2 Early Embryology

Question 1

Define the pre-embryonic period of human development.

Answer 1

From fertilisation to 2 weeks.

Question 2

Define the embryonic period of human development.

Answer 2

Weeks 3 to 8.

Question 3

Define the fetal periods of human development.

Answer 3

Weeks 8 to 38.

Note: Pregnancy calculated from date of last menstral period, with overvulation 2 weeks after that. Hence sometimes referred to as 40 weeks.

Question 4

Discuss the changes taking place as the fertilised human ovum (zygote) travels down the fallopian (uterine) tube towards the uterus.

Answer 4

Cleavage occurs, where the egg divides to form blastomeres. Continues until there are many cells enveloped in the zona pellucida, which results in the zygote being called a morula.

Question 5

What does cleavage mean?

Answer 5

A series of mitotic divisions where a zygote becomes a morula.

Question 6

What does zona pellucida mean?

Answer 6

A glycoprotein shell that surrounds the zygote, morula and blastocyst.

Question 7

Discuss the formation of the blastocyst and the initial stages of implantation.

Answer 7

The MORULA undergoes COMPACTION following CLEAVAGE of the initial zygote.

Forms the blastocyst - https://gyazo.com/9c53f7bec6889291a18d9f5492425420

• Trophoblast outer cell mass.
• Embryoblast inner cell mass.
• Blastocyst cavity (filled with fluid).

Totipotent (all cells) –> pluripotent (many cells) FOLLOWING COMPACTION.

HATCHING then occurs, where the blastocyst hatches from the zona pellucida.

Implants onto the uterus lining by breaching the uterine epithelium.

Blood flow is established within the placenta.

Question 8

Define sites for ectopic preganancy and placenta praevia and distinguish between these two implantation defects.

Answer 8

Ectopic: Implantation at the wrong site.
- life threatening if ruptures due to blood loss.
- commonly in the fallopian tube, but may also be periotoneum.

Placenta praevia: Implantation in the lower uterus.
- placenta blocks the birth canal.
- C section required.

Question 9

Describe the formation of the embryonic disc in the inner cell mass and initial cell differentiation within it.

Answer 9

Embryoblast and trophoblast cells further differentiate.

Embryoblast becomes epiblast & hypoblast, forming the bilaminar/embryonic disc. These 2 layers are referred to as germ layers.

Trophoblast becomes the cytotrophoblast and the syncytiotrophoblast.

Question 10

What is the trilaminar embryonic disk?

Answer 10

During GASTRULATION, the bilaminar disk becomes the trilaminar disk.

A PRIMITIVE STREAK forms on the epiblast surface during week 3.
https://gyazo.com/c979913e7a5a2f22731f615531c0bdd3

Cells divide by mitosis, and pile up on the epiblast. Invaginate through the primitive streak, forming the trilaminar disk. Consists of ectoderm, mesoderm and endoderm.
https://gyazo.com/ae7148b44422cd9c2b2b42aeca8cb812

Hypoblast pushed out the way. Trilaminar disk is formed from the epiblast ONLY.

Mesoderm/ectoderm has 2 gaps, which are the future mouth/anus.

Question 11

What key structures are derived from the ectoderm?

Answer 11

Structures that have contact with the outside:

• nervous system
• epidermis (skin)
• hair

Question 12

What key structures are derived from the mesoderm?

Answer 12

Supporting tissues:

• muscle
• cartilage
• bone
• vascular system (heart + vessels)

Question 13

What key structures are derived from the endoderm?

Answer 13

Internal structures:

• epithelial lining of the GI tract
• epithelial lining of the respiratory tract.
• liver

Question 14

Describe the formation and fate of the neural plate and outline the development of the neural tube.

Answer 14

Following GASTRULATION, NEURALATION occurs.

Notochord forms in the mesoderm - solid rod of cells.

Signalling role, causing the ectoderm above to thicken to form neuroectoderm (neural plate).

Thickens & curls, forming neural tube.

This will eventually become the brain and spinal cord.

Question 15

Describe the differentiation of mesoderm into the notochord, somites, intermediate mesoderm and lateral plate.

Answer 15

Following GASTRULATION, NEURALATION occurs.
https://gyazo.com/55c6e8a58e1b5596a99f957fd5265b4f

Notochord forms in the mesoderm - solid rod of cells.

Somites develop either side of the notochord in segments.

Intermediate mesoderm (between the somites and lateral plate) forms.

The lateral plate splits into somatic and splanchnic mesoderm, with a gap between called the intraembryonic coelom.

Question 16

List the derivatives of each region of the mesoderm.

Answer 16

Notochord becomes the vertebral column.

Neural plate becomes the brain (at the wide end) and the spinal cord (at the narrow end).

Somites - from paraxial mesoderm - form the vertebral column, ribs, and muscles.
https://gyazo.com/cc0e6183f3c7dfe7d1e9b78a5a94b925

Intermediate mesoderm forms the kidneys.

Lateral plate (either side of the mesoderm) forms the somatic mesoderm, a space called the intraembryonic coelom and the splanchnic mesoderm.

Somatic mesoderm forms the diaphragm and limb muscles.

The coelom becomes the body cavities.

Splanchnic mesoderm forms the heart muscle and viscera.

Question 17

Describe the differentiation of somites into dermatome, myotome and sclerotome and the fate of each of these in later development.

Answer 17

Myotome and dermatone disperse out.

Sclerotome becomes bones.

Dermatome becomes the dermis - skin.

Myotome becomes the muscles.

Question 18

Explain the use of the terms “dermatome” and “myotome” in describing the innervation of skin and muscles in the adult.

Answer 18

Dermatome refers to sections of skin that recieve innervation from a single spinal nerve. Myotome is the same, but just for muscles.

Question 19

Describe the formation of the head fold, tail fold and lateral body folds.

Answer 19

Ectoderm folds so no endoderm is on the outside.
https://gyazo.com/2043ee7210855b309939800f3fc2645c

This achieves:
- pulls the amniotic membrane around the trilaminar disk (and its derivatives).
- embryo therefore suspended in the amniotic sac.