Development & Ageing - Early fetal development

Question 1

What is fertilization age?

Answer 1

The age of a fetus measured from the time of fertilization

Question 2

What is gestational age?

Answer 2

• The age if calculated from the time of the beginning of the last menstrual period
• Fertilization period + 14 days

Question 3

What is the Carnegie stage?

Answer 3

• 23 stages of embryo development based on embryo FEATURES not TIME
• Covers the window of 0-60 days fertilization in humans

Question 4

What is the embryogenic stage?

Answer 4

• 14-16 days post fertilization
• Establishing the early embryo from the fertilized oocyte
• Determining to cell types:
  
  • Pluripotent embryonic cells
  • Extraembryonic cells

Question 5

What is the embryonic stage?

Answer 5

• 16-50 days post fertlization
• Establishment of the germ layers and differentiation of tissue types
• Establishment of the body plan

Question 6

What is the fetal stage?

Answer 6

• 50-270 days
• Major organs now present
• Migration of some organ systems to final location
• Extensive growth and acquisition of fetal viability (survival outside the womb)

Question 7

What stages occur in the first, second and third trimesters?

Answer 7

First:

• Embryogenic stage
• Embryonic stage

Second & third:
- Fetal stage

Question 8

What occurs in the maternal-to-zygotic transition?

Answer 8

• Embryo is dependent on maternal proteins and mRNAs to get through the first divisions
• These mRNA and proteins are synthesized and stored during oocyte development
• Transcription of of embryonic genes occurs at the 4-8 cell stage
• This increases protein synthesis and organelles are matured

Question 9

What occurs in compaction?

Answer 9

• Around the 8 cell stage
• Outer cells press against zona pellucida
• Outer cells change to wedge shaped from spherical
• Outer cells connect to each other through tight gap junctions and desmosomes
• This forms a barrier to diffusion between inner and outer embryo
• Outer cells become polarised (apical and basal polarity)
• Inner cells remain spherical

Question 10

Describe the structure of the blastocyst

Answer 10

• Zona pellucida outer shell
• Trophoectoderm under zona pellucida made of extra-embryonic cells
• Inner cell mass of pluripotent embryonic cells
• Blastocoel next to inner cell mass

Question 11

What is hatching?

Answer 11

• When the blastocyst escapes the zona pellucida

- Occurs by ezymatic digestion and cellular contactions

Question 12

Explain what occurs in the peri-implantation events

Answer 12

• The trophoectoderm separates into the syncitiotrophoblasts and the cytotrophiblasts
• Syncitiotrophoblast invade the endometrium and destroy blood vessels to allow them to be bathed in maternal blood
• The cytotrophoblasts continue to divide to provide more cells to the syncitiotrophioblasts
• The inner cell mass also differentiates into the epiblast and hypoblast
• The epiblast is what fetal tissue will be derived from
• The hypoblast will form the yolk sac

Question 13

Explain what occurs in the bi-laminar embryonic disc formation

Answer 13

• The syncitiotrophoblasts continue to expand into the endometrium
• Some of the epiblast cells become separated from the main block by the formation of the amniotic cavity

Question 14

What is gastrulation?

Answer 14

When the bi-laminar disc differentiates into 3 primary germ layers

Question 15

Explain the process of gastrulation

Answer 15

• After 25 days a thickened structure forms along the midline of the epiblast, called the primitive streak
• At the cranial end of the embryonic disc, the primitive streak expands and is called the primitive node which contains the primitive pit
• Once formed, cells of the epiblast migrate inwards to the caudal end, detatch from the epiblast and slip beneath it, known as invagination
• The hypoblast cells are eventually completely replaced by a proximal cell layer called the definitive endoderm
• The remaining cells of the hypoblast are called the ectoderm
• Some of the invaginated cells remain in the layer between the definitive endoderm and the ectoderm and form the mesoderm

Question 16

Where is the notochord formed?

Answer 16

It forms along the embryo midline, under the ectoderm, from the primitive streak to the head end

Question 17

What is the function of the notochord?

Answer 17

Acts as a key organiser centre for neuralation and mesoderm development

Question 18

What occurs in neurulation?

Answer 18

The notochord send signals to the neural plate to invaginate in the middle to form the neural groove and this creates two ridges on either side called the neural folds. Within the folds are neural crest cells.

Later on the neural folds rise and move in to meet eachother and eventually fuse over the neural groove which creates a hollow tube where the groove was, which is called the neural tube. The neural crest cells then migrate away

Question 19

On what days do the tail end and head end of the neural tube close?

Answer 19

Tail end - 27 days

Head end - 23 days

Question 20

What is somitogenisis?

Answer 20

The formation of somites

Somites arise from a paired block of paraxial mesoderm flanking the neural tube and notochord

Question 21

What tissue types do somites form?

Answer 21

Sclerotome:
- Forms vertebrae and rib cartilage

Dermomyotome:

• Sub divides to form:
  
  • Dermatome:
    
    • Forms the dermis of the skin, some fat and connective tissue of neck and trunk
  • Myotome:
    
    • Forms the muscles of the embryo

Question 22

How is the gut formed?

Answer 22

Ventral and lateral folding of the embryo cuases part of the yolk sac to be pinched off which forms the primitive gut

The primitive gut is the patterned into the foregut, midgut and hindgut

Question 23

What structures does each part of the gut form?

Answer 23

Foregut:
Esophagus, stomach, upper duodenum, liver, gallblader, pancreas

Midgut:
Lower duodenum and remainder of small intestine, ascending colon and first two thirds of the transverse colon

Hindgut:
Last thrid of transverse colon, descending colon, rectum and upper anal canal