Reproductive System

Reproductive Overview

The human reproductive system usually involves internal fertilization by sexual intercourse. During copulation, the male inserts his erect penis into the female’s vagina and ejaculates semen, which contains sperm. A small portion of the sperm pass through the cervix into the uterus and then into the fallopian tubes for fertilization of the ovum. Only one sperm is required to fertilize the ovum. Upon successful fertilization, the fertilized ovum, or zygote, travels out of the fallopian tube and into the uterus, where it implants in the uterine wall. This marks the beginning of gestation, better known as pregnancy, which continues for nine months as the fetus develops. When the fetus has developed to a certain point, pregnancy is concluded with childbirth or labor. During labor, the muscles of the uterus contract and the cervix dilates over the course of hours, and the baby passes out of the vagina. The female reproductive system has two functions: The first is to produce egg cells, and the second is to protect and nourish the fetus until birth. The male reproductive system has one function, and it is to produce and deposit sperm. Humans have a high level of sexual differentiation.

Female Reproductive System Overview

The human female reproductive system is a series of organs primarily located inside of the body and around the pelvic region of a female that contribute towards the reproductive process. The human female reproductive system contains three main parts: the vagina, the uterus and the ovaries.

It is in the ovaries that the female’s eggs or ova are produced.

The breasts are involved during the parenting stage of reproduction, but in most classifications they are not considered to be part of the female reproductive system. The vagina meets the outside at the vulva, which also includes the labia, clitoris and urethra; during intercourse this area is lubricated by secreted mucus. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the fallopian tubes. Each ovary contains hundreds of ova. Approximately every 28 days, the pituitary gland releases a hormone that stimulates some of the ova to develop and grow. One ovum is released and it passes through the fallopian tube into the uterus. Hormones produced by the ovaries prepare the uterus to receive the ovum. The lining of the uterus, called the endometrium, and unfertilized ova are shed each cycle through the process of menstruation. If the ovum is fertilized by sperm, it attaches to the endometrium and the fetus develops.

Female Reproductive Anatomy
By CDC, Mysid [Public domain], via Wikimedia Commons

Male Reproductive System Overview

The male reproductive system is a series of organs located outside of the body and around the pelvis region of a male that contribute towards the reproduction process. The primary direct function of the male reproductive system is to provide the male sperm for fertilization of the ovum.

The male reproductive system consists of a number of sex organs that play a role in the process of human reproduction. These organs are located on the outside of the body and within the pelvis. Sperm production takes place in the testes which are housed in the temperature regulating scrotum where the immature sperm then travel to the epididymis for development and storage. The ejaculatory fluid is produced by special glands which include the seminal vesicles, prostate, and the vas deferens. The penis, urethra, vas deferens and Cowper’s gland are structures of the reproductive gland that are those used for copulation and deposition of the spermatozoa (sperm) into the female by the male. An important sexual hormone of males is androgen, and particularly testosterone. The testes release a hormone that controls the development of sperm. This hormone is also responsible for the development of physical characteristics in men such as facial hair and a deep voice.

Use the acronym SEVEN UP to follow the path of semen.

  • S = Seminiferous tubules
  • E = Epididymis
  • V = Vas Deferns
  • E = Ejaculatory Duct
  • N = Nothing
  • U = Urethra
  • P = Penis
Male Reproductive Anatomy
By User:Indolences [Public domain or Public domain], via Wikimedia Commons

Role of Hormones/Menstrual Cycle

The hormone GnRH stimulates release of follicle stimulating hormone (FH) and leutinizing hormone (LH). Follicle stimulating hormone stimulates growth and maturation of a follicle, which is where an oocyte is housed, and it produces estrogen. Estrogen normally inhibits LH and FSH, but causes LH surge when it reaches a certain threshold. Leutinizing hormone stimulates the outer cells of the follicle which becomes a corpus luteum and the hormone maintains it. A surge in LH triggers primary oocyte transformation to a secondary oocyte causing the rupture of a follicle. A primary oocyte matures into a secondary oocyte every month. To prepare for it, the endometrium thickens. Corpus luteum makes estrogen and progesterone and maintains endometrium.

