Human Reproduction Summary Notes

 Male Reproductive System

• Primary sex organs: A pair of testes suspended in the scrotum
• Secondary sex organs: A pair of ducts each differentiated into rete testis, vasa efferentia, epididymis and vas deferens, ejaculatory duct and the associated glands
• External genitalia: Penis

Gametogenesis (Spermatogenesis)

• The process of formation of male gametes (sperms) in the testes
• Spermatogonia (immature male germ cells) undergo mitotic division to increase in number
• Spermatogonia differentiate into primary spermatocytes
• Primary spermatocytes undergo meiotic division to produce secondary spermatocytes
• Secondary spermatocytes undergo meiotic division to produce spermatids
• Spermatids are transformed into spermatozoa by the process of spermiogenesis

Hormonal Control of Spermatogenesis

• Gonadotropin releasing hormone (GnRH) is secreted by the hypothalamus
• GnRH stimulates the anterior pituitary to secrete luteinizing hormone (LH) and follicle stimulating hormone (FSH)
• LH acts on Leydig cells in the testes to stimulate the secretion of androgens
• FSH acts on Sertoli cells in the testes to support spermatogenesis

Structure of Sperm

• Sperm is a microscopic structure composed of a head, neck, middle piece, and tail
• The sperm head contains the elongated haploid nucleus, which is covered by an acrosome
• The acrosome contains enzymes that help the sperm penetrate the egg

Female Reproductive System

• Primary sex organs: A pair of ovaries
• Secondary sex organs: A pair of fallopian tubes, uterus, cervix, and vagina
• External genitalia: Mons pubis, labia majora, labia minora, clitoris
• Mammary glands: A pair of glands that produce milk

Gametogenesis (Oogenesis)

• The process of formation of female gametes (ova) in the ovaries
• Oogonia (gamete mother cells) are formed during embryonic development
• Oogonia divide by mitosis to produce primary oocytes
• Primary oocytes undergo meiotic division I and arrest in prophase I until puberty
• At puberty, a few primary oocytes begin to complete meiotic division I to produce secondary oocytes
• Secondary oocytes are ovulated from the ovaries and undergo meiotic division II only if they are fertilized by a sperm

Menstrual Cycle

• The menstrual cycle is a series of hormonal changes that occur in the female reproductive system each month
• The cycle begins with the menstrual phase, which is the shedding of the endometrium (lining of the uterus)
• The follicular phase follows, during which a follicle in the ovary develops and produces estrogen
• Estrogen stimulates the proliferation of the endometrium
• The ovulatory phase occurs when the follicle ruptures and releases an ovum (egg)
• The luteal phase follows, during which the corpus luteum (ruptured follicle) produces progesterone
• Progesterone maintains the endometrium in preparation for implantation of a fertilized egg
• If the egg is not fertilized, the corpus luteum degenerates and progesterone levels drop
• This drop in progesterone triggers menstruation

Fertilization

• Fertilization is the fusion of a sperm with an ovum
• Fertilization occurs in the ampullary-isthmic junction of the fallopian tube
• After fertilization, the zygote (fertilized egg) travels down the fallopian tube and implants in the endometrium of the uterus

Implantation

• Implantation is the process by which the zygote attaches to the endometrium of the uterus
• Implantation occurs 5-6 days after fertilization
• After implantation, the zygote begins to develop into an embryo

Embryonic Development

• The embryo undergoes cleavage to form a blastocyst.
• The blastocyst implants in the endometrium of the uterus.
• The inner cell mass of the blastocyst develops into the embryo.
• The ectoderm, endoderm, and mesoderm develop from the inner cell mass.
• The major organs and external genital organs develop by the end of 12 months.

Parturition and Lactation

• Parturition is the process of delivery of a fully developed fetus.
• Signals for parturition originate from the fully developed fetus and placenta.
• Oxytocin is released from the maternal pituitary to induce uterine contractions.
• The mammary glands of the female start producing milk at the end of pregnancy.
• The milk produced during the initial few days of lactation is called colostrum.

Polyspermy

• Polyspermy is the fertilization of an egg by more than one sperm.
• If polyspermy occurs, the resulting cell has too much genetic material to develop normally.