Lesson 2: Embryology

Question 1

The Prenatal Period
(first 38 weeks of human development)

Answer 1

Pre-embryonic period (weeks 1-2)
- ZYGOTE -> MORULA -> BLASTOCYST…

Embryonic period (weeks 3-8)
- EMBRYO

Fetal period (weeks 9-38)
- FETUS

Question 2

Describe the overall process of PRE-EMBRYONIC development. (5 Steps)

Answer 2

1. Fertilization
2. Cleavage and Morula Formation
3. Blastocyst Formation
4. Implantation
5. Bilaminar Disc Formation

Question 3

1. Fertilization

Answer 3

The process begins with the OOCYTE, or egg, being released from the ovary and entering the fallopian tube.

When the SPERM successfully penetrates the OOCYTE, fertilization occurs, resulting in the formation of a single-cell ZYGOTE.

Question 4

2. Cleavage and Morula Formation

Answer 4

The ZYGOTE undergoes rapid mitotic cell divisions known as CLEAVAGE, producing smaller cells called blastomeres.

3-4 days after fertilization, the ZYGOTE develops into a MORULA, a solid ball of cells.

Question 5

3. Blastocyst Formation

Answer 5

As the MORULA continues dividing, a fluid-filled cavity forms inside, creating the BLASTOCYST (or “BLASTULA”) by days 5-6.

The BLASTOCYST consists of:

1) The TROPHOBLAST, an outer layer of cells that will contribute to the formation of the CHORION and later the PLACENTA.
2) The INNER CELL MASS (ICM), or EMBRYOBLAST, is a group of cells that will become the embryo.

Question 6

4. Implantation

Answer 6

Around day 6-7, the BLASTOCYST (or “BLASTULA”) begins to implant into the uterine wall.

The TROPHOBLAST differentiates into:

1) The CYTOTROPHOBLAST, the inner layer of the trophoblast, and
2) The SYNCYTIOTROPHOBLAST, which invades the uterine lining and establishes nutrient exchange.

Question 7

5. Bilaminar Disc Formation

Answer 7

Within the implanted blastocyst, the inner cell mass (EMBRYOBLAST) differentiates into two layers:

1) The EPIBLAST, which will give rise to the embryo and forms the AMNIOM (a protective membrane), and
2) The HYPOBLAST, which contributes to the yolk sac.

These two layers together form the BILAMINAR DISC.

Question 8

What is an OOCYTE?

Answer 8

Unfertilized Egg

Question 9

What are SPERM?

Answer 9

Male Reproductive Cells

Question 10

What occurs during FERTILIZATION?

Answer 10

Sperm Cell Fuses With Oocyte

Question 11

What is a ZYGOTE?

Answer 11

A Fertilized Ovum
(just like wife changing her name:)

Question 12

What is CLEAVAGE?

Answer 12

Rapid Mitotic Division of Zygote

Question 13

What is the MORULA?

Answer 13

A Solid Ball of 16ish Cells
(mulberry haha)

Question 14

What is the BLASTOCYST? What is it composed of?

Answer 14

A Ball of 100ish Cells With A Cavity

Trophoblast (outer shell)
+
Embryoblast (inner cell mass)

Question 15

What does - BLAST - mean?

Answer 15

To Build

Question 16

What is the TROPHOBLAST? What does it divide into?

Answer 16

A thin layer of cells that surrounds the embryoblast

Divides into two layers (Cytotrophoblast + Syncytiotrophoblast) which
both play a vital role in the development of the chorion

Question 17

What is the CHORION? What is it composed of?

Answer 17

Fetal Portion of the Placenta

Between uterine wall and umbilical cord
- Composed of CYTOTROPHOBLAST (adjacent embryoblast) + SYNCYTIOTROPHOBLAST (helps embryo burrow into uterine lining)

Question 18

What is the PLACENTA? What is it composed of?

Answer 18

A temporary organ that connects a developing fetus to the mother’s uterus during pregnancy (where nutrient and gas exchange takes place)

Fetal Portion + Maternal Portion

Question 19

What is the EMBRYOBLAST? What does it divide into?

Answer 19

Inner Cell Mass (Future Embryo)

Divides into two layers (EPIBLAST cells + HYPOBLAST cells)

Question 20

What is the BILAMINAR DISC?

Answer 20

Two-Layered Disc Located Where Epiblast Cells Touch Hypoblast Cells

(between amnion and yolk sac)

Question 21

What is the AMNION?

Answer 21

Cavity Surrounded By Epiblast Cells

Question 22

What is the YOLK SAC?

Answer 22

Cavity Surrounded By Hypoblast Cells

Question 23

Describe the overall process of EMBRYONIC development (6 Steps)

i.e. Describe the basic events leading to nervous system development, body folding, and early organ formation.

Answer 23

1. Primitive Streak Forms
2. Gastrulation
3. Body Folding
4. Neurulation
5. Limb Buds Form
6. Organogenesis

Question 24

1. Primitive Streak Forms

Answer 24

The primitive streak appears in the epiblast layer of the bilaminar disc, marking the beginning of embryonic development (creation of “sink hole”)

This streak initiates the transformation of the flat bilaminar disc into a more complex structure.

