BGDA Practical 7 - Week 6

From Embryology

Introduction

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

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.

Week 6 Embryo - Stage 16 Movies  

Click Here to play on mobile device

These Week 6 movies shows the appearance of human embryo at Carnegie stage 16 (GA week 8).

Embryo Surface - Carnegie Stage 16

This movie shows an unlabeled MRI 3D volume embryo scan of the Kyoto embryo (stage 16, week 6). Compare this with the earlier stage 13 week 4 embryo.

Note:

  • Growth of limb buds
  • Size of the head
  • Pharyngeal arches
  • Size and position of the heart
  • Umbilicus containing cord with placental blood vessels

Click Here to play on mobile device

Neural Development

This labelled version shows the Stage 16 embryo rotating with the CNS spinal cord and the secondary brain vesicles and flexures labeled.

Note:

  • Growth of limb buds.
  • Development of the vision and hearing sensory vesicles (optic and otic) and their relative positions (optic at the diencephalon; otic at the metencephalon).
  • The lamina terminalis at the end of the neural tube (site of cranial/anterior neuropore closure).
  • The 5 secondary brain vesicles.
  • The large ventricular space within the tube, divided into future regions.
  • The lateral walls of the neural tube forming alar and basal plates (particularly obvious along spinal cord).
  • The top and bottom of the neural tube forming thin roof and floor plates (particularly obvious along spinal cord).
  • The large dorsal root ganglia along the length of the spinal cord.
  • The 3 brain flexures; cephalic (or mesencephalic), pontine and cervical.

Click Here to play on mobile device

Sagittal Sections

This movie shows an unlabeled MRI sagittal scan passing from right to left through the Kyoto embryo (stage 16, week 6). Compare this with the earlier week 4 stage 13 embryo CNS.

Note:

  • Growth of limb buds.
  • Development of the vision and hearing sensory vesicles (optic and otic) and their relative positions (optic at the diencephalon; otic at the metencephalon).
  • The lamina terminalis at the end of the neural tube (site of cranial/anterior neuropore closure).
  • The 5 secondary brain vesicles.
  • The large ventricular space within the tube, divided into future regions.
  • The lateral walls of the neural tube forming alar and basal plates (particularly obvious along spinal cord).
  • The top and bottom of the neural tube forming thin roof and floor plates (particularly obvious along spinal cord).
  • The large dorsal root ganglia along the length of the spinal cord.
  • The 3 brain flexures; cephalic (or mesencephalic), pontine and cervical.
Stage16 MRI 3D01 icon.jpg
 ‎‎Embryo Stage 16
Page | Play
Stage16 MRI 3D02 icon.jpg
 ‎‎Embryo CNS
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Stage16 MRI S01 icon.jpg
 ‎‎Embryo Stage 16
Page | Play
Stage 16 Movies: MRI Surface | MRI Surface (labeled) | MRI Central Nervous System | MRI CNS (labeled) | MRI Sagittal | MRI Transverse | MRI Coronal | EFIC Sagittal | EFIC Transverse | EFIC Coronal | EFIC Coronal (label) |


BGDA Week 4 Movies | BGDA Week 6 Movies | BGDA Week 8 Movies


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
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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 | Quiz



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.

Timeline

Week 6 - Human Embryo Stages and Events (GA week 8) 
Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9
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[1]

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[2]

Sense - Smell olfactory nerve fibres enter the brain[3] Neural - primordium of the epidural space appears first on the ventral part of the vertebral canal and develops rostro-caudally[4]

Pancreas - rotation of the developing stomach moves ventral bud dorsally and fuses with the dorsal bud.

42
  Heart - separation of common cardiac outflow (aortic arch and pulmonary aorta)
Note - the day timing of stages is only approximate, system names link to first page of that specific system, and events are based upon the literature cited below.
References
  1. F Müller, R O'Rahilly The human brain at stage 16, including the initial evagination of the neurohypophysis. Anat. Embryol.: 1989, 179(6);551-69 PubMed 2751117
  2. F Müller, R O'Rahilly The human brain at stage 17, including the appearance of the future olfactory bulb and the first amygdaloid nuclei. Anat. Embryol.: 1989, 180(4);353-69 PubMed 2802187
  3. F Müller, R O'Rahilly Olfactory structures in staged human embryos. Cells Tissues Organs (Print): 2004, 178(2);93-116 PubMed 15604533
  4. Magdalena Patelska-Banaszewska, Witold Woźniak The development of the epidural space in human embryos. Folia Morphol. (Warsz): 2004, 63(3);273-9 PubMed 15478101


