Talk:Timeline human development

Collapsible Tables

First Trimester Timeline

First Trimester Timeline  Links: human timeline | first trimester timeline | second trimester timeline | third trimester timeline Gestational Day GA Stage Event 1 Menstrual Phase menstrual cycle changes: uterus endometrium (loss), ovary (follicle development) 2   3   4     5 Proliferative Phase menstrual cycle changes: uterus endometrium (proliferation), ovary (Follicle Development) 6 7     8 Proliferative Phase 9   menstrual cycle - Mid proliferative 10     11     12     13   menstrual cycle - Late Proliferative 14 ovulation Capacitation Fertilization Day Stage Event 1 Secretory Phase Stage 1 fertilization, zygote, Secretory Phase 2 Stage 2 morula, Blastula 3   blastocyst 4 Stage 3 blastocyst Hatching (zona pellucida lost) 5   Late Secretory, blastocyst (free floating) 6 Stage 4 Adplantation 7 Stage 5 8 implantation 9     10     11     12     13 Stage 6 Chorionic Cavity 14     Day Stage Event 15 16 Stage 7 gastrulation, ectoderm, mesoderm, endoderm 17     18 Stage 8 neural neurogenesis, neural groove and folds are first seen 19   20 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 Neural Crest mesencephalic neural crest is visible 21   heart cardiogenesis, week 3 begins as paired heart tubes. Day Stage Event 22 Stage 10 Neural Crest differentiation at spinal cord level from day 22 until day 26 neural folds begin to fuse near the junction between brain and spinal cord, when Neural Crest cells are arising mainly from the neural ectoderm Neural Crest trigeminal, facial, and postotic ganglia components visible 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. 23   heart begins to beat in Humans by day 22-23, first functioning embryonic organ formed. 24 Stage 11 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. ventricular Optic ventricle appears and the neural groove/tube space is initially filled with amniotic fluid. 25 Stage 12 pituitary Week 4 hypophysial pouch, Rathke's pouch, diverticulum from roof liver septum transversum forming liver stroma and hepatic diverticulum forming hepatic trabeculae neural caudal neuropore takes a day to close (closure is approximately at future somitic pair 31/sacral vertebra 2) neural secondary neurulation begins ventricular onset of the ventricular system and separates the ependymal from the amniotic fluid. neural crest cardiac crest, neural crest from rhombomeres 6 and 7 that migrates to pharyngeal arch 3 and from there the truncus arteriosus neural crest vagal neural crest enter the foregut (20-25 somite stage) 26     27     28 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. liver epithelial cord proliferation enmeshing stromal capillaries smell Crest comes from the nasal plates integumentary 4 weeks - simple ectoderm epithelium over mesenchyme integumentary 1-3 months ectoderm- germinative (basal) cell repeated division of generates stratified epithelium; mesoderm- differentiates into connective tissue and blood vessels vision Optic vesicle lies close to the surface ectoderm. The surface ectoderm overlying the optic vesicle, in response to this contact, has thickened to form the lens placode. Diaphragm - pleuroperitoneal fold (PPF) first discernible in human embryos (CRL 6mm). 29 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) 30     31     32 Stage 14 Placodes sensory placodes, lens pit, otocyst, nasal placode, primary/secondary vesicles, fourth ventricle of brain somite 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. Abdominal Wall mesoderm of the primary body wall coalesced in the ventral midline to create the abdominal cavity. neural first appearance of the future cerebral hemispheres. Cerebellar plate differentiated to an intermediate layer, and future rhombic lip identifiable Neural - Vascular Development - basilar artery forms from the consolidation of the neural arteries. ventricular Subarachnoid space initially as irregular spaces on the ventral surface of the spinal cord. liver hepatic gland and its vascular channels enlarge, hematopoietic function appears vision lens placode is indented by the lens pit. 33 Stage 15 neural cranial nerves (except olfactory and optic) are identifiable in more advanced embryos 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 lens pit is closed. The lens vesicle and optic cup lie close to the surface ectoderm and appear to press against the surface. 34   35   vision 35 to 37 days retinal pigment present 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 respiratory 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 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. Abdominal Wall muscle cell migration about 25% of the hemicircumference of the abdominal cavity, the lateral plate mesoderm has become more condensed and thicker in the area around the myoblasts. 