1. Life History and Embryology of Vertebrates
Hazel Anne L. Tabo, MS

2. Embryology Study of development from fertilization to embryo.
The nature of relationship from: ancestor to offspring can be studied.
It reveals ancestry.
Haeckel: “Ontogeny recapitulates phylogeny.” (development repeats evolution).
De Beers: Recapitulation is not the only relationship between embryos and ancestors.

4. Embryo (De Beers, 1930)
Embryonic structures may or may not be present from their ancestors or descendants.
Embryonic structures - vestigial (lost); or retained to adulthood.
Developmental stages may be repeated in earlier stage but not in later stages.
Developmental sequence may or may not be altered in the descendant.

5. Gametes
Gametes (sex cells, haploid chromosomes) union to form diploid organism.
Gametes perform meiotic division  haploidy
Sperms (males) have various head appearance: spherical, spatulate, hooked, lancet-shaped, spiraled, capped (acrosome).
Sperm: Head (nucleus); Neck (mitochondria); Flagella (movement).
Egg cells: Microlecithal (small yolk); Mesolecithal (moderate yolk); Macrolecithal (abundant yolk).

6. Gametogenesis – formation of gametes

7. OOCYTE (egg cell; ovum)
Developed and released by ovarian follicle; vitelline membrane (nourishes the egg)
In therian mammals: egg is enclosed in thick zona pellucida, corona radiata, cells of ovarian follicle.
Other vertebrates: egg is enclosed after ovulation; Jelly layers (amphibians); albumen (birds); horny, membranous (fishes) or calcareous shells (reptiles, birds).
Microlecithal: Amphioxus; Eutherian mammals (placental nourishment)
Mesolecithal: Lampreys; some fishes; Amphibians
Macrolecithal: Most fishes; Reptiles; Birds; Monotremes (egg-laying mammals)

8. Fertilization
In therian mammals: Penetration of sperm from corona radiata to vitelline membrane of the egg. Fertilization involves enzymatic and physical interactions between sperm acrosome and egg cortex.
Union of gametes: restores the diploid # of chromosomes.
Activation of egg to refract additional sperm entry.
Zygote (fertilized egg)  Embryo (multicellular)

9. FERTILIZATION

10. Cleavage
Blastula – structure from series of cell division into multicellular (embryo).
Blastomeres – daughter cells
Blastocoel - coelom / cavity
Yolk – nutrition of developing embryo; process of cleaving & blastula is dependent on the yolk present.
Morula (blastula) – multiple numbered cells ready for uterine implantation.

11. Cleavage: 2-celled; 4-celled; 8-celled; 16-celled; 32-celled stages of blastula

12. Vertebrate blastula
Composed of single tissue layer with hundred of cells.
The cells exhibit cellular polarity related to the axis of the adult body.
Polarity: 1) animal pole - embryo
2) vegetal pole – yolk (gut)
Blastocoel – body cavity

13. (animal pole)
BLASTULA
(blastocoel)
(vegetal pole)

14. Blastula: Animal and vegetal poles
Animal pole – EPIBLAST or EPIMERE; developing embryo nourished by the vegetal pole (dorsal cells--ECTODERM)
Vegetal pole – HYPOBLAST or HYPOMERE or developing yolk; nourishes the embryo  GUT (ventral cells--ENDODERM)

15. Microlecithal (Amphioxus) – “holoblastic” (total cleavage furrows penetrate the entire yolk); equal-sized blastomeres
Animal pole
Blastocoel or Gut
Vegetal pole

16. Cleavage in Amphibians
Mesolecithal - holoblastic; unequal-sized blastomeres
Vegetal pole – larger blastomeres; nourishes the embryo; slower development
Animal pole – smaller blastomeres; developing embryo
Blastocoel – towards animal pole

17. Amphibians

18. Cleavage in Birds (Aves)
Macrolecithal – “meroblastic” (partial cleavage); unequal-sized blastomeres
Vegetal pole – large size yolk mass; too great to be penetrated by cleavage furrow
Animal pole – is relatively small (blastoderm)
Narrow blastocoel

19. Birds: Yolk > Embryo

20. Cleavage in Mammals
Microlecithal - holoblastic; unequal-sized blastomeres
Blastula  Blastocyst (inner cell mass formation)
Trophoblast – superficial layer of cells
Blastocoele – towards vegetal pole
Embryoblast – animal pole; epimeric cells
Endometrium – inner lining of the uterus (mother)

