Lecture - Week 1 and 2 Development

From Embryology

2010 Lecture | Zygote | Blastocyst | Implantation

Objectives

Week 1 and 2 Development (see animation)
  • Understand the events during week 1 of development (Zygote, Blastomeres, Morula, Blastocyst)
  • Understand the events during week 2 of development (Trophoblast, Syncytiotrophoblast, Cytotrophoblast, Embryoblast, Implantation)
  • Brief understanding of early placentation
  • Brief understanding of maternal changes

Week 1 and 2 Overview Animations

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 ‎‎Fertilization
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Week1 001 icon.jpg
 ‎‎Week 1
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Week2 001 icon.jpg
 ‎‎Week 2 - Implant
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Chorion 001 icon.jpg
 ‎‎Week 2 - Bilaminar
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Podcast icon.jpg Lectopia Lecture Audio

Lecture 3 - Week 1 and 2 Development Lecture Date: 2012-07-31 Lecture Time: 11:00 Venue: BioMed E Speaker: Mark Hill


References

The Developing Human: Clinically oriented embryology

The Developing Human, 9th edn.jpg Citation: The Developing Human: clinically oriented embryology 9th ed. Keith L. Moore, T.V.N. Persaud, Mark G. Torchia. Philadelphia, PA: Saunders, 2011.

Larsen's human embryology

Larsen's human embryology 4th edn.jpg Citation: Larsen's human embryology 4th ed. Schoenwolf, Gary C; Larsen, William J, (William James). Philadelphia, PA : Elsevier/Churchill Livingstone, c2009.

UNSW Embryology

Logo.png Hill, M.A. (2011) UNSW Embryology (11th ed.). Sydney:UNSW.

Fertilization

Antral Follicle and Oocyte
  • Fertilization usually occurs in first 1/3 of oviduct.
  • Fertilization can also occur outside oviduct, associated with In Vitro Fertilization (IVF, GIFT, ZIFT...) and ectopic pregnancy.
  • The majority of fertilized eggs do not go on to form an embryo.
Fertilization 001 icon.jpg
 ‎‎Mouse Fertilisation
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Fertilization - Spermatozoa

Fertilization- Oocyte

Zygote Formation

  • zygote is the first diploid cell formed following fertilisation.
  • male and female pronuclei, 2 nuclei approach each other and nuclear membranes break down.
  • DNA replicates, first mitotic division
  • sperm contributes centriole which organizes mitotic spindle
Stage1 size with ruler.jpg Human zygote two pronuclei 22.jpg Parental genome mix 01 icon.jpg

Movie - Pronuclear Fusion | Movie - Parental Genomes

Conceptus - the term refers to all material derived from this fertilised zygote, includes both the embryo and the non-embryonic tissues (placenta, fetal membranes).
Links: Carnegie stage 1

Cleavage of Zygote

  • cleavage of zygote forms 2 blastomeres and is also cleavage with no cytoplasm synthesis.
    • special "embryonic" cell cycle S phases and M phases alternate without any intervening G1 or G2 phases (MSMSMSMS, adult MG1SG2) therefore individual cell volume decreases.
  • cell division is initially synchronous, then asynchronously
    • slow- centre cells, larger fast- peripheral cells
  • zona pellucid still intact (division occurs within the ZP)

Human blastocyst day 1-6.jpg

Human Zygote to Blastocyst Development (day 1 to 6)


Links: Carnegie stage 2 | Movie - Early Cell Division

Morula

Human Zygote to Blastocyst Development (day 1 to 6)
Stage 2 Day 2 Stage 2 Day 3
Human Embryo (day 2) Human Embryo (day 3)
  • about day 4 is a solid ball of 16-20 cells with peripheral cells flattened against zona pellucida
  • compaction occurs forming a cavity and leading to the next blastocyst stage
Links: Figure 8.19. Changes in DNA methylation during mammalian development

Blastocyst

Blastocyst (day 5)
  • about day 5 have 2 identifiable cell types and a fluid-filled cavity (blastoceol)
    • outer cell layer - trophoblast, peripheral flattened cells, forms the placenta and placental membranes
    • inner cell mass - embryoblast, mass of rounder cells located on one wall of the blastocoel, forms entire embryo