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. Eyelid prior to the development of the eyelids, one small sulcus or groove forms above the eye (eyelid groove) and another below it. 38     39     40     41 Stage 17 neural telencephalon areas of the future archicortex, paleocortex, and neocortex, visible. Beginning of future choroid plexus ventricular primordium of the epidural space appears first on the ventral part of the vertebral canal and develops rostro-caudally smell olfactory nerve fibres enter the brain Eyelid sulcus (groove) above and below eye deepen and eyelid folds develop (below first and then above) Diaphragm - pleuroperitoneal fold (PPF) no longer separated from the diaphragm (CRL 14mm) Abdominal Wall muscle cells now migrated approximately 50% of the distance to the ventral midline, inner and outer layers were not discernible yet. 42   heart separation of common cardiac outflow (aortic arch and pulmonary aorta) Day Stage Event 43 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 44 Stage 18 limb bone forms by endochondrial ossification and throughout embryo replacement of cartilage with bone (week 5-12). neural smell vomeronasal fibres and nervus terminalis liverobturation due to epithelial proliferation, bile ducts became reorganized, continuity between liver cells and gut ventricular duramater appears and spaces surround the circumference of the spinal cord, which coalesce and contain many blood vessels. Female uterus opening of the paramesonephric (Müllerian) duct to the coelomic cavity formed as an invagination of the coelomic epithelium 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. 45     liver (stage 18 to 23) biliary ductules developed in periportal connective tissue produces ductal plates that receive biliary capillaries 46     47     48 Stage 19 vision - (stage 19 -22) eyelid folds develop into the eyelids and cover more of the eye as the palpebral fissure takes shape. The upper and the lower eyelids meet at the outer canthus in Stage 19. cardiovascular arterial system Chapter 18 fig. 447). aortic arches stages 11–19 (figs. 29–40). heart fusion of aortic and mitral endocardial cushion material Respiratory - first generation of subsegmental bronchi now complete, see bronchial tree reconstruction (plates 3 and 4). gastrointestinal tract - anal membrane defined. renal - Cloacal membrane ruptures from urinary pressure at stage 18 or stage 19, genital testis - Rete testis develops from the seminiferous cords at stages 19–23, and tunica albuginea forms.. ovary - Rete ovarii cords are developing. uterus Müllerian duct grows independently from the invagination of the coelomic epithelium during stages 19-23 musculoskeletal Sternum right and left sternal bars are present. (figs. 7-17 and 7-22) Abdominal Wall segregation of the myoblasts into four distinct muscle groups with unidirectional orientation of myoblasts. Myoblast migrated over half of the distance to the ventral midline, abdominal wall thickest where the muscles migrated and the outermost layer of connective tissue comprises approximately half of the total thickness of the abdominal wall. Rectus muscle completely separated after migrating over half the distance to the midline. neural rhombencephalon migration for olivary and arcuate nuclei begins. choroid plexus of the fourth ventricle present. stria medullaris thalami reaches the habenular nuclei. habenular commissure begins to develop. accessory olivary nucleus appears Neural - Vascular Development - middle cerebral artery becomes more prominent, the plexi fuse into a single artery and further branches pierce the cerebral hemisphere. 49     50 Stage 20 Head scalp vascular plexus visible limb upper limbs begin to rotate ventrally neural amygdaloid body has at least four individual nuclei 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 Eyelid the inner canthus is established. 51   gastrointestinal tract anal membrane perforates 52 Stage 21 neural cortical plate appears in the area of future insula Neural - Vascular Development - formation of the anterior communicating artery. 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. 53     54 Stage 22 neural neocortical fibres project to epithalamus, to dorsal thalamus, and to mesencephalon limb fingers and toes lengthen smell Stage 22 to early fetal period - migratory streams of neurons from the subventricular zone of the olfactory bulb towards the future claustrum Uterus Vagina fused duct (uterovaginal canal) bifurcated at the caudal portion at Carnegie stages 22 and 23 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 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 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) 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. Genital - Female Development paired paramesonephric (Müllerian) ducts contact each other and are fused into a single tube that separates again and returns to the mesonephric (Wolffian) ducts. The paramesonephric ducts have not yet reached the urogenital sinus. 57-63 Week 9 Beginning of Fetal Development Historic Embryology In 1949 the embryologist George Streeter used the replacement of cartilage within the humerus by bone marrow as an arbitrary definition of the embryo to fetus transition. "If the onset can be recognized in a given specimen, that specimen is straightway classed as a fetus." CRL 43 mm, femur length 6 mm 9 weeks CRL 50 mm - genital genitalia in both sexes look identical uterus - paramesonephric ducts come into apposition with the urorectal septum and begin to fuse Day Stage Event 64 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 outer ear 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. inner ear 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. 65     66   67     68   69     70 Week 10 - CRL 55 mm, femur length 9 mm, biparietal diameter 17 mm Day Stage Event neural - Cerebrum appearance of the first sulcus (week 11-15, GA 13-17 weeks) 71 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. 72     73   74     75   76     77 Week 11 - CRL 68 mm, femur length 12 mm, biparietal diameter 20 mm Systems   Systems: bone timeline | eye neural crest timeline | heart abnormality timeline | hearing EAM timeline | muscle timeline | ovary timeline | placental villi timeline | shoulder timeline | smell timeline | spleen timeline | ventricular timeline

