Post on 26-Dec-2015
The process by which the germ layers (lamina germinativa: ectoderm, endoderm, and mesoderm) develop into the
internal organs of the organism
Internal organs initiate development in humans: within the 3rd to 8th weeks in utero
The germ layers in organogenesis differ by three processes:
1. Folds 2. Splits 3. Condensation
Developing early during this stage in chordate animals:
1. Neural tube (tuba neuralis)2. Notochord (chorda dorsalis)
Vertebrate animals all differentiate from the gastrula the same way
Vertebrates develop a neural crest (crista neuralis) that differentiates into many
structures
(including some bones, muscles, and components of the peripheral nervous
system)
The coelom of the body forms from a split of the mesoderm along the somite axis
Germ layers: 1. Endoderm; 2. Mesoderm; 3. Ectoderm
HISTOGENESIS: the formation of different tissues from undifferentiated cells
GERM LAYER (LAMINA GERMINATIVA)
A collection of cells, formed during animal embryogenesis
Are only really pronounced in the vertebrates
All animals more complex than sponges (eumetazoans and agnotozoans) produce two
or three primary tissue layers (sometimes called primary germ layers)
Animals with radial symmatry (like cnidarians) produce:
Two called ectoderm and endoderm,
making them diploblastic
Animals with bilateral symmetry produce:
A 3rd layer in-between called mesoderm,
making them triploblastic
Germ layers will eventually give rise to all of an animal’s tissues and organs through process
called organogenesis
Gastrulation of a diploblast:
The formation of germ layers from a (1) blastula to a (2) gastrula. Some of the ectoderm cells (orange) move inward forming the endoderm (red)
MESODERM
Forms in the embryos of animals more complex than cnidarians, making them triploblastic
During gastrulation, some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm
This key innovation involved hundreds of millions of years ago and led to the evolution of nearly all large, complex animals.
The formation of a mesoderm led to the formation of a coelom
Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions protect them from shock
PRODUCTION
GERM LAYER PRODUCT
MESODERM 1. REPRODUCTIVE SYSTEM
2. URINARY SYSTEM
3. CHORDAMESODERM
4. PARAXIAL MESODERM
5. INTERMEDIATE MESODERM
6. LATERAL PLATE MESODERM
Mesoderm forms: skeletal muscles, skeleton, dermis of the skin, connective tissue, urogenital system, heart, blood (lymph cells),
and spleen (lien)
Endoderm Mesoderm Ectoderm
Blastocyst
Blastocyst:
Inner cell mass(embryoblast) Conceptus: Embryo, amnion, yolk sac, allantois(Conceptus: embryo + its membranes)
Trophoblast Placenta
Ectoderm
Endoderm
Endoderm
PRODUCTION: products produced by the endoderm
ENDODERM
one of the germ layers formed during animal embryogenesis
cells migrating inward along archenteron from the inner layer of the gastrula, which develop into
the endoderm
consists at first of flattened cells columnar
forms
1. the epithelial lining of the whole of the digestive tube
(excepting: part of the mouth and pharynx and the internal part of rectum – which are lined by
involutions of the ectoderm )
ENDODERM (cont.)
