Principles of Embryology (Anatomy) Flashcards
Human development days 4-7
- A few hours after the sperm and egg fuse, the zygote undergoes cleavage, dividing into 2 new cells. ‘blastomeres’.
- Blastomeres keep on splitting to loose clumps of 4 cells, then 8 cells then a structured mulberry-shaped 16 cell cluster called Morula, with inner and outer cell masses. The morula cell is held in place by the Zona Pellucida.
- The morula eventually develops an outer cell mass of trophoblast cells and an interior cell mass of embryoblasts with a fluid-filled cavity core called the blastocoel. Ending the morula, becoming the blastocyst
- Embryoblast cells cluster together at one end of the blastocyst, in the area called the Embryonic Pole, while the trophoblasts flatten out to for the epithelial wall
- Fully formed, the blastocyst ‘hatches’ from the ZP ~ day 4, and is ready to attach onto the wall of the uterus
- Trophoblast cells aid in uterus wall attachment by having L-selectin molecules on their surface (proteins bound to carbohydrates) which binds to carbohydrate receptors on the uterin wall
- Once attached, trophoblasts proliferate into the wall, called syncytiotrophoblasts, also during implantation, the mucosal wall of the uterus are in a secretory phase, where the Corpus Luteum (leftover cells where the egg was released) releases large quantities of progesterone = uterin ateries becoming large & coiled = more blood in the endometrial layer (~ day 7)
Human development week 2
During Week 2, the blastocyst attaches to the wall of the uterus
The outer cells of the blastocyst, trophoblasts, penetrate the uterus wall, establishing a connection between the blastocyst and the mother.
The inner layer of cells are called embryoblast cells and they turn into a new, flat, two-layered structure which eventually gives rise to all the organs and tissues
Day 7 or 8
By day 7 or 8, the blastocyst implants on the surface of the endometrial wall or decidua. The area that it implants into is called the decidua basalis
To snuggle deeper into the decidua basalis, the trophoblast cells from the outer layer of the blastocyst assemble into two layers of cells:
- Cytotrophoblast = mononucleated cells,
- Syncytiotrophoblasts = multinucleated cluster of cells
The syncytiotrophoblasts expand into the decidua basalis
Day 9
Syncytiotrophoblast has pushed deeper into the decidua basalis
Day 11
Syncytiotrophoblasts almost completely buried within it
Day 12
Decidua undergoes the decidual reaction.
High Progesterone = dedicdual cells enlarged & coated in a sugar-rich, fatty fluid which can get be absorbed by the syncytiotrophoblast and helps sustain the embryo early on
Initially, the decidual reaction only occurs at the decidual basalis, but eventually it spreads throughout all of the decidua
Day 14
Cells of the syncytiotrophoblast start to protrude out to form little protrusions called primary villi. These primary villi ‘trees’ form all the way around the fetus, and cells start to clear out from between the primary villi, leaving behind empty spaces called lacunae.
Meanwhile, arteris and veins grow into the decidual basalis
As the placenta develops, tiny arteries merge with the lacunae, and the empty spaces become filled with oxygenated blood.
Veins also merge with lacunae and bring blood back to the mother’s heart
Over time, more and more of these little pools of blood develop and they start merging to form a single large pool of blood with many arteries and vein
This large pool is called the junctional zone, submerging the primary villi trees
Meanwhile, the inner embryoblast cells assemble into two layers forming a flat structure called the bilaminar embryonic disc
- Hypoblast is the ventral layer, consists of cells that start to line the fluid-filled cavity containing the embryoblast cells, the blastocoel, which becomes known as the yolk sac. The yolk sac is filled with vitelline fluid, which washes across the embryo, nourishing it
- Epiblast is the dorsal layer of the bilaminar disc, and gives rise to all 3 germ layers of the embryo (endoderm, mesoderm and ectoderm)
- Amniotic cavity develops just above the bilaminar disk, and it gets lined with epiblast
Meanwhile, in the embryoblast, epiblast cells from epiblast layer start to differentiate into extraembryonic mesoderm cells, named because they are outside of the developing of the developing embryo
Some of the earliest mesoderm cells, and they start to form even while the embryoblast itself is a bilaminar disc
These mesoderm cells line the inside of the cytotrophoblast and syncytiotrophoblast and begin creating space to form what will eventually become the chorionic cavity
Describe, pre-embryonic, embryonic and foetal periods of human development
Pre-Embryonic Period
(Conception to week 2)
Fertilisation (12-24 hrs post ovulation):
- Male spermatozoa and female oocyte = ovum.
- Fertilisation usually occurs in the ampulla of the follopian tube
- The oocyte completes its second meiotic division, and the two pronuclei fuse (1 from sperm and 1 from oocyte).
