The development of the gonads is part of the

prenatal Prenatal development () includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal devel ...

development of the reproductive system The development of the reproductive system is the part of embryonic growth that results in the sex organs and contributes to sexual differentiation. Due to its large overlap with development of the urinary system, the two systems are typically ...

and ultimately forms the

testes A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoste ...

in males and the

ovaries The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. T ...

in females. The

gonads A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...

initially develop from the mesothelial layer of the

peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of meso ...

. The ovary is differentiated into a central part, the

medulla Medulla or Medullary may refer to: Science * Medulla oblongata, a part of the brain stem * Renal medulla, a part of the kidney * Adrenal medulla, a part of the adrenal gland * Medulla of ovary, a stroma in the center of the ovary * Medulla of t ...

, covered by a surface layer, the germinal epithelium. The immature ova originate from cells from the dorsal endoderm of the

yolk sac The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is ...

. Once they have reached the

gonadal ridge The genital ridge (or gonadal ridge) is the precursor to the gonads. The genital ridge initially consists mainly of mesenchyme and cells of underlying mesonephric origin. Once oogonia enter this area they attempt to associate with these somatic ce ...

they are called

oogonia An oogonium (plural oogonia) is a small diploid cell which, upon maturation, forms a primordial follicle in a female fetus or the female (haploid or diploid) gametangium of certain thallophytes. In the mammalian fetus Oogonia are formed in lar ...

. Development proceeds and the oogonia become fully surrounded by a layer of connective tissue cells (pre-granulosa cells). In this way, the rudiments of the

ovarian follicle An ovarian follicle is a roughly spheroid cellular aggregation set found in the ovaries. It secretes hormones that influence stages of the menstrual cycle. At the time of puberty, women have approximately 200,000 to 300,000 follicles, each with t ...

s are formed. In the testis, a

network Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...

of tubules fuse to create the

seminiferous tubules Seminiferous tubules are located within the testes, and are the specific location of meiosis, and the subsequent creation of male gametes, namely spermatozoa. Structure The epithelium of the tubule consists of a type of sustentacular cells known ...

. Via the

rete testis The rete testis ( ) is an anastomosing network of delicate tubules located in the hilum of the testicle (mediastinum testis) that carries sperm from the seminiferous tubules to the efferent ducts. It is the counterpart of the rete ovarii in fe ...

, the seminiferous tubules become connected with outgrowths from the

mesonephros The mesonephros ( el, middle kidney) is one of three excretory organs that develop in vertebrates. It serves as the main excretory organ of aquatic vertebrates and as a temporary kidney in reptiles, birds, and mammals. The mesonephros is included ...

, which form the efferent ducts of the testis. The descent of the testes consists of the opening of a connection from the testis to its final location at the anterior abdominal wall, followed by the development of the

gubernaculum The paired gubernacula (from Ancient Greek κυβερνάω = pilot, steer) also called the caudal genital ligament, are embryonic structures which begin as undifferentiated mesenchyme attaching to the caudal end of the gonads (testes in mal ...

, which subsequently pulls and translocates the testis down into the developing

scrotum The scrotum or scrotal sac is an anatomical male reproductive structure located at the base of the penis that consists of a suspended dual-chambered sac of skin and smooth muscle. It is present in most terrestrial male mammals. The scrotum co ...

. Ultimately, the passageway closes behind the testis. A failure in this process causes an

indirect inguinal hernia An inguinal hernia is a hernia (protrusion) of abdominal-cavity contents through the inguinal canal. Symptoms, which may include pain or discomfort especially with or following coughing, exercise, or bowel movements, are absent in about a third ...

Germ cell Germ or germs may refer to: Science * Germ (microorganism), an informal word for a pathogen * Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually * Germ layer, a primary layer of cells that forms during emb ...

s migrate from near the allantois and colonize the primordial gonads. In the female, the germ cells colonise the cortex and become oogonia. In the male, the germ cells colonise the seminiferous cords of the medulla, becoming

spermatogonia A spermatogonium (plural: ''spermatogonia'') is an undifferentiated male germ cell. Spermatogonia undergo spermatogenesis to form mature spermatozoa in the seminiferous tubules of the testis. There are three subtypes of spermatogonia in humans: ...

Before differentiation

The first appearance of the gonad is essentially the same in the two sexes, and consists in a thickening of the

mesothelial The mesothelium is a membrane composed of simple squamous epithelial cells of mesodermal origin, which forms the lining of several body cavities: the pleura ( pleural cavity around the lungs), peritoneum ( abdominopelvic cavity including the ...

layer of the

. The thick plate of epithelium extends deeply, pushing before it the mesoderm and forming a distinct projection. This is termed the

. The gonadal ridge, in turn, develops into a gonad. This is a

testis A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...

in the male and an ovary in the female. At first, the

and gonadal ridge are continuous, but as the embryo grows the gonadal ridge gradually becomes pinched off from the mesonephros. However, some cells of mesonephric origin join the gonadal ridge. Furthermore, the gonadal ridge still remains connected to the remnant of that body by a fold of peritoneum, namely the

mesorchium The testes, at an early period of foetal life, are placed at the back part of the abdominal cavity, behind the peritoneum, and each is attached by a peritoneal fold, the mesorchium, to the mesonephros. See also * mesentery The mesentery is an ...

mesovarium The mesovarium is the portion of the broad ligament of the uterus that suspends the ovaries. The ovary is not covered by the mesovarium; rather, it is covered by germinal epithelium. At first the mesonephros and genital ridge are suspended by a ...

