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Flashcards in Embryology Deck (30)
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1

Summarise Preimplantation development of the embryo

  • occurs within the Fallopian tube (oviduct) over a period of ~6 days
  • characterised by a series of cleavage divisions
  • double the number of cells in the conceptus (2, 4, 8, 16 cells)
  • produce a ball of undifferentiated cells (the Morula)
  • The Morula differentiates so that the inner cells differ from those on the outside 
  • This then develops into the Blastocyst, a structure that has an outer layer of trophectoderm, an inner cell mass, and a fluid-filled cavity.

2

How does the blastocyst initially develop?

  • 6 days post -fertilisation the Blastocyst hatches from the Zona Pellucida (within which it has developed up to this time)
  • begins to implant in the uterine lining - complete by 10 days post-fertilisation.
  • The inner cell mass differentiates to a bilayer disk, composed of hypoblast and epiblast cells
  • This bilayer disk gives rise to all the tissues of the human fetus, through a complex series of changes.

3

Describe gastrulation

  • converts the bilayer of hypoblast and epiblast cells into a trilaminar embryo, containing the three layers of Germ Cells (Ectoderm, Mesoderm and Endoderm)
  • occurs during days 14-18 post fertilisation
  • proliferation of epiblast cells,
  • which then differentiate to form mesoderm cells;
  • these move into the space between the epiblast and hypoblast.
  • These mesoderm cells differentiate further to generate the endoderm,
  • which replaces the hypoblast cells which are lost by apoptosis 

4

What does ectoderm give rise to?

  • skin
  • central nervous system

5

What does mesoderm give rise to?

  • muscles
  • blood
  • skeleton
  • heart
  • kidney

6

What does endoderm give rise to?

  • gut
  • lungs
  • liver

7

What is Neurulation?

  • the differentiation of the Ectoderm (Epiblast) to generate the central nervous system
  •  under the control of the notocord in the mesoderm

8

How does Neurulation proceed?

  • development of the neural plate;
  • this develops two folds, which increase in size until the meet over the neural groove
  • and fuse to form the neural tube 
  • This fusion process continues during week 4 of development as the central nervous system becomes a sealed tube.

9

What structures are present outside the embryo proper at week 3 of development?

  • the primordial germ cells (PGC) in the yolk sac endoderm at the caudal end of the embryo,
  • The cardiac and vascular progenitors in the primary heart field at the cranial end of the embryo

10

What does folding of the embryo create?

  1. Lateral folidng fuses the ventral midline (chest and abdomen) of the embryo
  2. Folding in the anterio-posterior direction folds the PGCs into the hind gut,
  • Also folds the developing heart progenitors under the head of the embryo

11

At which point are most of the internal structures laid down?

  • By the end of week 4 of development, the precursors of all internal tissues have been laid down, and many external structures are also developing.
  • Development during weeks 5-8 involves mostly the elaboration of the tissues generated during the early weeks

12

When do the limb buds begin and finish forming?

  • Forelimb bud appears at day 27/8
  • Hindlimb bud at day 29
  • Fully formed and patterned by day 56

13

What are the 3 phases of kidney development?

  • Pronephros is the most immature form of kidney
  • Mesonephros, an intermediate phase
  • Metanephros is most developed and persists as the definitive adult kidney.

14

When do the gonads begin developing?

7 weeks post-fertilisation

15

What do the gonads arise from?

  • intermediate mesoderm within the urogenital ridges of the embryo

16

What do the genital ducts arise from?

  • The mesonephric ducts give rise to MALE genital ducts (Wolffian system)
  • The paramesonephric ducts give rise to FEMALE genital ducts (Mullerian system)

17

What is required for the male genital ducts to arise?

  • the activity of sex-determining region Y (SRY) protein, coded for by the SRY gene on the Y chromosome.

18

What do the limbs develop from?

Grow out from lateral plate mesoderm rapidly

19

How does the kidney move during development?

  • Ascends from adjacent to the urinary bladder in the pelvis to its normal adult position
  • Forms and breaks down renal arteries as it ascends
  • ureters extent in length during this process, retaining the kidney-bladder connections

20

How does the indifferent reproductive system develop?

  • Within the mesonephros, the mesonephric and paramesonephric ducts develop (identifiable by week 5 post fertilisation) 
  • At the same time, the gonad precursor is developing from the mesonephric mesoderm, and is covered by coelomic epithelial cells.

21

How do the primordial germ cells develop?

  • PGC will give rise to the gametes within the gonads, 
  • They originate in the epiblast, but then migrate to the caudal part of the yolk sac 
  • Once the main caudal structures of the embryo proper have developed, the PGC migrate through the hind-gut and dorsal mesentery to the mesonephros and thence to the developing gonads.

22

When does the indifferent reproductive system become male or female?

Male = 7-8 weeks post fertilisation

Female = 8-9 weeks post fertilisation

23

How does the indifferent reproductive system develop into the male reproductive system?

  • The key regulator in male development is dihydrotestosterone (DHT), produced from the testis Leydig cells,
  • Stimulated by hCG from the maternal circulation. 
  • Testis Sertoli cells produce anti-Mullerian hormone (AMH), which causes the regression of the Mullerian (paramesonephric) ducts.
  • Testosterone support development of the Wolffian ducts, which give rise to the male reproductive tract.

 

24

How does the heart develop?

  • The cardiogenic cells develop in a U (or horseshoe) pattern outside the embryo proper.
  • These form a pair of heart tubes, which fuse to form a single heart tube by ~21 days post-fertilisation.
  • This tube is already able to pump blood unidirectionally.
  • Looping of the heart and septation give rise to the 4-chambered structure of the normal human heart
  • During this process the vascular connections are maintained, so that the major veins are connected to the atria, and major arteries to the ventricles.
  • Valves develop, to ensure that blood flows unidirectionally within the heart.

25

How does the fetal circulation differ from the adult?

  • The provision of oxygen to the embryo and fetus from the placenta means little blood flow to the lungs is needed
  1. There is a gap between the atria, the foramen ovale 
  • This allows blood returning to the heart (which is relatively high in oxygen) to pass from the right atrium to the left atrium, thence to left ventricle, from where it is pumped through the aorta to the body.
  1. The other major difference is that the pulmonary artery from the right ventricle is connected to the aorta by the ductus arteriosus
  • This diverts blood that would normally go to the lungs into the rest of the arterial system.

26

How does the face develop?

Strucutres all form on the side of the head 

They then move around to the front of the head to take their normal positions

 

27

What is the mechanism by which facial features can move during development?

  • Involves repeated formation of clefts in the face, and then filling in of the clefts
  • Leads to sequential loss of tissue from the centre of the face, and the movement of tissues to the correct places.

28

What lung structures are developing at each of the stages of lung development?

  1. Embryonic = bronchi and bronchioles
  2. Pseudoglandular = bronchioles and terminal bronchioles
  3. Canalicular = respiratory bronchioles
  4. saccular = Alveolar ducts and surfactant
  5. Alveola = alveolar sacs

29

When does lung surfactant start being produced?

aprox 25 weeks post fertilisation

30

What is role of the surfactant?

Reduce surface tension at air/liquid interface