Male and Female Reproduction

Bulbourethral Glands

  • The bulbourethral glands are located just off the spongy urethra and corpora cavernosa.
  • These glands produce a lubricant containing mucin, a viscous material.
  • This lubricant constitutes the pre-ejaculate fluid.
  • The three glands of the male reproductive tract (after leaving the vas deferens) are the seminal vesicle, the prostate gland, and the bulbourethral glands.

Penis

  • Root: The root is the anchor of the penis and is internal to the body.
  • Body: The main part of the penis.
  • Erectile Columns: The penis contains three erectile columns of tissue that fill with blood during an erection: two corpora cavernosa and one corpus spongiosum.
  • Glans: The tip of the penis (head).
  • Prepuce (Foreskin): Technically called the prepuce, it is not shown in the slide illustration.

Cylindrical Columns of Tissue

  • The cylindrical columns of tissue fill with blood during an erection.
  • Corpora Cavernosa: A pair located on the superior or ventral surface of the penis.
  • Corpus Spongiosum: A single column located underneath the corpora cavernosa; the urethra runs through it, leading to the urethral orifice at the tip.

Histological Cross-Section of the Penis

  • The cross-section reveals two large corpora cavernosa and a single corpus spongiosum.
  • The spongy or penile urethra passes through the corpus spongiosum.
  • The tissue has a lot of space and valves within the veins.
  • During arousal, the valves close off, causing the erectile columns to engorge with blood.
  • After climax, the valves relax, allowing the blood to drain out.
  • All three erectile tissues fill with blood, inflating the penis.
  • The dark purple circle is the epithelium lining the urethra.

Prepuce (Foreskin) Details

  • The prepuce is consistent with the skin covering the penis.
  • There is loose connective tissue around the columns, but no hypodermis.
  • The skin is thin.
  • The prepuce forms a hood over the glans, connected by the frenulum on the underside of the shaft.

Circumcision

  • Circumcision involves removing the prepuce or foreskin.
  • There is debate about circumcision due to it being performed on babies who cannot consent.
  • Health benefits are debatable; some claim it prevents infection, but evidence suggests this is rare (1 infection in 10,000 cases with foreskin).
  • Circumcision is largely a social, religious, or personal preference.

Fertilization

  • Fertilization starts before and continues after implantation as the embryo develops.
  • Scott Gilbert's quote: "Fertilization is the process whereby two cells, each on the verge of death, unite to create a new organism that will have numerous cell types and organs."
  • After ovulation, the oocyte lives about 24 hours; sperm live for 48-72 hours.
  • Fertilization is the beginning of cell interactions that define animal development.
  • Life is a continuum; a dead egg or sperm cannot create a living zygote.
  • The combination of DNA in a new organism has never existed before (unless identical twins).

Sperm and Fertilization

  • Up to 6 million sperm are involved in fertilization.
  • There is evidence of chemotactic processes attracting sperm (found in rabbits).
  • Many sperm go into the wrong fallopian tube due to random movement.
  • Fertilization usually occurs in the ampulla of the fallopian tube.
  • The embryo must be at a specific developmental stage for implantation.
  • Sperm may be viable for three days, oocytes for only 24 hours.
  • Success depends on timing: gamete meeting, embryo development, and uterine lining preparation.

Zygote vs. Embryo

  • Zygote: The fertilized egg after its first cell division (two cells).
  • Embryo: Develops when cells start differentiating after multiple divisions.

Sperm Capacitation

  • Sperm in a sample are initially incapable of fertilization without capacitation.
  • Capacitation occurs in the female reproductive tract.
  • Receptors and cholesterol on the sperm surface are scrubbed off, primarily by cervical mucus.
  • Calcium must be present.
  • These processes prepare the sperm to break through barriers and fertilize the egg.
  • Capacitation may take up to 7 hours and prolong sperm lifespan.

Oocyte Complex and Fertilization

  • The oocyte is in the second meiotic metaphase, paused with 23 sister chromatids.
  • Sperm must penetrate the corona radiata and zona pellucida.
  • The sperm head contains acrosomal enzymes that are released upon encountering cells of the corona radiata, dissolving a path through it, which allows the sperm to make its way through the zona pellucida.
  • The sperm attaches to receptors to penetrate the zona pellucida.
  • As soon as the first sperm penetrates the oocyte's plasma membrane, the zona reaction occurs, preventing polyspermy (fertilization by multiple sperm).
  • Polyspermy results in embryonic death because an extra set of chromosomes is lethal.
  • Example of Down syndrome (trisomy 21): an extra copy of chromosome 21 leads to significant phenotypic effects, even though the chromosome itself is healthy and contains no mutations.

Formation of Pronuclei and Chromosome Alignment

  • The oocyte completes the second meiotic division.
  • The process resembles mitosis after metaphase II.
  • Sperm entry triggers replication of sperm chromosomes into 23 sister chromatids.
  • The pronuclei membrane breaks down.
  • Microtubules align 46 sister chromatids (23 from each parent) at the equator, resembling metaphase of mitosis.
  • In anaphase, 46 individual chromosomes are pulled to each daughter cell, creating two diploid daughter cells.

Visual Representation of Fertilization

  • Male and female pronuclei come together, chromosomes replicate, and they meld into one before mitosis.
  • Cytokinesis results in two diploid daughter cells.

