Beginning of Human Development Weeks 1-3 Flashcards
Beginning of human development overview
Human development is a continuous process that begins when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male. Cell division, cell migration, apoptosis, differentiation, growth and cell rearrangement transform the fertilized oocyte into a highly specialized, Totipotent cell called a zygote into a multicellular human
Human embryology
The study of prenatal development
Gestation period in humans is
280 days from onset of LMP (overestimated by 2 weeks) or
266 days from fertilization
Pre-natal development is divided into three periods:
- Pre-embryonic period- fertilization to 2 weeks
- Embryonic period- 3-8 weeks
- Fetal period-9th week to birth
Fertilization results in these 5 processes
1.oocyte completing meiosis 2
2. Restoration of diploid chromosome number in zygote
3. Species variation
4. Determination of genetic sex
5 initiation of cleavage (mitosis) of zygote
Ootid
Secondary oocyte with 2 pronuclei- one from each parent
Gametogenesis
The process of forming and developing specialized generative cells called gametes
Gamete maturation in males is called spermiogenesis and in females is called oogenesis
Cell cycle
G0- cell cycle arrest G1- cellular contents duplicate S-Each of the 46 chromosomes duplicate G2- Cell double checks for duplication errors, and makes any repairs Mitosis Cytokinesis G0/ G1
Outside of mitosis and cytokinesis is interphase
Mitosis vs meiosis
Mitosis takes one diploid parent cell and creates two diploid identical daughter cells
Meiosis takes one diploid parent cell and creates 4 haploid daughter cells
Ad spermatogonia divide in which way
Asymmetrically
Can produce an identical Ad spermatogonia or a Ap spermatogonia (less potency)
Differences of spermatogeneis and oogeneis
- One diploid primary spermatocyte produces 4 haploid sperm. Whereas, only one mature oocyte results from maturation of a primary oocyte
- There is a follicle around oocyte
- Second meiosis completed after fertilization in females and before spermiogenesis in males.
- Sperm can be X or Y, oocyte is only X
- Polar bodies develop and degrade in females
- Tests vs ovaries
- Spermatogonium in soermatogeneis but no Oogonium in oogenesis
- Sperm are much smaller than eggs
Why no oogonium in oogenesis?
Oogonium develop into primary oocytes before birth (prenatal maturation). No more oocytes develop after birth. At birth you have 2M, which depletes to 40,000 by puberty, and only 400 become secondary oocytes for ovulation.
Gonium
Germ cell
High potency
Spermiogenesis phases
Rounded spermatid becomes an elongated sperm
- formation of the acrosome derived from the golgi region, contains enzymes that are released at the beginning of fertilization to assist in breaking down corona radiata and ZP.
- nucleus capped with lyric enzymes
- Mitochondria concentrate and arrange themselves end to end in form of a tight helix, forming a collar-like mitochondrial sheath.
- tail elongates
- centrioles on opposite side of golgi
- microtubules form to create cytoskeleton structure
- residual cytoplasm is shed
Parts of sperm
Principal piece of tail
End piece of tail
Middle piece of tail (mitochondrial sheath)
Stages of oogenesis
- Primary oocyte suspended in prophase and remain dormant until puberty (vulnerable to environmental agents)
- Follicle matures and shortly before ovulation the primary oocyte completes meiosis 1, to become secondary oocyte and first polar body. (Division of cytoplasm is uneven and secondary oocyte receives almost all and polar body none)
- at ovulation the secondary oocyte begins second meiosis but is arrested in metaphase 2
- if sperm penetrate it completes meiosis 2 and the cytoplasm is uneven and second polar body is made.
FSH and LH produce_____
Cyclic changes in the ovaries-the ovarian cycle that includes growth of follicles, ovulation and the corpus luteum formation
Development of follicles is characterized by
- growth and differentiation of primary oocyte
- proliferation of follicular cells
- formation of Zona Pellucida
- Development of the Theca Folliculi
Ovulation characterization
- Triggered by the surge of LH production
- LH surge is elicited by increased levels of Estrogen
- Formation of the stigma
HPO AXIS
Hypothalamus releases Gonadotropin-releasing hormone which causes the pituitary gland to release FSH and LH which causes the ovaries to mature follicles and develop corpus luteum
How do you know if an oocyte is a secondary oocyte?
Antrum is present
Theca folliculi
Produces??
Connective tissue surrounding a follicle that produces estrogen
Corpus luteum forms from…
During LH surge there is so much pressure build up the follicle ruptures and releases the secondary oocyte. The left over tissue folds and creates the corpus luteum which produces progesterone and estrogen
The inner layer of the uterus_____ goes through _____
Endometrium
Monthly changes (based off hormone levels)
What directs the secondary oocyte to enter the Fallopian tube?
