BGDA Practical 3 - Week 3 Summary

From Embryology
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Practical 3: Oogenesis and Ovulation | Gametogenesis | Fertilization | Early Cell Division | Week 1 | Implantation | Week 2 | Extraembryonic Spaces | Gastrulation | Notochord | Week 3 | Quiz

Introduction

This page is a overview of events that occur in human development up to week 3 post-fertilization. From this Practical understand concepts of: fertilization, blastocyst development, implantation, bilaminar and trilaminar embryo formation, development of embryonic cavities and brief understanding of early placenta development.

By the end of week 3, segmentation of the trilaminar embryo 3 germ layers has begun:

Human development timeline graph 01.jpg

  1. Ectoderm - central neural plate and lateral parts form epidermis
  2. Mesoderm - midline notochord, adjacent somites, formation of the internal embryonic space (intraembryonic ceolom)
  3. Endoderm - epidermal lining of gastrointestinal tract and yolk sac lining

Note

Use the links to Carnegie stage 7, Carnegie stage 8 and Carnegie stage 9 to see a number of different views of the human embryo in the third week of development.

The timeline at the bottom of this page should give you a better perspective of the sequence of early developmental events. You would not be expected to know exact days (as they are only approximate anyway) it is more important to get the weeks and sequence right.


Week: 1 2 3 4 5 6 7 8
Carnegie stage: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Stage 7

Stage7-sem2.jpg

Facts

Human embryonic stage 7 occurs during week 3 between 15 to 17 days.

The embryo is now 0.4 mm diameter in size.

The initial images are displayed unlabeled to allow you to explore the embryo for yourself, linked labeled versions are also available for some images.

Events

Gastrulation is continuing as cells migrate from the epiblast, continuing to form mesoderm.

Mesoderm lies between the ectoderm and endoderm as a continuous sheet except at the buccopharyngeal and cloacal membranes. These membranes have ectoderm and endoderm only and will lie at the rostral (head) and caudal (tail) of the gastrointestinal tract.

From the primitive node a tube extends under the ectoderm in the opposite direction to the primitive streak. This tube forms first the axial process then notochordal process, then finally the notochord.

The notochord is a key to embryonic folding and regulation of ectoderm and mesoderm differentiation. It lies in the rostrocordal axis and the embryonic disc will fold either side ventrally, pinching off a portion of the yolk sac to form the lining of the gastrointestinal tract.

Stage 7 - Implanted Conceptus

Stage7 bf5.jpg

 ‎‎Mobile | Desktop | Original

Stage 7 | Embryo Slides

Carnegie Stage 8

Stage8 SEM1.jpg

Facts

Human embryonic stage 8 occurs during week 3 between 17 to 19 days.

The embryo is now 1.0 - 1.5 mm in size.

Events

Gastrulation is continuing as cells migrate from the epiblast, continuing to form mesoderm.

Mesoderm lies between the ectoderm and endoderm as a continuous sheet except at the buccopharyngeal and cloacal membranes. These membranes have ectoderm and endoderm only and will lie at the rostral (head) and caudal (tail) of the gastrointestinal tract.

From the primitive node a tube extends under the ectoderm in the opposite direction to the primitive streak. This tube forms first the axial process then notochordal process, then finally the notochord.

The notochord is a key to embryonic folding and regulation of ectoderm and mesoderm differentiation. It lies in the rostrocordal axis and the embryonic disc will fold either side ventrally, pinching off a portion of the yolk sac to form the lining of the gastrointestinal tract.

Identify

  • embryonic disc
  • primitive node, primative streak, primative groove
  • connecting stalk
  • cut amniotic membrane

Carnegie stage 8


Carnegie Stage 9

Stage9 sem4c.jpg

Facts

Human embryonic stage 9 occurs during week 3 between 19 to 21 days.

The embryo is now 1.5 to 2.5 mm in size and somites have begun to form and number between 1 to 3 somite pairs during this stage.

The initial images are displayed unlabeled to allow you to explore the embryo for yourself, linked labeled versions are also available for some images.

Stage9 sem1.jpg

Events

Ectoderm - Neural plate brain region continues to expand, neural plate begins folding over the notochord. Gastrulation continues through the primitive streak region.

Mesoderm - Paraxial mesoderm segmentation into somites begins (1 - 3 somite pairs). Lateral plate mesoderm begins to vacuolate, dividing it into somatic and splanchnic mesoderm and to later form the intra-embryonic coelom. Prechordal splanchnic mesoderm begins to form the cardiogenic region, from which the primordial heart will develop.

Endoderm - Notochordal plate still visible which will form the notochord. Endoderm is still widely open to the yolk sac and germ cells form part of this layer. Extra-embryonic mesoderm on the yolk sac surface begins to form "blood islands".