Second Trimester Timeline

Second Trimester Timeline  Links: human timeline | first trimester timeline | second trimester timeline | third trimester timeline Week Stage Event 12 Clinical second trimester Week 12 - CRL 85 mm, femur length 15 mm, biparietal diameter 25 mm Hearing Week 12-16 - Capsule adjacent to membranous labrynth undegoes vacuolization to form a cavity (perilymphatic space) around membranous labrynth and fills with perilymph Genital male and female external genital differences observable Respiratory Month 3-6 - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant Tongue Week 12 - first differentiated epithelial cells (Type II and III) Genital female genital canal (80 days) formed with absorption of the median septum 13   tongue Week 12 to 13 - maximum synapses between cells and afferent nerve fibers hearing outer ear Week 13 - Meatal plug disc-like, innermost surface in contact with the primordial malleus, contributes to the formation of the tympanic membrane.   14 tongue Week 14 to 15 - taste pores develop, mucous ovary 100 days - primary follicles present nail toenails appear Head Development facial skeleton remodelling begins Hearing - Inner Ear Development Week 14 GA 16 - neural-crest-derived melanocytes, now intermediate cells of the stria vascularis, tightly integrate with Na+ /K+ -ATPase-positive marginal cells, which started to express KCNQ1 in their apical membrane. 15   Pancreas glucagon detectable in fetal plasma. spleen Week 15 -alpha-smooth muscle actin (alpha-SMA)-positive reticulum cells scattered around the arterioles. 16 14 cm Hearing Week 16-24 - Centres of ossification appear in remaining cartilage of otic capsule form petrous portion of temporal bone. Continues to ossify to form mastoid process of temporal bone. pituitary adenohypophysis fully differentiated respiratory Week 16 to 25 lung histology - canalicular Hearing - Outer Ear Development Week 16.5 - External auditory meatus is fully patent throughout its length, lumen is still narrow and curved. Hearing - Inner Ear Development Week 16 GA 18 - cells in the outer sulcus express KCNJ10 and gap junction proteins GJB2/CX26 and GJB6/CX30, but these are not expressed in the spiral ligament. gap junction cartoon neural - Cerebrum development of the periinsular sulci (week 16-17, GA 18-19 weeks) integumentary 4 months - basal cell- proliferation generates folds in basement membrane; neural crest cells- (melanocytes) migrate into epithelium; embryonic connective tissue- differentiates into dermis, a loose ct layer over a dense ct layer. Beneath the dense ct layer is another loose ct layer that will form the subcutaneous layer. Ectoderm contributes to nails, hair follictles and glands. Nails form as thickening of ectoderm epidermis at the tips of fingers and toes. These form germinative cells of nail field. Cords of these cells extend into mesoderm forming epithelial columns. These form hair follocles, sebaceous and sweat glands. primary follicles begin to form in the ovary and are characterized by an oocyte glandular urethra forms and skin folds present 17 Neural - Brain development histology week 17 Cerebellum Magnetic Resonance Imaging (MRI) can study the developing cerebellum from 17 to 18 weeks (GA 19 to 20 weeks). tooth Week 17 - First papilla of the permanent dentition appear (first molar) immediately behind the second milk molar, milk teeth are well advanced (Fetus 180 mm). 18 tongue Week 18 - substance P detected in dermal papillae, not in taste bud primordia integumentary vernix caseosa covers skin spleen Week 18 - alpha-SMA-positive reticulum cells increase in number and began to form a reticular framework. An accumulation of T and B lymphocytes occurred within the framework, and a primitive white pulp was observed around the arterioles. Hearing - Outer Ear Development week 18 - External auditory meatus is already fully expanded to its complete form. neural - Cerebrum central sulci and opercularization of the insula (week 18-20, GA 20-22 weeks) 19   neural week 19 neuronal migration ends and the radial glial cells that aided the migration now become transformed into astrocytes and astrocytic precursors. 20   pituitary week 20 to 24 growth hormone levels peak, then decline integumentary lanugo, skin hair integumentary 5 months - Hair growth initiated at base of cord, lateral outgrowths form associated sebaceous glands; Other cords elongate and coil to form sweat glands; Cords in mammary region branch as they elongate to form mammary glands. touch pacinian corpuscle begin to develop 21     22   Neural brain cortical sulcation - sylvian fissure, interhemispheric fissure, callosal sulcus, parietooccipital fissure, and hippocampic fissures present spleen - Week 22 - antigenic diversity of the reticular framework was observed, and T and B lymphocytes were segregated in the framework. T lymphocytes were sorted into the alpha-smooth muscle actin-positive reticular framework, and the periarteriolar lymphoid sheath (PALS) was formed around the arteriole. B lymphocytes aggregated in eccentric portions to the PALS and formed the lymph follicle (LF). The reticular framework of the LF was alpha-SMA-negative. neural - Cerebrum covering of the posterior insula (week 22-24, GA 24-26 weeks) 23     24   respiratory Week 24 to 40 lung histology - terminal sac spleen Week 24 - marginal zone appeared in the alpha-smooth muscle actin-positive reticular framework around the white pulp. tooth Week 24 - Permanent incisors and canines appear. Earliest potential survival expected if born ovary follicles can consist of growing oocytes surrounded by several layers of granulosa cells 25   respiratory end month 6 alveolar cells type 2 appear and begin to secrete surfactant neural - Cerebrum closure of the laeteral sulcus (Sylvian fissure or lateral fissure) (week 25-26, GA 27-28 weeks) 26   touch pacinian corpuscle well developed Systems   Systems: bone timeline | eye neural crest timeline | heart abnormality timeline | hearing EAM timeline | muscle timeline | ovary timeline | placental villi timeline | shoulder timeline | smell timeline | spleen timeline | ventricular timeline

Third Trimester Timeline

Third Trimester Timeline Links: human timeline | first trimester timeline | second trimester timeline | third trimester timeline Week Stage Event Clinical third trimester hearing 3rd Trimester - vibration acoustically of maternal abdominal wall induces startle respone in fetus. 27   28   respiratory Month 7 - respiratory bronchioles proliferate and end in alveolar ducts and sacs 29   tooth Week 29 - Permanent premolars (correspond to the milk molars) appear. 30     Genital male gonad (testes) descending 31   32   nail fingernails reach digit tip 33   neural brain cortical sulcation - primary sulci present 34   neural brain cortical sulcation - insular, cingular, and occipital secondary sulci present 35     36   Nail Development toenails reach digit tip Lens Development - lens growth and interocular distance plateaus after 36 weeks of gestation 37     38 Birth Clinical Week 40 Heart pressure difference closes foramen ovale leaving a fossa ovalis thyroid TSH levels increase, thyroxine (T3) and T4 levels increase to 24 h, then 5-7 days postnatal decline to normal levels adrenal - zona glomerulosa, zona fasiculata present Systems   Systems: bone timeline | eye neural crest timeline | heart abnormality timeline | hearing EAM timeline | muscle timeline | ovary timeline | placental villi timeline | shoulder timeline | smell timeline | spleen timeline | ventricular timeline

primordial germ cell
oocyte		spermatozoa
zygote
morula
blastocyst
trophectoderm		inner cell mass
syncytiotrophoblast	cytotrophoblast	hypoblast	epiblast
			ectoderm  mesoderm  endoderm  primordial germ cell

