Embryology 2 Flashcards
What process occurs after implantation and placentation?
- bilaminar and trilaminar disphormation
- gastrulation
What are the three primary germ layers?
- ectoderm
- mesoderm
- endoderm
Why are the ectoderm, mesoderm, and endoderm called the “primary cell layers”?
all origins of all organs can be tracked back to these layers
What is the function of the ectoderm/mesoderm/endoderm?
form specific parts or the organism
What does the ectoderm form?
- epidermis of the skin,
- epithelium of the oral cavity,
- epithelium of the nasal cavities,
- nervous system
- sense organs.
What does the mesoderm form?
- muscle,
- connective tissue,
- notochord
- bone,
- circulatory system components (eg. RBC),
- urinary system components (eg. kidney tubule),
- genital system components,
What does the endoderm form?
- mucosal epithelium
- glands of the respiratory system (eg. lung cell)
- glands of the digestive system (eg. stomach cell)
- pharynx (eg. thyroid cell)
What is gastrulation?
the morphogenic process that gives rise to three germ
layers: ectoderm, mesoderm, and endoderm.
What process gives rise to the three primary germ cell layers?
gastrulation
What are germ cells?
male: sperm
female: egg
What are the names of the three cells in order (from fertilization to tissue)?
zygote –> blastula –> gastrula
State the order of cell differentiation.
stem cell –> committed cells –> specialized cells
What is cell differentiation? When does it occur?
- result of cells expressing some genes and supressing others within a common genome
- cells differ because they produce different proteins/peptides
Why do cells differ?
because they produce different proteins/peptides
State examples of proteins/peptides involved in cell differentiation.
- structural components (cytoskeleton/extracellular components)
- enzymes (controlling cell metabolism)
- secretory products (hormones/digestive enzymes/etc.)
- channels & pumps (passage of mollecules across membranes)
- receptors (communication, etc)
What does gastrulation begin with? What cell?
blastocyst
Explain the process of gastrulation. How does it occur?
- proliferation of inner cell mass cells –> thickened embryonic disc (blastocyst surface)
- degeneration of trophoblast layer (overlaying the inner cell mass)
-
formation of the hypoblast: inner cell mass cells migrate forming a new cell layer
inside the trophoblast layer
(hypoblast will form a yolk sac)
(remaining inner cell mass=epiblast) -
primitive streak formation (epiblast surface) (differential cell growth
generates a pair of ridges separated by a depression) - coelom forms (deep to the primitive streak, b/w the hypoblast and epiblast)
- mesoderm filling of coelom
- mesoderm undergoes cavitation (gives rise to body cavities)
- proliferation of epiblast cells (along primitive streak margins)
-
migration of epiblast cells (streak –> coelom) forming endoderm & mesoderm layers.
(embryonic endoderm= cells joining the hypoblast layer)
(mesoderm=majority of migrating cells that enter the primary mesenchyme) -
migartion of the primary mesenchyme
(laterally and cranially)
(not along the midline region
directly cranial to the primitive
streak where notochord will form).
What does the primitive streak define? What does it indicate?
- defines the embryo’s longitudinal axis
- indicates the start of germ layer formation
What 2 new layers are formed as the trophoblast degenerates?
epiblast:
- on top,
- takes most of the remaining inner cell mass cells.
hypoblast:
- inner cell mass layer
- cells detatch
What is the name of the structure after the formation of the two layers? What are the two layers?
bilaminar disk:
- epiblast
- hypoblast
What is the location called which used to be the “embryoblast”/”inner cell mass”?
epiblast
What is the process of gastrulation initiated by?
the formation of the primitive streak
What happens as more cells are produced?
cells are pushed towards the primitive streak (center), then they must migrate due to the ‘push effect’ of the epiblastic cell division.
State the location of the ectoderm, mesoderm, prospective endoderm, and endoblast layers.
- ectoderm: on top
- mesoderm: in the center of the disk, and in the middle
- prospecive endoderm: center, going towards the bottom layer
- endoblast: bottom layer
What does the formation of the primitive streak allow us to do?
determine the location:
- where the head and tail will go,
- which is the right and left side,
- which is the top and ventral surface
How can we determine where the head will develop?
the frontal part doesn’t have the primitve streak!
(no line in the center)
What does most of the mesoderm volume transform into?
mesenchyme!!
