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EMBRYOLOGY SPRING 2012
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REPRODUCTIVE ANATOMY – FEMALES Ovaries –
Female gonad (2) Produces egg cells & reproductive hormones Surface of ovary is covered in follicles 400,000
found in most women; formed before birth Follicle –
A cluster of cells that surround and nourish a developing egg cell
Secretes estrogen (hormone) ~ 1 follicle matures and releases an egg cell in
women every 28 days from puberty menopause This releasing of the egg cell is called ovulation
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REPRODUCTIVE ANATOMY – FEMALES After ovulation, the remaining follicle tissue
continues to grow w/in the ovary Forms a solid mass called the corpus luteum The CL secretes:
Progesterone (hormone) that maintains the uterine lining during pregnancy
More Estrogen (hormone)
If egg is NOT fertilized CL disintegrates & a new follicle matures during next cycle
More later on if the egg is fertilized…
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REPRODUCTIVE ANATOMY – FEMALES Oviduct / Fallopian Tube –
“Tube” in which egg cell travels after ovulation Cilia push the egg cell toward the uterus Fertilization usually occurs in the oviduct
Uterus / womb – Site of pregnancy Lined with a rich supply of blood vessels –
endometrium Embryo implants in the endometrium linning
Embryo is considered a fetus at the ninth week of development
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REPRODUCTIVE ANATOMY
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REPRODUCTIVE ANATOMY – MALES Testes –
Male gonad (2) Produces sperm (in the seminiferous tubules w\
in) and reproductive hormones “Housed” outside the body in the scrotum
sperm cannot develop at human core temperature, therefore the scrotum placement allows sperm forming cells to function normally
Vas Deferens – Moves sperm away from the testes
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FORMATION OF SPERM CELLS Spermatogenesis –
Formation of sperm cells Takes 65-75 days in humans Cells differentiate into primary spermatocytes (diploid)
Undergo Meiosis Two secondary spermatocytes are produced (haploid)
Undergo Meiosis II Four sperm cells are produced (haploid)
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SPERM CELLS Acrosome –
Contains enzymes that help sperm penetrate the egg
Surrounds the head/nucleus
Tail – Flagellum
Neck & Middle piece – Contain mitochondria Mitochondria provide ATP for movement of the
tail
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OOGENESIS Oogenesis –
Formation of egg cells Majority of the process occurs in the ovaries, prior to birth
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FORMATION OF EGG CELLS At birth, each follicle contains a
dormant primary oocyte (diploid)
“Frozen” in Prophase I
At puberty, a hormonal change (FSH) triggers the primary oocyte to complete Meiosis I every 28 days
Haploid
Follicle enlarges primary oocyte completes Meiosis I and begins M2
Meiosis then STOPS again at Metaphase 2 (haploid)
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FORMATION OF EGG CELLS (CON’D) Larger haploid cell is now the Secondary oocyte
Smaller haploid disintegrates The secondary oocyte is the stage that is released by the ovary (ovulation) If a sperm cell enters the ova, the secondary oocyte completes meiosis 2 Meiosis 2 creates another polar body and the actual egg cell Egg + Sperm = zygote
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HORMONES FSH –
Follicle-Stimulating Hormone Increases sperm production (males) Stimulates the growth of the follicle (females)
LH – Luteinizing Hormone In males, stimulates sperm production In females:
Stimulates growth of follicle Production of secondary oocyte Promotes ovulation
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EMBRYONIC DEVELOPMENT If fertilization occurs:
The sperm and egg fuse zygote Remember: Chromosome number never changes after
this point!!!
First two major stages of development:1) Cleavage 2) Gastrulation
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CLEAVAGE Cleavage is the rapid chain of cell divisions
(differentiation) that create a multi-cellular embryo (blastula) from the single- celled zygote Zygote does not grow in size during cleavage
As cells divide, a solid ball of cells is formed morula
Morula
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CLEAVAGE Cell divisions split the zygote into many smaller cells
Divisions act as an organizer dividing the embryo into developmental regions (more on this…)
As cleavage continues, a fluid-filled cavity forms in the center called the blastocoel
The blastocoel is surrounded by 1+ layers of cells
This new “hollow ball” is called the blastula
AKA - blastocyst in humans
The blastula now enters the gastrulation stage
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GASTRULATION The 2nd major phase of embryonic
development Adds more cells to the embryo Cell division continues (now called a gastula) Cells are sorted into three distinct layers of
embryonic tissue:1) Ectoderm (outer most layer)2) Endoderm (inner most layer)3) Mesoderm (middle layer)
These three cell layers eventually develop into the various parts of our human body
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GASTRULATION A small groove (the Blastopore), forms at
one side of the gastula
Cells that will form the ectoderm spread across the surface of embryo; mesoderm cells spread inside the embryo
Endoderm cells create a digestive cavity called the archenteron
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GASTRULATION Gastrulation is complete when the embryo is
3-layered
• The endoderm & archenteron fill the blastocoel space
• The entire surface (except the yolk plug) is covered by the ectoderm
- Yolk plug will be future anus
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GASTRULA LAYERS Ectoderm –
Epidermis, nervous system, spinal cord, eyes, tooth enamel, hair
Endoderm – Lining of digestive tract, digestive organs, pancreas,
liver, respiratory system, lungs, tonsils, urinary tract, reproductive system, tongue
Mesoderm – Most organs (kidneys, heart), skin, bones, muscles,
blood, circulatory system, excretory system
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ORGAN FORMATION Once the 3-layers form, cells in each layer
differentiate into tissues and organs begin to form!
