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MCB 135 E
Discussion 2
September 13-17, 2004
Physiology of Human
Development
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GSI – Jason Lowry
E-Mail – [email protected]
Office Hours – Thursday 1-2pm in 208 Donner
Discussions:
– Thursday 11-12 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
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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
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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
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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
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Epidermis and lining cells of glands
Appendages of skin
Nervous system
Posterior Pituitary
Chromafin organs - adrenal medulla
Anterior Pituitary
Some Epithelium
• Entoderm
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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
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Remaining organs and tissues not made by Ectoderm or Entoderm
Connective tissue
Teeth
Musculature
Blood
Adrenal Cortex