds at top move and fuse together
Creates neural tube
One end of tube swells and develops into the brain, the rest becomes the spinal cord
Layers of Inner Cell Mass
Top layer: nervous system, nails, teeth, inner ear, lens of the eyes, outer surface of skin
Middle layer: muscles, bones, circulatory system, inner layers of the skin, other internal organs
Bottom layer: digestive system, lungs, urinary tract, glands
Amniotic Sac
Membrane filled with clear, watery fluid in which the fetus floats
Operates as protective buffer for developing fetus
Provides even temperature and cushion from jolting