Notochord develops (from mesoderm) between the digestive tract and nerve cord
Present only in embryos replaced by backbone
Neural tube also forms (from ectoderm) Gives rise to the brain and spinal cord
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EMBRYONIC DEVELOPMENT SEQUENCE
1. Zygote forms2. Cleavage of cells
3. Blastula forms4. Gastrulation
5. Organs form!
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GESTATION Another term for “pregnancy” Average of 40 weeks for humans Cleavage begins ~ 24 hours after fertilization By 7th day embryo blastocyst
Blastocyst implants in uterine wall Outer layer called the trophoblast
Aids in implantation and forms the placenta
Implantation occurs after ~ one week Gastrulation begins after implantation
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GESTATION Blastocyst & trophoblast give rise to the
extraembryonic membranes. This forms during 1st trimester and consists of: Amnion – encloses the amniotic sac containing embryo
and fluid; protects embryo; breaks just prior to birth (water breaks)
Chorion – forms part of placenta; secretes HCG hormone HCG VERY IMPT! W/O this menstration would occur emrbyo
would abort
Yolk Sac – produces embryos 1st blood cells
Allantois – forms part of umbilical cord (connects embryo to placenta via the “belly button”
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PLACENTA Provides nourishment and oxygen to the embryo; stores
embryonic waste Chorionic villi – contain embryonic blood vessels; villi
absorb nutrients & oxygen from mom’s blood and pass on to embryo NOTE: Mom’s blood & embryo blood NEVER touch
C.V. also carry waste away from embryo Placenta allows protective antibodies to pass from mom to fetusHOWEVER, placenta cannot protect against certain viruses (HIV) or drugs/alcohol serious injury to fetus
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3 TRIMESTERS 1ST Conception birth is divided into 3 trimesters First trimester (1-3 mo)
Multiple tissue layers and organs have developed
All organs and major body parts are present Fingers and toes are developing
9 weeks
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Second trimester (4-6 mo) Increases size and refines structures Placenta takes over progesterone secretion Has eyebrows, eyelashes, fingernails, toenails,
body hair Very active Eyes open and teeth form
3 TRIMESTERS 2ND
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Third trimester (7-9 mo) Gains weight and strength Respiratory and circulatory systems prepare for
air breathing Bones harden and muscles thicken Loses body hair
3 TRIMESTERS 3RD
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CHILDBIRTH Labor – rhythmic contractions of the uterus
Oxytocin – hormone that stimulates contractions of the uterus and mammary glands
Post-birth, babies lungs expand and are used for the first time!
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REPRODUCTIVE OPTIONS
Technology can help infertile couples In Vitro Fertilization (IVF)
Most common type of assisted reproductive technology
Woman’s eggs (secondary oocyte) are mixed with sperm in a culture dish and incubated to aid fertilization
After an embryo has been developed (8 cells), it is inserted into the woman’s uterus
Higher risk of birth defect/low birth rate with IVF babies Approx. $10,000 a try
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REPRODUCTIVE OPTIONS Fertility drugs are also commonly used Only available for women Drugs stimulate follicle development increase the number of eggs released Unfortunately, this is very risky 2+ fetusesincreases the risk of birth defect, low birth rate, mental retardation, etc.
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STEM CELLS
Unspecialized cells that can differentiate and create new, specialized cells These then become tissues/organs
Many types (amniotic, umbilical cord, etc.) Two we will focus on:
1) Embryonic 2) Adult
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STEM CELLS Embryonic –
Come from blastocysts (human embryos) Can differentiate into all types of specialized
cells (aka – can become the cell of any body tissue)
Adult – Come from three sources:
1) Bone marrow2) Adipose tissue (fat/lipid cells)3) Blood
Can only differentiate into some cell types
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STEM CELL POSSIBILITIES Under the right conditions (grown in a
culture), embryonic stem cells can reproduce indefinitely
Using Bob’s cells, the nucleus is taken out and inserted into the ES cell Since these stem cells are now genetically
identical to Bob’s own cells, his body will not reject these new cells!
Embryonic stem cells could cure/help many diseases: Diabetes, heart disease, Parkinson’s,
Alzheimer's, Lou Gehrig’s, arthritis, and/or organ failure
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STEM CELL CONTROVERSY Ethical issues –
ES cells can only come from discarded embryos (donated by infertile patients) Many fear that embryos will be created just to be
destroyed