Unit 7 - Embryogenesis

Question 1

what is “embryogenesis”?

Answer 1

weeks 1-8 of human pregnancy

-also “organogenesis” as organ primordia (earliest recognizable stage of development) are established

Question 2

what is compaction?

Answer 2

after the third cleavage of blastomeres

-max contact with each other, and form compact ball of cells, which form 16-cell morula

Question 3

what is the inner cell mass?

Answer 3

inner cells of morula

-give rise to tissues in embryo proper

Question 4

what is the blastocoel?

Answer 4

fluid penegrates into intercellular spaces of ICM to create a blastocyst

Question 5

when is it considered a blastocyst?

Answer 5

after the blastocoel is formed (outer cells flatten and form wall of blastocyst)

Question 6

how do trophoblasts attach to uterine wall epithelium?

Answer 6

trophoblast cells penetrate between epithelial cells of uterine mucosa (endometrium)
-L-selectins (CHO-binding PRO) on trophoblast cells interact with CHO receptors on uterine epithelium to mediate attachment of blastocyst to uterus

Question 7

what does the trophoblast do after implantation?

Answer 7

day 8; differentiates into 2 layers

1. cytotrophoblast - inner layer of mononucleated cells
2. syncytiotrophoblast - outer multinucleated layer that lacks distinct cell boundaries
   - continues to expand into uterine wall

on day 9, trophoblast invades maternal capillaries to establish uteroplacental circulation

Question 8

what does the ICM do after implantation?

Answer 8

day 9; differentiates into flat bilaminar germ disc with 2 layers

1. hypoblast - continue to extraembryonic tissues
2. epiblast - gives rise to all tissues in embryo proper
   - amniotic cavity forms within

Question 9

what are considered extraembryonic tissues?

Answer 9

trophoblast: cytotrophoblast –> syncytiotrophoblast
hypoblast: extraembryonic endoderm –> yolk sac

amniotic ectoderm (from epiblast, but mostly emryonic tissues)

Question 10

what are the goals of gastrulation?

Answer 10

1. bring inside the embryo areas destined to form endodermal organs
2. surround embryo with cells capable of forming ectoderm
3. place mesodermal cells in proper positions in between

Question 11

how does gastrulation begin?

Answer 11

formation of primitive streak on surface of epiblast

-primitive streak becomes narrow groove with primitive node surrounding a primitive pit at the cephalic end

Question 12

what is invagination? what does this cause?

Answer 12

the “second step” of gastrulation

• epiblast cells migrate toward primitive streak, then detach from epiblast and move through streak to slip beneath it
• some cells displace hypoblast to create endoderm
• other cells between epiblast and new endoderm become mesoderm
• cells remaining in epiblast become ectoderm

Question 13

what is the source of all germ layers that give rise to all tissues of the embryo?

Answer 13

the epiblast, through gastrulation

Question 14

what is neurulation? the steps?

Answer 14

process of forming a neural tube

1. formation and folding of neural plate
2. elevation of neural crest
3. convergence of neural folds
4. closure of neural tube when neural folds are brought in contact with one another

Question 15

what are the major domains of ectoderm after neurulation?

Answer 15

1. internally positioned neural tube (forms brain and spinal cord)
2. externally positioned surface ectoderm that primarily forms epidermis
3. neural crest cells that migrate to new locations to give rise to many cell types

Question 16

how many sites of closure are there on the neural tube? what happens before complete closure?

Answer 16

three

• neural folds proceed bidirectionally
• until fusion is complete, cephalic and caudal ends communicate with amniotic cavity via anterior and posterior neuropores

Question 17

spina bifida VS anacephalopathy VS craniorachischisis

Answer 17

SB: failure to close posterior neuropore (severity varies)
A: failure to close two other sites causes open anterior neuropore (forebrain remains in contact with amniotic fluid and degenerates)
C: complete failure of closure along entire neural tube

Question 18

what is neural tube closure dependent on?

Answer 18

interplay between genes, environment, and maternal nutrition

Question 19

how much folic acid can make a difference?

Answer 19

50-70% of NTD can be prevented if women take 400 micrograms of folic acid daily, starting 3 mo prior to conception and continuing through pregnancy

Question 20

what kind of ectoderm undergoes epithelial to mesenchymal transition?

Answer 20

neural crest cells

-happens at dorsal neural tube, then migrates to give rise to tissues

Question 21

what is mesenchyme?

Answer 21

loosely organized connective tissue of any origin

Question 22

what are the 4 types of regions the neural crest cells migrate to?

Answer 22

4 main but overlapping anatomical regions

1. cranial (cephalic) neural crest
2. cardiac neural crest
3. trunk neural crest
4. enteric (vacal and sacral) neural crest

Question 23

cranial/cephalic neural crest

Answer 23

produces craniofacial cartilage, bone, neurons, glia, and connective tissue
-cells enter pharyngeal arches to give rise to thymic cells, teeth, and bones of middle ear and jaw

Question 24

cardiac neural crest

Answer 24

subregion of cranial neural crest that develops into melanocytes, neurons, cartilage, and connective tissue
-also makes muscular connective tissue wall and septum of cardiac outflow tract that gives rise to large arteries (aorta and pulmonary artery)

Question 25

trunk neural crest

Answer 25

migrates to form dorsal root ganglia containing sensory neurons or sympathetic ganglia and adrenal medulla

Question 26

enteric (vagal and sacral) neural crest

Answer 26

form parasympathetic ganglia of gut

Question 27

where do human cardiac neural crest cells migrate?

Answer 27

to pharyngeal arches 3, 4, and 6

-enter truncus arteriosus (cardiac outflow tract) to make septum