Embryology Flashcards
How many chromosomes do primary spermatocytes contain?
46
How many chromosomes do secondary spermatocytes contain?
23
What are the 4 daughter cells produced from meiosis II called in spermatogenesis?
Spermatids
What is a spermatid?
An immature male sex cell formed from a spermatocyte, which may develop into a spermatozoon without further division.
What is an oogonium?
An immature female reproductive cell that gives rise to primary oocytes by mitosis.
During fetal development, when is oogenesis paused?
Prophase I of meiosis I
True or False:
Meiosis II is not completed until fertilisation.
True
Where does fertilisation usually occur?
Ampullary region of the fallopian tube
How long are sperm viable for?
Five days
How long is the secondary oocyte viable for?
12-24 hours
What does sperm require to be capable of fertilisation?
Capacitation, meaning the tail of the sperm moves faster, causing the plasma membrane to alter
What is embryonic age?
Time since fertilisation
What is gestational age?
Time since last menstruation
Embryonic age - 2 weeks
What is the germinal stage?
Time from fertilisation to the end of the second week
What is the embryonic period?
Time from the 3rd week to the end of the 8th week
What is the foetal period?
Time from the 9th week to birth at 38 weeks
At what day does implantation usually occur?
Day 6
What does the blastocyst have to lose before it can implant?
Zona pellucida
What is the process of losing the zona pellucida called?
Hatching
What is the zona pellucida thought to prevent?
The morula/blastocyst adhering to the oviduct
What is a morula?
A solid ball of cells resulting from division of a fertilized ovum, and from which a blastula is formed
What does the trophoblast become?
The yolk sac and placenta
What is the trophoblast?
A layer of tissue on the outside of a blastula, supplying the embryo with nourishment and later forming the major part of the placenta.
What is the normal implantation site?
Superior and posterior wall of the uterine cavity
Why is the 2nd week of development known as the weeks of 2s?
- The trophoblast differentiates into 2 layers (cytotrophoblast and syncytiotrophoblast)
- The embryoblast forms 2 layers (the epiblast and hypoblast - the bilaminar embryonic disc)
- The hypoblast contributes to the formation of 2 cavities (the yolk sac and chorionic cavity)
What is the period from weeks 3 to 8 called?
Embryonic period
What are some examples of things that can lead to major congenital malformation during the embryonic period?
Thalidomide, isotretinoin, warfarin, rubella
What are the key events and processes in early development?
Fertilisation, implantation, gastrulation, neurulation, segmentation, folding
What is the epiblast at the end of the second week?
A uniform disk
What marks the start of gastrulation?
Primitive streak
When does the primitive streak form?
Start of the 3rd week
What is gastrulation?
Gastrulation is a phase early in the embryonic development during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula
What are the three germ layers?
Ectoderm, mesoderm, endoderm
What does the ectoderm give rise to?
Organs and structures that maintain contact with the outside world
eg nervous system, epidermis
What does the mesoderm give rise to?
Supporting tissues
eg muscle, cartilage, bone, heart, vascular system
What does the endoderm give rise to?
Internal structures
eg epithelial lining of GI tract, respiratory tract, parenchyma of glands
When does gastrulation begin?
Third week
How does gastrulation begin?
Primitive streak forms in the caudal epiblast, leading to migration and invagination of epiblat cells
Why does gastrulation occur?
To ensure the correct placement or precursor tissues to allow subsequent morphogenesis to take place
What do notochord signals cause?
Overlying ectoderm to thicken
What is neurulation?
Neurulation refers to the folding process in vertebrate embryos, which includes the transformation of the neural plate into the neural tube.
What does organisation of mesoderm into somites give rise to?
Vertebrae, ribs, intercostal muscles, spinal cord segments
What does folding achieve?
Draws together the margin of the disk
- creating a ventral body wall
- pulling amniotic membrane around disk (embryo becomes suspended within the amniotic sac)
- pulling connecting stalk ventrally
What has happened by the end of the 4th week?
Nervous system has started to form, segments have appeared so assign specific tasks to specific cells, the embryo has folded putting everything in the right place
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 the 12th week on
What is placenta previa?
If you have placenta previa, it means that your placenta is lying unusually low in your uterus, next to or covering your cervix.
What is the syncytiotrophoblast?
Cells that merge together in the trophoblast
What is cleavage?
Cleavage is the division of cells in the early embryo with no significant growth (stay within zona pellucida)
After the zygote has undergone many cell divisions (cleavage) what is it now called?
Morula
What is the process of compaction?
Within a few days after fertilization, cells on the outer part of the morula become bound tightly together with the formation of desmosomes and gap junctions, becoming nearly indistinguishable
In the morula, what are the two distinct cell types?
Trophoblasts (outer layer)
Embryoblasts (inner cell mass)
What is the cavity in the blastocyst called?
Blastocoel
What does a morula become when the inner cell mass compacts further and the blastocoel forms?
Blastocyst
Define blastulation
Blastulation is the process following the morula and precedes the gastrulation. It is marked by the presence of a blastocoel.
When does hatching (loss of zona pellucida) occur?
Around 5 days after fertilisation
What replaces the zona pellucida?
Underlying trophoblast layer
Aside from the blastocoel, what other cavity is developed in the blastocyst?
(Hint - its within the inner cell mass)
Amniotic cavity
In the blastocyst, what two layers arise from the inner cell mass?
Hypoblasts and epiblasts
What are the layers of hypoblasts and epiblasts collectively known as?
Bilaminar disc
True or False:
The gastrula is bilaminar
False
It is trilaminar (3 layers)
What is the next step after gastrulation?
Neurulation
Where does the notochord form?
Mesoderm of the trilaminar disc
The formation of the notochord induces a change in another layer of the trilaminar disc. Which layer is this and what is the change?
The ectoderm - thickening, forming neural plate
What does the neural plate go on to form?
Neural tube
What are neural crest cells?
Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm cell layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia
What is situs inversus?
A congenital condition in which the major visceral organs are reversed or mirrored from their normal positions.
What does situs inversus commonly result from?
Immotile cilia
When does the first pair of somites appear?
Day 20 in the occipital regioin
How many pairs of somites appear a day?
3
How many pairs of somites are present by end of week 5?
42 to 44
As some somites disappear, how many pairs of somites are left in total?
31
How do somites appear?
As regular block of mesoderm cells arranged around a small cavity
What is a sclerotome?
The part of each somite in a vertebrate embryo giving rise to bone or other skeletal tissue.
What are the somite derivatives?
Dermatome, myotome and sclerotome
What is the future of the buccopharyngeal membrane?
Mouth
