Template:First Trimester Timeline collapsable table
|First Trimester Timeline|
|Timeline human development | First Trimester Timeline | Second Trimester Timeline | Third Trimester Timeline|
|Week 1 (GA Clinical Week 3)|
|Secretory Phase||Fertilization, Secretory Phase|
|Stage 2||Morula, Blastula|
|Stage 3||Blastocyst Hatching (zona pellucida lost)|
|Late Secretory, Blastocyst (free floating)|
|Week 2 (GA Clinical Week 4)|
|Stage 6||Chorionic Cavity|
|Week 3 (GA Clinical Week 5)|
|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 (1 - 3 somite pairs).
Neural the 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.|
|Week 4 (GA Clinical Week 6)|
Neural Crest differentiation at spinal cord level from day 22 until day 26
Neural Crest migration of vagal level neural crest cells begins (7-10 somite stage)
Neural rostral neural tube forms 3 primary brain vesicles (week 4)
Respiratory Week 4 - laryngotracheal groove forms on floor foregut.
|Heart begins to beat in Humans by day 22-23, first functioning embryonic organ formed.|
Thyroid thyroid median endodermal thickening in the floor of pharynx
Neural rostral (or cephalic) neuropore closes within a few hours; closure is bidirectional, it takes place from the dorsal and terminal lips and may occur in two areas simultaneously. The two lips, however, behave differently.
Pituitary Week 4 hypophysial pouch, Rathke's pouch, diverticulum from roof
Neural caudal neuropore takes a day to close (closure is approximately at future somitic pair 31/sacral vertebra 2)
Neural secondary neurulation begins
Neural Crest vagal neural crest enter the foregut (20-25 somite stage)
|Stage 13|| Neural the neural tube is normally completely closed, ventricular system now separated from amniotic fluid. Neural crest at spinal level is segregating, and spinal ganglia are in series with the somites. Spinal cord ventral roots beginning to develop.
telencephalon cavity appears
Neural - Vascular Development - hindbrain is supplied by two parallel neural arteries (or channels) that obtain their blood supply from carotid-vertebrobasilar anastomoses given by the pharyngeal arch arteries; trigeminal artery, the otic artery, hypoglossal artery, and the proatlantal artery.
Skin 4 weeks - simple ectoderm epithelium over mesenchyme
Skin 1-3 months ectoderm- germinative (basal) cell repeated division of generates stratified epithelium; mesoderm- differentiates into connective tissue and blood vessels
|Week 5 (GA Clinical Week 7)|
|Pituitary Week 5 elongation, contacts infundibulum, diverticulum of diencephalon
Heart Week 5 septation starts, atrial and ventricular
Respiratory Week 5 left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity)
Respiratory Week 5 to 17 lung histology - pseudoglandular
Hearing Week 5 cochlear part of otic vesicle elongates (humans 2.5 turns)
|Stage 14|| Placodes 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.
Ventricular System Subarachnoid space initially as irregular spaces on the ventral surface of the spinal cord. 16228957
Neural - Vascular Development - vertebral arteries form from transverse anastomoses between cervical intersegmental arteries, beginning with the proatlantal artery and proceeding downward to the 6th intersegmental artery,
|Vision 35 to 37 days retinal pigment present|
|Week 6 (GA Clinical Week 8)|
|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 - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes gustatory papilla, caudal midline near the foramen caecum (week 6 to 7 - nerve fibers approach the lingual epithelium)
|Stage 16|| Neural first parasympathetic ganglia, submandibular and ciliary, are identifiable
Neural - Vascular Development - development of the middle cerebral artery is first identified as small buds originating proximal to the anterior cerebral artery on the anterior division of the primitive internal carotid artery.
Limb 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.
|Heart separation of common cardiac outflow (aortic arch and pulmonary aorta)|
|Week 7 (GA Clinical Week 9)|
|Pancreas Week 7 to 20 pancreatic hormones secretion increases, small amount maternal insulin
Respiratory Week 7 - enlargement of liver stops descent of heart and lungs
Abdominal Wall separation of the myoblasts into distinct inner and outer layers, with unidirectional orientation. Abdominal wall thicker in the region where secondary structures were forming compared with the primary body wall region, dorsally outermost layer of connective tissue approximately half of this thickness.
Liver (stage 18 to 23) biliary ductules developed in periportal connective tissue
produces ductal plates that receive biliary capillaries
|Week 8 (GA Clinical Week 10)|
Head scalp vascular plexus visible
Limb upper limbs begin to rotate ventrally
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
|Gastrointestinal Tract anal membrane perforates|
Limb upper and lower limbs rotate
Intraembryonic Coelom pericardioperitoneal canals close
Abdominal Wall Myoblasts have reached the ventral midline and myotubes were present and oriented uniformly within all muscle groups. The rectus abdominis formed distinct bundles of muscle. Connective tissue layers comprised the majority of the thickness of the abdominal wall, outermost layer of connective tissue accounted for the majority of this thickness.
|Stage 22|| Neural neocortical fibres project to epithalamus, to dorsal thalamus, and to mesencephalon
Limb fingers and toes lengthen
|Genital 8 Weeks Testis - mesenchyme, interstitial cells (of Leydig) secrete testosterone, androstenedione
Genital 8 to 12 Weeks - hCG stimulates testosterone production
|Stage 23|| 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, 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
Abdominal Wall Rectus muscle forms 2 or 3 distinct layers with myotube orientation uniform in all muscles. The external oblique and internal oblique started to expand in thickness, transversus a thin layer of muscle.
|Week 8|| Stomach Week 8 - Gastrin containing cells in stomach antrum. Somatostatin cells in both the antrum and the fundus.
|Week 9 (GA Clinical Week 11)|
Hearing Week 9 - mesenchyme surrounding membranous labrynth (otic capsule) chondrifies
|Week 9 - CRL 43 mm, femur length 6 mm
uterus - paramesonephric ducts come into apposition with the urorectal septum and begin to fuse
|Week 10 (GA Clinical Week 12)|
Gastrointestinal Tract Week 10 intestines in abdomen
Pituitary growth hormone and ACTH detectable
Pancreas Week 10 glucagon (alpha) differentiate first, somatostatin (delta), insulin (beta) cells differentiate, insulin secretion begins
Tongue Week 10 shallow grooves above the taste bud primordium
Stomach Week 10 - Glucagon containing cells in stomach fundus.
Nail Development fingernails appear
Hearing - Outer Ear Development Week 10 - Meatal plug extends in a disc-like fashion, the meatus is boot-shaped with a narrow neck and the sole of the meatal plug spreading widely to form the future tympanic membrane medially. Proximal portion of the neck starts to be resorbed.
Hearing - Inner Ear Development Week 10 - neural-crest-derived melanocytes migrate into the cochlea. They penetrate the basement membrane of the lateral wall epithelium and develop into the intermediate cells of the stria vascularis.
|Week 10 - CRL 55 mm, femur length 9 mm, biparietal diameter 17 mm|
|Week 11 (GA Clinical Week 13)|
Thyroid colloid appearance in thyroid follicles, iodine and thyroid hormone (TH) synthesis
Stomach Week 11 - Serotonin containing cells in both the antrum and the fundus.
|Week 11 - CRL 68 mm, femur length 12 mm, biparietal diameter 20 mm|