Developmental Map

1. Gametes Meiosis
   1. Spermatozoa
   2. Oocyte Menstrual Cycle
      1. Zona pellucida
      2. Granulosa cell
2. Zygote Fertilization Mitosis
3. Morula Mitosis
4. Blastocyst
   1. Trophoblast
      1. Syncitotrophoblast
      2. Cytotrophoblast
      3. Extravillous Cytotrophoblasts
   2. Embryoblast
      1. Epiblast Gastrulation
         1. Ectoderm
            1. Neural Plate (Central Nervous System)
               1. Brain
               2. Spinal Cord
            2. Neural Crest
               1. Sensory Ganglia (Dorsal Root Ganglia)
               2. Sympathetic Ganglia
            3. Placodes
            4. Surface Ectoderm (Epidermis)
         2. Mesoderm
            1. Axial Mesoderm (Notochord)
            2. Paraxial Mesoderm
               1. Unsegmented Paraxial Mesoderm
               2. Segmented Paraxial Mesoderm (Somites)
                  1. Dermamyotome
                     1. Dermatome
                     2. Myotome
                        1. Epaxial
                        2. Hypaxial
                           1. Trunk Muscles
                           2. Limb Muscles
                  2. Sclerotome
                     1. Vertebra
                     2. Intervertebral Disc
            3. Intermediate Mesoderm
               1. Renal
               2. Genital
            4. Lateral Plate Mesoderm
               1. Somatic Mesoderm
               2. Intraembryonic Coelom
               3. Splanchnic Mesoderm
         3. Endoderm
         4. Germ Cells
      2. Hypoblast

Links are to the original UNSW Embryology webpages: Timeline Human Development (simple) | Timeline Human Development (detailed)

From a cell to a newborn infant in 9 months

Neural Crest

Links: Neural Crest Development

The development of the neural crest in the human

J Anat. 2007 Sep;211(3):335-51.

O'Rahilly R, Müller F.

School of Medicine, University of California, Davis, California, USA.

Abstract

The first systematic account of the neural crest in the human has been prepared after an investigation of 185 serially sectioned staged embryos, aided by graphic reconstructions. As many as fourteen named topographical subdivisions of the crest were identified and eight of them give origin to ganglia (Table 2). Significant findings in the human include the following.

(1) An indication of mesencephalic neural crest is discernible already at stage 9, and trigeminal, facial, and postotic components can be detected at stage 10.

(2) Crest was not observed at the level of diencephalon 2. Although pre-otic crest from the neural folds is at first continuous (stage 10), crest-free zones are soon observable (stage 11) in Rh.1, 3, and 5.

(3) Emigration of cranial neural crest from the neural folds at the neurosomatic junction begins before closure of the rostral neuropore, and later crest cells do not accumulate above the neural tube.

(4) The trigeminal, facial, glossopharyngeal and vagal ganglia, which develop from crest that emigrates before the neural folds have fused, continue to receive contributions from the roof plate of the neural tube after fusion of the folds.

(5) The nasal crest and the terminalis-vomeronasal complex are the last components of the cranial crest to appear (at stage 13) and they persist longer.

(6) The optic, mesencephalic, isthmic, accessory, and hypoglossal crest do not form ganglia. Cervical ganglion 1 is separated early from the neural crest and is not a Froriep ganglion.

(7) The cranial ganglia derived from neural crest show a specific relationship to individual neuromeres, and rhombomeres are better landmarks than the otic primordium, which descends during stages 9-14.

(8) Epipharyngeal placodes of the pharyngeal arches contribute to cranial ganglia, although that of arch 1 is not typical.

(9) The neural crest from rhombomeres 6 and 7 that migrates to pharyngeal arch 3 and from there rostrad to the truncus arteriosus at stage 12 is identified here, for the first time in the human, as the cardiac crest.

(10) The hypoglossal crest provides cells that accompany those of myotomes 1-4 and form the hypoglossal cell cord at stages 13 and 14.

(11) The occipital crest, which is related to somites 1-4 in the human, differs from the spinal mainly in that it does not develop ganglia.

(12) The occipital and spinal portions of the crest migrate dorsoventrad and appear to traverse the sclerotomes before the differentiation into loose and dense zones in the latter.

(13) Embryonic examples of synophthalmia and anencephaly are cited to emphasize the role of the neural crest in the development of cranial ganglia and the skull.