21. Cleavage patterns followed by holoblastic (total cleavage) and meroblastic (partial cleavage) eggs
Radial (sea urchin, amphioxus)
Bilateral (tunicates, amphibians)
Spiral (annelids, mollusks)
Rotational (mammals)
Discoidal (fish, birds, reptiles)
Superficial (insects)

22. Gastrulation: formation of three germ layers
Blastula  Gastrula
Animal pole – EPIBLAST or EPIMERE; developing embryo (dorsal cells--ECTODERM)
Vegetal pole – HYPOBLAST or HYPOMERE or Yolk; nourishes the embryo  GUT (ventral cells--ENDODERM)
Germ layers:
1) Ectoderm – from epiblast (animal pole); outermost layer
2) Mesoderm – middle layer; MESENCHYME
3) Endoderm – from hypoblast (vegetal pole); innermost layer

23. (animal pole)
BLASTULA
(blastocoel)
(vegetal pole)

24. Ectoderm
Endoderm
Mesoderm
Gastrula

25. Gastrulation
Cells migrate to the interior of the blastula, consequently forming two (in diploblastic animals) into three (triploblastic) germ layers.
The embryo during this process is called a gastrula.
Among the different animals, different combinations of the following processes occur to place the cells in the interior of the embryo:
Epiboly - expansion of one cell sheet over other cells
Ingression - cells move with pseudopods
Invagination - forming the mouth, anus, and archenteron
Delamination - the external cells divide, leaving the daughter cells in the cavity
Polar proliferation

27. In most animals a blastopore is formed at the point where cells are entering the embryo.
Two major groups of animals can be distinguished according to the blastopore's fate. In deuterostomes the anus forms from the blastopore (formerly the mouth), while in protostomes it develops into the mouth.

28. Blastopore
Blastopore – an opening that resulted from invagination of cells towards embryo

29. A. Protostomes B. Deuterostomes

30. Holoblastic Gastrula: mesoderm formation
Vegetal (yolk) pole – folds inward towards animal pole  double-walled cup
Cup opening – blastopore continues to another end
Gastrocoel – body cavity (coelom)
Ectoderm – uppermost layer of cells
Archenteron – primitive gut (yolk) lined by endoderm
Mesoderm – middle layer formed from dorsolateral outpocketing of archenteron
Notochord – anlagen of nervous system (primitive), formed from dorsal wall of archenteron

33. Ectoderm
Endoderm
Mesoderm
Archenteron (primitive gut)
Gastrula

34. Holoblastic gastrula
Notochord – anlagen of nervous system (primitive), formed from dorsal wall of archenteron
Chordamesoderm – roof of the archenteron, forms the notochord in the midline and somites (series of paired mesodermal tissue blocks)
No outpocketing from the gut tube - schizocoel
Coelom – formed by splitting of the hypoblast in the somites (schizocoel – pouch form)

36. Mammalian embryo
Blastocoel – indirectly filled with maternal fluid and enlarges  Blastocyst
Delamination of endoderm (hypoblast) from inner cell mass and cover the trophoblast
Blastoderm – (or embryoblast) flattened inner cell mass forms primitive streak (notochord  CNS)
Mesoderm – forms extraembryonic membranes (fetal membranes) and body of embryo (musculoskeletal and circulatory system); Mesenchymal tissue
Coelom – forms by schizocoel (pouch form)

38. Embryoblast or Blastoderm
inner cell mass + primitive streak
Primitive streak – notochord (anlagen of CNS); promotes neurulation

39. Germ layers
Ectoderm – Central nervous system, retina and lens, cranial and sensory, ganglia and nerves, pigment cells (melanocytes), head connective tissue, epidermis of skin, hair, mammary glands.
Mesoderm – musculoskeletal system, circulatory system, dermis of skin, connective tissue, urogenital system, heart, blood (lymph cells), and spleen.
Endoderm – gastrointestinal system (gut); stomach, colon, liver, pancreas, urinary bladder; lining of urethra, epithelial parts of trachea, lungs, pharynx, thyroid, parathyroid, intestine.

40. Neurulation
Overlaps with gastrulation establishing the central nervous system (CNS)
Gastrula  Neurula
Neurocoel – coelom
Chordamesoderm – induces thickening of the ectoderm into a neural plate
Neural crest cells – (Ectomesenchyme) considered as the 4th germ layer which arised from ECTODERM forms cartilage & bones of the head, pharyngeal cartilages; peripheral nerve ganglia, some glandular tissues; melanocytes

41. Neural plate – ectodermal cells
Neural folds – formed from neural plate; fold of cells that arches and meet at the mid-dorsal line and forms the neural tube.
Neural tube – encloses the neurocoel (cavity)
Neurocoel – anlagen of the dorsal hollow nerve cord  (+) chordates

43. Organogenesis Migration of primordial germ cells
Holoblastic embryo: Establishes the head and tail
Meroblastic embryo: 3 germ layers spread faced down on the uncleaved yolk
Embryo increase in length while the archenteron becomes part of the gut tube
Schizocoely  Enterocoelom (blastopore becomes anus while the mouth opens anteriorly.

