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
Placenta
Rich network of blood vessels (1lb) that extends into the tissues of uterus
90% of cells for placenta come from fetus
Semipermeable: permits exchange of materials carried in bloodstreams (Prevents blood of mother and fetus from mixing)
Umbilical Cord
Tube containing blood vessels that run to fetus
Connects the amniotic sac and the placenta
Embryo
Developing organism from 3rd-8th week
Major systems and internal organs begin (but don’t complete) their development (heart, limbs, sensory, organs, brain)
Cephalocaudal development: areas near the head develop earlier than areas farther from the head
Fetus
Developing organism from the 9th week until birth
9 Weeks
All major organs present
Rapid brain growth
Rudimentary eyes and ears are forming
Sexual differentiation begins
Ribs are visible
Fingers and toes emerge, nails are growing
16 Weeks
Development of lower half; external genitals may be visible
28 Weeks
Lungs and heart sufficiently developed that in cases of premature birth, possible the fetus could survive without medical intervention
Eyes can open and move (e.g. REM)
Auditory system functions
4th Week Post Conception
Embryo is curved so head and tail structure are nearly touching
Primitive heart is visible and is already beating and circulating blood
Leg and arm buds are slightly visible (more arm than leg)
Cephalocaudal Development
Areas near the head develop more quickly than extremities
5th Week Post Conception
Nose, mouth, and palate begin to differentiate into separate structure
3 weeks later, nose and mouth are almost fully formed (clef palate originates at this time)
11 Weeks
Heart at basic adult structure
Developing spine and ribs
Major divisions of the brain
5 months
Lower part of body growth accelerates
External genitalia are substantially developed
18 weeks
Covered with fine hair and greasy coating protecting skin from long immersion in liquid
Components of facial expressions are present
Last Three Months (6+ Months)
Fetus grows dramatically in size (triples weight)
Develops repertoire of behaviours and learns from experiences
