MCB 135 E Discussion 2 September 13-17, 2004

Physiology of Human Development GSI – Jason Lowry – Office Hours – Thursday 1-2pm in 208 Donner Discussions: –Thursday in 130 Wheeler –Friday 2-3 in 219 Dwinelle –Material covered is similar for each discussion Reviews –An organized review will be given prior to each mid-term and the final –Times and Locations to be announced

Fertilization Reproductive Techniques (Web-Site) Terminology Stages of Fertilization Roles of Ovum Roles of Sperm

Terminology Fertilization – Formation of zygote Capacitation – Changes that occur between sperm and ovum to allow only one sperm to successfully penetrate the ovum (Increase fertilizing capacity) Cleavage – Mitotic division of the zygote into a morula Morula – Ball of cells that will form blastula that will implant in uterus Zygote – Union of female and male sex cells into a combined cell

Stages of Fertilization Penetration –Random contact between sperm and egg –Sperm propel past corona cells and attach to zona pelucida –One sperm-One Egg Activation –Completion of Meiosis of ovum – forms female pronucleus –Sperm Changes – Sperm travels toward center of egg, head swells, becomes male pronucleus –Rearrangement in egg – biochemical changes necessary prior to nuclear fusion Nuclear Fusion –Union of two pronuclei –Loss of nuclear envelope, contribution of chromosomes to first mitotic cleavage

Roles of Ovum Contribute maternal complement of genes to the nucleus of the fertilized egg Reject all sperms but one Provide food reserves until the embryo begins to feed upon exogenous material

Roles of Sperm Reach and penetrate egg Activate the egg to nuclear and cytoplasmic division necessary to embryonic development Contribute the paternal complement of genes to the nucleus of the fertilized egg

Results of Fertilization Reassociation of male and female sets of chromosomes to full diploid number –Physical basis for inheritance and variation Determination of sex Activation of ovum into cleavage

Implantation Terminology Sequence of Events Changes necessary for implantation Time Course

Terminology Implantation – Process of preparation of the uterine zone for blastocyst embedding and the active penetration of this zone by the blastocyst Nidation – Latin for nest (alternate name for implantation) Blastocyst – Approximately 60 cells stage where morula has become fluid filled –Of the cells, about 5 clump together to form inner cell mass (ICM) that projects into blastocele

Implantation Events Days 1-8 –Fertilized egg begins to divide by cleavage into smaller blastomeres –Blastomere increase follows a double synchronous sequence initially, but later becomes asynchronous –Later stage cleavage forms a ball of cells or morula –Fluid begins accumulating in morula and a conversion occurs to the blastula (blastocyst) –Blastocyst attaches to uterine stroma –Outer layer of cells begin to proliferate and invade stroma of uterus

Implantation Events Days 8-16 –Blastocyst lodged in uterine stroma –Trophoblast (outer cells) invades uterine stroma and begins to form placenta –Blastocele becomes chorionic cavity –Inner cell mass begins to proliferate and form germ disc and primitive entoderm –Complex sequence of chemical and physical interactions occur between ovum and mother Only about 50% of fertilized eggs successfully implant

Uterine Preparation Estrogen –Involved in proliferation of uterus (hypertrophy) –3-5 Days post-ovulation E dominates and is thought to play a role in transport of ovum Progesterone –Increased Vascularization (hypervascularization) –5-7 days post-ovulation, P increases and counters the effect of E thus relaxing uterine contractions hCG –Prevents corpus luteum regression –Similar to LH –Maintains E+P during early embryonic development

Embryonic Development Terminology Germ Layer Theory Embryonic Stages Germ Layers and Their Systems

Terminology Germ Layer Theory –Embryo’s Method of sorting out its parts Ectoderm – Outer covering of embryo Entoderm (Endoderm) – Lies under the ectoderm and forms lining of the primitive gut cavity Mesoderm – Develops between ectoderm and entoderm Gastrulation – The process by which germ layers come to occupy their characteristic positions Embryonic Stages – Phases of gastrulation; stages of organ and tissue differentiation

Embryonic Stages Early Gastrulation –Occurs at implantation when cells on ICM arrange into the entoderm –Remaining cells that are neither trophoblast or entoderm become a plate containing the progenitors of future ectodermal and mesodermal cells –Referred to as 2-layered embryo and characterized by embryonic disc

Embryonic Stages Late Gastrulation (second phase) –Segregation of mesoderm and notochord occurs Notochord will for primitive vertebral column –Starts by end of second week and finishes by end of third week –2-layered embryo becomes 3-layered embryo –Primitive streak appears upon upper surface of embryonic disc over which mesoderm spreads –At conclusion of this process, upper layer of embryonic disc becomes ectoderm

Germ Layers and Their Systems Ectoderm –Epidermis and lining cells of glands –Appendages of skin –Nervous system –Posterior Pituitary –Chromafin organs - adrenal medulla –Anterior Pituitary –Some Epithelium Entoderm –Epithelial lining of alimentary canal –Lining cells of glands that open to alimentary canal –Epithelium of most of the urinary bladder and urethra –Epithelium of prostate Mesoderm –Remaining organs and tissues not made by Ectoderm or Entoderm –Connective tissue –Teeth –Musculature –Blood –Adrenal Cortex