Fimbriae- finger like projections on the infundibulum of the Fallopian tube
Cellular signalling in endometrial cells
Estrogen moves through the cell membrane into the cell to bind with an estrogen receptor. It phosphorylizes and dimerizes to another estrogen receptor and translocates to nucleus. The estrogen complex acts as a transcription factor to cause mitotic genes to grow rapidly to prepare for egg attachment
End of luteal phase sees crazy…
Proliferation with progesterone and progesterone receptors causing mitotic gene expression
When the corpus luteum degenerates (follicle is not fertilized) and stops releasing Progesterone and Estrogen it causes
Endometrium to shed= menstrual cycle
Iechemic phase
Cells can’t live with lack of progesterone and estrogen so they are sloughed and shed
As soon as a sperm contacts plasma membrane of a secondary oocyte it causes….
Zona Reaction
Protective layer forms so secondary oocytes are impermeable to any other sperm.
-maintains chromosome number
Phases of fertilization
- passage of sperm through corona radiata (hyalurinidase)
- penetration of ZP (acrosin)
- fusion of plasma membrane of the oocyte and sperm
- completion of second meiotic division of oocyte and formation of male pro nucleus
- oocyte containing two haploid pronuclei is called ootid
- fusion of pronuclei leads to the formation of zygote from ootid
Cleavage of Zygote
-time and phases
- cleavage begins about 30 hours after fertilization
- zygote becomes 2 identical blastomeres
- cleavage consists of repeated mitotic divisions of the zygote, resulting in a rapid increase in the number of cells. These embryonic cells(blastomeres) become smaller with each successive cleavage division and forms the morula (solid hall of 12-32 blastomeres) by day 3.
- by day 4, morula crimes a hollow ball called the blastocyst (now in uterus)
Formation of the blastocyst
- shortly after morula enters uterus (4days), a fluid filled space called the blastocystic cavity appears inside the morula. The fluid passes through the uterine cavity into the zona pellucida to form a space. As fluid increases in the bladtocystic cavity, it separates the blastomeres into two parts:
- trophoblast
- inner cell mass (embryo blast)
Trophoblast
Gives rise to?
A thin, outer layer which gives rise to the embryonic part of the placenta
Inner cell mass or embryo blast
A group of centrally located blastomeres, the inner call mass, which gives rise to the embryo; because it is the primordium of the embryo.
Hatching
When the ZP of the blastocyst begins degenerating.
-disappears by day 5
Is there increase in size of a developing embryo prior to ZP degrading?
No, blastomeres become smaller and compacted
Blastogenesis
- The conceptus of a blastocyst (day 4, morula becomes hollow ball)
- embryoblast now projects into the blastocystic cavity and the trophoblast forms the wall of the blastocyst.
- ZP degrades in uterine fluid after 2 days
- At day 6 since fertilization (day 20-28 of cycle), the blastocyst attached to the endometrial epithelium, usually adjacent to the embryonic pole.
- as soon as it attaches, the trophoblast starts to proliferate and differentiate into two layers.
What two layers does the trophoblast differentiate into?
Inner layer- cytotrophoblast
Outer layer- syncytiotrophoblast
First week development short overview of processes
- cleave begins about 30 hours after fertilization
- zygote becomes 2 identical blastomeres
- morula is a solid ball of 12-32 blastomeres (3 days)
- day 4, morula becomes a hollow ball called the blastocyst (now in uterus)
- blastomeres separate into trophoblast and embryoblast
- blastocyst has a fluid filled cavity called blastocyst cavity
- blastocyst begins to implant in the wall of the uterus by end of first week.
- trophoblast proliferated and differentiated into cytotrophoblast and syncyntiotrophoblast
What is the nuclear makeup of the two trophoblastic layers?
Cytotrophoblast is mono nuclear
Syncytiotrophoblast is multinucleated
ESC
Embryonic stem cells or Embryoblast cells
-can make all organs and tissues but are pluripotent because they can’t make extra embryonic structures like the placenta.
Syncytiotrophoblast _______ into epithelium of uterus
Invaginate
Highly regulated process unless cancer and parasites and stops at a point
Second week of development events
- implantation complete
- bilaminar embryonic disc forms
- 2 layers of trophoblast continue to develop
- extra embryonic structures from (amniotic cavity, amnion, umbilical vesicle, connecting stalk and chorionic sac)
Development of the bilaminar embryonic disc
- As implantation progresses, a small space appears in the embryoblast. This space is the primordium of the amniotic cavity. Morphological changes occur in the embryoblast the result in the formation of a flat, almost circular bilaminar plate of cells called the embryonic disc. The disk has two layers:
- Epiblast
- Hypoblast
Epiblast
The thicker layer, consisting of high columnar cells related to the amniotic cavity.
