S9-10) Embryology Flashcards
What is embryology?
Embryology is the study of the molecular, cellular and structural factors involved in the progression from a single cell to a baby in 9 months
Describe the process of oogenesis
Where does fertilisation normally occur?
The ampullary region of the fallopian tube
Sperm must be introduced into the female reproductive tract within 5 days before ovulation.
How long are the sperm and secondary oocyte viable?
- Sperm are viable for 5 days
- Secondary oocyte is viable for 12-24 hours
Why is there a need for capacitation?
- Sperm are initially incapable of fertilisation
- Capacitation allows the tail of the sperm to move faster, causing the oocytes plasma membrane to alter
Describe the process of fertilisation
Define the following terms:
- Embryonic age
- Gestational age
- Embryonic age: time since fertilisation
- Gestational age: time since last menstruation (embryonic age plus 2 weeks)
Define the following terms:
- Germinal stage
- Embryonic period
- Foetal period
- Germinal stage: time from fertilisation to the end of the 2nd week
- Embryonic period: time from the 3rd to end of the 8th week
- Foetal period: time from the beginning of the 9th week to birth at 38 weeks
Before the blastocyst can implant it has to lose the zona pellucida.
What is this process called and why does it happen?
Hatching is the process in which the blastocyst emerges from its zona pellucida as the zona pellucida prevents the morula/blastocyst adhering to the oviduct and enlarging
Distinguish between histiotrophic nutrition and haemotrophic nutrition
- Histiotrophic nutrition is the nutrition provided to the embryo which is not from the maternal blood and is important in humans up to the 12th week
- Haemotrophic nutrition is the nutrition provided by the mother’s blood and is important in humans from 12th week on
Where and when does the blastocyst implant?
Why is this important?
- Normal implantation of the blastocyst occurs in the posterosuperior wall of the uterine cavity on the 6th day
- The implantation site determines the site of formation of the placenta
Identify 7 abnormal implantation sites
Differentiate between the following terms:
- Trophoblast
- Synctiotrophoblast
- Cytotrophoblast
- Trophoblast is a layer of tissue on the outside of a mammalian blastula, supplying the embryo with nourishment
- Syncytiotrophoblast are cells that merge together in the trophoblast
- Cytotrophoblast are cells in the trophoblast
What is the bilaminar disc?
- The bilaminar embryonic disc refers to the epiblast and the hypoblast, evolved from the embryoblast
- These two layers are sandwiched between the primitive yolk sac and the amniotic cavity
Differentiate between the following terms:
- Hypoblast
- Epiblast
- Hypoblast is a tissue type, lying beneath the epiblast, that forms from the inner cell mass and consists of small cuboidal cells
- Epiblast is the outermost layer of an embryo before it differentiates into ectoderm and mesoderm
Differentiate between the follow structures:
- Amniotic cavity
- Yolk sac
- Chorionic cavity
- Amniotic cavity is the closed sac between the embryo and the amnion, containing the amniotic fluid
- Yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast adjacent to the embryonic disk
- Chorionic cavity is the space surrounding the primary yolk sac and amniotic sac (not around connecting stalk)
Why is the second week of development called the week of 2’s?
- The trophoblast differentiates into 2 layers (cytotrophoblast & syncytiotrophoblast)
- The embryoblast forms 2 layers (epiblast & hypoblast)
- The hypoblast contributes to the formation of two cavities (yolk sac & chorionic cavity)
Identify the 6 key events/processes in early development
- Fertilisation
- Implantation
- Gastrulation
- Neurulation
- Segmentation
- Folding
Gastrulation happens in the 3rd week.
What occurs during gastrulation?
During gastrulation, the bilaminar disk is converted to a trilaminar disk consisting of three germ layers: ecotoderm, mesoderm and endoderm and the primitive streak forms in the caudal epiblast
What is the importance of the primitive streak?
- The primitive streak forms in the caudal epiblast, leading to migration and invagination of epiblast cells
- This process ensures the correct placement of precursor tissues to allow sub sequent morphogenesis to occur
What is the germ layer derivative of the ectoderm?
