Cellular Development: Spermatogenesis and Chick Gastrulation
Spermatogenesis: Process and Stages
Spermatogenesis is the transformation of non-motile, rounded spermatids into functional, motile sperm. This process takes place in the seminiferous tubules of the testes. The tubules are lined by germinal epithelium, which contains cuboidal primary germ cells and tall Sertoli (nurse) cells.
Spermatogenesis involves two main stages:
Formation of Spermatids
This process consists of the following three phases:
Proliferative or Multiplication Phase
The undifferentiated germ cells present in the seminiferous tubules divide several times by mitosis to produce a large number of spermatogonia (sperm mother cells). Spermatogonia (2N) are of two types:
- Type A spermatogonia: Serve as stem cells and divide to form more Type A and Type B spermatogonia.
- Type B spermatogonia: Precursors of sperms.
Growth Phase
Each Type B spermatogonium actively grows into a larger primary spermatocyte by obtaining nourishment from nurse cells. They accumulate cytoplasm and replicate DNA. The growth phase is also called spermatocytogenesis, as it gives rise to spermatocytes.
Maturation Phase
The primary spermatocytes undergo the first meiotic division (reductional division) to produce haploid (N) secondary spermatocytes. This is immediately followed by the second meiotic division (equational division) to form spermatids. Thus, each diploid spermatogonium produces four haploid spermatids.
Formation of Spermatozoa (Spermiogenesis)
The transformation of spermatids into spermatozoa is called spermiogenesis or spermateleosis. The following changes occur during this process:
Changes in the Nucleus
The nucleus changes from the usual spherical shape to an elongated one. DNA is condensed, and RNA is greatly reduced.
Acrosome Formation
The Golgi body gathers in front of the nucleus to form the acrosome.
Changes in the Centrosome
The centrioles of the spermatid shift behind the nucleus. The anterior one is called the proximal centriole, and the posterior one is the distal centriole. The distal centriole gives rise to the axial filament.
Changes in Mitochondria
Mitochondria shift behind the centrioles. They form a mitochondrial spiral around the axial filament.
Changes in Cytoplasm
The cytoplasm of the spermatid is reduced to a condensed layer.
Changes in Plasma Membrane
The plasma membrane extends to surround the acrosome, nucleus, middle piece, and the main portion of the axial filament of the tail.
Gastrulation in Chick Embryo
Gastrulation in chick involves the following phases:
Formation of Endoderm
The endoderm is formed by delamination, where the lower layer of cells of the blastoderm separates or delaminates. The blastoderm now has two layers: the outer epiblast and the inner hypoblast (endoderm). The space between the epiblast and hypoblast is called the blastocoel, and that between the hypoblast and yolk is called the archenteron. The hypoblast is continuous with the extraembryonic endoderm formed by marginal cells.
Formation of Primitive Streak
After the formation of the endoderm, the presumptive mesoderm and notochord cells migrate towards the midline and pile up to form a longitudinal thickening called the primitive streak in the posterior region of the area pellucida. The circular area pellucida becomes pear-shaped as the presumptive mesoderm cells converge medially. At the anterior end of the primitive streak, a depression called the primitive pit appears, and in front of this depression, an elevation called the primitive knot or Hensen’s node develops. A narrow depression, the primitive groove, appears along the middle of the primitive streak. The primitive groove is the site through which the cells migrate into the blastocoel. There are two types of cellular movements: convergence on the surface of the embryo and divergence in its cavity.
Formation of Mesoderm
The presumptive extraembryonic mesoderm cells converge into the primitive groove. These cells migrate and form a sheet between the epiblast and endoderm on either side. The sheets of this mesoderm move outward as two horns with a wide space between them. Finally, the primitive somitic mesoderm cells also converge into the primitive groove and sink down into the blastocoel. They form two bands close to the midline and separated by the notochord.
Formation of Notochord (Notogenesis)
The presumptive notochordal cells roll over the tip of the primitive knot and sink into the blastocoel. They occupy the central position below the neural plate and between bands of somitic mesoderm. These cells form the notochord.
Formation of Neural Tube (Neurogenesis)
The two wings of presumptive neuroectoderm move towards each other and meet in the median line to form the neural plate. The lateral margins of the neural plate rise up as neural folds, which finally unite to form the neural tube. The cavity of the neural tube is called the neurocoel, and it opens out by a neuropore at the anterior end. Later, the neural tube differentiates into the brain and spinal cord.
Formation of Mesoblastic Somites
The somitic mesoderm lying close to the notochord segments transversely to form paired cubical blocks called mesoblastic somites, and these blocks lie on the sides of the notochord. During elongation of the embryo, more pairs of somites are added behind the first pair.
Formation of Coelom
The lateral plate mesoderm splits into two layers: the upper or outer somatic and the inner or lower splanchnic. The upper layer comes in contact with the ectoderm and forms the somatopleur, while the inner layer comes in contact with the endoderm and forms the splanchnopleur. The space or cavity formed between these two mesodermal layers is called the coelom.