First to Third Weeks of Development

Question 1

Fertilization

Answer 1

Oocyte (23 chromosomes)
Spermatozoon (23 chromosomes)
Oocyte + Spermatozoon = Zygote (46 chromosomes)

Question 2

Cleavage of the Zygote

Answer 2

30 hours following fertilization
Sequence of mitotic cell divisions of the zygote - gives rise to blastomeres
Takes place while zygote travels in uterine tube to get to the uterine cavity
At 12-32 cell stage –> called a morula

Question 3

Formation of Blastocyst

Answer 3

Morula enters the uterine cavity (4 days after fertilization)
Blastocystic Cavity - fluid filled inside the morula
Fluid causes blastomeres to divide into inner cell mass (embryoblast) and outer cell layer (trophoblast)

Question 4

Blastocyst consists of

Answer 4

Embryoblast
Blastocystic Cavity
Trophoblast
Zona Pellucida

Question 5

Implantation of the Blastocyst

Answer 5

6 days after fertilization
Embryonic pole implants in endometrial lining
Tophoblast surrounds blastocyst and when touches endometrial lining it gives rise to 2 layers - Cytotrophoblast (inner) and Syncytiotrophoblast (outer)

Question 6

Formation of hypoblast

Answer 6

Appears around day 7

early stage of primary endoderm

Question 7

2 embryonic layers in week 2

Answer 7

Bilaminar embryonis disc gives rise to
Epiblast - floor of amniotic cavity
Hypoblast - roof of exocoelomic cavity

Question 8

2 cavities in week 2

Answer 8

Amniotic cavity - new cavity
Blastocystic cavity - now called the exocoelomic cavity
Exocoelomic membrane gives rise to primary and secondary umbilical vesicles

Question 9

2 Trophoblast layers in week 2

Answer 9

Cytotrophoblast = inner layer
Syncytiotrophoblast = outer mass - secretes hCG (basis of positive pregnancy tests)

Question 10

Embryonic disc and amniotic sac and secondary umbilical vesicle are suspended in the…

Answer 10

Chorionic cavity by the connecting stalk

Week 2

Question 11

Implantation of Conceptus - Week 2

Answer 11

Blastocyst becomes completely implanted in the endometrial lining
Implantation occurs day 7-10
Closing plug - blood clot that forms days 11-12
Decidual reaction - Glycogen and lipids are accumulated to prevent fighting foreign paternal DNA
Endometrial cells are now decidual cells

Question 12

3 Germ Layers of Week 3

Answer 12

Epiblast of the bilaminar embryonic disc undergoes gastrulation to form 3 germ layers
Ectoderm
Mesoderm
Endoderm
Trilaminar embryonic disc is now known as the gastrula

Question 13

3 New structures

Answer 13

Primitive Streak
Notochord
Neural Tube

Question 14

3 Cavities

Answer 14

Amniotic cavity
2nd umbilical vesicle
Extraembryonic Coelom

Question 15

Primitive Streak

Answer 15

Intense mitotic activity in epiblast causes elevation to form in the midline and this is the primitive streak
Its cranial end is rounded - primitive node
Dimple in primitive node - primitive pit
Primitive groove - narrow groove in the primitive streak

Question 16

Primitive Streak gives rise to…

Answer 16

Mesenchyme - embryonic connective tissue

Mesenchyme cells can differentiate into different types

Question 17

Intraembryonic Mesoderm then gives rise to…

Answer 17

Connective tissues
Skeleton
Skeletal muscle, smooth muscle
Bone marrow, blood cells
Cardiovascular system
Reproductive organs
Excretory organs

Question 18

Intraembryonic Endoderm gives rise to…

Answer 18

Epithelium lining of the respiratory tract
Epithelium lining of the GI tract
GI glands

Question 19

Intraembryonic Ectoderm gives rise to…

Answer 19

Epidermis
Nervous system
Retina

Question 20

Notochord

Answer 20

eventually the vertebral column
is a cellular rod from the notochord process
persists as the nucleus pulposus of the intervertebral discs

Question 21

Notochordal Process

Answer 21

cellular cord

extends cranially between the ectoderm and endoderm

Question 22

Notochordal canal

Answer 22

lumen that develops in the notochord process

Question 23

Neurulation

Answer 23

during weeks 3 and 4
Embryo is now a neurula
Ectoderm = surface ectoderm forms the epidermis of the skin
Neuroectoderm forms:
Neural plate (neural groove, folds and tube)
Neural crest

Question 24

Neural plate gives rise to

Answer 24

Neural Groove - center of neural plate
Neural Folds - Elevate and fuse in midline
Neural Tube - CNS

Question 25

Neural Crest gives rise to

Answer 25

Structures in the head and neck
Sensory ganglia
Autonomic nervous system ganglia
Schwann cells
Meninges
Melanocytes
Adrenal medulla

Question 26

Intraembryonic mesoderm consists of

Answer 26

lateral mesoderm that forms the lateral and ventral body wall, gut wall, and lines some cavities

Question 27

Intraembryonic Coelom splits…

Answer 27

Lateral mesoderm into somatic and parietal layers

Somatic forms the somatopleure (embryonic body wall)

Question 28

Limb development begins during week..

Answer 28

4

Question 29

Histogenesis of Cartilage

Answer 29

Cartilage is derived from mesenchyme

Mesenchymal cells differentiate into chondroblasts

Question 30

3 types of cartilage

Answer 30

Hyaline (articular surfaces of bones)
Elastic (auricles of the ears and epiglottis)
Fibrocartilage (intervertebral discs and pubic symphysis)

Question 31

Histogenesis of Bones

Answer 31

Derived from mesenchyme

Form by osteogenesis

Question 32

Intramembranous Bone Formation

Answer 32

Bone forms in a membrane of mesenchyme
This is how flat bone forms and bone collar around diaphysis of long bones
Mesenchymal cells differentiate into osteoblasts
osteoblasts - deposit osteoids
Bone matrix mineralizes
Osteoblasts trapped in mineralized matrix - osteocytes
Bone spicules form network of spongy bone
When marrow spaces are filled - compact bone is formed

Question 33

Endochondral Bone Formation

Answer 33

Hyaline cartilage model of the bone forms
Cartilage model calcifies, dies and is replaced by bone
A periosteal bud of blood vessels grows into the calcified cartilage
Periosteal bud brings in stem cells
Bone increases in diameter via appositional bone formation on bones outer surface
Bones grow in length by addition of new cartilage on epiphyseal side of growth plate