PMID 17848161

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2375817

http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7580.2007.00773.x/abstract

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Week -2

(GA Clinical Week 1)

Day	Menstrual cycle	Event
1	Menstrual Phase	Menstrual Cycle changes: Uterine endometrium (loss), Ovary (Follicle Development)
2
3
4
5	Proliferative Phase	Menstrual Cycle changes: Uterine endometrium (proliferation), Ovary (Follicle Development)
6
7

Week -1

(Clinical Week 2)

Day	Menstrual cycle	Event
8	Proliferative Phase
9		Menstrual Cycle - Mid proliferative
10
11
12
13		Menstrual Cycle - Late Proliferative
14	Ovulation Capacitation

Week 1

Week 1 (GA Clinical week 3)
Week 1 of fertilisation age begins on the day of fertilisation, while the clinical gestational age (GA) is measured from the first day of the last menstrual period (LMP). Concepts Gametogenesis (Oocyte and Spermatozoa), Meiosis, Menstrual Cycle Fertilisation, Zygote, Zona Pellucida, Polar Body, Mitosis, X inactivation Blastomeres, Morula, Blastocyst (Inner cell mass and Trophoblast), Twinning	Week 1 summary
Week 1 Timeline   Day Carnegie Stage Concepts 1 1 Zygote, Menstrual Cycle - Secretory Phase 2 2 Morula, Mitosis 3-5 3 Blastocyst
Week 1 Links: stage 1 | stage 2 | stage 3 | menstrual cycle | fertilization | zygote | morula | blastocyst | Lecture - Fertilization | meiosis | mitosis | Lecture - Week 1 and 2 | menstrual cycle | oocyte | spermatozoa | twinning | Genetic risk maternal age | Trisomy 21 | Trisomy 18 | Trisomy 13 | hydatidiform mole | GA week 3

Day	Stage	Event
1	Secretory Phase Stage 1	Fertilization, Secretory Phase
2	Stage 2	Morula, Blastula
3		Blastocyst Development
4	Stage 3	Blastocyst Hatching (zona pellucida lost)
5		Late Secretory, Blastocyst (free floating)
6	Stage 4	Adplantation
7	Stage 5

Week 2

Week 2 (GA Clinical Week 4)

Day	Stage	Event
8		Implantation
9
10
11
12
13	Stage 6	Chorionic Cavity
14

Week 3

Week 3 (GA Clinical Week 5)

Day	Stage	Event
15
16	Stage 7
17
18	Stage 8	Neural neurogenesis, neural groove and folds are first seen
19
20	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 Neural Crest mesencephalic neural crest is visible
21		Heart cardiogenesis, week 3 begins as paired heart tubes.

Week 4

Week 4 (GA Clinical Week 6)

Day	Stage	Event
22	Stage 10	Neural Crest differentiation at spinal cord level from day 22 until day 26 Neural neural folds begin to fuse near the junction between brain and spinal cord, when Neural Crest cells are arising mainly from the neural ectoderm Neural Crest trigeminal, facial, and postotic ganglia components visible 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.
23		Heart begins to beat in Humans by day 22-23, first functioning embryonic organ formed.
24	Stage 11	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. Ventricular System Optic ventricle appears and the neural groove/tube space is initially filled with amniotic fluid.
25	Stage 12	Pituitary Week 4 hypophysial pouch, Rathke's pouch, diverticulum from roof Liver septum transversum forming liver stroma and hepatic diverticulum forming hepatic trabeculae Neural caudal neuropore takes a day to close (closure is approximately at future somitic pair 31/sacral vertebra 2) Neural secondary neurulation begins Ventricular System onset of the ventricular system and separates the ependymal from the amniotic fluid. Neural Crest cardiac crest, neural crest from rhombomeres 6 and 7 that migrates to pharyngeal arch 3 and from there the truncus arteriosus Neural Crest vagal neural crest enter the foregut (20-25 somite stage)
26
27
28	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. Liver epithelial cord proliferation enmeshing stromal capillaries Smell Crest comes from the nasal plates 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 Vision Optic vesicle lies close to the surface ectoderm. The surface ectoderm overlying the optic vesicle, in response to this contact, has thickened to form the lens placode. Diaphragm - pleuroperitoneal fold (PPF) first discernible in human embryos (CRL 6mm).

Week 5

Week 5 (GA Clinical Week 7)

Day	Stage	Event
29		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)
30
31
32	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. Abdominal Wall mesoderm of the primary body wall coalesced in the ventral midline to create the abdominal cavity. Neural first appearance of the future cerebral hemispheres. Cerebellar plate differentiated to an intermediate layer, and future rhombic lip identifiable Neural - Vascular Development - basilar artery forms from the consolidation of the neural arteries. Ventricular System Subarachnoid space initially as irregular spaces on the ventral surface of the spinal cord. 16228957 Liver hepatic gland and its vascular channels enlarge, hematopoietic function appears Eye - Lens the lens placode is indented by the lens pit.
33	Stage 15	Neural cranial nerves (except olfactory and optic) are identifiable in more advanced embryos 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, Eye - Lens the lens pit is closed. The lens vesicle and optic cup lie close to the surface ectoderm and appear to press against the surface.
34
35		Vision 35 to 37 days retinal pigment present

Week 6

Week 6 (GA Clinical Week 8)

