Spermatogenesis Overview

Spermatogenesis

• Males begin producing sperm at puberty (typically ages 10-16).
• Continuous sperm production occurs at a high rate (~200 million per day).
• Sperm production takes place in the seminiferous tubules within the testes.
• A blood-testis barrier (formed by Sertoli cells) isolates the seminiferous tubules from the systemic circulation.
• This barrier prevents blood-borne substances (hormones/waste products) from affecting developing sperm.
• It also prevents the male's immune system from recognizing sperm as foreign (sperm has different surface antigens).

Forming Functional Sperm

• Spermatogonia are the initial diploid cells that divide by mitosis to create new spermatogonia and primary spermatocytes.
• Type B spermatogonia divide by mitosis to create primary spermatocytes.
• Primary spermatocytes undergo meiosis I to produce secondary spermatocytes, which are haploid and linked by cytoplasm bridges.
• Secondary spermatocytes undergo meiosis II, creating four haploid spermatids per primary spermatocyte.

Maturation

• Cytoplasmic bridges between spermatids break down.
• Spermatids are released into the seminiferous tubules, undergoing spermiogenesis to transform into sperm.
• Spermatids travel to the rete testis for sperm concentration by removing excess fluid.
• Sperm then move to the epididymis where final maturation stages occur.
• For continuous sperm production, multiple spermatogenic cycles happen simultaneously within the same seminiferous tubule; every 16 days a new spermatogenic cycle begins.

Following Ejaculation

• Sperm undergoes capacitation in the female reproductive tract.
• Capacitation involves removing cholesterol and glycoproteins from the sperm head, enabling it to bind to the zona pellucida of the egg.

Oogenesis

• Oogenesis begins in the fetus before birth, in contrast to spermatogenesis.
• Primordial germ cells migrate to the primordial gonad.
• Replication by mitosis peaks around 7 million by mid-gestation (~20 weeks).
• Significant cell death occurs after this peak, leaving roughly 2 million cells.
• Primary oocytes are formed and arranged in clusters in the gonads, surrounded by flattened epithelial cells (primary follicle).
• Atypical atresia (cell death) occurs during childhood, decreasing the number of oocytes to approximately 40,000 at puberty.
• Several primary oocytes begin to mature each month, but only one reaches full maturation to become an oocyte.
• Primary oocytes undergo three stages: pre-antral, and antral.

Pre-antral Stage

• The primary oocyte remains in meiosis I.
• Follicular cells proliferate, forming a stratified cuboidal epithelium.
• Granulosa cells are formed, secreting glycoproteins that form the zona pellucida around the oocyte.
• Theca folliculi cells differentiate from surrounding connective tissue.

Antral Stage

• Fluid-filled spaces form between granulosa cells and combine to form antrum.
• Follicles are now called secondary follicles.
• One secondary follicle becomes dominant each month, developing under influence of FSH, LH, and oestrogen.

Pre-Ovulatory Stage

• LH surge triggers the completion of meiosis I.
• Two unequally sized haploid cells are created: a secondary oocyte and a first polar body.
• The secondary oocyte then undergoes meiosis II, but is arrested in metaphase II until fertilization.
• The follicle grows larger becoming a mature Graafian follicle.
• LH surge also increases collagenase activity.
• Collagen breakdown results in weakening of the follicular wall.
• Muscular contractions of the ovarian wall cause ovulation (release of the ovum)
• The fimbriae in the fallopian tube captures the ovum and transports it into the fallopian tube.

Fertilization

• Meiosis II is completed only with fertilization, releasing a third polar body.
• Fertilization results in a fertilized egg.
• If no fertilization occurs, the oocyte degenerates after 24 hours.
• Peristaltic movements of the fallopian tube transport the fertilized ovum to the uterus for implantation.

Prenatal Development

• Gestation is divided into three trimesters, each lasting 3 months.
• The first trimester involves cleavage, implantation, placentation, and embryogenesis.
• The second trimester is characterized by organ development completion.
• The third trimester shows rapid growth.

Blastocyst Formation

• Stages of cleavage and blastocyst formation occur during the first week after fertilization.
• Development culminates in implantation (7-10 days).

Stages of Implantation

• Stages in the implantation process (Days 6-9).

Blastodisc Organization & Gastrulation

• Blastodisc becomes two layers (epiblast and hypoblast).
• Epiblast migration leads to amnion formation.
• Hypoblast formation prepares for yolk sac development.
• Gastrulation (week 3): Formation of three germ layers (ectoderm, mesoderm and endoderm)

Fates of Germ Layers

Embryonic Membranes & Placenta

Extra Embryonic Mesoderm

Chorionic Villi

Placental Components

Uterus & Placenta

• Maternal placenta and fetal placenta components.

Chorionic Villi (details)

• Chorionic villi development and differences between primary, secondary and tertiary villus.

Primary, Secondary, and Tertiary Villi (details)