MALE ANATOMY INTRODUCTION - Anatomical differences between the sexes (Part II)

General Information -Developmental origin of sex-specific structures -Back to Contents

General Information

Males and hermaphrodites show substantial differences in their anatomy (summarized in MaleFig6-MaleFig10). Many structures are either sex-specific, being entirely unique to that sex (e.g. the hermaphrodite vulva, male sensory rays and fan), or are present in both sexes but have been sex-specifically modified (i.e. are sexually dimorphic, such as the tail tip, the anal depressor and sphincter muscles and some neurons). Currently, the only tissues thought to be sex-indifferent are the pharynx, excretory system and main body muscles (Sulston and Horvitz, 1977; Sulston et al., 1980; Hodgkin, 1988). Adult males and hermaphrodites also differ in somatic nuclei number (males: 1031; hermaphrodites: 959) and the proportion of these that are sexually specialized (40% of male and 30% of hermaphrodite nuclei). Additional male somatic cells arise through the division of cells, which in the hermaphrodite, either do not divide, undergo programmed cell death or produce alternative lineages (Sulston and Horvitz, 1977; Sulston et al., 1980). Male and hermaphrodite germ lines also differ both in nuclei number and the type of gametes they produce (Klass et al., 1976; Kimble and Hirsh, 1979).

Developmental origin of sex-specific structures

At hatching, male and hermaphrodite larvae are anatomically identical apart from a few cells (blue labels MaleFig5). Most sex-specific tissues of the adult are formed from the descendents of blast cells (black labels MaleFig5), that are present in both sexes but express different fates or lineages in each sex (MaleFig5; Sulston and Horvitz, 1977; Sulston et al., 1980).


The Epithelial System

The epithelial system includes the hypodermis, cells of the lateral seam and the interfacial cells that line the major openings to the outside (e.g., anus, lined by the rectal epithelium) or sensory openings (socket and sheath cells; for overview see General description - EPITHELIAL SYSTEM OF THE MALE). Major differences between the sexes are found in the posterior and the ventral body (MaleFig6). In the male, cells of the posterior lateral seam generate the support cells (and neurons) of the sensory rays, instead of alae (described in EPITHELIAL SYSTEM OF THE MALE-Part I and Part II). hyp cells of the tail tip reorganize to form a blunt tail morphology (EPITHELIAL SYSTEM OF THE MALE-Part I; for animation visit Fitch lab website; Sulston and Horvitz, 1977; Sulston et al., 1980; Nguyen et al., 1999). The hypodermis (or lateral seam) secretes the cuticular fan in which the sensory rays are embedded. Internally, rectal epithelial cells (B, F, U, Y, K and K') divide to produce the cloaca and spicule channels of the proctodeum, replacing the simple larval rectum (see THE PROCTODEUM - EPITHELIAL SYSTEM OF THE MALE, PARTIII). The B lineage also generates the support cells (and neurons) or the spicules. Ventrally, hypodermal P cells in the posterior generate support cells (and neurons) of the hook. The rectal epithelial cell Y produces the support cells (and neurons) of the PCS, which lies posterior to the hook. In hermaphrodites P cells in the mid-body make the vulval epithelium (and associated egg-laying neurons).


The Nervous System

The male nervous system contains 383 neurons (89 of which are sex-specific) and the hermaphrodite 302 neurons (8 of which are sex-specific) (MaleFig7A, 7B). Most male-specific neurons are located in the posterior (Sulston and Horvitz, 1977; Sulston et al., 1980) and many have been shown to have specific roles in male mating behavior (MaleFig2 MALE HANDBOOK-Introduction-Part I; Table 1- Emmons and Sternberg, 1997). The hermaphrodite sex-specific neurons are located in the VNC (VC1-6) and around the vulva (HSNs) and control egg-laying behavior (see HERMAPHRODITE HANDBOOK-Reproductive System). There are also neurons common to both male and hermaphrodite nervous systems that exhibit sexually dimorphic synaptic patterns or morphology (e.g. BDU, PLM, and PDB; White et al., 1986; Hodgkin, 1988; Sulston et al., 1980). Potentially, more neurons of this latter class may be discovered as the nervous system of the male is reconstructed (for recent reconstructions see Wormatlas INDIVIDUAL NEURONS pages or THE MALE WIRING PROJECT on the Emmons lab website).


