BGDA Practical 7 - Week 3
Practical 6: Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 |
Introduction
Key events of human development during the third week (week 3) following fertilization clinical week 5 (GA week 5).
Note that during this time the conceptus cells not contributing to the embryo are contributing to placental membranes and the early placenta. This page describes the mechanical events and changes occurring in each of the 3 germ layers (some concepts will also be covered in later weeks).
Gastrulation
Gastrulation is continuing through week 3 as cells continue to migrate through the primitive streak contributing to mesoderm.
As the embryonic disc grows overall in size, the primitive streak appears to become more caudal as it does not increase in size.
Folding
Endoderm, mesoderm and ectoderm layers. There are two major folding processes that take place during this time.
- Folding of the whole embryonic disc ventrally, separates the endoderm to form the epithelial lining of the gut. Folding of the embryonic disc occurs ventrally around the notochord, which forms a rod-like region running rostro-caudally in the midline.
- Folding of the ecoderm will form a neural groove, then closing to form a neural tube, separating the neural ectoderm from the embryo surface ectoderm.
Mesoderm Segmentation
Different regions of mesoderm form early intermediate structures.
- Cardiac development - forming the simple heart tube within splanchnic mesoderm.
- Somitogenesis - when part of the mesoderm layer segments during week 3 to form balls of mesoderm called somites. The later migration of cells forms the mesoderm germ layer. An embryonic connective tissue (mesenchyme) which forms nearly all the connective tissues of the body (the head is different). Somitogenesis is when part of this layer segments during week 3 to form balls of mesoderm called somites.
- Intraembryonic coelom - Within the embryonic disc lateral plate mesoderm a space (coelom) forms, it lies within the embryo and so is called the intraembryonic coelom. This single "horseshoe-shaped" space will form the 3 major body cavities: pericardial (around the heart), pleural (around the lungs) and peritoneal (around the GIT and visceral organs).
Ectoderm
The central portion of the embryonic disc forms the neural plate, the edge of this plate forms neural crest and outside of this again will contribute the epitheium of the skin. (this will be covered in more detail week 4).
Endoderm
|
| ||||||
The major folding processes that take place during this time, in relation to the notochord:
|
The ventral endoderm (shown yellow) has grown to line a space called the yolk sac. Folding of the embryonic disc "pinches off" part of this yolk sac forming the first primative GIT. |
Mesoderm
Mesoderm means the "middle layer" and it is from this layer that nearly all the bodies connective tissues are derived. In early mesoderm development a number of transient structures will form and then be lost as tissue structure is patterned and organised. Humans are vertebrates, with a "backbone", and the first mesoderm structure we will see form after the notochord will be somites.
Facts: Week 4, 22 - 23 days, 2 - 3.5 mm, Somite Number 4 - 12
View: This is a dorsal view of the human embryo, the amniotic membrane has been removed. Top embryo is an early stage 10, bottom is late stage 10.
Mesoderm Development
- epiblast -> mesoderm + axial mesoderm (notochord)
- lateral plate + paraxial mesoderm + axial mesoderm
- lateral plate + intermediate mesoderm + somites (body), paraxial mesoderm (head) + axial mesoderm
- somatic mesoderm + intraembryonic coelom + splanchnic mesoderm + intermediate mesoderm + somites (body), paraxial mesoderm (head) + axial mesoderm
Axial Mesoderm
The notochord
|
Paraxial Mesoderm
Adult - contributes vertebral column (vertebra and IVD), dermis of the skin, skeletal muscle of body and limbs |
Intermediate Mesoderm
Adult - metanephros forms the kidney |
Lateral Plate Mesoderm
|
Heart Development
Splanchnic mesoderm lying above the notochord (prechordal splanchnic mesoderm) forms a pair of simple tubes, that will fuse to form the primordia of the heart tube.
More: Primitive Heart Tube |
Somite Development
Somite initially forms 2 main components
- ventromedial- sclerotome forms vertebral body and intervertebral disc
- dorsolateral - dermomyotome forms dermis and skeletal muscle
Sclerotome
|
Myotome
Forms 2 muscle groups in body and limbs
| |
Development of the sclerotome and myotome components of the somite. |
Dermatome
- connective tissue underlying epidermis
- begins as a dorsal thickening
- spreads throughout the body
Note - Dermatome is the term also used clinically postnatally to describe the region of skin supplied by a single spinal nerve.
Week 2 and 3 Movies
Week 2
|
|
Week 3
|
|
|
| ||||||||||||
|
|
|
Practical 6: Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 |
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. |
Embryo Stages and Events
Day | Stage | Event |
Stage 7 | Primitive node (Hensen's node, primitive knot) The small circular region located at the cranial end of the primitive streak, where gastrulation occurs, and is a controller of this process. The second role is to act as an initial generator of the left-right (L-R) body axis. | |
Stage 8 | Neural neurogenesis, neural groove and folds are first seen | |
Stage 9 | Musculoskeletal somitogenesis - first somites form and continue to be added in sequence caudally
Neural - three main divisions of the brain, which are not cerebral vesicles, can be distinguished while the neural groove is still completely open
| |
Heart cardiogenesis - week 3 begins as paired heart tubes. |
BGDA: Lecture 1 | Lecture 2 | Practical 3 | Practical 6 | Practical 12 | Lecture Neural | Practical 14 | Histology Support - Female | Male | Tutorial
Glossary Links
- Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link
Cite this page: Hill, M.A. (2024, June 17) Embryology BGDA Practical 7 - Week 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/BGDA_Practical_7_-_Week_3
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G