Ectoderm – organs and structures that maintain contact with the outside world e.g. nervous system, epidermis
What is the germ layer derivative of the mesoderm?
Mesoderm – supporting tissues e.g. muscle, cartilage, bone, vascular system
What is the germ layer derivative of the endoderm?
Endoderm – internal structures e.g. epithelium of GI tract, respiratory tract and parenchyma of glands
Discuss the left-right asymmetry that arises from gastrulation
- Prior to gastrulation the embryonic disk is bilaterally symmetrical
- However, in the body there are clear left vs right differences:
I. Thoracic viscera – left lung has 2 lobes while the right has 3
II. Abdominal viscera – liver on right, stomach and spleen on the left
What is neurulation?
Neurulation refers to the folding process in vertebrate embryos, which includes the thickening of the ectoderm to form the neural plate and its transformation into the neural tube (neurectoderm)
What is the role of the notochord?
- The notochord is formed during gastrulation and drives neurulation
- It has no function in adult life and forms the nucleus pulposus of the intervertebral disc
Somites appear as regular block of mesoderm cells arranged around a small cavity.
Explain the degeneration of somites into the myotome and dermatome
- The ventral wall of the somites break down and the sclerotome forms
- Further organisation of the dorsal portion forms the dermomyotome
- Myotome proliferates and migrates & dermatome disperses
Differentiate between the dermatome, myotome and sclerotome
- Dermatome – skin section i.e. dermis
- Myotome – muscle section i.e. muscles
- Sclerotome – hard tissue section i.e. bones
What are the implications of segmentation?
- Organisation of mesoderm into somites gives rise to repeating structures e.g. vertebrae, ribs, spinal cord segments
- Guides innervation
What does embryonic folding do?
- Folding draws together the margins of the disk
- It creates a ventral body wall
- It pulls the amniotic membrane around the disk
- It pulls the connecting stalk ventrally
Briefly described how the intraembryonic coelem is formed
- The intraembryonic coelom is the cavity between the somatopleure (dorsal) and the splanchnopleure (ventral) which forms during the 3rd week of development from the splitting of the lateral mesoderm
- This space will give rise to the thoracic and abdominal cavities
Why do the buccopharyngeal membrane and the cloacal membrane have no mesoderm?
- The buccopharyngeal and cloacal membrane have no mesoderm as these membranes form the upper and lower parts of the GI tract
- The mesoderm derives into muscle, cartilage and bone; which are not part of the GI tract, hence not necessary
Saccrococcgeal teratomas are common in newborns. The developmental basis for these tumours is the excessive persistence of the primitive streak.
Why are these tumours composed of many tissue types?
- The primitive streak gives rise to the mesoderm which is a derivative of supporting tissues (muscle, cartilage & bone)
- Hence, saccrococcgeal tumours would consist of precursor cells of the mesoderm
What are embryonic stem cells?
Embryonic stem cells are pluripotent stem cells derived from the inner cell mass of a blastocyst
What can umbilical cord stem cells be used for?
Umbilical cord blood collected at birth is a rich source of stem cells that can be used in research and in the clinic to treat diseases of the blood and immune system
Where can adult stem cells be found?
Adult stem cells have been identified in many organs and tissues, including brain, bone marrow, blood, liver, gut, heart, skin and teeth
What is placenta praevia?
Placenta previa is a condition wherein the placenta covers all or part of the cervix during the last months of pregnancy and can cause severe bleeding before/during labour (emergency C-section may be necessary)
Why is it that ectopic pregnancy can quickly become a life-threatening emergency?
- The narrow fallopian tube cannot distended largely and with the progression of pregnancy, the fallopian tube ruptures
- This can cause heavy internal bleeding and abdominal pain
Identify 5 of commonest causes for maternal mortality globally
- Post-partum bleeding
- Complications from unsafe abortion
- Hypertensive disorders of pregnancy
- Postpartum infection
- Obstructed labour
Identify 5 of the commonest causes for maternal mortality in the UK
- Thrombosis and thromboembolism
- Antepartum and postpartum haemorrhage
- Amniotic fluid embolism
- Genital tract sepsis
- Ectopic pregnancy
