Cleavage to Trilaminar Germ Disc

Question 1

blastocyst formation

Answer 1

morula enters uterine cavity 4 days after fertilization
fluid from uterine cavity forms blastocystic cavity (Blastocoel)

blastomeres separate into 2 groups (compaction):
1. inner cell mass (Embryoblast): develop into Embryo
2. outer cell mass (Trophoblast): form Placenta, zona pellucida degenerates to interact with the endometrium

Question 2

placenta

Answer 2

respiratory and excretory organ of fetus

Question 3

is the morula totipotent or pluripotent

Answer 3

totipotent

Question 4

are the cells of the inner cell mass totipotent or pluripotent

Answer 4

pluripotent (no placenta)

Question 5

umbilical cord develops from the

Answer 5

connecting stalk

Question 6

fetus develops from the

Answer 6

inner cell mass

Question 7

placenta develops from the

Answer 7

outer cell layer; trophoblast

Question 8

implantation

Answer 8

trophoblast cells cling and attach tightly to the endometrium
decidual reaction: endometrial cells become metabolically active and secretory (Decidual cells)… glands enlarge and area is more vascular

Question 9

_________ secreted by _______ cells maintains pregnancy, also used in urine pregnancy tests

Answer 9

human chorionic gonadotropic (hCG), trophoblast

Question 10

first week of human development

Answer 10

fertilization- at the distal end of a uterine tube (Ampulla)
formation of zygote
cleavage blastocyst formation
penetration of endometrium

Question 11

changes in trophoblast

Answer 11

formation of cytotrophoblast (inner layer with mononuclear cells) and syncytiotrophoblast (outer layer without cell boundaries)

Question 12

changes in the embryoblast

Answer 12

1. formation of hypoblast (single layer of flat cells inferior to embryoblast)
2. formation of amniotic cavity (above embryoblast)… cavity will enlarge and enclose embryo
3. formation of bilaminar germ disc
   embryoblast differentiates (inferior hypoblast… flat cells and superior epiblast… tall cells)
   -two cavities:
4. amniotic cavity above epiblast
5. yolk sac/exocoelomic cavity below hypoblast

Question 13

formation of connecting stalk

Answer 13

bilaminar germ disc is separated from the cytotrophoblast by the formation of the extra embryonic mesoderm (developed from proliferation of cells of hypoblast)

Question 14

what is the extraembryonic coelom/chorionic cavity

Answer 14

large cavity in extraembryonic mesoderm which does not extend into the region of the connecting stalk

Question 15

what happens at week 3

Answer 15

gastrulation

Question 16

gastrulation

Answer 16

the bilaminar embryonic disc becomes the trilaminar embryonic disc with 3 germ layers: ectoderm, mesoderm, endoderm
embryo called a gastrula
formation of primitive streak

Question 17

what is the first sign of gastrulation

Answer 17

formation of primitive streak

Question 18

formation of primitive streak

Answer 18

epiblast cells in caudal end enlarge and migrate
hypoblast cells in cranial end enlarge and form prechordal plate (future mouth)
signals development of forebrain and eyes
establish cranial and caudal ends, dorsal and ventral surfaces and right and left sides of embryo

Question 19

what happens after primitive streak is formed

Answer 19

-proliferating cells of epiblast gives rise to 3 layers of cells
-forms mesenchyme until 4th week and degenerates after 4th week
-epiblast cells displace hypoblast and forms intraembryonic endoderm
-remaining epiblast –> intraembryonic ectoderm
-mesenchym –> connective tissue
-some forms mesoblast –> intraembryonic mesoderm

Question 20

derivative of 3 embryonic cell layers

Answer 20

ectoderm: epidermis, CNS, PNS
endoderm: epithelial linings of respiratory tract, gastro-intestinal tract, liver, pancreas
mesoderm: cardiovascular system, blood cells, marrow, bones, muscles, reproductive and excretory organs

Question 21

formation of notochord

Answer 21

mesodermal cells from primitive streak migrate cranially from cranial end of primitive streak to the prechordal plate forming a median cellular cord called the Notochord

Question 22

intraembryonic mesoderm

Answer 22

mesenchymal and mesodermal cells migrate cranially on each side of notochord and around prechordal plate
cranial to prechordal plate meet and forms cardiogenic mesoderm

Question 23

two sites where the ectoderm and endoderm remain fused maintaining the bilaminar nature preventing the migration of mesenchyme and mesoderm are

Answer 23

1. prechordal plate/oropharyngeal membrane- future site of mouth
2. cloacal membrane (caudal to primitive streak)- future site of anus

Question 24

notochord

Answer 24

-axis of embryo
-site of future vertebral bodies
-primary inductor for early embryo
-induces formation of overlying neural plate
-most degenerates, rest forms nucleus pulposus of inter vertebral disc