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Flashcards in Embryology Deck (62):

What is embryology?

The study of progression from a single cell to a baby in 9 months is called embryology. It includes molecular, cellular and structural factors


Where does fertilisation normally occur?

Ampullary region of the Fallopian tube


What is capacitation of a spermatozoa?

The tail of the sperm moves faster causing the plasma membrane to alter and allowing the sperm to become capable of fertilisation


What is the zone pellucida?

Thick transparent membrane surrounding an ovum before implantation


What is the corona radiata?

The outer protective layer of the ovum


What is the embryonic age?

Time since fertilisation


What is gestational age?

Time since last menstruation (embryonic + 2 weeks)


What is the germinal stage?

Fertilisation until the end of the second week


What is the embryonic period?

The time from the third to end of the eighth week


What is the foetal period?

The time from the beginning of the ninth week to birth at 38 weeks


What is the function of the zone pellucida?

Thought to prevent the morula/ blastocyst adhering to the oviduct and it stops the morula enlarging


What is hatching?

Before the blastocyst can implant it has to loose the zona pellucida. The process of losing the zona pellucida is called hatching


What is the trophoblast?

Layer of tissue within the zona pellucida which becomes the yolk sac and the placenta


What is histiotrophic nutrition?

nutrition provided to the embryo not from the
maternal blood. Important in humans up to the 12th week.


What is haemotrophic nutrition?

Nutrition provided by the mother’s blood. Important in humans from 12th week on.


Where does implantation usually occur?

Upper part of the body of the uterus, more often on the posterior wall


What is an ectopic pregnancy?

Implantation of the fertaliced oocyte in an abnormal. Site e.g. Fallopian tube,


What is placenta previa ?

Placenta lies unusually low within the uterus, next to the covering of the cervix. Can cause bleeding which can then lead to other complications


Why is the syncytiotrophoblast?

Cells that merge together in the trophoblast


What is the cytotrophoblast?

Cells within the trophoblast


When does the implantation of the blastocyst occur?

6-8 days


What is the cytotrophoblast?

The inner layer of cells in the trophoblast


What is the trophoblast?

Layer of tissue on the outside of a mammalian blastula, later forming major part of placenta


Where does the blastocyst burrow into the uterine wall?

Synytiotrophoblast (epithelial covering)


What does the hypoblast eventually form?

Forms the yolk sac


What does the epiblast eventually form?

Forms amniotic cavity


What is the yolk sac?

- Attaches outside developing embryo
- Connected by yolk stalk (to umbilical cord)
- Preliminary circulatory system
- Eventually gets absorbed into gut of embryo
- Sac lined by embryonic endoderm and mesoderm
- Purpose = nutrition during early stages, initial circulation and delivers nutrients via primitive aorta


What happens during the second week of development?

• The trophoblast differentiates into 2 layers - the cytotrophoblast and syncytiotrophoblast
• The embryoblast forms 2 layers - the epiblast and hypoblast - the bilaminar embryonic disc
• The hypoblast contributes to the formation of two cavities – the yolk sac and chorionic cavity


When is the embryonic period?

Weeks 3 to 8


What happens during the embryonic period?

– period of greatest change (lots of differentiation)
– all major structures and systems are formed
– greatest risk of major congenital malformation (i.e. teratogenesis) due to environmental exposure or drug therapy


What are the key events and processes in early development?

1. Fertilisation and Implantation
2. Gastrulation
3. Neurulation
4. Segmentation
5. Folding


What is the primitive streak?

Groove/ streak that cuts the epiblast disc bilaterally. There is a pit/node at the end away from the margin that contains the cells that coordinate the process of gastrulation.


Which structures lie either side of the bilaminar disc?

Amniotic cavity (epiblast side)
Yolk sac (hypoblast side)


When does the primitive streak appear?

Third week (day 16)
Dorsal surface of epiblast


Describe how the three germ layers are created?

Epiblast cells migrate to the edges of the primitive streak where they begin to push through intracellular boundaries of the bilaminar disc and invaginate under the epiblast layer, displacing the hypoblast layer, and giving rise to the three germ layers.


