Membranes / Bones

2. Serous Membranes

Serous membranes are very thin and transparent and line compartments of the central body cavity:

a. Pleura ( the lining on the *lungs* and the *pleural cavities* in which the lungs are found)

b. Peritoneum ( the lining of the *abdominopelvic cavity* and all the organs situated there)

Membranes

*Epithelial*and their underlying *connective tissue* form *membranes* that cover and protect other structures and tissues. There are four types of membranes

The areole connective tissue that attaches the epithelium to the underlying tissue is called

*Lamina Propria* (only in association to mucous membranes)

*Histological organization of mature bone*

*Matrix:*the matrix combines the properties of *compressional strength* due to the presence of *hydroxyapatite* crystals and *tensile strength* due to *collagen fibers*

*Periosteum and Endosteum* The outer and inner surfaces of hollow bones are covered by periosteum and Endosteum, respectively.

*Periosteum* The outer layer surfaces of bone are covered by periosteum everywhere except where one bone articulates with another. Periosteum consists of: a.*fibrous outer layer* and a b. *cellular inner layer* that contains osteoprogenitor cells

Fibers in the periosteum are interwoven with the tendons that attach muscle to bone Fibers of tendons become incorporated into bone as it grows and strengthens the attachment of the tendon to bone. These fibers are called *perforating or Sharpey's fibers

...

Serous membranes consist of simple squamous epithelium called *mesothelium* and a thin underlying layer of *areolar connective tissue*

...

Spongy bone is found wherever bone is * not heavily stressed* or where the *stresses come from many directions*. Spongy bone reduces the weight of the bone and the open trabeculae framework provides support and protection for cells of the bone marrow.

...

c. Pericardium ( the lining on the surface of the *heart* and the *pericardial sac* that surrounds the heart

...

*Osseous Tissue Forms Two Types of Bone:*

1. *Compact Bone*- is dense and solid and forms the walls of bone. The basic functional unit of compact bone is the *osteon*. The osteon consists of *concentric lamellae* of bone matrix arranged around a *central canal * containing blood vessels. The patrons are cylinder and aligned parallel to the long axis of the bone

Matrix:

1. *Hydroxyapatite* - result from the interaction between calcium phosphate with calcium hydroxide, these crystals may also incorporate ions sodium, magnesium, fluoride

Bone Tissue Functions:

1. *Support*-The skeletal system provides *structural support* and a *framework* for attachment of soft tissues and organs.

2. *Storage of Minerals*-Bones maintain a large reserved of *calcium* and *phosphate ions*

3. *Blood Cell Production*- The hollow spaces in some bones contain *red marrow* where blood cells are produced.

The periosteum: 1.*Isolates* and *protects* the bone from the surrounding tissue 2. Contains and supports the *blood vessels and nerves* that supply bone

3. Participates in *bone growth and repair* 4. Attaches bone to *deep fascia*

4. *Protection*-In many places bone protects soft tissues and organs

5. *Leverage*-The articulation of bones creates levers that change the magnitude and direction of the forces exerted by muscles

4.*Osteoclasts*

Are giant cells with 50 or more nuclei and are derived from the stem cells that also produce monocytes and macrophages. They secrete acids that dissolve bone matrix and release calcium and phosphorus into the body fluids. The process of bone erosion by the activities of osteoclasts is called *osteolysis*

2. *Osteoblasts*

Are precursor cells that are found on the inner and outer surfaces of bone. They secrete the organic components of bone matrix called *osteoid* The osteoid then becomes mineralized to form bone

*Structure of bone*

Bone is similar to other connective tissue in having specialized cells and a matrix with ground substance and fibers. It is unique in that calcium salts are deposited around the fibers to make bone less compressible.

1. Mucous Membranes

Mucous membranes line passageways into the body that are *continuous with the exterior*

4.Synovial Membrane

Found within joint capsules and is composed of areolar connective tissue incompletely lined by "not true epithelial cells". Secretory cells secrete *synovial fluid* that lubricates and nourishes the hyaline cartilage of the joint.

Blood vessels travel from the surface to the central canals and the bone marrow by means of *perforating canals* that run roughly perpendicular to the patrons and the bone surface. The collagen fibers spiral within each lamella and the orientation of the collagen fibers within each lamella changes to create a crisscrossing pattern that increases the strength of the entire osteon

Lamellae of bone form on the inner and outer surfaces of bone and form what are called *circumferential lamellae* *Interstitial lamellae* are found between osteons and represent lamellae that were once part of an entire osteon or circumferential lamellae

2.*Spongy Bone*-is found in the interior of normal bones. In this type of bone parallel lamellae form struts and thin branching plates called *trabeculae*. The trabeculae surround spaces that contain *bone marrow*. Bone marrow is loose connective tissue that is dominated either by adipocytes (*yellow marrow*) or *hemopoietic tissue* (*red marrow*)

Osteons are generally * not found* within spongy bone unless the trabeculae are large. Without blood vessels in a central canal to supply osteocytes nutrients reach the osteocytes by means of canaliculi that open onto the surface of the trabeculum

2. *Collagen Fibers*

Provides a framework for the deposition of mineral crystals and contribute about 1/3 of the weight of the bone

*Endosteum* The inside surfaces of the bone are lined by a cellular *endosteum* These inner surfaces include the lining of the central and perforating canals and the surfaces on the trabeculae

The Endosteum is an incomplete cellular lining and osteoprogenitor cells, osteoclasts and osteoblasts are present on the exposed surfaces.

The thinness of serous membranes allow fluid to seep through and form *transudate*

The transudate contributes to the smooth and slippery nature of the lining

3. *Osteoprogenitor Cells*

These cells are *mesenchymal, or *stem cells* that can divide and differentiate into osteoblasts

Cells of Mature Bone: 1. *Osteocytes*

These cells are mature bone cells that maintain the protein and mineral content of the surrounding bone matrix. They occupy spaces called *lacunae* that are sandwiched between mineralized layers of matrix called *lamellae* They communicate/ exchange nutrients by cytoplasmic processes that extend through narrow passageways in the bony matrix called *canaliculi*. Canaliculi permit the exchange of nutrients and waste products between the osteocytes and blood vessels and provide a means for signals to be transferred from one osteocyte to another

These passageways include *digestive, urinary, reproductive and respiratory tracts*

This lining is kept moist and lubricated at all times by secretions including the mucus secreted by *mucous glands*

The process of bone formation is called *osteogenesis*

When the osteoblasts is surrounded by bone matrix it differentiates into an osteocyte