Question

Describe with diagram the embryogenesis of a phallus.

Hint:

The development of the phallus is a complex process involving the genital tubercle, urethral folds, and androgenic influences, ultimately leading to the formation of the glans penis and shaft.

Answer 1

The development of the phallus occurs during embryogenesis as part of the formation of male external genitalia. The process involves both androgen-dependent and androgen-independent stages, with the contribution of the mesoderm, ectoderm, and genital tubercle.
Step 1: Formation of Genital Tubercle:
At around the 6th week of embryonic development, a structure called the genital tubercle forms from the mesoderm. This structure is the precursor to the phallus in males and the clitoris in females. In response to the presence of male hormones (androgens), particularly testosterone, the genital tubercle elongates and develops into the phallus.
Step 2: Development of Urethra:
The urethra forms from the endodermal tissue and begins as a solid epithelial cord within the genital tubercle. It gradually opens up to form the urethral groove on the underside of the phallus. This groove later closes to form the urethral tube, with the opening at the tip of the glans penis.
Step 3: Development of Glans Penis:
As the genital tubercle elongates, the glans penis begins to form at its distal end. The ectodermal tissue at the tip of the genital tubercle gives rise to the glans, which is the rounded, bulbous structure at the tip of the phallus. The glans penis is separated from the rest of the phallus by a groove, which later forms the preputial ring.
Step 4: Fusion of Urethral Fold:
The urethral folds, which form on either side of the urethral groove, eventually fuse in the midline to complete the formation of the penis shaft. This fusion is crucial for the formation of the complete urethra. The glans and shaft form the fully developed phallus by the 16th week of gestation.
Step 5: Androgen Influence:
The presence of androgens (such as testosterone) is crucial for the masculinization of the genital tubercle. In the absence of androgens, the genital tubercle develops into a clitoris. The action of dihydrotestosterone (DHT) also promotes the formation of the phallus.