Embryogenesis — formation of embryonic structures/organs
• embryogenesis involves
- cell proliferation (& selective cell death) and
- cell differentiation into different cell types
• cell differentiation results from activation/suppression of selective genes which produces the different
proteins/peptides that characterize the different cell types
• cell differentiation is unidirectional, its direction depends on a cell’s genetic history (commitment
to a cell line) and its progression in that direction depends on the cell’s current environment
(intercellular communication, chemical gradients, etc.)
Embryonic Period: from fertilization to initial organ formation
[30 days (dog, cat, pig, sheep) to near 60 days (cattle, horse, human)]
Fetal Period: period of organ growth and function until birth

Fertilization — union of haploid gametes (sperm & oocyte) to produce a diploid zygote
Spermatozoa (sperm) — several hundred million per ejaculate
- propelled by uterine contraction from cervix to uterine tube
- must undergo capacitation (removal of surface proteins)
- undergo acrosomal reaction (release of acrosomal enzymes)
Oocyte — selective follicles mature in response to FSH from the pituitary
- enveloped by zona peullucida & granulosa cells at ovulation
- following ovulation, primary oocytes resume Meiosis I (begun when the female was an embryo)
- secondary oocyte completes Meiosis II following fertilization
Fertilization sequence — fusion of a spermatozoan with a secondary oocyte
- fertilization takes place within the uterine tube, near the ovary
- spermatozoan must undergo capacitation (removal of surface proteins)
- spermatozoan binds to species specific glycoprotein on zona pellucida
- spermatozoan releases degradative enzymes (acrosomal reaction)
- spermatozoan is able to penetrates the modified zona pellucida
- plasma membranes fuse and spermatozoan enters oocyte
- oocyte precludes union with additional sperm by
cancelling membrane potential via Ca++ efflux, and
releasing enzymes to denature the zona pellucida
- male & female pronuclei meet and begin mitosis leading to zygote cell division

Cleavage — the large zygote undergoes cell division without cytoplasm growth
- each daughter cell is called a blastomere
- begins with a zygote, progresses to a morula, ends in a blastula (blastocyst)
- the first eight blastomeres are undifferentiated stem cells with unlimited potential
Morula: a solid ball of committed blastomeres within a zona pellucida
• outer blastomeres – become flattened, form tight junctions, secrete fluid, destined to become
trophoblasts (cells that form chorion & amnion fetal membranes)
• inner blastomeres – form gaps junctions for intercellular communication, destined to become
inner cell mass (cells that form the embryo plus two fetal membranes)
Blastula (blastocyst): a hollow conceptus capable of attachment to the uterine wall,
following zona pellucida rupture
• trophoblasts (surface cells) surrounding a large fluid-filled cavity (blastocoele)
• inner cell mass (embryoblast) the future embryo, cell mass localized to one pole within blastocoele
Note: During cleavage in birds, reptiles & fish, the large quantity of yolk impedes complete cell
division, resulting in a disc of cells (blastodisc) on the surface at one end of the yolk.

Regarding Twins — three inner blastomeres are sufficient to develop into an entire embryo
• monozygotic or identical twins – originate from one fertilization; same genetic composition (clones)
- more commonly: morula splits and each part becomes a separate conceptus
- less commonly: morula inner cells separate and each becomes an embryo, sharing same placenta
- (separations later in development can produce conjoined (Siamese) twins, double heads, etc.)
• dizygotic or fraternal twins – result from two fertilizations; both sharing the same pregnancy
- one fetus can impact the other via commingling of blood
(e.g., male hormones stunting a heifer genital tract is common in cattle twins)
- rarely, a chimera could occur
(two morulas merge and produce one individual containing cells with either of two genotypes)

Gastrulation — the morphogenic processs that gives rise to three primary germ layers
(all subsequent tissues arise from the three primary germ layers: ectoderm, mesoderm & endoderm)
• the process begins with degeneration of trophoblast cells overlying the inner cell mass,
putting the mass on the blastula surface as an embryonic disc
• from the interior of the inner cell mass, cells delaminate and migrate to establish an inner hypoblast
layer (the future yolk sac); the inner cell mass is called epiblast, the space between the
trophoblast and hypoblast is called coelom (celom)
• on the epiblast surface a primitive streak develops, defining the longitudinal axis of the embyo
• cells proliferate along the margins of the primitive streak and migrate, as primary mesenchyme,
through the streak into the coelom
• the initial migrating cells join the hypoblast layer and become endoderm
• the majority of migrating cells, called primary mesenchyme, fill the coelom and become mesoderm
• the remaining epiblast cells become ectoderm
• the mesoderm is divided into paraxial, intermediate, and lateral regions; within lateral mesoderm
cavitation occurs, re-establishing a coelom and splitting lateral mesoderm into two parts:
somatic mesoderm in contact with ectoderm, &
splanchnic mesoderm in contact with endoderm

