No information was available from the literature about the differentiation of stromal tissues of the penis complementary to the erectile structures. The present study demonstrates the special orientation of the fibers and their arrangement into the perineal raphe and septum and into the various fasciae. The analysis presented here demonstrates that the perineal raphe, perineal septum and body, and fasciae form a composite histological structure based on the diversity in tissues involved in its development. Information about this development is scarce and pertains to the raphe only.
The perineal raphe is generally considered to mark the line of fusion between the so-called urethral folds (Glenister 1954; Hamilton and Mossman 1972), or of a combination of urethral folds and scrotal swellings (Szenes 1925; Wartenberg 1993), or between urethral folds for the penile raphe and between scrotal swellings for the scrotal raphe (Arey 1965; Stephens et al. 1996; Moore and Persaud 1998a). However, Fleischman (1907) and Politzer (1932) rejected the idea of fusion as a developmental mechanism, with Polit-zer suggesting that the raphe is formed by embryonic tissue filling the temporary cloacal groove that had remained from the regressing middle part of the cloaca between the urogenital and anal compartments.
The present investigation indicates that the raphe cannot possibly mark the line of fusion between the urethral labia (syn. urethral folds) because such fusion does not take place, as has been discussed above. Other arguments against its formation by the fusing of "urethral folds" are: (a) the existence of a dorsal part of the raphe just ventral to the anal orifice, which region is never implicated in any fusion; (b) the distinct longitudinal orientation in the texture of most of the raphe, which structure does not fit into a pattern to be expected from supposedly fused urethral labia built of a much more irregularly structured fibrovascular tissue; and (c) the structural unity of the raphe and the superficial part of the septum, which part of the septum shows an intimate relationship to the deep septum and the dartos fascia laterally.
The subtle groove often observed in the midline of the ventral part of the raphe, which has also been illustrated before (Spaulding 1921; Szenes 1925;
Wartenberg 1993), is not the result of fusion but due to elongation of a split originally present between the diverging dorsal parts of the urethral labia.
The formation of the dorsal and ventral segments of the raphe is found to be directly related to the great lengthening of the midperineal region between the anal and urethral orifices. The current investigation indicates that both develop from the same type of tissue, namely, the mesenchyme that originally formed the cloacal labia. After the division of the cloaca, this fi-brovascular tissue becomes divided into components, related to the urogenital and the anal orifices, that meet where the originally connecting cloacal groove regressed. It is now shown that the component ventral to the anal orifice elongates into the dorsal raphe, while the component dorsal to the urogenital orifice becomes involved in the markedly proliferating superficial dorsal urogenital stroma and forms the ventral, or scrotal and penile, raphe. The derivation of these components from the labia is also illustrated by permanent histological characteristics, such as high vascularity and broad, markedly cornifying str.sq. epithelium, which in the midline of the raphe does not form appendages, in contrast to the strikingly large sebaceous glands in flanking zones. This histological pattern is similar to that developing around the anal orifice (both sexes) and vestibular opening in the female. The absence of a dorsal raphe in the female is due to a marked lateralward enlargement of the midperineal region between the orifices, which may later broaden the structure homologous to the dorsal raphe of the male into a convex, flat, or concave hairless area in the traditional "female perineum" (Stephens 1968). The structure in the female which is homologue to the ventral raphe of the male remains very short since no strong proliferation of superficial dorsal urogenital stroma takes place. It forms no more than the dorsal frenulum (fourchette) of the vestibular opening. It is surprising that the dual origin of the raphe has not been recognized before, although the two parts are well illustrated in works on the development of the external genitalia (Spaulding 1921; Szenes 1925; Glenister 1956; Wartenburg 1993).
The perineal septum including the perineal body proves to be a complicated structure derived from a number of primordial stromal tissues. It separates and at the same time unifies the structures of the right and left halves of the perineum. This strong fixation to the median septum is in the first place due to the weak demarcation of the median primordial stromal components, which have extended into all adjacent developing structures, including striated muscles such as the puborectalis and bulbospongiosus, the most caudal parts of the urethral sphincter musculature, external longitudinal bundles of the muscularis propria of the rectum, the bulbourethral glands, and the bulbar parts of the corpus spongiosum. This aspect is accentuated by the regression of the central area of the deep dorsal urogenital stroma and further thinning of the septal tissue, which causes approximation of the initially still widely separated bilateral structures, and results in a particularly strong coherence of the two sides in a rather rigid construction.
The process forms an interesting contrast with the events in the female perineum, which develops in an opposite direction, with the central area of deep dorsal urogenital stroma expanding into a broad fibromuscular mass widely separating the right and left perineal structures in an equally strong but much more flexible construction.
No data are available about the development of the dartos fascia. The present analysis of its development reveals a much more interesting basic structure than is suggested by the traditional description of the tissue as a thin layer of smooth muscle in the scrotum and penis. Attention is here drawn to its likely origin in the bilateral dorsolateral perineal complex, derived from the dermatomes of the same somites that provide the tissue for the striated muscles of the perineum. Focus is also on its double-layered construction. This construction is shown to be the result of an early division into two components, superficial and deep. The latter participates in the strong proliferation of the stroma dorsal to the urethral orifice, thereby forming a continuous layer of parallel bundles of smooth muscle cells extending from the external anal sphincter to the free margin of the prepuce. Meanwhile, the superficial component keeps its distinct structure and remains restricted to the skin of the scrotum. Together, the two components provide the dartos fascia of the scrotum with its remarkably intricate bilaminate histologic structure, as also recognized in the adult (Holstein et al. 1974).
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