BIO303: Vertebrate Anatomy

Chapter 5: Early Craniate Morphogenesis

  1. Craniate eggs, fertilization and parity types
  1. Types of eggs (see table 5.1)—affects type of cleavage of the early embryo
  1. Amount of yolk
  1. Microlecithal (cephalochordates, eutherian mammals)
  2. Mesolecithal (most fish and amphibians)
  3. Macrolecithal (sharks, reptiles, birds, monotremes)
  1. Distribution of yolk
  1. Isolecithal (cephalochordates, eutherian mammals)
  2. telolecithal (note: table says amphioxus, but means amphibian)
  1. Fertilization
  1. Internal: if egg/embryo is to be retained for internal development OR if a shelled egg is produced (some anamniotes, but all amniotes)
  2. External: confined to anamniotes whose embryos develop outside the body
  1. Birthing (parity)
  1. Oviparous- egg laying; most anamniotes, most reptiles, all birds and monotremes
  2. Ovoviviparous- retain eggs in body, but mother contributes very little in the way of nutrition; eggs have sufficient yolk for development; some anamniotes, some reptiles
  3. Euviviparous- retain eggs in body with the development of a nutrient system (placenta or modified yolk sac) from the mother; eutherian mammals
  1. Early developmental patterns
  1. Cleavage: mitotic cell division which reduces the size of the embryonic cells (blastomeres)
  1. Microlecithal eggs: holoblastic equal cleavage; complete cleavage with blastomeres of equal size; note: the amphioxus zygote and the mammalian zygote both divide this way
  2. Mesolecithal eggs: holoblastic unequal cleavage; these telolecithal eggs have a slower cleavage rate through the vegetal pole region, so the animal pole region divides faster and produces more and smaller cells than the vegetal pole region; amphibians are examples
  3. Macrolecithal eggs: meroblastic cleavage; these telolecithal eggs have so much yolk that it can not cleave, only the animal pole region cleaves; reptiles and birds are examples; a cap of cells forms at one end
  1. Blastula formation (blastulation: hollow ball whose center is the blastocoel)
  1. Microlecithal eggs- first form a solid ball (morula) which then fills with fluid to become a blastula
  1. amphioxus: true blastula
  2. mammals: blastocyst with trophoblast cells on the outside and inner cell mass (ICM) on the inside at one pole; the ICM is the future embryo, the trophoblast participates in implantation
  1. Mesolecithal eggs- animal pole eggs grow and eventually surround the yolky vegetal pole (epiboly); blastocoel forms between the animal pole cells and the vegetal pole cells
  2. Macrolecithal eggs- animal pole cells form two layers which lifts off the yolk and forms a subgerminal space between them and the yolk; the two layers (epiblast and hypoblast) separate and form a blastocoel between them (Fig 5.6)
  3. Question: which egg type does blastulation in the mammal most resemble?
  1. Gastrulation part I (formation of the archenteron: primitive gut)
  1. Microlecithal eggs
  1. amphioxus: one end caves into the blastocoel (involution) forming an opening into the new archenteron called the blastopore; the embryo also elongates and becomes bilaterally symmetric with a dorsal surface and a ventral surface, though it is not immediately obvious; the dorsal lip of the blastopore will give rise to the notochord and mesodermal tissues, the inner layer of cells lining the archenteron will become endodermal cells, while the outer layer of cells becomes the ectodermal cells (Fig. 5.2)
  2. mammals- see section C.4
  1. Mesolecithal eggs: during epiboly one part of the yolk is not covered by the animal pole cells; this yolk plug is surrounded by the cells of the blastopore; cells from the dorsal lip of the blastopore push inward and form the archenteron (Fig 5.4); the roof forms the notochord, the outer layer the ectoderm and the yolk cells closest to the archenteron the endoderm
  2. Macrolecithal eggs: the hypoblast cells slowly grow around the yolk and contribute to the yolk sac (not the embryo); the epiblast cells are destined to become the embryo; certain cells of the epiblast become thicker and form a primitive streak with a node (Hensen’s node) at one end; Hensen’s node is homologous to the dorsal lip of the blastopore and pushes cells inward and forward to form the notochord under the epiblast; primitive streak cells also push inward and laterally to form mesoderm, while epiblast cells from the periphery of the blastoderm (cap) push inward to form the endoderm layer
  3. Mammalian eggs: the ICM separates into a two layered blastodisk (epiblast and endoderm); the endoderm cells grow downward into the blastocoel and form an yolk sac; the epiblast cells thicken and form a primitive streak and node
  1. Gastrulation part II (formation of mesodermal structures)
  1. Amphioxus- (fig 5.3) bands of tissue lateral to notochord pouch outward and separate from the notochord, push ventrolaterally between endoderm and ectoderm. The pouching produces two layers of mesoderm: somatic and splanchnic with coelom between. The mesoderm immediately lateral to notochord thickens and forms somites (epimeres) which become segmented in craniocaudal direction. Endoderm separates from notocord and completes formation of gut tube.
