BGDA Practical 7 - Week 6

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Introduction

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Practical 6: Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8


Key Events of Human Development during the sixth week (week 6) following fertilization or clinical GA week 8.

There are 2 Carnegie stages that show external embryo development during this week.

Limb Development

Stage16-17-limbs01.jpg

  • By week 6 the upper limb bud has grown to the point where the primitive hand appears as a "paddle-like" structure at the end of an externally visible undifferentiated limb bud.
  • Within this primitive hand the beginning of the fingers can be seen as "digital rays".
    • Cell between the digits will die by a process called apoptosis (programmed cell death).
  • The lower limb bud appears less developed at each embryo stage. Features that appear on the upper limb appear on the lower limb about 2 days later.
  • Upper limb bud nerves (median nerve, radial nerve and ulnar nerve) entered into hand plate.
  • Myoblasts spindle shaped and oriented parallel to limb bud axis.


Note - Overall limb development will be covered in the week 8 section.

Face Development

  • During week 6 there is fusion of the upper lip.
  • Formed by the maxillary prominences of of the first pharyngeal arch and the frontonasal prominence.
  • Failure of this embryonic process leads to cleft lip, this will be covered in more detail in BGDB.

Cardiovascular Development

Outflow Tract Septation

The fused endocardial cushions are shown and the animation shows the growth of the spiral membranous septa that divide the single outflow tract into aortic and pulmonary outflows from the heart. Note the spiral nature of this septation process "swaps" the left/right ventricle to vessels anatomical relationship. Abnormalities in this process contribute to ventricular septal defects.

Outflow tract 001 icon.jpg
 ‎‎Outflow Tract
Page | Play
Heart1 ventricle icon.jpg
 ‎‎Outflow Septation
Page | Play

Endocrine Development

These organs mainly develop in the embryonic period and function in the fetal period. (Covered in next practical)

  • Thyroid - develops from thyroid median endodermal thickening in the floor of pharynx outpouch – thyroid diverticulum (day 24), functions from week 10
  • Pituitary - develops from surface (anterior) and neural tube (posterior) ectoderm, connecting stalk between pouch and oral cavity degenerates
  • Parathyroid - develop from endoderm of pharyngeal pouches, diverticulum elongate, hollow then solid, dorsal cell proliferation
  • Thymus - develops from 2 origins for lymphoid thymocytes and thymic epithelial cells, diverticulum elongate, hollow then solid, ventral cell proliferation
  • Adrenal - develops from 2 origins adrenal medulla (neural crest) and fetal cortex (surrounding mesenchyme), fetal cortex forms from mesothelium adjacent to dorsal mesentery, medulla neural crest cells from adjacent sympathetic ganglia
  • Gonad - develops from 2 origins, germ cells migrate into the genital ridge (mesothelium and underlying mesenchyme)
Links: Endocrine System Development

Sex Determination

At an earlier stage during week 4, the primordial germ cells began their migration into the region where the gonads will eventually form, the genital ridge, see this migration in the mouse.

Stage 13 image 085.jpg

Genital Ridge (Week 4)

  • Humans (week 5-6), these germ cells do not determine sex, it is the support cells regulated by the presence of an X or Y chromosome.
  • germ cells migrate into gonadal ridge
  • Gonads (male - testis, female - ovary) identical at this stage, termed "indifferent"


Note - Sex determination and development will be covered in detail in BGDB.



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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.

Limb Development

Mouse limb tissue development.jpg

Mouse Limb Tissue Development[1]

Species Embryonic Comparison Timeline
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
Human Days 1 2-3 4-5 5-6 7-12 13-15 15-17 17-19 20 22 24 28 30 33 36 40 42 44 48 52 54 55 58
Mouse Days 1 2 3 E4.5 E5.0 E6.0 E7.0 E8.0 E9.0 E9.5 E10 E10.5 E11 E11.5 E12 E12.5 E13 E13.5 E14 E14.5 E15 E15.5 E16
Rat Days 1 3.5 4-5 5 6 7.5 8.5 9 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5
Note these Carnegie stages are only approximate day timings for average of embryos. Links: Carnegie Stage Comparison
Table References  
Human

