BGDA Practical 7 - Week 5

Introduction

Key Events of Human Development during the fifth week (week 5) following fertilization or clinical week 7 (GA week 7).

By this week there are many different organs and tissues differentiating throughout the embryo, only a few select systems will be discussed.

Heart - septation starts, atrial septation and ventricular septation
Vascular - 3 vascular systems (systemic, placental, vitelline) extensively remodelled


Heart Looping	Heart Realign	Heart Atrial Septation	Heart Outflow Septation

See also Heart Development Timeline - Basic Cardiac Embryology

left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity)

Heart Development Timeline (see Basic Cardiac Embryology)

Respiratory Tract (stage 13)

Week 4 - laryngotracheal groove forms on floor foregut.

Week 5 - left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity)

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

Week 7 - enlargement of liver stops descent of heart and lungs.

Month 3-6 - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant.

Month 7 - respiratory bronchioles proliferate and end in alveolar ducts and alveolar sacs.

Neural tube - brain flexures (stage 13)

Neural - 5 secondary vesicles

Rapid growth folds the neural tube forming 3 brain flexures:

Day	Stage	Event
29		Pituitary - Week 5 elongation, contacts infundibulum, diverticulum of diencephalon Heart - Week 5 septation starts, atrial and ventricular Respire - Week 5 left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity) Respire - Week 5 to 17 lung histology - pseudoglandular Hearing - Week 5 cochlear part of otic vesicle elongates (humans 2.5 turns)
30
31
32	Stage 14	Ectoderm - sensory placodes, lens pit, otocyst, nasal placode, primary/secondary vesicles, fourth ventricle of brain Mesoderm - continued segmentation of paraxial mesoderm (somite pairs), heart prominence Head - 1st, 2nd and 3rd pharyngeal arch, forebrain, site of lens placode, site of otic placode, stomodeum Body - heart, liver, umbilical cord, mesonephric ridge visible externally as bulges. Limb - upper and lower limb buds growing Neural - first appearance of the future cerebral hemispheres. Cerebellar plate differentiated to an intermediate layer, and future rhombic lip identifiable^[1] Liver hepatic gland and its vascular channels enlarge, hematopoietic function appears^[2]
33	Stage 15	Neural - cranial nerves (except olfactory and optic) are identifiable in more advanced embryos^[3]
34
35		Eye - 35 to 37 days retinal pigment present