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Flashcards in First to Third Weeks of Development Deck (33):
1

Fertilization

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

2

Cleavage of the Zygote

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

3

Formation of Blastocyst

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)

4

Blastocyst consists of

Embryoblast
Blastocystic Cavity
Trophoblast
Zona Pellucida

5

Implantation of the Blastocyst

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)

6

Formation of hypoblast

Appears around day 7
early stage of primary endoderm

7

2 embryonic layers in week 2

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

8

2 cavities in week 2

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

9

2 Trophoblast layers in week 2

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

10

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

Chorionic cavity by the connecting stalk
Week 2

11

Implantation of Conceptus - Week 2

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

12

3 Germ Layers of Week 3

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

13

3 New structures

Primitive Streak
Notochord
Neural Tube

14

3 Cavities

Amniotic cavity
2nd umbilical vesicle
Extraembryonic Coelom

15

Primitive Streak

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

16

Primitive Streak gives rise to...

Mesenchyme - embryonic connective tissue
Mesenchyme cells can differentiate into different types

17

Intraembryonic Mesoderm then gives rise to...

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

18

Intraembryonic Endoderm gives rise to...

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

19

Intraembryonic Ectoderm gives rise to...

Epidermis
Nervous system
Retina

20

Notochord

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

21

Notochordal Process

cellular cord
extends cranially between the ectoderm and endoderm

22

Notochordal canal

lumen that develops in the notochord process

23

Neurulation

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

24

Neural plate gives rise to

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

25

Neural Crest gives rise to

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

26

Intraembryonic mesoderm consists of

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

27

Intraembryonic Coelom splits...

Lateral mesoderm into somatic and parietal layers
Somatic forms the somatopleure (embryonic body wall)

28

Limb development begins during week..

4

29

Histogenesis of Cartilage

Cartilage is derived from mesenchyme
Mesenchymal cells differentiate into chondroblasts

30

3 types of cartilage

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

31

Histogenesis of Bones

Derived from mesenchyme
Form by osteogenesis

32

Intramembranous Bone Formation

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

33

Endochondral Bone Formation

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