Notochord formation — the notochord induces formation of the head process, nervous system,
and somites
• the notochord is a rod-shaped cell mass that grows from the primitive node (the cranial end
of the primitive streak)
• it grows anteriorly in the coelom along the midline which is not invaded by primary mesenchyme
• it marks the future location of the vertebral column and floor of the cranium,
ultimately the notochord becomes the nucleus pulposus of an intervertebral disc

Neurulation — refers to the transformation of ectoderm into nervous tissue (induced by the notochord)
• cells superficial to the notochord become columnar neuroectoderm cells, forming a neural plate
• the neural plate becomes a neural groove (as apical ends of neuroectoderm cells undergo constriction)
• midline merger of neural groove shoulders produces a neural tube deep to surface ectoderm,
the neural tube becomes central nervous system (brain & spinal cord, retina and optic nerve)
• cells at the junction of the neuroectoderm and ectoderm detach from the neural tube and
form accumulations of neural crest; neural crest gives rise to neurons in sensory and autonomic
ganglia, lemmocytes, adrenal medulla cells, skin pigment cells, and ectomesenchyme (which
gives rise to bone fascia and teeth)

Somite formation — somites are bilateral blocks of mesoderm located beside the notochord
• a pair of somites develop for each adult vertebra, plus six somites form in the head
• somites develop chronologically in craniocaudal order; number of somites indicates embryo age
• somites arise as follows:
- paraxial mesoderm accumulates medially following migration of primary mesenchyme
- from rostral to caudal over time, transverse fissures divide paraxial mesoderm into blocks (somites)
- temporarily, cells within somites transform from mesenchyme to an epithelioid arrangement
(epithelioid cells reorient 90 ° from transverse to the notochord to longitudinal)
Note: mesenchyme = mesodermal cells dispersed within abundant extracellular matrix fluid
— favoring mesoderm migration; vs,
epithelioid cells (epithelium arrangement) = mesodermal cells in contact with one another
— favoring cell communication
• each somite gives rise to three components:
1] sclerotome: gives rise to axial skeleton & floor of cranium (ventromedial region of somite)
2] dermatome: gives rise to dermis of skin (lateral region of somite)
3] myotome: gives rise to skeletal muscle (dorsomedial region of somite)
Note: somites are induced to form by the notochord; rostral to the notochord, in the head, somitomeres
(reduced somites) develop and migrate to pharyngeal arches to form head muscles.

Cylindrical Body formation — the early embryo is flat, but the adult body features a cylindrical gut
within a cylindrical trunk; transition from a flat to a cylindrical embryo involves formation
of a head process, tail fold, and lateral body folds:
• head process: anterior to the primitive node, a cylindrical head process grows upward and forward
the head process overgrows the region anterior to the embryo causing endoderm to reflect back on
itself and form a blind-ended foregut, the future pharynx; a subcephalic pocket separates the embryo
head from the flat region below the head
• tail fold: similar to the head process but to a lesser extent, a tail fold and blind hindgut are formed
• bilateral body folds: as the embryo grows upward, body folds converge on the midline and merge
anterior and caudal to the yolk sac (umbilicus), the merger produces a tubular gut within a tubular
embryo, distinct from the flattened fetal membranes
• coelom: the cavity within the embryo becomes body cavities of the adult; the body wall
of the embryonic coelom is formed by somatopleure (somatic mesoderm plus ectoderm),
the gut wall is formed by splanchnopleure (splancnic mesoderm plus endoderm)
• flexures: the tube-shaped embryo undergoes three flexures (in the head, neck & tail)
that make it C-shaped

Pharyngeal Arches — temporary structures corresponding to branchial (gill) arches in fish
• three arches oriented dorsoventrally in the pharyngeal wall and demarcated by grooves (clefts) are
visible in the mammalian head
• each pharyngeal arch contains an artery (aortic arch) and ectomesenchyme located between
by ecotoderm and endoderm
• each arch receives myotomes (migrated from somitomeres) and is innervated by one cranial nerve
- the first aortic arch forms lower & upper jaws and muscles of mastication
- the second aortic arch forms hyoid bones and muscles of facial expression
- remaining arches form hyoid bone, larynx, and related muscles

Cardiovascular system — develops early, as the embryo enlarges beyond the distance capacity
of diffusion to nourish tissue
- angiogenesis (vessel formation) first begins in splanchnic mesoderm of the yolk sac
- blood islands are formed when selective mesenchyme cells transform into hemocytoblasts
that induce surrounding mesenchyme to become endothelium
- blood vesicles coalesce, bud, and enlarge to produce vessels, initially in splanchnic mesoderm
of the yolk sac and allantois