Microscopic Views of Fertilized Eggs

  • Female pronucleus visible.
  • Female and male pronuclei merge.
  • The mitotic spindle forms for the first division.
  • The sperm tail, containing microtubules, is sometimes visible during staining.

Outcomes of Fertilization

  • Completion of the oocyte's second meiotic division.
  • Establishment of a diploid cell.
  • Variation of species through crossing over and independent assortment that has already occurred.
  • Determination of the embryo's chromosomal sex (XX or XY).
  • Initiation of the cleavage division.

Cleavage Division

  • The first several generations of cell divisions; no growth one or growth two phase.
  • The fertilized egg divides repeatedly without growth, resulting in smaller cells.
  • Later developmental stages include a growth phase, where cells grow bigger.

Undifferentiated Cell Growth

  • Uncontrolled or uncoordinated cell divisions would hinder the right structures.
  • Checkpoints regulate cell division even without a replication phase.

Sex Determination

  • Sex is determined at fertilization.
  • The Y chromosome has genes that stop the female pathway and initiate male development.

Sex Selection

  • Centrifugation separates sperm by weight: X chromosome-bearing sperm sink, Y chromosome-bearing sperm float.
  • Artificial insemination can be used to select sperm based on desired sex (though it most likely isn't a routinely performed procedure).

Early Zygote Development and Transport

  • Fertilization occurs in the ampulla.
  • The zygote undergoes cleavage divisions (two-cell, four-cell, eight-cell stages).
  • The zona pellucida protects the early zygote as it travels down the fallopian tube.
  • Cells become smaller as they increase in number.

Blastomeres and Morula Stage

  • Early cells (first two cells) are called blastomeres.
  • At 18+ cells, the zygote reaches the morula stage (resembling a mulberry).

Totipotency

  • At the 12-15 blastomere stage, cells are totipotent.
  • Totipotent: Each cell can develop into an identical individual if separated and cultured, such with in vitro fertilization.
  • Identical twins result from early separation of totipotent cells.

Timing and Implantation

  • Early development occurs within the fallopian tube (days one through eight).
  • Implantation occurs around day eight in the uterus.
  • Fertilization may occur without implantation.

Blastocyst Formation

  • Cells produce a hollow space inside the morula, forming the blastocyst.
  • Cells differentiate: the inner cell mass forms the embryo, and the surrounding trophoblast becomes part of the placenta.
  • Trophoblast means to feed; trophoblastic cells become the embryonic part of the placenta.
  • The inner cell mass develops into the being (you).

Implantation Process

  • Timing is critical: fertilization, zygote arrival in the uterus, and uterine preparation must align.
  • Trophoblast cells have receptors that attach to pinopodes (projections) on the endometrium.
  • Contact anchors the zygote to the endometrium.

Trophoblast Differentiation

  • Outer cells divide and form a multinucleated syncytiotrophoblast.
  • Inner trophoblast remains as single cells (cytotrophoblast).
  • The syncytiotrophoblast invades the uterine lining using enzymes, breaking blood vessels.
  • Implantation can cause spotting or bleeding but should not cause a full period.

Chorion and Amnion Development

  • The amnion grows, pushing the chorion outward.
  • The outer layer of membrane is the chorion; the inner layer is the amnion, with the embryo in the middle.

Placental Development and Function

  • The syncytiotrophoblast and placenta develop, pushing out on the endometrium.
  • Chorionic villi project into the endometrium.
  • Maternal blood enters large spaces around the villi, bathing them in blood.
  • Gas and nutrient exchange occurs in the capillaries of the villi, transported via the umbilical cord.

Blood Flow and Exchange in the Placenta

  • Arterial blood squirts into inter-villous spaces under high pressure, perfusing villi.
  • Blood then drains into the venous system.

Unique Blood Flow in Umbilical Vessels

  • Arteries usually carry deoxygenated blood with the exception of the pulmonary circulation and umbilical circulation.
  • Umbilical vein: oxygen-rich blood to the fetal heart.
  • Umbilical arteries: deoxygenated blood away from the fetal heart.
  • The umbilical arteries and the veins are reversed (arteries= deoxygenated, veins = oxygenated).
  • Always view in perspective of the circulation of the individual.

Implantation Site

  • Fertilization occurs in the ampulla of the fallopian tube.
  • Implantation typically occurs in the upper portion of the uterus.

Placenta Previa

  • Low implantation near the cervix can lead to placenta previa where the placenta grows across the cervical opening.
  • Placenta previa can cause premature detachment, resulting in a need to receive surfactant.

Cervix and Labor: Normal Placement vs Low Placement

  • The cervix is muscular and resistant to dilation during pregnancy.
  • Effacement thins the cervical wall before dilation.

Placental Abruption

  • Placenta previa is a placental attachment over the internal cervical os.
  • Typically, the placenta is delivered after the baby.
  • Placental abruption, where the placenta detaches before delivery, requires an emergency C-section.

Ectopic Pregnancy

  • Ectopic pregnancy is when fertilization and implantation occur outside the uterus, often in the fallopian tube (tubal pregnancy).
  • The syncytiotrophoblast invades tissues, and if it erodes a major blood vessel, it can cause life-threatening internal bleeding.
  • Tubal pregnancies may require tube removal.