Comparative Anatomy Notes – Set 5
EARLY EMBRYO
- difficult to differentiate early embryos
- it allows us to see what structures gives rise to what
Egg Types
- amniotic egg - possess amnion
- reptilian egg possess shelled (cleidoic) egg shell
- based on distribution of yolk
Microlecithal egg - small amount of yolk
- Mammals
Mesolecithal egg - moderate amount of yolk
- amphibians
Macrolecithal egg - large amount of yolk
- reptiles, birds, sharks
- when egg divides, yolk is a barrier to division
Types of Cleavage
Holoblastic Cleavage - division into 2 equal sized cells
- holoblastic equal or holoblastic unequal
Meroblastic - restricted to periphery, rest is yolk
- in reptiles, birds and snakes
- amphibian egg is easiest egg to work with
- start with zygote
- 1st division, 2 cell egg
- 2nd division, 4 cell egg, 4 blastomeres
- 8 cell stage, cells on periphery are smaller
- micromeres vs. macromeres = size of blastomeres
- yolk is mostly distributed in macromeres
- 16 cell, animal pole, vegetal pole
- 32 cell stage - rapid division of animal pole cells because they are smaller
- 64 - solid mass of cells
- animal pole cells divide much faster and lift away from vegetal pole cells
- when this happens, a cavity is formed inside
32 celled stage = Morula - solid mass of cells
64 celled stage = Blastula
- cavity is called blastocoel
- micromeres are so small they lift up away from, then they can keep dividing
- cells push inward from outside, form new cavity = gastrula
- eventually blastocoel is disappears
- true gastrula stage
Late Gastrula
- archenteron (primitive gut) is formed
- notochord is present
- Blastopore
- gives rise to anus
= deuterostome
- giving rise to mouth
= protostome
- Dorsal Lip - involution of cells
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- cells are dividing so fast
- micromeres are dividing really fast
- move around yolk cells
- turns in and causes yolk plug to stick out
- Blastocoel lost
- Micromeres move interiorly
- have 3 primary germ layers
Ectoderm, Endoderm, Mesoderm
- Notochord forms from mesodermal cells
- flattening of ectoderm in top region begins
- thickening takes place
Neurulation - standard for all vertebrates
- neural plate thickens and pushes inward
- cells coming out of dorsal here are neurectoderm
- gives rise to nerve tube
- some cells pinch off and lie between nerve tube and ectoderm
= neural crest cells
- these are important cells
- this happens in all vertebrates
- gives rise to - dorsal root ganglion
- parasympathetic and sympathetic ganglia
- in head region, anterior most cells go forward to give rise to
all cartilages, dermis of face, cephalic armor (dermal bones),
sensory capsules (optic, nasal)
- mesoderm becomes regionally divided
Dorsal Mesoderm
- has subdivisions
1. Dermatome - part associated with ectoderm
- gives rise to dermis in most of body but head region
2. Myotomes - gives rise to most of skeletal muscle
3. Sclerotone - gives rise to skeletal and vertebral column
- associated with nerve tube
- as mesoderm grows, it splits laterally
- cavity forms between the two = coelom
- part of mesoderm becomes bound to ectoderm = somatic mesoderm
- part of mesoderm becomes bound to endoderm = splanchnic mesoderm
- when somatic mesoderm and ectoderm come together = somatopleure
- when splanchnic mesoderm and endoderm come together = splanchnopleure
- Dorsal mesoderm is also called somite
- extends outward
- Body is segmented, has serial homology
- as mesoderm continues to develop, we get differentiation between dorsal mesoderm and rest of body
= Intermediate and lateral plate mesoderm
Dorsal Mesoderm (somite) - Epimeric Mesoderm
Intermediate Mesoderm - Mesomeric Mesoderm
Lateral Plate - Hypomeric Mesoderm
- Splanchnology - everything that arises from splanchnic mesoderm
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REVIEW
- blastula goes to gastrula (ecto. endo. meso.)
Endoderm - Digestive Tube ( and all glands)
- Lungs
- Liver
Mesoderm - somites
- muscles (myotome)
- skeletal system ( portions of ribs) - sclerotome
- limbs
-circulatory system
- gonads, and ducts
- kidneys
- uterus
Ectoderm - epidermis
- Nervous system
Neural Crest - give rise to peripheral NS
- skull and jaw
- mesenchyme
- pigment cells
- adrenal medulla
- dermal bones
Extraembryonic Membranes
- Stomadeum - mouth formation
- Proctodaeum - anus formation
- some cells push inward at stomadeal plate and become nestled beside nerve tube and associate with brain
= makes Hypothalmus - anterior pituitary
- Rathke’s Pouch
Chief Membranes
- shell
- yolk sac
- allantois
- amnion
- chorion
- Amniotic egg
- amnion is on inside, lines amniotic cavity
- chorion is on outside, it is they layer that comes back
- near region of proctodaeum
- outpocketing of hindgut = allantois
Yolk sac - highly vascularized
- lipids and nutrients
- vessels are vitelline
- can function another way to become simple yolk sac placenta in viviparous organisms
(fish and amph.)
Amnion and Chorion
- amnion surrounds embryo
- is amniotic fluid
- chorion can become associate with allantois to become chorioallantoic placenta in viviparous
forms
- placental mammals
- other types of placentas
- in marsupials, chorion and yolk sac become associated with one another to become
choriovitelline placenta
- placenta lies near maternal uterus
- can be yolk sac itself (viviparous anamniotes)
- viviparous amniotes - chorioallantoic placenta
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