Embryogenesis terminology and timeline

Question 1

Where is sonic hedgehog gene produced? What patterning is it involved in?

Answer 1

Base of limbs in zone of polarizing activity. Anterior-posterior axis

Question 2

What other major development process is SHH present in?

Answer 2

CNS development; mutation can cause holoprosencephaly.

Question 3

Where is Wnt-7 produced? What patterning is it involved in?

Answer 3

Apical-ectodermal ridge: thickened ectoderm at distal end of each developing limb. Shapes dorsal/ventral axis.

Question 4

Where is FGF gene produced? What is it responsible for?

Answer 4

Apical-ectodermal ridge. Stimulates mitosis of underlying mesoderm, providing for lengthening of limbs.

Question 5

What are hox genes?

Answer 5

HOX genes are involved in segmental organization of embryo in a cranio-caudal direction. Codes for transcription factors. Hox mutations –> appendages in wrong locations.

Question 6

What is considered “Day 0” of embryogenesis?

Answer 6

Fertilization by sperm

Question 7

By what day does the zygote form?

Answer 7

2

Question 8

By what day does the morula form?

Answer 8

3

Question 9

By what day does the blastocyst form

Answer 9

5?

Question 10

When does implantation in the endometrium occur?

Answer 10

Day6-10 (within week 1)

Question 11

What occurs by the second week?

Answer 11

By 2 weeks, 2 layers: bilaminar disc

Question 12

What are the layers of the bilaminar disc?

Answer 12

epiblast, hypoblast

Question 13

What occurs by the third week?

Answer 13

3 weeks, 3 layers –> trilaminar disc.

Question 14

What is gastrulation?

Answer 14

The process that forms the trilaminar embryonic disc. Starts with epiblast invaginating to form the primitive streak.

Question 15

What are the layers of the trilaminar disc?

Answer 15

ectoderm, mesoderm, and endoderm germ layers

Question 16

When does the neural plate begin to form?

Answer 16

Week 3

Question 17

When is the neural tube closed by?

Answer 17

week 4

Question 18

What occurs during weeks 3-8?

Answer 18

Organogenesis. Extremely susceptible to teratogens.

Question 19

When does the hear begin to beat by?

Answer 19

Week 4

Question 20

When are limb buds formed by?

Answer 20

4 weeks, 4 limbs.

Question 21

By when is fetal cardiac activity visible by transvaginal ultrasound?

Answer 21

Week 6

Question 22

By when do fetal genitalia have male/female characterisitics?

Answer 22

Week 10

Question 23

What is the destination of ectoderm, mesoderm, and endoderm, generally?

Answer 23

Ectoderm: external/outer layer
Meso: Middle layer
Endoderm: Internal layer

Question 24

What are the three components of ectoderm?

Answer 24

surface ectoderm, neuroectoderm, and neural crest

Question 25

What are the derivatives of surface ectoderm?

Answer 25

``` Epidermis, anal canal below pectinate line. Gland related: -Adenohypophis (from rathke pouch) -parotid, sweat, mammary glands Sense related: - lens of eye, -epithelial linings of oral cavity, - sensory organs of ear - olfactory epithelium ```

Question 26

What are the derivatives of neuroectoderm?

Answer 26

Brain+spinal cord (inc neurohypophyis, pineal gland) | Retina/optic nerve

Question 27

What are the derivatives of neural crest?

Answer 27

PNS: - dorsal root ganglia, cranial nerves, celiac ganglion, schwann cells, ANS Melanocytes Chromaffin cells of adrenal medulla parafollicular C cells of thyroid pia, arachnoid, bones of skull, odontoblasts, aorticopulmonary septum

Question 28

What are the derivatives of mesoderm?

Answer 28

muscle, bone, connective tissue, serous linings of body cavities (eg peritoneum), spleen (derived from foregut mesentary), CV structures, lymphatics, blood, wall of gut tube, vagina, kidneys, adrenal cortex, dermis, testes, ovaries

Question 29

What is the purpose of the notochord? What is its post-natal derivative?

Answer 29

Induces ectoderm to from neuroectoderm (neural plate). Its only post-natal derivative is the nucleus pulposus of the intervertebral disc.

Question 30

What are the derivates of endoderm?

Answer 30

``` Gut tube epithelium (including anal canal above the pectinate line), most of urethra (derived from urogenital sinus), luminal epithelial derivatives (e.g., lungs, liver, gallbladder, pancreas, eustachian tube, thymus, parathyroid, thyroid follicular cells). ```

Question 31

What is a craniopharyngioma derived from?

Answer 31

Surface ectoderm: benign rathke pouch tumor with cholesterol crystals and calcifications.

Question 32

What does VACTERL stand for?

Answer 32

``` Mesoderm defects: Vertebral defects Anal atresia Cardiac defects Tracheo-Esophageal fistula Renal defects Limb defects (bone/muscle). ```

Question 33

What is agenesis?

Answer 33

Absent organ due to absent primordial tissue.

Question 34

What is aplasia?

Answer 34

Absent organ despite presence of primordial tissue.

Question 35

What is hypoplasia?

Answer 35

Incomplete organ development; primordial tissue present.

Question 36

What is deformation?

Answer 36

Extrinsic disruption; occurs after embryonic period.

Question 37

What is disruption?

Answer 37

2° breakdown of previously normal tissue or structure (e.g., amniotic band syndrome).

Question 38

What is malformation?

Answer 38

Intrinsic disruption; occurs during embryonic period (weeks 3–8).

Question 39

What is a "sequence"

Answer 39

Abnormalities result from a single 1° embryologic event (e.g., oligohydramnios Ž Potter sequence).