Session 2 ILO's - Embryology and early pregnancy Flashcards
Define the pre-embryonic, embryonic and fetal periods of human development
Pre-embryonic: 1-2 weeks after fertilisation (Cleavage, compaction, implantation - is essential in allowing us to embark on the embryonic period)
Embryonic: 3-8 weeks inclusive post fertilisation (All of the systems of the body are built - embryo will have brain, kidney, heart ect…)
Fetal: From 9 weeks post fertilisation to birth (Functional maturation of those structures built in embryonic phase)
Describe the fertilisation of the oocyte
- The oocyte gets fertilised by the sperm in the ampulla of the Fallopian tube
- The sperm passes through the zona pellucid and the plasma membrane
- It merges it’s genetic information with the ovum, to form a zygote
Describe the zona pellucida
- Thick glycoprotein shell
How long is a sperm viable for? How long is an oocyte viable for?
Sperm - up to 3 days
Oocyte - 1 day
Discuss the changes taking place as the fertilised human ovum (zygote) travels down the Fallopian (uterine) tube towards the uterus
- The zygote, is transported along the fallopian tube to the uterus, by the specialised cilia in the oviduct under the actions of progesterone.
- The zygote travels from the ampulla end of the Fallopian tube to the junction with the uterus (called the isthmus). This takes about 4 days.
- As it travels through the Fallopian tube, the zygote undertakes successive cell divisions, a process known as cleavage.
- Cleavage (week one):
- Begins 30 hours after fertilisation
- Cell divisions occur without an increase in cell size and cells are now known as blastomeres
- First division occurs to create two totipotent blastomeres of equal size
- Cells become smaller with each cleavage division
- Cells keep dividing, all within zona pellucida
Compaction:
- After the third cleavage blastomeres maximize their contact with each other, forming a compact ball of cells held together by tight junctions.
- The process of compaction, segregates inner cells from the outer cells and they communicate extensively by gap junctions.
- Compaction creates a ball of cells known as the morula.
- The spaces between the cells of the morula merge to form one large central space (blastocoele or blastocyst cavity) converting the morula into a hollow sphere of cells, which is now called the blastocyst.
- The blastocyst comprises two cell groups with different fates and functions:
- The inner cell mass and the outer cell mass
Hatching (week 1):
- Blastocyst hatches from zona pellucida
- No longer constrained and now free to enlarge
- Can now interact with the uterine surface to implant
Implantation begins next?
- The inner cell mass becomes the embryo itself.
- At 8-9 days, the inner cell mass shows the first evidence of cell differentiation, where the blastocyst has formed two layers, the epiblast and the hypoblast.
- At this stage, this layer is known as a bilaminar embryonic disc. It now has dorsal (ectodermal) and ventral (endodermal) surfaces, but as yet no real head to tail orientation.
- Whilst the outer cells collectively become the trophoblast, which is the major component of the fetal membranes, especially the placenta, that surround and sustain the developing embryo and fetus, respectively, until birth.
o The trophoblastic placenta develops a special cell layer, the syncytiotrophoblast, which first adheres to, and then invades, the modified (decidualised) endometrium.
o This process is known as implantation. The preimplantation stages of pregnancy are the most hazardous for life, with up to 30% of zygotes failing to develop to the blastocyst stage.
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State the steps that occur before implantation begins
1) Fertilisation
2) Cleavage
3) Compaction
4) Hatching
5) Implantation begins
All these stages occur in week 1
Where does the zygote travel from and to before implantation? How long does this take?
- The zygote travels:
- From the ampulla end of the Fallopian tube to the junction with the uterus (called the isthmus). This takes about 4 days.
Describe the process of cleavage
- As the zygote travels through the Fallopian tube, the it undertakes successive cell divisions, a process known as cleavage.
- Cleavage (week one):
- Begins 30 hours after fertilisation
- Cell divisions occur without an increase in cell size and cells are now known as blastomeres
- First division occurs to create two totipotent blastomeres of equal size
- Cells become smaller with each cleavage division
- Cells keep dividing, all within zona pellucida
Describe the process of compaction
Compaction (week 1):
- After the third cleavage blastomeres maximize their contact with each other, forming a compact ball of cells held together by tight junctions.
- The process of compaction, segregates inner cells (embryoblast) from the outer cells (trophoblasts) and they communicate extensively by gap junctions.
- Compaction creates a ball of cells known as the morula (16+ cells)
- The spaces between the cells of the morula merge to form one large central space (blastocoele or blastocyst cavity) converting the morula into a hollow sphere of cells, which is now called the blastocyst.
- The blastocyst comprises two cell groups with different fates and functions:
- The inner cell mass and the outer cell mass
How do cells before compaction differ to cells after compaction?
Before compaction - Totipotent (the capacity to become ANY cell type)
After compaction - Pluripotent (the capacity to become one of MANY cell types - multi lineage potential)
What helps the zygote get transported from the Fallopian tube to the uterus?
- The zygote, is transported along the fallopian tube to the uterus, by the specialised cilia in the oviduct under the actions of progesterone.
