The postnatal development and differentiation of the boundary tissue of the seminiferous tubule of the rat

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Abstract
The morphology of the boundary tissue of the seminiferous tubule of the rat has been investigated in testes from animals aged from birth to four months. In the adult, the boundary tissue consists of four layers: (1) an inner non‐cellular layer comprising a network of collagen fibrils sandwiched between two basement membranes (2) an inner cellular layer of flattened cells showing many of the characteristics of smooth muscle, including intracytoplasmic filaments and micropinocytotic vesicles, (3) an outer non‐cellular layer containing scattered colagen fibrils and associated usually with a single basement membrane and (4) an outer cellular layer, component cells of which contain no fine filaments. At birth, the seminiferous tubule rests upon a basement membrane which separates it from a layer of low cuboidal cells. External to this, there is a narrow intercellular zone containing scattered fibrillar material and a region consisting of numerous processes of mesenchyme cells. By ten days postnatal, four definite layers are established due to the differentiation of an outer cellular layer of flattened cells from the mesenchyme, and intracytoplasmic filaments have made their appearance within the inner cellular layer. In subsequent stages there is progressive thinning and specialization of the cellular layers and increased fibril content within the non‐cellular layers. By 22 days postnatal the boundary tissue appears virtually adult in morphology.