Pregnancy {gestation}| has embryonic and fetal stages. Human embryonic and fetal development takes 280 days or 40 weeks [Winick, 1978].
cells
Approximately 50 cell divisions happen from zygote to newborn, making 256 or so cell types and 10^15 cells.
DNA
DNA one-dimensional molecules can encode embryo development in three spatial dimensions and one time dimension, because embryonic development uses relative times and positions.
development genes
Development genes have many and long introns and many regulatory regions. Development genes express in same order as order on chromosome, which is also spatial order of organism tissues and organs. Development genes turn on and off in cascades. Successive stages depend on previous stages. Transcription factors and receptors cause different gene-regulation and gene-expression patterns and so different cell types.
fertilization
At fertilization, maternal-effect genes code transcription factors that establish top-to-bottom embryo polarity. Bicoid morphogen, at one pole, sets up top-to-bottom gradient. Follicle-cell maternal-effect genes code transcription factors that establish front-to-back embryo polarity. Nanos morphogen, at other pole, sets up gradient across embryo.
Dorsal protein transcription factor, similar to rel protein and NF-kappaB, concentrates in cell nucleus ventrally, and cytoplasm dorsally, in all embryo cells. Cactus and Toll genes can partition dorsal protein to cell locations. Perhaps, Toll proteins are receptors.
gap genes
After first cell divisions, gap genes code transcription factors, with zinc fingers, that make bands along embryo by working with maternal-effect genes and by repressing each other. Gap genes are hunchback, Kruppel, knirps, and hunchback-maternal. Gap genes also regulate genes expressed later. Transcription-factor binding sites are high-affinity or low-affinity, so transcription-factor concentration affects which genes transcribe and how much, leading to gradients and bands.
segmentation genes
After gap-gene expression, segmentation genes code transcription factors that make number of segments, pair segments, and give polarity to segments. Segmentation genes work with gap-gene products and interact with each other, using autofeedback, to sharpen segment boundaries. Segmentation genes include pair-rule genes, such as fushi tarazu gene, even-skipped gene, hairy gene, runt gene, and eve gene.
homeotic genes
After segmentation-gene expression, homeotic genes, such as vertebrate HOX genes, code transcription factors that determine body-part type, such as antenna or thorax. Homeotic genes work with pair-rule pairs to make segments differentiate. All homeotic genes evolved from one gene by gene duplication. Homeotic genes have homeoboxes, which make homeodomains, which bind to DNA promoter sequences to control transcription. All animals have homeotic genes.
torso and polehole genes
After segmentation-gene expression, head and tail develop. For transcription factors only in head and tail, torso gene makes protein-tyrosine-kinase membrane receptors. Polehole gene, similar to raf proto-oncogene, makes protein-serine, threonine kinase that acts on head and tail growth-factor receptors.
proneural genes
After head and tail develop, proneural genes, such as daughterless and achaete-scute, code for transcription factors, with helix-loop-helix, that make neural precursor cells to start brain development. da enhances achaete-scute. enc inhibits achaete-scute.
neurogenic genes
Then neurogenic genes, such as notch, split enhancer, big brain, mastermind, and neuralized, make cell-to-cell signal proteins for cell adhesion, signal transduction, membrane channels, and transcription factors. Neurogenic genes develop cells and inhibit nearby cells.
selector genes
Then selector genes, such as cut gene, code for homeobox transcription factors that make neuron types.
neurotrophic genes
After neurons creation, neurotrophic genes code for secreted neurotrophic factors that keep neurons alive, differentiate neurons, and make neurotransmitters, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), CNTF, and NT-3. Other genes code neurotrophic-factor receptor proteins.
information from mother
Fetus can use information about mother and environment, such as orientation information. Fetus needs to know relation to mother to aid survival.
1 day: Cell division makes egg cell into a many-celled ball {blastomere}, with same volume and mass as egg cell.
1 day: Cell division {cell cleavage} makes egg cell into blastomere.
4 days: Human embryos {morula} have 10 to 30 cells.
5 days: Human embryos have several hundred cells in one layer in a hollow sphere {blastocoel} {blastocyst}, with inside cavity filled with fluid. Human blastocyst has extra cells at one spot.
5 days: Lower-animal embryos have cell spheres {blastula}.