-amnion cells derived from it
Hypoblast
In the embryonic disc, the smaller cuboidal cells adjacent to the exocoelomic cavity (Primary Yolk Sac)
Extracellular mesoderm is derived from it
Why are cavities formed?
So cells have room to differentiate and migrate
Is the extra embryonic mesoderm part of the germ layers?
NO
extraembryonic means outside of the embryo
Amniotic cavity
Cavity between the inner cell mass and the cytotrophoblast
The blastocyst cavity becomes the
Umbilical vesicle (or yolk sac) or (primary then Smaller secondary)
The extraembryonic mesoderm splits into what?
- Extraembryonic somatic mesoderm:
- Extraembryonic splanchnic mesoderm
Spaces fuse and then Split creating chronionic sac cavity
Extraembryonic somatic mesoderm
Lines the trophoblast and covering the amnion
Extraembryonic splanchnic mesoderm
Surrounds the umbilical vesicle
The end of the second week is characterized by the appearance of _______
Primary chronionic villi
Proliferation of cytotrophoblastic cells produce cellular extensions that grow into the syncytiotrophoblast
What forms the chorion
The extra embryonic somatic mesoderm and the two layers of trophoblast
Chorion
Forms the wall of the chorionic sac, within which the embryo and it’s amniotic sac and umbilical vesicle are suspended by the connecting stalk
The extraembryonic coelom is now called the…… in the second week
Chorionic cavity
Where does Extraembryonic mesoderm develop
Between the trophoblast and the amnion and umbilical vesicle
Spaces appear within the extraembryonic mesoderm and form a cavity called the extraembryonic coelom which will become the
Chorionic cavity
The extraembryonic mesoderm is split into 2 layers:
- Extraembryonic somatic mesoderm
- which along with the 2 layers of trophoblast will become the chorion - Extraembryonic splanchnic mesoderm
- which covers the umbilical vesicle (yolk sac)
Connecting stalk is made up of
Extraembryonic mesoderm
The second week of development is called the week of 2’s why?
Trophoblast: syncytiotrophoblast and cytotrophoblast
Extraembryonic mesoderm: splanchnic and somatic
Embryonic disc: Epiblast and Hypoblast
Primary chorionic villi develop with a core of _____ and covering of ______
Cytotrophoblast
Syncytiotrophoblast
Primary chorionic villi are the primordia of:
Lacunar networks are the primordia of
Chorionic villi
Intervillous spaces
Prechordal plate
Flat, bilaminar embryonic disc develops a thickening at one end of Hypoblast that indicates future site of mouth
What prevents the decline of the endometrium
Blastocyst secreted human chorionic gonadotropin (hCG) to stimulate the corpus luteum to continue to produce estradiol and progesterone
-about 20 weeks in the placenta takes over hormone production
Clinical significance chorionic (gestational) sac diameter
Using trans vaginal ultrasonography, the measurement of the chorionic sac diameter can help determine the age of the embryo, as well as early development and pregnancy outcomes
How can implantation of a blastocyst be detected?
- ultrasonography
- highly sensitive radioimmunoassays of hCG as early as the second week (pregnancy blood test)
2 ways implantation goes wrong
- implantation if the lower segment of the uterus - placenta previa
- implantation sites other than the uterus (especially in the uterine tube)- ectopic pregnancy
When do blood filled lucanae appear in the syncytiotrophoblast
Day 9
At day 10, the blastocyst sinks be earth the endometrial epithelium and the _____ is filled by ____
Repaired?
Defect
A Closing plug
Defect Repaired day 13-14
Can be confused for menses
Lacunar networks form by ____ of adjacent lacunae (day )
Fusion
10-11
Uteroplacental circulation
The syncytiotrophoblast erodes endometrial blood vessels, allowing maternal blood to seep in and out of lacunar networks
Day 11-12
More nutrients by blood network than simple diffusion
Prechordal plate develops as a ______
Localized thickening of the Hypoblast, which indicates the future cranial region of the embryo and the future site of the mouth
Primary chorionic villi stages
Primary- contain syncytiotrophoblast and cytiotrophoblast
Secondary- contain above and extraembryonic somatic mesoderm
Tertiary-lined with blood vessels
Third week of development is most noted for
Formation of germ layers and early tissue and organ differentiation
- beginning of the embryonic period of development
- coincides with the first missed menstrual period (5th week)
- formation of primary streak which gives rise to 3 germ layers
- notochord, neural tube and somites
Gastrulation
The process by which the bilaminar embryonic disc is conveyed to a trilaminar embryonic disc
- occurs via the formation of the primitive streak
- develops on dorsal aspect, caudally along midline
- includes primitive streak, groove, bode and pit
-embryo is called the Gastrula
What creates the primitive groove
-what signalling pathway allows this movement
Differentiation and proliferation of cells on caudal end move to midline and create a groove by pushing against each other.
