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Connective and supportive tissues (textus connectivus) 69

cally active permeable barrier that regulates extracellular
VetBooks.ir transport of water-soluble molecules. The hydrophilic
properties of glycosaminoglycans also allow the fluid con-
tent of the connective tissue to be increased or decreased.
Connective tissue therefore also serves as a water reservoir
(turgor).
Structural glycoproteins are composed of conjugated
proteins bound covalently with carbohydrates (monosac-
charides). These occur in tendons, cartilage, bone, vessel
walls and basal laminae. They are involved in the regula-
tion of connective tissue regeneration and calcification.

Types of connective tissue
Common to most types of connective tissue is their
derivation from the mesoderm. Definitive classification
of the different types of connective tissue is difficult, as
they vary greatly and often manifest in transitional forms.
Nevertheless, connective tissue is conventionally catego-
3.11 Collagen fibre bundles (longitudinal and trans- rised as:
verse section) with enclosed elastic fibre (x6000).
· embryonic connective tissue:
− mesenchymal connective tissue,
Ground substance − mucous (or gelatinous) connective tissue,
The cells and fibres of connective and supportive tissues · specialised connective tissue:
are embedded in a viscous, amorphous ground substance. − lymphoreticular connective tissue (lymphatic
The nature of the ground substance contributes to the tissue),
characteristics of the tissue (e.g. the suppleness of con- − haemoreticular connective tissue (haemopoietic
nective tissue proper, the pliant yet pressure-resistant tissue),
nature of cartilage or the mechanical strength of bone). − adipose tissue,
All substances passing between the lumen of capillaries · connective tissue proper:
and the target organs of the body, be they solids, liq- − loose connective tissue (low fibre content) and
uids or gases, must traverse the ground substance of the − dense connective tissue (high fibre content)
connective tissue. (Figure 3.12).
Biochemically, ground substance is composed of
polyanionic proteoglycans and structural glycoproteins. Embryonic connective tissue
Proteoglycans contain a considerably greater propor- (textus connectivus embryonalis)
tion of carbohydrate than protein. They contain linear In the embryo, the mesoderm gives rise to mesenchymal
(unbranched) chains of disaccharide units comprising one and mucous connective tissue. Embryonic connective
of two acetylated sugars (glucosamine and galactosamine) tissue consists of relatively poorly differentiated, widely
and a uronic acid. Based on their high content of uronate spaced cells and gel-like ground substance.
and sulfate esters, these molecules are referred to as acid The cells of mesenchymal connective tissue (mesen-
mucopolysaccharides, or glycosaminoglycans. chymal cells) are predominantly stellate to polymorphous
Glycosaminoglycans vary considerably in both their (7–10 μm) with long processes that form a three-dimen-
sulfate content and degree of acetylation. Consequently, sional network (Figures 3.13 and 3.14). The nucleus is
there are various types (e.g. chondroitin sulfate, dermatan relatively large and heterochromatic. Cell division is com-
sulfate, keratin sulfate and [non-sulfated] hyaluronate) that mon. The cells, which are capable of phagocytic activity,
bestow different characteristics on particular connective can detach from the surrounding tissue and migrate
tissues. The synthesis of ground substance is hormonally through the typically amorphous ground substance in an
regulated. amoeboid fashion. Mesenchymal cells give rise to the vari-
The chemical structure of the glycosaminoglycans ous connective and supportive tissues and their derivatives,
permits binding of large quantities of cations and water most of the muscle cells, the vessels and the endo- and
molecules. In addition, the proteoglycan molecules are mesothelia.
extensively interconnected. The combination of ground Mucous connective tissue is derived from mesen-
substance and collagen fibres thus forms a metaboli- chymal tissue. It is found around the umbilical vessels

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