1 week: Embryos several days old have stem cells {embryonic stem cell, gestation}| (ES cell). Embryonic stem cells can uptake and insert genes by homologous recombination. Adding altered stem cells can change mice embryos. Mouse blastocysts have inner cell layer, which can culture with fibroblasts or with leukemia inhibiting factor to prevent further differentiation.
1 week: Cells that will make brain and spinal cord first roll into a hollow cylinder {periventricular germ layer}, then multiply around that cavity, and then migrate outwards to form neuroblasts.
1 week: Cells that will make brain and spinal cord first roll into periventricular germ layer, then multiply around that cavity, and then migrate outwards to form brain nuclei {neuroblast}. Later cells pass through earlier cells, so younger cells are on outside.
1 to 3 weeks: Neuroblasts make cell slab {cortical plate} in embryo upper layer. Two hemispheres form around ventricles.
10 days: Two fluid cavities, yolk sac and amnion sac, develop inside cell sphere {chorion} {gastrula}.
10 days: Two fluid cavities develop inside gastrula. Fluid cavities {amnion sac}| can have ectoderm inside.
10 days: Two fluid cavities develop inside gastrula. Fluid cavities {yolk sac}| can have endoderm inside, to later make primitive gut.
10 days: Yolk sac has endoderm {primitive gut} inside.
10 days: In hard-shell eggs, one cavity {subgerminal space} is away from yolk, under dividing cells.
14 days: Embryo has 800 cells. Ectoderm disk lies under endoderm. Mesoderm grows between endoderm and ectoderm and splits into two sheets, one on ectoderm and one on endoderm, with a fluid cavity {coelom, development}| between sheets.
14 days: Endoderm disk has line {primitive streak}| of cells along longitudinal body-axis top. At anterior end, primitive streak becomes more specialized and has no segments, like notochord.
2 to 3 weeks: Ectoderm above notochord makes first neural plate, then neural groove, then neural folds, and then neural tube {neural tube}|.
2 to 3 weeks: Neural tube has inner ventricular zone {ependyma} around central ventricle, intermediate-zone or mantle-layer gray matter, and marginal-zone or pia white matter. Ependyma becomes CNS neurons and glia.
2 to 3 weeks: Neural-tube mantle layer becomes dorsal sense neurons {alar plate} and ventral-motor-neuron basal plate.
2 to 3 weeks: Neural-tube mantle layer becomes dorsal-sense-neuron alar plate and ventral motor neurons {basal plate}.
2 to 3 weeks: Mesoderm {neural crest}| lies next to neural tube and makes adrenal medulla, sympathetic ganglia, and dorsal-root ganglia.
2 to 9 weeks: Embryo {embryo, animal}| grows first at head and then down sides. First, disk sides curve up and around notochord at head. Then disk sides curve down to tail. Body folding makes digestive-system foregut and hindgut.
2 to 9 weeks: In vertebrates, embryos are cylindrical bodies {neurula}, with ectoderm layer on outside reaching over notochord, mesoderm layer, and inner endoderm tube.
2 to 9 weeks: Ventral-body-wall constriction makes a tube {umbilical cord}| {umbilicus} come out from belly. Tube has outer ectoderm, middle mesoderm, and inner endoderm.
15 days: Egg yolk {yolk, egg}| can be throughout cell {isolecithal}, at one end {telolecithal} or in center {centrolecithal}. Mammalian eggs have little yolk.
3 weeks: Brain interneurons send axons down brainstem to spinal cord and up to forebrain {interneuron development}. Axons transmit messenger chemicals to other neurons to integrate central nervous system. Medulla oblongata appears at neural-tube first flexure. Soon after, diencephalon appears at second flexure. Last neuroblast cells, having left-right asymmetry, migrate.
4 weeks: Forebrain telencephalon and diencephalon, midbrain, and hindbrain pons and medulla differentiate {brain divisions}.
4 weeks: Embryo floats in amniotic sac, connected to uterus wall only by umbilical cord. Embryo has large head, gills, tail, somites on notochord sides, and beating heart {head and heart development}.
4 weeks: Guided by chemical or electrical signals with no learning, brainstem and spinal-cord ventral motor cells grow axons to trunk, limb, and viscera muscles {motor cell and movement}. Fetal birds and mammals have varied movements, even before sense nerves appear.
