embryology-03-third and fourth weeks

Start Studying!

Terms

undefined, object

copy deck

main developmental task during the third week?: gastrulation Â– third tissue layer appears
3rd & 4th weeks Â– events?: bilaminarà trilaminar (3rd tissue layer appears), complex three dimensional changes & organogenesis begin
When does the primitive streak form?: 13 Â– 14 days after fertilization
What is the primitive streak?: a thickening of epiblast layer in the midline
cause of primitive streak?: epiblast cell proliferation and movement caudally + medially, piling up in the midline
name given to the cranial end of the primitive streak?: Primitive knot, or HensenÂ’s node.
what occurs during the first stages of gastrulation?: epiblast cells ingress at primitive knot & primitive streak, and migrate rostrally & laterally
name of the layer formed by migration of epiblast cells between endoderm & the epiblast?: mesoderm
what do some of the ingressing epiblast cells displace the hypoblast to form?: single layered embryonic endoderm
layer of overlying epiblast tissue on the surface of the embryo, facing the amniotic cavity called, once the mesoderm has formed?: ectoderm
How long does the primitive streak continue to form mesoderm?: through the 3rd, 4th, & 5th weeks
what happens to the primitive streak later in gastrulation?: streak becomes smaller and more caudally situated as embryo grows. Usually disappears in 5th week.
where is the streak situated at the time of its disappearance?: in the area that will become the sacrococcygeal part of the trunk
if primitive streak doesnÂ’t disappear, what can happen: remnant causes sacrococcygeal teratoma
what do the cells that are first to migrate through the primitive streak contribute to?: tissues that contribute to rostral structures
what do those cells that migrate through the streak later contribute to?: progressively more caudal structures
what happens to the cells that migrated rostrally first, while the primitive streak is still forming mesoderm and endoderm caudally?: rostral tissues continue to grow, differentiate, and form organs
what is the name given to the gradient in developmental activity?: rostro-caudal gradient, because rostral structures develop first and are generally more developmentally well advanced than caudal structures.
what do the cells which first ingress through HensonÂ’s node and migrate rostrally in the midline condense to form?: rod-like structure called the notochord
importance of the notochord?: organizes subsequent embryonic development
what does the ectoderm contribute to?: nervous system, epidermis body surface tissues (epidermis, hair, nails), glands that connect by ducts to the body surface (sweat glands, sebaceous glands)
process of neural tube developing into ectoderm?: neurelation
what is neural plate induction?: notochord and adjacent paraxial mesoderm induce the overlying ectoderm to differentiate into a neural plate Â– a differentiated layer of ectoderm above the notochord
what is the neural plate made up of?: neuroectoderm
what forms the neural folds?: elevation of the edges of the neural plate, forming the neural fold
name the cell types associated with the neural fold: medial sides are neuroectoderm, non-neural ectoderm (surface) ectoderm is on lateral sides, mesoderm in core, neural crest cells at peak of crest
what folllows development of the neural fold?: neural folds contact, fuse at top, forming neural tube
what does the neural tube develop into?: brain and spinal cord
what role does the non-neural ectoderm play after the neural tube has developed?: comes together and seals over the neural folds, allows separation of the tube from the surface. this ectoderm becomes the skiin covering the brain & spinal cord
what migrates next to the area between the neural tube and the ectoderm, and what does it form?: mesoderm, which later forms the bones of the calvarium, covering the brain, and the vertebrae covering the spinal cord.
what happens to the neural crest cells in this process?: they migrate downwards, away from where the neural folds fused, and start to develop into the dorsal root ganglion
initial contact between neural folds occurs where?: site of the future cervical segment of spinal cord
in which direction does the neural fold fusion process proceed?: both cranially and caudally simultaneously
which parts of the neural groove are last to close?: the anterior and posterior neuropores
where is the developing brain?: anterior neuropore
timing of closure of anterior and posterior neuropores?: 23 days +/-
what occurs if the neural folds fail to close?: neurelation defect
describe consequences if neural folds donÂ’t close: failure of neural fold closure results in neural plate exposure on body surface: no skin covering the defect, therefore no space for the mesoderm to inhabit, so also so no bone development, no bony covering.