Blastula Cell Communication

Two forms of cellular junctions Junctions

  • gap junctions, allow electrically couple cells of epithelium surrounding a fluid-filled cavity
  • tight junctions, close to outer surface create a seal, isolates interior of embryo from external medium

Blastocyst Hatching

Hatching Blastocyst

Blastocyst Hatching - zona pellucida lost, ZP has sperm entry site, and entire ZP broken down by uterine secretions and possibly blastula secretions. Uterine Glands - secretions required for blastocyst motility and nutrition

Human blastocyst day 1-6.jpg

Human Blastocyst (day 3 to 6)
Human-blastocyst-day-3-6-icon.jpg
 ‎‎Day 3 to 6
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Human blastocyst day 5-6.jpg
 ‎‎Contractions
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 ‎‎Hatching
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Links: Carnegie stage 3 | Figure 21-69. The blastula

Week 2 - Implantation

The second week of human development is concerned with the process of implantation and the differentiation of the blastocyst into early embryonic and placental forming structures.

  • implantation commences about day 6 to 7
  • Adplantation - begins with initial adhesion to the uterine epithelium
    • blastocyst then slows in motility, "rolls" on surface, aligns with the inner cell mass closest to the epithelium and stops
  • Implantation - migration of the blastocyst into the uterine epithelium, process complete by about day 9
    • interaction between trophoblast cells and endometrial epithelium (apoptosis and decidualization)
  • coagulation plug - left where the blastocyst has entered the uterine wall day 12

Normal Implantation Sites - in uterine wall superior, posterior, lateral

Week2 001 icon.jpg
 ‎‎Week 2
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Endometrial Receptivity

  • In humans, receptivity occurs 6 days after the post-ovulatory progesterone surge and lasts about 2 to 4 days.
    • Similar "receptivity window" in other species (rat day 5 and mouse day 4.5).
  • Many studies have looked into identifying markers for this receptivity period both to optimise and to block this process.

Abnormal Implantation

Ectopic tubal pregnancy

Abnormal implantation sites or Ectopic Pregnancy occurs if implantation is in uterine tube or outside the uterus.

  • sites - external surface of uterus, ovary, bowel, gastrointestinal tract, mesentry, peritoneal wall
  • If not spontaneous then, embryo has to be removed surgically

Tubal pregnancy - 94% of ectopic pregnancies

  • if uterine epithelium is damaged (scarring, pelvic inflammatory disease)
  • if zona pellucida is lost too early, allows premature tubal implantation
  • embryo may develop through early stages, can erode through the uterine horn and reattach within the peritoneal cavity
Tubal pregnancy historic.jpg Abnormal implantation sites.jpg


Links: Implantation | Ectopic Pregnancy | Movie - Ectopic pregnancy ultrasound

Uterus

  • Endometrium - 3 layers in secretory phase of menstrual cycle: compact, spongy, basal
  • Myometrium - muscular layer outside endometrium, contracts in parturition
  • Perimetrium - tunica serosa of the uterus continuous with the peritoneal wall


Endometrial Layers

  • Compact - implantation occurs in this layer, dense stromal cells, uterine gland necks, capillaries of spiral arteries
  • Spongy - swollen stromal cells, uterine gland bodies, spiral arteries
  • Basal - not lost during menstruation or childbirth, own blood supply

Decidual Reaction

  • transformation of endometrial stromal cells
  • occurs initially at site of implantation and includes both cellular and matrix changes
  • reaction spreads throughout entire uterus, not at cervix
  • deposition of fibrinoid and glycogen and epithelial plaque formation (at anchoring villi)
  • presence of decidual cells are indicative of pregnancy

Other Uterine Changes

  • Cervix - at mouth of uterus, secretes mucus (CMP), forms a plug/barrier, mechanical and antibacterial
  • Vascular - increased number of blood vessels

Decidua

The endometrium becomes the decidua and forms 3 distinct anatomical regions (at approx 3 weeks)

  • Decidua Basalis at implantation site
  • Decidua Capsularis enclosing the conceptus
  • Decidua Parietalis the remainder of uterus
    • Decidua Capsularis and Parietalis fuse eventually fuse and uterine cavity is lost by 12 weeks

Uterus Abnormalities

Endometriosis endometrial tissue located in other regions of the uterus or other tissues. This misplaced tissue develops into growths or lesions which respond to the menstrual cycle hormonal changes in the same way that the tissue of the uterine lining does; each month the tissue builds up, breaks down, and sheds.

