1st month: Day by day

Question 1

1

Answer 1

FERTILIZATION

Question 2

2

Answer 2

ZYGOTE CLEAVAGE

· Zygote has undergone its 1st mitotic divisions => forms 2 smaller identical cells called BLASTOMERES (2-cell stage)

· Rapid cell divisions = cleavage

· Each step => nbr of blastomeres doubles, overall size doesn’t increase, cells become proportionally smaller

· End 2nd day: four-cell stage

Question 3

3

Answer 3

MORULA COMPACTION

· Zygote = ball of 8 cells of blastomeres (all equivalent and totipotent)

· later 12-16 cells => MORULA undergoes compation => increases contact between blastomeres + tight functions form between them

Question 4

4

Answer 4

BLASTOCYST

· Blastocyst cavity/ blastocoele

· Outer cells = Trophoblast => Main contribution to the placenta

· Inner cell mass (embryoblast) = Embryo

· Still inside zona pellucida, blastocyst floats in uterine fluid

Question 5

5

Answer 5

HATCHING FROM ZONA

· it slowly dissolves + falls away, if this fails the blastocyst “hatches” through a weak spot.

· Blastocyst can now interact with uterien wall

· Uterine endometrium prepares to receive it (progesterone rising secreted by luteal body/ corpus luteum)

Question 6

6

Answer 6

ENDOMETRIAL ADHESION

· surface blastocyst attaches to uterine epithelium

· region overlying inner cell mass => formation of multinucleated giant synciotiotophoblast cells

· Remaining trophoblast => cytotrophoblast

Question 7

7

Answer 7

ENDOMETRIAL INVASION

· Synciotiotophoblast cells increase + advance into uterine endometrium

· Trophoblast cells secrete human Chorionic Gonadotrophin (hCG) => hormone pregnancy tests
Rising blood levels of hCG act on LUTEAL BODY to mantain progesterone output
Feedback loop until placenta starts making progesterone

Question 8

8

Answer 8

BILAMINAR GERM DISK + AMNIOTIC CAVITY CREATION

· Inner cell mass attached to cytotrophoblast (outer cell mass)
2 distinct cellular layers separated by BASAL LAMINA:
- HYPOBLAST faces blastocoele
- EPIBLAST faces trophoblast (cytotrophoblast + Synciotiotophoblast)

·Hypoblast + Epiblast lying together = BILAMINAR GERM DISK

Question 9

9

Answer 9

AMNIOTIC & YOLK SACS

· Roof-lining of EPIBLAST = now AMNIOBLAST

· AMNION membrane= cells that separate amnioblast from cytotrophoblast

· AMNIOTIC SAC => sorrounds embryo & fetus until birth

· Lining of blastocoele = Primary yolk sac, form HEUSER’S MEMBRANE

· Synciotiotophoblast => still envading endometrium
Small spaces appear in Synci. = LACUNAE, fill with blood from ruptured maternal capillaries + secretions

Question 10

11

Answer 10

EXTRAEMBRYONIC TISSUES

· Exocoelomic membrane now complete => outlines primary yolk sac

· Its cells => secrete cell-free reticulum that pusches yolk sac away from cyto.

Question 11

12

Answer 11

LACUNAR CIRCULATION

· Maternal arterioles and venules are in open communication with each other through sinusoids

· Pressure difference between arterial and venous blood mantains steady convective flow through lacunar spaces

Question 12

13

Answer 12

PRIMARY CHORIONIC VILLI

=> cyto. grows + bulges out into sync. to form these
=> increase surface area for gas and nutrients exchange (embryonic tissue - maternal blood)

Question 13

14

Answer 13

EXTRAEMBRYONIC SPACES

· Fluid-filled spaces inside extraembryonic reticulum have been growing => create extraembryonic coelum
=> splits the reticulum into 2 layers:
- inner one envelopping yolk sac
- outer one lining cyto

· formation of seconday yolk sac

Question 14

15

Answer 14

PRIMITIVE STREAK + GASTRULATION

· 1st signs of major cell rearrangement in epiblast layer

· PS appears on amniotic surface of epiblast
=> streaks: cells that are changing their shape and orientation as they prepare to leave epiblast layer
=> forms in middle of embryonic disk towards particular point on its edge -> future head-to-tail axis

End of streak => PRIMITIVE NODE (cluster of knot of cells)
=> cells that grow out of the node will define CRANIAL END of body axis
=> streak runs towars the CAUDAL END

GASTRULATION
=> conversion of 2-layered bilaminar germ disk into a trilaminar disk from which every tissue of embryo will be derived

3rd layer = plane between epiblast and hypoblast

Internalization of epiblast cells happens all along the line of the PS, specially PRIMITIVE NODE (node marks cranial end of streak).

