Early human development Flashcards
Oogenis
- process involving meiotic division of a diploid germ cell, resulting in a single haploid female gamete (egg) and to two polar bodies
- cytoplasm is kept in 1 developing oocyte and the 2 polar bodies (nuclear biproducts of meoiosis) extruded and ulyimately degrated
- Fertilizationoccurs externally in fish/frogs (lower vertebrates) internally in higher vertebrates
Fertilization and first cell division in humans
- Once a singlr sperm enters the oocyte, the zona reaction (hardening of zona pellucida) occurs to block polyspermy
- The second meiotic division of the oocyte is completed at fertilization (generation of the 2nd polar body)
- formation of pronuclei and DNA replication follows
- pronuclei fuse into a single (diploid) zygote nucleus (genetically unique)
- Miotic division now occurs
Preimplantation
*The blastocyst is in the uterus for 2 days (day 4 and 5) prior to implantation
*During this time the blastocyst derives nourishment from secretions of the uterine glands
*The zona pellucida must degrade before implantation
*An immunosuppressant, early pregnancy factor is secreted
by the trophoblast (detected in the mother’s serum- helps to prevent rejection of the embryo by the mother’s immune system)
Early stages of implantation
The inner cell mass contains pluripotent stem cells
The trophectoderm is extraembryonic and is made up of:
* Cytotrophoblasts (individual cells)
* Syncytiotrophoblasts (invades the endometrial epithelium)
fertilization in human occurs at ?
the falopian tubes
When things go wrong: ectopic pregnancies
- X is the “normal”
implantation site
Second week of development overview
I. Implantation of the blastocyst is completed (6-10 days after fertilization)
II. Embryoblast becomes a bilaminar disc ( epiblast and hypoblast )
III. Amnion and amniotic cavity forms IV. Umbilical vesicle (yolk sac) forms
Hypoblast =
primitive/primary
What does extraembryonic mean?
outside the embryon
Third and fourth week of development overview
(here we can see the distinct parallels to lower vertebrate development)
I. Gastrulation: Establishment of the three germ layers II. Development of the notochord
III. Development of the somites
IV. Neurulation: formation of the neural tube
Gastrulation:
- This process gives rise to the three primary germ layers
- Epiblast cells migrate through the primitive streak (extends anteriorly)
forming the underlying mesoderm and definitive endoderm - The primitive streak than regresses posteriorly, while the forming
notochord (forms in the anterior to posterior direction) - Cells that remain in the epiblast will form ectoderm
Gastrulation: formation of three definitive germ layers
- The primitive streak identifies the craniocaudal axis and left/right sides of the embryo
- Epiblastic cells move through the primitive streak (ingression) to either form the mesoderm -or- displace hypoblast cells to form the definitive endoderm
- The remaining epiblastic cells form the ectoderm of the embryo
- Primitive streak diminishes in size by the fourth week
Formation of the neural plate
Elongation and regression of the primitive streak
- The primitive streak reaches an anterior limit forming a primitive node and begins to regress posteriorly
- The developing notochord induces the overlying ectoderm to thicken and form the neural plate
- The neural plate develops craniocaudally. The expanded cranial portion gives rise to the brain, and the tapered portion gives rise to the spinal cord
gastrulation starts at the
dorsal lips
When things go wrong: Sacrococcygeal teratoma
- due to the failure of the
primitive streak to regress *1:35,000 live births - male/female disparity (76% female)
- can include many different tissue types (e.g., neural tissue, skin, teeth)
Why do you think this happens? Most are benign- when is not regress
Neurulation: formation of the neural tube (NT)
The NT gives rise to the central nervous system and neural crest
o The neural plate invaginates to form a neural groove along the central axis
o The neural folds on both sides of the groove fuse to form the neural tube
o Cells at the crest of the neural folds (neural crest cells) detach and migrate dorsolaterally
Neurulation is completed by the end
Neural crest cells
of the fourth week
When things go wrong:
Neural Tube Defects (NTDs)
(neural tube fails to close
Disorders:
spina bifida occulta (least severe –tuft of hair)
spina bifida with myeloschisis (most severe –neural tissue is expose
Risk Factors
- use of anti-seizure medications (e.g. valproic acid)
- neural tube defect in a previous child (genetics)
- lack of folic acid
- maternal diabetes
- maternal hyperthermia (hot tubs, etc.)
Prevention:
Folic acid supplements before conception and during neural tube developmental period
At fertilization a single sperm must pass through the zona pellucida
yes, zone of reaction, prevents polysperm
The zona reaction prevents polyspermy
The zona pellucida persists until day 4 of development; once it degrades, the embryo can implant
because prevents inmanute implanation
what happens is something goes wrong with neurolation
neurotube is not going to cloose- spina bifida
The inner cell mass contains pluripotent stem cells, which can form any cell of the embryo proper
- extra embryonic, becomes part of the placenta
- ## neuro cell mass
The fallopian tube is the most common site for ectopic pregnancies
fertilization happens, implantation happens
Cells of the epiblast migrate through the primitive streak during gastrulation to form the three germ layers
migrate, during the process of fertilization
Sacrococcygeal teratoma results from a failure of the primitive streak to regress
Neural tube defects result when the neural tube fails to close properly
neurotube effects- spina bifida
