Anatomy

• Process by which male and female gametes fuse
• resulting in a 2n cell (generated from n + n)

• o Egg
• is arrested in prophase II until the sperm reach the egg, digest the ECM
• surrounding it (also results in solidification of zona pellucid)

• o After
• fertilization, meiosis completes, and the second polar body is “kicked out”

• o Pronucleii
• swell, chromatin structure changes → pronucleii fuze →
• beginning as zygote

• MITOTIC process; reductive cell divisions (the
• amount of cytoplasm stays constant)

o Zona pellucida can be seen through morula stage, inhibiting growth of cells

• After the fertilization and before the
• development into a fetus

o Does not yet show the characteristics of an adult

• An unborn or unhatched vertebrate showing signs
• of the mature animal

16 cell stage (mulberry)

o Sometime after compaction, differentiation of the cells occurs, so the inner cell mass and outer cell mass develop

• Compaction: Blastomeres maximize contact with each other, helps
• with segregation as the inner cells communicate extensively by gap junctions

• Fluid begins to enter the embryo and a single cavity forms
• After the formation of morula

o Cells from the inner cell mass (embryoblast) are all on one pole while the outer cell mass (trophoblast) flatten to form the epithelial wall

Embryoblast; develops after compaction (early blastocyst), forms the embryo

o Develops into epiblast (becomes all 3 germ layers) and hypoblast (develops into yolk sac)

• Embryo adheres to the wall of the uterus as the
• syncytio-trophoblasts invade

o Occurs as a blastocyst at the embryonic pole (end of wk 1)

• Abnormal implantation site that results in
• development in novel site that is often dangerous to both mother and baby

• Includes internal os of uterus, interstitium, uterine tube, ampulla,
• mesentery

• Outer cell mass; develops after compaction (early
• blastocyst); forms the extra embryonic tissue, later the placenta

• Inner most layer in wall of uterus; mucosa
• lining on inside of wall

• Middle layer in wall of uterus; thick layer of
• smooth muscle

• o Peri
• metrium: Outermost layer in wall of uterus; peritoneal covering in lining
• outside wall

• Chorion frondusum (bushy chorion) and deciduas
• parietalis (uterine wall); connects fetus to the uterine wall, supplies fetus
• with oxygen and food, and allows for fetal waste to be disposed of via maternal
• kidneys

o Develops from the trophoblast

• Process that establishes the three germ layers –
• ectoderm, mesoderm, and endoderm

o ALL layers derived from the inner cell mass

Furrow in the midline of the embryonic disc at the caudal end of the embryo as epiblast cells ingress

o These ingressed cells go on to form the endoderm and mesoderm

dorsal-most layer of the embryo, derived from epiblast

• o Goes on to develop
• Neuroectoderm: neural crest, neural tube, CNS, retina, pineal body, posterior part of pituitary gland
• Surface ectoderm: epidermis, hair, nails, cutaneous and mammary glands, anterior part of the pituitary gland, enamel of teeth, internal ear, lens of eye

Inner layer after gastrulation (on top of yolk sack), derived from epiblast

• o Goes on to develop trachea, bronchi, lungs,
• Epithelium of gastrointestinal tract, liver, pancreas, urinary bladder, urachus
• Epithelial parts of pharynx, thyroid gland, tympanic cavity, phryngotympanic tube, tonsils, parathyroid glands

Middle layer after gastrulation, derived from epiblast

• o Goes on to develop
• Paraxial Mesoderm: muscles of head, striated skeletal muscle, skeleton (except cranium), dermis of skin, connective tissue

Intermediate Mesoderm: Urogenital system (include gonads, ducts, and accessory glands),

Lateral Mesoderm: Connective tissue and muscle of viscera, serous membranes of pleura, pericardium, and peritonium, primordial heart, blood and lymphatic cells, spleen, suprarenal (adrenal) cortex

fluid filled cavity formed within the mesoderm

o Lateral mesoderm splits to form the intraembryonic coelum → splanchnic (visceral) and somatic (body wall) mesoderm

Membrane building the amniotic sac that surrounds the embryo, derived from epiblast

o First see amnionic cavity in the center of the bilaminar embryonic disc

o Cushions embryo

Structure located ventral to bilaminar germ disc, derived from hypoblast

o Site of origin of the first blood cells and the germ cells

Derived from prenotochordal cells that invaginate in the primitive pit and move cranially until they reach the prechordal plate (the plate is undisturbed), the cells intercalate into the hypoblast → midline consists of two layers that form the notochordal plate → proliferate and detach from endoderm to form the definitive notochord which underlies the neural tube and serves as the basis for the axial skeleton

o Notocord formation induces the neural plate, neural fold, and neural tube

formed when the lateral edges of the neural plate become more elevated → neural folds → depressed midregion forms the neural groove → neural folds approach each other and the neural tube is formed (begins in middle and “zips up” toward the ends

o Becomes the CNS, lumen becomes the brain and spinal cord

Formation of the neuroectorderm induced by the underlying notochord

o Epithelial mesenchymal interaction

o Neural plate is transformed into the neural tube

o Failure results in neural tube defects including anencephaly and spina bifida

cranial and caudal openings in the neural tube that exist from the beginning until the end of the time of closure of the neural tube

Derived from the edges of the neural folds and begin to migrate as the neural tube forms

o PNS, adreno-medulla cells, pigment cells, ganglia, portions of the cranial cartilages/skeleton

o Spinal ganglia, bones and CT of face, septa for outflow tract of the heart, cranial nerve ganglia, ganglia for the gut tube, melanocytes (vulnerable to teratogenic insult)

Formed by Paxaxial mesoderm: Epithelial ball of cells formed in segmental pairs along the NT

o Differentiate into vertebrae, muscles of the back and body wall, dermis of the skin

o Each somite forms a sclerotome (bone), myotome (muscle), and dermatome (skin)

Lateral plate mesoderm internal to the coelom plus the endoderm

o Gives rise to the inner tube, the side toward the viscera

Lateral plate mesoderm extranal to the coelom plus the ectoderm

o Gives rise to the outer tube, the side toward the body

Forms the somites -->

• Dermatome:
• Derived from somite, give rise to a segment of skin
• Sclerotome:
• Derived from somite, give rise to a segment of bone
• Myotome:
• Derived from somite, give rise to a segment of muscle

urogenital system (kidneys, ureters, gonads)

Splits and forming the intraembryonic coelom and forms the splanchnic (visceral) mesoderm and the somatic (body wall) mesoderm

The oocyte is released, and fertilization occurs in the antrum of the uterus. The male and female pronuclei fuse as the embryo progresses through the fallopian tube. By the time the embryo is at the advanced morula (compacted) stage, it reaches the uterine lumen. At the early blastocyst stage (d5), it is still travelling down in the uterus and eventually the zona pellucida is released (called hatching, d 6-7). When the blastocyst is 8-9 days old it begins implantation in the wall of the uterus, with the embryo implanting at the embryonic pole. The cells of the syncytiotrophoblast are invading the uterine wall and burrowing in.

Craniocaudal folding results in the folding in at the head and tail to make the “C” shape.

Lateral folding goes underneath the embryo to pinch off the yolk, so the embryo is only connected by the umbilical cord