4 weeks: Embryo has mesoderm muscle precursors {somite}| on notochord sides.
4.5 weeks: Spinal ganglia begin forming {spinal ganglia development}. Pons and cerebellum appear at neural-tube third flexure.
5 weeks: Sensory nerve tracts begin {sense-nerve development}. Epithelial sensory cells project to brainstem and spinal-cord dorsal half, which also receive from head sensory receptors. Old cortex, cerebral medulla, and basal ganglia appear at neural-tube fourth flexure.
6 weeks: Arms and legs appear, reflex-arc elements appear, sympathetic ganglia form segmental masses, hypothalamus and epithalamus begin, and cerebral hemispheres start {hypothalamus development}. Embryo is 12 millimeters long.
7 weeks: Thalamus, corpus striatum, and hippocampus begin {thalamus development}. Y-chromosome stimulates cell division and causes embryonic gonad medulla to differentiate into testis.
7.5 weeks: Spinal reflexes work, and limbic lobe begins {limbic lobe development}.
8 weeks: Embryo is 25 millimeters {cerebral cortex development}. Face has eyes, ears, and nose. Arms and legs have fingers and toes. Embryo has small tail and all internal organs. Brainstem has many projections into cerebral cortex to guide cortical-neuron migration and differentiation. Eight weeks ends embryonic period and begins fetal period.
9 weeks: Anterior commissure appears {anterior commissure development}. Testis secretes androgenic hormones to organize genitalia and brain. Fetus has red, wrinkled skin.
9.5 weeks: Hippocampal commissure appears {hippocampal commissure development}.
10 to 23 weeks: Corpus callosum appears {corpus callosum development}, but, in the next three months, it trims most callosal axons.
10.5 weeks: Spinal cord has internal structure {spinal cord development}. Fetus has localized movements. Neocortex parietal lobe starts.
10.5 weeks: Baby teeth {deciduous teeth}| appear.
12 weeks: Sex differences are present {brain layer development}. Fetus is 75 millimeters. Deepest cortical layers five and six appear.
12.5 weeks: Spinal cord has reached next organization stage {brain connection development}. Connections from neocortex to hippocampus start.
14 weeks: Long sensory tracts appear in spinal cord, and flocculonodular lobe appears {sensory tract development}.
15 to 25 weeks: First neocortical cells are around cerebral-hemisphere cavities and move into cortex {neocortex development}.
16 weeks: Spinal-cord ventral-root myelination begins {myelination development}, old cerebellar vermis is in position, corpora quadrigemina appear, neocortex has first layering stage, and parietal and frontal lobes separate.
18 weeks: Occipital lobe and temporal lobe separate {brain lobe development}.
19.5 weeks: Tract myelination from spinal cord to cerebellum and pons begins, and dorsal-root myelination begins {tract myelination development}.
20 weeks: Pyramidal tracts from cortex begin {pyramidal tract development}. Fetuses have REM sleep, indicating dreaming. Inner neocortex layers mature. Fetuses can have voluntary movements. Fetuses can move eyes, which aids eye development. Hair appears. Fetus is 250 millimeters. Malnutrition, narcotics, and emotional stress raise blood epinephrine level and cause hyperactivity.
22 weeks: Outer neocortical layers mature {outer neocortex development}. Bone ossification and teeth calcification begin.
24 weeks: Some left-temporal-lobe and parietal-lobe regions become asymmetric {brain asymmetry development}. Ventral commissure myelinates. Cranial nerves myelinate through midbrain. All cerebral-hemisphere commissures are complete. Brain has all cortical layers.
26 weeks: Brain areas that will attain mature tissue structure only at adolescence are the last to receive neurons {brain final development}.
28 weeks: Tract from spinal cord to thalamus myelinates through midbrain {brain convolution development}. Tract from spinal cord to cerebellar vermis myelinates. Cerebellum configuration is in place. Cerebral convolutions and fissures start.
32 weeks: Secondary and tertiary sulci start {brain sulci development}.
32 to 40 weeks: Brain weight increases four times during last two gestation months {brain weight development}. It continues to increase during first infancy months, while making more axons, dendrites, and synapses. After that, neuron number is constant. In fetus, sorting, editing, and removing sense and motor connections depends on maternal-environment external-stimuli timing and spatial arrangement.
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Date Modified: 2022.0225