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)