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 - 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)
37	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. Abdominal Wall muscle cell migration about 25% of the hemicircumference of the abdominal cavity, the lateral plate mesoderm has become more condensed and thicker in the area around the myoblasts. 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. Eyelid prior to the development of the eyelids, one small sulcus or groove forms above the eye (eyelid groove) and another below it.
38
39
40
41	Stage 17	Neural telencephalon areas of the future archicortex, paleocortex, and neocortex, visible. Beginning of future choroid plexus primordium of the epidural space appears first on the ventral part of the vertebral canal and develops rostro-caudally Smell olfactory nerve fibres enter the brain Eyelid sulcus (groove) above and below eye deepen and eyelid folds develop (below first and then above) Diaphragm - pleuroperitoneal fold (PPF) no longer separated from the diaphragm (CRL 14mm) Abdominal Wall muscle cells now migrated approximately 50% of the distance to the ventral midline, inner and outer layers were not discernible yet.
42		Heart separation of common cardiac outflow (aortic arch and pulmonary aorta)

Week 7

Week 7 (GA Clinical Week 9)

Day	Stage	Event
43		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
44	Stage 18	Limb Bone forms by endochondrial ossification and throughout embryo replacement of cartilage with bone (week 5-12). Neural Smell vomeronasal fibres and nervus terminalis Liver obturation due to epithelial proliferation, bile ducts became reorganized, continuity between liver cells and gut Ventricular System duramater appears and spaces surround the circumference of the spinal cord, which coalesce and contain many blood vessels. Genital - Female Development opening of the paramesonephric (Müllerian) duct to the coelomic cavity formed as an invagination of the coelomic epithelium 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.
45		Liver (stage 18 to 23) biliary ductules developed in periportal connective tissue produces ductal plates that receive biliary capillaries
46
47
48	Stage 19	Vision - (stage 19 -22) eyelid folds develop into the eyelids and cover more of the eye as the palpebral fissure takes shape. The upper and the lower eyelids meet at the outer canthus in Stage 19. Cardiovascular arterial system Chapter 18 fig. 447). aortic arches stages 11–19 (figs. 29–40). Heart fusion of aortic and mitral endocardial cushion material</ref>Teal SI., Moore GW. and Hutchins GM. Development of aortic and mitral valve continuity in the human embryonic heart. (1986) Amer. J. Anat., 176:447-460.</ref> Respiratory - first generation of subsegmental bronchi now complete, see bronchial tree reconstruction (plates 3 and 4). Gastrointestinal - anal membrane defined. Renal - Cloacal membrane ruptures from urinary pressure at stage 18 or stage 19, Genital Testis - Rete testis develops from the seminiferous cords at stages 19–23, and tunica albuginea forms.. Ovary - Rete ovarii cords are developing. Uterus Müllerian duct grows independently from the invagination of the coelomic epithelium during stages 19-23 Musculoskeletal Sternum right and left sternal bars are present. (figs. 7-17 and 7-22) Abdominal Wall segregation of the myoblasts into four distinct muscle groups with unidirectional orientation of myoblasts. Myoblast migrated over half of the distance to the ventral midline, abdominal wall thickest where the muscles migrated and the outermost layer of connective tissue comprises approximately half of the total thickness of the abdominal wall. Rectus muscle completely separated after migrating over half the distance to the midline. Neural rhombencephalon migration for olivary and arcuate nuclei begins. choroid plexus of the fourth ventricle present. stria medullaris thalami reaches the habenular nuclei. habenular commissure begins to develop. accessory olivary nucleus appears Neural - Vascular Development - middle cerebral artery becomes more prominent, the plexi fuse into a single artery and further branches pierce the cerebral hemisphere.
49

Week 8

Week 8 (GA Clinical Week 10)

Day	Stage	Event
50	Stage 20	Head scalp vascular plexus visible Limb upper limbs begin to rotate ventrally Neural amygdaloid body has at least four individual nuclei 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 Eyelid the inner canthus is established.
51		Gastrointestinal Tract anal membrane perforates
52	Stage 21	Neural cortical plate appears in the area of future insula Neural - Vascular Development - formation of the anterior communicating artery. 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.
53
54	Stage 22	Neural neocortical fibres project to epithalamus, to dorsal thalamus, and to mesencephalon Limb fingers and toes lengthen Smell Stage 22 to early fetal period - migratory streams of neurons from the subventricular zone of the olfactory bulb towards the future claustrum Uterus Vagina fused duct (uterovaginal canal) bifurcated at the caudal portion at Carnegie stages 22 and 23
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	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 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) 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. Genital - Female Development paired paramesonephric (Müllerian) ducts contact each other and are fused into a single tube that separates again and returns to the mesonephric (Wolffian) ducts. The paramesonephric ducts have not yet reached the urogenital sinus.

Week 9

(GA Clinical Week 11)

Day	Stage	Event
57	Fetal Period	Hearing Week 9 - mesenchyme surrounding membranous labrynth (otic capsule) chondrifies Smell Embryonic/Fetal transition - localized incomplete lamination of the olfactory bulb
58
59
60
61
62
63		Week 9 - CRL 43 mm, femur length 6 mm 9 weeks CRL 50 mm - Genital genitalia in both sexes look identical uterus - paramesonephric ducts come into apposition with the urorectal septum and begin to fuse

Week 10

(GA Clinical Week 12)

Day	Stage	Event
64		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.
65
66
67
68
69
70		Week 10 - CRL 55 mm, femur length 9 mm, biparietal diameter 17 mm

Week 11

(GA Clinical Week 13)

Day	Stage	Event Neural - Cerebrum appearance of the first sulcus (week 11-15, GA 13-17 weeks)
71		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.
72
73
74
75
76
77		Week 11 - CRL 68 mm, femur length 12 mm, biparietal diameter 20 mm