Describe the process of Hatching
Hatching (week 1):
- Blastocyst hatches from zona pellucida
- No longer constrained and now free to enlarge
- Can now interact with the uterine surface to implant
Describe the meaning of the following term zygote
A fertilised ovum/oocyte
Describe the meaning of the following term cleavage
The division of cells in the early development of the embryo, following fertilisation
Describe the meaning of the following term zona pellucida
The thick glycoprotein shell surrounding a mammalian ovum(/zygote?) before implantation.
Describe the meaning of the following term morula
A solid mass of blastomeres (16+) resulting from a number of cleavages of a zygote
Describe the meaning of the following term ovary
A female reproductive organ in which eggs are produced, present in humans and other vertebrates as a pair
Describe the meaning of the following term Fallopian tube
Either of a pair of tubes along which eggs travel from the ovaries to the uterus.
Describe the meaning of the following term uterus
A hollow, pear-shaped organ located in a woman’s lower abdomen between the bladder and the rectum, where offspring are conceived and in which they gestate before birth (AKA the womb)
Discuss the formation of the blastocyst
- The spaces between the cells of the morula merge to form one large central space (blastocoele or blastocyst cavity) converting the morula into a hollow sphere of cells, which is now called the blastocyst.
- The blastocyst comprises two cell groups with different fates and functions:
- The inner cell mass and the outer cell mass
(Compaction is what leads to formation of the blastocyst)
What is the difference between pluripotent and totipotent cells?
A totipotent cell has the potential to divide until it creates an entire, complete organism. Pluripotent stem cells can divide into most, or all, cell types in an organism, but cannot develop into an entire organism on their own.
CHECK DISCUSSION BOARD
Describe the initial stages of implantation
Hatching (week 1):
- Blastocyst hatches from zona pellucida
- No longer constrained and now free to enlarge
- Can now interact with the uterine surface to implant
- The inner cell mass becomes the embryo itself.
What stages of pregnancy are the most hazardous for life and why?
The preimplantation stages of pregnancy are the most hazardous for life, with up to 30% of zygotes failing to develop to the blastocyst stage.
Define sites for ectopic pregnancy and placenta praevia and distinguish between these two implantation defects * Why does it require C section delivery?
Placenta praaevia
- Placenta previa is a problem during pregnancy when the placenta completely or partially covers the opening of the uterus (cervix):
- Implantation of embryo in lower uterine segment
- Can result in placenta positioning itself across the cervix
- During labour, this can cause haemmorage as the placenta is a vascular structure
- Requires C-section delivery because…
- High risk but are still viable
Ectopic Pregnancy
- Implantation of embryo at a site other than the uterine body (most commonly Fallopian tube)
- Uterus has a muscular wall which can expand as the foetus develops
- Fallopian tube and other non-uterine areas don’t have the ability to expand, which can lead to rupture of Fallopian tube and haemmorage
- Can be peritoneal (abdomen) or ovarian
- Can quickly become life-threatening emergency
- Usually not viable
Define the meaning of the term blastocytst
By the fifth or sixth day, the fertilized egg is known as a blastocyst — a rapidly dividing ball of cells
Describe the meaning of the following term trophoblast
A thin layer of cells that helps a developing embryo attach to the wall of the uterus, protects the embryo, and later forms the major part of the placenta.
Describe the meaning of the term inner cell mass
The inner cell mass (ICM) is the part of the blastocyst that is fated to become the embryo, amnion, and yolk sac.
describe the meaning of the term syncytiotrophoblast
The syncytiotrophoblast, the outermost layer of the human placenta, is the main site of exchange for drugs and metabolites, nutrients, waste products, and gases between the maternal and fetal circulations
Describe the meaning of the term cytotrophoblast
Cytotrophoblast is the name given to both the inner layer of the trophoblast (also called layer of Langhans) or the cells that live there.
Describe the meaning of the term implantation
Implantation is when a fertilized egg cell attaches to the wall of the uterus in early pregnancy.
Describe the formation of the embryonic disc in the inner cell mass and initial cell differentiation within it (and implantation)
- At 8-9 days, the inner cell mass shows the first evidence of cell differentiation, where the blastocyst has formed two layers, the epiblast and the hypoblast.
- At this stage, this layer is known as a bilaminar embryonic disc. It now has dorsal (ectodermal) and ventral (endodermal) surfaces, but as yet no real head to tail orientation.
- Whilst the outer cells collectively become the trophoblast, which is the major component of the fetal membranes, especially the placenta, that surround and sustain the developing embryo and fetus, respectively, until birth.
- The trophoblastic placenta develops a special cell layer, the syncytiotrophoblast, which first adheres to, and then invades, the modified (decidualised) endometrium.
- This process is known as implantation.
Define the term bilaminar disc
The bilaminar embryonic disc is formed when the inner cell mass forms two layers of cells, separated by an extracellular basement membrane. The external layer is called the epiblast and the internal layer is called the hypoblast. Together, they compose the bilaminar embryonic disc.
Define the term germ layer
A germ layer is a group of cells in an embryo. Those layers interact with each other as the embryo develops and will eventually give rise to all of the organism’s organs and tissues. Almost all animals initially form two or three germ layers.