No fertilization results in the dropping of LH levels which causes the corpus luteum to die. This proceeds to cause a fall in estrogen and progesterone which results in the destruction of the thickened endometrium, causing menstruation. The cycle begins anew with FSH and LH levels re-rising. If fertilization does occur, the implanted embryo releases hCG which mimics LH to maintain the corpus luteum. The corpus luteum continues to produce estrogen and progesterone until this duty is taken over by the placenta later on.

Menstrual Cycle
Chris 73 / Wikimedia Commons [GFDL 1.3 (www.gnu.org/licenses/fdl-1.3.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Spermatogenesis

Spermatogenesis occurs in the seminiferous tubules and is the process by which new sperm cells (spermatozoa) are formed. Spermatogonium (2n) is the stem or starting cell. Mitosis of spermatogonium can either create more spermatogonium or create primary spermatocyte. This occurs after puberty. Primary spermatocyte (2n) undergoes meiosis I to form a secondary spermatocyte (n). The secondary spermatocyte (n) undergoes meiosos II to form a spermatid (n). Spermatid (n) undergo maturation to form mature sperm (n).

Oogenesis

Ooogenesis occurs in the ovaries, then fallopian tubes and it is how egg cells (ova) are formed. Oogonium (2n) is the stem cell and it undergoes mitosis to form a primary oocyte. The primary oocyte (2n) freezes at prophase I as this process occurs before birth. Every month, one comes out of its arrested state, between puberty and menopause. Primary oocyte (2n) undergoes meiosis I to form a secondary oocyte (n). It will then ruptures from the ovary follicle into the fallopian tube. Secondary oocyte (n) arrests at metaphase II and comes out of arrest if fertilization occurs. Secondary oocyte (n) undergoes meiosis II to form ovum (n).

Gametogensis
Note: Anaphase I has been omitted. Remember that the primary oocyte/spermatocyte will divide into 2 cells first (secondary oocyte/spermatocyte). These 2 cells will then divide by Meiosis II to create the 4 final cells.

Fertilization

Sperm are motile or capable of movement due to a tail like structure known as a flagella. The egg is non-motile and round. Sperm contributes DNA only becausethe the egg actively destroys sperm mitochondria upon fertilization. The egg contributes DNA as well as everything else such as mitochondria, organelles and epigenetics. Fertilization is when the sperm and egg fuse to become a zygote. At this point the egg completes meiosis II. Acrosomal reaction causes sperm to penetrate egg and cortical reaction causes egg to prevent additional sperm from penetrating it.

Gastrulation

The zygote becomes a morula or solid ball of cells as the cells of the sperm/egg fusion grows then divides, grows again, then divides. This then becomes a blastocyst (mammals) and the blastocyst is what is implanted in the endometrium. Cleavage is mitotic divisions without cell growth and it produces the morula. Cleavage produces the morula and the morula hollows out into the blastocyst. Over the period of nine months, the blastocyte undergoes organogenesis and gastrulation.

At birth, the baby switches from getting oxygen from mother’s blood to breathing on its own. The bay also switches from getting nutrients from mother’s blood to suckling on the breast. Fetal circulation (which bypasses lungs and liver) transitions to normal circulation by the closing off of ducts and openings. Cells from the surface migrate inwards. In mammals, the cells start migrating inward at the primitive streak and forms the primary germlayers (endoderm, mesoderm, ectoderm). The cells that migrate inwards form the endoderm, the cells that remain outside become the ectoderm and the cells in the middle become the mesoderm.

  • Ectoderm forms the brain and spinal cord and the does so by folding into a tube. Also forms outermost layers such as the skin, nerves, hair, and nails.
  • Mesoderm forms middle layers such as muscle, blood and bone tissues. Also creates the circulatory system, gonads, and kidneys.
  • Endoderm forms the innermost epithelial linings such as the gut, lungs, and digestive internal organs such as bladder, stomach, liver, colon, and pancreas. Also forms the lungs.
Gastrulation
By CNX OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

Attachments5

  • Female Reproductive Anatomy
  • Menstrual Cycle
  • Male Reproductive Anatomy
  • Gametogensis
  • Gastrulation
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