General Embryology 1 Flashcards
Oocytes
- 2 million at birth
- 40,000 at puberty
- 400 ovulated
- LH surge causes changes within the ovary and egg for it to rupture
Egg is surrounded by
- Zona pelucida
- Corona radiate
Zona pelucida
- Layer of glycoproteins
Corona radiate
- Granulosa cells
- Surround the ovary after ovulation
Early egg development
- Egg swept into oviduct (uterine tube)
- Fertilization
- Begins process of cellular division (cleavage)
- Enters uterus (day 4)
- Implantation into uterine wall
begins 6th embryonic day
Week 1 (days 1-6) of oocyte development
- Fertilization, day 1
- Cleavage, day 2-3
- Compaction, day 3
- Blastocyst formation, day 4
- Implantation begins, day 6
Fertilization usually occurs in
- Ampulla of the uterine tubes
Fertilization steps (8)
- Multiple sperm bind to corona radiate
- Sperm passes through the corona radiata
- Sperm binds to a ZP protein in the zona pellucida
- Cell membrane of 1 sperm fuses with cell membrane of oocyte
- Initiates calcium influx causing the release of cortical granules (cortical reaction)
- Completion of 2nd meiotic division of the oocyte
- Male and female pronuclei form fuse
- Arrangment of the chromososmes for mitotic cell division
Sperm binds to a ZP protein in the zona pellucida, causing
- Release of enzmes
- Allow it to burrow through ZP (acrosome reaction)
Initiation of calcium influx by fusion causes
- Release of cortical granules (cortical reaction)
- Blocks other sperm from fertilizing egg
Zygote
- Result of the union of the male/female gametes
- Restores 46
- Maternal/paternal chromosomes are mixed
Cleavage of zygote
- Repeated mitotic cell division
- Results in an increase in cell number (not size)
Blastomere
- A cell formed by cleavage of a fertilized ovum
- Becomes smaller with division
- Early blastomeres are totipotent (capable of giving rise to any cell type)
8 cell stage
- Compaction begins
- Blastomeres tightly align by increased cell adhesion
- Segregate inside vs. outside
Blastomere cell segregation (inside vs. outside)
- Outer cells become trophoblast (will form placenta)
- Inner cell mass will form embryo
Morula
- Conceptus with 16-32 blastomere
- Inner cells and outer cells
- Enters uterus ~ 4 days after fertilization
Blastocytic cavity
- Created by sodium and water being pumped into embryo
- Mostly separates the embryoblast and trophoblast
- Embryo is called a blastocyst
~ 6 days after fertilization
- Blastocyst attaches to endometrium
- Blastocyst “hatches” from zona pellucida prior to implantation
Blastocyst “hatches” from zona pellucida by
- Enzymatically bores a hole and squeezes out
Implantation in the wrong location (ectopic pregnancy) can be caused by
- Early ZP shedding
- Delayed zygote transport
Week 2: days 7-14
- Embryo becomes more deeply embedded within endometrium
- Development of trophoblast into placenta precursor
- Formation of bilaminar embryo, amniotic cavity
(~Day 7) Trophoblast proliferates and differentiates into
- Cytotrophoblast
- Syncytiotrophoblast
Cytotrophoblast
- Stem cell population that adds cells to the syncytioblast
Syncytiotrophoblast
- Derived from cytotrophoblast proliferation
- Cells lose membranes and form a syncytium
Syncytiotrophoblast is located
- At the embryonic pole
- Adjacent to embryoblast)
Syncytiotrophoblast on day 8
- Begins invading into the endometrium
(~Day 10) Blastocyst/embryo becomes
- Completely embedded within the endometrium
Synctytiotrophoblast contacts uterine vessels and glands, creating
- Primitive uteroplacental circulation
- Lacunae filled with maternal blood
- Lacunae fuse creating a lacunar network
Cytotrophoblast forms extensions that grow into overlying synctiotrophoblast to form
- Chorionic villi
- Villi become penetrated by extraembryonic mesoderm
- Eventually form blood vessels (week 2 and 3)
By day 8, the embryoblast differentiates into
- 2 epithelial cell layers
- Epiblast
- Hypoblast
Epiblast
- Columnar cells adjacent to amniotic cavity
- Dorsal side of embryo
Hypoblast
- Small cuboidal cells adjacent to exocoelomic cavity
- Primitive yolk sac
Epiblast + Hypoblast
- Forms into a bilaminar embryonic disc
- Located between amniotic cavity and the primary umbilical vesicle