Define the term ectoderm
The outermost of the three primary germ layers of an embryo that is the source of various tissues and structures (such as the epidermis, the nervous system, and the eyes and ears)
Define the term mesoderm
The mesoderm is a germ layer that arises during gastrulation, and is present between the ectoderm, which will turn into skin and central nervous system cells, and the endoderm, which will produce the gut and the lungs
Define the term primitive streak
- The primitive streak is a linear band of thickened epiblast that first appears at the caudal end of the embryo and grows cranially. At the cranial end its cells proliferate to form the primitive knot (primitive node).
- Appears on day 15 of human development, marks the start of gastrulation
Define the term gastrulation
Gastrulation is a crucial time in the development of multicellular animals. During gastrulation, several importance things are accomplished: The three primary germ layers are established. The basic body plan is established, including the physical construction of the rudimentary primary body axes.
Define the term trilaminar embryonic disc
A trilaminar embryonic disc is an early stage in the development of triploblastic organisms, which include humans and many other animals. It is an embryo which exists as three different germ layers - the ectoderm, the mesoderm and the endoderm.
List the important ectodermal, mesodermal and endodermal derivatives
Ectoderm - Organs and structures that maintain contact with the outside world:
- Nervous system
- Skin (epidermis)
Mesoderm - Supporting tissues:
- Muscle
- Cartilage
- Bone
- Vascular system (including heart and vessels)
Endoderm - Internal structures:
- Lining of GI tract
- Respiratory tract
- Parenchyma of glands
Describe the differentiation of mesoderm into the notochord, somites, intermediate mesoderm, and lateral plate.
Later development of the mesoderm creates an axial rod, the notochord, and a series of segments on either side of it, the somites.
The notochord precedes the vertebral column.
The somites form the vertebral column, ribs and extensor muscles of the
spine and muscles of the body wall.
Lateral to the somites there is a cord of mesoderm, the intermediate mesoderm that forms the kidneys. The most lateral part of the mesoderm is a sheet, the lateral plate. This splits into two layers, somatic and splanchnic, with a space, the coelom, between them.
The somatic layer forms the diaphragm and limb muscles
The splanchnic layer forms the muscle of the heart and viscera
The coelom becomes the body cavities.
Describe the formation of the intra-embryonic coelom
The intraembryonic coelom first appears as many small isolated coelomic spaces in the lateral mesoderm and cardiogenic mesoderm (between the 2 layers of the lateral plate mesoderm) which coalesce to form a horseshoe-shaped cavity, the intraembryonic coelom, which is lined by flattened epithelial (mesothelial) cells. It will become the pleuropericardial-peritoneal cavity.
List the derivatives of each region of the mesoderm
Notochord:
- Preceedes/comes before Vetebral column
Somites:
- Vertebral column
- Ribs
- Extensor muscles of the
spine
- Muscles of the body wall.
Intermediate mesoderm:
- Kidneys
Lateral plate:
This splits into two layers, somatic and splanchnic, with a space, the coelom, between them.
- The somatic layer forms the diaphragm and limb muscles
- The splanchnic layer forms the muscle of the heart and viscera
- The coelom becomes the body cavities.
Describe the formation and fate of the neural plate
At the same time that the mesoderm differentiates, changes affect the ectoderm. An oval thickening forms in the area overlying the notochord. It is the neural plate, the origin of the nervous system.
Its wide anterior end becomes the brain.
Its narrower posterior end the spinal cord.
Outline the development of the neural tube.
At the same time the endoderm becomes the lining of the primitive gut, the neural plate, on the dorsal aspect of the embryo, is folding into the neural tube. This process is known as neurulation. The embryo now possesses the cell types necessary for the differentiation of all specialised tissues of the adult, organised in a basic human body plan.
Describe the formation of the head fold, tail fold and lateral body folds.
Before day 14, the embryo is still disc shaped, with ectoderm dorsally, endoderm ventrally, and mesoderm sandwiched between them. The next stage converts it into a 3-dimensional, roughly cylindrical configuration, with ectoderm
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outside, endoderm inside, and mesoderm between. Head, tail and lateral body folds form, closing the embryo off from the yolk sac.
(watch lecture 4)
Describe the differentiation of somites into dermatome, myotome and sclerotome and the fate of each of these in later development.
The somites undergo further changes which underlie the pattern of outgrowth of spinal nerve roots from the spinal cord, the formation of the vertebral column and the innervation of muscles and skin.
Each somite separates into a dermatome, which gives rise to an area of skin, a myotome, which gives rise to a block of skeletal muscle (myotomes become the muscles of the thoracic and abdominal walls.) and a sclerotome, which gives rise to a vertebra and rib. Spinal nerve roots growing out from the neural tube enter and innervate the dermatomes and myotomes. The sclerotomes are rearranged to lie between the nerve roots and become the vertebrae.
Explain the use of the terms “dermatome” and “myotome” in describing the innervation of skin and muscles in the adult.
The terms ‘ dermatome’ and ‘ myotome’ are used clinically to define an area of skin or muscle supplied from a single spinal nerve root, and the spinal cord is described as having ‘ segments’, each of which gives rise to a nerve root.