- This fusion forms a diploid Zygote
Cleavage (Day 1-3):
- zygote cells undergo mitotic division (cleavage) to form smaller cells called “blastomeres”.
- By day 3, it forms a solid ball of 12-16 cells known as the morula
Blastocyst formation (Day 4-5):
- Morula continues to differentiate, fluid-filled cavities appear called the blastocoele, creating a blastocyst by day 4-5. The blastocyst consist of:
- Trophoblast: Outer layer that forms the foetal component of the placenta
- Embryoblast: Inner cell mass at one pole of the blastocyst that gives rise to the embryo
- Blastocoel: A fluid-filled cavity within the blastocyst
- By day 5, the blastocyst prepares for implantation and sheds its zona pellucida to allow direct contact with the uterine lining
Implantation (day 6-7):
- By day 6-7, the blastocyst attaches to the endometrium (lining of the uterus), typically near the fundus
- Cells from the outer trophoblast layer begin to differentiate into two types:
- Syncytiotrophoblast: invades the uterine lining to facilitate implantation and later forms part of the placenta
- Cytotrophoblast: These cells remain closer to the inner cell mass and help maintain the structural integrity of the blastocyst
marking the end of the pre-embryonic stage
Disc formation and germ layers:
- Implanation of blastocyst/embryo (day 6-13)
- invasion of uterine epithelium and endometrium
- Differentiation of trophoblast: Syncytiotrophoblasts (maternal side) and cytotrophoblast (embryo side)
- inner cell mass differentiation
Pre-embryonic
Week 2
- Fertilisation, formation of morula and blastocyst, blastocyst implantation
- Embryoblast differentation: Bilaminar disc (day 8-10)
- Inner cell mass: Epiblast and hypoblast
Epiblast = amnionic cavity (maternal side)
Hypoblast = exocoelomic membrane = primary yolk sac (nutrition)
Extra-embryonic coelom clefts form and merge = chorionic cavity, connecting stalk develops into the umbilical cord (disappears and merge with amnioic cavity) (Day 12-13)
Embryonic period
Week 2 - week 8
Disc formation and germ layers
Gastrulation: Trilaminar disc (week 3) (day 15-16)
- Epiblast – ectoderm of amnion and embryo
- Ectoderm cells migrate in response to the formation of the primitive streak – embryonic mesoderm, embryonic endoderm
- Hypoblast – endoderm of umbilical vesicle
- Germ layer development, placenta development, body system develop from trilaminar germ layers
- Ectoderm: Epidermis, viscerocranium and nervous system (including eyes and inner ear)
- Primitive streak (day 15) - midline depression in caudal end of bilaminar disc
Primitive node (day 16) - cephalic end of primitive streak
Notochordal plate/notochord (day 17-21): cranial migration of cells from primitive node, forms longitudinal axis of embryo
- Neurulation (day 18-25) = neural tube formation
neural tube : 3 primary vesicles and 5 secondary vesicles
Mesoderm: skeletal muscle, skeleton connective tissue, urogenital system, serous membranes
Differentiation of mesoderm (day 17):
- paraxial mesoderm
- intermediate mesoderm
- lateral plate mesoderm
paraxial mesoderm (): increase number of cells immediately lateral to neuro tube. Skeletal muscle, skeleton, dermis, connective tissue
- somites (day 20) 42-44 pairs; Dermatomyotomes & sclerotomes (connective tissue and skeleton)
Intermediate mesoderm:
Urogential system
lateral plate mesoderm: most lateral cell group from the neural tube
Serous membranes: somatic parietal layer = parietal membrane
- splanchnic/visceral layer = visceral membrane
smooth muscle
stroma
Endoderm:
- Epithelium of alimentary tract, epithelium and glands of respiratory tract
- parenchymal cells (liver, pancreas, thyroid gland, parathyroid glands)
- mucosa of bladder
Foetal periods
week 8 - birth (38 weeks)
- Growth and visceral development, MSK is functional
Describe the pre-embryonic periods of human development
1) Fertilisation (Day 0)
- Male spermatozoon penetrates the female oocyte, forming a zygote, a single diploid cell containing 46 chromosomes
- The oocyte completes its second meiotic division, and the two pronuclei fuse (1 from sperm and 1 from oocyte).
- This fusion forms a diploid Zygote
2) Cleavage (Day 1-3)
Describe the development of a morula and then blastocyst.
Describe the development of the bilaminar and trilaminar germ disc
Describe the basics of the folding of the embryo
Week 4:
Lateral folding (day 20) = somite development (paraxial mesoderm)
Longitudinal: day 21-24
- vesicles of brain development