What does the hypoblast eventually differentiate into?

The yolk sac


What are the layers of the bilaminar disk?



What are the three layers of the trilaminar disk?



Why are there two regions of the trilaminar disk where there is no mesoderm?

The ectoderm and endoderm lie next to each other with no mesoderm. The membrane eventually ruptures in these regions to form the future mouth and future cavity.


What does the ectoderm eventually form?

Forms organs and structures that maintain contact with the outside world e.g. Nervous system and epidermis


What is the function of the mesoderm?

Differentiates into supporting tissues e.g. Muscle, cartilage, bone and vascular system (including heart and vessels)


What are the functions of the endoderm?

Forms the internal structures e.g. Epithelial lining of GI tract, respiratory tract and the parenchyma of glands


What is situs inversus?

- Complete mirror-image viscer
- Commonly results from immortal cilia
- No associate morbidity (usually )
- Problems if there is both normal and mirror image disposition


Why is there left-right asymmetry in the body despite the embryonic disk being bilaterally symmetrical?

Cilia in the node of the primitive streak beat in a characteristic way causing the leftward flow of signalling molecules. This initiates the signalling cascade for "left-sidedness". The absence of these signals leads to "right sidedness".


What is the notochord?

A solid rod of cells running in the midline of the embryonic disk that laterally bisects it.


Describe the role of the notochord in neurulation

The notochord releases signals that direct the conversion of overlying ectoderm to neurectoderm.


Why do the signals released by the notochord only effect certain tissues?

Diffusion limited & cells require form of receptor


How is the neural tube formed?

Notochord signals cause the overlying ectoderm to thicken to form a slipper shaped neural plate. The edges elevate out of the plane of the disk and curl towards each other creating the neural tube.


Describe the types of mesoderm

1. Paraxial mesoderm - flanks and forms simultaneously with the neural tube (blocks either side)
2. Intermediate mesoderm
3. Somatic and splanchnic mesoderm - separated by intraembryonic coelom (cleft opened laterally in plate mesoderm)


How do somites form?

- Organisation of parochial mesoderm into segments, first appearing at 20
- Appear as regular blocks of mesoderm cells arranged around small cavity
- Followed by "organised degeneration" (ventral wall of somites breaks down leading to the formation of the sclerotome)


What does the sclerotome give rise to?

- Hard tissue of the MSK system
- Dorsal portion organises to form the combined dermomyotome
- Myotome proliferates and migrates(muscle precursor)
- Dermatome disperses (dermis precursor)


What does the organisation of mesoderm into somites give rise to?

1. Repeating structures e.g. Vertebrae, ribs, intercostal muscle, spinal cord segments etc
2. Guides innervation


How many somites are there in the human body?

31 segments = 31 pairs of spinal nerves


Describe the position of the cardiogenic area and buccopharyngeal membrane in the trilaminar embryonic disc

Between margin of disk and the end of the primitive streak sits the cardiogenic area (will form heart)

Buccopharayngeal membrane sits roughly underneath the end of the primitive streak


What causes the folding of the embryonic disk?

1. Neural tube growth forces folding of head and tail = cephalocaudal folding
2. Somite growth drives lateral folding

Opening remains for umbilical cord


What are the consequences of the folding of the embryonic disk?

Inner membrane of endoderm. Outer layer of ectoderm. Creates a ventral body wall and pulls amniotic membrane outside around the disk (embryo becomes suspended), pulling connecting stalk ventrally.


What has occurred by the end of the forth week?

– The nervous system has started to form
– Segments have appeared, assigning specific tasks to specific cells
– The embryo has folded, putting everything in the right place


What is the embryonic period?

Week 3 - 8


Which period of development is the period of greatest change?

Embryonic period (all major structures are formed)


What is a teratogenic agent?

An agent or factor which causes malformation of an embryo e.g. Thalidomide, warfarin, rubella etc


What is the primitive streak?

Grooved structure that forms on day 15 of human development, along the caudal midline of the bilaminar embryonic disk and is the first visible sign of gastrulation.


In which week does the folding occur?

4th week