  2. Frog- cordomesoderm pushes laterally between ectoderm and endoderm and splits to form coelomic cavity. Dorsolateral mesoderm forms somites.
  3. Bird/reptile- basically the same as the frog (mesodermal splitting) only restricted to the blastodermal cap; endodermal cells slowly grow around the yolk and produce a yolk sac; the dorsal region of the yolk sac pouches up and forms a separate gut tube
  4. Mammals- basically the same as bird/reptile
  1. Neurulation (only concerned about craniates, not protochordates)
  1. Ectoderm above the notochord is termed the neurectoderm. Under influence of notochord it thickens and invaginates to form a neural groove which eventually forms a neural tube with neurocoele.
  2. Neurectodermal cells lateral to the forming tube break away and form neural crest cells, which give rise to a variety of structures (neuronal, connective, glandular and pigment)
  3. Neurulation begins cranially and progresses caudally (spina bifida is a failure of the caudal neural tube to complete its closure, anencephally can result from failure of cranial tube to close)
  1. Idealized triploblastic embryo- developed from figure 5.11
  1. Ectoderm
  2. DHNC
  3. Neural crest cells
  4. Somite (epimere)
  1. dermatome
  2. sclerotome
  3. myotome
  1. mesomere
  2. hypomere
  1. splanchnic mesoderm (part of splanchnopleure)
  2. somatic mesoderm (part of somatopleure)
  3. coelom is the space separating the two
  1. Gut endoderm
  2. Dorsal mesentary
  3. Ventral mesentary
  1. Embryonic tissue fates (not a complete list)
  1. Ectoderm
  1. Neurectoderm
  1. neural tube: brain, spinal cord, retina, optic nerve
  2. neural crest: dorsal root ganglia and autonomic ganglia, pigment cells, adrenal medulla, branchial skeletal structures
  1. Epidermis of the skin with accessory structures (hair, feathers, some scales, sweat glands, mammary glands)
  1. Mesoderm
  1. Bone (mostly from sclerotome of epimere)
  2. Muscle (mostly from myotome of epimere)
  3. Dermis of skin (mostly from dermatome of epimere)
  4. Kidneys (from mesomere)
  5. Gonads (medial to mesomere)
  6. Heart (ventral lateral plate splanchnic mesoderm)
  7. Mesentaries (dorsal suspends gut tube from dorsal body wall; ventral suspends the liver)
  1. Endoderm
  1. Inner lining of the gut
  2. Lungs/gills of pharyngeal arches
  3. Pancreas
  4. Liver/gall bladder
  5. Urinary bladder
  6. Yolk sac
  7. Probably the future sperm and eggs (but some texts have them migrate there from neighboring ectoderm)
  1. Extraembryonic membranes
  1. Yolk sac
  1. anamniotes, reptiles and birds: endoderm surrounds yolk, blood vessels (vitelline arteries and veins) from the splanchnic mesoderm and surround yolk to pick up nutrients
  2. ovoviviparous fishes it supplies oxygen and is referred to as a simple yolk sac placenta
  3. mammals: it is empty but probably has a brief role (gamete development?); it can have a remnant (Meckel’s diverticulum) about 30 cm above the ileocolic valve in humans; it is double layered (endoderm and splanchnic mesoderm)
  1. Amnion and chorion- these arise as the somatic mesoderm grows above the embryo, eventually fusing (amniotes only)
  1. The fusion produces a cavity surrounding the embryo (amniotic cavity- surrounded by amniotic membrane: ectoderm interior, mesoderm exterior) which fills with fluids, and a
  2. Chorion which lies either against the inside of the shell or against the uterine wall (mesoderm interior, ectoderm exterior)
  1. Allantois- a separate outgrowth of the gut endoderm distal to yolk sac which pushes out and between chorion and shell or uterine wall; endoderm interior, mesoderm exterior
  1. in oviparous amniotes the chorioallantoic membrane is the respiratory membrane for the embryo
  2. in true mammals it forms the embryonic portion of the placenta
  3. a portion near the gut becomes a part of the urinary bladder
  1. Placenta- this acts as the source of nutrients, site of gas exchange, excretion site, endocrine source of important hormones of pregnancy
  1. maternal blood enters sinuses and comes in close contact with fetal blood vessels, so surface area is maximized and diffusion distance is minimized
  2. no exchange of fetal and maternal blood normally occurs
  3. umbilical cord is the connection of the allantois with the gut tube, its blood vessels come from iliac arteries and go to the hepatic portal vein
  4. there are a variety of placental types which vary according to the type of connection with the endometrium
  5. humans have a deciduous placenta- it is shed as afterbirth and has maternal tissues (decidua) attached
  6. Note: marsupials have a choriovitelline placenta between the yolk sac and chorion; it is short lived since the offspring are born in a very immature state (before hindlimb development is even completed