What is the basic difference between epithelial tissue and connective tissue? Why are they so different despite the fact that all tissues originate in embryo and an embryo arises from a single fertilised egg cell?

Epithelial tissues are made up of a particular type of cell that is arranged in single or multiple layers. The tissue sits on a thin non-cellular basement membrane that prevents blood vessels from directly penetrating epithelium. The exposed side of the epithelium could be ciliated, as seen in the lining of trachea, oviduct, etc. Epithelial tissues can either protect, cover, or line an organ. In modified conditions, they may form secretory glands, as seen in the wall of the stomach and small intestine.

Large blood vessels supply oxygen to connective tissue. Two or more types of cells, such as RBCs, WBCs, and platelets in blood, may be present in a single connective tissue. Connective tissues function as skeletal tissue, packaging material, and most importantly, blood. The matrix of a connective tissue can be solid, as in bones, jelly-like, as in adipose/areolar, or liquid, as in blood.

The genetic basis of all observed differences among tissues is highlighted by the fact that all connective tissues arise from embryonic mesoderm, while epithelial tissues can arise from any of the germ layers; thus, the peritoneal lining of the coelum is mesodermal in origin, the innermost mucosal lining of the gut is endodermal, and the epithelial layer of skin is ectodermal.

Different cells achieve different structural and functional characteristics. For example, some genes are kept inactive while others express themselves through protein synthesis. For example, only the chief cells of the gastric epithelium secrete HCl, only blood lymphocytes can produce antibodies, only fibroblasts can secrete fibers in tissue matrix, only chondroblasts can secrete cartilage components, and so on.

Tissue differentiation happens very early in the life of the embryo.

While all tissues originate from the embryo and an embryo arises from a single fertilized egg cell, they differentiate into various types of tissues through a process called histogenesis. The differentiation process involves genetic regulation and cellular interactions, leading to the specialization of cells into different types of tissues. The basic differences between epithelial tissue and connective tissue are in their structure, function, and location in the body. Epithelial tissue covers body surfaces and lines body cavities, providing protection, secretion, absorption, and sensation. On the other hand, connective tissue supports, connects, and divides different types of tissues and organs.