Question 25

2. Gastrulation

Answer 25

Through the primitive streak, epiblast cells migrate inward to form three primary germ layers. Gastrulation establishes the body’s blueprint by creating these layers, which will differentiate into all tissues and organs.

Question 26

3. Body Folding

Answer 26

The flat embryonic disc undergoes folding in two directions: 1) Transverse (lateral folding side-to-side): Wraps the embryo, enclosing the gut tube and creating a cylindrical body shape. 2) Cephalocaudal (longitudinal folding head-to-tail): Brings the cranial and caudal ends of the embryo closer, forming a tubular structure. This step converts the flat disc into a three-dimensional form with a defined internal cavity.

Question 27

4. Neurulation

Answer 27

The ectoderm forms the neural plate, which folds to create the neural tube—the precursor to the central nervous system (brain and spinal cord). This stage also involves the formation of neural crest cells, which contribute to structures like peripheral nerves, melanocytes, and facial cartilage. Proper neurulation is critical for normal development of the nervous system.

Question 28

5. Limb Buds Form

Answer 28

Small protrusions, known as limb buds, emerge from the lateral plate mesoderm. These buds grow and differentiate into the arms and legs, including their associated muscles, bones, and digits. This stage marks the beginning of recognizable external features.

Question 29

6. Organogenesis

Answer 29

The germ layers (ectoderm, mesoderm, and endoderm) differentiate into the body’s organ systems: - The heart starts beating early in this stage. - The lungs, liver, kidneys, intestines, and other organs begin to take shape. - Specialized tissues and functional systems develop, setting the stage for fetal growth and maturation.

Question 30

What is the process of GASTRULATION? What are the THREE RESULTS?

Answer 30

Epiblast cells move towards primitive streak then dive down, forming different layers between the hypoblast and the epiblast. The results are the primary germ layers: 1. ECTODERM (most superficial) 2. MESODERM 3. ENDODERM (most deep) * ALL THREE LAYERS COME FROM EPIBLAST CELLS! (not hypoblast cells)

Question 31

What are the THREE PRIMARY GERM LAYERS and their DERIVATIVES?

Answer 31

ECToderm: Forms the INTEGUMENTARY and NERVOUS systems MESoderm: Forms everything else in the middle (SKELETAL, MUSCULAR, ENDOCRINE, CARDIOVASCULAR, LYMPHATIC, URINARY & REPRODUCTIVE systems) ENDoderm: Forms the lining of the DIGESTIVE and RESPIRATORY systems. * THINK DONUT: A human is like a donut. The hole is the Endoderm (digestive & respiratory). The glaze is the Ectoderm (skin & senses). The dough is the Mesoderm (everything else in the middle).

Question 32

Which system are kidneys a part of?

Answer 32

The Urinary System

Question 33

Describe the embryo's TRANSVERSE and CEPHALOCAUDAL body folding.

Answer 33

Transverse: burrito folding (ectoderm on the outside, mesoderm in the middle, endoderm on the inside forming the gut tube) Cephalocaudal: fetal-position folding (head and tail ends coming together) * BOTH ARE HAPPENING AT THE SAME TIME (along with neurulation!!)

Question 34

Outline the steps involved in NEURULATION.

Answer 34

1. Formation of the NEURAL PLATE: The ectoderm, the outermost germ layer, receives signals from the underlying NOTOCORD (a structure derived from the mesoderm) to thicken and form the neural plate. 2. Shaping of the NEURAL GROOVE: The neural plate elongates and narrows. The edges of the neural plate begin to elevate, forming neural folds, while the central region (the neural groove) remains depressed. 3. Closure of the NEURAL TUBE: The neural folds continue to elevate and eventually converge at the midline, fusing to form the neural tube. * It's just like dating!! (once you've struck a notoCORD with someone, go get a PLATE of food, then go get your GROOVE on, then go home to watch the TUBE;)

Question 35

What is ORGANOGENESIS?

Answer 35

The Creation of Organs (organ-genesis): - Upper and lower limbs form - All rudimentary organ systems have developed by week 8 (WOAH!!) - Embryo is approximately 1 inch long by week 8

Question 36

What occurs during the FETAL PERIOD?

Answer 36

Growth and maturation of existing organs (weeks 9-38)

Question 37

What is considered as PREMATURE BIRTH?

Answer 37

Any birth before week 38 * Infants born as early as week 30 can survive without lifesaving measures * Infants can survive as early as week 22

Question 38

What is known about the CAUSES of BIRTH DEFECTS?

Answer 38

Causes of 50% of birth defects unknown 25% Multifactorial 10% Chromosomal 8% Monogenic 7% Major Environmental

Question 39

What is a TEROTAGEN?

Answer 39

Anything that causes a birth defect

Question 40

What is known about the PREVELANCE of BIRTH DEFECTS?

Answer 40

3 in 100 babies born in US have a serious structural birth defect.

Question 41

From which germ layer do kidneys originate?

Answer 41

Mesoderm

Question 42

From which germ layer does the liver originate?

Answer 42

Endoderm

Question 43

From which germ layer do lenses originate?

Answer 43

Ectoderm