Amniotic cavity
- Forms within the epiblast
- Epiblast cells migrate forming the amnion (which encloses the amniotic cavity)
Hypoblast migration
- Lines the blastocystic cavity
- Forms the exocoelomic membrane
Blatocystic cavity
- Referred to as the primary umbilical vesicle (primary yolk sac)
Extraembryonic mesoderm
- Located outside the embryo
- Produced by hypoblast and cytotrophoblast
- Proliferates and develops spaces within
Spaces within the extraembyronic mesoderm
- Fuse to form the extraembryonic coelom (chorionic cavity)
Extraembryonic coelom (chorionic cavity)
- Fluid filled cavity
- Surrounds the umbilical vesicle and amnion
- Except at connecting stalk (precursor to the umbilical cord)
Week 3: days 14-21
- Appearance of primitive streak
- Development of the notochord and neural induction
- Differentiation of 3 germ layers (gastrulation)
Gastrulation
- Process where the bilaminar embryonic disc is converted into a trilaminar embryonic disc
- Beginning of morphogenesis
Morphogenesis
- Development of the form and structure of organs and parts of the body
3 germ layers developed in gastrulation
- Ectoderm
- Endoderm
- Mesoderm
Ectoderm
- Outside layer
- Gives rise to skin and nervous tissue
Mesoderm
- Middle layer
- Generates most of the muscle, blood and connective tissue
Endoderm
- Epithelial lining and glands of the gut, lung, urogenital tract
Onset of gastrulation
- Beginning of the 3rd week with the formation of the primitive streak
Primitive streak location
- Appears caudally in the medial plane
- Dorsal aspect of embryonic disc
Primitive streak formation
- Epiblast proliferates and migrate toward the median plane of the embryonic disc
Epiblast dives within the primitive streak
- Pushes away hypoblast, forms definitive endoderm
- Forms mesoderm in between ectoderm and endoderm
- Remaining epiblast form the ectoderm
Elongation of primitive streak
- Addition of cells to the caudal end
Appearance of primitive streak allow identification of
- Cranial/caudal
- Dorsal/ventral
- Left/right
- Medial/lateral
Cell proliferation at the cranial end
- Forms the primitive node
- Primitive node functions as a signaling center
Mesoderm is patterned based on
- Where the epiblast pass through the primitive streak
- Closer to primitive node the more axial in location
- Notochord
- Paraxial mesoderm
- Intermediate mesoderm
- Lateral mesoderm
Notochordal process
- Cellular rod
- Formed by cell migration cranially from the primitive node/pit
- Formed by day 20
The notochord
- Defines embryo axis and provides support
- Serves as an important signaling center in the development of the gut, vertebral column and CNS
Notochord degeneration
- Degenerates as the vertebrae form
- Part persist as the nucleus pulposus
Neurulation
- Formation of the neural tube
Neurulation process
- Forming the neural plate
- Neural plate inviginates to form neural groove with lateral neural folds
- Neural folds fuse forming the neural tube (week 4)
- Neural tube separates from overlying ectoderm (week 4)
Folding and closure of the nueral plate
- Closure first begins in the cervical region
- “Zips” up toward the head and down toward the tail
2 openings of the neural plate upon closure
- Anterior neuropore (closes day 25)
- Posterior neuropore (closes day 28)
Errors in neurulation
- Raschischisis
- Anencephaly
- Spina bifida
Raschischisis
- Failure of neural tube folding
Anencephaly
- Failure of anterior neuropore closure
Spina bifida
- Failure of posterior neuropore closure (other causes)
Neural crest cells
- Group of cells that arise in the ectoderm at the margins of the neural plate
- Form a flattened mass on the dorsolateral aspect of neural tube
- Detach from the neural tube and become migratory
Neural crest cells contribute to many structures
- Neurons of the spinal ganglia, ANS ganglia, cranial nerve ganglia
- Enteric ganglia (gut)
- Melanocytes
- Schwann cells
- Adrenal chromaffin cells
- Pia and arachnoid
- Parafollicular cells of thyroid gland
- Also contribute to heart and face development
Neurons of the spinal ganglia, ANS ganglia, cranial nerve ganglia (contributed to by neural crest cells)
- Pseudounipolar sensory neurons and post-synaptic neurons of ANS
