Spermatogenesis is the differentiation of primordial germ cells into spermatozoon in males (assigned at birth) and begins at puberty. I will go through some basic embryology of the development of the testes and then the actual process of spermatogenesis.
Spermatogenesis produces 4 spermatids through meiosis: A = Spermatogonium which undergoes normal mitotic division, B = resting primary spermatocyte which undergoes the 1st meiotic division, C = two secondary spermatocytes which undergoes the 2nd meiotic division and D = 4 (haploid) spermatids.
Spermiogenesis is the morphological change (change in shape) from a spermatid to spermatozoa. E = shows the formation of the acrosome, F and G = shows condensation of the nucleus and formation of a neck, middle and tail piece. H = demonstrates shedding of majority of its cytoplasm.
Brief Overview of the Embryological Development of Testes
After fertilisation occurs the zygote undergoes cleavage (division) where the number of cells increase while the size of the zygote remains the same. The zygote becomes a blastocyst and eventually becomes a bi-laminar (with an epiblast and hypoblast- this will be explained in more detail in another segment!) disk and then through Gastrulation (second image) it becomes a tri-laminar disk (with the three layers: Ectoderm, Mesoderm and Endoderm). The intermediate mesoderm will eventually differentiate into the testes via several steps. Looking at a quick overview:
- Intermediate Mesoderm differentiates into the nephrogenic cord, mesonephric duct and tubules
- Germ cells will migrate to the nephrogenic cord along the gonadal ride (proliferation of epithelial cells of the nephrogenic cord) and becomes a series of primitive sex cords
- These primitive sex cords join together and form the Testis Cordis (seminiferous cords) as seen in the 3rd image above
- The Testis Cordis remains solid until just before or at puberty when they acquire a lumen
- The 3rd histological image below demonstrates the seminiferous tubules with no lumen (indicating this slide is of a prepubertal individual) and the 6th histological image shows a clear lumen
Histology of Seminiferous Tubules
The first three histological sections are from the same testis, with increased magnification. The 3rd image shows individual seminiferous tubules that are still underdeveloped (lack of clear sertoli and leydig cells, and there is no lumen present in the centre). The last three images are of an adult male (assigned male at birth) with clear differentiated cell types and large lumen allowing for the transport of spermatozoon (immobile cells) to travel to the epididymis to mature. The last annotated image shows the progression of spermatogenesis from spermatagonia to spermatids. A = myoid cell (aids in the contraction of muscle surrounding seminiferous tubules to move spermatozoon), B = sertoli cells (apex of cell points towards lumen, aids in the spermatogenesis process), C = Primitive germ cell or spermatagonia, D = Primary Spermatocytes, E = Round Spermatids (tails facing lumen), F = Lumen of Seminiferous Tubules.
All histological sections are from the Virtual Microscopy Database where these sections are owned by M Hortsch from the University of Michigan. Links to original images can be found below:
How I Taught Myself
I have previously learnt about spermatogenesis but I’ve never learnt histology of the embryonic origins of the testes and its parts. I’m definitely a visual learner so I like to draw structures first and then compare them to histological section or gross anatomy. After drawing structures I like to look at the actual structures themselves and there word origin (most anatomical words come from either latin or greek and share similar traits based off appearance or function) this allows me to understand their origins a little better.
When learning about a new topic such as the embryology behind seminiferous tubule development, histology of the testes or the process of spermatogenesis I learn from multiple resources. I have listed several amazing resources down below which I used to understand this topic.
Resources
Jamie Chapman is a Senior Lecturer in Human Anatomy and creator of 3 Min Histology YouTube Videos. I will usually go straight to his videos to understand a histology topic rather than trying to find answers on google, online textbooks or in research papers. Both the videos below talk about the appearance of the testis (in individuals assigned male at birth).
There are other resources that I used to investigate the origin and embryonic development of the seminiferous tubules but I will hopefully go through that in more detail in another segment!
- https://embryology.med.unsw.edu.au/embryology/index.php/Testis_Development
- https://discovery.lifemapsc.com/library/review-of-medical-embryology/chapter-99-development-of-the-testis
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