Identify

  • Neural groove and neural folds, the mesoderm showing first somite bulges, that segments beside the neural groove to form somites but extends laterally to margin of embryonic disc lateral plate mesoderm, where it merges with the covering extraembryonic mesoderm.
  • The intra-embryonic coelom develops in the middle of the lateral plate mesoderm.


Carnegie stage 9


Carnegie Stage 10

Human embryonic stage 10 occurs at the beginning of week 4 development (GA week 6). This week will be covered in detail in the next practical class. The virtual slide image below shows a dorsal view of the embryo at the beginning of week 4.


Stage 10 - Dorsal View

Stage10 bf10.jpg

 ‎‎Mobile | Desktop | Original

Stage 10 | Embryo Slides


Carnegie Stages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | About Stages | Timeline


Additional Information

Additional Information - Content shown under this heading is not part of the material covered in this class. It is provided for those students who would like to know about some concepts or current research in topics related to the current class page.

Human Development Timeline

The table below shows human development features and approximate timing during the menstrual cycle to fertilization and the first 3 weeks of development.

The timing assumes fertilization the day after ovulation and the "weeks" refer to embryonic development and differ from Gestational Age (GA, clinical weeks shown in brackets, from last menstrual period) and "stages" refer to Carnegie stages of development.


Week -2

(GA week 1, clinical week 1)


Day
Menstrual cycle
Event
1
Menstrual Phase
Menstrual cycle.png

Menstrual Cycle changes: Uterine endometrium (loss), Ovary (Follicle Development)

2
  Human- menstrual uterine endometrium.jpg
3
   
4
   
5
Proliferative Phase Smear- early proliferative.jpgOva41he.jpg Menstrual Cycle changes: Uterine endometrium (proliferation), Ovary (Follicle Development)
6
7
   

Week -1

(GA week 2, clinical week 2)


Day
Menstrual cycle Event
8
Proliferative Phase
9
  Smear- mid-proliferative.jpg Human- mid-proliferative uterine endometrium.jpg Ovary10x.jpg Ova20he.jpg Menstrual Cycle - Mid proliferative
10
   
11
   
12
   
13
  Smear- late-proliferative.jpg Human- late proliferative uterine endometrium.jpg Menstrual cycle.png Menstrual Cycle - Late Proliferative
14
Ovulation

Capacitation

Human oocyte.jpg Follicle 001 icon.jpg

Week 1

Week 1 (GA week 3, clinical week 3)


Day
Stage
Event
1
Secretory Phase
Stage 1
Early zygote.jpg Smear- secretory.jpg Human- secretory uterine endometrium.jpg Fertilization, Secretory Phase
2
Stage 2 Stage2.jpg Week1 001 icon.jpg Morula, Blastula
3
 
4
Stage 3 CSt3.jpg Blastocyst Hatching (zona pellucida lost)
5
  Smear- late secretory.jpg Human- late secretory uterine endometrium.jpg Late Secretory, Blastocyst (free floating)
6
Stage 4 Adplantation
7
Stage 5 Week2 001 icon.jpg

Week 2

Week 2 (GA week 4, clinical week 4)


Day
Stage
Event
8
Week2 001 icon.jpg Implantation
9
   
10
   
11
   
12
   
13
Stage 6 Chorion 001 icon.jpg Chorionic Cavity
14
   

Week 3

Week 3 (GA week 5, clinical week 5)


Day
Stage
Event
15
16
Stage 7 Stage7-bf1.jpg Stage7-sem2.jpg Stage7.jpg
17
   
18
Stage 8 Stage8 human.jpg Neuralplate 001 icon.jpg Neural neurogenesis, neural groove and folds are first seen
19
 
Stage8 SEM1.jpg
20
Stage 9 Stage9 bf2c.jpg Stage9 sem1b.jpg Musculoskeletal somitogenesis, first somites form and continue to be added in sequence caudally

Neural the three main divisions of the brain, which are not cerebral vesicles, can be distinguished while the neural groove is still completely open

Neural Crest mesencephalic neural crest is visible PMID: 17848161

21
  Heart cardiogenesis, week 3 begins as paired heart tubes.

Next

Finished Lab 3 !

If you have finished and would like to apply your knowledge, I have also included some Clinical Questions based around this period of development.

If you have finished and need some more help understanding this period of development, I have included some links to Online References.


If you have finished and are interested in looking at tissues involved in this period of development, I have included some links to Histology Images.


Note that this Practical has discussed mainly development of the embryo as placental development will be covered in detail in another practical (Placenta and Fetal Membranes).

The next Practical will continue on through embryonic development (Implantation to 8 Weeks).



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Cite this page: Hill, M.A. 2017 Embryology BGDA Practical 3 - Week 3 Summary. Retrieved November 23, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/BGDA_Practical_3_-_Week_3_Summary

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© Dr Mark Hill 2017, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G