V-shaped groove where epiblast cells stream towards the streak from either side and drive along it => as its BASE they break away from epiblast layer to become MESODERM cells

Question 15

16

Answer 15

THREE GERM LAYERS

MESODERM cells continue to spread in all directions from the base of the PS creating 3rd germ layer over entire area of embryonic disk
+ spread beyond disk to colonize extraembryonic reticulum

Cells that migrate through new MESODERM layer + infiltrate the hypoblast = convert that layer into ENDODERM (facing yolk sac)
-> this layer will later line the gut tube of embryo after folding

MESODERM will form most of the internal structural tissues of embryo
EXCEPT from 2 small patches:
- near the caudal end of embryonic axis => cloacal plate
_ near cranial end => prochordal plate

ECTODERM: layer of remaining epiblast cells that still faces into amniotic cavity

Question 16

17

Answer 16

NOTOCHORD

At the cranial end of primitive streak lies primitive node.
Many of the epiblast cells contribute to a still, intially rod-like structure called the notochord.

=> pushes forward in the same plane as the rest of the mesoderm, towards cranial margin of the embryonic disk.

=> directly determines the body axis

Question 17

18

Answer 17

NEURAL PLATE

=> thickened portion of ectoderm along the midline of the embryo

=> sends specific molecular signals to a broad patch of overlying ectoderm to help pattern the Nervous System

Later these notochordal signals help to organize surrunding mesodermal structures that’ll form the vertebral bodies and laminae.

Question 18

19

Answer 18

PRIMITIVE HEART TUBULES

Cardiovascular system = 1st system to function

Vasculogenesis: process by which tthe first blood vessls form

Creation of heart itself= fusion of a symmetrical pair of vessels known as endocardial tubes or primitive heart tubes, one from each side of the disk

Question 19

20

Answer 19

NEURAL FOLDS

Neural plate begins to form nervous system by developing longitudonal ridges called neural folds along its borders, to create important midline structure called the neural groove. Crests of the neural folds rise slowly out of the main plane of the disk.

Strip nearest to notochord on each side is** paraxial mesoderm** = will be chopped lenghtwise into compact blocks called somites that form segmented structures all along the body axis.
FIRST somites appear close to the cranial end of notochord.
Somite generation = 3 or 4 new pairs each day

Question 20

21

Answer 20

COELUM CAVITIES

Intraembryonic coelom is formed. => pericardial, pleural and peritoneal cavities will be derived from

Question 21

22

Answer 21

NEURAL TUBE

Neural folds = very prominent
At the level of somites 4-6, they draw together and begin to fuse along the midline.

As neural folds draw together, an important population of neural crest cells is extruded into the adjacent mesoderm.

Question 22

23

Answer 22

FIRST CIRCULATION

Capillaries of yolk sac wall have formed a network of vitelline vessels, converging on the heart tube. These vessels contain the embryo’s first blood cells.

Peristaltitc heartbeat quickly becomes strong enough to move the blood cells around this simple circulation.

Question 23

24

Answer 23

PLACENTAL CIRCULATION

When heart begins to beat => blood circulates whough villi, shortening the diffusion path for gases, nutrients and waste products between embryonic tissues and maternal blood.

Question 24

25

Answer 24

LONGITUDONAL FOLDING

Folding of the embryonic disk is fundamental to body form.

Embryonic foling:
* Creates a foregut and a hindgut from the yolk sac
* Carries the heart ventral to the foregut
* Carries the** septum transversum** caudal to the heart
* Carries the connecting stalk ventral to the hindgut

Question 25

26

Answer 25

LATERAL FOLDING

• Makes the foregut and hindgut tubular
• Wraps the intraembryonic coelum around the gut
• Draws together the entire margin of the disk
• Creates the ventral body wall
• Causes the amniotic sac to sorround the embryo

Question 26

27

Answer 26

UMBILICAL CORD

Product of embryonic folding

• Brings the yolk sac and connecting stalk side by side
• Narrows the yolk sac to form the vitelline duct
• Encloses both structures within a sheath of amnion that “shrink-wraps” them into a single compact package

Question 27

28

Answer 27

AXIAL SEGMENTATION

Metameric segmentation: serial repetition of certain body parts along the cranio-caudal axis that all vertebrates and many invertebrates show

First segmented structures = somites (come from paraxial mesoderm)
=> will form vertebrae and ribs