Principles on embryology

Question 1

What are the time periods and names of the 3 trimesters?

Answer 1

Weeks 0 to 3 = conceptus/Embryo
Weeks 3 to 8 = Embryonic period
Weeks 9 to 40 = fetal period

Question 2

What period has the highest risk of birth defects being induced?

Answer 2

The embryonic period has the highest risk, specifically around week 5

Question 3

Name the 6 phases of embryogenesis

Answer 3

1. Gametogenesis
2. Fertilisation
3. Cleavage
4. Gastrulation
5. Formation of the body plan
6. Organogenesis

Question 4

Explain what gametogenesis is and how it differs in females and males

Answer 4

Gametogenesis is the formation of a gamete (a haploid cell with 23 chromosomes. The formation of a gamete consists of mitosis and meiosis which leads to genetic variation.

In females, gamete production is known as oogenesis (as it is production of an oocyte). It starts in development and ends at menopause. By week 28 to 30, the primary oocytes begin meiosis but then stop in prophase until puberty.

In males, gamete production is known as spermatogenesis. It starts at puberty and continues throughout life however the production rate of sperm decrease with age. The male gamete can either be 22+X or 22+Y so determines the sex of the child

Question 5

When is reproductive life span determined?

Answer 5

Reproductive life span is actually determined in fetal life

Question 6

Explain phase 2 - fertilisation

Answer 6

Fertilisation is when the gametes (sperm + oocyte) fuse together to form a zygote, which is a diploid, single cell embryo. The sperm is attracted to the egg by chemicals

Question 7

What happens in phase 3 (cleavage)?

Answer 7

Cleavage is a period of rapid cell division but with NO increase in size. As the cells divide, they are known as blastomeres and once there are at least 16 cells, this is known as a morula (looks like a mulberry). A blastocyst is then formed and has a blastocyst cavity inside the zona pellucida.

Question 8

What is Gastrulation?

Answer 8

Gastrulation is the formation of germ layers, usually in week 3. There are 3 primary layers of cells here known as the ectoderm (outer layer - skin, nervous), the mesoderm (middle layer - skeletal + muscle tissue) and the endoderm (inner layer - internal lining). The body axes are also established during this phase

Question 9

What happens during the formation of the body plan (phase 5)?

Answer 9

Embryonic folding occurs at this phase. Tube-within-a-tube so the gut folds into the skin/body wall

Question 10

What is organogenesis?

Answer 10

This is the formation of organs and organ systems. The basis of these are all in place by the end of the embryonic period and will continue to develop through foetal period

Question 11

What happens during the foetal period and when does it occur?

Answer 11

The foetal period is from weeks 9 to 40. During this period, it is mainly growth and weight gain that occurs. Mainly length growth during 2nd trimester and weight gain during 3rd trimester. Here, tissues mature and become functional, bone is laid down and connections are made in the CNS

Question 12

What are the anatomical terms used when describing an embryo or foetus?

Answer 12

Cranial and caudal are used to distinguish head and tail, respectively and dorsal and ventral are used to distinguish back and front, respectively

Question 13

What are the primary processes which occur when a zygote is becoming a human?

Answer 13

1. Cell division - increase in number of cells
2. Differentiation - Specialisation, change in appearance/structure and adoption of new functions
3. Cell attachment - physical/functional linkages and formation of tissues
4. Apoptosis - programmed cell death
5. Induction - Ability of one cell to cause another to differentiate
6. Cell migration - The movement from one location to another

Question 14

What are the secondary processes?

Answer 14

1. Axis formation/polarity - This is when cells figure out where they are in relation to each other and the embryo as a whole
2. Folding/rotation - This is gives the embryo a 3D form and allows the formation of complex organs such as the heart and the gut. The entire embryo and structures within it are included in this process

Question 15

What are the controls of development?

Answer 15

The controls of development are:

1. Genetic - gene expression which is tightly regulated in time and space
2. Epigenetic (effects phenotype) - epigenetic changes can switch genes on or off and so can result in expression of either the maternal or paternal copy of a gene. E.g Angelman syndrome (deletion on maternal chromosome 15) and Prader-Willi Syndrome (deletion on paternal chromosome 15)