. About the seventh week the distinction of sex in the gonadal ridge begins to be perceptible.

Ovary

The ovary is thus formed mainly from the genital ridge and partly from the mesonephros. Later the mass is differentiated into a central part, the

medulla of ovary The medulla of ovary (or Zona vasculosa of Waldeyer) is a highly vascular stroma in the center of the ovary. It forms from embryonic mesenchyme and contains blood vessels, lymphatic vessels, and nerves. This stroma forms the tissue of the hilum ...

, covered by a surface layer, the germinal epithelium. Between the cells of the germinal epithelium a number of larger cells, the immature ova, are found. The immature ova, in turn, are carried into the stroma beneath by bud-like ingrowths (''genital cords of the germinal epithelium''). The surface germinal epithelium ultimately forms the permanent epithelial covering of this organ. Furthermore, it soon loses its connection with the central mass. Instead, the tunica albuginea of the ovaries develops between them.

Origin of ova

During early embryonic development, cells from the dorsal endoderm of the

migrate along the hindgut to the gonadal ridge. These

primordial germ cells Primordial may refer to: * Primordial era, an era after the Big Bang. See Chronology of the universe * Primordial sea (a.k.a. primordial ocean, ooze or soup). See Abiogenesis * Primordial nuclide, nuclides, a few radioactive, that formed before ...

(PGCs) multiply by mitosis and once they have reached the gonadal ridge they are called

(diploid stem cells of the ovary). Once oogonia enter this area they attempt to associate with the other somatic cells, derived from both the peritoneum and mesonephros. Development proceeds and the oogonia become fully surrounded by a layer of connective tissue cells (pre-

granulosa cell A granulosa cell or follicular cell is a somatic cell of the sex cord that is closely associated with the developing female gamete (called an oocyte or egg) in the ovary of mammals. Structure and function In the primordial ovarian follicle, and la ...

s) in an irregular manner. In this way, the rudiments of the

ovarian follicles An ovarian follicle is a roughly spheroid cellular aggregation set found in the ovaries. It secretes hormones that influence stages of the menstrual cycle. At the time of puberty, women have approximately 200,000 to 300,000 follicles, each with t ...

are formed.

Origin of granulosa cells

The embryological origin of

s remains controversial. In the 1970s, evidence emerged that the first cells to make contact with the oogonia were of mesonephric origin. It was suggested that mesonephric cells already closely associated with the oogonia proliferated throughout development to form the granulosa cell layer. Recently this hypothesis has been challenged with some thorough histology. Sawyer et al. hypothesized that in sheep most of the granulosa cells develop from cells of the mesothelium (i.e., epithelial cells from the presumptive surface epithelium of the ovary).

Descent of the ovaries

Just as in the male, there is a

in the female, which effects a considerable change in the position of the ovary, though not so extensive a change as in that of the testis. The gubernaculum in the female lies in contact with the fundus of the uterus and adheres to this organ, and thus the ovary can only descend as far as to this level. The part of the gubernaculum between the ovary and the uterus ultimately becomes the proper

ovarian ligament The ovarian ligament (also called the utero-ovarian ligament or proper ovarian ligament) is a fibrous ligament that connects the ovary to the lateral surface of the uterus. Structure The ovarian ligament is composed of muscular and fibrous tissu ...

, while the part between the uterus and the labium majus forms the

round ligament of the uterus The round ligament of the uterus is a ligament that connects the uterus to the labia majora. Structure The round ligament of the uterus originates at the uterine horns, in the parametrium. The round ligament exits the pelvis via the deep inguina ...

. A pouch of peritoneum analogous to the vaginal process in the male accompanies it along the inguinal canal: it is called the

canal of Nuck __NOTOC__ The canal of Nuck, first described by Anton Nuck ( de) in 1691, is an abnormal patent (open) pouch of peritoneum extending into the labia majora of women. It is analogous to a patent processus vaginalis in males (see hydrocele testis, in ...

Pathology

In rare cases, the gubernaculum may fail to contract adhesions to the uterus, and then the ovary descends through the inguinal canal into the

labium majus The labia majora (singular: ''labium majus'') are two prominent Anatomical terms of location, longitudinal Skin, cutaneous folds that extend downward and backward from the mons pubis to the perineum. Together with the labia minora they form the ...

; under these circumstances, its position resembles that of the testis.