(embryonic connective tissue giving rise to adult connective tissue)
What is the function of the ectoderm? Where is it located?
ectoderm:
- forms outermost layer of the embryo
- forms the outermost layer of the adult organism/fetus (epidermis), nervous system,
What is the function of the mesoderm? Where is it located?
mesoderm:
- major embryo contributor
- cells used to build bones, cartilage, c.t. components, skeletal system, lymphatic system, circulatory system, reproductive system, urinary system, epidermis
mesenchyme (–> adult connective tissue) is a product of mesoderm
What does the fact that the ectoderm develops into the nervous system reflect?
reflects the evolutionary process:
- external sensory apparatus sensing chemicals, nutrition, light
What is the function of the endoderm?
- epithelia of the respiratory system, gastrointestinal tract
- glands (eg. pancreas, liver)
What is the notochord? Where is it located? What does it occupy?
What?
- rod-shaped aggregate of cells
Location:
- between ectoderm
and endoderm
- anterior to the primitive streak
Occupation?
- midline coelomic space (not invaded by migrating primary
mesenchyme)
What are the principal functions of the notochord?
- formation of the head process,
- development of the nervous system,
- formation of somites
What is associated with the caudal and cranial part of the embryo? (oversimplified)
caudal: primitive streak
cranial: notochord
What does the notochord mark?
the future location of:
- the vertebral column
- cranium base
What is the ultimate fate of the notochord? Become which part of the intervertebral disks?
become nucleus pulposus of
intervertebral discs
What does the notochord develop from? Where does this happen?
- develops from the primitive node,
- located at the cranial end of the primitive streak
What are somites?
blocks of the mesoderm, adjacent to the notochord.
What is neurulation?
- notochord-induced transformation of ectoderm into
nervous tissue
When does neurulation begin?
third week
What is the location of neurulation?
begins: region of the future brain
progresses: caudally into the region of the future spinal chord
State what happens to cells during neurulation.
- ectodermal cells surrounding the notochord become tall columnar (neuroectoderm);
- form a thickened area designated the neural plate.
- other ectodermal epithelium is flattened.
What is formed during neurulation? How is it formed?
- neural groove
- edges of the neural plate
become raised on each side of a midline depression.
What is formed from the neural groove? How is it formed? What does it seperate?
- formed as the neural groove undergoes midline
merger of its dorsal edges. - the tube separates from non-neural ectoderm
How does non-neural ectoderm unite to the tube?
dorsally
Where does tube formation begin? Where does it continue?
begins: central nervous
system (cranial cervical region)
continues: cranially and caudally
What does the neural tube become?
the central nervous system!
- brain
- spinal cord
How is the neural crest formed?
-
cells detach bilaterally as the neural groove closes;
(where the neural groove is joined to non-neural ectoderm) - cells proliferate
- cells assume a position dorsolateral to the neural tube
Why are neural cells special?
form a remarkable range of structures:
- pigment cells of the skin (dorsal migration)
- neurons and glial cells of the PNS (peripheral nervous system)
- adrenal medulla cells
What does the neural crest form in the head? What does this become?
- neural crest forms the mesenchyme (ectomesenchyme)
- becomes meninges, bones, fascia, and teeth.
What do mesoderm blocks induce? Where are they located?
mesoderm blocks:
- located lateral to the notochord,
- induced somite development.
How many somites does an individual have?
- pair of somites for every vertebra,
- half dozen somite pairs in
the head. - somites # in an embryo is indicative of age,
- individual somites develop chronologically, in craniocaudal order.
How do individual somites develop?
- chronologically,
- craniocaudal order.
How many regions does each somite differentiate into? What are they?
3:
- sclerotome (ventromedial)
- dermatome (lateral)
- myotome (intermediate)
What does the sclerotome region of the somite give rise to? Where is this region located?
- ventromedial region
gives rise to:
- vertebrae,
- ribs,
- endochondral bones (base of skull)
What does the dermatome region of the somite give rise to? Where is this region located?
- lateral region
gives rise to:
- skin dermis
What does the myotome region of the somite give rise to? Where is this region located?
- intermediate region
gives rise to:
- skeletal muscles
What is the structure of the early embryo? What must it develop into?
early embryo = flat,
vertebrate body
plan = cylindrical
What does the trasition from a flat to cylindrical embyo involve?
1) lateral body folds
2) head process formation
3) tail fold formation
Explain the formation of lateral body folds.
- embryo grows (dorsal elevation),
-
lateral body folds adduct and join together ventrally,
—> tubular embryo separated from extra-embryonic tissue (fetal membranes).
Explain the formation of the head process. Which end of the embryo?
- dorsal and forward growth of embryo (cranial end)
- projects above the region originally in front of the embryo.
- cylindrical head process elongates by additional growth from its base
Explain the formation fo the tail fold. Which end of the embryo?
- embryo (caudal end)
- cylindrical tail fold is formed
- similar to head process formation
- dorsal and backward growth of embryo
- cylindrical tail process elongates by additional growth from its base