Forms:
2. the lining cells of all glands (which open into the digestive tube), including:
- liver and pancreas
- the epithelium of the auditory tube and tympanic cavity,
- the trachea, bronchi, and air cells of the lungs,
- the urinary bladder and part of the urethra,
- that which lines the follicles of the thyroid gland and thymus
GERM LAYER PRODUCT
ENDODERM 1. GASTROINTESTINAL TRACT2. RESPIRATORY TRACT3. ENDOCRINE GLANDS AND ORGANS (liver, pancreas)
ECTODERM
Ectoderm is the start of a tissue that covers the body surfaces
It emerges first and forms from the outermost of the germ layers
In vertebrates, it has three parts:
1. External ectoderm
2. Neural crest
3. Neural tube
1. EXTERNAL ECTODERM
(1). Skin (along with the glands, hair, nail)
(2). Epithelium of the mouth and nasal cavity, salivary glands, and glands of mouth and nasal cavity
(3). Enamel (in teeth) – as a side note dentin and dental pulp are formed from ectomesenchyme which is derived from ectoderm (specially neural crest cells and travels with
mesenchymal cells)
(4). Epithelium of pineal and pituitary glands
(5). Lens and cornea of the eye
(6). Apical Ectodermal Ridge inducing development of the limb buds of the embryo
(7). Sensory receptors in epidermis
2. NEURAL CREST*
(1). Pigemnt cells in the skin
(2). Ganglia of the autonomic nervous system
(3). Schwann cells
(4). Facial cartilage
(5). Spiral septum of developing heart
(6). Ciliary body of the eye
* Due to the great importance it has been referred to as the fourth germ layer
3. NEURAL TUBE
(1). Brain (rhombencephalon, mesencephalon, and prosencephalon)
(2). Spinal cord and motor neurons
(3). Retina
(4). Posterior pituitary
(5). Adrenal medulla
EMBRYOGENESIS
Process of cell division and cellular differentiation of the human embryo during early prenatal development
(It spans from the moment of fertilization to the end of the 8th week of gestational age, where it is called a fetus)
One cell: zygote
Ovum Spermatozoon
8 cellsCompact sphere
16 cells
(morula)
(compaction) (cavitation)
Trophoblast
(secretes water)
Blastocoel
(fluid-filled cavity)
VolumeBlastulaBlastocyst
(differentiation)
Blastocyst
(differentiation)
Inner cell mass(embryoblast)
Trophoblast
Placenta
Embryo
proper
Yolk sac Allantois
INNER – CELL MASS
(EMBRYOBLAST)
TWO-LAYERED EMBRYO
EPIBLAST HYPOBLAST
Columnar cells
(Primitive ectoderm)
Cuboidal cells
(Primitive endoderm)
Gastrulation
(day 16 after fertilization)
3 GERM LAYERS
ECTODERM
MESODERM
ENDODERM
CRITICAL PERIODS IN HUMAN DEVELOPMENT
The most critical period in the development of an embryo or in growth of a particular tissue or organ:
DURING THE TIME OF MOST RAPID CELL DIVISION
The critical period varies in accordance with the timing and duration of the period of increasing cell
numbers for the tissue or organ concerned
SUSCEPTIBILITY
Toxic exposures during the first two weeks following fertilization
(2nd and 3rd weeks of gestational age)
May cause prenatal death
(but do not cause developmental defects)
The body performes a miscarriage
Subsequent toxic exposures in the embryonic period
Often cause major congenital malformation
(since the precursors of the major organ systems are developing)
Fig. 8-14 Schematic illustration of the critical periods
During the first two weeks of development:
the embryo is usually not susceptible to tertogens.
During these predifferentiation stages: a substance either damages
all or most of the cells of the embryo, resulting in its death,
or its damages only a few cells, allowing the embryo to recover without developing defects.
(Red: highly sensitive periods
Yellow: stages that are less sensitive to teratogens)
Teratogens: agents that may induce congenital malformations
when the tissues and organs are developing.
CONGENITAL MALFORMATIONS
Congenital malformations:
anatomical abnormalities present at birth,
macroscopic or microscopic, on the surface or
within the body
(L. congenitus: born with)
Teratology: the study of abnormal development
and the causes of congenital malformations
Until 1940’s: generally accepted that human embryos were protected from environmental agents by:
-Fetal membrane
- mother’s abdominal walls
- uterus
Gregg (1944): presented the first well-documented evidence
that an environmental agent (rubella virus) could produce
congenital abnormalities if present during the critical stages of development
Lentz (1961) and MacBride (1961):
focussed attention on the role of drugs in the etiology of human congenital abnormalities
It is now estimated that nearly 10% of human developmental abnormalities result from the actions of drugs, viruses, and other environmental factors
(Persaud, 1979)
About 20% of deaths in the perinatal period:
Are attributed to congenital malformations (Mac Vicar, 1976)
Malformations are observed in about 2.7% of newborn infants, and, during infancy, congenital
abnormalities are detected in additional 3% (McKeown, 1976)
CONGENITAL MALFORMATIONS
1. GENETIC FACTORS
(chromosomal abnormalities or mutant genes)
2. ENVIRONMENTAL FACTORS
(but many common congenital malformations are caused by a number of genetic and environmental factors acting together:
MULTIFACTORIAL INHERITANCE)
(causes)