Conceptus - Bilaminar Embryoblast

  • about day 8 to 9
  • The outer trophoblast and inner embryoblast layers now both differentiate to form two distinct cellular layers.
  • The trophoblast layer forms the syncitotrophoblast and cytotrophoblast layers.
  • The embryoblast (inner cell mass) forms the epiblast and hypoblast layers.
    • Epiblast - will form the 3 germ layers.
    • Hypoblast - transient layer replaced by endoderm.
  • This early stage of embryo development is referred to as the bilaminar embryo.
File:Chorion 001 icon.jpg</wikiflv>

Movie - Week 2 Bilaminar Embryo

Conceptus - Bilaminar Trophoblast

Syncitiotrophoblasts

  • secrete proteolytic enzymes, enzymes break down extracellular matrix around cells
  • Allow passage of blastocyst into endometrial wall, totally surround the blastocyst
  • generate spaces that fill with maternal blood- lacunae
  • secrete Human Chorionic Gonadotropin (hCG), hormone, maintains decidua and Corpus Luteum, basis of pregnancy diagnostic test, present in urine is diagnostic of pregnancy
    • levels peak at 8 to 10 weeks of pregnancy, then decline and are lower for rest of pregnancy
    • 1-2 months: 5,000-200,000 mIU/ml; Non-pregnant females: <5.0 mIU/ml; Postmenopausal females: <9.5 mIU/ml)
    • Later in development placenta will secrete hCG

Pregnancy Test Kit Ovary corpus luteum

Cytotrophoblasts

  • form a continuous cellular layer that covers the developing placental villi.

Twinning

Twinning can be due to two separate fertilization events (dizygotic twins) or as an abnormality of a single fertilization (monozygotic twins) event during the early weeks of development.

Dizygotic Twinning

Dizygotic twins (fraternal, non-identical) arise from separate fertilization events involving two separate oocyte (egg, ova) and spermatozoa (sperm).

  • In dizygotic twinning the genetic material is different and implantation and placentation is also different.

Monoygotic Twinning

  • In monozygotic twinning the genetic material is initially identical and degree of twinning will depend upon the timing (early to late) from separate fetal membranes and placenta to conjoined twins.
    • morula stage (diamniotic dichorionic), early blastocyst (diamniotic monochorionic), late blastocyst to bilaminar (monoamniotic monochorionic), bilaminar to trilaminar embryo (conjoined)
  • Monozygotic twins are a unique research resource for comparing environmental effects on development and health.
  • Congenital abnormality statistics for twins is generally increased in various conditions.

Monoygotic twins (identical) produced from a single fertilization event (one fertilised egg and a single spermatazoa, form a single zygote), these twins therefore share the same genetic makeup. Occurs in approximately 3-5 per 1000 pregnancies, more commonly with aged mothers. The later the twinning event, the less common are initially separate placental membranes and finally resulting in conjoined twins.

Week Week 1 Week 2
Day 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Cell Number 1 1 2 16 32 128 bilaminar
Event Ovulation fertilization First cell division Morula Early blastocyst Late blastocyst

Hatching

Implantation starts X inactivation
Follicle 001 icon.jpg Early zygote.jpg Human embryo day 2.jpg Human embryo day 3.jpg Human embryo day 5.jpg CSt3.jpg Week2 001 icon.jpg
Monoygotic

Twin Type

Diamniotic

Dichorionic

Diamniotic

Monochorionic

Monoamniotic

Monochorionic

Conjoined

Table based upon recent Twinning Review.[1]


  1. <pubmed>12957099</pubmed>


Links: Twinning | Australian Twin Registry


Now watch the Week 1 and 2 animation overview.


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Cite this page: Hill, M.A. (2024, March 28) Embryology Lecture - Week 1 and 2 Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Lecture_-_Week_1_and_2_Development

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G