Neural Development

Human Stage16 neural02.jpg
Cranial Nerves 
Nerve Number Name Type Origin Function
CN I Olfactory sensory telencephalon smell placode
CN II Optic sensory retinal ganglial cells vision
CN III Oculomotor motor anterior midbrain extraocular muscles eye movements and pupil dilation (motor)
CN IV Trochlear motor dorsal midbrain extraocular muscles (superior oblique muscle)
CN V Trigeminal motor/sensory pons proprioception, mastication
CN VI Abducent motor extraocular muscles control eye movements (lateral rectus muscle)
CN VII Facial motor/sensory pons facial expression, taste (tongue anterior and central regions) regulate salivary production.
CN VIII Acoustic sensory vestibular and cochlear nuclei hearing/balance placode
CN IX Glossopharyngeal motor/sensory medulla swallowing and speech, taste (tongue posterior region)
CN X Vagus motor/sensory medulla larynx and pharynx muscles (speech and swallowing), regulates heartbeat, sweating, and peristalsis
CN XI Accessory motor motor neurons sternocleidomastoid and trapezius muscles
CN XII Hypoglossal motor motor neurons tongue muscles (speech, eating and other oral functions)

Summary of the cranial nerves.

Right lateral view of the central nervous system of embryo at Carnegie stage 16. Scale bar is 1 mm

Mouse Limb Development

Mouse limb tissue development.jpg

Mouse Limb Tissue Development[1]

Species 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 4 5.0 6.0 7.0 8.0 9.0 9.5 E10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 15.5 16
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
References  
Human

R O'Rahilly Early human development and the chief sources of information on staged human embryos. Eur. J. Obstet. Gynecol. Reprod. Biol.: 1979, 9(4);273-80 PubMed 400868


Mouse
The House Mouse: Atlas of Mouse Development by Theiler Springer-Verlag, NY (1972, 1989). | online book
E M OTIS, R BRENT Equivalent ages in mouse and human embryos. Anat. Rec.: 1954, 120(1);33-63 PubMed 13207763


Rat
Witschi, E. (1962) Development: Rat. In: Growth Including Reproduction and Morphological Development. Altman, P. L. , and D. S. Dittmer, ed. Fed. Am. Soc. Exp. Biol., Washington DC, pp. 304-314.
Francisco J Pérez-Cano, Àngels Franch, Cristina Castellote, Margarida Castell The suckling rat as a model for immunonutrition studies in early life. Clin. Dev. Immunol.: 2012, 2012;537310 PubMed 22899949

Endocrine Development

Endocrine Links: Introduction | BGD Lecture | Science Lecture | Pineal | Hypothalamus‎ | Pituitary | Thyroid | Parathyroid | Thymus‎ | Pancreas‎ | Adrenal‎ | Gonad‎ | Placenta‎ | Other Tissues | Stage 22 | Abnormalities | Hormones | Category:Endocrine
Historic Embryology - Endocrine  
1903 Islets of Langerhans | 1904 interstitial Cells | 1908 Pancreas Different Species | 1912 Suprarenal Bodies | 1914 Suprarenal Organs | 1915 Pharynx | 1916 Thyroid | 1918 Rabbit Hypophysis | 1920 Adrenal | 1935 Mammalian Hypophysis | 1926 Human Hypophysis | 1937 Pineal | 1938 Parathyroid | 1940 Adrenal | 1941 Thyroid | 1950 Thyroid Parathyroid Thymus | 1957 Adrenal
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Cite this page: Hill, M.A. 2017 Embryology BGDA Practical 7 - Week 6. Retrieved September 24, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/BGDA_Practical_7_-_Week_6

What Links Here?
© Dr Mark Hill 2017, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
  1. Leila Taher, Nicole M Collette, Deepa Murugesh, Evan Maxwell, Ivan Ovcharenko, Gabriela G Loots Global gene expression analysis of murine limb development. PLoS ONE: 2011, 6(12);e28358 PubMed 22174793 | PMC3235105 | PLoS One.