Second Trimester

(GA Clinical Week 14) Second Trimester

Week	Stage	Event
12	Clinical second trimester	Week 12 - CRL 85 mm, femur length 15 mm, biparietal diameter 25 mm Hearing Week 12-16 - Capsule adjacent to membranous labrynth undegoes vacuolization to form a cavity (perilymphatic space) around membranous labrynth and fills with perilymph Genital male and female external genital differences observable Respiratory Month 3-6 - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant Tongue Week 12 - first differentiated epithelial cells (Type II and III) Genital female genital canal (80 days) formed with absorption of the median septum
13		Tongue Week 12 to 13 - maximum synapses between cells and afferent nerve fibers Hearing - Outer Ear Development Week 13 - Meatal plug disc-like, innermost surface in contact with the primordial malleus, contributes to the formation of the tympanic membrane.
14		Tongue Week 14 to 15 - taste pores develop, mucous Ovary Development 100 days - primary follicles present Nail Development toenails appear Head Development facial skeleton remodelling begins Hearing - Inner Ear Development Week 14 GA 16 - neural-crest-derived melanocytes, now intermediate cells of the stria vascularis, tightly integrate with Na+ /K+ -ATPase-positive marginal cells, which started to express KCNQ1 in their apical membrane.
15		Pancreas glucagon detectable in fetal plasma. Spleen Week 15 -alpha-smooth muscle actin (alpha-SMA)-positive reticulum cells scattered around the arterioles. Fetal Timeline
16	14 cm	Hearing Week 16-24 - Centres of ossification appear in remaining cartilage of otic capsule form petrous portion of temporal bone. Continues to ossify to form mastoid process of temporal bone. Pituitary adenohypophysis fully differentiated Respiratory Week 16 to 25 lung histology - canalicular Hearing - Outer Ear Development Week 16.5 - External auditory meatus is fully patent throughout its length, lumen is still narrow and curved. Hearing - Inner Ear Development Week 16 GA 18 - cells in the outer sulcus express KCNJ10 and gap junction proteins GJB2/CX26 and GJB6/CX30, but these are not expressed in the spiral ligament. gap junction cartoon Neural - Cerebrum development of the periinsular sulci (week 16-17, GA 18-19 weeks) Skin 4 months - basal cell- proliferation generates folds in basement membrane; neural crest cells- (melanocytes) migrate into epithelium; embryonic connective tissue- differentiates into dermis, a loose ct layer over a dense ct layer. Beneath the dense ct layer is another loose ct layer that will form the subcutaneous layer. Ectoderm contributes to nails, hair follictles and glands. Nails form as thickening of ectoderm epidermis at the tips of fingers and toes. These form germinative cells of nail field. Cords of these cells extend into mesoderm forming epithelial columns. These form hair follocles, sebaceous and sweat glands. primary follicles begin to form in the ovary and are characterized by an oocyte glandular urethra forms and skin folds present
17		Neural - Brain development histology week 17 Cerebellum Magnetic Resonance Imaging (MRI) can study the developing cerebellum from 17 to 18 weeks (GA 19 to 20 weeks). Tooth Development Week 17 - First papilla of the permanent dentition appear (first molar) immediately behind the second milk molar, milk teeth are well advanced (Fetus 180 mm).
18		Tongue Week 18 - substance P detected in dermal papillae, not in taste bud primordia Skin vernix caseosa covers skin Spleen Week 18 - alpha-SMA-positive reticulum cells increase in number and began to form a reticular framework. An accumulation of T and B lymphocytes occurred within the framework, and a primitive white pulp was observed around the arterioles. Hearing - Outer Ear Development week 18 - External auditory meatus is already fully expanded to its complete form. Neural - Cerebrum central sulci and opercularization of the insula (week 18-20, GA 20-22 weeks)
19		Neural week 19 neuronal migration ends and the radial glial cells that aided the migration now become transformed into astrocytes and astrocytic precursors.
20		Pituitary week 20 to 24 growth hormone levels peak, then decline Skin lanugo, skin hair Skin 5 months - Hair growth initiated at base of cord, lateral outgrowths form associated sebaceous glands; Other cords elongate and coil to form sweat glands; Cords in mammary region branch as they elongate to form mammary glands.
21
22		Neural brain cortical sulcation - sylvian fissure, interhemispheric fissure, callosal sulcus, parietooccipital fissure, and hippocampic fissures present Spleen - Week 22 - antigenic diversity of the reticular framework was observed, and T and B lymphocytes were segregated in the framework. T lymphocytes were sorted into the alpha-smooth muscle actin-positive reticular framework, and the periarteriolar lymphoid sheath (PALS) was formed around the arteriole. B lymphocytes aggregated in eccentric portions to the PALS and formed the lymph follicle (LF). The reticular framework of the LF was alpha-SMA-negative. Neural - Cerebrum covering of the posterior insula (week 22-24, GA 24-26 weeks)
23
24		Respiratory Week 24 to 40 lung histology - terminal sac Spleen Week 24 - marginal zone appeared in the alpha-smooth muscle actin-positive reticular framework around the white pulp. Tooth Development Week 24 - Permanent incisors and canines appear. Earliest potential survival expected if born ovarian follicles can consist of growing oocytes surrounded by several layers of granulosa cells
25		Respiratory end month 6 alveolar cells type 2 appear and begin to secrete surfactant Neural - Cerebrum closure of the laeteral sulcus (Sylvian fissure or lateral fissure) (week 25-26, GA 27-28 weeks)