Testis

The

is developed in much the same way as the ovary, originating from mesothelium as well as mesonephros. Like the ovary, in its earliest stages it consists of a central mass covered by a surface epithelium. In the central mass a series of cords appear, and the periphery of the mass is converted into the tunica albuginea, thus excluding the surface epithelium from any part in the formation of the tissue of the testis. The cords of the central mass run together toward the future hilum and form a network which ultimately becomes the

. On the other hand, the

seminiferous tubule Seminiferous tubules are located within the testes, and are the specific location of meiosis, and the subsequent creation of male gametes, namely spermatozoa. Structure The epithelium of the tubule consists of a type of sustentacular cells known ...

s are developed from the cords distal to the hilum, and between them connective-tissue septa extend. Via the rete testis, the seminiferous tubules become connected with outgrowths from the mesonephros, which form the efferent ducts of the testis.

Descent of the testes

The descent of the testes consists of the opening of a connection from the testis to its final location at the anterior abdominal wall, followed by the development of the gubernaculum, which subsequently pulls and translocates the testis down into the developing scrotum. Ultimately, the passageway closes behind the testis.

Opening of connection

At an early period of fetal life the testes are placed at the back part of abdominal cavity, behind the peritoneum, and each is attached by a peritoneal fold, the

, to the mesonephros. From the front of the mesonephros a fold of peritoneum termed the inguinal fold grows forward to meet and fuse with a peritoneal fold, the inguinal crest, which grows backward from the antero-lateral abdominal wall. The testis thus acquires an indirect connection with the anterior abdominal wall. At the same time, a portion of the peritoneal cavity lateral to these fused folds is marked off as the future vaginal process.

Development of gubernaculum

Also, in the inguinal crest a structure, the

gubernaculum testis In the inguinal crest of a peculiar structure, the gubernaculum testis makes its appearance. This is at first a slender band, extending from that part of the skin of the groin which afterward forms the scrotum through the inguinal canal to the bod ...

, makes its appearance. This is at first a slender band, extending from that part of the skin of the

groin In human anatomy, the groin (the adjective is ''inguinal'', as in inguinal canal) is the junctional area (also known as the inguinal region) between the abdomen and the thigh on either side of the pubic bone. This is also known as the medial comp ...

which afterward forms the

through the

inguinal canal The inguinal canals are the two passages in the anterior abdominal wall of humans and animals which in males convey the spermatic cords and in females the round ligament of the uterus. The inguinal canals are larger and more prominent in males. ...

to the body and epididymis of the testis. As and contains the upper part of the gubernaculum, and ultimately also the internal spermatic vessels; the one below, the plica gubernatrix, contains the lower part of the gubernaculum. The gubernaculum grows into a thick cord. It ends below at the abdominal inguinal ring in a tube of peritoneum, the vaginal process, which protrudes itself down the inguinal canal. By the fifth month the lower part of the gubernaculum still is a thick cord, while the upper part has disappeared. The lower part now consists of a central core of smooth muscle fibers, surrounded by a firm layer of

striated muscle Striations means a series of ridges, furrows or linear marks, and is used in several ways: * Glacial striation * Striation (fatigue), in material * Striation (geology), a ''striation'' as a result of a geological fault * Striation Valley, in An ...

elements, connected, behind the peritoneum, with the abdominal wall.

Translocation

As the testes develops, the main portion of the lower end of the gubernaculum is carried, following the skin to which it is attached, to the bottom of this pouch. Other bands are carried to the medial side of the thigh and to the perineum. The tube of peritoneum constituting the vaginal process projects itself downward into the inguinal canal, and emerges at the

external inguinal ring The inguinal canals are the two passages in the anterior abdominal wall of humans and animals which in males convey the spermatic cords and in females the round ligament of the uterus. The inguinal canals are larger and more prominent in males. T ...

, pushing before it a part of the

obliquus internus The abdominal internal oblique muscle, also internal oblique muscle or interior oblique, is an abdominal muscle in the abdominal wall that lies below the external oblique muscle and just above the transverse abdominal muscle. Structure Its fiber ...

and the aponeurosis of the obliquus externus, which form respectively the

cremaster muscle The cremaster muscle is a paired structure made of thin layers of striated and smooth muscle that covers the testis and the spermatic cord in human males. It consists of the lateral and medial parts. Cremaster is an involuntary muscle, responsib ...

and the external spermatic fascia. The vaginal process forms a gradually elongating pouch, which eventually reaches the bottom of the scrotum, and behind this pouch the testis is drawn by the growth of the body of the fetus, for the gubernaculum does not grow proportionately with the growth of other parts, and therefore the testis, being attached by the gubernaculum to the bottom of the scrotum, is prevented from rising as the body grows, and is instead drawn first into the inguinal canal and eventually into the scrotum. It seems certain also that the gubernacular cord becomes shortened as development proceeds, and this assists in causing the testis to reach the bottom of the scrotum.

Closing of connection

By the end of the eighth month the testis has reached the scrotum, preceded by the vaginal process, which communicates by its upper extremity with the peritoneal cavity. Just before birth the upper part of the vaginal process, at the internal inguinal ring, usually becomes closed, and this obliteration extends gradually downward to within a short distance of the testis. The process of peritoneum surrounding the testis is now entirely cut off from the general peritoneal cavity and constitutes the tunica vaginalis.

=Pathology

= If the internal inguinal ring does not close properly, then there is a risk that other contents of the abdominal cavity protrudes through the passageway and cause

References

{{Development of urinary and reproductive systems Embryology of urogenital system