O'Rahilly R. (1979). Early human development and the chief sources of information on staged human embryos. Eur. J. Obstet. Gynecol. Reprod. Biol. , 9, 273-80. PMID: 400868
Otis EM and Brent R. Equivalent ages in mouse and human embryos. (1954) Anat Rec. 120(1):33-63. PMID 13207763

Mouse
Theiler K. The House Mouse: Atlas of Mouse Development (1972, 1989) Springer-Verlag, NY. Online
OTIS EM & BRENT R. (1954). Equivalent ages in mouse and human embryos. Anat. Rec. , 120, 33-63. PMID: 13207763

Rat
Witschi E. Rat Development. In: Growth Including Reproduction and Morphological Development. (1962) Altman PL. and Dittmer DS. ed. Fed. Am. Soc. Exp. Biol., Washington DC, pp. 304-314.
Pérez-Cano FJ, Franch À, Castellote C & Castell M. (2012). The suckling rat as a model for immunonutrition studies in early life. Clin. Dev. Immunol. , 2012, 537310. PMID: 22899949 DOI.

Endocrine Development

Endocrine Links: Introduction | BGD Lecture | Science Lecture | Lecture Movie | pineal | hypothalamus‎ | pituitary | thyroid | parathyroid | thymus | pancreas | adrenal | endocrine gonad‎ | endocrine placenta | other tissues | Stage 22 | endocrine abnormalities | Hormones | Category:Endocrine
Historic Embryology - Endocrine  
1903 Islets of Langerhans | 1903 Pig Adrenal | 1904 interstitial Cells | 1908 Pancreas Different Species | 1908 Pituitary | 1908 Pituitary histology | 1911 Rathke's pouch | 1912 Suprarenal Bodies | 1914 Suprarenal Organs | 1915 Pharynx | 1916 Thyroid | 1918 Rabbit Hypophysis | 1920 Adrenal | 1935 Mammalian Hypophysis | 1926 Human Hypophysis | 1927 Adrenal | 1927 Hypophyseal fossa | 1930 Adrenal | 1932 Pineal Gland and Cysts | 1935 Hypophysis | 1935 Pineal | 1937 Pineal | 1935 Parathyroid | 1940 Adrenal | 1941 Thyroid | 1950 Thyroid Parathyroid Thymus | 1957 Adrenal

Embryo Stages and Events

Timeline

Day
Stage
Event
36
Pituitary - Week 6 connecting stalk between pouch and oral cavity degenerates

Parathyroid - Week 6 diverticulum elongate, hollow then solid, dorsal cell proliferation

Thymus - Week 6 diverticulum elongate, hollow then solid, ventral cell proliferation

Adrenal - Week 6 fetal cortex forms from mesothelium adjacent to dorsal mesentery, medulla neural crest cells from adjacent sympathetic ganglia

Respire - Week 6 - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes

Tongue Week 6 - gustatory papilla, caudal midline near the foramen caecum (week 6 to 7 - nerve fibers approach the lingual epithelium)

37
Stage 16 Stage16 bf1c.jpg Neural - first parasympathetic ganglia, submandibular and ciliary, are identifiable[2]

Limbs upper limb bud nerves median nerve, radial nerve and ulnar nerve entered into hand plate, myoblasts spindle shaped and oriented parallel to limb bud axis.

Heart - outflow tract elliptical configuration with four cushions, the two larger fusing at this stage. Semilunar valve leaflets form at the downstream end of the cushions

Head lip and palate components of the upper lip, medial nasal prominence and maxillary process present, median palatine process appears.

38
   
39
   
40
   
41
Stage 17 Stage17 bf1c.jpg

Neural - telencephalon areas of the future archicortex, paleocortex, and neocortex, visible. Beginning of future choroid plexus[3]

Sense - Smell olfactory nerve fibres enter the brain[4] Neural - primordium of the epidural space appears first on the ventral part of the vertebral canal and develops rostro-caudally[5]

42
  Heart - separation of common cardiac outflow (aortic arch and pulmonary aorta)

References

  1. <pubmed>22174793</pubmed>| PMC3235105 | PLoS One.
  2. <pubmed>2751117</pubmed>
  3. <pubmed>2802187</pubmed>
  4. <pubmed>15604533</pubmed>
  5. <pubmed>15478101</pubmed>



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Cite this page: Hill, M.A. (2024, March 28) Embryology BGDA Practical 7 - Week 6. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/BGDA_Practical_7_-_Week_6

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