The Reproductive System

The reproductive system consists of the somatic gonad, germ line (MaleFIG8) and associated muscles (MaleFIG9) and neurons (MaleFig7A). In both sexes, the somatic gonad and germ line arise from the gonadal primordium (GP), which consists of 4 cells at L1, Z1-Z4 (Kimble and Hirsh, 1979). These cells execute sex-specific lineages during development establishing a distinct reproductive system in each sex. The adult male gonad consists of a single J-shaped arm that connects to the cloaca (rectal epithelial tube) of the proctodeum. The cloaca opens to the environment at the anus (cloacal opening) (for details see MALE REPRODUCTIVE SYSTEM and THE PROCTODEUM - EPITHELIAL SYSTEM OF THE MALE, PARTIII; Sulston and Horvitz, 1977; Sulston et al., 1980; MaleFIG6). The male germ line produces only male gametes (sperm) (Hirsh et al., 1976; Klass et al., 1976). By contrast, the adult hermaphrodite gonad consists of 2 U-shaped arms that join at the mid-body to a common uterus. The uterus opens to the exterior via the vulva. The hermaphrodite germ line produces female gametes (oocytes) and a limited number of sperm (see HERMAPHRODITE HANDBOOK-Reproductive System).


The Muscles

In this system, males and hermaphrodites differ most in their sex muscles (MaleFig9). The sex muscles of the male are necessary for several steps in mating behavior (see MUSCLES OF THE MALE-Part I and Part II; Loer and Kenyon, 1993; Garcia et al., 2001). In hermaphrodites, the sex muscles control egg-laying behavior. In both males and hermaphrodites sex muscles are descended from M, which executes different lineages in each sex (Sulston and Horvitz, 1977; Sulston et al., 1980). In the male M generates 41 muscles (of 7 types) that are distributed throughout the posterior; in the hermaphrodite 16 sex muscles (of 2 types) are produced and these are centered on the uterus and vulva.


The Coelomocyte System

At hatching males and hermaphrodites have the same number of coelomocytes (ccs) but the position of two differ with respect to the gonadal primordium (see MaleFig5). During larval development M generates additional ccs in both sexes. In the male the M lineage produces a single cc, located towards the tail; in the hermaphrodite M generates a bilateral pair of ccs, located in the dorsal posterior half of the animal (see HERMAPHRODITE HANDBOOK-Coelomocyte System; Sulston and Horvitz, 1977; Sulston et al., 1980).

The Alimentary System

The alimentary system consists of the pharynx, the intestine, muscles and neurons required for defecation, the rectum and anus. In the male the simple larval rectum is replaced by the proctodeum (MaleFig10; for details see THE PROCTODEUM - EPITHELIAL SYSTEM OF THE MALE, PARTIII). This structure is a common outlet for both the alimentary tract and the genital tract. It includes the non-sex-specific rectal glands and valve and several male-specific structures: the cloaca, spicule channels and spicules. These latter structures are formed by the progeny of B, F U, K and P12. In hermaphrodites most of these cells do not divide. Muscles and neurons are also associated with the proctodeum. Some are sex-specific and while others are present in both sexes but modified in the male (the sphincter and anal depressor muscles) to accomodate the dual role of the proctodeum in defecation and ejaculation (Sulston and Horvitz, 1977; Sulston et al., 1980). In the hermaphrodite, but not the male, the intestine produces yolk protein from L4 onwards, which is taken up by developing oocytes in the gonad.


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