most common neurulation defects?: anencephaly when it occurs at the anteriormyeloschisis when it occurs posteriorly
what happens to babies born with anencephaly?: no skin over brain, no cranial vault, brain usually degenerates. usually die within hours of birth from infection
what do the neural crest cells form?: many structures related to the nervous system
structures formed by the neural crest cells: sensory neurons of dorsal root ganglia of spinal nerves; post-ganglionic autonomic neurons in sympathetic chain ganglia, visceral ganglia, etc... (Aurbachs and MeissnerÂ’s plexuses); endocrine cells in medulla of adrenal gland; schwann cells of peripheral nerves; melanocytes of skin; etc.
endoderm form: lining of the GI tract, and lining of ducts that open to tract, as well as their secretory cells (parenchyma) and glands
what causes the embryo to fold during the 4th week of development?: different rates in the growth of tissue
what causes folding in the long axis?: elongation of the neural tube and the notochord, producing the head and tail folds
endoderm does what as the head and tail folds develop?: endoderm rolls into the primitive foregut and hindgut
primitive foregut forms?: parts of the mouth, esophagus, stomach, and duodenum, as far as the opening of the common bile duct
other imp. structures formed: prim. foregut outgrowth?: respiratory airways, lungs, liver, gallbladder, pancreas
what does the primitive hindgut form?: large intestine from the distal part of the transverse colon to the anal canal and parts of the bladder and urethra
when the tailfold rolls around, what is formed?: it is flared Â– wider in elsewhere than rest of tube, so creeates bladder, rectum
what causes the sides of the embryo to fold?: enlargement of the somites
endoderm does what during transverse folding?: rolls into a primitive midgut
what connects the primitive midgut to the remains of the yolk sac?: vitelline duct (or yolk stalk)
what does the primitive midgut form?: small intestine from the opening of the common bile duct to the ileocecal junction, and the large intestine from the cecum to the distal part of the transverse colon
mesoderm forms what in the embryo?: most of the muscle and connective tissues
subdivisions of mesoderm that run length of embryo?: paraxial, lateral plate, and intermediate mesoderm
what does the paraxial mesoderm do?: blocks itself out into somites
what forms the intraembrionic parietal coelum: intercellular spaces in lateral plate Â– cavities coalescing
what does the intermediate mesoderm form?: kidneys and gonads
what does the intraembryonic (visceral (splanchnic) mesoderm form: connective tissues, vessels of digestive system and outgrowths
where is the paraaxial mesoderm: either side of the notochord
in what order do somites form?: craniocaudal sequence as paired blocks of cells in paraxial mesoderm
what do the somites flank: notochord and neural tube
what do the somites develop into?: vertebrae, intervertebral discs, ribs, skeletal muscles of the body wall, limbs, and contribute to body wall connective tissue
where is the lateral plate mesoderm found?: at periphery of embryo
what forms in the lateral plate: a cavity called the intraembryonic coelom
what does this cavity divide the mesoderm into?: somatic (parietal) and splanchnic (visceral) mesoderm
what does the parietal mesoderm support?: ectoderm
what does the splanchnic mesoderm support?: endoderm
what does the somatic mesoderm contribute to?: connective tissue, blood vessels, smooth muscle of body wall and limbs
what does splanchnic mesoderm become?: smooth muscle, blood vessels, connective tissue of GI tract and associated organs
what does the intraembryonic coelom develop into?: main body cavities: pleural, pericardial, and peritoneal
where does the intermediate mesoderm lie?: between the paraxial mesoderm and the lateral plate mesoderm
what does the intermediate mesoderm contribute to?: formation of urinary and reproductive systems
when is most structure complete?: by end of 8th week after fertilization
embryonic period: 0-8 weeks
first two weeks: transport & implantation
3 Â– 8 weeks: morphogenesis, organogenesis
Fetal Period: 9 weeks to term
during fetal period?: growth and physiological maturation
if adverse environment during 1-2 weeks: dev. response: death or live, normal birth
if adverse environment during gastrulation: dev. response: multiple profound abnormalities
if adverse environment 4 Â– 8 weeks: dev. response: organ specific malformations
if adverse environment fetal period: dev. response: growth retardation and/or functional deficit(s)

Start Studying!

Deck Info

Number of cards 84