Embryology

Question 1

Zygote

Answer 1

Union of egg + Sperm

Question 2

Cleavage

Answer 2

Process by which zygote divides via mitotic division. Rapid cell cycles w/ no significant growth. Product of cleavage is called Blastomeres

Question 3

Blastomeres

Answer 3

Cells produced by cleavage

Question 4

Morula

Answer 4

At 16 blastomeres (approx 4 days) zygote becomes morula. Named after mulberries. Continues to move toward uterus.

Question 5

Blastocystic Cavity

Answer 5

Uterine fluid invades the morula, forming a cavity

Question 6

Blastocyst

Answer 6

Morula transitions to blastocyst when blastomeres begin to arrange into masses/structure. Hollow structure. Two components: Embryoblast and Trophoblast

Question 7

Embryoblast

Answer 7

Inner cell mass. Future embryo

Question 8

Trophoblast

Answer 8

Outer cell mass. Future embryonic placenta

Question 9

Hypoblast

Answer 9

Layer of embryoblast becomes the hypoblast (primitive endoderm)

Question 10

Events of implantation

Answer 10

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.

Question 11

Cytotrophoblast

Answer 11

Inner layer of trophoblast. Contributes to burgeoning trophoblast

Question 12

Syncytiotrophoblast

Answer 12

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

Question 13

Ectopic Pregnancy

Answer 13

Implantation usually occurs in posterior uterine wall, but ectopic pregnancies can occur (usually in uterine tube [tubal], rare cases in abdominal cavity [intraabdominal])

Question 14

Bilaminar Disc

Answer 14

Occurs at week 2. 2 layers form: Epiblast and Hypoblast in addition to several cavities.

Question 15

Epiblast

Answer 15

Floor of the amnitotic cavity, continuous with the amnion. Upper level of bilaminar disc

Question 16

Amnion

Answer 16

Thin membrane that closely covers the embryo, surrounds and forms the amniotic cavity

Question 17

Amniotic Cavity

Answer 17

fluid-filled cavity that surrounds the developing embryo.

Question 18

Hypoblast during Implantation

Answer 18

roof of the exocoelomic cavity (was the blastocyst cavity), and is continuous with the exocoelomic membrane. Lower level of bilaminar disc

Question 19

Exocoelomic Cavity

Answer 19

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).

Question 20

Exocoelomic Membrane

Answer 20

Lines exocoelomic cavity along with hypoblast.

Question 21

Primary Umbilical Vesicle

Answer 21

Primary Yolk Sac. Develops from exocoelomic cavity around day 12. Fully implanted by this point

Question 22

Extraembryonic Mesoderm

Answer 22

Mesoderm not part of the embryo. Mesoderms of the amnion, chorion, umbilical vesicle

Question 23

Extraembryonic Coelem

Answer 23

arises within the extraembryonic mesoderm. It will become the chorionic cavity.

Question 24

Chorionic Cavity

Answer 24

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).

Question 25

Secondary Umbilical Vesicle

Answer 25

Largery than Primary umbilical vesicle. Forms around end of 2 weeks and the primary umbilical vesicle pinches off and disappears. Surrounded by splanchnic mesoderm

Question 26

Splanchnic mesoderm

Answer 26

surrounds umbilical vesicle. Will form gut wall.

Question 27

Connecting Stalk

Answer 27

Will become umbilical cord. Attaches

Question 28

Uteroplacental Circulation

Answer 28

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

Question 29

Decidual Cells

Answer 29

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

Question 30

Primary Chorionic Villi

Answer 30

Appear from the cytotrophoblast, become the embryonic contribution to placenta.

Question 31

Gastrulation

Answer 31

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

Question 32

Trilaminar Disc

Answer 32

Ectoderm, Mesoderm, and Endoderm

Question 33

Ectoderm

Answer 33

outer layer, stay where they are after migration (previously the epiblast) will become the epidermis & nervous system

Question 34

Mesoderm

Answer 34

middle layer, will become various muscle & connective tissues (mesenchyme

Question 35

Endoderm

Answer 35

Inner layer, replacing the hypoblastwill become the lining of the gut tube

Question 36

Notochord

Answer 36

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

Question 37

Formation of Neural Tube (Neuralation)

Answer 37

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).

Question 38

Spina bifida

Answer 38

Improper fusion of the neural tube caudally results in spina bifida, a disease of varying degrees and symptoms.

Question 39

Spina Bifida Oculta

Answer 39

Mildest and frequently asymptomatic. Neural tube does not fuse completely, but spinal cord does not poke through

Question 40

Spina bifida meningocele

Answer 40

Least common. Vertebrae/spinal cord develop normally, but meninges poke through gap in spinal cord.

Question 41

Spina Bifida Myelomeningocele

Answer 41

More severe. Unfused portion of spinal cord allows spinal cord AND meninges to protrude through

Question 42

Spina bifide myeloschesis

Answer 42

Most severe. Spinal cord passes through unfused portion with no meningeal protection leaving it exposed

Question 43

Paraxial Mesoderm

Answer 43

Most medial to notochord. will form most of the axial skeleton, muscles, & dermis. Gives rise to somites.

Question 44

Somites

Answer 44

Differentiate into myotomes, dermatomes, sclerotomes

Question 45

Intermediate Mesoderm

Answer 45

Between paraxial and lateral mesoderms. will form the urogenital system

Question 46

Lateral Mesoderm

Answer 46

continuous with the extraembryonic mesoderm. will form the body walls. It divides into two layers: the somatic (parietal) and the splanchnic (visceral) layer.

Question 47

Somatic (Parietal) Layer

Answer 47

somatopleure (somatic mesoderm + embryonic ectoderm) will form the outer body wall

Question 48

Sphlanchnic Mesoderm

Answer 48

Splanchopleure (splanchnic mesoderm + embryonic endoderm) will form the gut wall.

Question 49

Intraembryonic Coelem (Body Cavity)

Answer 49

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.