Union of egg + Sperm
Process by which zygote divides via mitotic division. Rapid cell cycles w/ no significant growth. Product of cleavage is called Blastomeres
Cells produced by cleavage
At 16 blastomeres (approx 4 days) zygote becomes morula. Named after mulberries. Continues to move toward uterus.
Uterine fluid invades the morula, forming a cavity
Morula transitions to blastocyst when blastomeres begin to arrange into masses/structure. Hollow structure. Two components: Embryoblast and Trophoblast
Inner cell mass. Future embryo
Outer cell mass. Future embryonic placenta
Layer of embryoblast becomes the hypoblast (primitive endoderm)
Events of implantation
By end of week one (day 6). Blastocyst attaches to endometrium (lining of uterine wall). Trophoblast contacts endometrium and begins differentiation into cytotrophoblast and syncytiotrophoblast.
Inner layer of trophoblast. Contributes to burgeoning trophoblast
Outer layer of trophoblast. Means "without cells". Undifferentiated, invades into uterine wall. Releases many enzymes that allows it to penetrated aggressively further/deeper into endometrium. Produces HCG: Human coreanic gonadotropin, + on pregnancy tests, important for maintain endometrium of uterus so not expelled
Implantation usually occurs in posterior uterine wall, but ectopic pregnancies can occur (usually in uterine tube [tubal], rare cases in abdominal cavity [intraabdominal])
Occurs at week 2. 2 layers form: Epiblast and Hypoblast in addition to several cavities.
Floor of the amnitotic cavity, continuous with the amnion. Upper level of bilaminar disc
Thin membrane that closely covers the embryo, surrounds and forms the amniotic cavity
fluid-filled cavity that surrounds the developing embryo.
Hypoblast during Implantation
roof of the exocoelomic cavity (was the blastocyst cavity), and is continuous with the exocoelomic membrane. Lower level of bilaminar disc
Artist formerly known as blastocystic cavity. lined by hypoblasts and the exocoelomic membrane. It will become the primary umbilical vesicle (aka the “yolk sac” in older texts).
Lines exocoelomic cavity along with hypoblast.
Primary Umbilical Vesicle
Primary Yolk Sac. Develops from exocoelomic cavity around day 12. Fully implanted by this point
Mesoderm not part of the embryo. Mesoderms of the amnion, chorion, umbilical vesicle
arises within the extraembryonic mesoderm. It will become the chorionic cavity.
Formed by extraembryonic coelom. part of the chorionic sac, whose walls are formed by the chorion (extraembryonic mesoderm + trophoblasts). The embryo, amniotic sac, and secondary umbilical vesicle hang within the chorionic sac via the connecting stalk (future umbilical cord).
Secondary Umbilical Vesicle
Largery than Primary umbilical vesicle. Forms around end of 2 weeks and the primary umbilical vesicle pinches off and disappears. Surrounded by splanchnic mesoderm
surrounds umbilical vesicle. Will form gut wall.
Will become umbilical cord. Attaches
As the syncytiotrophoblast burrows into uterine wall, lacunae appear and fill with maternal blood. Syncitiotrophoblast produce HCG hormone. iii. Decidual reaction (cells) in the endometrial connective tissue will respond to the invading syncytiotrophoblast, providing an immunological privileged site. These cells will become the maternal contribution to the placenta. Primary chorionic villa appear from the cytotrophoblast, and will be the embryonic contribution to the placenta
swell with glycogen and lipid to form a boundary in order to protect embryo from immune response. This becomes the mother’s contribution to placenta
Primary Chorionic Villi
Appear from the cytotrophoblast, become the embryonic contribution to placenta.
Conversion of bilaminar disc into trilaminar disc. Begins with formation of primitive streak, formed from migrating epiblasts. Epiblasts migrate medially and “dive into” primitive pit, migrating to different layers. Near the cranial end, the primitive node and primitive pit form, with the primitive groove extending caudally
Ectoderm, Mesoderm, and Endoderm
outer layer, stay where they are after migration (previously the epiblast) will become the epidermis & nervous system
middle layer, will become various muscle & connective tissues (mesenchyme
Inner layer, replacing the hypoblastwill become the lining of the gut tube
mesenchymal cells thicken into the notochordal plate within the mesoderm, and will fold into a rod known as the notochord. Defines embryo's axis, forms basis for axial skeleton, induces ectoderm to form the nervous system. Remains part of intervertebral discs in adults
Formation of Neural Tube (Neuralation)
After induction of the ectoderm by the notochord, the neural tube will start forming via the process neurulation. Neural tube will become the CNS. Neural crest will become the PNS and other structures (melanocytes).
Improper fusion of the neural tube caudally results in spina bifida, a disease of varying degrees and symptoms.
Spina Bifida Oculta
Mildest and frequently asymptomatic. Neural tube does not fuse completely, but spinal cord does not poke through
Spina bifida meningocele
Least common. Vertebrae/spinal cord develop normally, but meninges poke through gap in spinal cord.
Spina Bifida Myelomeningocele
More severe. Unfused portion of spinal cord allows spinal cord AND meninges to protrude through
Spina bifide myeloschesis
Most severe. Spinal cord passes through unfused portion with no meningeal protection leaving it exposed
Most medial to notochord. will form most of the axial skeleton, muscles, & dermis. Gives rise to somites.
Differentiate into myotomes, dermatomes, sclerotomes
Between paraxial and lateral mesoderms. will form the urogenital system
continuous with the extraembryonic mesoderm. will form the body walls. It divides into two layers: the somatic (parietal) and the splanchnic (visceral) layer.
Somatic (Parietal) Layer
somatopleure (somatic mesoderm + embryonic ectoderm) will form the outer body wall
Splanchopleure (splanchnic mesoderm + embryonic endoderm) will form the gut wall.
Intraembryonic Coelem (Body Cavity)
begins as small spaces within the lateral mesoderm that coalesce. (It is these spaces that cause the lateral mesoderm to divide into the 2 layers.) Eventually divides into pleural, pericardial, and peritoneal cavities.