Embryology Flashcards
What happens in pre-implantation development of a human conceptus? !
- Preimplantation development normally occurs within the Fallopian tube (oviduct) over a period of around 6 days.
- It is characterised by a series of cleavage divisions, which subsequently double the number of cells in the conceptus to 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.
- The Blastocyst then hatches from the Zona Pellucida (within which it has developed up to this time, about day 6 after fertilisation – the zona pellucida surrounds ovum), and begins to implant in the uterine lining, a process which is complete about 10 days post-fertilisation.
- By this time the inner cell mass, which was a group of undifferentiated cells, has become a bilayer disk, composed of hypoblast and epiblast cells. This bilayer disk gives rise to all the tissues of the human foetus, through a complex series of changes.
What is gastrulation?
It is the series of changes that occur upon implantation of the blastocyst into the uterine lining.
What happens in gastrulation? !
- The blastocyst bilayer of hypoblast and epiblast cells is converted into a trilaminar embryo containing the three layers of germ cells (ectoderm, mesoderm and endoderm).
- This is shown in the photo: there is proliferation (P) of epiblast cells (ectoderm), which then differentiate (D) to form mesoderm cells; these move (M) into the space between the epiblast and hypoblast. These mesoderm cells are thought to differentiate further to generate the endoderm, which replaces the hypoblast cells which are lost by apoptosis (A). i.e. epiblast cells differentiate into the three germ cell types.
- Hypoblast cells differentiate into extraembryonic endoderm such as the yolk sac and placenta.
What do the three germ layers differentiate into from the embryo? Exception?
□ Ectoderm gives rise to skin and the CNS. □ Mesoderm gives rise to muscles, blood, skeleton, heart and kidney. □ Endoderm gives rise to gut, lungs and liver. □ Muscular and vascular tissue are generally of mesodermal origin, so tissues are normally a mixture of germ layer types (e.g. muscle in the skin and gut). □ NB: all are derived from epiblast cells.
What is neurulation?
Occurs before the end of gastrulation. It comprises of the development of the ectoderm to generate the CNS, under the control of the notochord in the mesoderm of the developing embryo.
What happens in neurulation?
- There is development of the NEURAL PLATE.
- The neural plate develops two folds, which increase in size until they meet over the neural groove and fuse to form the NEURAL TUBE.
- This fusion continues during week 4 of development and produces a sealed neural tube.
- Note that the structure of the neural folds is much more complex at the upper (cranial) end of the embryo; brain development has started by this stage.
What cells develop outside of the embryo? (x2)
□ 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.
What folding happens in the embryo to produce the chest and abdomen?
□ The embryo FOLDS LATERALLY, which fuses the ventral midline of the embryo (photo 1)
□ The cells that develop outside of the embryo are brought inside the embryo through ANTERIOR-POSTERIOR FOLDING: PGCs are folded into the hindgut, and the developing heart progenitors under the head of the embryo (photo 2).
□ This occurs from days 19-28 post-fertilisation.
□ Urogenital, cardiac, facial and lung development all proceed rapidly during the second month of development.
What is mosaicism?
It is the presence of two population of cells with different genotype in one individual. The genotypes arise from a single zygote. Occurs because the DNA in one of the cells of the embryo becomes mutated. As the cells of the growing embryo continue to divide, the number of both the cells with the mutated and unmutated genotype expand and contribute to the formation of organs and tissues.
What is chimerism?
Fused multiple non-identical zygotes, so individual is made up of cells in their tissues and organs with differing genotypes.
What is the genetic composition of people with Down’s syndrome?
Extra Ch21.
What are birth defects?
A congenital malformation or congenital abnormality which changes the PATTERN of development. It does not usually have a genetic basis.
What is a teratogen?
Any agent that can disturb the development of an embryo or foetus.
What examples of teratogens? (x3 (x5, x1 and x5))
□ INFECTIOUS AGENTS e.g. Rubella, Herpes, HIV, Syphilis and Zika virus. □ PHYSICAL AGENTS e.g. X-rays and other ionising radiation. □ CHEMICAL AGENTS e.g. thalidomide, lithium, amphetamines, cocaine and alcohol.
What is the pathophysiology of spina bifida?
□ There are two separate parallel tissues either side of the spine itself. The defect is often found towards the lower part of the back and therefore affects the lower limbs (downstream of the spina bifida determines the limbs that have compromised function).
□ Formation of the neural tube, the precursor of the CNS occurs in early pregnancy, about 3 weeks after fertilisation. Fusion of the neural tube should occur, and spinal bifida occurs if this process is not completed (see photo) – faulty neurulation.
What are the causes of spina bifida? (x4)
□ Low folic acid in maternal diet. The timing of foetal development is early, so folate needs to be given before conception, preferable about three months beforehand. □ Family history. □ Some medications have been linked to spina bifida such as valproate and carbamazepine. □ Obesity and diabetes in mother are risk factors.
What is the pathophysiology of Anencephaly?
Compromised development of the head and skull – the aetiology is thought to be similar to spina bifida, though the lack of neural tube fusion occurs at the anterior neuropore of the neural tube.
What happens in the process of facial development?
Initially, facial structures form on the SIDES of the head. From 5-10 weeks, these structures move centrally until they reach the expected positions on the face.
What is the pathophysiology of cleft palate?
□ Figures B, D and H show the LIP.
□ Normally, grooves in the mesenchyme are filled in during development (see D).
□ In cleft palate, the groove is not filled in and the ectoderm pierces into the mesenchyme too deeply, creating a persistent labial groove.
□ If this groove is too deep, it can manifest as penetrating the lip completely –> complete cleft lip.
What is the difference between cleft lip and cleft palate?
Cleft lip is asymmetric and occurs only on one side; cleft palate is usually symmetric as the halves of the palate do not feet and fuse correctly.
What is the process of heart development?
□ Two endocardial tubes fuse into a primitive heart tube. A primate atrium forms at the base, and ventricle in the middle.
□ The cardiogenic cells develop into a U pattern outside the embryo proper.
□ Looping and septation gives rise to the 4-chambered structure of the normal human heart.
□ Valves develop to ensure that blood flows unidirectionally.
How does the foetal circulation differ from a human circulation? (x2)
As little blood flow to the lungs is needed, there is a gap between the atria – the foramen ovale. This allows blood returning to the heart to pass from the right to the left atrium, thence to the left ventricle, from where it is pumped to through the aorta to the body. The other major difference is that the main artery from the right ventricle is connected to the aorta by the ductus arteriosus, diverting blood that would normally go to the lungs into the rest of the arterial system.
What is Tetralogy of Fallot?
□ Describes a congenital heart condition presenting a combination of FOUR DEFECTS in the heart:
□ (1) Pulmonary stenosis – thickened, narrow pulmonary outflow tract.
□ (2) Thickened right ventricle wall.
□ (3) Ventricular septal defect.
□ (4) Aorta overrides septal defect.
What is the congenital heart defect: Transposition of the Great Arteries?
The aorta is usually connected to the left ventricle, and the pulmonary artery to the right ventricle. In transposition of the great arteries, these are swapped such that the pulmonary artery is attached to the left ventricle, and the aorta from the right ventricle. This generates two separate blood flows; oxygenated blood is cycled through the left side of the heart via the lungs; de-oxygenated blood through the right side of the heart to the rest of the body. This isn’t much of an issue before birth as the foramen ovale and ductus arteriosus allows mixing of the blood to sustain foetal growth. After delivery, when the foramen close, this leads to Blue Baby Syndrome (cyanosis).
