Embryology: Germ Layers and Limb Development

Question 1

What marks the beginning of human development?

Answer 1

Fertilization marks the start when a sperm fuses with an oocyte to form a single cell, a zygote

Question 2

What are the phases of fertilization?

Answer 2

1. The sperm penetrates the oocyte
2. The zona pellucid becomes impenetrable to other sperm
3. The male and female pronucleus both form
4. Formation of the zygote

Question 3

What are the stages of zygote cleavage?

Answer 3

2 cells -> 4 cells -> 8 cells -> Morula: (16- 32 cells) which enter the uterus and the fluid filled space appears -> Blastocyst: which has inner cell mass (embryoblast) & outer cell mass (trophoblast)

Question 4

What is the blastocyst?

Answer 4

The zygote has divided into a structure that has formed a layer of outer cells surrounding an inner cell mass.

Question 5

When does the implantation of the blastocyst occur?

Answer 5

Occurs at the beginning of the 1st week and ends by the 2nd week.

Question 6

How and where does the implantation of the blastocyst occur?

Answer 6

Occurs in the posterior wall of the superior part of the uterine body as zona pellucida disappears, with the trophoblast differentiating into cytotrophoblast & syncytiotrophoblast.

Question 7

How and when does gastrulation occur?

Answer 7

It occurs during the 3rd week of development and leads to the formation of the epiblast that forms all three germ layers.

Question 8

What does gastrulation mark the start of?

Answer 8

Morphogenesis

Question 9

How do chorionic villi form and what is their function?

Answer 9

Chorionic villi form as outward projections from the trophoblast and act as a barrier between the maternal and fetal blood and function in their exchange.

Question 10

What is the primitive streak and how is it important?

Answer 10

It is the thickening of the epiblast cells and occurs during the third week which leads to formation of the epiblast and gastrulation.

Question 11

What is the primitive node and how is it important?

Answer 11

It is swelling at one end of the primitive streak and leads the cranial migration of mesenchymal cells and is followed by the formation of the notochord.

Question 12

A 20-year-old woman presented to ER with severe abdominal pain on the right side with signs of internal bleeding, she missed her last menstrual period. What is a possible condition?

Answer 12

Ectopic Tubal Pregnancy - the zygote is implanted in the fallopian tube instead of in the uterus

Question 13

What must degenerate for the implantation of the blastocyst to occur?

Answer 13

Zona Pellicuda

Question 14

During the later stages of pregnancy what separates the maternal blood from the fetal blood?

Answer 14

Syncytiotrophoblast and fetal endothelium

Question 15

At the beginning of the 3rd week of development, what is the composition of the embryonic disc?

Answer 15

Ectoderm, mesoderm & endoderm

Question 16

What future site does does the prechordal plate mark?

Answer 16

The mouth

Question 17

A 28-year-old pregnant woman experiences repeated episodes of red vaginal bleeding at week 28, 32 & 34 of pregnancy. The bleeding spontaneously subsided each time. Use of ultrasound shows that the placenta is located in the lower right portion of the uterus over the internal os. What is the diagnosis?

Answer 17

Placenta previa - the placenta is inserted partially or wholly in the lower uterine segment

Question 18

What is the maternal component of placenta?

What is the fetal component of placenta?

Answer 18

Decidua basalis.

Villous chorion.

Question 19

Describe the proximal-distal growth of the limb bud.

Answer 19

Limb growth is initiated by the apical ectodermal ridge (AER) and proceeds from the proximal to distal parts. (Shoulder to hand)

Question 20

Explain the relationship between the cutaneous segmental nerve supply of the limbs and limb rotation.

Answer 20

There is a strong relationship between the growth and rotation of the limbs and the cutaneous segmental nerve supply of the limbs.

The lower limb rotates medially 90 degrees and the upper limb rotates 90 degrees laterally.

Question 21

What are some anomalies of limb development?

Answer 21

Amelia - absence of an entire limb.
Meromelia - absence of part of a limb.
Phocomeila - short, poorly formed limb.
Polydactyly - extra digit.
Syndactyly - fusion of digits.
Lobster Claw deformity - middle digit is absent.


Question 22

When does the patterning and growth of the limbs occur?

Answer 22

During weeks 4-8 of development

Question 23

What are the major events during weeks 1-3 of development?

Answer 23

Week 1: Implantation of the blastocyst

Week 2: The inner cell mass differentiates into epiblast & hypoblast (bilaminar disc) and the outer cell mass differentiates into cytotrophoblast & syncytiotrophoblast.

Week 3: Primitive streak forms then primitive node forms. The mesoderm differentiates into 3 regions, somite, intermediate and lateral.