Organs & Tissues

Epithelium: is a basic tissue type that lines all body surfaces, cavities and tubes. All epithelia are supported by an avascular connective tissue layer called the basement membrane which contains collagen type IV and laminin. Epithelium is classified according to shape as follows:

• simple squamous epithelium: is 1 layer and flat.

• simple cuboidal epithelium: is 1 layer and square; The ovary is simple squamous to cuboidal

• simple columnar epithelium: is 1 layer and columnar; lines most of digestive track, oviduct/fallopian tube (where fertilization occurs), uterus.

• stratified squamous epithelium: is more than 1 cell layer and flat. Examples include the skin, vagina

Organs & Tissues derived from Ectoderm

Integument/skin: Skin consists of two main parts: the epidermis on the outside and a lower layer of connective tissue which includes the two layered dermis. Skin is sometimes categorized as either thick such as that found on the foot or palm of the hand or thin. Thick skin has 5 layers in the epidermis whereas thin skin contains 4 layers. The various cells of the skin are attached to each other through desmosomal type junctions and to the basement membrane through hemidesmosomes.

• epidermis is made up of a stratified (multilayed) squamous (flattened) epithelium composed largely of keratinocytes. The epidermis is renewed by stem cells lying in its basal layer. Other cells associated with the epiermis are merkel cells which function as mechanoreceptors and langerhan cells which are phagocytic and present processed antigens to lymphocytes.

• dermis consists of the 1) papillary layer which is a loose connective tissue with fine collagen fibers and 2) reticular layer.

Hair and sweat glands which penetrate into the dermis are actually down growths from the epidermal layer. Glands can have ducts (exocrine) or no ducts (endocrine). Sweat glands are made up of a cuboidal (shaped like a cube) epithelium surrounded by myoepithelial cells.  One modified type of sweat glands are the mammary glands whose growth and development of the epithelium of the glands is regulated by steroid hormones.

Sensory Epithelium:The sensory epithelium of the eye and ear do not regenerate while the olfactory epithelium will regenerate.

(1) Neural retina:  consists of several cell layers. Retinal ganglion cells lie closest to the external world so that light must pass through them to reach the photoreceptor cells which are classified as rods or cones which contain different rhodopsins. Rods are especially sensitive at low light levels while cones deter color and fine detail. Photoreceptors, like auditory hair cells, are permanent cells that do not divide and thus are not replaced if destroyed but the rhodopsin molecules are continually replaced.

(2) Ears:  are the sensory epithelium responsible for hearing. Ears have auditory hair cells with protruding microvilli which held in an elaborate epithelial structure of supporting cells and overlaid by a mass of extracellular matrix. The hair cells convert mechanical stimuli into electrical signals. Sounds vibrations tilt the microvilli and open or close gated ion channels . The flow of electric charge carried into the cell by the ions alters the membrane potential and controls the release of neurotransmitter at the cell's basal end, where the cell synapses with a nerve ending. Auditory hair cells, unlike olfactory neurons have to last a lifetime.

(2) Olfactory epithelium of the nose:  contain olfactory sensory neurons. Basal stem cells in the epithelium generate replacements for the olfactory neurons that are lost.

Organs & Tissues derived from Endoderm

Simple Columnar Epithelium of the Digestive Track: is made up of secretory mucous cells, absorptive cells as well as stem cells.

Colon: is a simple columnar epithelium whose function is water absorption. The colons has goblet cells which sythesize and secrete mucus which is a fecal lubrication.

Lungs: are formed from an outpocketing of the gut lining in the embryo. Repeated branching of a system of tubes terminate in several hundred million air filled sacs called alveoli which which contain Types I & II alveolar cells (pneymocytes).  Macrophages which engulf foreign matter and bacteria are associated with the lungs.

Lungs: is the site where nutrients that have been absorbed from the gut and then transferred to the blood are processed by hepatocytes. Liver cell loss stimulates liver cell replacement.

Organs & Tissues derived from Mesoderm

Blood vessels: are lined with endothelial cells. New vessels originate as capillaries which sprout from existing small vessels in response to specific signals in a process called angiogenesis. Angiogenesis is stimulated by hypoxia which results in the release of growth factors. Stimulation of cells around endothelial cells called pericytes may play an important role in the process.

Blood cells: are considered a connective tissue.

Muscle: There are 3 types of muscle 1) skeletal (which is attached to bones and responsible for skeletal movements is composed of skeletal muscle cells), 2) smooth (composed of smooth muscle cells and found in organs like blood vessels, the GI tract) and 3) cardiac.

Cartilage and Bone: is considered a 3rd connective tissue (the other two being blood and muscle). Cartilage consists of chondrocytes. Bone matrix is secreted by osteoblasts.

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