Embryologic Timeline JK

Development week 1

1. Fertilization

2. Two cell stage (Cleavage)

3. Morula (morulation/blastulation)

4. Blastocyst formation- Zona Pellucida disappears

5. Beginning of implantation

Development week 2

1. Implantation

2. Bilaminar Germ Disc formation

a. Inner cell mass

b. Outer cell mass

3. Exocoelomic cavity (primary yolk sac)

a. Heuser's membrane

b. Cytotrophoblasts all around

4. Chorionic Cavity formation

a. Bilaminar disc attached to cytotrophoblast by connecting stalk.

b. Uteroplacental circulation begins due to lacunae in syncytiotrophoblastic layer

5. Appearance of Primitive Streak and Prechordal Plate

Development week 3

1. Primitive streak

2. Trilaminar germ disc formation

a. Invagination by ectodermal epiblasts to form mesoderm around primitive streak.

3. Mesoderm Migration

4. Formation of CNS

a. Neurulation

5. Appearance of somites

Development week 4

1. Neural groove closes (zips in both directions)

2. Cranial and Caudal Neuropores still open to amniotic fluid

3. Villus Formation

a. Primary

b. Secondary

c. Tertiary

4. Branchial arches

a. Five pairs of pharyngeal pouches (endodermal) and inversion of ectoderm

b. Gills

c. Forms heart bulge

5. Ear placode, eye anlage, and arm bud

Development week 5

1. Arm and leg bud development

2. Facial development

3. Tail bud

4. Branchial arches and clefts differentiate

Development week 6

1. Umbilical cord herniation from loops of intestine

2. Development of facial characteristics

3. Finger and foot rays

4. Chorionic villi

Development week 7

1. Fingers

2. Development of Face

3. Umbilical herniation still present

4. Toes

5. Well developed placenta

FROM CHAPTER 6 (She told us to self-study this)

Development week 9

1. Eyes move ventrally

2. Ears in definitive position

3. Arms reach relative length

4. Legs still short

Development week 12

1. Primary ossification centers in skull and long bones

2. Intestinal loops withdraw to cavity

3. Muscles have reflex action

4th and 5th month

1. Fetus lengthens rapidly

2. Covered with hair (lanugo hair)

a. Eyebrows and Head Hair

In 5th month, mother recognizes movement by fetus

6th month

1. Skin reddish and wrinkled because of lack of connective tissue under it.

2. Although several organ systems are function, CNS and Respiratory not fully diff, and not coordinated to work together

Last 2 months

Fetus obtains well-rounded contours- subcutaneous fat.

End of intrauterine life- Vernix Caseosa- whitish fatty substance on babies- result of sebaceous glands

28 weeks, able to survive although with great difficulty

9th month

1. Skull have largest circumference of any part

2. Sex characteristics pronounced- testes have moved into scrotum.