Third Trimester

(GA Clinical Week 28) Third Trimester

Week	Stage	Event
	Clinical third trimester	Hearing 3rd Trimester - vibration acoustically of maternal abdominal wall induces startle respone in fetus.
27
28		Respire Month 7 - respiratory bronchioles proliferate and end in alveolar ducts and sacs
29		Tooth Development Week 29 - Permanent premolars (correspond to the milk molars) appear.
30		Genital male gonad (testes) descending
31
32		Nail Development fingernails reach digit tip
33		Neural brain cortical sulcation - primary sulci present
34		Neural brain cortical sulcation - insular, cingular, and occipital secondary sulci present
35
36		Nail Development toenails reach digit tip Lens Development - lens growth and interocular distance plateaus after 36 weeks of gestation
37
38	Birth	Clinical Week 40 Heart pressure difference closes foramen ovale leaving a fossa ovalis Thyroid TSH levels increase, thyroxine (T3) and T4 levels increase to 24 h, then 5-7 days postnatal decline to normal levels Adrenal - zona glomerulosa, zona fasiculata present

Postnatal

Week	Stage	Event
+1		Normal Newborn Neural Exam Movies   Normal Newborn Neural Exam Movies The newborn neuromuscular system can be initially assessed by 6 quick tests (posture, square window, arm recoil, popliteal angle, scarf sign and heel to ear). The following short videos show clinical examination of these assessments and a number of neonatal reflexes. Later developmental assessment includes behaviour, reflexes (primitive and postural), muscular tone, and motor (gross, fine, co-ordination). Normal Newborn Neural Exam Movies Newborn Behaviour  ‎‎Behaviour Page | Play  ‎‎Cranial Nerves Page | Play Newborn Tone  ‎‎Resting Posture Page | Play ‎‎ Tone Upper Limb Page | Play ‎‎ Arm Traction Page | Play  ‎‎Arm Recoil Page | Play  ‎‎Scarf Sign Page | Play  ‎‎Hand Position Page | Play  ‎‎Lower Limb Page | Play  ‎‎Leg Traction Page | Play  ‎‎Leg Recoil Page | Play  ‎‎Popliteal Angle Page | Play  ‎‎Heel to Ear Page | Play  ‎‎Neck Tone Page | Play  ‎‎Head Lag Page | Play  ‎‎Head Control Page | Play Newborn Positions  ‎‎Prone Position Page | Play  ‎‎Ventral Suspend Page | Play  ‎‎Vertical Suspend Page | Play Newborn Reflexes  ‎‎Tendon Reflexes Page | Play  ‎‎Plantar Reflex Page | Play  ‎‎Reflex Suck,Root Page | Play  ‎‎Moro Reflex Page | Play  ‎‎Galant Reflex Page | Play  ‎‎Stepping Reflex Page | Play  ‎‎Grasp Reflex Page | Play Newborn Head ‎‎ Head Shape Page | Play ‎‎Head Circumference Page | Play Neural Exam Movies: normal behaviour | cranial nerves | Newborn Tone - resting posture | upper extremity‎ | arm traction | arm recoil | scarf sign | hand position | lower extremity | leg traction‎ | leg recoil‎‎ | popliteal angle‎ | heel to ear | neck tone | head lag‎ | head control | Newborn Positions - prone | ventral suspension | vertical suspension | Newborn Reflexes - deep tendon reflexes | plantar reflex‎ | suck, root | Moro | Galant | stepping‎ | grasp | Newborn Head - head shape and sutures‎ | head circumference‎ | Neonatal Diagnosis
+2
+3
+4		Vision Development - eye globe growth plateaus after 42 weeks of gestation
+5
Month		Testis | Spermatozoa - about 2 months of age, primordial germ cells (gonocytes) are replaced by adult dark (Ad) and pale (Ap) spermatogonia forming the spermatogonial stem cell (SSC) population that at puberty will commence differentiation into spermatozoa.
Year
Year 1		Neural Hearing (6 months to 5 years) thalamocortical afferents to the deeper cortical layers mature and are the first source of input to the auditory cortex Normal 12 Month Neural Exam Movies   Normal 12 Month Neural Exam Movies The following short videos show clinical examination of neural development assessments and a number of reflexes. 12 Months Neural Exam Movies: Shy | Social and Language | Cranial Nerves | Tone - Tone | Reflexes - deep tendon reflexes | plantar reflex‎ | Parachute | Pincer Grasp | Beads in the Cup | Play Ball | Transition in and out of Sitting | Creeping | Stoop and Recover | Motor/Gait - Stand, Walks with Support | Toddler’s Gait | Head Circumference | Neural Exam Movies
Year 2
Year 3		Adrenal - Year 3 zona reticularis present. Normal 30 Month Neural Exam Movies   Normal 30 Month Neural Exam Movies The following short videos show clinical examination of neural development assessments and a number of reflexes. Neural Exam Movies: Establishing Relationship | Follows Commands | Points to Pictures | Names Pictures | Response to Questions | Pointing to and Naming Body Parts | Motor/Coordination - Using Puppets | Using Measuring Tape | Block Tower | Drawing | Tone | Deep Tendon Reflex | Kicking and Throwing a Ball | Walking, Running
4
5		Neural Hearing - (5 to 12 years) commissural and association axons in the superficial cortical layers allows communication between subdivisions of the auditory cortex
7-13		Puberty - Female
9-15		Puberty - Male