-E-Cadherin is a cell adhesion molecule that is down regulated by Fibroblast Growth Factor (FGF-8) produced by Eli last cells. This allows cells to be less attached and move
Cells of epiblast origin migrate down the primitive groove and displace Hypoblast cells to establish the _________ and _________ then the Epiblast is renamed the
Intraembryonic mesoderm
Endoderm
Ectoderm
Ectoderm is the germ layer that creates
Skin, CNS, PNS, eyes, internal ears, neural crest cells (bones and connective tissue if the face and part of skull)
Mesoderm is the germ layer that creates
Bones, connective tissue, Urogenital system and cardiovascular system
Endoderm is the germ layer that creates
Gut and gut derivatives (liver, pancreas, lungs), epithelial linings, respiratory and digestive tracts
When does the primitive steaks regress
End of the 4th week (Sacroccygeal teratoma)
Notochordal process grows cranially from the ______ to the ____
Primitive pit
Prechordal plate
Notochord serves as
- an axis for the development of the axial skeleton
- induced the formation of the bodies of the vertebrae
- part persists as the nucleus purposes of the intervertebral discs
- induced the overlying ectoderm to form the neural plate
Neurulation
- neural tube formation begins in the 3rd week and is complete by end of 4th week
- ectoderm thickens to form the neural plate
- day 18, neural tube invaginated to form the neural groove with neural folds on either side
- by end of third week, neural folds fuse converting the neural groove into the neural tube
- begins in the Center and progressed cranially to caudally
- neural crest cells form from margins of folds
Neuropore closure dates
Anterior neuropore closes day 25
Posterior neuropore closes day 27
The cardiogenic field is established cranial to the ____
Neural plate (brain)
Cariogenic field is within the
Mesoderm
Blood islands
What happens if neuropore as soon not close?
Neural tube defects (NTD)
Development of the somites
Paraxial mesoderm forms paired, cubed shaped blocked called somites (day 20-30)
-somites give rise to vertebrae (sclerotomes, muscles of neck, body wall and limbs (myotomes), and dermis of scalp, neck and trunk (dermatomes)
Neural plate secreted _____ to regulate nodal (GF) in left side of the embryo
5-HT (serotonin)
Primitive node released _____ to cause epiblast to release E-Cadherin
FGF-8 and ZIC3
What ventralizes mesoderm
BMP4
What inhibits the left side genes of the developing embryo from activating right side? What happens if this doesn’t occur
Shh
Situs Inversus
Notochord causes ectoderm cells to create
Notochord plate
What signalling pathways cause the formation of a cranial and caudal end
Up regulation of FGF and down regulation of BMP’s in cranial end
Upregulation of both FGF’s and WNT3a in caudal end
Delveopment of the intrembryonic coelom
Spaces appear in the lateral mesoderm and coalesce to form the intraembryonic coelom
-this divides the intraembryonic meoderm into a somatic layer and a splanchnic layer
Intraembryonic somatic mesoderm plus the ectoderm form the ____
Intraembryonic splanchnic mesoderm plus the endoderm form the ____
Somatopleure (body wall)
Splanchnipleure (gut wall)
Intrembryonic coelom will give rise to
The pericardial (heart) , pleural (lungs) and peritoneal (abdominal) cavities
The cardiogenic field is established _____ to the neural plate (brain)
Cranial
Vasculogenesus
Angiogenesis
The formation of new vascular channels by assembly of individual cell precursors called angioblasts
The formation of new vessles by budding and branching from preexisting vessels (secondary structures)
Decelopment of chorionic villi
- Primary villi appeared in week 2
- in 3rd week, mesoderm grows into the prinary villi converting them to secondary villi
- mesoderm differentiates to form blood vessels converting villi to tertiary
- by end of 3rd week, there is circulation between the chorionic villi and the embryo
-primordual cardiovascualr system forms in a similar manner
The cardiovascular system is the____functional system in the embryo
First
Somite parts what they become
somites give rise to vertebrae (sclerotomes, muscles of neck, body wall and limbs (myotomes), and dermis of scalp, neck and trunk (dermatomes)
What cell type creates endoderm, mesoderm and ectoderm?
Epiblast
