Talk:2010 BGD Practical 3 - Week 3 Summary

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.

Trilaminar Embryo

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

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

Carnegie Stage 8

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

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.

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, which 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. Note amniotic ectoderm covered by extra-emebryonic mesoderm (empty spaces above and below the mesoderm are artefacts, as are the lateral folds in the ectoderm).

The first two images using bright field microscopy approximate the orientation of the scanning electron micrographs below. There are additional scanning electron micrographs showing selected features in detail.

Carnegie stage 9

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

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 clinical weeks (shown in brackets, from last menstrual period) and "stages" refer to Carnegie stages of development.

Week -2

(Clinical Week 1)

Day	Menstrual cycle	Event
1	Menstrual Phase	Menstrual Cycle changes: Uterine endometrium (loss), Ovary (Follicle Development)
2
3
4
5	Proliferative Phase	Menstrual Cycle changes: Uterine endometrium (proliferation), Ovary (Follicle Development)
6
7

Week -1

(Clinical Week 2)

Day	Menstrual cycle	Event
8	Proliferative Phase
9		Menstrual Cycle - Mid proliferative
10
11
12
13		Menstrual Cycle - Late Proliferative
14	Ovulation Capacitation

Week 1

Week 1 (Clinical Week 3)

Day	Stage	Event
1	Secretory PhaseStage 1	Fertilization, Secretory Phase
2	Stage 2	Morula, Blastula
3
4	Stage 3	Blastocyst Hatching (zona pellucida lost)
5		Late Secretory, Blastocyst (free floating)
6	Stage 4	Adplantation
7	Stage 5

Week 2

Week 2 (Clinical Week 4)

Day	Stage	Event
8		Implantation
9
10
11
12
13	Stage 6	Chorionic Cavity
14

Week 3

Week 3 (Clinical Week 5)

Day	Stage	Event
15
16	Stage 7
17
18	Stage 8	Neural neurogenesis, neural groove and folds are first seen
19
20	Stage 9	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.