45. Organogenesis: Endoderm
Endoderm form the lining of the gut.
Archenteron: Foregut, Midgut & Hindgut
Gut
Major Structures
Foregut
Epithelium: oral cavity,
nasal cavity, pharynx, gill, esophagus, lung
Midgut
Visceral organs: stomach, liver, gall bladder, pancreas, intestines, germ cells of gonads (ovary & testis), part of the yolk sac membrane, part of the allantois
Hindgut
Urinary bladder; cloaca / anus

46. Organogenesis: Mesoderm
Mesoderm layer
Major structure
Outer Epimere – Dermatome
Skin Dermis
Middle Epimere – Myotome
Muscles: axial, appendicular, branchiomeric, hypobranchial
Inner Epimere – Sclerotome
Vertebral column
Chordamesoderm
Notochord
Intermediate mesoderm
Kidney; Urogenital ducts
Somatic hypomere
Bones: ribs, sternum (breast plate), appendicular skeleton, some appendicular muscle, parietal peritoneum (outer abdominal wall); genital ridge; part of amnion and chorion
Splanchnic hypomere
Blood, heart, gut, smooth muscle, visceral peritoneum (inner abdominal wall); yolk sac and allantois

47. Organogenesis: Ectoderm
Ectoderm layer
Major structure
Somatic ectoderm
Skin epidermis; Teeth enamel; stomodeum (mouth); proctodeum (anus/cloaca); gill epithelium; part of amnion and chorion
Neural plate ectoderm
Brain; Spinal cord
Epidermal placodes
Capsules: olfactory (nasal); optic (eye); otic (ear); epibranchial capsules; neuromast (electroreceptors organs); ganglia of some cranial nerves
Ectomesenchyme (Neural crest)
Spinal ganglia; splanchnocranium; neurocranium (part); dermatocranium (part); teeth dentine; eye cornea; chromatophores (pigment cells); branchiomeric muscles (part); aortic arches; heart septum (divides the right and left side of heart)

48. Extraembryonic membranes (fetal membranes) and Placenta
Amphibians – yolk cleaves and directly incorporated into somatic cells (body cells)
Yolk supply – limited
Early hatching  Larva
No fetal membranes required to sustain larva

50. Fishes
Yolk remains uncleaved, eggs are laid in water; yolk sac is the only fetal membrane formed.
Yolk sac – formed from 3 germ layers to absorb the yolk into the body
Respiration and excretion are direct contact with the environment

51. Reptiles and Birds (Avians)
Primitive streak  extraembryonic mesoderm splits  forms extraembryonic coelom  SPLANCHNOPLEURE
Splanchnopleure + adjacent Yolk = Yolk sac
Allantois (vascular) – under the shell; near the chorioallantoic membrane. Allantois (respiration, receives excretory wastes; absorbs albumen; takes some minerals from the shell)
Somatopleure + head fold of amnion = Chorion (outer) and Amnion (inner)
Amnion – “water bag” contains amniotic fluid that bathes the embryo; cushions the embryo (shelter for growth)

54. Eutherian mammals
Placental mammals – nourish their young in the uterus, exchange and nutrition of fetus occurs between fetal and maternal bloodstream by PLACENTA
Yolk sac and Allantois – vascularized (blood vessels)  supplies nutrition to placenta
Chorion and Amnion – avascular (no blood vessels)  cannot support placenta

55. Allantoic circulation (birds & reptiles) – homologous to umbilical circulation (mammals)
Amnion – sac filled with amniotic fluid bathes the fetus; allows the early fetus to move freely and protects the fetus from pressure of the maternal abdomen
Amniotes – collective terms for reptiles, birds and mammals because the possess amnion.
PLACENTAL FORMATION
Maternal contribution: Chorion (lies between allantois & uterus) incorporated into the placenta  CHORIOALLANTOIC Membrane
Fetal contribution: Allantois (in most mammals)  as fetus grows, allantois decreases in size

57. Prototherians – egg-laying mammals; deposits egg in a pouch (oviparous)
Metatherians – marsupials (pouched animals) – no typical placenta
Eutherians – placental mammals (viviparous)