Week 8

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Key Events of Human Development during the eighth week (week 8) following fertilization or clinical gestational age GA week 10 (LMP). At the end of the embryonic period in week 8 about 90% of the adult anatomical structures have formed.

This is the final week of embryonic development, after this we describe all future development as Fetal Development.

Carnegie Stages: 20 | 21 | 22 | 23
Week 8 Links: stage 20 | stage 21 | stage 22 | stage 23 | Lecture - Head | Outflow Tract | Heart Valves | Category:Week 8 | GA week 10
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

Timeline

Template:8 Embryo Stages and Events table

Day
Stage
Event
50
Stage 20 Stage20 bf1c.jpg

Head scalp vascular plexus visible

Limb upper limbs begin to rotate ventrally

Neural - amygdaloid body has at least four individual nuclei[1]

oculomotor nerve shows a dorsolateral and a ventromedial portion

rhombic lip (rhombencephalon) formation of the cerebellum (intermediate layer) and of the cochlear nuclei

cerebellum cell layer (future Purkinje cells) develops

choroid plexuses of the fourth and lateral ventricles

51
  Gastrointestinal Tract anal (cloacal) membrane perforates
52
Stage 21 Stage21 bf1c.jpg

Neural - cortical plate appears in the area of future insula[2]

Limb upper and lower limbs rotate

Intraembryonic Coelom pericardioperitoneal canals close

53
   
54
Stage 22 Stage22 bf1c.jpgNeural - neocortical fibres project to epithalamus, to dorsal thalamus, and to mesencephalon[2]

Limb fingers and toes lengthen

Sense - Smell Stage 22 to early fetal period - migratory streams of neurons from the subventricular zone of the olfactory bulb towards the future claustrum[3]

55
Genital 8 Weeks Testis - mesenchyme, interstitial cells (of Leydig) secrete testosterone, androstenedione

Genital 8 to 12 Weeks - hCG stimulates testosterone production

Tongue Week 8 - nerves penetrate epitheilai basal lamina and synapse with undifferentiated, elongated, epithelial cells (taste bud progenitor cell)

56
Stage 23 Stage23 bf1c.jpg Stage 23 defines the end of the embryonic (organogenesis) period

Mesoderm heart prominence, ossification continues

Head nose, eye, external acoustic meatus, eyelids, external ears, rounded head

Body - straightening of trunk, heart, liver, umbilical cord, intestines herniated at umbilicus

Limb upper limbs longer and bent at elbow, hands and feet turned inward, foot with separated digits, wrist, hand with separated digits

Extraembryonic Coelom chorionic cavity is now lost by fusion with the expanding amniotic cavity

Neural - rhombencephalon, pyramidal decussation present, nuclei and tracts similar to those present in the newborn cerebellum present as only a plate connected to midbrain and hindbrain through fibre bundles[4]

Axial Skeleton vertebral column 33 or 34 cartilaginous vertebrae (20-33 mm in total length), vertebral pedicles, articular and transverse processes identifiable (no spinous processes)[5]

 
Week 8 Stomach - Week 8 - Gastrin containing cells in stomach antrum. Somatostatin cells in both the antrum and the fundus.

References

  1. F Müller, R O'Rahilly The human brain at stages 18-20, including the choroid plexuses and the amygdaloid and septal nuclei. Anat. Embryol.: 1990, 182(3);285-306 PubMed 2268071
  2. 2.0 2.1 F Müller, R O'Rahilly The human brain at stages 21-23, with particular reference to the cerebral cortical plate and to the development of the cerebellum. Anat. Embryol.: 1990, 182(4);375-400 PubMed 2252222
  3. F Müller, R O'Rahilly Olfactory structures in staged human embryos. Cells Tissues Organs (Print): 2004, 178(2);93-116 PubMed 15604533
  4. F Müller, R O'Rahilly The human rhombencephalon at the end of the embryonic period proper. Am. J. Anat.: 1990, 189(2);127-45 PubMed 2244584
  5. R O'Rahilly, F Muller, D B Meyer The human vertebral column at the end of the embryonic period proper. 1. The column as a whole. J. Anat.: 1980, 131(Pt 3);565-75 PubMed 7216919



Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 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


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Cite this page: Hill, M.A. 2017 Embryology Week 8. Retrieved April 27, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/Week_8

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