Embryology Spring 2012

1. EmbryologySpring 2012

2. 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

3. 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…

4. 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

5. Reproductive Anatomy

6. 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

7. 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)

8. 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

9. Oogenesis • Oogenesis – • Formation of egg cells • Majority of the process occurs in • the ovaries, prior to birth

10. Formation of Egg Cells • At birth, each follicle contains a • dormant primaryoocyte (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)

11. 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

12. 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

13. 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: • Cleavage • Gastrulation

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Embryonic Development Sequence • Zygote forms • Cleavage of cells • Blastula forms • Gastrulation • Organs form!

22. 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

23. 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”

25. 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 fetus HOWEVER, placenta cannot protect against certain viruses (HIV) or drugs/alcohol  serious injury to fetus

26. 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

27. 3 Trimesters  2nd • 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

28. 3 Trimesters  3rd • 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

29. 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!

30. 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

31. 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+ fetuses increases the risk of birth defect, low birth rate, mental retardation, etc.

32. 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

33. Stem Cells • Embryonic – • Come from blastocysts (human embryos) • Can differentiate into all types of specialized cells (aka – can become the cell of anybody tissue) • Adult – • Come from three sources: 1) Bone marrow 2) Adipose tissue (fat/lipid cells) 3) Blood • Can only differentiate into somecell types

34. 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

35. 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