Embryology

Meiosis II and Mitosis

• Meiosis I, II = haploid
• Mitosis = diploid

Meiosis I

Does NOT!

Equal cytokinesis, continuous source of sperm, occurs in waves. Primary spermatocyte -MeioI-> secondary spermatocyte -MeioII-> spermatid 

• Oocyte hogs most of cytoplasm (unequal cytokinesis)
• Meiosis I begins before birth and arrests by 5th month of fetal life at synapsis...
• Meiosis I finishes at ovulation
• Meiosis II arrests at metaphase, then finishes at fertilization
• No replenishment of oogonia
• Primary oocyte -MeioI-> Sec. oocyte+polar body -MeioII-> Mature oocyte + 3 polar bodies

Trisomy 13,18,21 (13 and 18 usually die soon after birth)

XO (only survivable monosomy), XXY, XYY

advancing maternal age

• 1. Defects in crossing-over (chiasmata region) 
• 2. Premature degradation of cohesins (hold sister chromatids together)
• 3. limited region of homology that supports synapsis of X and Y
• 4. spindle assembly "checkpoints" function poorly 
• 5. mosaicism (some cells have normal chromosome # and others are aneuploid) from early mitotic divisions in zygote
• 

• 1. growth of flagellum
• 2. conversion of golgi into acrosome
• 3. condensation of nucleus
• 4. excess cytoplasm discarded

Sex chromosome aneuploidy (same rate for gametes derived from men and women)

Autosomeal aneuploidy (over 80% occur in eggs rather than sperm)

• Acrosome - sac of digestive enzymes needed to bore through Zona Pellucida
• Nucleus - tightly condensed DNA
• Tail - mitochrondrial sheath to provide energy for flagellum

Describes changes in sperm that occur upon exposure to female reproductive tract, receptors on ZP, and inducing rapid motility

• thich matrix around ooctye which allows:
• - sperm binding, triggers acrosome reaction
• - block polyspermy
• - prevents premature/ectopic implanation

** oocyte offers nutrients for embryonic life for several days

• -Expose receptors for corona cells and ZP
• -greatly increase motility
• -chemotaxis toward egg-cumulus complex

• -They prevent capacitation by making both sperm and egg unwell.
• -99.5% effective
• -Cause inflammation in uterus to discourage sperm from fertilization

• 1. Cortical Reaction: release of cortical granules that cross-link the ZP 
• 2. Depolarization of egg membrane upon sperm fusion

3 days after fertilization, zygote looks like a grape-like cluster of cells

When the zygote forms a smooth ball of cells ~3 days after fertilization

• Inner cell mass = Hypoblast (lining of epiblast)+ Epiblast
• Trophoblast = outside lining

• Epiblast = embryo proper and amniotic sac
• Hypobast = extraembryonic endoderm that forms yolk sac
• Trophoblast = fetal chorion, or placenta

Two eggs ovulated and sep. fertilized. They share same uterus but not any membranes.

• ~25% splitting of zygote first few cleavages (share nothing but uterus; dichorionic diamnionic)
• ~75% complete splitting of inner cell mass (share uterus, placenta; monochorionic diamnionic) 
• Conjoined Twins: incomplete splitting of inner cell mass (share chorion/placenta, amniotic sac; monochorionic monoamnionic)

If implantation occurs too close to cervix, but within uterus (prevents safe vaginal delivery) 

• Cytotrophoblast - single layer of cells that arises from trohphoblast during implantation
• Synctiotrophoblast - multi-nucleated cells that branch out

Enzymes secreted from synctiotrophoblast that degrade uterine wall and pull entire blastocyst into wall

Production of glycogen and lipids of stromal cells in uterus to nourish embryo

The formation of new blood vessels

• - Pelvic Inflammatory Disease
• - Cigaretta Smoking (affects smooth muscle perstalsis/cilia that move egg)
• - In vitro fertilization
• - Maternal age greater than 24

Epiblast and hypoblast

• Epi = dorsal
• Hypo = ventral

Formed within inner cell mass by fluid secretion (it is dorsal to epiblast). It acts as a shock-buffer for embryo.

Extraembryonic mesoderm forms from epiblast cells and forms the chorionic cavity and blood vessels of the chorion, definitive yolk, and connecting stalk. 

hypoblast; epiblast

The chorion is formed by the two trophoblast layers (cytotrophoblast/synctiotrophoblast) and extraembryonic mesoderm.

The lacunae fill with materal blood around the synctiotrophoblast. The early embryo gets nutrients via this method until primary chorionic villi form.

Outgrowths form the cytotrophoblasts into the synctiotrophoblast that become blood-filled lacunar network

Primitive node

• - the depression of the middle of the primitive streak 
• - endoderm

Cell fate becomes restricted...they become determined.

• Epiblast is dorsal; Hypoblast is ventral
• Cranial part of primitive streak is also known as anterior
• Caudal part of primitive streak is also known as posterior

position along the streak and distance from the node

FGF+ BMP4 = ventralize mesoderm into intermediate and lateral plate mesoderm

FGF + BMP4 antagonist (form primitive node) = dorsalize mesoderm to form notochord and somitic mesoderm

• Dorsal ectoderm's lateral edges fold to form neural tube
• 

Fifth somite (i.e. mid-region) then is zipped up and down simultaneously

Incomplete fusion of the neuropores (ends of the neural tubes are being "zip-tied")

Neuropores close

Defines body axis, signaling center; MESODERM!

Notochord (directly below NT) induces floor plate (i.e. ventral) by expressing sonic hedgehog (SHH).

• The ectoderm expresses BMPs that cause dorsal NT fates
• 

Neural Crest

Neuroectoderm cells

A wide variety of tissues/organs

• 1. Cranial neural crest
• 2. Cardiac neural crest
• 3. Vagal/sacral neural crest - parasympathetic ganglia of gut
• 4. Trunk neural crest -
• consisting of dorsolateral migrating trunk neural crest giving rise to melanocytes and ventromedial migrating neural crest cells forming dorsal root ganglia/nerves

Ectoderm!

1. axial (chordamesoder) = notochord

2. paraxial mesoderm = head, myotome = skeletal muscle (myotome), dermatome (dermis), sclerotome (cartilage)

3. intermediate mesoderm = kidney (nephrotomes, nephrogenic chord), gonads (urogenital structures)

4. lateral mesoderm = parietal mesoderm (somatic) = body walls & thin serous membrane around body cavities, visceral mesoderm (splanchnic) = gut wall, & thin serous membranes around each organ 

GI tract, liver, gall blader, pancreas

Anterior forgut (cranial region) and posterior hindgut (caudal) are connected by midgut and attached to vitelline duct which is attached to yolk sac

Nucleus Polposus (jelly-like susbtance in the middle of spinal chord)

Folate is used to synthesize DNA (purin synthesis) therefore it is important for reducing Neural Tube Defects.