72. Male gametes. Spermatogenesis. Structure of the spermatozoon.
Covers male gametes and how spermatozoa are produced in the seminiferous tubules, following spermatogonia through primary and secondary spermatocytes to spermatids via mitosis and meiosis. It then describes spermiogenesis, including acrosome formation, nuclear condensation, flagellum growth and mitochondrial rearrangement, before outlining the head, middle piece and tail of the mature spermatozoon. Sertoli cell support and the blood-testis barrier are also included.
Topic 72 of 127 in the MU Sofia syllabus for Cytology (Dental Medicine, Year 1). The free AceMed note covers it in 5 sections, about 716 words (5 min of reading), with 14 flashcards.
In this note
Male Gametes, Spermatogenesis, and the Structure of the Spermatozoon
Male Gametes
Spermatogenesis
Spermiogenesis
Structure of the Spermatozoon
Male Gametes
Male gametes are spermatozoa (sperm cells). They are haploid cells, meaning they contain half the diploid number of chromosomes, having one chromosome from each pair present in somatic cells.
In humans, germ cells are biological cells that give rise to gametes. These germ cells originate in the primitive streak of the embryonic disk and migrate to the developing gonads.
The testis (or testicle) is the male gonad where sperm production occurs.
Spermatogenesis
Definition: Spermatogenesis is the process by which spermatozoa are produced from male primordial germ cells through a series of mitotic and meiotic divisions. It is the first part of sperm production, including stem cell mitosis and meiosis.
Location: This process takes place in the seminiferous tubules of the male testes.
Timing: Spermatogenesis begins at puberty and continues throughout adult life. The cellular events from the final mitoses of spermatogonia to the formation of spermatids take about 2 months.
Process Steps:
1. Spermatogonia (stem cells): Small, round diploid cells (46 chromosomes) located near the basement membrane in the basal compartment of the seminiferous tubule.
Type A spermatogonia act as stem cells, dividing by mitosis to produce new stem cells and progenitor cells.
These progenitor cells, after several mitotic divisions, become Type B spermatogonia.
Type B spermatogonia undergo a final mitotic division to produce primary spermatocytes.
2. Primary Spermatocytes: These are the largest cells of the spermatogenic lineage, diploid (46 chromosomes) and with 4N DNA content. They replicate their DNA and enter Meiosis I.
Meiosis I prophase (Prophase I) is an extended stage, lasting about 3 weeks in humans, during which homologous chromosomes come together in synapsis, and DNA recombination (crossing over) occurs.
Primary spermatocytes disassemble tight junctions of the blood-testis barrier to move from the basal compartment to the adluminal compartment of the tubule.
The first meiotic division produces two secondary spermatocytes.
3. Secondary Spermatocytes: These are haploid cells (23 chromosomes, 2N DNA content), but each chromosome still consists of two chromatids. They are short-lived and quickly undergo Meiosis II.
4. Spermatids: Division of each secondary spermatocyte in Meiosis II separates the chromatids, producing two haploid cells, each containing 23 chromosomes (1N DNA content).
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